Waverider Makes Hypersonic History

http://www.gizmag.com/waverider-fourth-test/27382/

by bifrost • 13 years ago

18 14 13 years ago

jessriedel• 13 years ago

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?

Gravityloss3• 13 years ago

You are right, it's mostly orthogonal to space launch technology. But what would the public know?

It's mostly useful for weapons at this point.

RomP• 13 years ago

That's not necessarily a bad thing. A little factoid: out of ~4000 space launches that have happened so far, about 1500 (~40%) were done using delivery vehicles derived from an ICBM. This includes about 40% of the human spaceflights as well. http://en.wikipedia.org/wiki/R-7_%28rocket_family%29

dexen• 13 years ago

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.

Someone• 13 years ago

Another challenge for scramjets is that you can only use hem once you are hypersonic. So, even if you have a perfectly working one, you will need something else to accelerate your scramjet and load to Mach 3-4 or so.

Gravityloss3• 13 years ago

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).

ChuckMcM• 13 years ago

Lets see if I can do it in 5.

Horizontally launched rockets, such as Pegasus [1], are launched from a parent aircraft at 50,000' altitude. The mass fraction that you can get into orbit is limited by the weight of the oxidizer the rocket is required to carry. A scramjet on this rocket would allow the rocket to accelerate to Mach 15 or 20 while in the upper atmosphere, and then use an oxidized insertion motor to establish the desired orbit.

[1] http://en.wikipedia.org/wiki/Pegasus_(rocket)

mapt• 13 years ago

You might also want a small disposable rocket-assist jumpstart on your scramjet, just to get it up to supersonic speeds from what is presumably a subsonic launch platform.

edit: ...which I see is mentioned in the actual article, once I got around to reading it.

maaku2• 13 years ago

New York to Sydney in 45 minutes.

maaku2• 13 years ago

Wow, why the harsh downvotes? I answered the question succinctly. That is where this line of research is headed:

http://en.wikipedia.org/wiki/Rockwell_X-30

rrrrrraul• 13 years ago

FWIW, from my understanding, I don't think the X-51A is being developed to improve access to space but rather as a proof of concept for an advanced hypersonic transport within the atmosphere.

lifeformed• 13 years ago

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

jared314• 13 years ago

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

Waverider Makes Hypersonic History

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