Mar 20 2012

SuperJump

Felix Baumgartner is attempting to break the world record for the highest skydive ever. He recently completed a test dive from 71,500ft (22km), putting him in the top three. The purpose of the dive was to test out all of his equipment before he attempts the record breaking dive of 120,000ft later in 2012.

The current record is held by Joe Kittinger, a US Air Force Colonel who jumped from 102,800ft in 1960. At that altitude the air is very thin and cold, so the equivalent of a space suit is required to survive the trip up and down.

Baumgartner, a 42 year old Austrian, said after his test jump that the cold was “hard to handle” and that they may need to work on that problem before the big jump.

When discussing the upcoming jump on a recent episode of the SGU we commented that the high altitude jump is very dangerous. Kittinger, in fact, spun out of control at one point and blacked out. The details, however, are a bit different from what we discussed.

Actually,  Kittinger made three jumps as part of Project Excelsior, two test jumps and then the third for the standing world record. During the first jump is when he spun out of control and blacked out. Here is a description from Gregory Kennedy:

As he prepared to jump, Kittinger noted the balloon had climbed to 76,400 feet, more than three miles higher than intended. Standing in the door of the gondola, he pulled the lanyard that activated the barometric release for the main canopy and the timer for the pilot chute. It took three tries to pull the lanyard clear. Unknown to Kittinger, the first pull started the timer. The result of this was the pilot chute deployed only 2.5 seconds after he left the balloon.

Without sufficient airspeed to create adequate dynamic pressure, the pilot chute flopped around in the thin air and eventually wrapped around his neck. He began spinning. At first, Kittinger could correct the spin, but soon he could no longer compensate for it and eventually blacked out. He didn’t regain consciousness until he was floating beneath the reserve parachute several thousand feet above the ground. The pilot parachute had not deployed the drogue because it was tangled around his neck, so the main parachute never deployed. After Kittinger lost consciousness, the spin continued and eventually reached 80 RPM. A barometric release triggered the reserve parachute at 10,000 feet, but due to the spin, at first it tangled around him. Beaupre had planned for such a contingency and installed the reserve parachute pilot chute with a break-away cord that would enable the canopy to clear itself. Thankfully, the reserve cleared itself at 6,000 feet and inflated.

It seems that the spinning out of control had nothing to do directly with the altitude, but rather the early deployment of the pilot chute. However, the thin atmosphere is what made the timing of the chute deployment critical and ultimately resulted in the dangerous spin out.

Our discussion received some feedback from a listener who is an experienced sky diver that led to an interesting physics question. His point is this – wind resistance is independent of altitude because it is determined by by the wind resistance force at terminal velocity. In higher thinner atmosphere terminal velocity is faster, and at lower thicker atmosphere it is slower, but these two factors exactly balance out and the resulting wind resistance will be the same – enough to counteract the accelerating force of gravity. The experience (and danger) to the skydiver will be the same.

I don’t quite buy this, however. My counter point is this – at higher altitude the atmosphere is thinner and terminal velocity is much faster. It is estimated that Kittinger went a top speed of 625.2 mph during his third and highest jump. Typical terminal velocity for a skydiver (depending on various aerodynamic factors, such as position) is about 117-125 mph. A skydiver in a bullet position can go as fast as 210 mph. So Kittinger was going much faster than that.

I agree that once a skydiver reaches a terminal velocity the force they feel is constant, regardless of speed vs atmosphere density. But as they descend into a thicker atmosphere the terminal velocity will decrease. This means that the skydiver not only is maintaining terminal velocity, they are slowing down to the slower terminal velocity. Therefore the forces on them would be greater. I could not find a figure for how fast Kittinger was going when he deployed the parachute, but peak velocity was reached at 90,000 ft, and so he was slowing down from there until his chute deployed at much lower altitude.

The question then becomes – how much greater are the wind resistance forces due to this deceleration and how does that affect the riskiness of the jump.

I have another question as well, related to Kittinger’s first jump where he blacked out, which was related to the high RPM of the spin. If the skydiver does go into a spin at higher altitude and therefore higher velocity, will the increased momentum mean that the spin is likely to be faster, increasing the risk of blacking out?

I don’t have any definitive answers to these questions. I have chatted with a couple of physicists about it, but no one seems to have a final answer.

So I open it up to my astute readers – how does the higher terminal velocity alter the danger and experience for the super high altitude skydiver? What risks is Baumgartner going to face, other than the oppressive cold? I will continue to search for some appropriate expertise in the meantime, but it seems like a fun thought experiment, so have at it.

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