Apr 9, 2012, 8:32 PM
Post #34 of 38
Re: [Chris-Ottawa] RIG for the upcoming supersonic skydive
[In reply to]
Normally, round parachutes open harder at high altitude. Say, three to four times as hard at 40,000' than low altitude. [Reference: Knacke] It might not work quite the same with reefed ram air parachutes; it might be not quite so bad.
Still I can say:
At high altitude a jumper will take longer to get to the local terminal velocity. (One outside analysis of the planned Red Bull jump suggested 45 seconds to the peak speed.) But once at it, the dynamic pressure at say 800 mph is going to be the same as when the jumper is doing 120 mph or whatever near sea level.
So that's 6.7 times as much actual speed, and thus by squaring it, 44 times as much actual kinetic energy to deal with. (Well, one isn't slowing to a canopy descent rate of zero, certainly at high altitude, but we're just looking at rough numbers here.)
Whether all that energy matters depends on how fast the parachute opens -- it would be no problem only if the canopy takes a lot longer to open, with a long long snivel. On a regular skydive, if a canopy inflates fully in a very short time, the G forces get excessive when at terminal. Don't go slider off at terminal velocity. Deceleration loads are only acceptable when the full inflation starts at lower speed. So normally we need that reefing from the slider so the partially open parachute slows us somewhat before it snaps fully open.
To reiterate, the wind pressure felt on a high altitude jump at high altitude terminal will be like 120 mph down low -- so there's plenty of force trying to open the canopy.
The part of the deployment process where the canopy fills with air is less of a factor for squares than rounds, but in any case that inflation is affected more by the volume of air (even though it is very thin air), than the density. So the tendency, at high speed & altitude, is for much more rapid canopy filling, because there's a lot more cubic feet of air going by every second.
Thus overall you still have plenty of wind pressure like usual, tons more energy, and faster canopy filling. This sounds like it should lead to much harder openings. Unless the slider somehow guaranteed a really long snivel in some manner I don't understand, a high altitude, high airspeed opening would be extremely hard.