

Waverider Makes Hypersonic History - bifrost
http://www.gizmag.com/waverider-fourth-test/27382/

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jessriedel
Can anyone give me 5 sentence argument about why this is useful for getting to
space? My understanding is that the whole idea should be to get out of the
atmosphere asap since it's so thick. This only gets you to, what, Mach 10?
While LEO speed is Mach 25ish? So what's the point?

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dexen
With current launch technology -- chemical rockets -- flying out of atmosphere
takes the most of fuel and oxidizer, because for almost two minutes, the
rocket literally stands on flame, fighting both gravity and air resistance.
You cannot shorten this time much by speeding up, because air friction goes up
with speed in roughly quadratic relation. Aside of fuel and oxidizer, an
atmospheric rocket stage has to carry very strong, and thus bulky, engines, to
give the whole vehicle Thrust-to-Weight Ratio > 1, typically about 2. Only
when you are at, or close to, LEO, you can use smaller/lighter engines with
lower TWR.

A long sought after alternative is to launch and go to high altitude with
atmospheric engines (thus saving mass of oxidizer) and aerodynamic lifting
force (so you can use lighter engines, having overall TWR < 1), basically an
aeroplane. Only after reaching high altitude and speed switch to rocket mode,
or separate a rocket-based next stage. Scaled Composites' SpaceShipTwo goes
far into that direction, but it is still limited to low-supersonic speeds by
using jet engines. The problem is, plain jet engines are not very efficient at
high Mach, due to both having to slow air down to sub-sonic speed and thermal
limitations.

The next logical step is to use scramjet engines, which perform in high
supersonic (`hypersonic') flight regime. This is technically challenging,
because you have to work with high supersonic airflow inside engine
(hypersonic gas behavior is significantly harder to model than at the usual
subsonic), and also you have to maintain steady flame in its hypersonic
airflow.

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Gravityloss
This is wrong for a multitude of reasons.

1\. Rocket engines have high thrust to weight ratio, typically around 100.
Jets can have about 10. Scramjets can be really bad.

2\. You can speed up quite quickly since the atmosphere is there for the first
30 km or so only.

3\. White Knight Two that launches Spaceshiptwo is subsonic, not supersonic.
The air launch is beneficial roughly:

\- primarily because your rocket engine's expansion ratio can be bigger and
you get more thrust for same fuel flow

\- secondarily because then you can launch flexibly by flying to a location,
that for example has less population or air traffic or is easy to launch to
the right orbit from

\- thirdly because you can always glide to a landing if something goes wrong
with the engine. With a vertical takeoff there's a time right after takeoff
where it's hard to do an engine out abort with something like a parachute.

Note all the above advantages are _not_ dependent on carrier aircraft speed.
In fact supersonic separation is a hard problem, never mind something at Mach
3. (See D-21)

If you look at something like dry mass, rockets will practically always win
over air breathers in studies because of the lightness and simplicity of
simple tanks and high thrust to weight ratio of rocket engines.

This whole scramjet affair is very misguided if you're trying to save
something like liquid oxygen which is extremely cheap. Even when it weighs
something, it doesn't matter since tanks are light and simple and (did I
mention already:) rocket engines have high thrust to weight ratio.

I'll choose that any day over a huge scramjet inlet that needs to adjust to
variable speeds by having big movable ramps, a big burner and a big exhaust,
all experiencing large aerodynamic heating problems, multiple load paths (a
rocket has basically only vertical loads).

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lifeformed
Aw, based on the name, I thought it was going to be a hypersonic boat. That
would be pretty silly.

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jared314
Not silly. There are supercavitating and ground effect vehicle designs that
reach subsonic speeds(300+ km/s). Most are torpedos, like the VA-111
Shkval[0], or military craft, but the potential is there for a very fast boat-
like thing.

[0] <http://en.wikipedia.org/wiki/VA-111_Shkval>

