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From: kwan@smeagol.UUCP (Richard Kwan)
Newsgroups: net.space
Subject: Re: Scramjets (specific impulse)
Message-ID: <623@smeagol.UUCP>
Date: Tue, 4-Mar-86 17:28:59 EST
Article-I.D.: smeagol.623
Posted: Tue Mar  4 17:28:59 1986
Date-Received: Fri, 7-Mar-86 06:47:06 EST
References: <344@vger.UUCP> <617@smeagol.UUCP> <34@petrus.UUCP>
Organization: Spacecraft Data Systems, JPL, Pasadena, CA
Lines: 61

In <617@smeagol.UUCP> I originally said... 
> > 1.  Specific impulse (Isp): thrust per pound of propellant.  At least,
> >     that's the way I learned it, a carry over from non-metric
> >     engineering.  (Thrust per unit mass is probably more meaningful.)
> >     Propellant naturally includes both fuel and oxidizer.  You are
> >     correct that in the case of air breathers, they get their oxidizer
> >     from the atmosphere.  Thus, their Isp's are higher.  Rockets tend
> >     to have Isp's in the low 100's; turbojets in the 3000's (?), and
> >     ramjets somewhere in between....

And in <34@petrus.UUCP>, Phil Karn responded:
> I have several problems with this.


With good reason.  I blew it.

> Specific impulse is often erroneously specified in "seconds"; the correct
> units should be "meters/sec", i.e., velocity.
I don't know what got into me.  You are correct.  "Seconds" is the
accepted units in the English system.

> This way of expressing specific impulse has a much more elegant and
> straightforward meaning: it is simply the velocity of the rocket exhaust
> relative to the rocket.
Some other classical examples:  How much thrust do you get if you burn
one pound of propellant for one second?  Or, how many seconds can you
burn one pound of propellant if you maintain one pound of thrust?
Hence, we get pound(thrust)-seconds per pound of pound of propellent.
The numerator is impulse; thus, the per pound measure is termed
"specific impulse."  But as you say, the metric version, meters/sec,
is probably clearer.

> ... It's not clear to me that "specific impulse" has any meaning,
> though, for an air-breathing (and air-pushing) aircraft, nor for an
> automobile.
Perhaps so.  Some clarifications are in order.
1.  All the figures I gave for specific impulse were in seconds.  I have
    not worked with the metric form.
2.  Certainly for air breathers, the velocity analog does not work.
    The figures I gave for air breathers are for
	thrust x time / *fuel*.
    Thus, the unusually high performance rating is due to not carrying
    oxidizer.  (Not my idea; sorry, can't remember the source.)

By the way, Hank Walker <dmw@UNH.CS.CMU.EDU> reminded me that there
IS such a thing as a variable geometry engine.  He points out:

    ...you can convert a scramjet to a ramjet, and perhaps start
    as low as Mach 0.5.  Scramjets are really only useful above
    Mach 5...

You propulsion specialists can take it from here.

-- 
		Rick Kwan
		JPL Spacecraft Data Systems
		sdcrdcf!smeagol!kwan (UUCP)
		ia-sun2!smeagol!kwan@csvax.caltech.EDU (ARPA)
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