Nov, 2010 DEPARTMENT OF REUSABLE LAUNCH SYSTEMS AEROSPACE ENGINEERING Soumik Bose Keshav Kishore Anand Indian Institute Of Technology, Kharagpur 1 INTRODUCTION A hundred years ago, on December 17, 1903, Wilbur and Orville Wright successfully achieved a piloted, powered flight. Though the Wright Flyer I flew only 10 ft off the ground for 12 seconds, traveling a mere 120 ft, the aeronautical technology it demonstrated paved the way for passenger air transportation. Man had finally made it to the air. The Wright brother’s plane of 1903 led to the development of aircrafts such as the WWII Spitfire, and others. In 1926 the first passenger plane flew holiday makers from American mainland to Havana and Bahamas. In 23 years the world had moved from a plane that flew 120 ft and similar planes that only a chosen few could fly, to one that can carry many passengers. In October 1957, man entered the space age. Russia sent the first satellite, the Sputnik, and in April 1961, Yuri Gagarin became the first man on space. In the years since Russia and United States has sent many air force pilots and a fewer scientists, engineers and others. But even after almost 50 years, the number of people who has been to space is close to 500. The people are losing interest in seeing a chosen few going to space and the budgets to space research is diminishing. The space industry now makes money by taking satellites to space. But a major factor here is the cost. At present to put a single kilogram into orbit will cost you between $10000 and $20000. This is clearly prohibitively high and a major objective for the coming years is to drop the cost to a fraction of today's value. Despite the fact that the space shuttle has regularly gone into orbit over the last two decades there is still no tourist business. This is due to the fact that to build an orbital hotel under present conditions will cost 100's of billions of dollars (at least). It is clear why there has been so little progress in orbital developments. The development of a plane which can fly to space at lower cost, which is reusable and can take more payloads, is very much required for further development of space industries. The Reusable Launch Vehicle, usually called Space plane or Hyper plane which can take crew and payload into orbit is being developed by various space agencies and private companies. The Space plane would make space travel cheap and will help in increasing space tourism and just like in the aviation industry, within a few decades, the space tourism industries would be worth billions. Reusable Launch System (or reusable launch vehicle, RLV) It is a launch system which is capable of launching a launch vehicle into space more than once. This contrasts with expendable launch systems, where each launch vehicle is launched once and then discarded. RLV's, due to the fact that they are re-used, will dramatically reduce the cost of access to low earth orbit. However, the technical challenges of designing a system to fly to orbit and return are monumental. For example, the entire Saturn V rocket was expended while sending humans to the Moon. On the other hand, the Space Shuttle, which transports astronauts to Low Earth Orbit and back, is reused over and over again. A number of government research projects, most notably the X-33, X-34, X-37 and X-38, were initiated to develop and test new RLV technologies to reduce the risk of developing a next generation RLV. At the same time, a number of entrepreneurial companies have developed their own RLV concepts in an effort to reduce launch costs and undercut established launch vehicle providers. Orbital RLVs are thought to provide the possibility of low cost and highly reliable access to space. However, reusability implies weight penalties such as non-ablative reentry shielding and possibly a stronger structure to survive multiple uses, and given the lack of experience with these vehicles, the actual costs and reliability are yet to be seen. No true orbital reusable launch system is currently in use. The closest example is the partially reusable Space Shuttle. The orbiter, which includes the main engines, and the two solid rocket boosters, are reused after several months of refitting work for each launch. The external fuel drop tank is typically discarded, but it is possible for it be re-used in space for various applications. Advantages The rockets which take satellites and other payloads have to carry the fuel and oxidizer with them as it uses conventional rocket engines. The combined weight of the fuel and oxidizer is very large due to the fact that a lot of energy is expended pushing the plane forwards. This is why today's rockets launch vertically as it maximizes the rocket's potential by allowing all the energy expended to be focused in the direction we want to go-upwards. With present technology it is the easiest and cheapest method of reaching space. Clearly then the way forward is to utilize jet engines in some manner. The main advantages of jet engines over rocket engines are that they do not need to carry their own oxidizer; instead they suck in air and use the oxygen present in the air as their oxidizer. This will greatly remove the need to carry oxidizer, as it will only be needed when at an altitude that the air contains insufficient oxygen for jets to operate. At this point the rocket engines will fire and burn the much smaller quantity of onboard oxidizer. This will dramatically reduce the take-off weight and also the cost of the craft. Reduction in take-off weight means the payload can be increased. Further to this the use of jet engines will make a substantial saving on the expensive rocket fuel. As a comparison to produce the same thrust, jet (air-breathing) engines require less than one seventh the propellants (fuel + oxidizer) that rockets do. For example, the space shuttle needs 143,000 gallons of liquid oxygen, which weighs 1,359,000 pounds (616,432 kg). Without the liquid oxygen, the shuttle weighs a mere 165,000 pounds (74,842 kg). Another advantage of jet-engine craft is that as they rely on aerodynamic forces rather than on rocket thrust, they have greater maneuverability, which in turn provides better flexibility and safety, for example missions can be aborted mid-flight if there is a problem. This is not the case for staged vehicles, which typically have complex "range safety" requirements as the stages detach and fall back to earth. Range safety is one of the main reasons that the US launches from Florida, where the rocket's flight path takes it out over open water almost immediately. The lack of such abort modes on the Shuttle requires incredible failure avoidance costs and massive overhauls. The space shuttle used by NASA is partially reusable. It still has to take off vertically with the help of multistage rocket and solid boosters. The use of rockets increases the cost of manufacturing parts for each launch as some rocket parts are not reusable. Furthermore, using rockets increases the amount of fuel and oxidizer required. Some of the components of the rocket get added to the space debris and continue orbiting the earth. This causes unwanted collisions with other debris or satellites. Thus using a jet-engine craft as a reusable launch vehicle is faster, efficient, and has increased affordability, flexibility and safety for ultra high-speed flights within the atmosphere and into Earth orbit. 2 THE DIFFERENT REUSABILITY CONCEPTS The difference is in way the RLV is launched. This difference results in differing levels of complexity in design. The major concepts are: Two Stage To Orbit (TSTO) Two stage to orbit requires designing and building two independent vehicles and dealing with the interactions between them at launch. Usually the second stage in launch vehicle is 5-10 times smaller than the first stage, although in biamese and triamese approaches each vehicle is the same size. In addition, the first stage needs to be returned to the launch site for it to be reused. This is usually proposed to be done by flying a compromise trajectory that keeps the first stage above or close to the launch site at all times, or by using small airbreathing engines to fly the vehicle back, or by recovering the first stage downrange and returning it some other way (often landing in the sea, and returning it by ship.) Most techniques involve some performance penalty; these can require the first stage to be several times larger for the same payload, although for recovery from downrange these penalties may be small. The second stage is normally returned after flying one or more orbits and reentering. An advantage of such a system over single-stage-to-orbit is that the entire mass of the spacecraft is not carried into orbit. This reduces the difficulty involved in reaching orbital velocity. An advantage over three or more stages is reduction in complexity and fewer separation events, each of which reduces cost and risk of failure. One and a Half Stage To Orbit (OHSTO) A classical rocket has one, two, three, four etc. stages to make orbit, each smaller than the previous, each burning for a time, before the next one lights. However most rockets, such as The Space Shuttle, have more than one set of engines running in parallel at lift off. At some altitude one set (the half stage) of engines (and often associated fuel tanks) are dropped off, and the remaining stage continues burning all the way to orbit.