Subject:
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Re: Spacecraft propulsion (was: Ship Power Core)
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Newsgroups:
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lugnet.space
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Date:
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Thu, 16 Dec 1999 21:34:02 GMT
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Reply-To:
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jsproat@io.comNOSPAM
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Viewed:
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836 times
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"John J. Ladasky Jr." wrote:
> For a conventional rocket engine -- say, the Space Shuttle's
> liquid-hydrogen, liquid-oxygen fuel system -- the specific impulse is
> about 475 seconds, at sea level. The efficiency drops off with
> increasing altitude.
This is due to the shape of the engine bell -- how gases tend to collect
inside instead of just spewing out, increasingly so the farther you get from
optimal pressure. This waste gas not only robs from the total thrust mass,
but also introduces drag within the bell.
> There's little room left for improvement in
> chemical thrusters. I've found some web pages that refer to a change in
> the exhaust nozzle geometry that they call the "linear aerospike"
> engine. This will eliminate the loss of efficiency with altitude.
Aerospike is totally elegant/cool. It solves the bell shape problem because
there is no bell. Instead, the jet exhaust is directed against the upper and
lower slopes of a truntcated wedge, merging together at the narrow end. The
errant gases are trapped and compressed between these jets of exhaust and the
wedge. (1) Instead of introducing drag, they essentially become part of the
hardware. The shape of the "spike" is determined by the ambient pressure, and
maintains an optimal streamlining form from sea level to orbit. The result is
that less thrust mass is wasted.
A side benefit of using an aerospike instead of a bell is that it's much
easier to vector the thrust. Think of an aerilon.
Aerospike engines are currently being developed for one of the next-generation
Space Shuttles -- the Venturestar, I believe.
Cheers,
- jsproat
1. Crude ASCII Figure 1:
exhaust -------->
====== -------->
wedge ===========***-------->
===========***-------->
====== -------->
exhaust -------->
The *'s represent excess gases trapped between the wedge and the merging jet
exhaust.
--
Jeremy H. Sproat <jsproat@io.com> ~~~ http://www.io.com/~jsproat/
"Hello. My name is Jeremy, and I'm a...a brick chewer."
(in unison) "Hi, Jeremy."
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Message is in Reply To:
 | | Spacecraft propulsion (was: Ship Power Core)
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| Hi there, You've got to love these fantasy-tech discussions that spring out of the desire to model spaceships in Lego! To all of you who have posted pictures: love your designs. They have the no-nonsense look of real machines. (...) O.K., Tobias, (...) (25 years ago, 16-Dec-99, to lugnet.space)
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