On the Development of Affordable Space Travel

On the Development of Aﬀordable Space Travel

William Earley

26th March 2013

Abstract

Space travel is a dream to many, but there has been little change to the prohibitive expenses involved. By examining the history and current state of space travel it is possible to identify the reasons why we strive for space and why progress has slowed. Looking at research into cheaper, more aﬀordable launch vehicles, as well as using mathematical analysis and computer simulations to evaluate the costs involved, the transition from a mainly governmental to private space industry can be considered, and the prospects of new space startups is evaluated. Whilst it is too soon to tell for sure, private space companies are having a lot of preliminary successes, and it would appear that the already impressive cost reductions will continue as supply and demand catch up. It is important to recognise the limits of rocket-based launches though, and that even cheaper space travel needs more radical solutions, however there is currently little interest in these techniques. Nevertheless, the future looks hopeful for aﬀordable space travel and it may not be long before extraterrestrial colonies and space tourism become commonplace.

1 Contents

1 The Current State of Spaceﬂight3 1.1 History...... 3

2 Motivations 5 2.1 Opinions...... 8

3 Preliminary Analysis 10

4 Technology 13 4.1 Current...... 15 4.2 In Development...... 18 4.3 Unmanned Spaceﬂight...... 22 4.4 Theoretical...... 24

5 Conclusion 27

A Interstellar Travel 29

B Derivations 32 B.1 Rocket Equation...... 32 B.2 Adjustments...... 33 B.3 Solid-State Travel Equation...... 34 B.4 Rocket fuel eﬃciencies...... 35

C Evaluation 36 C.1 Sources...... 36 C.1.1 Academic Journals...... 36 C.1.2 Other Publications...... 37 C.1.3 Books...... 37 C.1.4 Online Technical References...... 37 C.1.5 Interviews...... 38 C.1.6 Internet...... 38

List of Figures 38

List of Tables 39

References 39

2 1 The Current State of Spaceﬂight

During a recent trip to the airport, I came across an advert from Virgin promising the opportunity to ‘travel in space from just £30’. Unfortunately, the advert was referring to upgrading to more leg room, however with the rise in new space industry startups such as Virgin Galactic and SpaceX, and the ubiquity of space tourism and exploration in popular culture, I was inspired to consider the cost of getting into space, and how far away many people’s visions of space seem to be. It’s been over forty years since the last mission to the moon, and the space shuttle has just recently been retired. Can these new startups take over from where NASA left off, and can they make any substantial reductions to costs? Even though space exploration is in its infancy, prices are still exorbitantly high and have shown little improvement, with space tourists paying upwards of $20 million, and space shuttle launches costing $1.5 billion on average. This raises questions about its viability as a commercial industry, and whether or not abundant and affordable space travel will ever be a reality. While for many, the reasons for going into space are obvious, it is important to identify the motivations for our potential future exploration of space if we are ever going to achieve a significant subsidy and reduction in costs, and it is also necessary to seriously consider alternatives to primarily rocket based launches, such as space elevators. Few would doubt that space travel could be cheaper, but by just how much? There is a substantial energy cost in moving even small masses from the surface of the Earth into orbit, and then even more so for attaining a suitably fast trajectory for interplanetary missions. There are also expensive technology and materials considerations for protecting astronauts and equipment on the journey from radiation and debris; However, by looking at aerodynamics, thinking laterally, and investigating alternative fuels, it may be possible to theorise about a space-based economy in the future, where going into space is as easy and as common as boarding an aeroplane.

1.1 History

The history of spaceflight should be seen as a culmination of thousands of years of dreams, myths, experimentation and scientific discovery. It begins with the ancient Babylonian legend of Etana and the ancient Greek myth of Icarus. Whilst these stories don’t directly address the topic of spaceflight, and have more important over-arching interpretations about origins and ambition, the fact that flight into the heavens is featured at all shows a distinct example of the human spirit and our desire to explore. Nevertheless, the first important breakthrough didn’t occur until

3 the first century, when the Chinese developed a crude form of gunpowder, and real advancements were not seen until the last millenium. The first documented use of rockets occurred in 1232, when the Chinese repelled a group of Mongolian invaders with ‘arrows of flying fire’. This battle inspired the Mongolians to develop rockets of their own, and later spread the technology to the West, where the gunpowder recipe was improved to increase the range, and new methods of rocket launching were developed to improve accuracy (Benson 2010). The next major breakthrough came in the sixteenth century, with the invention of multistage rockets by a German fireworks maker, an idea common to most modern rockets. Finally, after centuries of use in warfare and fireworks displays, a Chinese official by the name of Wan-Hu linked the use of rockets and flight, and attempted to realise the dream by strapping forty-seven rockets to his chair. Whilst his attempt was unsuccessful, it marked the first recorded use of rocketry in transportation, and was a significant milestone. With the establishment of Newton’s Laws of Motion in the seventeenth century (Newton 1687), the basic underlying science of rocketry was discovered. Meanwhile, rockets were becoming more advanced, as Germans and Russians began experimenting with rockets exceeding forty-five kilograms of mass, and the British Rocket Corps exhibiting a range of almost two kilometres. This was accompanied by a rise in the popular culture surrounding rocketry and an acknowledgement of its potential for space travel, with Jules Verne’s book ‘From the Earth to the Moon’, and H. G. Well’s ‘The War of the Worlds’. It wasn’t until the late nineteenth and early twentieth centuries, however, with Konstantin Tsiolkovky’s publication of his theories on rocket propulsion (Tsiolkovsky 1903), that the ability to escape the confines of Earth was proved, and the dream of spaceflight finally validated (Tsiolkovsky 1911). Tsiolkovsky’s influence went much further still though, as he also proposed designs for numerous critical aspects of modern spaceflight, including airlocks, multistage boosters, and even space stations. For his pioneering works, Tsiolkovsky is often considered the father of astronautics and he laid the foundations for the groundbreaking technology developed over the twentieth century. He enabled the space race, including the launch of the first men into space. The final prerequisite innovation came with the first successful launch of a liquid-fuel rocket in 1926 by Robert Goddard. Following this, the German’s produced the V-2 liquid-fuel rocket, and then, in a 1944 test flight, the first rocket reached the 100 km altitude mark, and then surpassed it by another 89 km (Reuter 2000), signifying the success of thousands of years of dreams, experimentation and scientific discovery. What followed was an eighteen year space race in which the USA and Russia competed extensively to beat each other to successive space-based

4 milestones.

1957 - Sputnik-1 becomes the ﬁrst artiﬁcial satellite, heralding the start of a world-changing communications and monitoring revolution.

1957 - Sputnik-2 takes the ﬁrst animal into space, proving the dream is possible.

1961 - Yuri Gagarin becomes the ﬁrst man into space, inspiring hundreds more to follow.

1969 - Apollo 11 takes man to the moon, and humanity walks on another world.

1976 - Viking 1 becomes the ﬁrst spacecraft to successfully land on Mars, another planet.

Over the next few years, the first spacewalks, spacecraft rendezvous, and dockings were achieved. Both sides also succeeded in launching space laboratories. Then in 1998, the first permanently inhabited structure, the International Space Station, started construction, and become a symbol for international cooperation. Unfortunately, despite all these achievements, most have simply been in the name of competition and national pride, with little attention to cost effectiveness or prolonging and advancing our exploration of space. With the end of the space race, it has been over forty years since a man landed on the moon, and repeated budget cuts and program cancellations have delayed numerous NASA missions and even led to the retiring of the Space Shuttle (Day 2011), leaving the USA without a means to transport astronauts between the ISS and the Earth, with Russia left responsible for their welfare. Although NASA plans to send men back to the moon by 2020, and newcomers China, India and Japan are also heavily invested in advancing their space programs and too reaching the moon, it is clear that our space ambitions have stagnated. Without a strong motivation, a reduction in costs, and an influx of new players to the field of spaceflight, we risk becoming despondent and withdrawing from space. How much more could we insult the legacy of Neil Armstrong, if we end up taking back that big leap forward.

2 Motivations

With the slow progress that has been seen recently in space travel, it is diﬃcult to see the potential beneﬁts that can be obtained with a more powerful space industry. For this reason, it is important to keep the motivations for space travel fresh in mind, as by developing the space industry, major advancements to the economy and technology are possible. In addition, there are many persuasive arguments for the pursuit of space travel in regards to the human spirit, fostering new businesses

5 and industries, and for future-proofing our civilisation and species against unseen threats, such as asteroid strikes. Human nature has driven us as a species to pursue numerous options which are not immediately beneficial, involving considerable cost and risk, with uncertain or even completely unknown outcomes. Nevertheless, our perseverance has enabled us to rise up from our hunter-gatherer roots to become masters of agriculture, metalwork, philosophy, art, language, and more. Not only that, but we have also developed science and mathematics as a means to explain the world around us, and through it we have invented the modern world, so that we can now communicate at the speed of light and travel around the world in mere hours. Whilst not everyone has the ambition and drive to bring about revolutionary new changes to humanity, it is a defining characteristic of our species that we are able to cooperate and push forwards into uncharted territory just because we can, and we have reaped innumerable rewards because of it. When asked why he wanted to climb Mount Everest, George Mallory replied, “because it’s there” (Mallory 1923), and if nothing else, this should be just as powerful and desperate a need as any to motivate us to explore space. Similarly, this attitude took air travel from a madman’s pursuit to the vibrant global industry it is today due to the perseverance of early investors and pioneers with flying machines. When we chronicle the history of human civilisation, we divide it into periods based on their defining characteristic. As we mastered stone and metals, we made huge advancements, and clearly we wouldn’t be in the economic position we are today without these foundations. Currently, we are living in the information age, however it is clear that, just as the renaissance and industrial revolutions came before us, space travel is to be the next defining transitionary period for humanity. While space travel may currently be prohibitively expensive and risky to consider human missions to Mars in the near future, it is vital to work on making it cheaper and safer, so that in ten or fifteen years, we will have the capability to attempt these exciting missions. Few can claim to know the meaning of life, but we have the gift of intelligence, and unless we take chances like this, we will forever be stuck on Earth, it would be a shame if we don’t push ourselves to our limits. Nevertheless, there are other genuine concerns, such as social problems and the recession, that cause some to question the validity in spending money on a space program which is certainly not essential. The problem with this view, however, is that, at least in the USA, it is practically impossible to argue that NASA is wasting valuable fiscal resources which could be better put to use on social reform. In 1966, NASA was allocated almost five percent of the federal budget, which, despite the space race, could be considered overzealous, but in 2007, NASA received just

6 0.6% of the federal budget, whilst social spending totalled at least 57%, and defence cost the USA 23% (Brooks 2007). In fact, in 2007 over 14% of the budget was spent on paying off the interest on their national debt. Clearly the idea of NASA siphoning off funds from other more worthwhile programs is misguided at best, and at worst, it is holding back what could be an incredibly powerful transition in human history. Evidence of the economic benefits derived from the aerospace industry is already available. There are thousands of artificial satellites in orbit around the Earth, and these have become a critical part of modern day infrastructure. The satellite industry has enabled continuous and accurate weather monitoring, GPS, high-speed worldwide communications, and even orbital laboratories and telescopes. According to the Organisation for Economic Co-operation and Development, the 2005 value of artificial satellites totalled between $170 and $230 billion, whilst net profit is an estimated $65 to $75 billion, with trends indicating growth in the space economy (Jolly and Razi 2007). This is accompanied by the numerous technology breakthroughs and inventions which have been developed as byproducts of the industry, ranging from water filtration systems which have been applied in aid programs in third world countries, to the highly-efficient and relatively cheap solar panels which are installed all over the world (Jones 2011). It is undeniable that the aerospace industry has already had a significant positive impact on the world economy and also on technology, and while some may argue that the benefits could be achieved in other ways without resorting to space travel, the fact is that they weren’t until we launched these satellites. It is also important to note that we have now become reliant on satellites for numerous day-to-day activities, and even temporary outages attributed to solar flares cause appreciable damages. We need to reduce the costs attributed to the space industry and increase our launch capabilities so that we can build on this infrastructure and enact quick repairs when necessary. Reasons for exploring space can also be found by looking into the future. In the short-term, there are numerous economic benefits that can be conceived. For one, there is a fledgling space tourism industry which has massive potential, just as the air travel industry has exploded over the last century. There are also more useful opportunities as well, asteroid mining offers the opportunity to extract large amounts of rare metals and resources, that though scarce on Earth, can be found in more abundant quantities in the asteroid belt. For example, Planetary Resources estimates that an asteroid just thirty metres in length can potentially provide up to $50 billion in platinum (Klotz 2012), and numerous other resources are also speculated to be present. By taking advantage of these opportunities, space travel can be reduced in costs through mining subsidies, and also by extracting rocket fuel such as hydrogen and oxygen in vast quantities.

7 Just as the gold rush attracted hordes of prospectors in the nineteenth century, so too does asteroid mining offer a massive potential to investors in space exploration, and there are already several companies working on mining projects, including Planetary Resources and Deep Space Industries. Unfortunately, however, space travel is currently prohibitively expensive for this kind of venture to be profitable, with NASA’s OSIRIS-REx mission scheduled to return just 60g of asteroid material for a mission cost of over $1 billion (“Plans for asteroid mining emerge” 2012). With increased investment and interest, however, it should be possible to consider such economic activities. This would also allow for experiments with other industries, such as space-based solar power, which could help contribute to the solution of the energy crisis by providing a powerful and renewable energy source. By establishing new settlements, we can also nurture an environment for a space-based transportation industry as well. Looking long term, more morbid scenarios become relevant. It is well known that space is a dangerous place, and events such as the asteroid impact responsible for the extinction of the dinosaurs are not uncommon. In addition, we have solar flares, supervolcanoes and resource crises to contend with, whilst our population seems to be unsustainable and numerous social problems abound. By accepting our urge to explore and encouraging the space tourism industry, we have the potential to indirectly address most of these concerns via permanent settlements on another celestial body. This would provide a destination for excess populations to go to, an increase in the availability of resources, mass new economic opportunities, and a refuge for our species from potential disaster. Furthermore, by augmenting our space presence, we will also gain experience and technology crucial to diverting and preventing these potential cataclysmic events, by for example developing satellites capable of changing the course of an asteroid. These situations may seem extreme, however they are very real possibilities, and if other reasons are insufficient, perhaps impending doom will motivate us to pursue space travel.

2.1 Opinions

The recent rise in space startups over the last decade has led to increased innovation in propulsive technologies, and promises of cheap, abundant and affordable space travel in the near future. In fact, during a recent live Google+ Hangout Interview with Elon Musk (CEO of SpaceX), Charles Bolden (NASA Administrator), and hosted by John Yembrick (Director of Strategic Communications at NASA), regarding the first cargo resupply mission of the Dragon spacecraft, I was fortunate enough to participate by sending in a question to John regarding Elon’s opinion on the affordability of space travel.

8 John: And the next one is from, on Twitter [sic YouTube], @1tswill1: “How cheap can space travel become? Will it ever be as aﬀordable and ubiquitous as air travel?”

Elon: I sure hope so, it’s fundamentally more expensive to go to space, I mean energy requirements are much greater and everything, so there is certainly a difference. The base difference is, well, I mean it’s like a thousand times more expensive, if not more to go to space than to take an air trip, and a thousand is a huge difference, I mean perhaps it can be brought down to being only ten times more expensive, I think that should be achievable, but that of course would require a two order of magnitude improvement in space transport, but that’s what I think, that needs to happen, and I think it can happen if we can make rapidly and fully reusable spacecraft and rockets. (Musk 2012)

Elon later went on to say that for this to happen, a pivotal breakthrough in rocket reusability was needed. Others are not as optimistic. In an article for Forbes, Michio Kaku expressed his despondency on the current state and cost of space exploration (Kaku 2009). He notes that at the beginning of the space race, president Eisenhower laid out a roadmap for the exploration of space, with the intention of developing a rapidly reusable and efficient spaceplane for agile access to space. Unfortunately, NASA has not made much progress in driving down the cost of space travel, and its infrastructure has been largely dismantled in the wake of the end of the Apollo missions, with space travel left prohibitively expensive. Kaku seemed pessimistic on the capacity of the commercial sector to take on the entrepreneurial spirit. Nevertheless, Kaku’s pessimism may be misguided. Virgin Galactic’s plans for renewing the space tourism industry is well underway, with the possibility of flights beginning as early as 2013, and many passengers have already purchased tickets. According to Business Insider, a trip on Virgin Galactic’s SpaceShipTwo is just $200 thousand, compared to between $20 and $35 million a decade ago (Lubin 2012); however, whether this price can be delivered, and how much further prices can be reduced, is uncertain. Will Whitehorn, President of Virgin Galactic, believes that prices can be further cut from $200 to $100 thousand in a period of just 3-5 years, but that anything further is unlikely, especially to $10, 000 (Whitehorn and Lam 2009), the level Elon Musk thought to be achievable. More recently, though, Richard Branson, founder of Virgin Galactic proposed that, in the near future, “space travel could be nearly as commonplace as, say, travelling to another continent is” (Lubin 2012), implying that the $10, 000 mark was a possibility. In an interview with CBS, he also considered the possibility of exploiting advances in space propulsion to achieve hypersonic air travel, for the possibility of flights from the US to

9 Australia in less than two hours (“Richard Branson on space travel” 2012). The problem is that the majority of space startups are still primarily focused on suborbital flights. Even though they are breaching the edge of space for significantly cheaper prices, costs to reach orbit remain exceedingly high, and trips to the ISS still come in around $50 million (Lubin 2012). This, however, is most likely attributed to the lack of commercial competition in this area. If innovation and competition can be stimulated, then we can also hopefully look forward to similar improvements in orbital flight costs and efficiencies.

3 Preliminary Analysis

To compare and analyse the effectiveness and costs of different space launch technologies, it is useful to establish a standard basis case around which analysis of different technologies can be conducted and compared. Please note, I have derived a set of equations and commented on their applications and limitations in the appendix, sectionB, which may be worth reading before this section and beyond to better understand the mathematics involved. Low Earth Orbit is an ideal standard to use for this as it is a highly advantageous position for a spaceport to be placed. The reasons for this are numerous:

It is fairly close to the Earth, so it is quick to get to, and if destinations further away are needed then it is an ideal place to put a refuelling station.

Most satellites orbit in LEO, so any calculations will have real-world applications to a common scenario.

LEO is the closest stable equilibrium point in space to the Earth, and once in a stable equilibrium, other gravitational ﬁelds can be largely ignored, greatly simplifying missions to elsewhere in the solar system.

There are other positions near the Earth that are also being considered for positioning spaceports, most interestingly perhaps, the first Lagrangian (L1) point of the Earth and the Moon, a gravitationally neutral point in which there is no attraction of an object to either the Earth or the Moon. This point is not as stable as LEO orbit however. LEO is also useful for this analysis as it represents the most costly part of any space mission: Overcoming Earth’s gravitational field and thick atmosphere. The energy expenditures against gravity and in entering orbit are quite easily calculated as the Earth is approximately spherical and thus the specific energy is given by:

10 GM r V = 2 − 2r r + h Energy lost due to drag is not so easily accounted for, however, as it varies depending on the surface area and aerodynamic qualities of the vehicle, air temperatures and densities, angle, speed, weather and numerous other difficult to predict variables. To simplify things, I have used data from a simulated Saturn V launch (Braeunig 2010), and one of a set of equations I have derived to analyse spaceflight, equation6 in the appendix, to generate a drag force profile for a typical rocket. Further, this profile can be used to calculate the energy expended against drag,

ED, which can then be used in equation7. The original data only included the drag force due to atmospheric friction and did not include the drag force due to pressure differences at the engine nozzles. To account for this, I developed my own generalised spacecraft launch simulator in the Python programming language (Earley 2013), using the Akima (1970) method for interpolating the reference data, and entered in the launch events as calculated by Braeunig (2010). The data I produced was then integrated using a R trapezium rule interpolation in order to compute the total energy lost due to drag, ED = D ds. This launch profile is shown in figure2, and the launch trajectory for the first 712 s in figure3. Now it is possible to generate a number of characteristic values for this basis case, as shown in figure1. The final detail is the altitude, which for LEO can vary from 200 km to 2000 km, translating to a specific energy from 32.2 MJ kg−1 to 38.7 MJ kg−1. Taking the altitude as 1160 km:

Quantity Value Altitude h 1160 km −1 Speciﬁc Mission Energy sj 36.0 MJ kg

Drag Energy ED 97.0 GJ

Figure 1: LEO Proﬁle

From this data, it is then possible to analyse different spacecraft by their fuels and efficiencies, using equations5 and7 depending on the desired accuracy of the comparison, and8 for technologies like ion drive where the energy generation is internal and the exhaust mass is negligible. The given ED value should be a good estimate for any large rocket, and the impact is small enough that any deviations shouldn’t be too significant; however, for other types of launch vehicle, such as space planes, new simulations must be carried out in order to maintain accuracy. With these suitable values calculated, the mf equations can then be adapted to compare costs - taking sp as

11 1 2 Figure 2: Saturn V Typical Drag Proﬁle, D = 2 CDAρv + RsAeρT 5

2 Drag Force/MN

0 0 50 100 150 200 250 300 Distance Travelled/km

t=700 t=600 t=500 t=400 t=300

t=200

t=100

0 50 100 150 200 250 Altitude/km

Figure 3: Saturn V Launch Trajectory between t = 0 and t = 712 s. The small central circle represents the Earth, with the polar gridlines equidistant from the Earth. The trajectory is annotated with the time at which each point was attained, in seconds.

12 the speciﬁc price of fuel per kilogram:

k p0 = sp e − 1 Launch cost per kilogram of payload (1)

sp k p0 pD = ED e − 1 = ED Additional ﬁxed launch cost due to drag (2) sj sj ED pΣ = p0 1 + Combined launch cost per kilogram (3) sjm0 These equations can also determine the impact of drag on payload launch prices, and thus can show where improvements need to be made, whether in fuel choice and eﬃciency, or in aerodynamic qualities. It can be shown that the percentage increase in price due to drag is

ED/sjm0. So, taking the Saturn V as an example, and with the simulated 64 300 kg LEO payload mass, the impact is shown to be just 4.19 %, but if launch costs are already in the thousands of dollars per kilogram, even a four percent impact can increase the budget by many millions of dollars, so it is important to minimise drag when designing a rocket or spacecraft, even though most of its mission will take place in a vacuum.

4 Technology

Since the invention of the liquid-fuel rocket and the space race, there has been little innovation in space travel. We’re still using technology from the 1960s for practically all of our rockets, and most of the fuels currently in use today date from the mid-1950s. We have gained significant experience and made improvements to the designs, but overall, there has been a lack of innovation or drive, leading to unsustainable costs, increasing evermore with the economic inflation over this period. Numerous proposals have been put forth over the years, such as the Eisenhower spaceplane, for agile and cost efficient launch systems, but the most widely used rocket today, the Soyuz, dating from 1966, is based on the Russian R-7 rocket (which is thirteen years older still) and uses RP-1 rocket fuel developed in the 1950s. Nevertheless, the actual method of propulsion is not the most significant problem in achieving affordable space travel. Taking the Saturn V as an example and applying the simulation to a mission to Low Earth Orbit instead of the Moon, we can calculate the following values for the costs due to fuel: Using a weighted average, we get the value $7.057 kg−1 for a LEO mission, and surprisingly, using the actual fuel values for the complete lunar mission, the total launch cost is $7.206 kg−1. Of course, these are 1969 prices; adjusting for inflation, we get $44.28 kg−1 and $45.22 kg−1 respectively. Although these prices seem high, costing nearly $3000 just to launch an average person into space, it is nothing when compared to the actual launch costs. For this same Saturn

13 −1 −1 Stage m0 / kg sj / MJ kg cf / MJ kg Isp / s k pΣ / $/kg S-IVB 64 283 19.9 23.43 435 1.3886 1.836 S-II 174 456 17.3 20.89 429 1.4368 1.766 S-IC 655 584 3.08 12.05 304 0.4049 0.042

Table 1: Calculated Saturn-V per-stage launch costs, sp values extrapolated from Wade (2011a), individual EDs recalculated from the simulation

V launch, the total cost was $185 000 000 (Heppenheimer 1998), adjusting for inflation and taking the Apollo 11 payload mass, this translates to a launch cost of $18 059 kg−1. Even if we use instead the maximum supported payload mass of 118 000 kg (Wade 2011c), the launch cost is still very high at $9838 kg−1. These values show the cost for launching just one person is between $610 000 and $1 120 000, let alone the supplies needed to keep each alive. To explain the discrepancy, we must look at what else goes into launching a rocket. Whilst the fuel costs on their own are fairly expensive in themselves, with the most costly part of the above analysis due to the price of hydrogen, these expenses can be expected to fall in the near future due to the progress towards a hydrogen economy, catalysed by the potential for clean energy in cars. In addition, there are many cheaper fuels, and despite the lower efficiency, they are still cost effective, such as RP-1. The extra several thousand dollars per kilogram, however, comes from a combination of the following:

Materials - The cost of materials which can withstand the wild temperature and pressure extremes experienced by a spacecraft, and which are used in bulk, can be high. These can be reduced through recycling and improving reusability in spacecraft, which is already becoming a focus of the space industry.

Labour - Building rockets is a very specialised profession due to the expense and slow nature of the space industry currently. This is likely to fall on its own, however, as startups such as SpaceX create manufacturing lines and enforce cost eﬃciency.

Technology - Spacecraft are very high-tech vehicles, needing unparalleled reliability and advanced computing resources to ensure that no problems are encountered, the correct trajectory is achieved, and life support systems remain functional. As was seen in the Challenger disaster, even a small crack can cause catastrophic damage, so redundancy, monitoring, and smart software is essential. These costs will likely follow the trends that have been seen in computing, and will fall as demand increases.

14 Research and Development - Space travel is still a relatively new innovation, and as such there is still a lot of experimentation to do, experience to be gained, and technology to develop, which is part of the reason for high launch costs as nearly all space launches are performed by government agencies. New space companies, however, are more proﬁt-focused and so will be more eﬃcient and help drive down costs for routine missions, whereas governmental space agencies can continue to develop new technologies to advance our capabilities in space, such as by developing new propulsion systems.

That these account for amounts sometimes exceeding a billion dollars, however, is hard to comprehend. Much of this seems to be due to high labour and infrastructure costs - Russian rockets can have LEO launch costs as low as $3409 kg−1 (Space Transportation Costs 2002) as these expenditures are cheaper over there (Note, the Shtil has been excluded because its launch costs are heavily subsidised and anomalous).

4.1 Current

These four areas of high expenditure are confirmed by London (1994). In his report, London elaborates on the difference between Russian and US spacecraft to explain the significant difference between their launch costs. Whilst US rockets are built to a specification, minimise weight, and maximise performance, Russian rockets are constructed by mixing and matching a number of stages which have been in production for decades with minimal changes. These stages may be described as crude and bulky, however due to the rapid production times and simplicity of design, these four major expenditures are in fact much lower than for US rockets. Meanwhile, the increase in weight results in a negligible increase in price due to the low cost of fuel, especially as Russia tends to use RP-1 instead of liquid hydrogen, which is available much more readily and is cheaper. Overall this gives a significantly lower launch cost, and also a much higher reliability. London quotes 99.73 % reliability for Vostok boosters, compared to 90 % for US Atlas boosters. This is despite Russian launch sites experiencing temperature ranges from −48 ◦C to 41 ◦C, blizzards, and high winds, whereas US launch conditions are highly specific and launches can be cancelled for 30 mph winds. Nevertheless, it still costs $63 000 000 for Russia to launch a man to the ISS (Atkinson 2013), or at least that’s how much they are charging NASA, so clearly there is still major room for improvement in costs, and a combination of Russia’s simplicity, reliability and rapid production levels with US optimisation and efficiencies should be used to reduce costs. All current space launch vehicles are combustive reaction engines, and these are the launch system we have the most experience with. Therefore, the near-future development of cheaper

15 space travel will naturally revolve around optimising the use of these motors. There are three main fuel types:

Solid fuel rockets are the oldest type of rocket and date back to at least the twelfth century with the Chinese use of rockets. They are the simplest type of rocket, and almost all model rockets made today are solid fuel due to this fact, and the relatively low cost of making them. They are the least powerful however, and the most dangerous. Their entire casings must be able to withstand the incredibly high temperatures and pressures experienced during combustion, and must be designed to be both suﬃciently strong and ﬂexible, or else catastrophic explosion is possible, as typically happens with 1 % of solid rocket boosters. Finally, once constructed and cast, a solid rocket is susceptible to spontaneous ignition, and once ignited, they are practically impossible to extinguish.

Liquid fuel rockets were first seriously proposed by Tsiolkovsky (1903), but the first successful liquid-fuelled rocket was not launched until 1926. Liquid fuel rockets are much more complicated than solid rockets, due to the need to maintain low, sometimes cryogenic, temperatures of the fuel and oxidiser, keep them in separate tanks, and pump them quickly enough to the motor, among other engineering difficulties. There are, however, myriad benefits to liquid rockets. Liquid fuels have much higher specific impulses, they are typically far cheaper than solid fuels, liquid oxidisers are more efficient, and the only part of the motor that needs to withstand high temperatures is the nozzle. In addition, liquid fuel rockets are much more suitable for reusable rocket designs.

Hybrid rockets are the most recent concept, and are being tested by some space startups. They combine a solid fuel with a liquid oxidiser to try to compromise between the two other rocket types. The liquid oxidiser helps maintain a high speciﬁc impulse, while the solid fuel makes the rocket much simpler to design. Nevertheless, the rocket still requires some of the high engineering to support the oxidiser, the entire rocket must be built to withstand extreme conditions due to the solid fuel, and reusability isn’t a realistic option.

Overall, despite the greater engineering complexity, it is fairly obvious that liquid fuel rockets are safer, more powerful, more efficient, and potentially more cost effective than other types, which is why nearly all rockets launching into space use liquid fuels, the most common of which were developed between the mid-1950s and the mid-1960s: RP-1 (a highly refined form of kerosene), liquid hydrogen, UDMH and hydrazine. Of these, liquid hydrogen with a liquid oxygen oxidiser

16 (LOX/LH2) is the most powerful, and is used in most US rockets, while RP-1 is the cheapest and is used in nearly all Russian rocket stages. Most Russian rockets follow a design developed in the 1950s with the R-7 missile. Although the R-7 wasn’t very effective as a weapon, it was much more useful for space launches, and rockets such as the Vostok and Soyuz are considered to be part of the R-7 rocket family (Wade 2011b). This is consistent with Russian rocket design, to develop a series of suboptimal boosters and stages, and combine them together in various configurations for different missions (London 1994). In general, these rockets have two stages, however occasionally a third stage is developed, such as the Molniya and Fregat, for reaching Molniya and GSO orbits respectively. This modular design has enabled lower development costs, and production rates peaked at 60 Soyuz rockets per year in the 1980s. It is no surprise, then, that they are considered the most reliable and widely used rockets in the world (ESA 2004). Although Russian launch vehicles have seen steady improvement, US technology has been erratic. In the 1960s, the US rapidly developed the Saturn V for use in the Apollo missions, and set several world records, as it was the tallest, heaviest rocket built and supported launching the biggest payload, up to 120 000 kg. These records, however, have not since been broken, even by NASA. With its decommissioning in 1972, NASA was left without a powerful rocket, and had to use an older Saturn IB in order to operate their Skylab space station. The next big development came with the Space Shuttle, intended to be a mostly reusable launch vehicle providing quick and frequent access to space, with missions estimated to cost just $9.3 million per flight ($55 million, adjusting for inflation) (“The Space Shuttle” 1973), with a relatively low launch cost of $1400 kg−1 (adjusted); However, in the end the budget became bloated, and the program was extended, keeping the 1980s technology in service through 2010, giving an ultimate cost of around $1500 million per flight (Pielke and Byerly 2011), and a launch cost of over $66 000 kg−1 (adjusted). Nevertheless, the Space Shuttle was a powerful addition to the US space industry, with a maximum payload capacity of 22 700 kg (Wade 2011d), capable of routinely transporting astronauts to the ISS, enabling space based repairs of satellites and telescopes, and was the first spacecraft to land on a runway. It was also fairly reliable, with a launch success rate of 99.26 %. With NASA’s dependence on the Shuttle, however, and the numerous extensions to its original 15 year planned lifetime, its retirement in 2011, and the delays in developing a replacement, have left NASA without any means to transfer astronauts between the Earth and the ISS, and thus reliant on the Russian Soyuz.

17 4.2 In Development

The current most promising developments in space travel originate with the termination of the Space Shuttle program. To replace it, NASA has put into development the Space Launch System (SLS), NASA’s next multipurpose heavy launch vehicle, which will eventually surpass the Saturn V in capability. NASA is also investing in and working with numerous private aerospace startups in order to promote the commercial space industry and support NASA’s future missions. The private space sector is where cheap and affordable space travel gets most interesting, as rockets such as the SpaceX Falcon are already providing far cheaper access to space than NASA has been able to achieve. These startups are also promising audacious plans to open up wider access to space, and deploy cheap missions to Mars. The Space Launch System, which began development in 2011 and is scheduled to first launch in 2017, is NASA’s continuation of the Apollo program and its solution to future exploratory missions. It is to be a two stage non-reusable rocket, and similarly to the Soyuz, there will be a variety of upper stage configurations enabling many different mission profiles. Initially, it will have a 70 000 kg payload (Kyle 2013), and its first mission will be an unmanned trip to the Moon, followed by a manned lunar flyby (Bergin 2011). After this, new upper stages are to be developed, eventually extending its launch capacity to 130 000 kg with the Earth-Departure Stage, in preparation for manned Martian missions around 2030. This new payload capacity would surpass that of the Saturn V. The SLS is also designed for missions to near-Earth asteroids and Lagrange points, with development costs estimated at $18 billion, and launch costs targeted at $500 million for a possible $7100 kg−1 launch cost. If successful, this program will be very exciting and will provide the same, or even greater, inspiration as the Apollo missions in the mid-20th century. Unfortunately, many see the program as flawed by US Senate requirements, and predict development costs will balloon to over $40 billion, with the viability of scheduled missions uncertain (Strickland 2011). In addition, although the US plans for manned missions to Mars would beat other countries’ plans, they may not be the first to reach Mars. With the accelerating progress and achievements of commercial space startups, plans for missions to Mars as early as 2018 cannot be ignored. It must be remembered, however, that the only organisation to have currently succeeded in landing spacecraft on the surface of Mars is NASA, and overzealous development of manned missions should be carefully scrutinised for safety and reliability. As for more local missions, NASA’s encouragement of private spaceflight is developing a low-cost infrastructure for orbital launches, satellite repair, and ISS crew transportation. NASA has partnered with Boeing, Sierra Nevada Corporation, and SpaceX via its Commercial Crew

18 Figure 4: An artist’s rendition of the SLS at the launchpad (NASA Announces Design for New Deep Space Exploration System 2011)

Development program (CCDev) in order to achieve these goals (Turnbough 2012), but the most prominent of these has been SpaceX. SpaceX, headed by CEO Elon Musk, has been very successful in developing a cost-effective and as-of-yet reliable launch vehicle. The Falcon 9 rocket, a two stage RP-1 fuelled rocket with up to a 13 150 kg payload to LEO (Kyle 2012) has already been launched five times. Its costs are very impressive - while NASA estimated SpaceX would need $3.6 billion to develop the rocket, an amount which NASA then pledged to fund the company with, SpaceX came in at around just $300 million, and are able to launch for just $54 million (Musk 2011b). Elon also plans on improving these prices by making both stages fully reusable. On top of developing the Falcon 9, SpaceX is also famous for their Dragon spacecraft. Dragon was the first private spacecraft to rendezvous with the ISS and to be recovered from orbit, and since October 2012, has been fulfilling a NASA contract for regular cargo re-supply missions to the ISS. This ambition is accompanied by a US grant to develop a crewed Dragon variant, which is scheduled for first launch in 2015 with a cost of $20 million per seat (Musk 2011a), compared to the Soyuz’s $63 million. What is more impressive is the fast manufacturing schedule: SpaceX is currently building a new Dragon and Falcon 9 every three months, and there are plans to double the rate of Dragon builds in the near future (Chow 2010). SpaceX is a case study of ambition and success in the Space industry, and they are leading the way in affordable space travel, along with

19 Virgin Galactic’s plans for suborbital travel, showing that even though governments can struggle to keep to their budgets, mass reductions in launch costs are feasible, whilst remaining safe and redundant, as is the fault-tolerant design of the Dragon. This view is bolstered by SpaceX’s rapid development cycle, which intends to launch Falcon Heavy, a rocket with a 53 000 kg payload and $2200 kg−1 launch cost by late 2013, and a low-cost, 1-tonne payload, Mars lander (Red Dragon) in 2018 for a sample return mission (Wall 2011), which with its current successes, is not unreasonable. It seems that Elon’s view that space travel costs can be brought down to just ten times that of air travel is certainly a possibility if commercial development continues at this pace.

Figure 5: A photograph of a SpaceX Falcon 9 launch from Cape Canaveral, Florida, carrying a Dragon spacecraft (Falcon 9 Overview 2012)

The other NASA contracts have not been as publicised, however they too are developing cheaper launch vehicles. SpaceDev, a subsidiary of Sierra Nevada Corporation is developing a crewed spaceplane, the Dream Chaser, for transferring crew to and from the ISS, similar to the Space Shuttle. Boeing is also developing a reusable spaceplane, the X-37, but it is a remotely controlled vehicle designed for satellite maintenance and repairs, among other missions, and has already had several successful launches, although these appear to be contracted by the US military, and much of their mission details remains classiﬁed. Development of space propulsion systems is also being pursued in the UK, for both cheaper access to space and for faster, potentially hypersonic, air travel. Reaction Engines Limited has been pursuing the design of a single-stage-to-orbit, fully reusable launch vehicle with mass

20 scalability and enormous efficiencies of up to 3500 s specific impulse (Hempsell and Longstaff 2010). This is being achieved through the use of a single, multipurpose engine named SABRE. SABRE is a liquid hydrogen burning engine, however instead of obtaining its oxygen internally, it gets most of its oxidiser from the atmosphere, operating as a jet engine whilst climbing to space, before transitioning to a rocket engine after reaching speeds of Mach 5. The secret to SABRE’s development was in creating a precooler capable of reducing the temperature of incoming air to just −150 ◦C in a mere 0.01 s. With this innovation, Skylon, with a maximum payload of 15 t or up to thirty astronauts at a time, is expected to have a launch cost of £650 kg−1, or under $1000 kg−1, even lower than that of SpaceX. Of course, it is still short of funding, and so these figures are simply projected and won’t necessarily come to fruition, but it shows that by pursuing alternate methods and using lateral thinking, major improvements are possible. There are a number of other theoretical launch concepts that could reduce the costs still further, which I will discuss in the next section, that avoid rocket-based propulsion altogether. The other eminent UK-based enterprise is Virgin Galactic. Building off of Scaled Composites’ SpaceShipOne, a reusable vehicle which enabled the first privately funded human spaceflight, Virgin Galactic is working with The Spaceship Company (TSC), to develop SpaceShipTwo (twice the size of SS1), and possibly SpaceShipThree later on. These vehicles are carried to high altitudes by a carrier aircraft, from which they use a hybrid rocket motor to achieve speeds of over Mach 3 and travel suborbitally. The intention is for SpaceShipTwo to provide space tourists with cheap access to the edge of space. In fact, tickets are being sold for just $200 000 (Space Tickets - Virgin Galactic 2013), and there are already hundreds of paid passengers, with the first flight scheduled for 2013. If successful, this is to be followed by SpaceShipThree, which will take advantage of suborbital spaceflight to offer unprecedentedly quick intercontinental travel times, with a trip from London to Sydney estimated at just two hours. Ultimately, Richard Branson wishes to use these spacecraft to increase the availability of space travel and make it affordable for middle class families. Assuming an average mass of 62 kg per person, giving a current launch cost of $3200 kg−1, this seems feasible. With SpaceShipOne and Two, TSC has presented another interesting solution to cheaper space travel, this time with tourism in mind, and the enterprising innovations obtained during development are further evidence that the future of space travel is likely to be cheap and prosperous. The influx of new space travel solutions has prompted a number of realistic, yet ambitious plans for space missions in the near future. Among these are two for manned travel to Mars. Dennis Tito is in talks with current commercial aerospace companies to arrange for a 2018 manned

21 Mars ﬂyby with a married couple, whilst Mars One plans to establish a permanently habited Martian base by 2023 using current technology. Additionally, Virgin’s Richard Branson has stated a desire to start a Martian colony within his lifetime (“Richard Branson on space travel” 2012). What this shows, is that the dream of manned exploration of the solar system and colonisation has by no means deteriorated in the wake of the cancellation of the Apollo program, and in the short time that private space companies have been operational, plans to achieve these goals have been quickly put into eﬀect. It is thus very probable that we will see rapid growth in the aerospace sector in the near-future, and extraordinary missions like these will become commonplace.

4.3 Unmanned Spaceﬂight

When considering unmanned spaceflight however, there are a number of exciting technologies in development that have already had successful tests and hold promise for cheaper space-based technology, such as satellites and space probes. Technology that’s currently making major headway is in the form of CubeSats. Whereas most satellites are fairly large and use specially developed components, raising costs into the millions or even billions, CubeSats allow for those without as much expendable funding, such as universities, to also gain a presence in space. The CubeSat specification defines these as small cubes of side 10 cm, with a mass ≤ 1.33 kg (CubeSat Design Specification 2010). This enables launch costs to be brought down to just $40 000 on average but the major benefit is the encouragement of using off-the-shelf components. Using technology such as smartphones and other pre-manufactured components means that typical design to launch costs range from just $65 to $80 thousand (David 2004). This is possibly the cheapest available method to launch a satellite currently available, and many can be launched at a time. CubeSats represent the appeal of Space travel to many, cheap and affordable access to the frontier, although the current applications are mostly academic. Another technology with major potential which is starting to be used in real space missions is ion drive. The problem with chemical propulsion is that it is difficult to use very efficiently, and the amount of energy stored is fairly low, compared to sources such as nuclear or solar. Electric propulsive methods hold a lot of promise for highly efficient engines, and ion drives have been built to satisfy this need. Whilst maximum specific impulse for rockets is typically 450 s, for ion drives the impulse typically reaches thousands of seconds, up to 11 200 s with MPDT thrusters. They work via various methods to ionise, accelerate and expel a gas at high speed, thus accelerating the spacecraft due to Newton’s third law of motion, and can typically run continuously for many thousands of hours. The problem is that, unlike rockets which typically have thrusts on the

22 order of millions of Newtons, ion drives are much less powerful and rarely exceed even 1 N. The advantage is in the high efficiency and long run times, allowing for high speeds to be built up over a long period. The other benefit is that, because of the high efficiency, very little reaction mass has to be carried to be expelled, and the low powers can be supplied by solar energy instead of an internal mechanism, allowing satellites and spacecraft to be lighter, and cheaper to launch. The most common electric propulsion method currently in use is the Hall Effect Thruster, which powers hundreds of satellites to stabilise their orbits and reaches up to 75 % efficiency. It is also smaller than the original gridded ion thruster design. More powerful motors are also being designed; the MPDT thruster mentioned before has been tested up to 3.8 MW (compared to kilowatts for most engines), producing an 89 N thrust. The true test of this technology was with NASA’s Deep Space 1 mission, designed to test many new space technologies. It was the first long ion drive mission, and used it to perform flybys of an asteroid and comet despite generating just 92 mN of thrust. Although experiencing several problems, it also had artificially intelligent software which it used to fix itself each time and succeed in its missions (Deep Space 1 2013).

Figure 6: A depiction of the VASIMR ion drive mechanism (VASIMR System 2011)

If these highly eﬃcient yet low power engines are going to become more commonplace, perhaps in cargo missions, then a complementary part of it will likely be the Interplanetary Transport Network (ITN). This network is a map of gravitational assists and Lagrange points present in

23 the Solar System which allow for very low-energy transfer orbits between diﬀerent objects. An example of its use was in the Japanese Hitlen probe - although having only 10 % of the fuel needed for a trans-lunar injection, it was able to reach the Moon using an ITN transfer (Nerlich 2010). The main problem is the long duration of trips, a trans-lunar injection takes just 3 days to reach the Moon, but the ITN transfer used extended the trip to 5 months. There are uses for such long journeys, however, when considering cargo missions. Cargo ships often take months to complete their journeys, yet are sent regularly enough to provide a continuous supply of their relevant products, and the same could be useful when permanent settlements are established throughout the Solar System to provide cheaper access to resources and products needed by the pioneers.

4.4 Theoretical

While it is true that reaction engines are currently our best method of propulsion in space, rocket- based space launch costs can only be reduced so far in the near future, and there are numerous theoretical launch systems which anticipate greatly reduced costs and improved efficiency for the transition from Earth to orbit which should be considered. In addition, there are also a number of proposals for slower but more efficient propulsive devices in space. If successful, these will prove crucial in our exploration of space, by allowing for cheaper transport, at least for cargo. Human missions may still require faster trips due to the high amounts of radiation present in space, though. The main problem with rocket-derived launch systems, is that because of the low energy densities of chemical fuels, the exponential term in equations5 and7 can become quite large as more fuel begets more fuel due to increasing mass. Ultimately, as well as increasing launch costs due to fuel costs reaching as high as $45 per kg of payload as calculated in section4, costs then increase to accommodate a self-sufficient, reliable fuel tank and rocket motor to house this fuel and turn it into kinetic energy. If the majority of the necessary energy could instead be provided from an external system based on Earth, then much of this could be avoided:

As most of the fuel is not carried by the spacecraft, far less energy needs to be supplied.

There are a lot fewer restrictions on the energy supplies that can be used: A rocket must use a chemical fuel which burns well and produces a large amount of hot gas to expel at high velocities in order to provide substantial thrust, whereas an external system can take advantage of any existing energy infrastructure present on Earth, such as the electrical grid,

24 or an on-site nuclear power station, for example.

This also greatly improves the safety and reliability, as there is no chemical fuel to explode, making catastrophic failures unlikely, although some launch systems may be more risky than others.

Unlike reusable rockets, which must be refurbished after use to ensure no damage has been sustained under the extreme conditions experienced during ﬂight, an external launch system is much easier to design low-maintenance reusability into, and thus is another vector for reduced launch costs.

While these beneﬁts are generic to most ground-based launch systems, there are a number of problems associated with each proposal which have reduced the incentives for their developments. Chief among these is that, for most of the proposed systems, technology and materials currently fall short of the required speciﬁcations, as well as needing high levels of funding for research and development not currently available to such risky and untested designs. Nevertheless, they hold promise for improving our space launch capabilities.

One of the systems that is more well known is the space elevator. This concept involves constructing either a tower or suspending a tether that runs from the surface of the Earth to an altitude above geostationary orbit. Along this, a craft would be able to climb up and down to efficiently deliver payloads and transport humans to geostationary orbit. The major impediment to actually constructing one on Earth, however, is that until the late 20th century, no known materials existed with the compressive strength necessary for a tower, or the tensile strength to support a tether on a planet with as strong a gravitational field as Earth, rendering the concept infeasible on this planet, although it may have been possible to consider lunar or Martian elevators. Recently though, with the development of kevlar, carbon nanotubes and boron nitride nanotubes has prompted new discussion, including by NASA (Price 2007), on the possibility of using space elevators on Earth. Nevertheless, many obstacles remain, including the difficulty in constructing nanotubes around a hundred million meters in length, and the likelihood of collisions with orbiting satellites requiring extensive maintenance. If these problems, among others, can be solved though, prices as low as $22 kg−1 are possible (Foundation 2008), or even just $1.50 kg−1 according to the NASA article. Some of these problems have already been solved by modifying the concept, such as with the space fountain design.

25 An alternative to ground-based launch systems involves linear accelerators. The mass driver is a conceptual rail gun which uses powerful magnetic fields to quickly accelerate payloads to orbital velocities. The launch track would be between one and several hundred kilometres long, and lie flat on the ground with a ramped section at the end to launch the payload upwards. With this system, electricity costs could be as low as $1 kg−1, assuming high efficiencies, a 36 MJ kg−1 energy requirement as calculated for an LEO mission profile, and a typical $0.10 /kWh electricity cost. Of course, prices would be higher to accommodate maintenance and development costs, but without the overhead of expensive new nanotube materials, and with the technology needed already well established, costs could prove much cheaper than those posited for space elevators. There are problems however, a vacuum must be maintained to eliminate drag, and a crash at the high velocities experienced could be catastrophic. A similar method, the launch loop, may be less problematic, using a maglev-type interface instead of a railgun, although it has its own problems, in that the amount of energy stored whilst in operation is the same as released by a 350 kiloton bomb.

Whilst the above was not an extensive list of all ground-based launch proposals, some of the most popular and well-researched are detailed. These methods only provide for launching payloads into orbit however, for interplanetary missions, eﬃcient propulsion systems are needed. Whilst these would not be suitable for human missions, due to the minuscule accelerations available (refer back to the discussion of ion drives), they would be ideal for a cargo transport network, for stabilising orbits, or for altering trajectories of dangerous objects, due to the long-term results.

Solar sails have been pondered in science ﬁction since Jules Verne’s ‘From the Earth to the Moon’. They operate on the constant radiation pressure provided by the Sun, as the momentum carried by the light produced can be transferred to a spacecraft if a large enough area is intercepted. Because of the low power of this method of propulsion, however, to obtain any usable thrust, lightweight materials are essential. Ideally, the material should be very thin, strong enough to support a large surface area without ripping, and opaque enough to capture most of the incident radiation. Even still, accelerations are typically measured in millimetres per square second for any realistic proposals, but for applications where this technology is relevant, the costs are only for development and construction - no further fuel is needed, as the energy is provided by the Sun for free, although it is less powerful further from the Sun.

A more active approach is possible with magnetic sails, which harness the charge of the

26 solar wind using a powered magnetic field to generate thrust. More exotic technological innovations are also possible, such as M2P2 (“Mini-magnetospheric Plasma Propulsion” 2000), which generates a magnetosphere not dissimilar to that around geologically active planets, and can sustain consistent levels of thrust anywhere in the solar system due to the adaptive nature of the plasma bubble, as well as being more energy efficient than current ion drives and having a high specific impulse.

A more local and more powerful concept is in laser propulsion. By aiming a high-power laser at propellant released from a spacecraft, the propellant can be heated to high temperatures and produce considerable thrust, enabling for highly efficient travel (up to several thousand seconds of specific impulse) wherever the laser is able to focus on the vehicle. Additionally, because all the energy is supplied from Earth, efficiencies are even higher, and as much power can be supplied as is possible with current laser technology. It may even be possible to use lasers to launch a vehicle into space or along intercontinental hypersonic missions by heating the air behind it, as is proposed for ‘LightCraft’ (Hsu 2009).

5 Conclusion

With regard to the development of affordable space travel, the future is uncertain yet optimistic. It is unlikely, however, that these developments will originate with governmental space agencies, such as NASA. This is because the somewhat legitimate objections to government spending are overshadowing the appeal of the benefits of space travel and constricting the abilities of NASA to operate effectively. Nevertheless, those who are motivated by the opportunities available are enthusiastically entering the private sector to be a part of the burgeoning space industry, and the changes that are precipitating are clear. While the US and Russia have pursued cheaper space travel, and quoted such figures as $1400 kg−1, for example for the Space Shuttle in 1973, launch costs have failed to breach $3409 kg−1 for LEO, and this has been discouraging to the dream of space travel. On the other hand, SpaceX, in its eleven years of operation, has managed to achieve launch costs for its Falcon 9 rocket of between $54 and $1300 million, and its promised launch costs of just $2200 kg−1 for its Falcon Heavy rocket in 2013 seem attainable with its track record, especially considering its Dragon development costs fell well short of the NASA estimation. If SpaceX is successful in its endeavour, this will be an exciting development, as it will show that affordable space travel is possible, and will hopefully prompt greater investment and interest in the space industry, with a rise in space startups. It must be remembered though that $2200 kg−1

27 still doesn’t represent an affordable launch cost for many people. Whilst governments, large businesses and millionaires can account for such large expenditures, the cost for space tourism for an average 62 kg person is at least $136 000 , an amount greater than the annual salary of 99.58 % of the world’s population (Global Rich List 2013), let alone disposable spending money. Clearly costs need to fall much further, however it is too premature to worry that costs won’t continue to fall, considering only one private company has gone into operation so far. Taking into account Skylon’s estimate of $650 kg−1, and Richard Branson’s goal to make Space Travel affordable to middle class families, potentially on a whim, it may not be too unreasonable to expect these prices to fall sharply in the next decade or two, as the economics of supply and demand balance each other out in the new industry. Certainly, as supply increases we can expect launch costs to get considerably closer to the estimated $45 kg−1 for purely fuel costs. This will require greater investment in fully reusable spacecraft, however this is already Elon Musk’s plan for the Falcon 9, and considering that a rocket is inherently simpler than a jet engine, and fuel costs will decrease as rockets improve in efficiency, unit costs approaching the $30 to $90 million of a Boeing 737 is a realistic figure for a reusable launch vehicle, especially taking into account that the current non-reusable Falcon 9 costs $54 million. As has been established, though, rockets aren’t necessarily the most efficient launch vehicle, due to the need to carry vast amounts of heavy fuel with the spacecraft, and so it is important for even cheaper space travel to be investigated once a basic space industry is established. For this to occur, I would recommend that investment and research into Earth-based launch vehicles and orbital fuel depots is prioritised within the space industry, as it will greatly boost our capabilities and capacities in space and accelerate our exploration. If we visualise a future of routine spaceflight, interplanetary missions, lunar and Martian colonies, with manned missions taking place to explore the rest of the solar system, we begin to see the importance of cheap space travel to humanity. We need to be more ambitious and enthusiastic, like the pioneers of the New World, and develop the technology and infrastructure to take advantage of the vast universe around us. Millions were captivated and inspired by the Moon landing, but disasters and cutbacks have slowed our progress, I believe it is important that we re-engage with our exploratory instincts, and with the current state of private space travel, it appears that we are still striving for the future.

28 Appendices

A Interstellar Travel

Whilst the prospects of cheap space travel throughout our solar system are exciting, the really interesting aspects of space travel are in the solutions we are investigating in order to achieve interstellar travel. Travelling beyond the solar system is an intriguing thought to many for numerous reasons. Obviously there is the need to explore, but there are also scientific opportunities and prospects for discovering alien life. There is literally an entire universe to explore! The problem with this, however, is that it is difficult to travel these long distances. To overcome this problem, we must examine Einstein’s theory of relativity, or look at theoretical methods of sidestepping the issues. Consider a journey to our nearest star system, Alpha Centauri, a distance of 4.37 light years away. To understand how large these interstellar distances are, we can calculate that, travelling at a motorway speed of 70 mph would take almost 42 million years, while Voyager 1, one of our fastest space probes, would take over 70 thousand years to reach this destination. In fact, even going at the fastest speed possible in this universe, 671 million miles per hour, would take 4.37 years, and reaching this speed is physically impossible due to relativistic effects described by Einstein. Clearly, this presents a problem - using any technologies we currently know of, interstellar travel is still guaranteed to take many years at least, and there is certainly little hope of these spacecraft returning due to the immense amounts of time that will have passed, assuming we travel to a more meaningful destination many more light years away. However, if we accept that these voyages are likely to be one-way, there are ways to make the trip easier for the crew and passengers. Another of Einstein’s relativistic effects is time dilation; what this means is that as a spacecraft approaches the speed of light, time appears to slow down. Travelling at 50% of the speed of light, c, would reduce the apparent travel time to 87% of the actual journey time, at 90%c, the time is 44%, and at 99.9%, it is just 4.5%. In fact, the closer you get to c, the less time you experience, and it is possible to reduce the travel time to however little you want, the only problem is the vast amounts of energy needed. In fact, it is impossible to reach the speed of light, as an infinite amount of energy is needed, however, there are a number of proposed solutions, as have been elaborated in New Scientist (Marshall 2009). We have already discussed ion drives, however interstellar travel is where this technology really shines. With its ability to run continuously for a long time, providing a constant acceleration

29 with highly efficient use of fuel. This constant acceleration is the important bit, as over a long time the results can be powerful. For an engine providing a constant acceleration of 1g, and travelling to Alpha Centauri, with half the trip spent accelerating and the other half decelerating to stop at the system, the maximum speed that can be achieved is over 95%c. This is incredible, and even though current ion drives are incapable of accelerations this large, future research on megawatt and gigawatt engines may make this feasible someday. In addition, the 1g acceleration provides the added benefit of artificial gravity. This low acceleration would mean that reaching these high speeds would take a long time, however, and so even though the top speed gives a time dilation of 31%, the journey will actually take much longer. More powerful nuclear propulsion methods could be more effective, by taking advantage of some of the most energy dense fuels in the universe, apart from antimatter, to reach these speeds faster. Of course, this would result in a constant multi-g acceleration, and it is unknown what the effects on the human body are of constant high accelerations, although there is some research going into this at NASA (The Pull of Hypergravity 2003); ultimately, it may not be worth it for the few extra percent of c that can be achieved. There is still a problem, however, in that even using the most efficient fuel sources, reaching speeds close to c requires quantities of energy that are impractical to take on board in fuel form. This is where more exotic methods come in (Chown 2009). The dark matter starship discussed involves a hypothetical method to annihilate the substance theorised to be abundant, though almost imperceptible, in the universe. It would be a kind of ramjet, moving at high speeds into this mysterious medium, and forcing it to release its energy, propelling the spaceship forwards. Unfortunately, however, it must be accepted that we know little of dark matter, and so it is difficult to theorise on whether this is even possible. In contrast, the other New Scientist article mentions the Bussard ramjet, which goes for a more conventional nuclear option. Unfortunately, calculations doubt its effectiveness. The other interesting idea is a black hole starship, taking advantage of the Hawking radiation produced by a million tonne black hole, though while our technology is closer to making this a possibility, it is still an immensely difficult undertaking. Ultimately, however, I think it is disappointing to just accept that current technology occludes the possibility of travelling faster than the speed of light to other destinations, with relatively efficient fuel consumption and the ability to return to Earth, as without the possibility of remaining in contact with the rest of humanity, the benefits become less clear and there are far fewer incentives. Luckily, there has been considerable research into the theoretical possibility of faster than light (FTL) travel. One of the most exciting is the Alcubierre drive (Alcubierre 2000).

30 Figure 7: A depiction of a warp bubble - the mechanism of an Alcubierre drive

Inspired by the warp drive in Star Trek, Alcubierre developed a theoretical model for creating a warp bubble and using it for space travel. The original design has several complications, however. Warp travel works by bending the fabric of spacetime, which is allowed by Einstein’s theory of general relativity, to condense the space in front of the vehicle and expand it behind, causing motion (see ﬁgure7 for a simple depiction of the method). The most interesting part is that because there is no acceleration, and spacetime can move faster than light, the vehicle is able to reach its destination faster than light would, and there are no time dilation eﬀects, so Earth wouldn’t have advanced thousands of years further into the future if the travellers decided to return. Many problems have been encountered trying to bring it into reality, though, as the energy requirements seemed astronomical in size, and it seems that on reaching its destination, the warp bubble would cause catastrophic destruction of its immediate environment (McMonigal, Lewis, and O’Byrne 2012). Nevertheless, NASA seems less than deterred, and is actively researching warp drive (White et al. 2011). They have already developed theories to reduce the energy requirements, and are using interferometry to attempt to detect microscopic instances of warp bubbles. If there’s any hope left for realistic interstellar missions, this is it, as the theory seems to be legitimate and the problems are slowly being eliminated through research, although currently it seems to be highly impractical. The only other option is wormholes, and these are generally considered too unstable to use without the discovery of some unknown kind of matter. Here’s

31 hoping for developing a usable method of interstellar space travel, as otherwise we may be stuck within our own solar system for the rest of our existence, unable to experience and explore the wonders that exist beyond.

B Derivations

B.1 Rocket Equation

Suppose there is a rocket of mass M = mf + m0, where mf is the mass of fuel, and m0 is the mass of the actual rocket and payload. The fuel can provide a specific energy, sf , and the rocket has a fuel to energy efficiency η. The rocket undertakes a journey, and it is known that the specific energy required is given by sj, and it is desired to minimise the amount of fuel needed.

Let cf = ηsf be a constant denoting the amount of recoverable energy from a given fuel, v denote the initial rocket velocity, ∆v denote the change in velocity of the rocket,

ve = Ispg0 denote the eﬀective exhaust velocity (Benson 2008):

where Isp is the speciﬁc impulse,

and g0 is the acceleration due to gravity at the surface of the Earth.

First, it is useful to establish the conservation of momentum, which will come in useful later.

m0v + mf v = m0(v + ∆v) + mf (v + ve)

0 = m0∆v + mf ve

Now let’s look at the conservation of energy. We have the kinetic energy of the rocket before plus the gain in energy from the fuel is equal to the new sum of kinetic energies. We also note that we want to provide the rocket with an energy of E = m0sj to enable the journey.

1 2 1 2 1 2 1 2 2 m0v + 2 mf v + cf mf = 2 m0(v + ∆v) + 2 mf (v + ve) 1 2 1 2 cf mf = 2 m0∆v + v(m0∆v + mf ve) + 2 mf ve 1 2 1 2 = 2 m0∆v + 2 mf ve m s m = 0 j f 1 2 cf − 2 ve s k = j (4) 1 2 cf − 2 ve

32 We can now apply the identity for the rocket constant we have just deﬁned in equation4.

Also, as mf is the change in mass, and is a loss, we have the identity mf = − dm, and m0 is the ﬁnal mass, so if we replace it with m, we can then integrate from M to m0 to derive an expression for the entire journey.

− dm = km Z m0 1 M − dm = k = ln M m m0 k M = m0e

k mf = m0 e − 1 (5)

Note that for these equations to provide a meaningful result, the denominator of k must be 1 2 positive, that is, cf > 2 ve , which basically means the amount of energy that can be obtained per kilogram of fuel must be greater than the kinetic energy lost to each kilogram of fuel, or else there will be no thrust. It is also assumed that the rocket will burn fuel at a fast enough rate to provide enough thrust to overcome gravity and drag.

B.2 Adjustments

The rocket equation that has been derived is valid for any situation in which the energy expenditures are proportional to the mass of the object. There are three factors which this situation neglects to consider, however:

Drag due to air resistance

Variance in thrust, and hence speciﬁc impulse

The reduction in the eﬀective speciﬁc impulse due to atmospheric pressure

1 2 The drag due to air resistance can be expressed as FD = 2 CDAρv , where CD is the drag coeﬃcient, A is the projected area of the vehicle, ρ is the ﬂuid density, and v is the vehicle velocity. Clearly this force is not proportional to the rocket’s mass. Once ignited, solid fuel rocket thrust cannot be controlled (Braeunig 1996-2008); however, the thrust can be varied by shaping the grain before ignition, resulting in separate burn phases with a short transition period. This is similar to the use of multiple stages in a rocket, and so this variance in thrust can be handled by using the equation multiple times for each phase, and this technique can also be used to model a liquid fuel rocket in a similar way Another consideration is

33 the transition period between phases and in stage ignition, in which there is also a variance in thrust. This has been shown by Braeunig (2010) in his simulation of a Saturn V launch to be negligible, as despite ignoring its effect, the results were still accurate to real-world data, and so it shall also be ignored in this equation. The final adjustment concerns the change in specific impulse. The thrust of a rocket is described by the following equation (Benson 2008):

dm T = v + (p − p )A dt x e 0 e

dm Where dt is the mass flow rate, vx is the exhaust velocity, pe is the exhaust pressure, p0 is the atmospheric pressure, and Ae is the area of the nozzle opening. Specific impulse is ratio of thrust to mass flow rate, and is typically quoted for vacuum and sea level conditions. By factoring in our assumption of full thrust, the ideal gas law P = ρRsT , and acknowledging that a vacuum implies an atmospheric pressure of 0, we can now restate the thrust as two separate forces:

dm T = g I F = p A = R A ρT dt 0 spvac P 0 e s e

As the thrust is now effectively constant, we can now consider specific impulse and hence effective exhaust velocity, ve, to be constant. Notice that FP is also not proportional to the rocket’s mass. This refactor of the rocket situation gives us a new term involving all the forces which do not vary with mass: 1 2 D = 2 CDAρv + RsAeρT (6) R S The work done to overcome this force, ED = 0 D ds, where s is the distance travelled and S is the final distance, cannot be factored in to the original rocket equation as it is independent of the varying mass, and so we must derive a new set of equations thus: ED k ED ED k M = m0 + e − mf = m0 + e − 1 (7) sj sj sj

B.3 Solid-State Travel Equation

If the fuel is not expended after use, then the amount required is diﬀerent and much simpler as we don’t have to consider the change in mass over time.

The energy required to launch a mass mn is given by E = sjmn, and the energy provided by

sj fuel of mass mn+1 is E = ηsf mn+1, so equating the energies and solving, we get mn+1 = mn . ηsf

34 s Let k = j be a constant deﬁning the mass of fuel needed to launch a given mass ηsf

We can now obtain a geometric progression by adding fuel for the initial mass, m0, and fuel for this new mass, and for that, etc.

M = m0 + m1 + m2 + m3 + ...

2 3 = m0 + km0 + k m0 + k m0 + ...

2 3 = m0 1 + k + k + k + ... m = 0 ∀|k| < 1 1 − k m k m = 0 (8) f 1 − k This provides an interesting result. Unlike with the changing mass rocket equation, where 1 2 if a fuel can satisfy the inequality ηsf > 2 ve then it can power a rocket on any journey, there is a restriction on the types of fuel that can be used for a given mission. In the changing mass equation, a higher sj will simply require more fuel, for this new equation however, if the value of k is greater than or equal to one, i.e. sj ≥ ηsf then the value of M will diverge to inﬁnity!

lim M = ∞ sj →ηsf

B.4 Rocket fuel eﬃciencies

In order to calculate the rocket equations above, it is necessary to obtain the efficiency at which potential energy is converted into useful work done. Unfortunately, for rockets, these figures are usually unavailable, as rockets are compared using specific impulse, which ignores the differences in chemical potential energy between fuels. We can calculate a rough estimate however, by modelling a rocket as a heat engine. If we use the Carnot cycle model, then we can obtain a maximum possible efficiency, ηc, however in practice this is often unattainable. Alternatively, we can use a Novikov engine model to get a more realistic efficiency (Callen 1985), ηn. r TC TC ηc = 1 − ηn = 1 − TH TH

Where TC is the temperature of the cold sink in kelvin, which is either the ambient temperature, or the boiling point of the propellant, whichever is lower,

TH is the combustion temperature of the propellant.

35 One final consideration is how close the rocket engine can come to these figures. Typically, due to the very high temperatures in the engine and the high amount of engineering, the actual efficiency comes very close to these limits, over 90% of ηn can be expected for solid fuels, and perhaps even 99% for liquid propellants due to the high amount of control possible over the combustion. Therefore, by looking up the relevant temperature data (Wade 2011a; Martinez-Sanchez 2012; Braeunig 1996-2008), we can calculate typical expected efficiencies for several different rocket propellants.

Temperatures / K Fuel Type ηc ηn Expected η Cold Sink Combustion Solid 298 ∼3500 0.915 0.708 0.65-0.70

LOX / LH2 20 2958 0.993 0.918 0.90-0.92 Kerosene 90 3670 0.975 0.843 0.78-0.83 Hydrazine 294 3250 0.910 0.699 0.65-0.70

Table 2: A table to show the calculation of a few expected rocket propellant eﬃciencies.

C Evaluation

C.1 Sources

I used a variety of sources for this report, and by classifying them by type, their reliabilities can be eﬀectively assessed.

C.1.1 Academic Journals

Some of the articles from academic journals I referred to include Pielke and Byerly (2011), “Mini-magnetospheric Plasma Propulsion”(2000) and “The Space Shuttle”(1973). I consider these to be some of the most reliable sources used in my essay, as the journals each of these articles have been posted in are peer-reviewed, and thus have gained acceptance and credibility within academia. Using peer-reviewed sources is important, as they have been checked for mistakes and errors by a number of anonymous, independent and (usually) impartial experts to verify their validity. In addition, they are primary sources, and so even if they have errors, they can give a good insight into the research going on at time of publication.

36 C.1.2 Other Publications

I have also cited from other publications, such as Universe Today, Popular Science, and The Space Review, among others. Although these are not peer-reviewed, and there is a possibility of bias, I have included them because they are generally considered to be high quality publications, and their articles seem to be fairly reliable. Whilst Universe Today is a blog, it has gained a popular following among space and astronomy enthusiasts, and can be relied upon to provide accurate reports on current events. The Space Review, on the other hand, is published online weekly, and its essays and articles are reviewed before publication. Whilst not having the inherent reliability of a peer-reviewed journal, its works often come from well-respected authors in their ﬁelds. Popular Science falls somewhere in the middle in terms of reliability, although it is not necessarily the most impartial source. Other publications I have referred to have a reliability somewhere inside this range, and so I consider these citations to be quite useful, although for the most part they are secondary sources.

C.1.3 Books

Some of the books cited include Newton (1687) and Tsiolkovsky (1903). These too are primary sources, and given the more visible nature and their presence in academia, are often more scrutinised for their scientiﬁc value by publishers, authors, and others than in peer-reviewed articles. They also contain more content, and usually represent a long research project or academic work, and so can often provide much more useful information than a brief article. It is easier, however, for a less reputable author to publish inaccurate information, and so it was important to ensure that the author is respected in their ﬁelds before citing them, which all the authors of the books I have referenced appear to be.

C.1.4 Online Technical References

For some of my analyses, I have found it necessary to obtain data and mechanical equations. For this purpose, I have made several references to http://braeunig.us, a collection of data and rocket mechanics for use in simulation, compiled by Robert Braeunig from 1997 onwards. Similarly, Encyclopaedia Astronautica (http://astronautix.com) has been compiled by Mark Wade and lists numerical and historical data for numerous rockets, launches, and other technology. Both of these references include full bibliographies and source data, and though secondary, I consider them to be highly reliable. In addition, I have also referenced mechanics from NASA’s Glenn Research Centre, published by Tom Benson (see Benson (2008) and Benson (2010)), and

37 considering NASA’s long history in space research and development, and being a primary source, it can be considered to be highly accurate. I have also referenced non-technical information from NASA’s and ESA’s (the European Space Agency) websites, and for the same reasons I consider these sources to be reliable.

C.1.5 Interviews

I have included information from a few interviews in this report, and though they should be considered primary sources, the reputation of the publication has to be considered as well, as some of the interviews may be paraphrased, out of context, or summarised. The most reliable of these is that of Musk (2012). This was a Google+ Hangout, published on youtube, and so it is easy to observe that the interview is real and uncut, especially as I participated in it live. An interview with Richard Branson, see “Richard Branson on space travel”(2012), I also consider fairly reliable as it is also accompanied by a video clip. On the other hand, the interview with Mallory (1923) is less reliable as the original article is unavailable, and only a summary is provided, however as I cited this source merely for a non-essential opinion, this lack of reliability is not too important. The citation from Whitehorn and Lam (2009) is also less reliable and useful, as it has been published on an obscure technology blog, which has had legal problems in the past. In addition, the views apparently expressed by Will Whitehorn seem to have been superseded by Richard Branson’s statements in the CBS interview. Nevertheless, it provided an interesting commentary on the evolution of opinions, and is not a crucial part of this report.

C.1.6 Internet

Finally, I have also cited several papers and speciﬁcations only available online, such as CubeSat Design Speciﬁcation (2010), however these papers have been written at reputable universities or posted on websites of important space industry companies, and so they are primary sources and also very useful. Furthermore, I have published my simulation software (see Earley (2013)) online, and have included a citation for future reference, and as I am the author, I consider it a highly useful source.

List of Figures

1 LEO Proﬁle...... 11 1 2 2 Saturn V Typical Drag Proﬁle, D = 2 CDAρv + RsAeρT ...... 12

38 3 Saturn V Launch Trajectory between t = 0 and t = 712 s. The small central circle represents the Earth, with the polar gridlines equidistant from the Earth. The trajectory is annotated with the time at which each point was attained, in seconds. 12 4 An artist’s rendition of the SLS at the launchpad (NASA Announces Design for New Deep Space Exploration System 2011)...... 19 5 A photograph of a SpaceX Falcon 9 launch from Cape Canaveral, Florida, carrying a Dragon spacecraft (Falcon 9 Overview 2012)...... 20 6 A depiction of the VASIMR ion drive mechanism (VASIMR System 2011).... 23 7 A depiction of a warp bubble - the mechanism of an Alcubierre drive...... 31

List of Tables

1 Calculated Saturn-V per-stage launch costs, sp values extrapolated from Wade

(2011a), individual EDs recalculated from the simulation...... 14 2 A table to show the calculation of a few expected rocket propellant eﬃciencies.. 36

References

Akima, Hiroshi (October 1970). “A new method of interpolation and smooth curve ﬁtting based on local procedures”. In: Journal of the Association for Computing Machinery 17.4, pp. 589–602. url: http://student.ndhu.edu.tw/~u9111023/akima.pdf (Retrieved: February 22, 2013). Alcubierre, Miguel (September 2000). “The warp drive. hyper-fast travel within general relativity”. In: Classical and Quantum Gravity. doi: 10 . 1088 / 0264 - 9381 / 11 / 5 / 001. url: http : //arxiv.org/abs/grqc/0009013 (Retrieved: March 7, 2013). Atkinson, Nancy (March 2013). “How Will Sequestration Aﬀect NASA?” In: Universe Today. url: http://www.universetoday.com/100377/how-will-sequestration-affect-nasa/ (Retrieved: March 1, 2013).

Benson, Tom (July 2008). Speciﬁc Impulse. NB: Benson uses Veq for the eﬀective exhaust velocity,

which here is denoted by ve. url: http://www.grc.nasa.gov/WWW/k-12/airplane/specimp. html (Retrieved: November 8, 2012). — (February 2010). Brief History of Rockets. url: http : / / www . grc . nasa . gov / WWW / k - 12/TRC/Rockets/history_of_rockets.html (Retrieved: January 7, 2013).

39 Bergin, Chris (July 2011). “Preliminary NASA plan shows Evolved SLS vehicle is 21 years away”. In: url: http://www.nasaspaceflight.com/2011/07/preliminary-nasa-evolved-sls- vehicle-21-years-away/ (Retrieved: March 7, 2013). Braeunig, Robert A. (1996-2008). Basics of Space Flight: Rocket Propellants. url: http://www. braeunig.us/space/propel.htm (Retrieved: October 20, 2012). — (July 2010). Saturn V Launch Simulation. url: http://www.braeunig.us/apollo/saturnV. htm (Retrieved: November 18, 2012). Brooks, Jeff (July 2007). “Putting NASA’s budget in perspective”. In: The Space Review. url: http://www.thespacereview.com/article/898/1 (Retrieved: February 19, 2013). Callen, Herbert B. (October 1985). Thermodynamics and an Introduction to Thermostatistics. 2nd ed. John Wiley & Sons, Inc. isbn: 978-0471862567. Chow, Denise (December 2010). “Q & A with SpaceX CEO Elon Musk. Master of Private Space Dragons”. In: url: http://www.space.com/10443- spacex- ceo- elon- musk- master- private-space-dragons.html (Retrieved: March 7, 2013). Chown, Marcus (November 2009). “Dark power. Grand designs for interstellar travel”. In: New Scientist. url: http://www.newscientist.com/article/mg20427361.000-dark-power- grand-designs-for-interstellar-travel.html (Retrieved: March 7, 2013). CubeSat Design Specification (March 2010). Design Specification. California Polytechnic State University. url: http://www.cubesat.org/images/developers/cds_rev12.pdf (Retrieved: March 2, 2013). David, Leonard (September 2004). “Cubesats: Tiny Spacecraft, Huge Payoffs”. In: Space.com. url: http://www.space.com/308-cubesats-tiny-spacecraft-huge-payoffs.html (Retrieved: March 2, 2013). Day, Dwayne A. (July 2011). “The decision to retire the Space Shuttle”. In: The Space Review. url: http://www.thespacereview.com/article/1887/1 (Retrieved: February 18, 2013). Deep Space 1 (February 2013). url: http://science.nasa.gov/missions/deep-space-1/ (Retrieved: March 7, 2013). Earley, William (February 2013). Space Launch Simulator. Python. Version 0.1α. url: https: //dl.dropbox.com/u/1760961/SLS/index.html. European Space Agency (April 2004). Soyuz Launch Vehicle. The most reliable means of space travel. url: http://www.esa.int/Our_Activities/Human_Spaceflight/Delta_Mission/ Soyuz_launch_vehicle_The_most_reliable_means_of_space_travel (Retrieved: March 1, 2013).

40 Falcon 9 Overview (June 2012). url: http :/ /www . spacex. com/ falcon9. php (Retrieved: March 26, 2013). Foundation, The Spaceward (2008). The Spaceward Foundation. url: http://www.spaceward. org/elevator-ride (Retrieved: March 4, 2013). Global Rich List (March 2013). url: http://www.globalrichlist.com/how.html (Retrieved: March 18, 2013). Hempsell, Mark and Roger Longstaﬀ (January 2010). Skylon Users’ Manual. url: http:// www.reactionengines.co.uk/tech_docs/SKYLON_User_Manual_rev1-1.pdf (Retrieved: February 26, 2013). Heppenheimer, Thomas A. (June 1998). The Space Shuttle Decision. Chap. 6. isbn: 978- 1588340146. Hsu, Jeremy (July 2009). “Laser-Powered Lightcraft ‘At the Cusp of Commercial Reality’”. In: Popular Science. url: http://www.popsci.com/military- aviation- amp- space/ article/2009-07/high-powered-lightcraft-experiments-hint-future-space-travel (Retrieved: March 5, 2013). Jolly, Claire and Gohar Razi (November 2007). The space economy at a glance 2007, pp. 14–15. isbn: 978-9264031098. Jones, John (May 2011). NASA Technologies Beneﬁt Our Lives. url: http://spinoff.nasa. gov/Spinoff2008/tech_benefits.html (Retrieved: February 19, 2013). Kaku, Michio (July 2009). “The Cost Of Space Exploration”. In: Forbes. url: http://www. forbes.com/2009/07/16/apollo-moon-landing-anniversary-opinions-contributors- cost-money.html (Retrieved: February 17, 2013). Klotz, Irene (April 2012). “Tech billionaires bankroll gold rush to mine asteroids”. In: Reuters. url: http://www.reuters.com/article/2012/04/24/us- space- asteroid- mining- idUSBRE83N06U20120424. Kyle, Ed (December 2012). SpaceX Falcon Data Sheet. url: http://www.spacelaunchreport. com/falcon9.html (Retrieved: March 7, 2013). — (January 2013). Space Launch System Data Sheet. url: http://www.spacelaunchreport. com/sls0.html (Retrieved: March 7, 2013). London III, Lt. Col. John R. (October 1994). LEO on the Cheap. Methods for Achieving Drastic Reductions in Space Launch Costs. Research Report. Air University. Chap. 5. url: http: //www.dunnspace.com/leo_on_the_cheap.htm (Retrieved: March 1, 2013).

41 Lubin, Gus (September 2012). “Commercial Spaceﬂight Is Getting Really Cheap, Really Fast”. In: Business Insider. url: http://www.businessinsider.com/commercial-spaceflight- is-getting-really-cheap-really-fast-2012-8 (Retrieved: October 4, 2012). Mallory, George (March 1923). “Climbing Mount Everest is work for Supermen”. In: New York Times. An interview with George Mallory following his Mount Everest expedition. Marshall, Michael (December 2009). “Engage the x drive. Ten ways to traverse deep space”. In: New Scientist. url: http://www.newscientist.com/article/dn18283-engage-the-x- drive-ten-ways-to-traverse-deep-space.html (Retrieved: March 7, 2013). Martinez-Sanchez, Manuel (August 2012). “Lecture 17-18: Solid Propellants: Other Topics”. url: http://ocw.mit.edu/courses/aeronautics-and-astronautics/16-512-rocket- propulsion- fall- 2005/lecture- notes/lecture_17_18.pdf (Retrieved: October 20, 2012). McMonigal, Brendan, Geraint F. Lewis, and Philip O’Byrne (February 2012). “The Alcubierre Warp Drive. On the Matter of Matter”. In: url: http://arxiv.org/abs/1202.5708v1 (Retrieved: March 7, 2013). “Mini-magnetospheric Plasma Propulsion” (2000). “Tapping the energy of the solar wind for spacecraft propulsion”. In: Journal of Geophysical Research 105 (A9). Musk, Elon (April 2011a). SpaceX wins NASA contract to complete development of successor to the Space Shuttle. url: http://www.spacex.com/press.php?page=20110419 (Retrieved: March 7, 2013). — (May 2011b). Why the US can beat China. The facts about SpaceX costs. url: http://www. spacex.com/usa.php (Retrieved: March 7, 2013). — (October 2012). “NASA & SpaceX Hangout”. From a transcript of a live interview featuring Elon Musk, in which he answered my question approximately 22 minutes in. url: https: //www.youtube.com/watch?v=ogJee-ys5Bw (Retrieved: October 5, 2012). NASA Announces Design for New Deep Space Exploration System (September 2011). url: http://www.nasa.gov/exploration/systems/sls/sls1.html (Retrieved: March 2, 2013). Nerlich, Steve (March 2010). “The Hitchhikers Guide To The Solar System”. In: Universe Today. Astronomy Without A Telescope. url: http://www.universetoday.com/58934/ (Retrieved: October 4, 2012). Newton, Isaac (July 1687). Philosophiæ Naturalis Principia Mathematica. isbn: 978-0674664753. Pielke, Roger and Radford Byerly (April 2011). “Shuttle programme lifetime cost”. In: Nature 472 (7341), p. 38. doi: 10.1038/472038d.

42 “Plans for asteroid mining emerge” (April 2012). In: BBC News. url: http://www.bbc.co.uk/ news/science-environment-17827347 (Retrieved: February 20, 2013). Price, Steve (September 2007). Audacious & Outrageous: Space Elevators. url: http://science. nasa.gov/science- news/science- at- nasa/2000/ast07sep_1/ (Retrieved: March 4, 2013). Reuter, Claus (May 2000). The V2, and the Russian and American Rocket Program, p. 170. isbn: 978-1894643054. “Richard Branson on space travel” (September 2012). In: CBS News. url: http://www.cbsnews. com/8301-505263_162-57514837/richard-branson-on-space-travel-im-determined- to-start-a-population-on-mars (Retrieved: October 1, 2012). Space Tickets - Virgin Galactic. url: http://www.virgingalactic.com/overview/space- tickets/ (Retrieved: March 7, 2013). Space Transportation Costs (September 2002). Trends in Price Per Pound to Orbit 1990-2000. White Paper. Futron Corporation. url: http : / / www . futron . com / upload / wysiwyg / Resources/Whitepapers/Space_Transportation_Costs_Trends_0902.pdf (Retrieved: February 28, 2013). Strickland, John (November 2011). “The SLS: too expensive for exploration?” In: The Space Review. url: http://www.thespacereview.com/article/1979/1 (Retrieved: March 3, 2013). The Pull of Hypergravity (February 2003). url: http://science.nasa.gov/science-news/ science-at-nasa/2003/07feb_stronggravity/ (Retrieved: March 7, 2013). “The Space Shuttle” (February 1973). “NASA’s White Elephant in the Sky”. In: Bulletin of the Atomic Scientists, p. 39. Tsiolkovsky, Konstantin (1903). The Exploration of Cosmic Space by Means of Reaction Devices. isbn: 978-5458581592. — (1911). “Investigation of Outer Space Rocket Devices”. In: Vestnik Vozdukhoplavaniva. Turnbough, Linda (March 2012). CCDev Information. url: http://www.nasa.gov/offices/ c3po/partners/ccdev_info.html (Retrieved: March 7, 2013). VASIMR System (August 2011). url: http://www.adastrarocket.com/aarc/ImageGallery (Retrieved: March 3, 2013). Wade, Mark (November 2011a). Encyclopedia Astronautica. url: http://www.astronautix. com/props/index.htm (Retrieved: October 20, 2012).

43 Wade, Mark (November 2011b). R-7. url: http : / / www . astronautix . com / fam / r7 . htm (Retrieved: March 2, 2013). — (November 2011c). Saturn V. url: http : / / www . astronautix . com /lvs / saturnv . htm (Retrieved: October 20, 2013). — (November 2011d). Shuttle. url: http://www.astronautix.com/lvs/shuttle.htm (Re- trieved: March 2, 2013). Wall, Mike (July 2011). “‘Red Dragon’ Mission Mulled as Cheap Search for Mars Life”. In: url: http://www.space.com/12489-nasa-mars-life-private-spaceship-red-dragon.html (Retrieved: March 7, 2013). White, Harold et al. (December 2011). “Eagleworks Laboratories. Advanced Propulsion Physics Research”. In: url: http://ntrs.nasa.gov/search.jsp?R=20110023492 (Retrieved: September 23, 2012). Whitehorn, Will and Brian Lam (May 2009). “Virgin Galactic’s Boss Says Space Travel Will Never Be Cheap”. In: Gizmodo. An unveriﬁed telephone interview transcript. url: http: //gizmodo.com/5239883/virgin-galactics-boss-says-space-travel-will-never-be- cheap (Retrieved: October 4, 2012).