The Aerospike Engine

The aerospike rocket engine represents the epitome of efficiency in the pure rocket. Although it makes no direct use of the atmosphere, it nevertheless operates efficiently and in harmony with local pressure conditions, including the zero pressure of space. Aerospike engines do not look like rockets at all. They are, in essence, inside-out rockets. The familiar bell nozzle configuration is replaced by a curved ramp with either a linear or a circular shape. A series of small thrust cells is arranged along or around the ramp, onto which the hot gases from those cells are expelled. Or a doughnut-shaped combustion chamber may be used, with a circular slit serving as an annular nozzle. The exhaust gases expand in the region adjacent to the ramp and provide an optimum "altitude compensation" from the ground to the vacuum of space. Exhaust products are allowed to expand optimally between the ramp and the ambient atmosphere, instead of being expected to expand properly inside a fixed nozzle. The ramp takes the place of the nozzle walls and provides structure for the rocket exhaust gases to push against, thereby providing thrust for the launch vehicle. While conventional bell nozzles are inherently either overexpanded or underexpanded

M.A. Bentley, Spaceplanes: From Airport to Spaceport, 105

doi:10.1007/978-0-387-76510-5_8, © Springer Science + Business Media, LLC 2009

under virtually all conditions, the unique inside-out ramp of the aerospike ensures optimum expansion of the rocket exhaust products at all times. Hence, ambient pressure conditions allow continuous altitude compensation and peak efficiency, whether the rocket is operating at sea level or in the vacuum of space. Because of this, the specific impulse of an aerospike engine may reach 430 s or more, significantly better than conventional designs, which yield only around 350 s (Fig. 8.1).1

Because of the greater efficiencies built into aerospike engines, both the X-33 testbed and its planned successor, VentureStar, were to have used them. VentureStar would have been the world's first single-stage-to-orbit launch vehicle. It was intended to lift off vertically like the Space Shuttle but without an external tank or strap-on solid propellant rockets. Instead, it would have carried all of its liquid propellants internally. Later, it would land on a runway, again like the Shuttle. It would have been a fully reusable single-stage-to-orbit spaceplane. We will revisit the aerospike concept later in this chapter. But first, let us look at some other ideas that make more direct use of the free air around us.

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