Whether incorporated into a rotating turborocket or not, future rocket engines will need to maximize their efficiency. Let us now take a closer look at the aerospike rocket engine, and consider whether it can be combined with other concepts as well.
Conventional bell-shaped nozzles are efficient at one altitude only, because the ambient atmospheric pressure affects the expansion of the rocket exhaust. For peak efficiency, the exhaust products must expand within the bell such that they fill the nozzle and emerge "just right." Nozzles are therefore built for certain conditions. Low-altitude nozzles are relatively short, because the rocket exhaust does not need to expand much to match the surrounding air pressure. High-altitude nozzles are much larger, allowing the escaping gases to expand to the much lower ambient pressures at those altitudes. The exhaust gases perform efficient work in pushing the vehicle forward as long as they maintain the proper expansion inside the nozzle. And yet these nozzles are continually afflicted with inefficiencies at every altitude but one, because the ambient conditions are continuously changing during the boost to orbit. They are either "overexpanded" at low altitude or "underexpanded" at high altitude. In overexpansion, the nozzle is too big for the ambient pressure, allowing the exhaust gases to separate from the nozzle walls before they emerge from the nozzle exit plane. In underexpansion, the nozzle is too small at high altitudes or in space, and the exhaust gases therefore balloon out behind the nozzle. Both conditions reduce the efficiency and the specific impulse of a rocket engine.
Aerospike engines solve the problems inherent in conventional convergent-divergent thrust chambers. They do this by turning the nozzle inside-out, so that the expanding gases are directed along a ramp between the atmosphere and the ramp. This results in automatic and continuous altitude compensation, and superior efficiency at all altitudes, including the vacuum of space. Aerospike designs can be either linear or annular. The VentureStar spaceplane was slated to use linear aero-spikes to enhance its ability to reach low Earth orbit with a single stage. These engines were considered a vital design feature of the SSTO VentureStar and its unmanned testbed, the X-33.
The annular aerospike engine has "thrust cells" arranged in a circle around a roughly cone-shaped truncated ramp. By angling these thrust cells or using canted vanes in the exhaust similar to those originally used by William Hale over a century ago, the entire engine or parts of it can be rotated. In the Hale rocket, the entire rocket was spin-stabilized using this technique. In the annular aerospike, only certain parts of the engine need to spin, notably the air intake and compressor sections for flight within the atmosphere. The idea is to use a single engine functioning more like a jet in the atmosphere and as a pure rocket in space. This reduces the weight of the spaceplane when compared with designs that use separate jet and rocket engines.
For takeoff and "spike-jet" operations only a portion of the rocket engines need to be used, or alternatively, only a portion of the small thrust cells within the annular aerospikes need to be activated. The number in operation would depend on the total power required to operate essentially as a conventional jet aircraft or to liquefy ambient air for later use during the ascent to orbit. These power requirements are very low as compared with those necessary to achieve spaceflight. As in conventional jet aircraft, the main function of the engines is to overcome drag, once the climb to altitude has been accomplished. The thrust-to-weight ratio during this part of the flight can be less than 1, because the wings are providing the lift aerodynami-cally, reducing gravity losses to zero.
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