Future Design Concepts

Advanced spaceplanes will be powered by air-breathing turborockets of some kind. The specific impulse and delta V (AV) advantage of this kind of engine makes it essential. It is useful to keep the rocket equation in mind:

To review, the AV, or velocity increment required for spaceflight, depends on rocket exhaust velocity c and mass ratio R. The exhaust velocity is the product of specific impulse / and the standard acceleration of gravity ge, while the mass ratio is the initial fueled mass divided by the final mass of the rocket-powered vehicle after burning all propellants.

An air-breathing engine generates much of its thrust by admitting, compressing, and expelling the available atmosphere. This greatly increases the thrust without increasing the onboard propellant consumption. Specific impulse, you will remember, is the thrust divided by the onboard propellant usage rate. Dimensionally, this is pounds divided by pounds per second, yielding specific impulse in seconds. Because air-breathing engines utilize the atmosphere as propellant, the specific impulse can be as high as 10,000 s, compared to conventional chemical rockets, which have values in the 250-450-s range. The propellant consumption is low in air-breathers, because it involves a fuel only. Most of the working mass used to produce the thrust is the air itself, which also contains the oxidizer.

What features might a future air-breathing turborocket - one capable of powering a spaceplane - include? It will undoubtedly incorporate elements of the aeros-pike for altitude compensation, some sort of turbine for low-speed operations, and possibly ramjets for high-speed operation. Clever designs and methods will have to be used to keep weights to a minimum.

Imagine several small ramjets mounted tangentially at the perimeter of a turbine, so that when the engine shaft is rotated, the ramjets eventually encounter supersonic conditions, even when the vehicle is parked on the ramp. Spinning up the turbine by pneumatic, electrical, or magnetic means, the ramjets could then be started, to provide continuous rotation of a conventional compressor. In this manner, ramjets could provide the turbine-power for spaceport taxi and low-airspeed operations. While the forward speed of the spaceplane is zero or subsonic, the speed at the turbine tips would be supersonic, ideal for ramjet operation. Since ramjets have no moving parts, this type of engine would be very reliable. Now imagine those same ramjets mounted on pivots that could be rotated forward for supersonic flight. By simultaneously rotating the several ramjets forward, slowing and then halting the shaft, supersonic operations could then be sustained in a nonrotating air-breathing engine with no moving parts. The engine is the same. It has simply changed configuration. The ramjets are now "looking forward." They can now take the spaceplane up to around Mach 5, or about 5,000 ft/s, before the actual rocket engines are powered up.

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