Human factors in commercial suborbital flight: Impact acceleration: an extreme skydiving experience
by John Jurist
|Frefall from 100 kilometers would be quite a memorable ride, but thermal protection at extremes of cold and high stagnation temperatures (over 500°C) for the five- or six-minute ride will be required.|
Consider a commercial spaceflight passenger who bails out of a suborbital vehicle at apogee (effectively zero speed), 105 kilometers above the ground, and falls free. This could potentially be a result of some type of failure mode or elective as a form of extreme skydiving. Further, ignore the problems of maintaining stability but instead assume that the spread-eagled passenger either faces up to enjoy the sky and experience +Gx or faces down to enjoy the view of the approaching ground and experience –Gx. What kinds of dynamic loading does this individual experience as a result of air drag?
For the first 73 seconds, he or she experiences microgravity at less than one percent of normal gravity. During that time, he or she descends to about 81,300 meters and accelerates to a speed of about 695 meters/second. As the descent continues, the intrepid skydiver accelerates up to a maximum speed of about 1,000 meters/second (Mach 3) 114 seconds into the fall at an altitude of about 45,400 feet. He or she then decelerates until crossing 9,000 meters into modestly decent atmosphere 243 seconds into the fall at a speed of about 90 meters/second. The skydiver crosses 3,000 meters at 63 meters/second 325 seconds into the jump.
During the fall, 22 seconds are spent at acceleration levels above 2 G, 18 seconds are above 3 G, 13 seconds are above 4 G, and 6 seconds are above 5 G. Maximum acceleration is about 5.8 G at 137 seconds into the fall and an altitude of about 26,500 meters.
That would be quite a memorable ride, but thermal protection at extremes of cold and high stagnation temperatures (over 500°C) for the five- or six-minute ride will be required. A pressure suit is also required. Stability during the free fall is a problem, particularly during the high acceleration phases of entry into denser atmosphere. If the skydiver enters a flat spin, blood is effectively centrifuged into the extremities. The human tolerance to a flat spin is about 140 RPM, and uncontrolled spin rates of up to about 200 RPM can occur. Spinning can produce vertigo and result in nausea and vomiting. This is both unpleasant and dangerous in a pressure suit and/or oxygen mask.
The first extreme skydiver was Joseph Kittinger in the late 1950s as part of the USAF Project Excelsior. He jumped from balloons at altitudes of about 30,000 meters.
The next column will consider some failure modes and their dynamic implications for a potential passenger.