Is it posible to avoid sonic boom

[Yaveh]

Is it posible to avoid the sonic boom using some kind of magnetic field around the object?
Is there any investigation been carried away by USAF or anyone?

 

[mathman]

The answer is probably not. As for a magnetic field, it wouldn't have any effect. Sonic boom is an air compression effect. There is no electrical or magnetic force involved.

 

[russ_watters]

There are many things you can do to REDUCE the shock wave, but there is no way to completely eliminate it.

 

[enigma]

A sonic boom is just a shock wave which forms when an object goes faster than the speed of sound. It is caused by the air not being able to "get out of the way" fast enough.

If there were a way to use magnets or something similar to keep the shock wave from forming, then that would mean that there was no (or little) air touching the plane.

No air touching the wings means no lift.

No lift means you end up with a rocket, not a plane. If the engines were airbreathing, they would shut down, and the plane would fall to earth.

 

[Andersen]

there are proposals to use lasers to superheat air in front of the nosecone of craft. Hotter air has a higher speed of sound, so the craft would (locally) break sound barrier at higher speed. also, heat -> lower density, so maybe the sound waves would be weaker, reducing the boom.

If you imagine reducing the density low enought that most wavelengths were less than the particle seperations, then the wave idea breaks down (but this is meters or more, so probably not practical)

Joe

 

[enigma]

Originally posted by Andersen
there are proposals to use lasers to superheat air in front of the nosecone of craft. Hotter air has a higher speed of sound, so the craft would (locally) break sound barrier at higher speed. also, heat -> lower density, so maybe the sound waves would be weaker, reducing the boom.

That's an interresting idea. Do you have any sources for that?

I'm curious how they would dissipate the extra heat from the craft. Even if there is no shockwave, the total temperature felt in the boundary layers remains the same. Increasing the ambient temperature increases the total temperature linearly, and for SS jets it's already high enough that expansion of the airframe is an issue.

 

[FZ+]

Isn't it simpler just to go higher? If not into space, then where the sonic boom isn't too noticable.

 

[Loren Booda]

A jet stream of 500 MPH enclosing a commercial airliner with local airspeed 500 MPH in the same direction appears from the ground to be going 1000 MPH, yet without sonic boom. A gradual velocity gradient of air therefore does not conduct sound as well as static air. What is the minimum air velocity gradient needed to negate the effect of such relative supersonic speed?

 

[enigma]

Hey Loren,

I remember on PF 2.0 when you asked that question, and I didn't remember anyone finding an answer. Did you find a source which said it was the case? I'm interrested to find out why... it would seem to me that weird things would occur once the surrounding flow itself went supersonic.

FZ+, yes, it is simpler (relative to changing the flow properties) to get into space. That is exactly what current plans for hypersonic transports are looking to do. The OP asked if there was a way to avoid the boom in air, though.

 

[damgo]

^^^ It's not that the fast-moving air conducts sound worse; there is no sonic boom. You could have an arbitrarily small gradient over a large space and this would be the case.

You could ask what the maximum gradient is before you get a sonic boom... I would guess the characteristic length would depend on both the fluid properties of air like the density, *and* the size of the object creating the sonic boom. Not sure though... you could probably find the answer in a fluid dynamics book...

 

[Loren Booda]

enigma, damgo-

My guess is that the answer lies in the Reynolds number. The increase in apparent sonic length of the aircraft from stem to stern, due to gradient diagonalization in the jet stream, is the same effect as diminished boom at an equivalent diagonal with no jet stream. The transition from boom with no jet stream to no boom with jet stream (neglecting changes in atmospheric density) would be given by

(v_J+v_A)/v_A=L/L(sin(_[the]))=1/(sin(_[the]))

Where v_J is the jet stream velocity, v_A is the velocity of the aircraft, L the length of the aircraft, and _[the] the angle from the ground to the plane.

 

[enigma]

Integral, could you pull this discussion out, since it appears we have hijacked the thread.

The increase in apparent sonic length of the aircraft from stem to stern, due to gradient diagonalization in the jet stream, is the same effect as diminished boom at an equivalent diagonal with no jet stream.

Loren, can you explain what you mean by this, please? Also, how did you derive that equation?

I am missing what you mean about "angle from the ground to the plane". Angle from where on the ground? Do you mean the aircraft angle of attack?

******************************

damgo, while looking at the airplane relative to the wind gives no shock wave (a wind tunnel test would confirm this), I still have this nagging suspicion that we're missing something. The speed of sound is just a function of the random kinetic energy (Temp) of the gas. If you give a fixed volume of gas a specific velocity, that doesn't affect its ability to transfer kinetic energy one bit (unless I'm completely missing something here...).

My aerodynamics textbooks aren't providing any insight to this special case, and a 5 minute database search only gave articles on wind tunnel test data.

I'll keep looking.

 

[Loren Booda]

enigma-

In some sense I am claiming that the sonic velocity of the plane as perceived on the ground is relative to the jet stream velocity. I believe the angle is that of attack is what I describe, the angle between the flat ground and the airplane, with observer at the vertex.

(v_J+v_A)/v_A=L/L(sin(_[the]))=1/(sin(_[the]))

Where v_J is the jet stream velocity, v_A is the velocity of the aircraft, L the length of the aircraft, and _[the] the angle from the ground to the plane [with observer at its vertex].

This equation is both empirical and derived from the Reynolds condition

R_[nu]~(v_J+v_A)(L(sin(_[the]))=v_AL

where R is the Reynolds number and _[nu] is the viscosity of air, assumed constant.

The effective sonic length of the plane is stretched (in some respect similar to dilation in relativity) by the relative velocity (1/sin(_[the])) of the jet stream, so what is visually apparent as supersonic is, on its way to the ground, sonically attenuated.

 

[Andre]

There seems to be a tendency explaining how a digital atomic clock works correcting for relativity when anybody asks what time it is
Never mind great job, but

The answer is no, to the question "Is it posible to avoid the sonic boom".

Very impractical that heating up of the air.