Comments on: way better than sky-diving

Question: way better than sky-diving

clockzero — Tue, 06 Feb 2007 18:53:49 -0800

How could someone avoid burning up in the atmosphere while falling to Earth from orbit?

Assuming they were not in a space shuttle. With the right sort of space-suit? Diving skills, aerodynamism? Is it possible?

By: fvox13

fvox13 — Tue, 06 Feb 2007 19:05:44 -0800

It all has to do with the right angle.

By: fvox13

fvox13 — Tue, 06 Feb 2007 19:06:14 -0800

"right" as in "correct," not as in "90 degree"

By: b1tr0t

b1tr0t — Tue, 06 Feb 2007 19:09:14 -0800

Velocity matters too, 9.8 meters per second. Per second.

By: b1tr0t

b1tr0t — Tue, 06 Feb 2007 19:09:47 -0800

[yes, that's acceleration, you have to integrate to get velocity]

By: LobsterMitten

LobsterMitten — Tue, 06 Feb 2007 19:28:35 -0800

There are some relevant test cases:
highest freefall ever
highest skydive ever

By: LobsterMitten

LobsterMitten — Tue, 06 Feb 2007 19:30:49 -0800

From the Forbes article about Joe Kittinger:

On Aug. 16, 1960, as research for the then- fledgling U.S. space program, Air Force Captain Joseph Kittinger rode a helium balloon to the edge of space, 102,800 feet above the earth, a feat in itself. Then, wearing just a thin pressure suit and breathing supplemental oxygen, he leaned over the cramped confines of his gondola and jumped into the 110-degree-below- zero, near-vacuum of space. Within seconds his body accelerated to 714 mph in the thin air, breaking the sound barrier. After free- falling for more than four and a half minutes, slowed finally by friction from the heavier air below, he felt his parachute open at 14,000 feet, and he coasted gently down to the New Mexico desert floor.

And previously on MeFi.

By: LobsterMitten

LobsterMitten — Tue, 06 Feb 2007 19:38:21 -0800

Oops - The second link in my first comment was about Cheryl Stearns, a skydiver, who wants to break Kittinger's record using modern equipment, and move the record up from 102K feet to 130K feet (or 110K, depending which article you read about the project). She hasn't yet gotten the funding to do it, though.

By: ROU_Xenophobe

ROU_Xenophobe — Tue, 06 Feb 2007 19:46:10 -0800

The real killer isn't going to be your height, it's that you're going orbital velocity. Smack into an atmosphere going ~8 km/s and you get real fuckin' hot real fuckin' fast.

So what you'd want to do is use a retro-rocket to slow down to the same speed as the ground, and another thruster to hold you up as you lose orbital speed, and then fall.

If someone had a beanstalk or orbital elevator, that would be an excellent place to BASE-jump from. But even then, you'd want a retro-rocket for really big jumps. If you were jumping from 5000km, you'd be going *googles* 1300km/h relative to the ground.

By: Chuckles

Chuckles — Tue, 06 Feb 2007 19:52:03 -0800

It doesn't have much to do with the acceleration of gravity.. It is about burning off the kinetic energy of orbital velocity. The shuttle is going over 20,000 kph - that is some serious energy!

Well, more specifically:

Gravitational Potential Energy at 390km is about:

mass x 1/3(9.8 N/kg) x 390 km
--> 1.3 J/kg

but, kinetic energy at 20,000 km/h is about:

1/2 x mass x (20e6 m/h / 3600 s/h)² J/kg
--> 1.5e7 J/kg

Okay, I'm thinking that proves my theory by too much.. Anybody see a flaw in the math?

There will also be massive scale effects that I haven't thought through. A small object has relatively more surface area compared to its kinetic energy and mass/heat-capacity.. So reentry for a small object is probably less troublesome.

By: LobsterMitten

LobsterMitten — Tue, 06 Feb 2007 19:53:01 -0800

And from the discussion on the MeFi post, here's a site that explains the best physics behind freefall speeds from very great heights. They say that at Kittinger's height, the atmosphere is so thin that air resistance is "negligible", so the freefall speed is a good approximation of an actual fall from space. They say 274 mph is the most reliable estimate of how fast Kittinger would have been falling at his fastest.

Here are quotes relevant to your question:
About posture:

Terminal velocity is often reported to be approximately 60 m/s for a typical skydiver in free fall. Exceptional skydivers are able to increase this value considerably by diving head first with their arms against the sides of their bodies, legs held firmly together, and toes pointed. This posture presents a minimal projected area perpendicular to the direction of motion thus reducing aerodynamic drag. Special helmets and slick body suits reduce drag even further.

About air resistance and his airspeed:

The density of air at 30 km is roughly 1.5 % that at sea level and thus drag is essentially negligible. ... This is not true for skydivers at ordinary altitudes, which is why they reach terminal velocity and cease to accelerate... According to Captain Kittinger's 1960 report in National Geographic, he was in free fall from 102,800 to 96,000 feet and then experienced no noticeable change in acceleration for an additional 6,000 feet despite having deployed his stabilization chute. This gave him an unprecedented 3900 m (12,800 feet) over which to accelerate. At such extreme altitudes the acceleration due to gravity is not the standard 9.81 m/s2, but the slightly lower value of 9.72 m/s2. Using these numbers, it is possible to calculate the maximum theoretical velocity experienced during this record-setting jump. [Which they do in the article] ... Captain Kittinger most likely did not exceed the speed of sound on 16 August 1960. To do so would have required an additional 1,300 m (4,200 feet) of free fall.

The site also has calculations about other planned high-altitude jumps, and updates on their current status.

By: LobsterMitten

LobsterMitten — Tue, 06 Feb 2007 19:55:18 -0800

Of course, when I say "274 mph" what I mean is "274 meters per second". Because I have stupid fingers.

By: ROU_Xenophobe

ROU_Xenophobe — Tue, 06 Feb 2007 20:35:12 -0800

Okay, I'm thinking that proves my theory by too much.. Anybody see a flaw in the math?

I don't think so. Kinetic energy is (mv^2)/2, and orbital velocity at shuttle altitudes is 7.7 km/s.

For a 100 kilo person+gear, this gave me a kinetic energy of about 3 gigajoules, in line with what you said.

More to the point:

You have to give away all this energy somehow on the way down; usually through heat.

If you take a leisurely hour to fall 300 km, that means you need to get rid of energy at the rate of about 800,000 watts of power. So it's like spending 50 minutes in a one-megawatt heater, which is probably uncomfortable.

By: Fat Guy

Fat Guy — Tue, 06 Feb 2007 21:18:09 -0800

You could deploy your Inflatable Emergency Atmospheric-Entry Vehicle. The instructions for your Inflatable Emergency Atmospheric-Entry Vehicle are located on the card in the seatback in front of you.

By: aubilenon

aubilenon — Tue, 06 Feb 2007 21:23:16 -0800

Kittinger "stars" in one of my favorite music videos ever.

By: Steven C. Den Beste

Steven C. Den Beste — Tue, 06 Feb 2007 22:16:16 -0800

Orbital velocity (depending on the orbit) is something like 25 times the speed of sound. The irreducible problem is that you have a hell of a lot of both kinetic energy and potential energy to shed, and the only way to do it is as heat and turbulence.

Getting your outside hot is easy. Keeping the inside cool is not. Something the size of a human, plus some sort of suit, simply cannot insulate enough to prevent you from cooking, let alone being outright incinerated.

By: randomstriker

randomstriker — Wed, 07 Feb 2007 00:37:12 -0800

Something the size of a human, plus some sort of suit, simply cannot insulate enough to prevent you from cooking, let alone being outright incinerated.

Well, it depends how you define a "suit". If you expand it to include hardsuits analogous to those used in deep-sea diving, then a human-sized re-entry capsule is certainly within our current means. After all, the cone-shaped Apollo command modules were only 10' tall and 13' across; accommodating 3 people plus equipment.

By: maxwelton

maxwelton — Wed, 07 Feb 2007 01:54:56 -0800

As an aside, how freaking cool would it be to freefall from 114K feet?

By: veedubya

veedubya — Wed, 07 Feb 2007 04:36:32 -0800

This actually happens in Orbital Decay, a great Sci-Fi book by Allen Steele.

A character falls to Earth from a space station, not just from the upper reaches of the atmosphere, and so had to deal with re-entry heat. The apparatus used, as I recall, was something that looked similar to an umbrella, where the top of the 'umbrella' was an ablative heat shield. The user stood on the shield part, held onto the handle, and prayed. During the re-entry, the shield provided a tiny bubble of protection, if the user got the angle right.

The device was supposed to be used in only the direst of emergencies, and the thought of it actually being used was said to (I can't remember the exact quote), "Make the most hardcore of test pilots go pale and shake their heads."

By: NortonDC

NortonDC — Wed, 07 Feb 2007 07:55:07 -0800

To reinforce the critical distinction: dropping from space is (relatively) no biggie, dropping from ORBIT is your ticket to the all-over tan. That's because of all the extra kinetic energy that you would have due to your orbital speed, and that kinetic energy gets turned into heat when you hit the atmosphere.

Bonus fact: reentry heating has little to do with friction with the air. Rather, it comes from compressing the air in front of the object. Gasses heat up when they are compressed.

By: ShooBoo

ShooBoo — Wed, 07 Feb 2007 08:22:36 -0800

See: ... among them the famous General Electric MOOSE and the Space General FIRST. These gave the suited pilot the chance to step out into the void from a crippled craft, pull the ripcord, and manually cannonball or glide to the earth's surface.

See also Loose Moose: One Way to Bail Out of Orbit