Fastest Human Reentry Speeds Possible

[Aeneas]

Hi all,

do you know or can estimate what the fastest possible human reentry would be? Both, survivable for humans and in regard to technology?

For example an early return from Mars that intercepts with Earth with, let's say, 16 km/s. Is that possible without dying or do we have the material?

Thanks!

[laszlo]

There are so many possible variables that your question is too open-ended for a real answer. For example, are you looking for a purely aerobraking re-entry? Or is propulsive deceleration allowed? Or should it be a combination? Is it a simple capsule or a true spacecraft? You need to refine the conditions a bit.

The Galileo probe that ballistically entered Jupiter's atmosphere went in at 48 km/s and sustained 15,500 °C and 228 g so non-living systems can handle at least that. From a mechanical point of view, your 16 km/s is practical with 1990's technology.

For people, the speed and heat is not the main problem, it's the deceleration. The g forces need to be kept low enough so that people aren't damaged. If the spacecraft can reduce the deceleration rate by multiple skips in and out of the atmosphere, propulsive deceleration, force-absorbing support structures, etc. there's probably no theoretical limit other than the speed of light. But for an actual meaningful answer you need to set bounds for the problem.

[edzieba]

Ignoring material limitations, and assuming a requirement of purely aerodynamic braking into at least a stable orbit (i.e. no skip-off-the-atmosphere-and-then-circle-the-sun-to-brake-a-little-more-next-year sillyness), the limit would be the prolonged-duration g limit of the passengers combined with the maximum braking distance, which would be a combination of linear distance through the atmosphere without intersecting the surface and a curved negative-lifting path wrapping around the planet (with the path curvature contributing its own additional vector to the combined g load). The combination of going low enough to keep within the atmosphere with negative lift but high enough to avoid over-g gives you your longest possible braking path.

[Aeneas]

There are so many possible variables that your question is too open-ended for a real answer. For example, are you looking for a purely aerobraking re-entry? Or is propulsive deceleration allowed? Or should it be a combination? Is it a simple capsule or a true spacecraft? You need to refine the conditions a bit.

The Galileo probe that ballistically entered Jupiter's atmosphere went in at 48 km/s and sustained 15,500 °C and 228 g so non-living systems can handle at least that. From a mechanical point of view, your 16 km/s is practical with 1990's technology.

For people, the speed and heat is not the main problem, it's the deceleration. The g forces need to be kept low enough so that people aren't damaged. If the spacecraft can reduce the deceleration rate by multiple skips in and out of the atmosphere, propulsive deceleration, force-absorbing support structures, etc. there's probably no theoretical limit other than the speed of light. But for an actual meaningful answer you need to set bounds for the problem.

Ah, sorry, purely aerobraking is the assumption. And I would see a capsule in form comparable to Apollo/Orion and in size maybe 6 m at the base to comfortably house 6 people. An approach to aerocapture first (like 16 down to 9 or 10 km/s) into an elliptical orbit then reenter the capsule. If a spaceship like the Starship works better (with aerocapture first), so be it. Upper limit on the body is probably at 7 or 8 g, the less, the better. CMs from the Moon hat a peak at 6.5 as far as I know.

[gparker]

Upper limit on the body is probably at 7 or 8 g, the less, the better. CMs from the Moon hat a peak at 6.5 as far as I know.

NASA design limits are shown below (from NASA-STD-3001 volume 2). A nominal re-entry longer than about two minutes should be no more than 4 g. Emergency situations can push harder: 9 g for almost two minutes, for example.

Survivable limits will be above this. As described in the text, NASA wants conservative limits that will leave the astronauts conscious and able to manipulate controls if necessary.

Another data point is Soyuz ballistic descent. That can peak over 8 g, although I don't know its duration. (Presumably less than six minutes because that's the duration of the Soyuz non-ballistic 4 g descent.)

[Robotbeat]

There are so many possible variables that your question is too open-ended for a real answer. For example, are you looking for a purely aerobraking re-entry? Or is propulsive deceleration allowed? Or should it be a combination? Is it a simple capsule or a true spacecraft? You need to refine the conditions a bit.

The Galileo probe that ballistically entered Jupiter's atmosphere went in at 48 km/s and sustained 15,500 °C and 228 g so non-living systems can handle at least that. From a mechanical point of view, your 16 km/s is practical with 1990's technology.

For people, the speed and heat is not the main problem, it's the deceleration. The g forces need to be kept low enough so that people aren't damaged. If the spacecraft can reduce the deceleration rate by multiple skips in and out of the atmosphere, propulsive deceleration, force-absorbing support structures, etc. there's probably no theoretical limit other than the speed of light. But for an actual meaningful answer you need to set bounds for the problem.

Ah, sorry, purely aerobraking is the assumption. And I would see a capsule in form comparable to Apollo/Orion and in size maybe 6 m at the base to comfortably house 6 people. An approach to aerocapture first (like 16 down to 9 or 10 km/s) into an elliptical orbit then reenter the capsule. If a spaceship like the Starship works better (with aerocapture first), so be it. Upper limit on the body is probably at 7 or 8 g, the less, the better. CMs from the Moon hat a peak at 6.5 as far as I know.

The fastest limits are achievable using the highest hypersonic lift to drag ratio. Lifting upside down.

You can do it in multiple passes provided your last first pass gets you below escape velocity.

Assuming you have as much hypersonic lift to drag as you want (infinite), you can essentially glide upside down in a circle around the Earth at like 50km altitude. 4gee sustained is your limit, but you get an extra gee thanks to gravity. Acceleration in a circle is v^2/radius, so solving the equation for a =50m/s^2 and r as earth’s radius gets you about 18km/s.

There are other strategies to get even higher, since you can slow down on approach at a shallower curve. That might get you to maybe 25km/s? Maybe slightly higher like 30km/s since you just need to be down to 11km/s escape velocity? I dunno.

Also, you can do better than 4 gees by using special g suits, restraints, or being surrounded by fluid (but still breathing air). That gets you to maybe 24gees. So maybe 40-60km/s?

Actually breathing liquid (some sort of oxygenated fluid?) gets you to 100gees and maybe more, so about 80-120km/s.

[Robotbeat]

The sun is 10 times the radius of Jupiter which is 10 times the radius of the earth...

So with liquid breathing humans have an aerobraking limit of about 100 km/s for earth 1000 km/s for Jupiter and 10,000 km/s for the sun. That’s about 3% the speed of light

[rpapo]

You can do it in multiple passes provided your last pass gets you below escape velocity.

Don't you mean first pass?  Because if you don't get below escape velocity in the first pass, you'll never come back.  Without that silly option of a second pass in a year or so, that is.

[Robotbeat]

You can do it in multiple passes provided your last pass gets you below escape velocity.

Don't you mean first pass?  Because if you don't get below escape velocity in the first pass, you'll never come back.  Without that silly option of a second pass in a year or so, that is.

LOL, oops, yeah, you’re right.

[Aeneas]

There are so many possible variables that your question is too open-ended for a real answer. For example, are you looking for a purely aerobraking re-entry? Or is propulsive deceleration allowed? Or should it be a combination? Is it a simple capsule or a true spacecraft? You need to refine the conditions a bit.

The Galileo probe that ballistically entered Jupiter's atmosphere went in at 48 km/s and sustained 15,500 °C and 228 g so non-living systems can handle at least that. From a mechanical point of view, your 16 km/s is practical with 1990's technology.

For people, the speed and heat is not the main problem, it's the deceleration. The g forces need to be kept low enough so that people aren't damaged. If the spacecraft can reduce the deceleration rate by multiple skips in and out of the atmosphere, propulsive deceleration, force-absorbing support structures, etc. there's probably no theoretical limit other than the speed of light. But for an actual meaningful answer you need to set bounds for the problem.

Ah, sorry, purely aerobraking is the assumption. And I would see a capsule in form comparable to Apollo/Orion and in size maybe 6 m at the base to comfortably house 6 people. An approach to aerocapture first (like 16 down to 9 or 10 km/s) into an elliptical orbit then reenter the capsule. If a spaceship like the Starship works better (with aerocapture first), so be it. Upper limit on the body is probably at 7 or 8 g, the less, the better. CMs from the Moon hat a peak at 6.5 as far as I know.

The fastest limits are achievable using the highest hypersonic lift to drag ratio. Lifting upside down.

You can do it in multiple passes provided your last first pass gets you below escape velocity.

Assuming you have as much hypersonic lift to drag as you want (infinite), you can essentially glide upside down in a circle around the Earth at like 50km altitude. 4gee sustained is your limit, but you get an extra gee thanks to gravity. Acceleration in a circle is v^2/radius, so solving the equation for a =50m/s^2 and r as earth’s radius gets you about 18km/s.

There are other strategies to get even higher, since you can slow down on approach at a shallower curve. That might get you to maybe 25km/s? Maybe slightly higher like 30km/s since you just need to be down to 11km/s escape velocity? I dunno.

Also, you can do better than 4 gees by using special g suits, restraints, or being surrounded by fluid (but still breathing air). That gets you to maybe 24gees. So maybe 40-60km/s?

Actually breathing liquid (some sort of oxygenated fluid?) gets you to 100gees and maybe more, so about 80-120km/s.

Ok, given we'd stay in the 12 - 16 km/s range for just a little bit earlier return form Mars, this might be possible. I wouldn't go that far to let the astronauts drink oxygenated fluid...

[cdebuhr]

The sun is 10 times the radius of Jupiter which is 10 times the radius of the earth...

So with liquid breathing humans have an aerobraking limit of about 100 km/s for earth 1000 km/s for Jupiter and 10,000 km/s for the sun. That’s about 3% the speed of light

Impressive ... although if you're "aerobraking" on the sun, I'd thing max g-loading would be the least of your troubles!

[Robotbeat]

The sun is 10 times the radius of Jupiter which is 10 times the radius of the earth...

So with liquid breathing humans have an aerobraking limit of about 100 km/s for earth 1000 km/s for Jupiter and 10,000 km/s for the sun. That’s about 3% the speed of light

Impressive ... although if you're "aerobraking" on the sun, I'd thing max g-loading would be the least of your troubles!

Well... with a big enough ship, the Surface-to-volume ratio thing saves you from radiation and heatflux...

[Robotbeat]

There are so many possible variables that your question is too open-ended for a real answer. For example, are you looking for a purely aerobraking re-entry? Or is propulsive deceleration allowed? Or should it be a combination? Is it a simple capsule or a true spacecraft? You need to refine the conditions a bit.

The Galileo probe that ballistically entered Jupiter's atmosphere went in at 48 km/s and sustained 15,500 °C and 228 g so non-living systems can handle at least that. From a mechanical point of view, your 16 km/s is practical with 1990's technology.

For people, the speed and heat is not the main problem, it's the deceleration. The g forces need to be kept low enough so that people aren't damaged. If the spacecraft can reduce the deceleration rate by multiple skips in and out of the atmosphere, propulsive deceleration, force-absorbing support structures, etc. there's probably no theoretical limit other than the speed of light. But for an actual meaningful answer you need to set bounds for the problem.

Ah, sorry, purely aerobraking is the assumption. And I would see a capsule in form comparable to Apollo/Orion and in size maybe 6 m at the base to comfortably house 6 people. An approach to aerocapture first (like 16 down to 9 or 10 km/s) into an elliptical orbit then reenter the capsule. If a spaceship like the Starship works better (with aerocapture first), so be it. Upper limit on the body is probably at 7 or 8 g, the less, the better. CMs from the Moon hat a peak at 6.5 as far as I know.

The fastest limits are achievable using the highest hypersonic lift to drag ratio. Lifting upside down.

You can do it in multiple passes provided your last first pass gets you below escape velocity.

Assuming you have as much hypersonic lift to drag as you want (infinite), you can essentially glide upside down in a circle around the Earth at like 50km altitude. 4gee sustained is your limit, but you get an extra gee thanks to gravity. Acceleration in a circle is v^2/radius, so solving the equation for a =50m/s^2 and r as earth’s radius gets you about 18km/s.

There are other strategies to get even higher, since you can slow down on approach at a shallower curve. That might get you to maybe 25km/s? Maybe slightly higher like 30km/s since you just need to be down to 11km/s escape velocity? I dunno.

Also, you can do better than 4 gees by using special g suits, restraints, or being surrounded by fluid (but still breathing air). That gets you to maybe 24gees. So maybe 40-60km/s?

Actually breathing liquid (some sort of oxygenated fluid?) gets you to 100gees and maybe more, so about 80-120km/s.

Ok, given we'd stay in the 12 - 16 km/s range for just a little bit earlier return form Mars, this might be possible. I wouldn't go that far to let the astronauts drink oxygenated fluid...

BTW, while reading about this, apparently they did actual experiments with suspending someone in fluid (but breathing air). The peak of 31gees for 5 seconds (during a 25 second cycle) was achieved by the researcher just holding his breath while in the water capsule: https://history.nasa.gov/SP-4201/ch2-4.htm

Kind of interesting, but you can get a similar result by using nylon netting and proper restraints. So maybe we can do even better than this but without the weight penalty of water. Like a gee-loading-optimized mechanical counter-pressure suit. (but with something different than nylon, which has a springing effect... may need active compensation?). Could allow tolerance beyond 10gees for significant lengths of time.

At this point, we're talking g-loading higher than a typical spacecraft structure could handle. You're potentially more worried about the spacecraft failing than the person.

https://history.nasa.gov/SP-4201/ch2-4.htm


I kind of think we have more space to explore this area in terms of biomedical adaptation. A lot of these experiences are 70 or even 90 years old, and we haven't had the opportunity to try modern techniques.

I'm reminded of the high-gee adaptations and countermeasures in The Expanse. Gotta get some "juice"...

[Johnnyhinbos]

Felix Baumgartner. He wins. Fastest human reentry. Case closed - lock thread. You’re welcome.

[Robotbeat]

Felix Baumgartner. He wins. Fastest human reentry. Case closed - lock thread. You’re welcome.

MUCH slower than orbital.

[chopsticks]

Felix Baumgartner. He wins. Fastest human reentry. Case closed - lock thread. You’re welcome.

I just have to comment that Felix doesn't deserve this title - it should be Alan Eustace - he broke Felix's record without all of the media attention. Poor guy doesn't get any recognition so I had to correct you here.

Sorry for the OT.

Sent from my Redmi Note 7 using Tapatalk

[Johnnyhinbos]

Felix Baumgartner. He wins. Fastest human reentry. Case closed - lock thread. You’re welcome.

I just have to comment that Felix doesn't deserve this title - it should be Alan Eustace - he broke Felix's record without all of the media attention. Poor guy doesn't get any recognition so I had to correct you here.

Sorry for the OT.

Sent from my Redmi Note 7 using Tapatalk

You’re quite right - I knew about his achievement and then forgot! Credit to him.

Okay, back on topic - fastest human speed INSIDE a spacecraft. Definitely less exciting tho... :-)

[edzieba]

Okay, back on topic - fastest human speed INSIDE a spacecraft. Definitely less exciting tho... :-)

I'll add the obligatory comment mentioning MOOSE and Paracone. Not quite inside a spacecraft, but plenty exiting!

[chopsticks]

This idea of breathing liquid oxygen: is this a serious thing people are considering? Has anyone ever "breathed a liquid"? That in and of itself sounds pretty weird to me, and how does it make higher G loads less problematic? Is it because the lungs won't collapse if full of liquid? My apologies if I'm a bit dense but this sounds kind of nutty to me (I don't mean that in a bad way).

[webdan]

Still a bit off topic, but in the British TV show "U.F.O.", the aliens were breathing a liquid in their suits. Also, "The Abyss" comes to mind.

There was research on mice in a liquid: