It is a
hypersonic jetliner that takes off and lands as a
conventional plane, but is capable of achieving approximately
mach 25 (the velocity needed to escape the Earth's gravitational
pull) so that it may also enter orbit and dock with an orbital
space station.
Such a development will be crucial to any advance we make in
space, as it will provide a fully reusable craft that will give
cheap, reliable and safe access into space.
These planes cannot rely on rocket power alone, using current technology it is not possible. The combined weight of the fuel and oxidiser (remember rockets must carry their own oxidiser) is very large due to the fact that a lot of energy is expended pushing the plane forwards. This is why today's rockets launch vertically as it maximises the rocket's potential by allowing all the energy expended to be focused in the direction we want to go…upwards. With present technology it is the easiest and cheapest method of reaching space.
Clearly then the way forward is to utilise jet engines in some manner. The main advantages of jet engines over rocket engines is that they do not need to carry their own oxidiser, instead they stuck in air and use the oxygen present in the air as their oxidiser. This will greatly remove the need to carry oxidiser, as it will only be needed when at an altitude that the air contains insufficient oxygen for jets to operate. At this point the rocket engines will fire and burn the much smaller quantity of onboard oxidiser. This will dramatically reduce the take-off weight and also the cost of the craft. Further to this the use of jet engines will make a substantial saving on the expensive rocket fuel. As a comparison to produce the same thrust, jet (air-breathing) engines require less than one seventh the propellant (fuel + oxidiser) that rockets do.
Another problem with horizontal take-off that must addressed is that the plane must use wings to generate lift. These wings can be problematic on re-entry, as they will heat up dramatically making the craft less stable. Further to this there is no use for wings in space where there is no air. On the flip side however as jet-engine craft rely on aerodynamic forces rather than on rocket thrust, they have greater manoeuvrability, which in turn provides better flexibility and safety, for example missions can be aborted mid-flight if there is a problem.
Horizontal LandingHorizontal landing's are already the best method with current technology and this method is utilised by the shuttle.
Current Proposals - 2-Stage| A 2-Stage Spaceplane Proposal |
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| Courtesy NASA |
There are two main proposals to develop a vehicle able to launch horizontally and reach space, they are imaginatively called the 1-stage and the 2-stage approach. We shall look at the 2-stage first.
This is the easiest method, firstly a large aeroplane takes off carrying a smaller rocket engine craft (called the orbiter) and reaches a fairly high altitude. Then the smaller craft launches from the carrier and as it is already at high altitude before firing its engines, the need for fuel is minimised. It also means that the wings on the orbiter can be made smaller. There is no doubt that this option certainly creates less engineering problems. This type of technology was around in the 1960's and was used during the testing of the X-15.
Current Proposals - 1-Stage| 1-Stage Proposal (Skylon) |
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| Courtesy This is Rocket Science |
A reusable launch vehicle (RLV) that takes off and lands horizontally like a conventional plane, this is what is referred to as a true spaceplane. It is generally regarded that this method will be more efficient and safer than the 2-stage model, though that is not to belittle the 2-stage method which would be a considerable improvement on the vertical take off craft of today. It is also felt that while the 2-stage idea would be easier, the 1-stage would almost certainly be more commercially viable and would achieve a higher level of success in the objectives of a spaceplane.
What is required here is further development of jet
engines.
The only possibility at the moment are
ramjet working together with
scramjet.
The major problem is that the scramjets are far from fully developed,
offering many difficult aerodynamic problems. These, however,
offer the only current hope of sustained
hypersonic flight.
Even with the advance of scramjet development there are still many problems to be addressed with horizontal take off of spaceplanes. This is because a Scramjet will only function at hypersonic speeds and a ramjet will only function at supersonic speeds. The design will therefore require:
- A turbojet, once the air intake reaches to mach 1 (supersonic) the ramjet would fire.
- The ramjet would accelerate the plane to mach 4 (hypersonic) then the scramjet would fire.
- The scramjet is expected to be able to reach speeds of mach 15, when finally the rocket engine would fire.
- The rocket engine would accelerate the plane to mach 25 (escape velocity) and would be used in space operations.
While this sounds very good in theory, in practice it is very
doubtful whether such vehicles will have the efficiency to reach
orbit, due to the excessive weight and complexity of such a
system. Further to this such a design will not solve the other
problem of heat build-up.
These problems have not, however, removed the interest in this
system and several proposals are currently being tested by
NASA.
What we are really looking for is the development of a combined jet engine, that operates across the range, with maybe a switch to a rocket engine for the last stage and for space operations. The difficulties of designing a jet engine to perform at these levels are such that it can not even be seen how it could be done with present technology. The differences between the engines are how they physically take the air in. Nanotechnology could solve the problem by allowing the engine to reshape itself in flight, whether it could be shaped fast enough remains to be seen.
The other major advance that is required is the development of advanced materials such as composite alloys that can handle the heat associated with hypersonic flight. Once again nanotechnology could be instrumental.
Current Proposals - Hybrid proposalsThere are various 'One and a Half Stages' ideas that are certainly innovative ideas and deserve mention. The most promising is that of mid-air fuelling, taking on the fuel and oxidiser for space once at a high altitude. These ideas do not overcome the problems of commercially viability that the 2-stage models suffer from, however it could be a good temporary measure.
Current Spaceplane Designs There are several interesting projects out there right now, the main ones being:- The Lockheed VentureStar - the planned successor to the space shuttle, the future of payload lifting and space tourism?
- NASA's X-33 - A RLV that is a prototype for a space shuttle replacement (the VentureStar prototype)
- NASA's X-34 - A sub-orbital vehicle that will test technologies to reduce cost, time and personnel for space launches
- NASA's X-37 - which will test many space plane technologies, including re-entry capabilities. Delivered to orbit by the shuttle
X-33 |
X-37 |
| Courtesy NASA | |
Nasa's X-Planes are projects testing various features that
will be required to provide a fully reusable launch vehicle that
allows routine, regular and cheap spaceflight.
The initial plan is to drop the cost of lifting payloads to space
by a factor of at least 10.
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| Courtesy NASA | |
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Future Orbital Developments
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