PDA

View Full Version : Skydiver Felix Baumgartner seeks to break sound barrier



beskeptical
2010-Jan-22, 08:47 PM
I'd like some help with what looks like 'bad physics' to me here. But I don't know if my understanding is correct.

Skydiver Felix Baumgartner seeks to break sound barrier (http://news.bbc.co.uk/2/hi/science/nature/8475288.stm)
It is likely that in his long freefall of more than five minutes, he will exceed the speed of sound - the first person to do so without the aid of a machine.

"No-one really knows what that will be like," he said.

"The fact is you have a lot of different airflows coming around your body; and some parts of your body are in supersonic flows and some parts are in transonic flows. What kind of reaction that creates, I can't tell you," he told BBC News.

Isn't the claim about exceeding the sound barrier ** since a falling body with only gravity as the acting force (and maybe wind resistance) reaches terminal velocity and in this case that would be less than the speed of sound.

And what's with all the talk about supersonic and transonic flows? It sounds like he's claiming one part of his body will be going at a different speed than another part. How does the airflow speed over the object vs the object's true speed impact a sonic boom event?

schlaugh
2010-Jan-22, 08:59 PM
He might since he's planning to free fall from 120,000 feet. In 1960 Col. Joe Kittinger reached a speed of 614 mph after free falling from 108,000 feet. (Wikipedia source). Kittinger trailed a drogue chute but I'm not sure how this fellow plans to remain in a stable position. generally, the speed of sound in air drops with reduced pressure, so technically he might hit "the speed of sound" but it may not be as measured at sea level or 29,000 feet. Temperature plays a role as well.

swampyankee
2010-Jan-23, 12:47 AM
He might since he's planning to free fall from 120,000 feet. In 1960 Col. Joe Kittinger reached a speed of 614 mph after free falling from 108,000 feet. (Wikipedia source). Kittinger trailed a drogue chute but I'm not sure how this fellow plans to remain in a stable position. generally, the speed of sound in air drops with reduced pressure, so technically he might hit "the speed of sound" but it may not be as measured at sea level or 29,000 feet. Temperature plays a role as well.

For a perfect gas -- a very good approximation for our atmosphere -- speed of sound depends only on temperature.

beskeptical
2010-Jan-23, 08:53 AM
OK, so Wiki has the following (http://en.wikipedia.org/wiki/Terminal_velocity):
Based on wind resistance, for example, the terminal velocity of a skydiver in a free-fall position with a semi-closed parachute is about 195 km/h (120 mph or 55 m/s).[2] This velocity is the asymptotic limiting value of the acceleration process, because the effective forces on the body balance each other more and more closely as the terminal velocity is approached. In this example, a speed of 50% of terminal velocity is reached after only about 3 seconds, while it takes 8 seconds to reach 90%, 15 seconds to reach 99% and so on. Higher speeds can be attained if the skydiver pulls in his or her limbs (see also freeflying). In this case, the terminal velocity increases to about 320 km/h (200 mph or 90 m/s),[2] which is also the terminal velocity of the peregrine falcon diving down on its prey.[3] And the same terminal velocity is reached for a typical 150 grain bullet travelling in the downward vertical direction — when it is returning to earth having been fired upwards, or perhaps just dropped from a tower — according to a 1920 U.S. Army Ordinance study.[4]

Competition speed skydivers fly in the head down position reaching even higher speeds. The current world record is 614 mph (988 km/h) by Joseph Kittinger, set at high altitude where the lesser density of the atmosphere decreased drag.[2]

If you exceed the speed of sound in the upper atmosphere where there are not very many air molecules, I take it no sonic boom would be made. Is that correct?

Tom Servo
2010-Jan-23, 09:39 AM
I would have to agree that a less dense atmosphere would require something to be a lot faster to create a sonic boom.

Also Isnt he afraid of buring up on re entry (JK)

BigDon
2010-Jan-23, 02:04 PM
How is he going to hold himself together? People tend to come apart in airstreams approaching those speeds.

schlaugh
2010-Jan-23, 04:06 PM
For a perfect gas -- a very good approximation for our atmosphere -- speed of sound depends only on temperature.

Interesting. My recollection is incorrect. And apparently I'm not alone (according to Wikipedia):

Some textbooks mistakenly state that the speed of sound increases with increasing density.

Density in the atmosphere would seem to decrease with altitude; i.e. the air is less dense at 30,000 feet than at sea level. But even so, that's apparently irrelevant to the speed of sound.

I learn something new here every day. :)

cjl
2010-Jan-24, 12:31 PM
OK, so Wiki has the following (http://en.wikipedia.org/wiki/Terminal_velocity):

If you exceed the speed of sound in the upper atmosphere where there are not very many air molecules, I take it no sonic boom would be made. Is that correct?
A sonic boom would be made, but it would be much quieter.

cjl
2010-Jan-24, 12:32 PM
How is he going to hold himself together? People tend to come apart in airstreams approaching those speeds.
Dynamic pressure is the key criterion, and although the airspeed will be very high, the dynamic pressure should be no higher than at 120mph at sea level. Because of this, the forces should not be excessive, and he should be fine.

Craigboy
2010-Jan-25, 10:49 AM
Isn't the claim about exceeding the sound barrier ** since a falling body with only gravity as the acting force (and maybe wind resistance) reaches terminal velocity and in this case that would be less than the speed of sound.

"Up there where the air is rarefied..."

EDIT: Also here's the official site http://www.redbullstratos.com/

"HD" video of the one on the site.
http://www.youtube.com/watch?v=PyGmTV0q2kY&feature=player_embedded

Jens
2010-Jan-26, 02:55 AM
Though this is a bit morbid, I wonder if some of the astronauts of Columbia might not have been still alive (though surely not conscious) when they were ejected from the disintegrating cabin. If so, then they would, albeit for a brief moment, have very significantly exceeded the speed of sound outside of a machine.

But on second though, I suppose it's unlikely that they were still alive when the cabin broke apart.

JohnD
2010-Jan-26, 12:54 PM
Not morbid, fast and too quick to be painful.

Do you know "For Johnny"?
Written 'in memoriam' of WW2 pilots, but just as applicable here:

Do not despair
For Johnny-head-in-air;
He sleeps as sound
As Johnny underground.
Fetch out no shroud
For Johnny-in-the-cloud;
And keep your tears
For him in after years.
Better by far
For Johnny-the-bright-star,
To keep your head,
And see his children fed.

by John Pudney

Pudney used the male pronoun, but it works as well for female astro/cosmonauts.

John

Craigboy
2010-Feb-27, 09:19 AM
Couple of updates:
The guy now has a blog http://felix-baumgartner.blogspot.com/
And a new press release http://www.marketwire.com/press-release/Red-Bull-Stratos-Science-Team-Reveals-How-Felix-Baumgartner-Will-Try-Achieve-1120880.htm

mugaliens
2010-Mar-01, 08:12 AM
For a perfect gas -- a very good approximation for our atmosphere -- speed of sound depends only on temperature.

The speed of sound may depend only on temperature, but terminal velocity is a function of density altitude, and DA is far, far less at 120,000 feet than at sea level, thus, terminal velocity is far higher at those altitudes than at sea level.

Also, the mach drag rise effects are far greater at sea level than at high altitudes, as well, as they're largely DA effects related to the speed of sound, but the magnitude of the effects are dependant on DA, not the speed of sound itself.


How is he going to hold himself together? People tend to come apart in airstreams approaching those speeds.

The forces involved are dependant upon both the Mach number as well as the DA. We crack whips all the time at sea level, but does the leather, essential animal hide, turn to dust as the tip breaks the sound barrier? Of course not. Neither does the end of a thin damp kitchen towel. Sure left a mark on my back when my older brother nailed me with it back in high school, though! Yeouch...


Dynamic pressure is the key criterion, and although the airspeed will be very high, the dynamic pressure should be no higher than at 120mph at sea level. Because of this, the forces should not be excessive, and he should be fine.

Bingo! Again, congrats on your entry into the program at Boulder. Now if we can just work on the pumpkin hurling issue...

mugaliens
2010-Mar-01, 08:19 AM
Another thought...

He could also tuck inside a weighted fairing and if it's designed right, easily exceed mach at lower altitudes as well.

NEOWatcher
2010-Jul-13, 04:25 PM
Update:
Equipment tested up to 26000 feet.
Skydiver plans supersonic space jump (http://www.msnbc.msn.com/id/38224278/ns/technology_and_science-space/).

trinitree88
2010-Jul-13, 06:18 PM
Beskeptical also wondered how parts of his body could experience supersonic airflow while other parts experienced transonic. That depends on the convolution of his body to the relative wind. When they wind-tunnel test wings of different camber and angle of attack, it affects the airflow over and under the wing. On a real plane, the air usually flows a little faster over the top of the wing, if the plane is flying straight and level at constant power...( I know, planes fly upside down too, if you adjust the angle of attack, so it's not the only determinant). Due to that, the air passing over the wing actually reaches the speed of sound, before the body of the plane does. A shock wave forms over the wing before it appears on the nose of the fuselage and the leading edge of the wings/elevators/tail. This is the transonic region of velocities. When you reach the speed of sound the shock forms on all of them. Because the wing shock interferes with flight near the speed of sound, by immobilizing the control surfaces, rookies learn to pass quickly through this speed zone to move the wave to the rear of the ailerons....thereby restoring stability. So you don't travel just below the speed of sound....you cut in the afterburners, boost your acceleration, and zip through it....just like passing a car on the highway. Safer(lots of pilots endured mishaps before they figured it out). There might be a Youtube video on Schlieren photography in wind tunnels that shows it. The different pressure/density of the air has a different refractive index, and a backlit two toned card shows the shock as if colored with false coloring.
As for the diver, he roughly approximates a crude wing and some areas will experience near-sonic (transonic) effects, while others will experience supersonic. Hope he has a Kevlar/bodysuit, or the windchill might frosty him up a bit. pete

SEE:http://www.youtube.com/watch?v=lSFwH0BVd3Q

SEE:http://www.ian.org/Schlieren/HowTo.html

mugaliens
2010-Jul-16, 05:18 AM
Modern supersonic aircraft are designed such that the shock wave propogation over control surfaces is angled on those surfaces, such that only part of the surface is in the transonic range at any given moment.

Similarly, even at Mach 1.2, a space-suited skydiver will have several areas remaining in subsonic flow.