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From: kwan@smeagol.UUCP (Richard Kwan)
Newsgroups: net.space
Subject: Re: Scramjets
Message-ID: <617@smeagol.UUCP>
Date: Mon, 24-Feb-86 18:39:08 EST
Article-I.D.: smeagol.617
Posted: Mon Feb 24 18:39:08 1986
Date-Received: Sat, 1-Mar-86 00:44:02 EST
References: <344@vger.UUCP>
Organization: Spacecraft Data Systems, JPL, Pasadena, CA
Lines: 51

> A while back there was a discussion of scramjets on the net.  Unfortunately,
> I wasn't paying much attention at the time.  My impression of them is
> that they use atmospheric oxygen to make major reductions in weight
> for most of the launch phase.  Is this true?

I wasn't on the net when that discussion took place, and since I haven't
seen much comment yet, I'm gonna stick my neck out a bit.

(...attempting to revitalize old memory cells... sputter.. reboot.....)

Many years ago (197X?), I attempted to study various types of propulsion
technology.  As I remember, there are a couple significant parameters
in picking your engine type.

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.

2.  Engine thrust-to-weight (T/We) ratio:  thrust per pound of engine.
    (How about thrust(newtons)/engine-mass(kilograms), T/Me?) As I
    remember, turbojets were around 6 (T/We), ramjets higher, rockets
    ... well, way up there.

Thus, although jet engines give you much higher Isp's than rockets,
they also require a lot more massive machinery to function.  Of course,
turbojets are much more massive than ramjets.

The problem with ramjets is that you need something else to get them up
to a functioning velocity, i.e., rocket (lots of extra propellant) or
turbojet (lots of extra machinery).

Scramjets need an even higher startup velocity than most ramjets.
I would presume greater than Mach 1 (unless there is some mixed mode
tricks that can be played; any propulsion scientists care to comment?).
Given the engine mass, you get into the rocket/scramjet tradeoff area.

...and then, there are proposals for turbo-ram-rockets...

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