Japan to Talk With NASA on Supersonic Jet
Associated Press May 8, 2006
1% ?!?!?!?! How in the world?????? That has to be a type-o. Even though the Concorde used turbo jets, any high bypass engine will still put out a large enough noise to be more than 1% of the previous.
Yeah how can they do that? Got to be pie in the sky stuff.
I can't help suspecting that if someone came up with a silent scram-jet, it would be a military secret.
Keep in mind that that is just the engine they are talking about. While I'll agree that it does sound improbable, even if you have a silent engine, you will still be generating shocks which are LOUD, and the characteristic pressure N-wave will still be developed as the body moves through the flow at super sonic speeds. I recall an article I read, perhaps in popular science, or aviation weekly (possibly an AIAA jounral as well but less likely, as I have only recently begun reading those) which talked about reducing the noise levels generated by supersonic craft by maintaining a certain volumetric axial profile. Ultimately no matter how quiet the engines you will hear a "sonic boom" as any craft flies over head at super sonic speeds. This is why supersonic airline flights have been historically limited to ocean crossing routes. A quieter engine would be advantageous during take off and landing when the plane is subsonic, and in populated areas (aka near land). I think ruling out a RAM jet is a fair assumption, as they only operate at supersonic speeds (they use shocks and expansion waves to accomplish many of the thermodynamic functions of a typical engine without the moving parts) and need a different type of propulsion system to accelerate the vehicle to supersonic velocities. I agree that 1% is probably a typo but using helmholtz resonators, other muffling devices ducted jets etc could significantly lower the noise generation. I wouldn't be suprised if there is a new engine produced that is significantly quieter than the concords 30+ year old engines. Furthermore, high bypass engines are not really an option for supersonic vehicles as the exhaust velocity must exceeed the free stream velocity. This is difficult to do in the case of a bypass as the nature of the inlet air velocity is likely to be subsonic (inlets are typically behind the oblique shock generated by the nose etc. of the aircrafts airframe, partially in the boundary layer of other aircraft components, and if the air infront of the inlet is supersonic, a normal shock will form slowing the air to subsonic levels). It would be difficult to then increase the stagnation pressure of this internal flow to a point where it could be accelerated out of a nozzle to supersonic speeds. Sorry for this rant, its a bit late but I feel inspired and I'd rather be talking about fluid mechanics and propulsion than studying for my vibrations and heat transfer exam tomorrow.
Noise attenuation is just a bit tougher than adding a Helmholz resonator here and "other muffling devices" there. We have been working on that since the advent of the turbine engine. And why do people immediately start thinking about ramjets, especially when we are talking about a stated noise reduction of mamoth proportions?
How'd you know? It's true, they've been secretly developing stealth-flying pies. It's part of their ongoing synthesis of the traditionally disjoint aerospace and food industries.
I suspect they are referring to above Mach 1, i.e. the noise requirements it exclude the sonic boom.
It's hard to imagine 1% the noise of Concorde, i.e. eliminating 99% of the noise.
Well, the link goes somewhere else now, but I read it pretty carefully the first time. The article very specifically stated that they're scramjets. So those must be secondary engines used only after regular jets have gotten the thing up to operational speed. (Or is there some way to accelerate incoming air to supersonic while the engine is stationary?)
I get a different article now too. Even with scramjets, the combustion process is extremely loud. To get the scramjets to an operational range, they would still need something on the range of M4+. That's faster than the SR-71. They must be counting on some sort of atmospheric effect at very high altitudes. I know there is what is refered to as an acoustic duct at about the 20 km (65,000 ft) altitude mark.
I've never heard of an 'accoustic duct' before. Does that involve ambient air pressure confining the shockwave to a tubular form behind the plane? If so, is it reliant upon extreme speed?
The 24 hr window to edit is closed, and apparently ASM moved the article, so here is the article on Yahoo.
Japan to Talk With NASA on Supersonic Jet
I have discovered that one shortcoming (actually a major inconvenience) of the internet is linking bits of info on other sites, and invariably the site disappears, or someone reformats it, or removes or relocates the bit of information. I mean - the nerve of some people. :grumpy:
Actually it is an acoustic occurrance that is due to changes in the speed of sound at varying altitudes. The speed of sound usually drops with altitude. There is a location where the speed starts to increase again with continuing altitude. If you look at a plot of speed of sound vs altitude, the section would look like a sideways "V". If a soundwave experiences a change in medium that results in the speed of sound changing, the sound waves refract (bend) towards the area of the lower speed (Snell's Law). To make a long story short, the sound waves get bounced up and down, but never refract to the ground. They are channeled like they are in a duct, hence the name. The only thing is that now the sound will be able to travel very long distances.
It's just a guess on my part. It is probably way off the mark.
Thanks, Fred. That's a really interesting link.
The closest thing to crackpottery I own is a little book about the Aurora spyplane ( :uhh: ). One of the conjectures in it is that you could use retractable rocket combustion chambers inside the scramjet (using the scramjet nozzle as the rocket nozzle) for takeoff and acceleration. That'd require oxygen, but that's ok - I think this proposal is too far out there anyway. :tongue:
Sounds reasonable to me. It looks like the temperature (and therefore speed of sound) decreases up to about 10km, holds steady, then starts increasing again at 20km: http://perso.wanadoo.fr/ballonsolaire/en-theorie2.htm
Oh - here's the speed of sound itself vs alt: http://www.aerospaceweb.org/question/atmosphere/q0112.shtml
You've gotta give them points for imagination. It looks good on the surface (to an amateur), but I'm guessing that you've got Fred's brain scratching itself.
Seems to me that it would be easier to put retractible engines elsewhere on the plane.
With Dreams of Super-Sonic Jets Dancing in Their Heads
Five years after the crash of a Concorde in Paris, French and Japanese researchers are planning the next generation of super-sonic civilian jets. But without a huge leap forward in technology, their plans may never take flight.
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