Scramjets !!!! It works

Hi there!

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

ANTIcarrot replied:
>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.

I think I wasn't clear enough. You could rocket into orbit at 1/10g,
taking your time (and your granny), /if/ fuel wasn't an issue. That
rockets do it as fast as possible is to reduce the fuel requirements
imposed on them by the presence of a gravity well. These are the gravity
costs I was referring to.

>Ugh. Okay, let's assume you're right and they *fly* into space.

You're perhaps being a little disingenuous. They will fly (as in
generate lift in an atmosphere) for as long as possible, before a final
rocket kick gets them into true orbit. This last part will be ballistic,
as the atmospheric density will be virtually nil.

>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.)

The scramjet, at Mach 15 - possibly the upper limit to air-breathing
engines - will have gained 2/3rds of its orbital velocity. At these
speeds that old centripetal acceleration actively /reduces/ the lift
required from the wings. In a near-horizontal flight at Mach-15 the
gravity component is - what? - down to about 6m/s2.

>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.

That's fine: after all, it doesn't need to carry the oxidiser to achieve
a whole 2/3rds of its orbital velocity, nor does it need to carry the
fuel and oxidiser to propel that oxidiser. It's win-win.

Look at any LOX/LH mix: the SSME has a listed mixture ratio of 6.03:1.
The oxidiser is the "6" part. Imagine saving 60%-80% of the
fuel/oxidiser currently burned per kilo to reach orbit. The vehicle
would be smaller, presumably more durable, and able to commit a larger
fraction of its flying weight to payloads.

>> 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.

No. The designed lift required for flight in the hypersonic envelope
will mean more lift when the vehicle returns unpowered (as it'll have
less mass). Why assume a powered fly back? The shuttle is more than a
little lacking in lift during its landing approach, but it's energy-rich
(has lots of PE to shed) and has had no landing accidents to date. Why
assume a scramjet needs to fly back under power to its launch site, or
need to do better than a shuttle glide-in?

>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.

Well, you'll have to insulate those items that reach 430C (leading
edges). Is that difficult? Not if you're rich in a large mass of LH to
start with. So cryogenic cooling would be one answer - and it'd help by
warming the LH for subsequent combustion, too.

>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.

I agree with you there. Don't get me wrong - I'm all for the
man-inna-can option that's worked well for decades, but I wouldn't rule
out a potentially safer and more efficient route to space for the masses
in a 20-30 year timeframe.

Andy Goddard