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From: (GCHudson)
Subject: Re: Altitude Compensating idea
Date: 5 Aug 1998 00:14:46 GMT

Robert Lynn wrote:

>>As I understand it 'altitude' testing of rocket engines is performed by
running the high speed exhaust of a jet engine past the the nozzle in
order to create a lower pressure region.  If that is indeed correct then
would it be possible to use this in the design of an SSTO?  Use a jet
engine to eliminate the need for an altitude compensating nozzle by
simply running its exhaust around the conventional bell nozzle.  A jet
is useful for landing but is of only marginal use in the acceleration
phase.  Could this augmentation produce sufficient performance
advantages to make it worthwhile?  Not really of much use for a HTOL
because you're probably better off using just the jet but could this
work for a VTOL concept?  For a ROTON sized vehicle using an F119 engine
(roughly 160kN or '16 tonnes' thrust) massing maybe 2-3 tonnes, saving
perhaps 160kg of rocket engine, + 60 seconds * 16000(1/310-1/2000) =
2500 kg of fuel + the rotor system + tankage for 2500 kg of fuel +
perhaps an Isp improvement of 20s for the engine for the first 60
seconds (say 2MN at 330s vs 310s) = 60*2e5(1/310-1/330) = 2340 kg
Adding up to maybe 5000-6000 kg of gains for an added mass of 3000 kg of
jet engine and ducts.

I'm pretty sure I am making a major mistake here, but what?


This is a version of a ducted rocket, and is similar to a fan-rocket Phoenix
design I proposed to the USAF in the mid-1980s.  I think it makes lot of sense
for both VTOL and for HTOL vehicles, including tankered concepts.  I don't
think you're mising anything.  ;)

Gary C. Hudson

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