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I do not mean multiple combusters as in the linear aerospike. I mean place multiple aerospikes next to each other. Imagine a Falcon 9 or Saturn V with aerospikes instead of conventional bell nozzles. Is it possible? Are there some serious downsides?

I thought that aerospikes kind off expand against the ambient air. But that ambient air would be disturbed if there were multiple other aerospikes, no?

Firefly Beta, if it ever arrives, would cluster three cores each with it's own aerospike. Any disturbance between the three aerospikes to be expected?

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    $\begingroup$ Interesting question. Exhaust recirculation is a big design challenge for boosters using normal engines, seems like it would have some effect on aerospikes. Check out this picture of the Saturn V with the plumes climbing up the sides of the first stage. friends-partners.org/mwade/graphics/0/10074832.jpg $\endgroup$ Sep 1 '16 at 4:07
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    $\begingroup$ Interesting picture. Raises a side question: why the **** do the side plumes "want to" climb up the sides? $\endgroup$
    – Gulango
    Sep 1 '16 at 6:14
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    $\begingroup$ The base pressure environment on launch vehicles is very complicated. Preflight predictions of what it would be for Shuttle were far enough off that the first flight's ascent trajectory was significantly different than expected. ntrs.nasa.gov/search.jsp?R=19840002053 $\endgroup$ Sep 1 '16 at 13:28
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I think you're right, the separate aerospikes would interfere with each other and, depending on the placement, might not benefit from the aerodynamics of the vehicle while in atmosphere. Note that the linear aerospike you mention, intended for the X-33, fills out nearly the entire rear surface of that vehicle. Philip Bono's aerospike designs from the early Sixties (e.g. the Hyperion) also feature large plug nozzles completely covering the underside. Clustering the engines might require some fancy aerodynamics to get the right effect. And besides, it doesn't give you any redundancy benefit that isn't already present in the plug nozzle concept, which already uses multiple pumps and so on.

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  • $\begingroup$ The nozzle(s) covering nearly the entire underside of a rocket isn't really specific to aerospikes. $\endgroup$ Mar 8 '18 at 14:59
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http://www.astronautix.com/s/saturnv-3b.html

This proposed modification of the Saturn V would have clustered seven 400klbf thrust toroidal engines in its second stage and one more in the third stage. These engines would all operate in low-pressure or vacuum conditions, so there would certainly be plume interference among the engines. The benefit of the aerospike over the bell engine in this case was actually the drastically shorter nozzle length, which would have allowed for a 1.45m tank stretch and over 40 tonnes more propellant in the second stage. The extra propellant would allow an extra 7 tonnes to TLI compared to a similarly-modified variant, the MLV-V-3, even though this vehicle used a bell nozzle engine with higher Isp.

The potential 2.8 million pounds of thrust from the engine cluster meant that a two-stage variant (Saturn INT-17) without the MS-IC was studied - and in that case the seven toroidal engines would need to operate both at low and high altitudes. While it was never built, the design indicates that such an arrangement was possible. However, with such close packing of the engines - 2.8m gimbal separation - it is likely that the low-level altitude compensation effect would be reduced.

Remember that part of the effect on truncated toroidal nozzles (ie. the nozzle does not taper to a single point) comes from a recirculation zone under the engine. The pressure within this region remains higher than the ambient pressure and essentially extends the nozzle, hence "aero-spike." As such, aerospike engines can make small efficiency gains from this trapped pressure region when in a vacuum. Presumably such an effect would not be interrupted even by close-mounted engine clusters.

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The middle engines wouldn't see much befit, since they see the low pressure environment from the surrounding nozzles, rather than ambient pressure in the case of overexpansion.

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