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:

[edzieba]

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).

Not breathing LOX (that would likely be fatal) but breathing an fluid, usually a perfluorocarbon, that readily dissolved Oxygen and CO2.

[laszlo]

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"...

From Wikipedia article on Colonel John Stapp, USAF flight surgeon:

By riding the decelerator sled, in his 29th and last ride at Holloman Air Force Base in New Mexico, Stapp demonstrated that a human can withstand at least 46.2 g (in the forward position, with adequate harnessing). This is the highest known acceleration voluntarily encountered by a human, set on December 10, 1954.[7][8]

[WTF]

From Wikipedia article on Colonel John Stapp, USAF flight surgeon:

By riding the decelerator sled, in his 29th and last ride at Holloman Air Force Base in New Mexico, Stapp demonstrated that a human can withstand at least 46.2 g (in the forward position, with adequate harnessing). This is the . known acceleration voluntarily encountered by a human, set on December 10, 1954.[7][8]
[/quote]

'This is going to hurt'
Reportedly overheard as he prepared for that last rocket sled run.

Eyecandy?
GOOGLE: Rocket Sled, Sonic Wind I

An uncle. When visiting me at the maternity ward, my mother stated he told me to wear my seatbelt.

[chopsticks]

Not breathing LOX (that would likely be fatal) but breathing an fluid, usually a perfluorocarbon, that readily dissolved Oxygen and CO2.

Yes, I wasn't thinking of breathing the liquid oxygen that's used as rocket propellant!

Even with a breathable liquid, how does that help with surviving higher G forces?

[Asteroza]

Not breathing LOX (that would likely be fatal) but breathing an fluid, usually a perfluorocarbon, that readily dissolved Oxygen and CO2.

Yes, I wasn't thinking of breathing the liquid oxygen that's used as rocket propellant!

Even with a breathable liquid, how does that help with surviving higher G forces?

It's not just keeping the lungs from collapsing, it provides some physical pressure to resist crushing of the rib cage as a liquid is incompressible.

[Pete]

Not breathing LOX (that would likely be fatal) but breathing an fluid, usually a perfluorocarbon, that readily dissolved Oxygen and CO2.

Yes, I wasn't thinking of breathing the liquid oxygen that's used as rocket propellant!

Even with a breathable liquid, how does that help with surviving higher G forces?

Your goal is, as far as possible, to make the human body neutrally buoyant. Both in total and in parts!
So you don't have extra heavy bits that try to sink to the bottom, and extra light bits that try to rise to the top.
And so the whole is gently floating in fluid, rather than being supported on a concrete slab under you, flattening you out to straberry-jam-filled-pancake.

Now obviously we cannot do much about the denser bones being dense. But they are reasonably strong, and the weight is spread so bruising under the bones is not do bad.
But the lungs present a HUGE air void right in the middle of your body, that under heavy gravity wants to float up through your ribs and out. By filling them with something that is the same density as the rest of your body, you prevent this problem. Also, a fluid-filled interior will better resist compression from external forces. Provide interior support. Really, filling you lungs with liquid is *great* for you, aside from that slight issue of, well, breathing.

A grape, on a nice soft sponge bed, "dies" under less than 10g in a centrifuge.
The same grape, in a saline bath, is fine up to 120g, and then only breaks when the seeds float out through the skin.
Strange, i really expected them to sink, but apparently live grape seed are less dense than grape pulp.

[darkenfast]

Reminds me of the COPV tearing loose and shooting up through the LOX tanks on the failed SpaceX CRS mission. Once it is explained, it's obvious, but at first it sounds odd. I wonder what other little surprises would await a liquid-breathing astronaut trying a very high-g maneuver?

[Hog]

Not breathing LOX (that would likely be fatal) but breathing an fluid, usually a perfluorocarbon, that readily dissolved Oxygen and CO2.

Yes, I wasn't thinking of breathing the liquid oxygen that's used as rocket propellant!

Even with a breathable liquid, how does that help with surviving higher G forces?

It's not just keeping the lungs from collapsing, it provides some physical pressure to resist crushing of the rib cage as a liquid is incompressible.

But would add tremendous mass to delicate system that has evolved dealing with gases..  Perhaps more mass coupled with accelerations could possibly cause more issues inside the actual lung structures.
 
Incompressible fluids "could" be detrimental as they transmit forces rather than dissipate them. This is the basis of hydraulics.  If you are relying on the contents of your lungs to resist the crushing of you chest cavity, you've got very big issues at hand.

Breathing liquids can be accomplished.  It's having the human survive after switching back to air.  Respiratory Acidosis and I'd guess the stripping of the surfactants of the lungs could be problematic.

Very interesting topic.

[Genial Precis]

Incompressible fluids "could" be detrimental as they transmit forces rather than dissipate them. This is the basis of hydraulics.  If you are relying on the contents of your lungs to resist the crushing of you chest cavity, you've got very big issues at hand.

Breathing liquids can be accomplished.  It's having the human survive after switching back to air.  Respiratory Acidosis and I'd guess the stripping of the surfactants of the lungs could be problematic.

So, transmitting forces is actually what you want. A contained column of water doesn't rearrange itself. If you add material of different density, such as a bubble, it tries to float. Filling the lungs with a density-matched liquid is a win for acceleration tolerance.

I don't believe existing liquid breathing candidates are density matched. So they are maybe of interest for diving, but not for acceleration tolerance.

ISTM that it would be more realistic to use an artificial o2-co2 exchange rig hooked into the person's blood. That is an existing medical device. Then the requirement is just for the lungs to tolerate whatever they're filled with. And to get enough blood through the device, presumably.
Edit: Ah, the wikipedia state of the art is that you need to administer lots of anti-clotting agents (heparin) the whole time, and that's quite dangerous.

Medical interest seems to be in partially filling lungs with perfluorocarbon fluid to improve gas exchange, somehow, either in premature infants or adults with acute respiratory distress syndrome. Partial liquid ventilation doesn't kill adults with ARDS more effectively than the ARDS, at least, but survival was no different from control. On the other hand partial liquid ventilation isn't relevant here.

The surfactant stripping thing apparently isn't a totally unsolvable problem, because people do sometimes survive partial liquid ventilation.

Edit: This approach with micro/nano capsules full of high pressure oxygen suspended in saline seems more promising than perfluorocarbons:
https://link.springer.com/article/10.1007/s10047-015-0835-z
https://www.nature.com/articles/s41598-020-73768-9

[Robotbeat]

We are born with fluid in our lungs.

It’ll be like being born again.

[Paul451]

We are born with fluid in our lungs.
It’ll be like being born again.

With blood and screaming.

[Genial Precis]

We are born with fluid in our lungs.

It’ll be like being born again.

I mean, that's kind of prescriptive. A fetus gets everything by blood, right? So that's at least one place to look for what you could do to:
1) Ensure tolerance of fluid in the lungs without needing them for gas exchange
2) Exchange gases by extracorporeal exchange with blood without pumping the recipient full of heparin

Biology is hard, so the amount that that prescription solves on its own is nil, but it's a place to look.

[Robotbeat]

We are born with fluid in our lungs.
It’ll be like being born again.

With blood and screaming.

No one said it’d be a picnic.