Rocket trust and sound barrier breaktrough

[TeV]

Friend asked me this question ,and I wasn't about answer.
Not my area of expertise).
I guess rocket gets easier trough the sound barrier than millitary jet aircraft due to somewhat "less" problematic geometric shape.
I mean there is no such problems with wings and drag like in aircrafts?

 

[drag]

Greetings !

Well, it depends on what exactly "easier" means,
but the answer is yes in general. An aircraft
is more complex in shape and there're lots of different
processes going on as you get close to the sound barrier.
In some areas the air moves faster and in some slower
and various shock waves interfere with each other.
For a rocket it's all much simpler.

Live long and prosper.

 

[LURCH]

Additionally, a rocket doesn't have to deal with the supersonic or transonic intake of air and its effect on combustion.

 

[TeV]

Originally posted by LURCH
Additionally, a rocket doesn't have to deal with the supersonic or transonic intake of air and its effect on combustion.

Can you specify this ,give link or something?
You mean there's difference in means of accomplishing thrust in aircrafts and rockets and this is additional problem for aircraft due to engines used (jet propulsion issues)?
Yet again ,I thought the first aircraft that went thrugh the barrier was with typical rocket engine.Not sure about that but I think I saw one movie showing that.
Thanks for Your replies.

 

[enigma]

Yes, you're right.

The http://www.californiasciencecenter.org/Exhibits/AirAndSpace/AirAndAircraft/BellX1.php was the first aircraft to break the sound barrier and it was rocket powered.

The difference between rockets and traditional airplanes is how they get the oxygen to burn their fuel. A rocket carries all of its oxygen in a separate tank, while a jet engine takes oxygen from the air.

Designing a jet engine to take air in while it is supersonic is very difficult. If you consider than a jet engine is a few meters long at most and the plane is zooming through the air at a mile every few seconds, you'll see that there isn't much time for air to be inside the engine to get burned before it shoots out the back.

If you look at the engine intakes of an http://www.wvi.com/~lelandh/srspee~1.htm , the reason they're shaped that way is to form a sequence of shockwaves which guide and slow down the air as it enters the combustion chamber so that it is going well below the speed of sound, but still stays cool enough and with enough workable pressure to gain positive thrust from the combustion.

Scramjets (for Supersonic Combustion ramjets) are a theoretical engine which doesn't slow the air in the engine down to subsonic speeds. The biggest problem they're having is getting the fuel and the oxygen to mix properly while it's still inside the engine. NASA is planning on doing a second test (first one failed due to loss of their first stage Pegasus rocket) of the X-43 in a few weeks. The X-43 uses a scramjet engine. Another promising scramjet project is being run by the University of Queensland.

Regarding your original question,another reason that rockets don't have as much difficulty in supersonic flight is because they aren't built to rely on lift from wings. Airplanes use much less fuel, because their wings keep them aloft, reducing the required power of the engines. Rockets, on the other hand, have huge honking engines which provide all of the thrust required to get them where they're going.

 

[LURCH]

I followed your link, Enigma, and it does indeed lead to the first supersonic rocketplane. But that's the Bell X-1 (just a typo, I realize, but I'm a big X-15 fan).

Did you see the news about the X-43? Was supposed to happen yesterday (Saturday the 21^st), but it's been delayed at least a month! Again the Pegasus booster is the problem.

 

[enigma]

Ach. Fixed, thanks. X-15 on the brain, I suppose...

 

[russ_watters]

One important thing to remember here is thrust to weight ratio: at takeoff, the space shuttle is accelerating at about 3g, giving it a thrust to weight ratio of 4:1. The best fighter planes have a thrust to weight ratio of about 1.2:1. Thats a lot of power to accelerate through any drag issues.

The second most important thing to remember is rockets aren't merely designed for supersonic flight, they are designed for hypersonic flight and though the aerodynamics is about the same, the problems are different. The sharp leading edges of a fighter plane wing are great at mach 2, but will melt off in seconds at mach 5. A blunt nose will push a shock wave in front of it instead of attaching to the leading edge and greatly reduce frictional heating. It also forms a bubble of sorts around the craft, inside which conditions are a lot more docile and easier to manage.

 

[TeV]

Originally posted by russ_watters
One important thing to remember here is thrust to weight ratio: at takeoff, the space shuttle is accelerating at about 3g, giving it a thrust to weight ratio of 4:1. The best fighter planes have a thrust to weight ratio of about 1.2:1. Thats a lot of power to accelerate through any drag issues.

The second most important thing to remember is rockets aren't merely designed for supersonic flight, they are designed for hypersonic flight and though the aerodynamics is about the same, the problems are different. The sharp leading edges of a fighter plane wing are great at mach 2, but will melt off in seconds at mach 5. A blunt nose will push a shock wave in front of it instead of attaching to the leading edge and greatly reduce frictional heating. It also forms a bubble of sorts around the craft, inside which conditions are a lot more docile and easier to manage.

Also,lucky thing is that space lounching rockets go basicaly verticaly with respect to the ground surface.
So the carried Mach cone shock wave,when the rocket flyes supersonic, actually *never* hits the ground.Imagine what sonic boom that would be...
They are loud so much (to the point where sound energy start creating earthquakes) even without that.

 

[Clausius2]

Enigma, I am not an expertise in this area (or in any area, I suppose), but why don't put a convergent-divergent nozzle in the air intake, in order to make a subsonic flow at combustion chamber?.

What about combustion by detonation?. It would be possible a supersonic flow in the combustion chamber.What is the technological problem?

 

[enigma]

Originally posted by Clausius2
Enigma, I am not an expertise in this area (or in any area, I suppose), but why don't put a convergent-divergent nozzle in the air intake, in order to make a subsonic flow at combustion chamber?.

They do, but you will still be getting a normal shockwave in front of the entrance to the combustion chamber. The greater the mach number of the flow before a normal shockwave, the less the work you can get out of the flow behind the shock. The reason they use the oddly shaped inlet is to funnel and slow down the flow with oblique (slanted) shockwaves which don't kill the performance as much. This way, the flow in front of the normal shockwave is just over mach 1 before the flow goes into the converging/diverging section.

What about combustion by detonation?.

What do you mean by this?

 

[russ_watters]

Originally posted by Clausius2
It would be possible a supersonic flow in the combustion chamber.What is the technological problem?

That's a scramjet - supersonic combustion ramjet. I'm not sure what the problems are, but apparently they're daunting.

Remember the SR-71? The big cones in front of the engine nacelles moved forward and backward by up to like 10 feet depending on the speed in order to shape the shock wave they produced to strike just inside the intake. Those cones actually provide most of the compression of the engine and therefore most of the thrust.

 

[LURCH]

Originally posted by Clausius2


What about combustion by detonation?. It would be possible a supersonic flow in the combustion chamber.What is the technological problem?

Pulse detonation are currently a hot topic of propulsion research. This site has some good information regarding the potential of such a system. It also contains links to the research being done by NASA and Caltech. The pulse-detonation engine (PDE) will probably be the next step after the scramjet. It has tremendous potential, but the difficulty is in converting regular combustion or conflagration to supersonic detonation.

During WW2, the Germans were attempting to achieve pulse-detonation with their V-1 "buzzbomb", but the problems of timing, achieving a supersonic shockwave, and materials with adequate structural integrity were beyond their capabilities at that time.

Officially, no one has yet developed a PDE that works. Unofficially, it is widely rumored that the U.S. has a an as-yet-classified reconnaissance aircraft (the so-called "Aurora") that uses such a propulsion system.