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From: todd@sphinx.UChicago.UUCP (Todd Nugent)
Newsgroups: net.aviation
Subject: Re: Pulsejets
Message-ID: <1172@sphinx.UChicago.UUCP>
Date: Tue, 8-Oct-85 21:21:40 EDT
Article-I.D.: sphinx.1172
Posted: Tue Oct  8 21:21:40 1985
Date-Received: Sat, 12-Oct-85 13:20:13 EDT
References: <3024@mhuxd.UUCP>
Organization: U. Chicago - Computation Center
Lines: 39

>Could one of the technically knowledgeable people on this net
>enlighten the rest of us about pulsejets?  

The pulse jets I have played with were rather simple: you somehow get
a combustion chamber red-hot and then drip some sort of liquid fuel
into it while you control the fuel flow so that the expanding
vaporized fuel mixture detonates at a reasonable distance down an
exaust tube.  The "compression" is provided by a slight constriction
just before the exaust tube. It is the same principle as a gas
backpackers stove. (If you have ever used an MSR/XGK backpacking
stove you have probably noticed that it sounds like a pulse jet!).
The drops of vaporizing/detonating fuel provide the distinctive "pulse".

Maintenance isn't so much a problem as is the fuel flow:  too much
fuel and the expanding gas drives the detonation front down and out
of the exaust port causing a "flame-out".  To little and the
detonation moves up into the evaporation area leaving you with an
expensive hand warmer.  I have used one powered with liquid propane
and one powered with avgas.  In both cases the fuel flow was
controlled with a needle valve (the only moving part!) and in both
cases they were more than just "touchy": I would not like to try and
adjust 8 of those things at once!

Presumably, a computer controlled fuel meetering system would
provided greater thrust, at least according to NASA Lewis it will
provide about 40% more thrust for a typical gas turbine aviation
engine--then again who wants their engine to flame out when the
contoll system rolls-over?

Pulse jets are relatively expensive--considering the single moving part--in
materials and welding owing to the requirement that the combustion
chamber be able to glow cherry-red hot for long periods of time.

Todd.

P.S. Is anyone else out there working on a homebuilt airplane?  I am
currently in the early stages of a KR2 project--NOT pulse jet
powered--and would be interested in talking to others who prefere to
build their own. (other than a few times a year at EAA events!)