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Skylon illustration
Skylon is one example of air-breathing space propulsion systems under development, although often at very low levels and very early stages. (credit: Reaction Engines)

Current strategies towards air-breathing space launch vehicles

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In 1988 I wrote an article “Airbreathers To Orbit: The Best Way To Go!”, which presented a host of arguments in favor of air-breathing launchers, most of which are still valid. In 1988, an air-breathing launcher was conceived of by many as a large supersonic airplane that would drop the orbiting part of the vehicle off, just like the White Knight Two drops the SpaceShipTwo during a test flight. Today, no true air-breathing spaceplanes or reusable boosters yet exist, but there is now renewed interest in air-breathing technology. At the same time, remarkable launch cost reductions in more conventional boosters are imminent due to the efforts of SpaceX and other firms. It is beneficial to everyone to explore alternate technological paths, since no one can predict which paths will pan out and produce an economical and reliable vehicle. In addition, the impending launch cost competition will stimulate new ways of thinking in the industry worldwide. What is happening now in the air-breathing launcher field and what strategies should companies and countries pursue in the face of these diverging space launch paths?

First: there are currently four reasonably well defined and significant market (payload) classes for launchers.

  1. Commercial communication and other satellites built and operated by and for commercial companies;
  2. Government unmanned launches, occurring at a significantly higher rate than private launches due to continued use of military style cost-plus contracts;
  3. Manned launches of crew to the space station or for deep space missions;
  4. Launch of future heavy government or private payloads (70 tons or more to LEO) which need a true heavy lift vehicle (HLV).

The announcement of the Falcon Heavy by SpaceX has created a virtual fifth payload class intermediate between an HLV and a manned spacecraft booster, which remains to be defined or used since no one has existing payloads in that weight class. The existing booster classes fall into two rough classes: small to medium satellite launchers and launchers big enough to launch a crew vehicle or larger. There is some overlap in capabilities.

Today, no true air-breathing spaceplanes or reusable boosters yet exist, but there is now renewed interest in air-breathing technology.

The most important potential current markets for an air-breathing launcher are classes 1 and 3: commercial comsats, due to the relatively large number of launches; and crew launches, to improve crew safety. A large part of the cost of a comsat is the launch itself, and since no orbital repair service yet exists, the satellites must be highly reliable to justify the launch costs. Crews are currently launched on vertical rocket systems, which are vulnerable to pad accidents and thus need a heavy and expensive launch escape system. (The shuttle needed such a system but never got it due to cost cutting and bureaucratic arrogance.) Horizontal launches would do away with that risk and cost. Heavy lift size payloads are conceivable for air-breathers but the launcher airframe size would have to be very large.

Different technical approaches

Since 1988, a lot more engineering thinking has gone into designs for air-breathing launchers. The original conceptual designs, as early as the 1960s, simply used a very large supersonic airplane for the first stage and a rocket-powered second stage. The main point was to get rid of the huge mass of heavy liquid oxygen (LOX) needed by a first stage rocket while still in the atmosphere. For a single stage to orbit (SSTO) air-breather the vehicle simply carried all of the LOX along that would be used after it was too high to operate air-breathing engines. Powered hypersonic flight was still a distant dream.

There is now steady progress in actual testing of hypersonic engines. Antonio Ferri of the Guggenheim Aerospace Laboratory in New York first conceived the scramjet (supersonic combustion ramjet) in 1964. Since then most of the scramjet engine designs have used liquid hydrogen (LH2) as fuel. In 2010, the unmanned X-51A flew for about 200 seconds at near hypersonic speed while accelerating during the test. Previous tests had lasted only a few seconds—achieving hypersonic flight has proved to be much harder than we imagined. Hypersonic and scramjet (supersonic combustion ramjet) engines of various types will probably play a critical role in air-breathing boosters, while work on combined cycle concepts (jet and rocket all-in-one engine) continues. Most or all of the concepts currently under consideration do use winged vehicles to avoid vertical takeoffs and to reduce the required engine size for takeoff. Work on more advanced concepts is also underway.

One English company, Reaction Engines, has recently achieved a go-ahead from the ESA for a major test of critical engine components of its Skylon vehicle concept (see “Skylon: ready for takeoff?”, The Space Review, June 13, 2011). As air enters the intake, slows to subsonic speed and is compressed, it becomes very hot, which would reduce or eliminate engine power if it were not cooled. For that vehicle’s engine, which does not operate as a scramjet, the air is cooled to 133 kelvins (–140°C) using the liquid hydrogen fuel via a heat exchanger. The cooled air is used only when the engine is operating in air-breathing mode, up to about Mach 5, after which the vehicle would switch to pure rocket power using LOX which is loaded before launch. One development fear was that a thick layer of frost would rapidly form on the heat exchanger and interfere with the rapid cooling process. The company has apparently overcome this potential problem. If the next test is successful, there is a good chance for the project to proceed to build a complete test engine.

The next steps up the air-breathing tech ladder, including condensing the cold air into a liquid during flight, are ones that have been researched primarily in Japan and India and are extremely challenging. There is no current work proceeding in this area at large scale system R&D level, though significant work at subscale technology level continues.

Though considerable research was carried out in the US in the 1960s on such concepts, to develop a complete propulsion system which actually condenses the air and separates LOX from the air stream would require much additional research and testing.

Creating a purely mechanical condensation system using an air-breathing engine’s power that would condense the incoming air in a fraction of a second and still is light enough to put on a vehicle, would be virtually impossible. To advance the use of liquid hydrogen as a propulsion fuel, the “precooler” concept was originated by Robert P. Carmichael in 1955, using the hydrogen fuel (at 70 kelvins) to help condense the air into a liquid, not just to cool it. This was followed by the liquid air cycle engine (LACE) idea which was originally explored by Marquardt and General Dynamics in the 1960s as part of the US Air Force’s aerospaceplane efforts. This process also heats up the hydrogen and makes it ready to ignite in the engine. The liquid hydrogen is much colder, much less dense and viscous than liquid oxygen.

Cryogenic heat exchangers based on liquid hydrogen (LH2) for condensation require a larger heat exchanger surface, calling for compact, lightweight cryogenic heat exchangers with high surface-to-volume ratios as demonstrated by the Skylon team. Engineering design and construction of these heat exchangers between the air intake and the engine must be guaranteed to be free of flaws that could let the propellants mix accidentally. (It would be extremely dangerous to mix the oxidizer and fuel even when liquid before they enter the engine). Any ingested object which might damage the heat exchanger such as a bird strike could be disastrous.

The Japanese effort using the LACE concept began in the 1980s and uses a rocket engine for both the air-breathing and pure rocket phases of flight. During atmospheric flight, the intake air is condensed, pumped and injected into the rocket engine directly. It is not stored for use during the exo-atmospheric phase of flight—that LOX is loaded before launch. This allows a single engine type to cover both flight phases and keeps the engine simple.

The Indian propulsion system (engine and fuel system) uses a concept called FLOX which takes part of the cooled air coming into a scramjet engine when it is still in the atmosphere, condenses it into a liquid, and stores it in a LOX tank. Such a SSTO starts flying with an essentially empty LOX tank, uses the rest of the cooled air during the scramjet phase, and, as it reaches the height where it cannot use air for combustion any more, switches to the condensed LOX oxidizer and to a pure rocket engine mode.

With a separate turbojet engine for runway takeoff and landing and flight to Mach 2.5, a ram-scramjet engine thereafter to Mach 8, and a rocket engine for ascent thereafter to orbit, the addition of FLOX as a new feature of the fuel system of the aerospace-vehicle makes the propulsion system more complex. The matching strategy for a two stage to orbit (TSTO) vehicle would be for the first stage to condense and pump the extra LOX into the rocket-powered second stage attached to its belly. The upper stage for such a vehicle would still need airbreathing engines for landing unless it uses the shuttle’s engineless system of energy management. Massive fuel transfers between vehicle components which later separate took place during every shuttle launch, and the Falcon Heavy will use a similar fuel transfer system to achieve its remarkable performance.

Though considerable research was carried out in the US in the 1960s on such concepts, to develop a complete propulsion system which actually condenses the air and separates LOX from the air stream would require much additional research and testing. Then the system will still have to deal with the inert liquid nitrogen, which is 77% of air and is not useful as a propellant. However this “waste nitrogen” is also at cryogenic temperatures and can be used to enhance the cooling capabilities on board the vehicle to increase the effective hydrogen/oxygen liquefaction ratio significantly beyond what liquid hydrogen alone when used as a cryogenic coolant. The warmed “waste nitrogen” then can be fed through in the after-burner contributing to additional thrust and fuel efficiency to speeds up to about Mach 6. Once you can condense the air and separate oxygen from it, however, the process of storing and transferring it as LOX is fairly standard. Such a system was proposed in India many years ago: the Hyperplane/Avatar spaceplane concepts. It is reported that such vehicles which carry no LOX at launch can be significantly scaled down in launch mass (by factors of five to eight) and size thus reducing the risks and costs of developing and flying full-scale vehicles straight from the drawing board.

Five classes of vehicles

There would seem to be five classes of potential air-breathing launchers based on oxidizer source, ignoring whether they are SSTO or TSTO, and whether the vehicle uses a ramjet or scramjet type engine during any part of its air-breathing phase.

  1. Carries all the LOX at launch for use during the latter pure rocket phase, is boosted to hypersonic speeds by a rocket booster, and then uses atmospheric oxygen during the air-breathing (scramjet) phase. Example: conceptual launcher derived from current scramjet test vehicles.
  2. Carries all LOX at launch for rocket phase but cools incoming air with LH2 during the air-breathing phase for use in a gaseous air-hydrogen rocket engine that then doubles up as a LOX/hydrogen rocket engine. Example: Skylon
  3. Carries all LOX at launch for rocket phase, cools and condenses incoming air with LH2 during air-breathing phase and uses the liquefied air directly as the oxidiser during the same air-breathing phrase. Uses the same rocket engine during both flight phases. Example: Japanese vehicle powered by a LACE type engine system.
  4. Carries no LOX at launch. Uses separate ram air intakes with embedded heat exchangers for ram air collection, liquefaction and LOX separation. This condenses the entire amount of the required incoming air, with LH2 in hypersonic flight, then separates and stores LOX on board for use during the rocket phase. The engines used are turbojet/turbo-ramjet, switching over to a scramjet in hypersonic flight and finally to a rocket engine. Example: Avatar/Hyperplane from India. The benefits of in-flight air liquefaction and lox separation/storage (said to yield payload fractions up to 30%) were reported earlier in mid-1960s from studies at the Applied Physics Laboratory of John Hopkins University.
  5. A final variation. Carries no LOX at launch but with a separate intake provided for air collection, liquefaction, and LOX separation/on-board storage as in (4) above. Uses a single LACE (rocket) engine all the way to orbit, simplifying the propulsion system. The collected LOX is thus used for propulsion both in the atmosphere and above it. This is yet an unexplored concept, a possible integration of the Japanese LACE concept and the FLOX concept which emerged in India in the mid-1990s.

The air-breathing phase only of a class 1 type vehicle is currently being tested by at least several different scramjet programs in the US, Australia, and India. Funding for any further classes has been meager and very intermittent from all of the countries who have done any work on such vehicles. Research into materials for uncooled scramjet engine combustors and airframe hot structures needs to be intensified.

The first step would be a standard fully reusable hypersonic test vehicle that could be recovered after each flight and thus used as a flying test bed. Getting the tested hardware back to examine wear patterns in the engine would be very valuable.

The interest and emphasis on air collection, cooling/condensing and LOX separation has received intermittent attention in spurts in 1960s and again in 1990s. Research on the addition of nanoparticles for enhancing cooling capabilities of industrial coolants is a recent trend but has not been applied so far to liquid hydrogen. This potentially could become a key basic research area for advanced spaceflight systems. Individual companies have put some effort into advanced air-breathing engine design. Other than scramjets the only significant work involving actual precooler heat exchanger hardware development and testing seems to be that currently under way by Reaction Engines.

What is the best strategy for advancing the role of air-breathing vehicles in the face of the imminent cost reductions for vertically launched rockets and current low funding levels for all air-breather research?

The first step would be a standard fully reusable hypersonic test vehicle that could be recovered after each flight and thus used as a flying test bed. Landing on a runway or by parachute are equally valid methods of recovery depending on the size of the vehicle and where it is tested. Getting the tested hardware back to examine wear patterns in the engine would be very valuable. The same type of unmanned but reusable test vehicle could be used to test sub-scale versions of the more advanced engine concepts. Feasibility of this concept would be determined by how integrated into the airframe structure the engine systems are.

Companies should continue their focus on specific hardware component development to demonstrate proof of concept, but any hardware should be developed in an integrated way with the rest of the vehicle it would power, and flight-tested in a standard hypersonic flight test vehicle. Previously, entire vehicles were designed on paper without having any proof that individual parts of the system would even function or that the parts could function as a whole system. A good example is the National Aerospace Plane or NASP program which existed mostly on paper and collapsed as it was not going anywhere.

Far better are the smaller scale efforts like the current X-51A, which can prove out working engine concepts using small expendable unmanned vehicles or small scale vehicles that carry no LOX at launch. Such X programs, which design, build, test, and fly, then repeat the cycle until the result is “good enough”, have proved to be one of the best ways to develop new technology, since each hardware step must be proved to work before advancing to the next step. This saves vast amounts of money which would otherwise be wasted on non-workable paper concepts.

To reduce risk levels for advanced airbreathers, Technology Readiness Levels (TRL) need to be raised by extensive flight testing in a standard hypersonic flight test vehicle. Concurrently, separate efforts should focus on independent designs of TSTO and SSTO orbit vehicles. Both design types should be based on flight-proven technologies as they enter high TRL levels in the standard hypersonic flight test vehicle. This may be a better path than aiming for developing either a full scale TSTO or a full-scale SSTO right away directly from the drawing board. Thus, for the TSTO, the upper stage can be a pure rocket based on existing hydrogen engines and the first stage engine can be less complex. The only significant TSTO barrier other than the engine and heat exchanger is the fuel transfer technology and that has already been demonstrated in the Space Shuttle.

For pro-space organizations pushing for reduced launch costs, media generated “space races” created by real or imagined competition between countries are always beneficial, as long as the competition does not become a real race and thus turn into a wasteful crash program.

For governments, one obvious path for funding is to push for scramjet development for use as a standard hypersonic flight test vehicle. It is conceivable this may be of military interest as well provided the scramjet engine is based on kerosene fuel to enable the missile’s smaller size and allow a functional but modest size engine to be developed. An example of this is the current X-51A program. Once such an engine exists, it can then be converted to operate on hydrogen fuels and scaled up for use in space launchers, as long as the engine details are not totally classified, rather than just handled as a proprietary trade secret. Due to political pressures from the competition for funding, it is difficult to get new propulsion efforts started and sometimes even harder to keep them funded once started. Due to the difficulty and high expense in constructing hypersonic wind tunnels, small-scale test programs like the X-51A, where the vehicle is initially accelerated by a rocket, are probably more cost effective.

For pro-space organizations pushing for reduced launch costs, media generated “space races” created by real or imagined competition between countries are always beneficial, as long as the competition does not become a real race and thus turn into a wasteful crash program. In my view, the best outcome is for companies to compete and for countries to cooperate. However, in the real world, the best outcome is often the least likely outcome. To counter that, we need to keep reminding those in government that getting launch costs down should be our top priority right now.

With the very low launch costs potentially offered by such airbreathing space launch vehicles, a whole new range of space activities would become possible, such as large scale planetary exploration, cheaper space tourism and the construction of space solar power stations that are able to both compete with existing energy plants and actually have the capacity to stop global warming without breaking all of our banks. After 50 years of high launch costs, the last thing we need is another 50 years of the same. Fortunately, a real market competition now seems to be starting up, which hopefully will change all that. It is unfortunate that it did not happen 40 years earlier.