Sub-orbital skydiving?

[Ar-Pharazon]

I'm not sure what distance constitutes "sub-orbital", like what Capt. Kirk did in Generations, but this guy jumped from 96,000 feet in preparation for a jump from 125,000 feet.

[YT]http://www.youtube.com/watch?v=0m4IbyWIv0I[/YT]

He's hoping to break a record as well as break the sound barrier.

 

[YellowSubmarine]

I'm not sure what distance constitutes "sub-orbital"

Anything above 100 km is sub-orbital (except for regions directly above US territory, where it is 80 km).

 

[M'Sharak]

I'm not sure what distance constitutes "sub-orbital"...

There are more specific definitions, but for something like this, anything starting from less than 100 km/62 mi altitude would be considered sub-orbital.

For comparison, low-Earth orbit would lie between 160 km and 2,000 km above surface/sea level.

96,000 feet is already pretty close to the existing record and, I read elsewhere, quite a bit farther up than was originally planned for this jump.

 

[gturner]

Well, technically jumping out of bed is sub-orbital, but the term generally implies supersonic and near zero-G ballistic.

 

[YellowSubmarine]

I'm not sure what distance constitutes "sub-orbital"...

There are more specific definitions, but for something like this, anything starting from less than 100 km/62 mi altitude would be considered sub-orbital.

Any successful space jump that doesn't rely on propulsion or atmospheric drag for re-entry is sub-orbital, which means that you can have a sub-orbital space jump from anywhere up to the altitude of geosynchronous orbits. Technically, if you managed to do a propulsion-less space jump above that, it would still be sub-orbital, but it's kinda impossible (you would have to accelerate yourself towards the Earth after crossing geosynchronous orbits, which means that at least at one moment you were in orbit and you de-orbited).

 

[Trekker4747]

Call me crazy, but I don't think a human body moving at the speed of sound outside of a vehicle is a good idea.

 

[gturner]

Call me crazy, but I don't think a human body moving at the speed of sound outside of a vehicle is a good idea.

It's not a big deal at all for a jump like this. He'll probably just barely go supersonic, and since he's not going to go much faster than that, it means that around Mach 1 his speed will be creeping up slowly and then creeping back down slowly. That means he's not accelerating very much, which means his velocity is relatively constant, which means his body will be near 1G, just like he was laying in bed. The only forces he'll feel are a result of his body weight, since that's all that is driving him through the air. So even though he's supersonic, he's doing it in air thin enough that only a body-weight's worth of thrust is involved, and a body-weight's worth of drag.

It could get uncomfortable if he came in from much, much higher, where he'd be hitting the atmosphere's layers much faster, producing much higher deceleration rates, and thus much higher forces. Or if he was low in the atmosphere, such as ejecting from a supersonic fighter.

He could probably make it much more exciting by jumping out with an ejection seat rocket motor to speed him up.

 

[Trekker4747]

Yeah, I wasn't worried so much about acceleration forces so much as I was stresses on the body from flying at such a speed and what will happen when a sonic boom occurs from the pressure waves (assuming he goes super-sonic, if only even a little bit.)

Again, it just strikes me as... Risky. No one could probably really say what will happen when a human body goes that speed and experiences that. It'd be wicked cool when he does it, I just wonder if he won't come out of it with ruptured ear drums or something.

I also wonder if the reverse effect of diving to great depths won't happen? Something like the opposite of the bends/nitrogen narcosis.

 

[Knight Templar]

Call me crazy, but I don't think a human body moving at the speed of sound outside of a vehicle is a good idea.

It's not a big deal at all for a jump like this. He'll probably just barely go supersonic, and since he's not going to go much faster than that, it means that around Mach 1 his speed will be creeping up slowly and then creeping back down slowly. That means he's not accelerating very much, which means his velocity is relatively constant, which means his body will be near 1G, just like he was laying in bed. The only forces he'll feel are a result of his body weight, since that's all that is driving him through the air. So even though he's supersonic, he's doing it in air thin enough that only a body-weight's worth of thrust is involved, and a body-weight's worth of drag.

It could get uncomfortable if he came in from much, much higher, where he'd be hitting the atmosphere's layers much faster, producing much higher deceleration rates, and thus much higher forces. Or if he was low in the atmosphere, such as ejecting from a supersonic fighter.

He could probably make it much more exciting by jumping out with an ejection seat rocket motor to speed him up.

Well remember that the speed of sound falls as you gain altitude.

I remember reading about incidents where the pilot of a Soviet Mig-25 Foxbat ejected at Mach 3. Also where an SR-71 pilot was thrown from a disintegrating Blackbird at Mach 3. Both survived without critical injuries.

I believe the term is "dynamic air pressure". That is the amount of air pressure actually acting on the body. The pilot of the Mig-25 was at more than 70,000 feet IIRC but due to the altitude, his "Mach 3" ejection was only the equivalent of ejecting at about 600 mph near sea level.

Incidentally, it has been estimated that the highest speed ejection survived at low level and in the thickest possible air was that of an F-15 pilot who ejected near sea level over the Gulf of Mexico at about Mach 1.4. He was very seriously injured IIRC.

This site explains it somewhat well

http://www.ejectionsite.com/ejectfaq.htm#1

 

[Ar-Pharazon]

Does "less air" allow him to go faster than terminal velocity?

 

[Trekker4747]

Does "less air" allow him to go faster than terminal velocity?

That's sort of what I'm thinking is happening, as he steadily takes on air as he gets deeper into the atmosphere he'll slow down more and more, though I don't know if he'd reach terminal velocity before hitting the ground without the aid of the drag chute. (Chutes usually slowing someone down from TV to a safer landing speed.)