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I mean, it's called an aerospike engine for a reason, I guess, but are there any other advantages to the engine that accrue in space? Is the bell configuration better in a vacuum?

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    $\begingroup$ Some people use the term aerospike only for truncated spike nozzles with gas generator exhaust exiting at the base of the spike. $\endgroup$ – Everyday Astronaut Jan 2 '20 at 19:55
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The built-in altitude-compensating nature of the aerospike is its primary advantage.

tl;dr: It's better than a bell at low altitudes, and as good as one at high altitudes.

However, many claim that the ultimate strength of the aerospike nozzle is its inherent altitude compensation capability, as shown below.

enter image description here

This ideal behavior results from the fact that the outer plume boundary of the primary flow is acted upon only by the ambient pressure of the atmosphere.

Recall from our discussion of aerospike thrust characteristics, high ambient pressure at low altitudes forces the exhaust inward increasing the pressure on the "centerbody" and the centerbody component of thrust. In addition, the base region is open to high ambient pressure resulting in a greater "base" thrust component. At design pressure, the flow becomes column shaped, much like a bell nozzle, for maximum efficiency. When operating at low ambient pressure (at high altitude or in a vacuum), the flow is constrained by expansion/compression waves that direct the exhaust axially to maintain the thrust force on the centerbody.

At low pressures, however, the nozzle operates in a "closed wake" state. Since the base is not subject to a high ambient pressure, there is no altitude compensation benefit, and the aerospike behaves like a high area ratio bell nozzle. Thus, in theory at least, the aerospike nozzle meets or exceeds the performance of the bell nozzle at all operating pressures.

(emphasis mine)

Source

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