### Re: Is it possible to break the speed of sound in a freefall?

Date: Sun Nov 14 00:35:37 1999
Posted By: Matthew B. Weyerich, Technical Coordinator,ES&R Dept., CPI Corp.
Area of science: Physics
ID: 941571910.Ph
Message:
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Jeff:

I got so involved in my business trip to Canada I completely forgot to send
the answer to your question. I really apologize for the delay! Hope you
still have some use for this.

I believe the answer is "yes", and that it's actually been done by Col. Joe
Kittinger, Jr., USAF. (See this page for pictures & details:  http://www.math.ohio-state.edu/~kerler/BSBD/cult/kitt.html . Don't miss the
HALO sections, either, because it was sitting on a plane at 35K feet
wondering what it would be like to jump out which helped remind me I had

I haven't done any calculations, but there would be a few things to
consider before attempting a jump of this sort. For example, as you note,
what are you going to breathe? At altitudes over 10,000 feet, the air is
just too thin for humans to get enough oxygen efficiently. That's why
passenger planes are pressurized, and why the flight attendant always shows
us how to use the goofy-looking yellow plastic mask should the pressure
system suddenly fail. (If that system fails at over 30,000 feet you'll only
have a few seconds to get the mask on before you pass out. Pay attention to
the demonstration!) We'll have to take some oxygen bottles and masks on our
jump.

We'll also probably want some kind of a full-body pressure suit. This will
keep us from suffering the "bends" etc., as we ascend, then fall. It will
also help keep us warm, as you note, because it is VERY cold that close to
space. (Oh, by the way, pressurized "earmuffs", or a pressurized helmet
would be a must. I'm pretty sure the pressure differential between the air
we breathe and ambient at 100K feet is enough to cause our eardrums to
rupture, in which case our breathing air would leak uselessly out our ears.
At the very least, it would be incapacitatingly painful and
disorienting…something you don't want to be bothered with on a supersonic
leap!)

Assuming we have the gear to get up that high and step out the door of our
balloon, plane, etc., what will happen to us? Imagine it with me. We'll
fall like rocks…FAST ones!

Just after stepping out the door gravity will pull us back toward the earth
as usual. Key thing here is, there isn't as much air at those great
heights, so we'll be able to accelerate faster. "Terminal velocity" depends
on air friction. More space between the air molecules means less friction,
higher terminal velocity AND lower speed of sound. (Sound waves are
disturbances in the air "fluid" around us. If we whack an air molecule at
sea level it will "whack" into another one in short order, and so on, and
so on. If the air molecules are much farther apart, as they are at 100K
feet, it takes longer for the "whack" to propagate, so sound doesn't travel
as quickly.) Basically, we'll be doing over 600 miles per hour in very
short order.

To maximize our acceleration we might want to curl up in a ball, or, point
our head at the ground and try to reduce drag. At 19-20 miles up we have a
few miles to fall before we hit significantly thicker air, so we can get
going pretty fast…supersonic should be possible, and it will happen much
more quickly than you'd think. Can we survive that, or, will we be ripped
to shreds?

I think we can make it. Most "sound barrier" problems only apply to
aircraft. (Ask any bullet, or artillery shell.) As we fall, we are more
closely approximating high-speed "rocks", so we don't really have to worry
about the finer points of aerodynamics. We won't really have much control
in air that thin, though, so we have to worry about tumbling and spinning
creating g-forces which might make us pass out, or, worse. We need
something to keep us stable as we fall. A drogue chute, or streamer would
do the trick, but it will slow us down.

Speaking of chutes, this is probably the wrong point in our imaginary leap
to start thinking about them! We probably need some pretty good ones, as
does any skydiver. I'm going to assume we've thought this out in advance.

We can't just pop a regular chute at high velocity. It would rip to shreds,
tangle, etc., probably killing us in the process. What we need is a way to
bleed off excess velocity to the point where a robust "regular" chute can
open, survive, and not tear us to pieces in the process.

One way to slow down is to deploy a streamer. Basically, just a long ribbon
of cloth which creates some drag. That will bleed off some speed without
jolting us too much. Next we could try a drogue chute, which is a small
parachute used for slowing down. A good design might be like those used on
the shuttle: bands of cloth sewn into a parachute shape, but with plenty of
room for air to pass through. A cruciform parachute might work, too.
Whatever design we pick, the concept remains the same: maintain stability
and slow down. Otherwise, we encounter g-forces we can't handle, pass out,
and make a crater when we land.

Once we have a stabilizing chute open, we have plenty of time…minutes…to
decide when to pop the main. The thing to remember here is, once we are
stable under a drogue, we are always slowing down, because the air we are
falling through is becoming more dense. At a certain point we'll even out
to a speed somewhere around that of a "normal" skydiver and stay there.
Then we just decide how high we want to pop the main chute. Under 20,000
feet is a must, or the chute won't necessarily open properly. Under 10,000
will allow us to take off our mask and breathe "normal" air. Both might be
too high if we don't want to drift a long time on the winds. Maybe we'd
like to open at under a mile, like most skydivers. No problem. We can even
wait until we're under 3,000. All we need is time for the chute to open,
and time to cut it and deploy a reserve if something goes wrong.

There's a neat computer simulation of Col. Kittinger's flight and jump at
PBS's Nova site: http://www.pbs.org/wgbh/nova/escape/skydive.html

There are other answers in the MadSci Archives. Just look up "terminal
velocity" with the search engine.

Again, sorry to take so long. Hope this helps!

-Matt

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