Speed of sound?

[airdvr 210]

Caught this on CNN today http://www.cnn.com/2010/US/05/21/supersonic.skydive/index.html?hpt=T3

I'm questioning whether you can actually break through the sound barrier in freefall. I'm not a scientist but I would imagine you would be able to approach the wall but not break through regardless of altitude.

Please don't dent the planet.

Destinations by Roxanne

[jakee 1,594]

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I'm not a scientist but I would imagine you would be able to approach the wall but not break through

Why?

Do you want to have an ideagasm?

[airdvr 210]

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I'm not a scientist but I would imagine you would be able to approach the wall but not break through

Why?

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Military aircraft routinely accelerate to speeds greater than the local sound speed. Historically, this was referred to as "breaking the sound barrier". The passage from subsonic to supersonic speeds is accompanied by some unusual phenomena which lie in the realm of "nonlinear" mechanical events - events involving some degree of chaos.

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/soubar.html

I am imagining that it requires some source of energy other than gravity. I don't know the answer...that's why I'm asking.

Please don't dent the planet.

Destinations by Roxanne

[jakee 1,594]

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I am imagining that it requires some source of energy other than gravity.

I'm just wondering why you make that assumption?

Do you want to have an ideagasm?

[davelepka 4]

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Why?

I would guess drag. It increases significantly as you approach the speed of sound and this would 'encourage' you to hit terminal velocity sooner, as terminal is simply when drag equals the force of gravity. Even if the thin air allows gravity to have it's way with you, the increased drag would smack gravity back into line as you get close to the sound barrier.

Keep in mind that the sound barrier is not just another notch on an airspeed indicator, it's when you literally begin to go faster than the sound waves around you. These waves build up into a pressure 'wall' that you have to surpass in order to continue accelerating.

I have no idea if this will play a factor here, but I would assume that's what the OP was thinking.

[airdvr 210]

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Why?

I would guess drag. It increases significantly as you approach the speed of sound and this would 'encourage' you to hit terminal velocity sooner, as terminal is simply when drag equals the force of gravity. Even if the thin air allows gravity to have it's way with you, the increased drag would smack gravity back into line as you get close to the sound barrier.

Keep in mind that the sound barrier is not just another notch on an airspeed indicator, it's when you literally begin to go faster than the sound waves around you. These waves build up into a pressure 'wall' that you have to surpass in order to continue accelerating.

I have no idea if this will play a factor here, but I would assume that's what the OP was thinking.

Yea...that's it. I doubt that without some external energy source gravity alone will pull a body in freefall through that barrier. I guess we'll find out.

What I think we might be told is that the person in freefall exceeded X miles per hour which doesn't necessarily mean the local speed of sound was surpassed. Anyone know what the speed of sound is at given altitudes?

Please don't dent the planet.

Destinations by Roxanne

[robinheid 0]

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Caught this on CNN today http://www.cnn.com/2010/US/05/21/supersonic.skydive/index.html?hpt=T3

I'm questioning whether you can actually break through the sound barrier in freefall. I'm not a scientist but I would imagine you would be able to approach the wall but not break through regardless of altitude.

that's what they said before Chuck Yeager did it in the x-1.

SCR-6933 / SCS-3463 / D-5533 / BASE 44 / CCS-37 / 82d Airborne (Ret.)

"The beginning of wisdom is to first call things by their right names."

[airdvr 210]

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Caught this on CNN today http://www.cnn.com/2010/US/05/21/supersonic.skydive/index.html?hpt=T3

I'm questioning whether you can actually break through the sound barrier in freefall. I'm not a scientist but I would imagine you would be able to approach the wall but not break through regardless of altitude.

that's what they said before Chuck Yeager did it in the x-1.

Yea...but I seem to remember something about a pretty big set of rocket motors attached to his ass.

Please don't dent the planet.

Destinations by Roxanne

[davelepka 4]

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that's what they said before Chuck Yeager did it in the x-1.

Those same guys, around that same time also thought that cigarettes and bacon were good for you.

The whole point of Yeager's flight was to learn about the sound barrier. Now we know, and have reason to believe that an unpowered craft might not be able to punch through that sucker.

This actually brings up a good point about the jump, does anyone know what sort of data logging they're going to be doing, and what they're using for that?

[Andrewwhyte 1]

Meteoroids are usually post mach; they are gravity powered.

[rdufokker 6]

The speed of sound is not related to altitude or air density at all. It is only affected by temperature. This is why he will be able to break the sound barrier.

I'm sure there are some Aeronautical Engineers on here that will give the equations and explanations in further detail.

Burke

Irony: "the History and Trivia section hijacked by the D.B. Cooper thread"

[kallend 2,146]

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I'm not a scientist

Yep.

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The only sure way to survive a canopy collision is not to have one.

[kallend 2,146]

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Meteoroids are usually post mach; they are gravity powered.

So is the returning space shuttle.

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The only sure way to survive a canopy collision is not to have one.

[kallend 2,146]

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that's what they said before Chuck Yeager did it in the x-1.

Those same guys, around that same time also thought that cigarettes and bacon were good for you.

The whole point of Yeager's flight was to learn about the sound barrier. Now we know, and have reason to believe that an unpowered craft might not be able to punch through that sucker.

This actually brings up a good point about the jump, does anyone know what sort of data logging they're going to be doing, and what they're using for that?

The big problem with "the sound barrier" wasn't the force needed to go supersonic, it was the interaction of the shock waves with the aircraft controls.

The Grand Slam bombs of WWII went supersonic in freefall. They tended to wobble as they encountered the shocks. en.allexperts.com/e/g/gr/grand_slam_bomb.htm

www.youtube.com/watch?v=R-Mm-zFW_nA

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The only sure way to survive a canopy collision is not to have one.

[airdvr 210]

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Meteoroids are usually post mach; they are gravity powered.

So is the returning space shuttle.

True, but it starts it's decent at 17,000+ mph. It required some serious energy to reach that speed to begin with. I'm still not convinced.

Please don't dent the planet.

Destinations by Roxanne

[kallend 2,146]

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Meteoroids are usually post mach; they are gravity powered.

So is the returning space shuttle.

True, but it starts it's decent at 17,000+ mph. It required some serious energy to reach that speed to begin with. I'm still not convinced.

See post #14

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The only sure way to survive a canopy collision is not to have one.

[airdvr 210]

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Meteoroids are usually post mach; they are gravity powered.

So is the returning space shuttle.

True, but it starts it's decent at 17,000+ mph. It required some serious energy to reach that speed to begin with. I'm still not convinced.

See post #14

Interesting post. It appears then that what needs to be determined is how well bone and flesh hold up in an enviroment that steel and aluminum have struggled with?

Please don't dent the planet.

Destinations by Roxanne

[Calvin19 0]

mach has been broken in freefall and it will be again by Felix.

-SPACE-

[airdvr 210]

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mach has been broken in freefall and it will be again by Felix.

When?

Please don't dent the planet.

Destinations by Roxanne

[quade 4]

The biggest thing you're missing in your thinking is air density. At about 18,000 ft MSL the air is only 1/2 the density; only 1/2 the amount of air molecules to push out of the way. Go up another 18,000 ft and you've cut the density down to 1/2 of that. By the time you're up to 100,000 ft, there's almost, but not quite, any drag at all.

quade -
The World's Most Boring Skydiver

[muff528 3]

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The biggest thing you're missing in your thinking is air density. At about 18,000 ft MSL the air is only 1/2 the density; only 1/2 the amount of air molecules to push out of the way. Go up another 18,000 ft and you've cut the density down to 1/2 of that. By the time you're up to 100,000 ft, there's almost, but not quite, any drag at all.

Also, the actual speed for Mach 1 decreases by over 100 mph from 761.2 at MSL up to about 40000 ft, remains static up to 65000, and then increases to 675.6 mph from 65000 ft to 100k. Reduced drag at higher altitudes + less actual speed needed to achieve Mach 1 = looks possible.

interesting charts here-no math needed

[pchapman 279]

Yeah, the notion of a "wall" or "barrier" at Mach 1 is a bit of a myth, that comes from the simplistic way that it has been historically described.

It was a barrier of sorts, but the words used in English don't help provide real understanding about it.

Are there big differences in air foil aerodynamics that make it tough if you haven't designed your plane for transonic flight? Yes. Which resulted in loss of control and crashes.

Is there ever more rapidly increasing drag as speed goes up? Yeah, just like normal when going faster and faster. That was a problem for aircraft back when they were underpowered for the needs of transonic flight.


And if one is thinking about "the massive forces on the body when trying to go that fast":

The dynamic pressure at terminal velocity still has to equal the jumper's weight. Let's say some stratospheric jumper with all their gear weighs 300 lbs. If on a normal skydive that puts them at 150 mph (to pick a number), then at terminal velocity way up high the pressure forces on the body are going to be the same 150 mph basically -- even if the guy is going 700 mph true air speed.

(Similarly, that an airliner might have a red line at 280 kts -- yet fly at 600 mph up high -- because the pressure of the less dense air being hit is still less than that 280 kt red line at sea level. (Mach number limits are a separate matter.)

So, at a first level of approximation, the jumper isn't going to feel anything much different. Waving his hands in the breeze would still feel like 150 mph. That's a pretty disappointing barrier; the jumper probably wouldn't even know he crossed it until the data were analyzed on the ground.

Let's check actual air density up high: At 60000', for just one example, the air density is about one tenth that at the surface. The basic equation for dynamic pressure means to get the same pressure force from movement, you have to be going 3.2 times faster. So if terminal for the jumper down low is 150 mph, it'll be 480 mph at 60,000'. Sounds like if you want to break Mach 1 on your belly, you might expect only to do it much higher up in the jump.


Calvin 19 wrote:

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mach has been broken in freefall and it will be again by Felix.

I'm not sure about that breaking Mach 1 in freefall thing??
I don't recall the exact conclusions from all the Joe Kittinger threads, but I thought there were some data that was just wrong (eg, reports with a typo of 718 mph not 618 or something like that), while the other data (618 mph at whatever altitude) didn't seem to make sense as being above Mach 1 when the numbers were worked out. Close, but nothing to clearly show he had actually done it. Plus I'd want to know more about the accuracy of the speed measurements. But that's just off the top of my head.

[edited a bunch for clarity]

[JohnMitchell 16]

If you started from a standstill maybe 300,000 miles up you'd be going almost 25,000 mph when you hit the atmosphere, supersonic in any book. So it's definitely possible. Is it possible from 125,000 feet? I think so, but I don't have the math skills to prove it.

Like Kallend said, freefall ordinance can go supersonic if the weight and altitude are high enough and the ballistic coefficient good enough. Nothing magical about it at all.

[bfilarsky 0]

Drag doesn't just go up as normal when approaching the sound barrier - as soon as you get close, your coefficient of drag increases exponentially due to the shock wave you begin to generate (called wave drag).

You are correct about the big problem being controllability in aircraft. As an aircraft passes through its critical mach number (where supersonic flow begins on parts of the aircraft, but the total flow is not yet supersonic - usually around 0.7-.85 Mach), its center of pressure moves aft, causing a downwards pitching moment, known as mach tuck. Not really an issue with a jumper, as they create drag, not lift.

I think the sound barrier can be broken from 125,000' in freefall - can't really give any math to prove it, but it sounds reasonable given the air density.

Also, the point about the speed of sound in air being based on temperature only is correct. In Meters/second, It is the square root of (1.4*287*T) where T is the temperature in kelvin.

At 125,000 feet (temp 246 kelvin), that gives 314 m/s, or 611 knots.

[shropshire 0]

How does the fact that the temperature of the layer of air around the moving object increases with the object speed through it ... add to the complication of the maths for determining the actual local speed of sound?

Lots, I bet.

(.)Y(.)
Chivalry is not dead; it only sleeps for want of work to do. - Jerome K Jerome