[Tres Juicy]

Hi all,

I have just read this

http://www.bbc.co.uk...onment-16908004

Quote

He will fall so fast that he becomes the first person to go faster than the speed of sound unaided by a machine.

Is this possible from that height?

I would have thought that he would be slowed by the increasing air resistance before reaching that kind of speed?

[Leader Bee]

Isn't the terminal velocity of a person ~130 m/h? I know he's wearing the suit but surely the extra weigth is negligible.

on a side note, the last person to do this ended up getting some horrible injury to his had didn't he? due to a leak in his suit.
Good luck to him.

[imatfaal]

Terminal velocity at 36k above the earth will be a huge amount higher than 130mph - there is precious little air up there to slow him down.

The air pressure at 36k is about 1/1000 of that at sea level - the temp will be pretty much the same (around freezing point) - so the density will be about 1/1000. Thus the terminal velocity would be around 30 times higher than at sea level (well I guess at sea level you have crashed and made a mess - but you get the idea)

[Tres Juicy]

imatfaal, on 8 February 2012 - 04:43 PM, said:

Terminal velocity at 36k above the earth will be a huge amount higher than 130mph - there is precious little air up there to slow him down.

The air pressure at 36k is about 1/1000 of that at sea level - the temp will be pretty much the same (around freezing point) - so the density will be about 1/1000. Thus the terminal velocity would be around 30 times higher than at sea level (well I guess at sea level you have crashed and made a mess - but you get the idea)

Ok, but can he break the sound barrier before the increasing air pressure slows him down too much?

Obviously he will not remain at 36k above the earth for very long

[imatfaal]

Tres Juicy, on 8 February 2012 - 04:52 PM, said:

Ok, but can he break the sound barrier before the increasing air pressure slows him down too much?

Obviously he will not remain at 36k above the earth for very long

Even with idealised situation it would need better maths than mine to work that out precisely - I might try to approximate it

and of course it depends which speed of sound. will he break the sound barrier? or will he go faster than the speed of sound at sea level - but when he is at a much greater height

[TonyMcC]

Leader Bee, on 8 February 2012 - 04:32 PM, said:

Isn't the terminal velocity of a person ~130 m/h? I know he's wearing the suit but surely the extra weigth is negligible.

on a side note, the last person to do this ended up getting some horrible injury to his had didn't he? due to a leak in his suit.
Good luck to him.

Terminal velocity depends on air density. As he is going to jump from the edge of space he is expecting almost no air resistance at first.
Joseph Kittinger was the man with a faulty glove, his injury wasn't too bad. In fact the glove failed on the ascent, but he decided to jump anyway. For more info you can google Project Excelsior.
No doubt he will try to maximise his speed by reducing his air resistance by remaining as vertical as possible (quite likely a head down dive).

This post has been edited by TonyMcC: 8 February 2012 - 05:21 PM

[imatfaal]

Basis constant acceleration (ie no air resistance at all) it would take 5.4k to get to speed of sound at ground level. But he would not have broken the sound barrier because the speed of sound at 31k is higher.

edit - I think it is higher at such altitude - but I am struggling to prove it or find a reference. It varies inversely with the root of the density - but the bulk modulus will also vary and might lessen/wipeout changes from denisty

From what I can gather the bulk modulus varies almost directly with the pressure. The speed of sound varies with the root of the pressure and varies inversely with the root of the density - these will cancel and the only changing factor left will be the temperature. The temperature of the stratosphere is around freezing - so speed of sound up there is around 330 m/s.

This post has been edited by imatfaal: 8 February 2012 - 05:14 PM

[TonyMcC]

imatfaal, on 8 February 2012 - 05:23 PM, said:

Basis constant acceleration (ie no air resistance at all) it would take 5.4k to get to speed of sound at ground level. But he would not have broken the sound barrier because the speed of sound at 31k is higher.

edit - I think it is higher at such altitude - but I am struggling to prove it or find a reference. It varies inversely with the root of the density - but the bulk modulus will also vary and might lessen/wipeout changes from denisty

From what I can gather the bulk modulus varies almost directly with the pressure. The speed of sound varies with the root of the pressure and varies inversely with the root of the density - these will cancel and the only changing factor left will be the temperature. The temperature of the stratosphere is around freezing - so speed of sound up there is around 330 m/s.

You might be surprised to know how little the speed of sound is affected bt altitude:- http://www.fighter-p...om/jetmach1.htm

[imatfaal]

TonyMcC, on 8 February 2012 - 05:25 PM, said:

You might be surprised to know how little the speed of sound is affected bt altitude:- http://www.fighter-p...om/jetmach1.htm

If you saw how my post above progressed in stages I was very surprised. If I have got it straight the change is solely due to the temperature - and frankly I was pretty surprised that termperature in the high stratosphere was warmer than london was last night!

[DrRocket]

imatfaal, on 8 February 2012 - 05:32 PM, said:

If you saw how my post above progressed in stages I was very surprised. If I have got it straight the change is solely due to the temperature - and frankly I was pretty surprised that termperature in the high stratosphere was warmer than london was last night!

Take a look here http://en.wikipedia....tigue_(material)

Read down far enough to see that applying the ideal gas law and some thermodynamics that the speed of sound in a given gas depends on the temperature and ratio of specific heats (they call it the adiabatic index in the Wiki article, but it is still Cp/Cv). When you change gases the molecular weight also enters the equation.