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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."
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-