Upper Altitude Parachuting Survivability

[Jack_Reynolds]

@Stranger_On_A_Train

What is variable is the amount of oxygen different people carry in their blood; very aerobically fit people will have more red blood cells and oxygenation capacity, and will probably have a smaller difference between systolic and diastolic pressure, which means a drop in external pressure will have less of an immediate effect. However, the loss of pressure will still result in overall reduced diastolic pressure (regardless of how efficient or powerful the heart is) and it is that loss of pressure that results in interruption of oxygen perfusion.

What about the peoples with genetic adaptations towards high altitudes? The Tibetans, Peruvians and Ethiopians who have the ability to avoid altitude sickness through higher blood oxygenation or increased haemoglobin. Would that effect someone’s conciousness or the effects they would sustain following entry into low altitude?

I did previously establish that the space marine had spent a year on a low-oxygen world, “like fighting at 10,000 feet while at sea level.” Would that give any benefits? I know such conflicts occur, currently on the Indian/Pakistani border, and in 1900 in the Himalayas.

[LSLGuy]

@Melbourne

@Chronos

Once you get to terminal velocity (or even reasonably close to it), there is a “down”.

After ~10 seconds. Too late according to the table cited above.

But it’s a possibility.
(1) The system normally maintains enough pressure to pump blood to your head. At ground level you can maintain a greater pressure by inverting your posture.
(2) Outer space is only a short distance away. Gravity is still around the same value.
(3) Total unconsciousness takes longer than the ~10 seconds cited above, and perhaps you could learn to maintain a head-down posture while in free-fall and drifting out of consciousness. And brain use of oxygen is variable: perhaps some people are better at remaining mentally calm while falling out of a sub-space aircraft.
(4) Although the internet reports that it takes around 14 seconds to reach terminal velocity, I don’t know how long it takes until acceleration is slow enough that gravitational force on (essentially) a 6’ column of water will be enough to maintain perfusion (itself something I’ve never considered before)

I’ll take a different tack on this rather than discussing perfusion.

“Terminal velocity” is when air drag due to velocity has increased to equal the force of gravity and the object ceases accelerating downwards. Ref your sorta-cite that may take 10-14 seconds for a human shaped object at the low altitudes and thick atmospheres where recreational skydiving is done.

Starting from an altitude of WAG 30 miles, the atmophere up there is very, very thin. Such that terminal velocity is much higher, well above the local speed of sound. As you say, gravity is not appreciably different at the top of the atmosphere vs. the bottom. So we still have substantially the same downward force, but trying to accelerate against much less drag to a much higher speed.

[Stranger_On_A_Train]

@Jack_Reynolds

What about the peoples with genetic adaptations towards high altitudes? The Tibetans, Peruvians and Ethiopians who have the ability to avoid altitude sickness through higher blood oxygenation or increased haemoglobin. Would that effect someone’s conciousness or the effects they would sustain following entry into low altitude?

Tibetans, Peruvians, and others who are acclimatized to altitudes above 8000 ft have a number of adaptations; in addition to an increase in the quantity of red blood cells to deliver more oxygen, they also have modifications to central chemoreceptors, an increase the quantity and output of the mitochondria, and cells expressing more of the enzymes required for anaerobic glycolysis, all of which support more effective metabolism with a lower oxygen intake. The adaptations are actually not genetic modifications but changes to gene expression via altering epigenetic factors such that anyone living at high altitudes for an extended period of time will develop these adaptations (to a greater or lesser degree depending upon age, prior conditioning, and other confounding factors).

However, this doesn’t change the fact that once ambient pressure drops below a minimum threshold it just isn’t physiologically possible for the body to maintain sufficient average blood pressure such that it is possible to get sufficient diffusion of air through the cell membrane even if there is a large surplus of oxygenated blood cells. Your blood will not literally boil in your vessels (at least, not until you experience gross rupture of major vessels of the cardiovascular system) but it will be impossible for the body to maintain sufficient internal pressure. This is what a partial pressure suit does; by constricting the ability of the body to expand, it ensures that the cardiovascular system can maintain adequate internal pressure.

Stranger

[Whack-a-Mole]

At 30 miles up will you go fast enough to worry about friction heating you up and cooking you?

[Stranger_On_A_Train]

There would be some heating; going from that altitude down to 35 kft (basically, the entire range of the stratosphere) in absence of drag would result in a speed of about 860 m/s or almost 2000 miles/hr. That would exceed the speed of sound at that altitude, so you’d certainly get some compression heating at some point as the air density starts to increase significantly around 75 kft, but that would also cause you to lose speed as you transfer excess momentum to compress the air and eventually you’d come to some equilibrium speed, e.g. “terminal velocity” which would continue to decrease as you go down. The amount of heating from ‘skin friction’, e.g. the attachment and separation of ambient air would be minimal just because the density of the air is so low, and the dynamic viscosity actually drops slightly through the stratosphere. In doing hazard limit calculations from a flight termination or breakup, we generally assume no additional breakup due to heating from altitudes below 300 kft, but then, we’re generally considering solid metallic or high temperature composite components, not human bodies. I suspect the total amount of heat flux you would get would not be enough to offset the fact that an unprotected body would be subjected to very cold equivalent temperature.

The reason spacecraft heat up so much upon reentry is because they are coming from orbit (or for ballistic missiles, very high arcs) and are moving much faster than something falling with no initial velocity, so they have a lot of momentum to shed, virtually all of which is converted into thermal energy in the shock wave during reentry. They therefore require thick shields that protect the reentering spacecraft from both radiative heating and the ionized gases created by such heating.

Stranger

[Whack-a-Mole]

@Stranger_On_A_Train

There would be some heating; going from that altitude down to 35 kft (basically, the entire range of the stratosphere) in absence of drag would result in a speed of about 860 m/s or almost 2000 miles/hr.

At those speeds(ish) the SR-71 Blackbird could heat to 500+F and the Concorde (around 1300mph) would get over 212F. Would a person falling be different?

[Stranger_On_A_Train]

That calculation was assuming falling without drag. A body would actually experience drag as is fell, siphoning off momentum, so it would not achieve that speed.

Stranger

[Jack_Reynolds]

Could the turbulence of a rescue vehicle or another skydiver potentially bump a faller out of that dangerous flat spin? Say the main character is in a flat spin, and the hoverbike passes by beneath him. Would the turbulence possibly knock him out of the spin, or is there not enough air pressure for that?

I’ve been looking for causes of the flat spin, and how to avoid or halt it. It seems like a drogue shoot attached to the right spot will work (which is how I intend to have the unconscious emperor survive, a long strip of fabric acts as a drogue and stabilizes them), but for the space marine, I’m having trouble even determining why he would start spinning, and how Felix got out of it.

[Stranger_On_A_Train]

A “flat spin” occurs because the body experiences lift without enough forward speed to retain orientation control, e.g. the vehicle is stalled but still has significant buoyancy. The result is that any disturbing force contributes to increasing angular momentum in yaw, and the body spins faster and faster about the yaw axis. There is no way to “bump” a body out of a spin; it has acquired angular momentum and it has to shed it, either via the use of aerodynamic control surfaces or by deploying spin-recovery devices (ballutes or parachutes) to slow the rate of rotation and allow the body to reassert orientation control. In the case of your story, I wouldn’t worry about it; it isn’t an issue the casual reader would pick up on, and you can basically dismiss it by saying the jumper maintained a rigid, low lift posture and no spin occurred. It is a problem for HALO jumpers not because they’ll spin fast enough to break apart or pass out but just because the rate of spin can cause them to become disoriented and and cause the canopy lines to become tangled upon deployment.

Stranger

[Jack_Reynolds]

Well, I’m about to write the first draft of the chapter now. If it’s completely wrong, I can change it later. That’s what edits are for!

[John_DiFool]

@Stranger_On_A_Train

The reason spacecraft heat up so much upon reentry is because they are coming from orbit (or for ballistic missiles, very high arcs) and are moving much faster than something falling with no initial velocity, so they have a lot of momentum to shed, virtually all of which is converted into thermal energy in the shock wave during reentry. They therefore require thick shields that protect the reentering spacecraft from both radiative heating and the ionized gases created by such heating.

Hmm. Has there been any science fiction describing a single-person reentry suit or such? [Yes, from orbital velocity]

[Stranger_On_A_Train]

The Marauder Suit from Starship Troopers deployed from orbit within a protective shell, then broke out and flew down, but it is more of a large mecha (complete with nuclear grenade launcher) than just a vacuum suit. I’m sure someone has portrayed a ‘descent suit’ somewhere, and the MOOSE concept referenced above is a real world proposal that is at least conceptually plausible. The biggest problem with reentry isn’t actually the heat shield or protection but assuring it can maintain a stable orientation during reentry so your spacecraft doesn’t flip ass over teakettle and expose the nonprotected parts of your spacecraft to heating or huge shear aeroloads that will tear it apart.

Stranger