NASA Wants to Take the Blast Out of Sonic Booms

coondoggie writes to tell us that NASA and JAXA (the Japan Aerospace Exploration Agency) have announced a partnership to study the sonic boom. Hoping to find the key to the next generation of supersonic aircraft, the research will include a look at JAXA's "Silent Supersonic Technology Demonstration Program." "The change in air pressure associated with a sonic boom is only a few pounds per square foot -- about the same pressure change experienced riding an elevator down two or three floors. It is the rate of change, the sudden onset of the pressure change, that makes the sonic boom audible, NASA said. All aircraft generate two cones, at the nose and at the tail. They are usually of similar strength and the time interval between the two as they reach the ground is primarily dependent on the size of the aircraft and its altitude. Most people on the ground cannot distinguish between the two and they are usually heard as a single sonic boom. Sonic booms created by vehicles the size and mass of the space shuttle are very distinguishable and two distinct booms are easily heard."

Comments

this is not quite new

[emagery]

They've been working on this for a while, actually: See - http://www.nasa.gov/vision/earth/improvingflight/supersonic_jousting.html That particular project was wrapped up.. but maybe the plan to expound upon it =)

Re:Carefully

[rcw-work]

But if you can go that fast, why bother with a compressor, aside from using it to accelerate for takeoff? Just use a ramjet, no moving parts, who cares how fast it goes (as long as you can still get the fuel mixed into the air before it's out the back.)

Jet turbines and ramjets share the same problem - they are only capable of subsonic combustion and must slow the supersonic airflow before they can burn fuel in it and reaccelerate it. Thus the recent experiments with scramjets (supersonic combustion ramjets). They aren't ready for use yet.

Re:Does a bullet make a sonic boom?

[onkelonkel]

You seem to be labouring under the common misconception that a sonic boom is caused when an object "breaks the sound barrier". As long as an object is moving through the air at greater than the speed of sound it will create a shock wave (cone shaped, think of a boat wake rotated in 3-d) behind it. As the object flies by you, the shock wave passes you and you hear the "sonic boom" So the answer is yes, bullets have a sonic boom.

Re:Carefully

[AikonMGB]

Couple of problems with this.. First, the internal surfaces of a divergent (subsonic) duct experience adverse pressure gradients. This means you need to very gradually increase the duct area in order to prevent flow separation. Subsequently, you would need an extremely long duct to achieve an appreciable reduction in flow velocity, all of which is subject to friction and viscous drag. All in all, not good.

The second major problem with this is that a divergent duct in supersonic flow actually increases the flow velocity. You may notice in engines that possess a throat (i.e. the exhaust stream is supersonic), the duct area increases, accelerating the flow (take rocket engines for example). In order to slow down supersonic flow, you need a converging duct.

Aside from that, a couple other points.. shockwaves don't make flow turbulent. In fact, nearly all flow through a jet engine is turbulent, as opposed to laminar. This is actually desirable in most cases, because although turbulent flow causes an increase in skin friction drag, it is highly beneficial in delaying flow separation, which is very bad in most cases.

Finally, with respect to the ramjet, there are some serious issues still to overcome, especially for slower speeds. First and foremost, it can generate no static thrust, meaning you need an alternative means for propulsion to get your bird off the ground. This adds weight and takes up volume, both of which are very bad things.

And as for how fast it goes.. The faster a ramjet travels, the higher the increase in stagnation temperature of the flow. This affects how combustion occurs, and it actually reaches a point that by adding fuel and combustion it, you are cooling off the flow, which is the opposite effect that you desire. This upper limit on speed depends a great deal on the inlet design and the materials used, but in general it is sub-hypersonic (as in hypersonic speeds are too high).

Work is being done to develop a scramjet (supersonic combusition ramjet), which is essentially the same as a ramjet except that the combustion occurs while the flow is travelling at supersonic velocities (meaning less of an increase in stagnation temperature, less pressure loss, etc.), as well as schramjets, which again are similar, however use detonation waves to ignite the fuel/air, reducing profile drag due to burners and flameholders etc.

I hope this at least answered parts of your questions..

Aikon-

Re:Does a bullet make a sonic boom?

[icebrain]

You may be correct but I have always heard/believed that the sonic boom was a single event happening as an object passes the speed of sound (and I am a bit of a Discovery chan junky). I have seen planes fly by at greater than the speed of sound and heard no sonic boom, they are loud but not that loud. You might watch the Discovery channel, or even stay at the Holiday Inn Express... but I'm an aerospace engineer. GP is right; the boom is not a singular event, but rather it's the perceived sound when the "wake" of the shockwaves passes by the observer.

Also, how are you sure that the aircraft you claim to have seen were indeed supersonic? I've heard a real one (and many recreated F-18 and Concorde ones in Gulfstream's sonic boom demo trailer), you definitely notice it.