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The Space Review: Current strategies towards air

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Current review space strategie

   

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 by John K. Strickland
Monday, August 1, 2011

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.

Commercial communication and other satellites built and operated by and for commercial companies;

Government unmanned launches, occurring at a significantly higher rate than private launches due to continued use of military style cost-plus contracts;

Manned launches of crew to the space station or for deep space missions;

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.

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