Highest Survivable Skydive

[Kromlech444]

So I recently survived a 144,555m skydive, thinking, well, imma die, but I lived. So how high can one skydive and survive, nothing but a parachute, no heatshields or anything, free fall. I tried 1,000,000m, twice, died, twice.

[Zhetaan]

You have my apologies for this response, but ...

... It depends.

Your trajectory has a lot to do with it.  I assume you mean a personal parachute, not a pod with a parachute--but the case is still the same.  There are two ways to approach re-entry problems:  you can choose a steep re-entry or a shallow one, and each has its own trade-offs.  A steep re-entry gets you close to ground level quickly, but it also has high shock heating, which is to say that the maximum temperature that you experience is higher.  A shallow re-entry is slower and does not reach those high temperature spikes, but it leaves you in the heat for longer periods of time, which allows that heat to 'soak' into whatever is re-entering.

For practical purposes, the preferred method relates to the thermal tolerance of the part.  If the skin temperature limit is higher than the part temperature limit, then that implies that the part was designed to handle high shock heat--the assumption being that while the skin gets hotter than the main body of the part can tolerate, the re-entry will be completed and the skin can return that heat to the atmosphere before too much soaks into the part and destroys it.  On the other hand, if the skin temperature limit is the same as the part temperature limit, then the part was likely designed for shallower re-entry such that the skin has time to sink heat into that part--though some parts are set to very high temperature limits for both so that you have your choice of how to re-enter.  On the gripping hand, some parts have low values for both skin and part temperature limits:  these are designed not to re-enter at all, so use a shield or some other method to get it to the ground intact.

So far as I recall, Kerbals have temperature limits for both skin and body of 800 K.  That's quite low, so there is definitely a maximum re-entry speed and angle combination for them.  (Though I cannot help but point out that my body does not have a thermal limit anywhere even close to 800 K.)

There is another factor involved relating to heat conduction; some parts have intimate thermal contact with their skins, which means that heat in the skin (from re-entry) is conducted into the body of the part quickly.  Others have low skin conduction:  in a sense, the skin conducts heat so poorly that it can almost be considered a separate part for thermal purposes.  Heat shields are supposed to work this way, though they have a different method to manage the heat (ablation).  I have no idea whether Kerbals are good conductors.

Lastly, there are two other things to consider for purposes of trajectory calculation:  drag and wind speed.  Kerbals falling through air have drag, and although I don't know anything about a Kerbal body's flight characteristics, I can say that all Kerbals are treated equally by the physics.  Whether that means that orientation matters or that Kerbals experience body lift is nothing I can answer, but all EVAKerbals are considered identical parts by the game, so far as I understand.  Wind speed is obviously not a major concern on a planet that has no weather, but I think it important to note that the atmosphere rotates with the planet; otherwise, your rocket would be blown off the pad by 175 m/s winds before you had a chance to launch.  Therefore, you may experience differences in re-entry depending on whether you re-enter with the planet's rotation or against it.  I've never tested to see whether there is a significant difference, but it's something to consider.

[bewing]

In previous versions I found that the point of greatest danger was around 40km altitude. If the kerbal was traveling faster than 1400m/s (surface speed) at that altitude, they would die. To get down to that speed from any Ap, I needed to retroburn every last drop of the kerbal's RCS jetpack fuel. Attempting to rely on atmospheric drag to slow my kerbals always resulted in their deaths, reentering from any Ap to any Pe.

I haven't tried again in the current version, but it sounds like things must have changed.

 

[Kromlech444]

1 hour ago, Zhetaan said:

You have my apologies for this response, but ...

... It depends.

Your trajectory has a lot to do with it.  I assume you mean a personal parachute, not a pod with a parachute--but the case is still the same.  There are two ways to approach re-entry problems:  you can choose a steep re-entry or a shallow one, and each has its own trade-offs.  A steep re-entry gets you close to ground level quickly, but it also has high shock heating, which is to say that the maximum temperature that you experience is higher.  A shallow re-entry is slower and does not reach those high temperature spikes, but it leaves you in the heat for longer periods of time, which allows that heat to 'soak' into whatever is re-entering.

For practical purposes, the preferred method relates to the thermal tolerance of the part.  If the skin temperature limit is higher than the part temperature limit, then that implies that the part was designed to handle high shock heat--the assumption being that while the skin gets hotter than the main body of the part can tolerate, the re-entry will be completed and the skin can return that heat to the atmosphere before too much soaks into the part and destroys it.  On the other hand, if the skin temperature limit is the same as the part temperature limit, then the part was likely designed for shallower re-entry such that the skin has time to sink heat into that part--though some parts are set to very high temperature limits for both so that you have your choice of how to re-enter.  On the gripping hand, some parts have low values for both skin and part temperature limits:  these are designed not to re-enter at all, so use a shield or some other method to get it to the ground intact.

So far as I recall, Kerbals have temperature limits for both skin and body of 800 K.  That's quite low, so there is definitely a maximum re-entry speed and angle combination for them.  (Though I cannot help but point out that my body does not have a thermal limit anywhere even close to 800 K.)

There is another factor involved relating to heat conduction; some parts have intimate thermal contact with their skins, which means that heat in the skin (from re-entry) is conducted into the body of the part quickly.  Others have low skin conduction:  in a sense, the skin conducts heat so poorly that it can almost be considered a separate part for thermal purposes.  Heat shields are supposed to work this way, though they have a different method to manage the heat (ablation).  I have no idea whether Kerbals are good conductors.

Lastly, there are two other things to consider for purposes of trajectory calculation:  drag and wind speed.  Kerbals falling through air have drag, and although I don't know anything about a Kerbal body's flight characteristics, I can say that all Kerbals are treated equally by the physics.  Whether that means that orientation matters or that Kerbals experience body lift is nothing I can answer, but all EVAKerbals are considered identical parts by the game, so far as I understand.  Wind speed is obviously not a major concern on a planet that has no weather, but I think it important to note that the atmosphere rotates with the planet; otherwise, your rocket would be blown off the pad by 175 m/s winds before you had a chance to launch.  Therefore, you may experience differences in re-entry depending on whether you re-enter with the planet's rotation or against it.  I've never tested to see whether there is a significant difference, but it's something to consider.

So basically, as long as you have a shallow enough descent, you could skydive from eloo and survive?

[Zhetaan]

1 hour ago, Kromlech444 said:

So basically, as long as you have a shallow enough descent, you could skydive from eloo and survive?

Not quite.  Orbital speed is a major factor because that kinetic energy inherent in your velocity has to be shed as heat--that's what re-entry heating is.  You collide into the atmosphere and compress it, and that compression heats the air in the energy transfer from your moving space vehicle to moving molecules of air.  The more velocity you have, the more compression, and the more heating--entry angle doesn't change that.

The steepness of the re-entry profile matters because the atmosphere is not of uniform density, so by choosing the correct path of travel through the density gradient as you progress to thicker and thicker air, you can alter the distribution of the heat load to best suit the actual construction (in the sense of the mechanical and thermal properties) of the vessel:  shallower angles trade high temperatures for long soak times, and steeper angles do the opposite.  In a sense, it's an inversion of the gravity turn problem:  as the right choice of ascent paths minimises drag and gravity losses on your rocket as you reach orbit, but that right choice still nevertheless depends on the actual construction of that rocket, so too the right choice of descent paths minimises the types of heating that your rocket is least capable of dissipating safely, but that right choice also nevertheless depends on the actual construction of that rocket.  'Actual construction' means two things:  first, it refers to the design, but second, it refers to the limits of the materials.  To wit, a rocket with 100 thrust-to-weight on the pad flying horizontally at 1000 metres is going to tear itself apart in the atmosphere.  Theoretically, there is an ideal gravity turn for such a rocket, but it is practically useless because there's no real material that can handle that sort of stress.  In the same way, there probably is an ideal minimum-heat re-entry profile for a vessel returning directly from Eeloo, but it's still useless because whatever that minimum is is still too high for a vessel with no heat shield.

In other words, there is definitely a maximum survivable altitude for orbital skydiving, but it's an optimisation problem with variables that I do not fully understand.  There was a challenge here a few years ago called The Goddard Problem that dealt with optimising the altitude of a provided rocket by using throttle and attitude to minimise drag losses.  This, I think, is similar:  we want to optimise the return altitude of a provided part (the Kerbal) by using entry angle to minimise destructive heating.

[asherha]

I jumped from 11,000,000m. I used Bill as a shield and Jeb was the one flying behind him. That is so entry burn does not kill jeb.