Real Estate in Frequency Space

coverMARCH2012-x_Layout 1 2/13/12 2:51 PM Page 1 AEROSPACE AMERICA 3

March 2012 MARCH 2012 MARCH

REAL ESTATE IN FREQUENCY SPACE

The ephemeral ‘advanced propulsion’ Strategic bombers—relevant again A PUBLICATION OF THE AMERICAN INSTITUTE OF AERONAUTICS AND ASTRONAUTICS VIEWPOINT-X1-0312_Layout 1 2/21/12 11:58 AM Page 2

Viewpoint

The ephemeral ‘advanced propulsion’ requent letters to the editor and com- mentaries in space journals have de- cried current and past deep-space mission concepts as being doomed by New technologies with the promise Finefficient propulsion systems. They call upon NASA or DARPA to develop ‘advanced of more aﬀordable, more eﬃcient, propulsion’ technologies that will make and safer propulsion for space those difficult missions more efficient, more affordable, more capable, more whatever. launch currently seem to be out of The term ‘advanced propulsion,’ prop- erly, has been applied primarily to in-space reach. That, however, does not mean technologies, not those suitable for Earth- based launches. These have included vari- that we should stop searching. ous electric propulsion methods (electrosta- tic, electromagnetic, electrothermal, magne- toplasmadynamic), nuclear thermal rockets, various forms of catapults (railguns, tether ‘slingshots’), laser-heated propellants, pho- ton sails (solar or laser), charged-particle sails, or ‘way out’ concepts using nuclear fusion or antimatter-based energy sources. Unfortunately, advanced propulsion with sufficient thrust for Earth-based launchers requires concepts involving esoteric ma- terials (often denoted as ‘unobtainium’) or other new (or as yet unknown) principles of physics such as antigravity, modifying the structure of space-time, employing electromagnetic zero-point energy, faster-than- light drive, or ‘wormholes.’ None of these is likely to be operational in the foreseeable future. So, for Earth launch, we are stuck with the few high-thrust technologies avail- by Jerry Grey able within our current understanding of Editor-at-Large physics: liquid-propellant and solid-propel-

lant rockets, combined-cycle systems in- pioneer Konstantin Tsiolkovsky in the 19th volving air-breathing engines and rockets, century—the oxygen-hydrogen rocket. True, guns, and nuclear-thermal rockets. we can get slightly better performance from Neither guns nor nuclear-thermal rock- fluorine-hydrogen or ozone-hydrogen, but ets are suitable for space launch from Earth; only with unacceptable cost, hazard, and guns because of their need to achieve or- complexity issues (both have been tried in bital or escape velocity—over 7 km/sec— the past). Other improvements in liquid- ’ while still in the high-density atmosphere and solid-propellant rockets are, of course, (even high-altitude launch sites or airborne possible, and are indeed likely to be pur- gun-launch platforms have been studied sued, but they are equally likely not to pro- and found to be eminently impractical and vide game-changing breakthroughs. economically disastrous), and nuclear-ther- Nevertheless, in December 2011 the Air mal booster rockets because of valid envi- Force announced funding of the first major ronmental concerns. research phases of a reusable booster sys- Indeed, the most advanced but still tem intended to replace its costly expend- practicable Earth-launch propulsion available launch vehicles, with initial contracts able to us today or in the foreseeable future issued to Boeing, Lockheed Martin, and An- remains the one first conceived by rocket drews Space.

THE PROMISE OF THE COMBINED CYCLE

Most past efforts to improve launch per- 9.8, and the first powered flight of Boeing’s formance via air-breathing engines com- X-51A Waverider, which reached a Mach bined with rockets (the so-called combined- number of 4.87 in May 2010 and boasts the Dr. Jerry Grey, an honorary cycle systems) have never been able to longest operating time to date of a scramjet fellow of the AIAA and a fellow demonstrate practical, operationally suit- engine: 143 sec. The engine was developed of Britain’s Royal Aeronautical able results, although there are still several and built by Pratt & Whitney Rocketdyne. Society, was professor of aerospace engineering at Princeton University, such concepts currently being pursued—at Several other potential combined-cycle director of science and technology funding levels too low to possibly produce approaches are also worthy of note. For ex- policy for the AIAA, and the much in the way of operationally useful ample, Aerojet has proposed a three-engine founder and first publisher of systems for years to come. However, there concept, the TriJet, which combines the Aerospace America. He was are a few recent developments in this cate- two classical combined-cycle designs—tur- instrumental in completing gory that appear to be worth following up bine-based and rocket-based—to achieve a successful research on preventing actively, if sufficient funding can be made smooth transition from start to over Mach 7. rocket combustion instability, founder and director of Princeton’s available. Lockheed Martin’s axisymmetric scramjet, Nuclear Propulsion Laboratory, Of the many research efforts seeking to based upon a design conceived during and organizing chairman of the demonstrate a practical high-speed air- DARPA’s canceled Blackswift project, has University Space Research breathing engine that might be adaptable to been proposed as the turbine-based com- Association’s Center for Space space launch, only two have achieved sig- bined-cycle powerplant for a new Air Force Nuclear Research. He was president nificant flight demonstrations: the third flight Research Laboratory prototype of a long- of the International Astronautical of NASA’s X-43A in November 2004, whose range strike missile, planned for flight test- Federation and deputy secretary-general of the United supersonic combustion ramjet (scramjet) ing in 2016. Boeing’s successor to the X-51 Nations’ Unispace-82. He now engine operated for 10 sec and boosted the is another candidate for that mission. consults on space policy and craft to a new world speed record of Mach Whereas current U.S. high-speed com- space propulsion.

AEROSPACE AMERICA/MARCH 2012 25 VIEWPOINT-X1-0312_Layout 1 2/13/12 2:32 PM Page 4

The black X-43A rides on the oxygen rocket. Germany’s Sharp Edged front of a modified Pegasus Flight Experiment (Shefex-2), whose design booster rocket hung from the speed is Mach 11, is a pathfinder for subor- special pylon under the wing of NASA's B-52B mother ship. bital reentry tests in 2020, building on the prior Shefex-1 flight at Mach 6 in 2005. But despite all this activity, it is still far from clear that any of these efforts eventu- ally can produce a practical, low-cost, operational space-launch capability. Some will ask, “What about the Holy Grail of space transportation: fully reusable Lapcat has flight test targets single-stage to orbit (SSTO)?” Unfortunately of Mach 5 and Mach 8 using a hydrogen-fueled dual-mode the physics of orbital launch (or at least, ramjet-scramjet. our present knowledge of physics) simply does not allow us to attain this ultimate goal. With our highest performance Earth- to-orbit launcher, the hydrogen-oxygen rocket, the rocket equation tells us that we need a mass ratio (propellant mass divided by takeoff mass) of about 0.9; that is, the total mass of engine, tanks, structure, con- SpaceLiner is a two-stage trols, return and landing vehicle, and pay- all-rocket-propelled vehicle load can total only about one-tenth of the launched vertically from launch vehicle’s initial mass. For compari- the ground for ultra-fast long-range flights. son, 0.9 is about the mass ratio of a hen’s egg, if we consider the contents to simulate the propellant and the shell to contain everything else. Past efforts to beat those odds, even with the benefit of an air-breathing boost engine, haven’t even come close; for example, the X-30, the X-33, and Lockheed Mar- bined-cycle engine concepts are aimed at tin’s VentureStar. Britain’s Skylon project, military applications, Europe has been es- another single-stage-to-orbit wannabe, is pecially active in research on combined-cy- still in its very early stages and will not be cle engines for high-speed transport that able to prove its worth (if any) for a long, could be adaptable to space launch. ESA’s long time. Lapcat-II concept, a study conducted under And if we were to forgo advanced- the 4-year Long-Term Advanced Propulsion technology Earth-launch concepts and de- Concepts and Technologies program, has vote our attention to reducing space trans- flight test targets of Mach 5 and Mach 8 us- portation costs by using advanced higher ing a hydrogen-fueled dual-mode ramjet- thrust in-space propulsion for upper stages scramjet named Scimitar. This engine,de- and space ‘cruise’ operations, we would signed by the U.K.’s Reaction Engines, face a nearly insurmountable cost and mass employs air cooling and a shaftless air-com- barrier: the need by all such systems (other pression system. One Lapcat-II vehicle de- than nuclear thermal rockets) for high elec- sign is derived from Reaction Engines’ sin- tric power. This requirement, which calls gle-stage-to-orbit Skylon concept; another for multikilowatt or even megawatt nuclear is based on a waverider under study by (or less practical solar) powerplants, im- ESA’s Estec and the U.K.’s Gas Dynamics; a poses such severe mass penalties on the third is being explored by France’s ONERA craft as to make any mission that requires and the Universities of Brussels and Rome. both high performance and high thrust A separate project under Lapcat is the both impractical and much too costly. Sev- Future High-Altitude High-Speed Transport eral studies have explored the prospect of 20XX (FAST20XX), a two-stage vertical- using beamed power from another satellite takeoff SpaceLiner concept by ESA and serving as a ‘power depot,’ but this option, Germany’s DLR. Both of its recoverable although its basic technology is reasonably stages land horizontally; its upper stage well advanced, would require considerable, uses a new staged-combustion hydrogen- expensive development.

26 AEROSPACE AMERICA/MARCH 2012 VIEWPOINT-X1-0312_Layout 1 2/13/12 2:33 PM Page 5

Stratolaunch Systems, a new company based in Hunstville, Alabama, will develop and operate a new carrier aircraft bigger than a 747.

INNOVATION AND RISK

So with what options does this somewhat USAF Quick Reach booster, were both can- discouraging picture leave us? For Earth celed shortly after inception. launch in the foreseeable future, there is Nevertheless, in November 2011 DARPA really only one: Find ways to reduce the reinstated the prospective use of airborne cost of space transportation by seeking ma- launch for small (45-kg) payloads in a new jor improvements in development practices, program named Airborne Launch Assist manufacturing and testing, and perhaps Space Access (ALASA). most important, flight operations. Such im- Also, operational practices being pio- provements—in all three areas—require both neered by The Spaceship Company, a joint innovative thinking and, even more impor- venture of Burt Rutan’s Scaled Composites tant, greater willingness to accept risk. and Richard Branson’s Virgin Galactic, Innovative development practices have along with their innovations in develop- already begun to be pioneered in the U.S. ment and testing, could lead to significant by the new commercial entrepreneurs, cost reductions using airborne launch. In- most notably SpaceX and Scaled Compos- deed, in December 2011 Paul Allen, co- ites. These companies, in contrast to the founder of Microsoft, announced a new other ‘new space’ wannabes, have demon- Huntsville, Alabama-based launch company strated initial operational success in flight, named Stratolaunch Systems that will de- with development funding that is signifi- velop and operate a new carrier aircraft cantly lower than that of NASA or of the bigger than the Boeing 747. The aircraft will legacy launch-service providers. As cited in be designed and built by Scaled Compos- Aviation Week last August, “…SpaceX is ites; the rocket launcher it carries, able to ramping up plans to become the world’s orbit payloads up to 6,100 kg, will be de- largest producer of rocket engines in less signed and built by SpaceX. Although the than five years, manufacturing more units current plan is to fly only unmanned pay- per year than any other single country.” loads, the company’s future prospects envi- One of the newly revived operational sion a human-rated launcher. concepts is airborne launch, which could Allen’s impressive design team includes be significant mainly for the abovemen- Burt Rutan, his collaborator on X-Prize win- tioned combined-cycle propulsion systems. ner SpaceShipOne; Elon Musk, the CEO of Prior limited use for small payloads carried SpaceX; former NASA Administrator Mike by rocket-powered orbital launchers, such Griffin; David King, a former director of as the highly successful DARPA/NASA Peg- NASA Marshall; and Stratolaunch Systems’ asus launch system developed by Orbital current president and CEO, Gary Wentz, a Sciences and ATK, has not demonstrated former NASA chief engineer. significant cost reduction. Indeed, two more Another interesting operational innova- recent programs for such launches, DARPA’s tion is orbital refueling, currently being pur- 2003 Responsive Access Small Cargo Af- sued as low-level research by both NASA fordable Launch and the 2008 DARPA/ and DARPA, with a relevant but low-budget

AEROSPACE AMERICA/MARCH 2012 27 VIEWPOINT-X1-0312_Layout 1 2/13/12 2:33 PM Page 6

NASA demonstration project ($2.4 million Space Agency. ESA is considering the de- in several study contracts) being considered velopment of an Ariane 6; Russia is devel- by the Office of the Chief Technologist for oping the Angara and Phoenix families to 2016. Several commercial efforts to refuel launch, among other payloads, a new hu- and refurbish satellites have been aban- man-carrying space vehicle (to replace the doned, however, and NASA has recently tried-and-true Soyuz launcher and capsule), downplayed orbital refueling as a low-per- and a brand-new Siberian launch site, Vos- centage option. For human space missions, tochny. China is designing several new human-rating-proven legacy launchers such high-payload versions of the venerable as Atlas V and Delta IV are another pros- Long March family. India is upgrading its pect, but one that does not offer much in Geostationary Satellite Launch Vehicle, and the way of major cost reduction. Japan has the new H-2B. Although none of Outside the U.S., improvements in these developments can be categorized as launch effectiveness (although not specifi- employing ‘advanced propulsion,’ their im- cally in propulsion) are being pursued by provements in development, test, and oper- all the spacefaring countries—Russia, China, ations will contribute to some launch cost Japan, and India—as well as the European reduction and/or capability enhancement.

THE NUCLEAR AND SOLAR-ELECTRIC OPTIONS

Aside from these potential improvements, of the solar-electric options. Neither is ap- which are certainly worth pursuing but do plicable to Earth launch, the most costly not lead to the game-changing dreams of component of space transportation; they are advanced-propulsion proponents, there ap- suitable only for upper-stage or in-space pear to be only two prospects with any re- operations. alistic near- to midterm hope of offering The nuclear thermal rocket, in which a significant gains in cost and/or capability: relatively small nuclear fission reactor is the nuclear thermal rocket and one or more used to heat hydrogen propellant to very

December 1, 1967, the first ground experimental nuclear rocket engine is seen in ‘cold flow’ configuration as it arrives at the Nuclear Rocket Development Station in Jackass Flats, Nevada.

28 AEROSPACE AMERICA/MARCH 2012 VIEWPOINT-X1-0312_Layout 1 2/13/12 2:33 PM Page 7

high temperatures, offers reasonably high thrust (on the order of 75,000 lb) and about double the specific impulse of the best chemical rockets. It saw extensive development in the 1950s and 1960s, undergoing a series of quite successful ground tests. Its primary application was seen as a prospective propulsion system for a Mars mission, but when that mission faded from NASA’s view in the early 1970s so did the nuclear thermal rocket. However, with renewed recent interest in human flights to Mars, the prospect of using the nuclear thermal rocket in an upper stage has seen some re- vival. NASA Marshall is currently conduct- ing research on simulated nuclear-thermal rocket configurations, using electric heating to simulate the nuclear reactor’s energy. A November 2009 Aerospace America com- mentary (“Nuclear propulsion—the affordable alternative”) identified two key points: •“Planning for human solar system exploration has stubbed its toe, badly, on a simple bit of reality: The performance of Skylon is a grandchild of the chemical rocket propulsion is inadequate. and Mars after 2020, NASA is now consid- early British single-stage-to-orbit The mass ratio required to deliver some- ering the prospects for multi-hundred-kilo- HOTOL concept. Currently being thing to Mars is over 20 times greater than watt solar-electric propulsion systems, with planned by Alan Bond of the U.K.’s Reaction Engines, it uses with nuclear propulsion. The added costs projected savings of required mass in low the Sabre engine,which combines of necessary ferry flights and on-orbit inte- Earth orbit of up to 60% for such missions. turbomachinery using pre-cooled gration are fatal.” But the engineering obstacles for even air with a hydrogen-oxygen rocket •“To resuscitate this option, major deci- the smallest of these prospects (300 kW) to enable ﬂight from standstill to orbital speed. sions must be made, beginning with recov- are daunting: building an 800-m2, high-volt- ery of the engineering data and equipment age (~300 volt), radiation-protected (glass- still available from remnants of the extensive covered) solar-cell array that is deployable Rover/NERVA nuclear rocket testing and de- in space and can withstand the Earth- velopment programs in the 1950s and 1960s. departure acceleration. Moreover, getting A fast-track program ranging over six or budget approval of the development cost seven years to flight appears feasible.” for such systems may be difficult: Even a Electric propulsion has seen not only small 15-30-kW demonstration project, be- extensive development in the past half-cen- gun by NASA in 2010, had a $1-billion-plus tury or so, but also a large number of actual price tag before being cut back to a less mission applications, ranging from comet ambitious undertaking. and asteroid explorers to operational use for station-keeping in commercial commu- QQQ nication satellites to orbit-raising of military All in all, the near- to-midterm prospects for satellites. Offering proven reliability and applying ‘advanced propulsion’ to create a specific-impulse performance orders of new era of space exploration are not very magnitude higher than chemical or nuclear- good. Nevertheless, there is every reason to thermal propulsion, it nevertheless has the continue seeking breakthrough technolo- principal drawback of all electric propul- gies as an investment in the future, for ex- sion systems, as noted earlier: very low ample, via recently initiated programs such thrust in the absence of onboard megawatt- as DARPA’s 100-year Starship project and level electric powerplants. NASA’s Innovative Advanced Concepts, an However, if flight time is not of the outgrowth of the former highly successful essence, solar-electric propulsion can de- NASA Institute for Advanced Concepts that liver reasonably high payloads much more was terminated in 2007 after 10 years. efficiently than other propulsion options. But don’t expect anything approaching For example, as an enabling technology for Star Trek’s faster-than-light ‘warp drive’ for future human flights to near-Earth objects many years to come.

AEROSPACE AMERICA/MARCH 2012 29