🚀 go-pugleaf

There has been a lot of speculation as to the reasons why the SDIO SSRT 
program  is not using the aerospike engine for propulsion as originally
envisioned by the SSTO proponents in the 60s and later.

I saw a summary of the report done by The Aerospace Co. for the Air Force on
the SSX proposal that was made by Max Hunter, et.al.  This proposal was
the one that lead to the SSRT program being started by SDIO.

A couple of caveats emerged concerning the SSX design that were felt to be
potential show stoppers in meeting the SSX goals, all related to the 
use of aerospike engines.  First, the "engine first" reentry profile 
required active cooling of the heat shield/engine plug assembly.  It was
felt that this complicated plumbing could be a nightmare to repair if
a leak developed and hindered access to part of the structure.  Since
low maintenance was a goal of SSX, this was considered a risk.  Second,
no aerospike engine of the size  proposed had ever been built, much
less tested.  This was also a risk, as "simple scaling" of rocket engines
is an oxymoron.  Third, the exact performance characteristics of
aerospike engines at high thrust and multiple altitudes are somewhat
unknown.  The aerospike work NASA had funded had limited data from
vacuum tests, concentrated on lower thrust engines (since the test
stands at Lewis? in the vacuum chambers can only evacuate a limited
amount of exhaust products).  Given the uncertainty about large aerospikes,
I'd agree that they are a risky choice with multiple points of failure. 
My opinion is that Marshall or Lewis ought to be working on them (as well
as ion engines, arcjets (although DOD may fly some soon), solar sails,
and nuclear thermal engines) right now, rather than only trying to tweak an
additional 3 Isp out of H2O2 engines. 

The study was cautiously favorable to SSX, recommending use of conventional
engines if possible, and watching weight margins carefully.  (The major
warning was that if the mass of the vehicle rose only slightly from
projections, the useful payload would be wiped out.  From what I have
seen, the DC-X team is very aware of this.)

I would also like to second Bill Higgins' call for the SSTO 
"discussion" to be calmed down a bit. There has been more heat than
light generated recently.  It is pointless to argue about what the
DC-? can or can't do at this point.  Let's watch the DC-X flight this
spring and lobby, lobby, lobby for more money.

-- 
Edmund Hack - Lockheed Engineering & Sciences Co. - Houston, TX
hack@aio.jsc.nasa.gov - I speak only for myself, unless blah, blah..
"You know, I think we're all Bozos on this bus."
"Detail Dress Circuits" "Belt: Above A, Below B" "Close B ClothesMode"