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From: weltyc@nysernic (Christopher A. Welty)
Newsgroups: sci.space.shuttle
Subject: Re: Throttling the Challenger
Message-ID: <471@nysernic>
Date: Mon, 26-Oct-87 17:04:16 EST
Article-I.D.: nysernic.471
Posted: Mon Oct 26 17:04:16 1987
Date-Received: Wed, 28-Oct-87 22:28:41 EST
References: <340@ablnc.ATT.COM> <3143@ames.arpa> <6563@ut-ngp.UUCP> <8796@utzoo.UUCP> <201@splut.UUCP>
Sender: nobody@nysernic
Reply-To: weltyc@nisc.nyser.net.UUCP (Christopher A. Welty)
Organization: RPI Computer Science Dept.
Lines: 36
Keywords: Challenger Throttle

In article <201@splut.UUCP> stu@splut.UUCP (Stu Cobb) writes:
>In article <8796@utzoo.UUCP>, henry@utzoo.UUCP (Henry Spencer) writes:
>> Engines, like lightbulbs, generally do not have a single specific setting
>> for which they are designed, in the sense that all other settings work
>> poorly or not at all.  
>
>Sorry, but I don't believe this is true.  The Shuttle engine is rather rare
>in that it can be throttled over almost a 2:1 range.  Most rocket engines
>are, in fact, designed to run at a particular setting.  This is primarily
>because most rocket engines were designed before flexible control systems
>were available.  They use pneumatic/hydraulic/fluidic controls (the
>pinnacle of late-50's technology) which are fussy about their set-points.
>

	In fact, as I recall, the X-15 was the first vehicle with
"throttleable" rocket engines.  I saw a clip once of that famous
guy...ummm...Scott Crossfield (something like that, I can't believe I
don't remember...) testing the X-15 engines new throttle.  It blew up
- big explosion, too.  Scotty was OK, walked away in fact. Turned out
to be a faultly valve.   Anyway, it ended up working.  The camera view
of the test was right in back, so you could see the rocket engine
throttling up and down as he announced the current rate.  They were
able to take it from 0% up to 100%.  I believe the shuttle *main*
engines are based on the same idea.  Of course the SRBs are not
dynamically "throttleable", although I believe you can preset a
certain thrust pattern by playing with the solid fuel.

	Anyway, this was not Henry's point.  He is correct in that
certain thrust settings are very inefficient, and that most engines
are built for optimal performance at a specific thrust.  For the
Enterprise, I believe this was Warp 6 (although it is arguable that
*warp* drive generates any thrust).


Christopher Welty  ---  Asst. Director, RPI CS Labs
weltyc@cs.rpi.edu       ...!rutgers!nysernic!weltyc