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neomax
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Feasibility of high altitude commercial aircraft

Tue Feb 13, 2018 6:08 am

A lot of the problem of getting SST aircraft to work involves the sonic boom. Instead of securing approval for supersonic overflight for every new route, wouldn't it be more effective to engineer quieter sonic booms and fly aircraft at substantially higher altitudes to make sonic booms virtually unnoticeable? I was at the F9H SpaceX launch with a friend, and we were discussing why you couldn't hear the sonic boom for the launch (but you could hear it on reentry) and he said something to effect of the fact that after a certain altitude, you can't effectively hear a sonic boom. Concorde was advantageous for its ability to cruise at high altitudes, how effective will even higher cruising altitudes be at dampening the effect of the sonic boom?
 
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VirginFlyer
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Re: Feasibility of high altitude commercial aircraft

Tue Feb 13, 2018 7:56 am

Could the sonic boom from the SR-71 be heard when it was at altitude? The height it flew at (perhaps up to 100,000ft) is surely pushing the limits for gas turbine powered aircraft before needing to move to rocket propulsion.

At high altitudes like that you run into a number of issues, in particular the Armstrong Limit, the pressure altitude at which the boiling temperature for fluids is equal to the temperature of the human body. This occurs around 62,000ft. If you are exposed to the environment above this altitude, the exposed fluids in your body will begin to boil, and survival beyond a couple of minutes is extremely unlikely. Basically above this altitude you need to be wearing a pressure suit to survive a depressurisation incident. Beyond this, you would also need to consider exposure to cosmic radiation, effects of airframe heating from the speed needed to maintain high speed flight at that altitude (including the expansion issues which made the SR-71 leak like a sieve on the ground in order to allow it to work well at altitude), aerodynamic performance limits, and the general cost involved.

If the goal is to limit sonic boom, I think there are a number of other more promising technologies than simply trying to fly higher.

V/F
It is not for him to pride himself who loveth his own country, but rather for him who loveth the whole world. The earth is but one country, and mankind its citizens. —Bahá'u'lláh
 
PerVG
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Re: Feasibility of high altitude commercial aircraft

Thu Feb 15, 2018 11:24 pm

neomax wrote:
and we were discussing why you couldn't hear the sonic boom for the launch (but you could hear it on reentry) and he said something to effect of the fact that after a certain altitude, you can't effectively hear a sonic boom.


I'm no expert or anything, but might be that a big factor in not hearing the boom at take off is that it was drown out by the "boom" of 27 rocket engines... ;)
FH went supersonic at about 9~10km (30~33000ft) of altitude. That's not THAT high...
Just my 2 cents.
 
Andre3K
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Re: Feasibility of high altitude commercial aircraft

Fri Feb 16, 2018 1:56 am

Not to mention you have to be inside the boom carpet to hear it. If it went supersonic instantly when it left the pad you might hear it, but by the time its supersonic in an actual launch then you are effectively outside the boom carpet (vertically inclined in this case). Physically you still might be in it but the edges of the shockwave are where the noise comes from, not the middle of the shock cone.
 
WIederling
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Re: Feasibility of high altitude commercial aircraft

Fri Feb 16, 2018 12:45 pm

Well the boom is where the shock cone ( initially rather flat gaining conicity with speed )
intersects the ground ( resp. the listener. :-)
My guess would be that FH shock cone never touches populated areas.
the initial ascend is vertical and only later turning slowly towards horizontal.
Murphy is an optimist
 
planecane
Posts: 45
Joined: Thu Feb 09, 2017 4:58 pm

Re: Feasibility of high altitude commercial aircraft

Sat Feb 17, 2018 4:28 am

PerVG wrote:
neomax wrote:
and we were discussing why you couldn't hear the sonic boom for the launch (but you could hear it on reentry) and he said something to effect of the fact that after a certain altitude, you can't effectively hear a sonic boom.


I'm no expert or anything, but might be that a big factor in not hearing the boom at take off is that it was drown out by the "boom" of 27 rocket engines... ;)
FH went supersonic at about 9~10km (30~33000ft) of altitude. That's not THAT high...
Just my 2 cents.


Also, when rockets launch, they don't go straight up. They are launched down range and turn pretty quickly. They did this to take advantage of the Earth's rotation. By the time they go supersonic they are several miles east (or Northeast or southeast) of the launch pad.

On a space shuttle approach that came from the south, I heard the Sonic boom west of ft Lauderdale as it passed overhead. I don't remember exactly but I'm pretty sure the shuttle was well above commercial aviation cruise altitude at that point.

I think the solution to supersonic travel is what Elon musk proposed. Fly on a rocket through space but don't go into orbit. Basically an ICBM with a passenger cabin instead of a warhead.
 
planecane
Posts: 45
Joined: Thu Feb 09, 2017 4:58 pm

Re: Feasibility of high altitude commercial aircraft

Sat Feb 17, 2018 4:28 am

PerVG wrote:
neomax wrote:
and we were discussing why you couldn't hear the sonic boom for the launch (but you could hear it on reentry) and he said something to effect of the fact that after a certain altitude, you can't effectively hear a sonic boom.


I'm no expert or anything, but might be that a big factor in not hearing the boom at take off is that it was drown out by the "boom" of 27 rocket engines... ;)
FH went supersonic at about 9~10km (30~33000ft) of altitude. That's not THAT high...
Just my 2 cents.


Also, when rockets launch, they don't go straight up. They are launched down range and turn pretty quickly. They do this to take advantage of the Earth's rotation. By the time they go supersonic they are several miles east (or Northeast or southeast) of the launch pad.

On a space shuttle approach that came from the south, I heard the Sonic boom west of ft Lauderdale as it passed overhead. I don't remember exactly but I'm pretty sure the shuttle was well above commercial aviation cruise altitude at that point.

I think the solution to supersonic travel is what Elon musk proposed. Fly on a rocket through space but don't go into orbit. Basically an ICBM with a passenger cabin instead of a warhead.

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