Scramjets !!!! It works

> From: Andrew Goddard [mailto:a.goddard@...]
> With respect, that's a deeply biased argument. Rockets go up for two
> reasons:
>
> 1/ to get out of the bulk of the atmosphere as efficiently as possible
> since it's nothing but drag for them.

True.

> 2/ to reduce gravity costs on a non-lifting body.

Not true. Rockets 'reduce their weight' through orbital speed, not height.
You have to thousands of km up before such a reduction is even noticable.

> 1/ flying your scramjet up and through Mach10 to Mach20ish requires an
> atmosphere. ...Preferably a very thin high altitude one. ;-)

Ugh. Okay, let's assume you're right and they *fly* into space. Flying is
when the wings provide a substantially larger thrust vector than the engines
do. (On a 747 they produce 3x more. On a F-15 they can produce 10x more.) By
nature they have poor acceleration when compaired to rockets. This means
that it will take you much longer to get into orbit that a rocket. Because
you're trying to shove through the atmosphere, a huge chunk of your
potential acceleration is going to dissapear. So it's going to take you much
much much longer to get into orbit.

An hour or more (instead of 6 minutes) is probably not unreasonable when you
compare the 0-Mach3.0 times of any rocket based missile and any jet capable
of that speed. I'll come back to this in a minute.

> So it must be possible to develop a scramjet airframe
> for which the drag (and hence the heating) is comparable to modern jet
> aircraft, by simply flying it high enough.

Yes it would be. But you'd have lots of problems getting the same aiframe
generating any lift at landing speeds. It's be the same problem of the
dog-fight wing VS the landing-wing that have plagued jet fighters for
decades. Except much worse of course.

> Here's an example: the SR-71 flew at (a sustained) Mach-3 over forty
> years ago, at an altitude of 85000' (~22mb) and it reached skin
> temperatures of 430C.

Which sounds impressive, until you are told that the SR-71 was intended to
go half a mach number higher, but that they could never get it to work that
way. Flying very fast through an atmosphere is terribly difficult. In all
those decades the best a jet-aircraft has managed is Mach 9.9 - and only
within the past week. The X-15 was IIRC flying before the SR-71, and beat
its never exceeded speed by a good 100%.

But lets say you can build a scam-jet that can throttle between mach 3 and
mach 25 (and throttable for the associated thrust levels!) and that you can
find some way to fly it up through the air without exceeding a skin
tempreature of 430C. You've got to insulate your passengers, your payload,
your RCS/OMS fuel, and your cryogenic scamjet fuel from that tempreatures
for the length of your flight to orbit. This is also terribly difficult.
Then come the horrid part. Getting back down again.

Now in theory, you can fly down BUT ONLY if you have enough cryogenic
scram-jet fuel to power you for another hour; since flight at such safe
speeds requires your scram-jets to be working. Otherwise you generate more
drag than lift, fall into the lower atmosphere and melt. With an established
refueling station in orbit this would be easy, but otherwise you have to
carry that fuel up from the ground, keep it in orbit, and then keep it
protected from the same one hour long oven tempreatures of 430C until you
land again. Of course that will also be rather fun considdering the problems
of lift.

Of course you could carry a 5000C rated heatshild like the shuttle does and
reenter the hard way, but that has *major* problems of its own, and whatever
super-light super-strong TPS you can come up with can just as easily be
applied to capsule designs. Which will be simplier and cheaper for the next
few decades at least.

John