[AR] Re: Aerospike
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Tue, 29 Mar 2022 16:09:56 -0400 (EDT)
On Tue, 29 Mar 2022, Terry McCreary wrote:
...Instead of expanding against an external cone, the gases expand
against the central aerospike to create that additional thrust.
Properly, that's a "plug" or "spike" nozzle -- it's only an aerospike if
the central spike has been severely truncated, with (usually) a small feed
of low-velocity gas added to replace the solid spike, creating an
"aerodynamic spike" -- hence "aerospike". However, nowadays the term is
often used more loosely for any nozzle vaguely along those lines.
The central spike of an aerospike is much lighter than an exit cone of
the same length. I'm sure that there are other advantages but I'm not an
expert.
More importantly, because the outer surface of the flow is the surrounding
atmosphere, to some degree the flow adapts itself to the ambient pressure,
giving a variable expansion ratio, something that the conventional nozzle
can't do. How effective that is, is a complicated question which really
needs to be resolved by instrumented flight tests; those unfortunately
haven't yet happened much.
A disadvantage of an aerospike is that the spike is right in the middle
of the hot exhaust, and must be cooled in some way, or made of materials
that can withstand such high temperatures for a long time.
A proper aerospike eliminates much (not all) of the heating problem by not
having a long solid spike; that also makes it lighter. The bad news is
that it typically needs a bit of low-velocity gas flow from somewhere, and
its behavior and performance are complicated and poorly understood.
(There have been lots of CFD calculations and simulations and small-scale
wind-tunnel tests, yes, but nobody's sure how well those would translate
to an actual full-scale flight system.)
Henry
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