Re: Scrapping Scram

From: John Thingstad (john.thingstad_at_chello.no)
Date: 11/01/04 

Date: Mon, 01 Nov 2004 14:20:49 +0100

On Mon, 01 Nov 2004 01:03:09 GMT, redneckj <redneckj@tampabay.rr.com>
wrote:

>
> "John Thingstad" <john.thingstad@chello.no> wrote in message
> news:opsgraqgu4pqzri1@mjolner.upc.no...
>> On Sun, 31 Oct 2004 15:31:46 -0600, Paul F. Dietz <dietz@dls.net> wrote:
>>
>> > It works out otherwise, I understand.
>> >
>> > Air breathing is most useful early in the launch, so the oxidizer
>> > it replaces would have been dumped almost immediately anyway.
>> > The Isp of airbreathing engines declines with increasing
>> > speed, so where Isp really matters there is not much, if any,
>> > advantage.
>> >
>> > The scramjet will use much more *fuel* (hydrogen) than the
>> > rocket would. Since LH2 is low in density, you actually end
>> > up with a much *larger* vehicle, even if you manage to reduce
>> > the total propellant mass.
>> >
>> > Paul
>>
>> A scramjet would want to stay in the upper regions of the atmosphere
>> longer.
>> A rocket want's to get out of the atmosphere as fast as possible.
>> At approx 100 km there is no more atmosphere.
>> A scramjet would accelerate linearly at about 25 km.
>> Even here the air resistance is much diminished.
>> A scramjet could pull maybe mack 10. After that you would need a
>> rocket to accelerate to say mack 25. (depends on orbit)
>> logic dictates that there are two stages.
>> Ther scramjet could just as easely run on keroserine.
>> There dosn't seem to be a requirement that it runs on hydrogen.
>>

You might be right about scram-jets or you might be wrong.
Can't say I follow your numbers at I don't know how they are computed.
The military plans to test a scram-jet at Mach 7 in 2005 and
at Mach 10 by 2007. (I think)
To now hypersonic operation of scram-jets has only been tested in wind
tunnels.
And only for short durations. Certainly more experience with real
craft operating in the atmosphere is needed before any definite conclusion
is reached.
(Remember 100 years ago most scientist agreed that rockets could never
reach orbital velocity
because the weight of the fuel would be greater than the lifting force
generated.)
I find it entirely possible that advances in design and material
technology might allow
scram-jets to operate at much higher temperatures than today.

I am sure scram-jets have a position in the future.
Werther this is super fast interception jets for the military or if
it finds a application in space launch vehicles remains to be seen.
In 2015-2020 maybe I have a better idea.
I agree that building such a vehicle today would be premature.
Indeed advances in rocketry like the new (in the USA) Russian
rocket engine may outperform any scram-jet. 

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