coverMARCH2012-x_Layout 1 2/13/12 2:51 PM Page 1 3 AMERICA AEROSPACE March 2012 MARCH 2012 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 launch- ers 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 elec- tromagnetic 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- 24 AEROSPACE AMERICA/MARCH 2012 Copyright ©2012 by the American Institute of Aeronautics and Astronautics VIEWPOINT-X1-0312_Layout 1 2/13/12 2:32 PM Page 3 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 avail- able 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, op- erational 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 exam- ple, 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.