Introduction

Few technologies have been anticipated as long as the scramjet engine, or seen more breakthroughs and setbacks along the way. With the potential to provide propulsion at speeds of Mach 10 (7,000 miles per hour) or higher, the scramjet promises such luxuries as two-hour trans-Pacific flights, reusable space-launch vehicles as easy to launch as airplanes, and near-instantaneous global missile strike capacity – the “next generation” of the aerospace age.

Yet after half a century of research, scramjets are still powering little more than daydreams. Almost 50 years passed between the first demonstration of combustion in a supersonic airstream, the defining aspect of scramjet technology, and the first, highly experimental scramjet-powered craft to tear away from its booster rocket. In the intervening years, developments in rocketry and conventional jet engine design brought about such achievements as the moon landing, the commercial jumbo jet, the Space Shuttle age and supersonic passenger transport, while scramjet technology suffered a series of setbacks and program cancellations.

Today, however, scramjet technology appears to be at the dawn of a new era. The last five years have seen a rush of milestones traversed, from wind tunnel tests to passive test flights to the first, record-breaking flights of scramjet-propelled vehicles – even as unexpected program cancellations continue to plague NASA’s scramjet research.

And in the coming decades, new fundamental technologies such as dynamic, shape-shifting materials and customized catalysis techniques may offer powerful solutions to the challenges which have plagued scramjet design since its inception.

But even as scramjets once again rise to the surface of long-term strategic speculation, questions surround many of the technology’s potential applications. While mature scramjet technology could certainly help realize the U.S. military’s dream of “rapid and precise attack on any target on the globe,” it is unclear what strategic impact, if any, such an improvement to the U.S. military’s current quick-strike capability would have. Though the prospect of shorter intercontinental flights certainly appeals to any regular traveler, the commercial failure to date of supersonic passenger aircraft such as the Concorde raises serious doubts about the market for hypersonic passenger aircraft. And while the military, scientific and commercial sectors are all hungry for a convenient, safe reusable space-launch vehicle (RLV), the close association between RLV applications and military cruise-vehicle applications in today’s scramjet research raises concerns over the potential of the technology to blur the line between conventional airborne weapons and weapons in space.

The technology

A ramjet is a high-speed propulsion system which contains no moving parts, unlike a traditional jet engine, and requires no oxidizer (such as liquid oxygen), like a rocket. Because a ramjet (like a traditional jet) obtains oxygen for combustion from the atmosphere, rather than from an oxidizer, it is said to be “air-breathing.”

Whereas a traditional jet engine uses fan blades powered by the airstream to compress incoming air and improve combustion, a ramjet uses scoop-shaped ducts to “ram” air into its engine. This technique works only when the unit is already traveling through the air faster than the speed of sound; a ramjet-powered vehicle or missile therefore needs a supplementary power supply, either a rocket or a turbine jet, to boost it to supersonic speeds.

A ramjet uses a diffuser, a structure which rapidly increases the volume in which the air-flow is contained, to slow the incoming air to subsonic speeds before it is mixed with the fuel. While the subsonic air-flow makes combustion easier to control, slowing down the air increases drag on the ramjet and heats the air to the point where combustion becomes less efficient. At hypersonic speeds – above Mach 5 or so – a ramjet no longer produces net thrust.

A supersonic combustion ramjet (scramjet) is an experimental propulsion system which eliminates the diffuser from the ramjet design. Without the diffuser, air flows through a scramjet at supersonic speeds. By not slowing the airflow below the speed of sound before combustion, the scramjet overcomes the problem of overheating, and should maintain efficient combustion while traveling well above Mach 5.

Supersonic combustion in an engine poses several significant technical challenges, however, which have long held up the development of scramjet engines. The most important of these challenges are combustion rate and flow control.

A scramjet, like any jet or rocket, manufactures thrust by deflecting rapidly expanding gases backwards, thereby absorbing an equal and opposite change in momentum, as stated in Newton’s Third Law. For the combustion reaction to be useful in producing thrust, it must therefore take place while the reactants are still in the engine, rather than after they have been expelled from the nozzle. Because the reactants in a scramjet (the oxygen and the fuel) are traveling through the engine at supersonic speeds, the combustion must occur very quickly in order for the engine to function efficiently. Attaining very fast but controlled combustion is therefore a major challenge of scramjet design.

A traditional jet engine maintains consistent performance by controlling strictly the thermodynamic environment in which the combustion reaction takes place. A major aspect of this control is regulating the flow of air through the combustion space, a task achieved by the structure of the engine upwind of the combustion space. Supersonic fluid flows like the air in a scramjet react to upstream impediments in vastly different ways than subsonic streams; this makes the task of controlling conditions inside the scramjet’s combustion space extremely difficult.

Scramjet milestones

1913: French scientist Rene Lorin comes up with the concept of a ramjet.

1952: Experiments under the aegis of the National Advisory Committee on Aeronautics, the forerunner of NASA, demonstrate combustion in a supersonic airstream. These earliest experiments, in which the air flows along a flat surface, mark the first experiments involving the scramjet concept. Several years pass, however, before research begins to focus exclusively on containing the flame within a ramjet-like engine body.

1961: The Navy launches the program which comes to be known as the Supersonic Combustion Ram Missile (SCRAM) program. SCRAM targets propulsion based on combustion within an enclosed supersonic airstream, and is therefore considered the first full-fledged scramjet program (though the term “scramjet” had long been used in theoretical discussions). During its existence, SCRAM demonstrates that a scramjet can produce net thrust in a wind tunnel.

1977: SCRAM is cancelled because of the perceived impracticality of its scramjet technology. The impracticality resides in part in its reliance on exotic hydrocarbon fuels. The same year, the Dual Combustor Ramjet (DCR) program, a successor born out of SCRAM, is launched. In spite of its name, DCR is a type of scramjet, using a pilot flame in a subsonic airstream to “precondition” its more traditional hydrocarbon fuel and improve its combustibility.

1986: In spite of demonstrating some potential, DCR is cancelled by Congress.

Jan. 15, 2001: The Air Force’s Aerospace Propulsion Office tests the HyTech scramjet engine in a wind tunnel, simulating conditions at Mach 4.5.

June 2, 2001: The first of NASA’s three planned X-43A scramjet launches is aborted due to loss of control shortly after ignition of the booster rocket, long before the scramjet itself is to take over. The X-43A uses liquid hydrogen for fuel and is launched, with a Pegasus rocket to boost it to its initial altitude and speed, from the underside of a B-52 bomber. NASA’s hypersonics research is performed under the aegis of its Next Generation Launch Technology Program.

May 30, 2002: The Office of Naval Research (ONR) and the Department of Defense’s Defense Advanced Research Projects Agency (DARPA) test the HyFly scramjet engine in a wind tunnel, simulating conditions at Mach 6.5 and 90,000 feet. HyFly is a hydrocarbon-burning scramjet engine designed for potential use in a cruise missile.

July 30, 2002: The Centre for Hypersonics at the University of Queensland, Australia, performs the first successful test-flight of a scramjet. The HyShot program uses a conventional rocket to lift the test vehicle to an altitude of about 200 miles, then ignites the scramjet towards the end of the descent to test its ability to generate thrust. The scramjet does not actually power the vehicle’s flight at any point.

March 27, 2004: NASA’s second X-43A launch succeeds. The X-43A achieves a speed of Mach 6.8, setting a new speed record for an air-breathing engine.

Nov. 16, 2004: NASA launches its third and final X-43A scramjet. This flight reaches Mach 9.8 at 110,000 feet, breaking the previous speed record set by NASA.

U.S. government scramjet research today

The X-43C program, the successor to the X-43A jointly run by NASA and the U.S. Air Force, was cancelled as part of NASA’s reevaluation of priorities in March 2004. X-43B, which was to have followed X-43C, will therefore not be launched. Rumors about the resumption of some parts of the X-43C research program, albeit on a more modest scale, continue to circulate.

Several ongoing programs at NASA involve technologies intended to be integrated with scramjet technology, including the Revolutionary Turbine Accelerator, a jet engine which would accelerate a vehicle to the hypersonic speeds at which a scramjet could take over.

The U.S. Navy is continuing its HyFly program, which aims to develop scramjet-powered hypersonic cruise missiles. An unpowered flight test in January 2005 demonstrated successful separation from the booster, while unclassified budget information from February 2005 indicates that two powered flights are scheduled for FY 2006 and four more for FY 2007.

The U.S. Air Force is also still interested in the possibility of incorporating scramjets into space launch vehicles for military use. An unclassified paper recently produced at the Air Force Institute of Technology describes a configuration known as turbine-based-combined-cycle-powered booster, rocket-powered orbiter (TBCC-Rkt), a four-mode launch configuration in which a turbine engine, a ramjet, a scramjet and a rocket are all used sequentially to attain successively higher velocities and altitudes. This paper concluded that TBCC-Rkt is the most promising configuration for horizontal-takeoff-horizontal-landing RLVs.

DARPA is currently running a long-term project known as Force Application and Launch from the Continental U.S. (FALCON) aimed at developing an unmanned hypersonic transport, space launch and strike vehicle. This program is discussed at length below.

Applications

Space launch: Because scramjets, unlike rockets, do not need to carry along oxidizers, using a scramjet to boost payloads into orbit would confer a significant saving in mass overhead and therefore cost. Eliminating the need for oxidizer tanks also makes it more likely that a scramjet-powered vehicle will be fully reusable – unlike the Space Shuttle, which must jettison its booster fuel tank into space. Space launch is the most technically challenging of the commonly-discussed applications of scramjet technology, however, and is likely to take the longest to mature.

One problem with using a scramjet for space launch is that a scramjet is inherently limited to operating within the middle part of a launch trajectory: after the vehicle has reached hypersonic speeds, but before the atmosphere thins out too much to support an air-breathing engine. For this reason, Air Force theorists have envisioned the TBCC-Rkt configuration, in which a conventional mode, in this case a turbine engine, is used to attain supersonic speeds, a ramjet mode accelerates the vehicle from supersonic to hypersonic speeds, a scramjet boosts the vehicle through the atmosphere, and finally a rocket mode takes the vehicle into orbit.

A system utilizing a scramjet would therefore be enormously complex, and a significant advantage -- in oxidizer mass or other considerations -- would be needed to offset the cost of such complexity.

This problem is compounded by the fact that the performance of the scramjet is extremely sensitive to the dynamics of the airflow within the engine, and therefore to the aerodynamics of the vehicle as a whole. The Federation of American Scientists Space Policy Project reports that “the body of the [X-43A] aircraft itself forms critical elements of the engine.” Integrating a scramjet engine into a complex multimode vehicle will therefore be a far greater challenge than simply attaching an engine to the outside of a craft.

Similarly, the unusual qualities of supersonic fluid flows make a scramjet’s performance extremely sensitive to changes in air pressure and velocity. A traditional air-breathing engine or a ramjet can compensate for changes in the pressure of the surrounding atmosphere by throttling the airflow before the combustion space. As mentioned above, however, supersonic flows respond in a vastly different way to impediments than subsonic flows, and are therefore difficult or impossible to control using a throttle. The combustion conditions therefore vary dramatically with the vehicle speed, the atmospheric pressure, and the geometry of the scramjet.

One possible solution to this problem is to vary the geometry of the intake ducts and the engine space as the altitude and velocity change. A scramjet engine would therefore be an ideal application for advanced materials whose physical properties, such as their shape and dimensions, change in response to an electric or magnetic signal. Developing such dynamic materials, for a broad range of uses, is a high priority of many scientific bodies, including DARPA. Currently, however, building a scramjet engine which is robust to the differences in pressure experienced at different altitudes is a significant challenge.

A final consideration in using a scramjet for space launch is shared by all air-breathing engines. While air-breathing engines require less fuel weight than oxidizer-consuming rockets, they also produce significantly less thrust per unit engine weight. For this reason, air-breathing engines are traditionally used in different ways than rockets. Rockets fire downwards, while air-breathing engines fire backwards: that is, a rocket-powered vehicle uses the thrust of the rocket as its source of lift, while an air-breathing vehicle traditionally uses wings to translate the thrust of the engine into aerodynamic lift.

An air-breathing engine, such as a scramjet, would therefore not generally be used for a vertical-takeoff space launch. Instead, a space launch powered by a scramjet will follow a shallow trajectory through the atmosphere, gaining altitude like an airplane rather than like a rocket.

A horizontal takeoff would probably require much less complex infrastructure on the ground than a vertical takeoff, representing a certain savings to operational costs. A horizontal takeoff trajectory is also generally considered safer, since a winged flight is easier to control and abort than rocket flight. On the other hand, it exacts a high price in terms of the added heat-resistance and additional fuel required to travel for so long through the dense, dragging parts of the Earth’s atmosphere.

While the Air Force Institute of Technology paper previously mentioned ultimately recommends an entirely rocket-propelled vertical-takeoff-horizontal-landing configuration for an Air Force RLV, its alternate choice is a horizontal-takeoff configuration incorporating a scramjet. In general, there is a widespread sense today that a preference for simple infrastructure and low operating cost will make the fully reusable launch vehicle of the future a horizontal-takeoff vehicle.

High-speed transport: Using a scramjet for level flight through the atmosphere seems to work around several of the more serious challenges to scramjet-based RLVs. A scramjet application which remains inside the atmosphere would reduce the four modes which must be integrated into a TBCC-Rkt design to three, or even two, if an advanced turbine engine such as NASA’s notional RTA could attain high enough speeds to skip the ramjet mode altogether. Furthermore, a trajectory which aimed for distance rather than altitude would require less flexibility of the scramjet engine, since it would experience a narrower range of dynamic pressures. Finally, a trajectory of this sort is, of course, optimal for winged flight, as opposed to vertical takeoff.

The possibility of a commercial or military transport vehicle which could travel at Mach 10 – and circle the globe in under seven hours – is intriguing. Putting aside the fact that scramjet technology is years or decades away from commercial viability, however, it is unclear whether a broad market will ever exist for such a vehicle. Human physiology imposes strict limits on how fast a passenger vehicle can accelerate, complicating enormously the business of getting a scramjet-powered passenger vehicle up to hypersonic speeds. Even supersonic transport, most famously represented by the Concorde program, has not proved itself commercially viable to date.

Cruise missiles and unmanned vehicles: Given all the considerations above, perhaps the most appropriate use for a scramjet engine would be to power a missile or unmanned aerial vehicle (UAV) during the cruise phase of its flight. The successful wind-tunnel test of a cruise-missile scramjet engine by ONR and DARPA in May 2002 was described by the sponsoring agencies as a step towards “a future …hypersonic strike weapon” which could ultimately be launched from a surface ship or submarine.

Scramjet technology has also been mentioned frequently in conjunction with DARPA’s FALCON program. This program aims to develop a “reusable, Hypersonic Cruise Vehicle … capable of delivering 12,000 pounds of payload a distance of 9,000 nautical miles from [the continental U.S.] in less than two hours.” The program also aims to develop a vehicle usable as a small RLV. While DARPA’s website does not specifically mention scramjets, or any other propulsion technology, in its description of FALCON, its mention of “leveraging technology developed under the … HyFly program” buttresses the widely held assumption that scramjets play a role in the imagined system.

Implications

As a feat of engineering, a fully functional, scramjet-powered vehicle would be a tremendous achievement, requiring seminal advances in several fields of science and technology. As a strategic weapon in the U.S. arsenal, a scramjet-powered cruise missile or UAV would be a tremendous asset, providing a rapid global response capacity which the military has been eager to obtain.

It is unclear, however, whether tapping the potential of scramjets would alter the capacities of the U.S. military qualitatively, as opposed to quantitatively. “Rapid global response” is, after all, a relative term: the U.S. military’s global fleet of surface ships, submarines and aircraft, together with the global strike capabilities of North American- or Pacific-based B-52 bombers, already provide the ability to strike anywhere in the world quite “rapidly.” Absent a Cold War-style balance of power, shaving hours off the current response time is unlikely to tip the balance in a future conflict.

More troubling, perhaps, is the potential of scramjet technology to blur the line between weapons in the air and weapons in space. While the parameters of any future scramjet-powered craft are vague, DARPA’s short-term plans for the FALCON program include developing a novel booster, known as a Small Launch Vehicle (SLV), which will not only launch FALCON test-vehicles, but itself be usable both to launch small satellites into orbit and, in conjunction with an early-generation test-vehicle, to “provide a conventional prompt global strike capability from the [continental U.S.] in the 2010-2015 timeframe.”

While designing a single vehicle capable both of delivery of payload to space and of striking remote targets on the ground would certainly solve two significant technological priorities of the U.S. government at once, it may also raise legitimate concern among states and multilateral bodies already worried about possible U.S. plans to place weapons in space.

While traditional space-launch technology is often described as “inherently dual-use” due to its technical similarity to ballistic missile technology, the FALCON launch vehicle, as described by DARPA, raises the unique prospect of a single vehicle capable of both space-launch and military transport, surveillance, and even munitions delivery. This prospect raises various concerns: how will such a vehicle be categorized for purposes of current or future arms-control treaties? Will other countries, nervous about the possibility of a U.S. airstrike, deny access to their airspace for space launches using such a vehicle? Will the United States in turn claim that such a denial violates the principle of free access to outer space enshrined in the Outer Space Treaty of 1967? Will FALCON pave the way for other aircraft whose ability to inhabit the regions between suborbital and orbital space will finally blur the line between the regions traditionally inhabited (at least in worst-case scenarios) by long-range ballistic missiles and the regions long considered off-limits to weapons technology?

On the other hand, an RLV which provides low-cost, reliable access to space may, to some extent, reduce the perceived need for weapons in space. The enormous cost and complexity of space launch using today’s technology is a major component of the monetary and logistical value of orbital assets such as communications and global positioning satellites. As the cost of launching satellites into space goes down, so too will the inherent value of an orbiting satellite. If scramjet technology can make replacing a damaged satellite significantly easier and cheaper than it is today, an attack against U.S. satellites may pose less of an apocalyptic prospect than it does today and military planners may feel less pressured to declare outer space a U.S. military zone.

Conclusions

While the first military and commercial applications of scramjet engine technology are probably still several years away at least, it is possible that the technology has already crossed the point of no return, and that hypersonic air-breathing propulsion can now be said for certain to be possible. At the same time, it is unclear whether scramjet-powered transport across the globe will ever prove viable as a commercial product, or strategically significant as a military asset.

On the commercial side, the challenges inherent in making hypersonic travel safe for passengers and the vicious price wars which are decimating the airline industry seem to be likely to restrict the appeal of hypersonic travel. On the other hand, by the time scramjet technology is mature for passenger transport, the economic model of the air travel industry may be ready for a significant paradigm shift towards incorporating a new class of high-speed luxury travel. Furthermore, the thriving market for rapid global mail and package delivery may offer an opening for unmanned hypersonic cargo transport.

On the military side, evidence seems to indicate that sharing and collecting intelligence poses a greater challenge to the war on terror than a lack of quick-strike capacity. Meanwhile, the U.S. military’s current conventional quick-strike capacity already offers it considerable leverage against any hostile state.

Scramjet-powered space launch, while likely to take longer to mature as a technology than suborbital transport, offers greater prospects for viability. While scramjets seem to offer little to the commercial travel industry other than faster transport, the space launch industry is hungry for revolutionary technology. Scramjet-powered vehicles are highly suitable for horizontal launch, with its significant reduction in ground infrastructure and between-flight turnover time, and for reusability, with its obvious savings in launch cost.

Unfortunately, the U.S. military’s most significant current hypersonics research program, FALCON, combines the aims of high-speed suborbital transport and reusable space launch in a way which may dangerously undercut the distinction between conventional aerial weaponry and space weapons. This may occur because future successor programs could develop technologies that occupy the middle ground in a still-unforeseen way, because hypersonics research will bring the U.S. military and its civilian space program into increasingly close collaboration, because FALCON technology will be dual-purpose to an unusual extent, or simply because foreign governments and the civilian population will be left confused by Defense Department pronouncements regarding the FALCON program and its purpose – the last of which is a perennial danger, and already a very real one.

It is of critical importance, therefore, that the U.S. government take unusual pains to clarify what the purpose of the FALCON program is and what ancillary aims it hopes the program will achieve, provide a clear way to distinguish military and civilian uses of the resulting technology, and, of course, clarify as much as possible once and for all its position regarding the placement of weapons in space. Until these clarifications are given, it is inevitable that scramjet technology will generate more paranoia, unease and misinformation than real benefit to humanity.

Sources

“Aircraft Propulsion and Power: The Revolutionary Turbine Accelerator,” National Aeronautics and Space Administration Scientific and Technical Aerospace Reports, vol. 43 (2005).

Barry J. Hurewitz, “Non-Proliferation and Free Access to Outer Space: The Dual-Use Conflict between the Outer Space Treaty and the Missile Technology Control Regime,” Berkley Technology Law Journal 9.2 (1994), pp. 212-245.

Caldwell, Richard A., Weight Analysis of Two-Stage-To-Orbit Reusable Launch Vehicles for Military Applications, Master’s Thesis, Department of Aeronautics and Astronautics, Air Force Institute of Technology, 2005. http://stinet.dtic.mil/cgi-bin/GetTRDoc?
AD=A434888&Location=U2&doc=GetTRDoc.pdf

“Falcon,” Tactical Technology Office – Force Application and Launch from CONUS (FALCON), May 9, 2005, Defense Advanced Research Projects Agency Tactical Technology Office. http://www.darpa.mil/tto/programs/falcon.html

“Faster Than a Speeding Bullet: Guinness Recognizes NASA Scramjet,” NASA News, June 20, 2005, National Aeronautics and Space Administration.

“Fully Integrated Scramjet Missile Engine Tested at Mach 6.5,” ONR Media Area, June 12, 2002, U.S. Navy Office of Naval Research. http://www.onr.navy.mil/media/article.asp?ID=11

“FY2006/2007 RDT&E,N Budget Item Justification Sheet, February 2005,” U.S. Navy. http://www.dtic.mil/descriptivesum/Y2006/Navy/0603114N.pdf

Gallimore, Scott D. Jr., “Section 2: History, Development, Obstacles and Proposed Solutions of Hypersonic Technology,” Operation of a High-Pressure Uncooled Plasma Torch with Hydrocarbon Feedstocks, Master’s Thesis, Department of Mechanical Engineering, Virginia Polytechnic Institute and State University, 1998. http://scholar.lib.vt.edu/theses/available/etd-71998-13553/

Hearing of the Subcommittee on Emerging Threats and Capabilities of the Senate Armed Services Committee, March 9, 2005, testimony of Ronald M. Sega, Director of Defense Research and Engineering.

“HyShot,” Centre for Hypersonics – HyShot Scramjet Test Programme, June 2, 2003, The University of Queensland, Australia, Department of Mechanical Engineering. http://www.mech.uq.edu.au/hyper/hyshot/#top

“NASA Facts: Hypersonics Research and Development, January 2003,” National Aeronautics and Space Administration. http://www.nasa.gov/centers/marshall/pdf/
100410main_hypersonics.pdf

“NASA's X-43A Scramjet Breaks Speed Record,” Missions: News Releases, Nov. 16, 2004, National Aeronautics and Space Administration.

P.J. Waltrup, et al., “History of U.S. Navy Ramjet, Scramjet and Mixed-Cycle Propulsion Development,” American Institute of Aeronautics and Astronautics Meeting Papers on Disc 96-3152 (1996).

“Propulsion Directorate Monthly Accomplishment Report, March 2001,” U.S. Air Force Research Laboratory. http://www.pr.afrl.af.mil/mar/2001/mar2001.pdf

Richard Varvill, et al., “A Comparison of Propulsion Concepts for SSTO Reusable Launchers,” Journal of the British Interplanetary Society, vol. 56 (2003), pp.108-117. http://www.reactionengines.co.uk/downloads/
JBIS_v56_108-117.pdf

Steven Walker. “Quest for Hypersonic Flight,” DARPATech 2004 Symposium. http://www.darpa.mil/DARPAtech2004/pdf/scripts/WalkerScript.pdf

“Strategic Master Plan FY06 and Beyond, October 2003,” U.S. Air Force Space Command. http://www.peterson.af.mil/hqafspc/library/AFSPCPAOffice/
Final%2006%20SMP--Signed!v1.pdf

“The U.S. Air Force Transformation Flight Plan, November 2003,” U.S. Air Force. http://www.af.mil/library/posture/AF_TRANS_FLIGHT_PLAN-2003.pdf

“X-43A Hypersonic Experimental Vehicle,” NASA Dryden X-43A Photo Collection, Nov. 26, 2004, National Aeronautics and Space Administration Dryden Flight Research Center. http://www.dfrc.nasa.gov/Gallery/Photo/X-43A/

“X-43 Hyper-X Program,” FAS Space Policy Project, Dec. 25, 1998, Federation of American Scientists. http://www.fas.org/spp/guide/usa/launch/x-43.htm