Author Topic: Starship heat shield  (Read 1186477 times)

Offline InterestedEngineer

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Re: Starship heat shield
« Reply #3440 on: 05/16/2023 03:46 pm »
You are posting your supposition about the law as if it is fact. The clear wording of the law does not say it applies to failed launches which never reach space.
The clear working of the law is in fact clear, Article I Parts (b) and (d):
Quote
(b) The term "launching" includes attempted launching;
[...]
(d) The term "space object" includes component parts of
a object as well as its launch vehicle and parts threof.

I asked this question in the licenses thread, where it is more on topic than here.

Please take the legal discussions over there:

https://forum.nasaspaceflight.com/index.php?topic=58853.msg2487242#msg2487242

Online Robotbeat

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Re: Starship heat shield
« Reply #3441 on: 05/16/2023 03:52 pm »
You are posting your supposition about the law as if it is fact. The clear wording of the law does not say it applies to failed launches which never reach space.
The clear working of the law is in fact clear, Article I Parts (b) and (d):
Quote
(b) The term "launching" includes attempted launching;
[...]
(d) The term "space object" includes component parts of
a object as well as its launch vehicle and parts threof.
I stand corrected! Thank you.
« Last Edit: 05/16/2023 03:52 pm by Robotbeat »
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline FutureSpaceTourist

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Re: Starship heat shield
« Reply #3442 on: 05/22/2023 06:24 am »
Elon liked this tweet:

https://twitter.com/iniallanderson/status/1660391871900729345

Quote
Awesome video here of someone torching a piece of Starship Heatshield Tile! Look how quickly that heat is dissipated! 🔥
📸: @ Keith Yates | facebook.com/groups/1541938

Offline TheRadicalModerate

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Re: Starship heat shield
« Reply #3443 on: 06/27/2023 11:06 pm »
I think for thermal you have 2 primary constraints, not 1:

1. Heat pulse, i.e. the one you mentioned. This is the amount of energy which gets to your ship.
2. Peak heating. This is the maximum heating rate. Among other things this dictates skin temperature.

Ablative heat shields are primarily concerned with the first (1). But non-ablative ones are concerned with both (1 & 2), and it's easier to adjust them for 1 if a need arises (generally you improve the insulation layer). But adjusting for a higher peak heating (2) essentially means coming up with a different material which is not an easy change, and often means redoing your heatshield from scratch. And If you ramp peak heating just a tad too much we're simply out of known materials at all.

The thing about (2) is that it depends not just on velocity. It depends on deceleration as well. If you replace 2g braking with 4g braking you essentially doubled your peak heating.

At the same time you have likely reduced(!) heat pulse! This is counterintuitive, because kinetic energy to lose is the same regardless of how quickly you drop it. But the thing is, at certain point the more rarefied the ambient atmosphere the larger fraction of heat gets to the vehicle (and less is taken away by the ambient air). So when you pass entry interface heating initially grows much faster than deceleration. Just look at Shuttle reentry heating and deceleration profiles. Or some (SpaceX?) materials about Starship reentry. You're dropping speed slowly while heating is already high.

So if you have an ablative heatshield, you want to punch through the too thin air fast into high deceleration region. Total heat pulse is less that way, so you need less material to ablate. And you don't care that much about peak heating (within limits), as the temperature is primarily set by your ablative material's phase changes.

But if you have non-ablative heatshield, you prefer skidding on top of the atmosphere. The heat pulse is larger (often significantly), but you keep peak heating in check by keeping the whole kinetic energy drop rate in check. But if you have to generate more (negative) lift to hold onto a small low planet, you need to increase drag according to L:D ratio of your vehicle, which means higher deceleration which means higher kinetic energy drop rate and higher heating.

First, I never thanked you for this post.  It's a very clear problem statement, and I refer back to it quite often.

So how do things change if Starship intends to aerocapture instead of doing a direct EDL?  (This question applies for both Mars and Earth entry.)

I can think of a few of things:

1) You don't have to kill all the speed in a single direct EDL.  You only need enough to get you captured into an eccentric orbit.  That presumably makes the path through the atmosphere shorter, which reduces the heat pulse.

2) The Starship can cold-soak or barbecue roll to radiate away the heat pulse before beginning the actual EDL.

3) You also obviously get a chance to null out any guidance irregularities, allowing changes to both periapse altitude and inclination post-aerocapture.

4) The radius of curvature of the path requiring negative lift can be substantially greater, resulting in lower lift, which in turn reduces the amount of drag that's required.

It's #4 that I'm most curious about.  (See attachment.)  The deeper you can dip into the atmosphere before pulling out, the larger the radius of curvature of the path where substantial drag is produced.  But that means that the Starship has to endure a very brief period of high drag, and therefore high peak heating.  You obviously can't exceed a dynamic pressure that causes so high a peak heating that the tiles fail, but they're not going to soak at that temperature for very long.  This is the kinda-in-between case between pure peak heating and total heat pulse.

Is there a pony in here somewhere?  As always, the name of the game is the highest tolerable arrival v∞.

Offline Asteroza

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Re: Starship heat shield
« Reply #3444 on: 06/28/2023 12:33 am »
Heat pulse cooling post aerocapture is still non-trivial. You gotta deploy radiators fairly quickly once you pull out of the reentry dive, plus do a burn at apogee to keep you out of atmosphere for the period necessary to get cooling into a stable state in your elliptical loiter orbit until you decide to come down for another atmosphere run or EDL.

There's also the weird trade space of skip glide reetries post-aerocapture, where the heat pulse problem magnifies but you may be able to reduce peak heating.

Offline livingjw

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Re: Starship heat shield
« Reply #3445 on: 06/28/2023 06:54 pm »
Heat pulse cooling post aerocapture is still non-trivial. You gotta deploy radiators fairly quickly once you pull out of the reentry dive, plus do a burn at apogee to keep you out of atmosphere for the period necessary to get cooling into a stable state in your elliptical loiter orbit until you decide to come down for another atmosphere run or EDL.

There's also the weird trade space of skip glide reetries post-aerocapture, where the heat pulse problem magnifies but you may be able to reduce peak heating.

Why would you need to deploy radiators? Post-aerocapture the heat will radiate away from the tiles and also into the tanks. Tanks would be vented to control pressure. I don't see how radiators would help.

John

Offline sebk

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Re: Starship heat shield
« Reply #3446 on: 06/28/2023 07:23 pm »
I think for thermal you have 2 primary constraints, not 1:

1. Heat pulse, i.e. the one you mentioned. This is the amount of energy which gets to your ship.
2. Peak heating. This is the maximum heating rate. Among other things this dictates skin temperature.

Ablative heat shields are primarily concerned with the first (1). But non-ablative ones are concerned with both (1 & 2), and it's easier to adjust them for 1 if a need arises (generally you improve the insulation layer). But adjusting for a higher peak heating (2) essentially means coming up with a different material which is not an easy change, and often means redoing your heatshield from scratch. And If you ramp peak heating just a tad too much we're simply out of known materials at all.

The thing about (2) is that it depends not just on velocity. It depends on deceleration as well. If you replace 2g braking with 4g braking you essentially doubled your peak heating.

At the same time you have likely reduced(!) heat pulse! This is counterintuitive, because kinetic energy to lose is the same regardless of how quickly you drop it. But the thing is, at certain point the more rarefied the ambient atmosphere the larger fraction of heat gets to the vehicle (and less is taken away by the ambient air). So when you pass entry interface heating initially grows much faster than deceleration. Just look at Shuttle reentry heating and deceleration profiles. Or some (SpaceX?) materials about Starship reentry. You're dropping speed slowly while heating is already high.

So if you have an ablative heatshield, you want to punch through the too thin air fast into high deceleration region. Total heat pulse is less that way, so you need less material to ablate. And you don't care that much about peak heating (within limits), as the temperature is primarily set by your ablative material's phase changes.

But if you have non-ablative heatshield, you prefer skidding on top of the atmosphere. The heat pulse is larger (often significantly), but you keep peak heating in check by keeping the whole kinetic energy drop rate in check. But if you have to generate more (negative) lift to hold onto a small low planet, you need to increase drag according to L:D ratio of your vehicle, which means higher deceleration which means higher kinetic energy drop rate and higher heating.

First, I never thanked you for this post.  It's a very clear problem statement, and I refer back to it quite often.

So how do things change if Starship intends to aerocapture instead of doing a direct EDL?  (This question applies for both Mars and Earth entry.)

I can think of a few of things:

1) You don't have to kill all the speed in a single direct EDL.  You only need enough to get you captured into an eccentric orbit.  That presumably makes the path through the atmosphere shorter, which reduces the heat pulse.

2) The Starship can cold-soak or barbecue roll to radiate away the heat pulse before beginning the actual EDL.

3) You also obviously get a chance to null out any guidance irregularities, allowing changes to both periapse altitude and inclination post-aerocapture.

4) The radius of curvature of the path requiring negative lift can be substantially greater, resulting in lower lift, which in turn reduces the amount of drag that's required.

It's #4 that I'm most curious about.  (See attachment.)  The deeper you can dip into the atmosphere before pulling out, the larger the radius of curvature of the path where substantial drag is produced.  But that means that the Starship has to endure a very brief period of high drag, and therefore high peak heating.  You obviously can't exceed a dynamic pressure that causes so high a peak heating that the tiles fail, but they're not going to soak at that temperature for very long.  This is the kinda-in-between case between pure peak heating and total heat pulse.

Is there a pony in here somewhere?  As always, the name of the game is the highest tolerable arrival v∞.

Generally braking force (at a fixed velocity) is ~linear with atmospheric density, so over a single scale height the force will change by a factor of e. In effect about 3/4 (more precisely 73.1%) of the whole braking will occur in a 1 scale height band of the atmosphere, 88% will take 2 scale heights band, and 3 scale heights will cover over 95% of braking.

If your velocity is closeish to parabolic, your ballistic braking path through a single scale height band (i.e. responsible for about 3/4 of braking) will be very roughly 2x the horizon distance from 2x scale height (if I'm not badly mistaken). Plugging in Mars radius (3389.5km) and atmospheric scale height (11.1km) we get ~777km path for ~3/4 of aerocapture ∆v. If speeds are deeper into the hyperbolic territory the path will get shorter, down to about 660km at really high speeds. So say ~730-750km of path staying withing one scale height. This means ~1000km path equivalent for 100% ∆v. But remember that said 660~777km path still has a factor of e difference in density, so the peak deceleration would be √e ≈ 1.6x higher than average over that single scale height band.

So all in all, if you don't produce lift, for Mars your peak deceleration will be about 1.6x the uniform deceleration you'd need to achieve your target ∆v over 1000km path.

NB, on Earth this equivalent ballistic braking path is ~1250km.
« Last Edit: 06/28/2023 07:25 pm by sebk »

Offline TheRadicalModerate

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Re: Starship heat shield
« Reply #3447 on: 06/28/2023 11:53 pm »
Heat pulse cooling post aerocapture is still non-trivial. You gotta deploy radiators fairly quickly once you pull out of the reentry dive, plus do a burn at apogee to keep you out of atmosphere for the period necessary to get cooling into a stable state in your elliptical loiter orbit until you decide to come down for another atmosphere run or EDL.

There's also the weird trade space of skip glide reetries post-aerocapture, where the heat pulse problem magnifies but you may be able to reduce peak heating.

Why would you need to deploy radiators? Post-aerocapture the heat will radiate away from the tiles and also into the tanks. Tanks would be vented to control pressure. I don't see how radiators would help.

John

It does bring up some interesting questions about how to cool the payload bay / garage / crew module.  I guess they could sink that heat into the main tanks as well.

It'd be an interesting calculation to figure out how many tonnes of prop you lose dealing with the heat pulse.  Of course, you'd need to know what the heat pulse was in the first place.
« Last Edit: 06/29/2023 09:02 pm by TheRadicalModerate »

Offline InterestedEngineer

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Re: Starship heat shield
« Reply #3448 on: 06/29/2023 12:55 am »
.

4) The radius of curvature of the path requiring negative lift can be substantially greater, resulting in lower lift, which in turn reduces the amount of drag that's required.

It's #4 that I'm most curious about.  (See attachment.)  The deeper you can dip into the atmosphere before pulling out, the larger the radius of curvature of the path where substantial drag is produced.  But that means that the Starship has to endure a very brief period of high drag, and therefore high peak heating.  You obviously can't exceed a dynamic pressure that causes so high a peak heating that the tiles fail, but they're not going to soak at that temperature for very long.  This is the kinda-in-between case between pure peak heating and total heat pulse.

Is there a pony in here somewhere?  As always, the name of the game is the highest tolerable arrival v∞.

Here's a possible but very small pony:

http://mae-nas.eng.usu.edu/MAE_5420_Web/section3/appendix3.pdf

Rough back of the envelope calculations say there's about 8GJ of heat capacity for the tiles of Starship (LI-2200).

So some of that heat capacity is available for the "deep dip" the heat radiating capability of the tiles can be exceeded for however much heat capacity is left before Bad Stuff Happens.

Let's say there's 4GJ left.   For 300t vehicle running at 7km/sec that's a deltaV of 2m/sec.   

A tiny, tiny little pony.   Might be better off looking at film cooling hot spots with CH4.

Offline Asteroza

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Re: Starship heat shield
« Reply #3449 on: 06/29/2023 01:08 am »
Heat pulse cooling post aerocapture is still non-trivial. You gotta deploy radiators fairly quickly once you pull out of the reentry dive, plus do a burn at apogee to keep you out of atmosphere for the period necessary to get cooling into a stable state in your elliptical loiter orbit until you decide to come down for another atmosphere run or EDL.

There's also the weird trade space of skip glide reetries post-aerocapture, where the heat pulse problem magnifies but you may be able to reduce peak heating.

Why would you need to deploy radiators? Post-aerocapture the heat will radiate away from the tiles and also into the tanks. Tanks would be vented to control pressure. I don't see how radiators would help.

John

Well, I suppose that's a question of how much you're willing to vent. If you need to do an apogee burn anyways and using primary propellant tanks as a temporary heatsink, I suppose you could try to kill two birds with one stone by using gaseous RCS to both relieve pressure and do the burn... not the most efficient method but it is expedient plus uses existing equipment. Hrm, I wonder if you could front end the inputs to RCS with a vortex tube cooler to get some free cooling with your burn...

But the impression I get is you will end up needing to run the cryocooler to deal with the remaining heat to prevent all the propellant going to gas and exceeding tank pressure, plus the Raptors expect liquid propellants so you can't let it all go to gas even if you could keep it in the tank. Well, assuming you don't offload any remaining propellant to some other storage system, since Starship isn't ideal for long term propellant storage anyways. The consensus is Starship will have a cryocooler for interplanetary cruise, but a cooler sized for that is much smaller than one trying to overcome structural heat pulses.

Offline livingjw

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Re: Starship heat shield
« Reply #3450 on: 06/29/2023 04:25 pm »
Heat pulse cooling post aerocapture is still non-trivial. You gotta deploy radiators fairly quickly once you pull out of the reentry dive, plus do a burn at apogee to keep you out of atmosphere for the period necessary to get cooling into a stable state in your elliptical loiter orbit until you decide to come down for another atmosphere run or EDL.

There's also the weird trade space of skip glide reetries post-aerocapture, where the heat pulse problem magnifies but you may be able to reduce peak heating.

Only a small fraction (~5%?) of the heat makes it into the tanks.

Why would you need to deploy radiators? Post-aerocapture the heat will radiate away from the tiles and also into the tanks. Tanks would be vented to control pressure. I don't see how radiators would help.

John

Well, I suppose that's a question of how much you're willing to vent. If you need to do an apogee burn anyways and using primary propellant tanks as a temporary heatsink, I suppose you could try to kill two birds with one stone by using gaseous RCS to both relieve pressure and do the burn... not the most efficient method but it is expedient plus uses existing equipment. Hrm, I wonder if you could front end the inputs to RCS with a vortex tube cooler to get some free cooling with your burn...

But the impression I get is you will end up needing to run the cryocooler to deal with the remaining heat to prevent all the propellant going to gas and exceeding tank pressure, plus the Raptors expect liquid propellants so you can't let it all go to gas even if you could keep it in the tank. Well, assuming you don't offload any remaining propellant to some other storage system, since Starship isn't ideal for long term propellant storage anyways. The consensus is Starship will have a cryocooler for interplanetary cruise, but a cooler sized for that is much smaller than one trying to overcome structural heat pulses.


Only a small fraction (~5%?) of the heat makes it into the tanks.

John

Offline brussell

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Re: Starship heat shield
« Reply #3451 on: 07/14/2023 04:52 pm »
What exactly is the TPS material in Starship?

Apologies in advance since it's probably been discussed to no end already, but this thread is gigantic and I couldn't find it.

Offline rsdavis9

Re: Starship heat shield
« Reply #3452 on: 07/14/2023 10:36 pm »
What exactly is the TPS material in Starship?

Apologies in advance since it's probably been discussed to no end already, but this thread is gigantic and I couldn't find it.

Some kind of fused/sintered silica.
Same as or very similar to the space shuttle tiles.

Light weight and insulating and able to get hot.
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Offline edzieba

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Re: Starship heat shield
« Reply #3453 on: 07/17/2023 05:15 pm »
What exactly is the TPS material in Starship?

Apologies in advance since it's probably been discussed to no end already, but this thread is gigantic and I couldn't find it.
Sintered silica fibre bricks, with an outer coating of borosilicate reaction-cured glass, methyl-trimethoxy silane (MTMS) waterproofing agent [1], with embedded metallic 'track' inserts (metal unknown [2]) used to latch onto the pins for the flat tiles - the curved tiles (e.g. for the nose tip, flap leading edges, etc) that are glued in place using silicone adhesive lack the metal inserts.

[1] This is burnt off on first entry, the rewaterproofing agent SpaceX intend to use after recovery is unknown.
[2] On that note: does anyone who picked up one of the scattered tiles from OFT-1 happen to have access to an XRF gun?

Source is an FDEP report on the tile manufacturing facility in Florida (attached).
« Last Edit: 07/17/2023 05:16 pm by edzieba »

Offline Alberto-Girardi

Re: Starship heat shield
« Reply #3454 on: 07/17/2023 07:02 pm »
I think for thermal you have 2 primary constraints, not 1:

1. Heat pulse, i.e. the one you mentioned. This is the amount of energy which gets to your ship.
2. Peak heating. This is the maximum heating rate. Among other things this dictates skin temperature.

Ablative heat shields are primarily concerned with the first (1). But non-ablative ones are concerned with both (1 & 2), and it's easier to adjust them for 1 if a need arises (generally you improve the insulation layer). But adjusting for a higher peak heating (2) essentially means coming up with a different material which is not an easy change, and often means redoing your heatshield from scratch. And If you ramp peak heating just a tad too much we're simply out of known materials at all.

The thing about (2) is that it depends not just on velocity. It depends on deceleration as well. If you replace 2g braking with 4g braking you essentially doubled your peak heating.

At the same time you have likely reduced(!) heat pulse! This is counterintuitive, because kinetic energy to lose is the same regardless of how quickly you drop it. But the thing is, at certain point the more rarefied the ambient atmosphere the larger fraction of heat gets to the vehicle (and less is taken away by the ambient air). So when you pass entry interface heating initially grows much faster than deceleration. Just look at Shuttle reentry heating and deceleration profiles. Or some (SpaceX?) materials about Starship reentry. You're dropping speed slowly while heating is already high.

So if you have an ablative heatshield, you want to punch through the too thin air fast into high deceleration region. Total heat pulse is less that way, so you need less material to ablate. And you don't care that much about peak heating (within limits), as the temperature is primarily set by your ablative material's phase changes.

But if you have non-ablative heatshield, you prefer skidding on top of the atmosphere. The heat pulse is larger (often significantly), but you keep peak heating in check by keeping the whole kinetic energy drop rate in check. But if you have to generate more (negative) lift to hold onto a small low planet, you need to increase drag according to L:D ratio of your vehicle, which means higher deceleration which means higher kinetic energy drop rate and higher heating.

First, I never thanked you for this post.  It's a very clear problem statement, and I refer back to it quite often.

So how do things change if Starship intends to aerocapture instead of doing a direct EDL?  (This question applies for both Mars and Earth entry.)

I can think of a few of things:

1) You don't have to kill all the speed in a single direct EDL.  You only need enough to get you captured into an eccentric orbit.  That presumably makes the path through the atmosphere shorter, which reduces the heat pulse.

2) The Starship can cold-soak or barbecue roll to radiate away the heat pulse before beginning the actual EDL.

3) You also obviously get a chance to null out any guidance irregularities, allowing changes to both periapse altitude and inclination post-aerocapture.

4) The radius of curvature of the path requiring negative lift can be substantially greater, resulting in lower lift, which in turn reduces the amount of drag that's required.

It's #4 that I'm most curious about.  (See attachment.)  The deeper you can dip into the atmosphere before pulling out, the larger the radius of curvature of the path where substantial drag is produced.  But that means that the Starship has to endure a very brief period of high drag, and therefore high peak heating.  You obviously can't exceed a dynamic pressure that causes so high a peak heating that the tiles fail, but they're not going to soak at that temperature for very long.  This is the kinda-in-between case between pure peak heating and total heat pulse.

Is there a pony in here somewhere?  As always, the name of the game is the highest tolerable arrival v∞.


I wonder if since SS will be the first vehicle heading to mars with a reusable heat shield it will not do a capture burn, but instead only aerobrake.
IIRC all missions to mars have either 1) directly plunged in the atmosphere and EDLed, because they had ablative heat shields   2)did a capture burn aided by aerobraking

This maneuver would save a lot of dV, but the heating pulse and peak heating would need to be carefully studied. Also SS could aerobrake in a very  elliptical orbit, but that orbit might be month long, and so they would probably want to get deeper in the atmosphere to scrub more velocity.
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Offline edzieba

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Re: Starship heat shield
« Reply #3455 on: 07/18/2023 11:24 am »
I think for thermal you have 2 primary constraints, not 1:

1. Heat pulse, i.e. the one you mentioned. This is the amount of energy which gets to your ship.
2. Peak heating. This is the maximum heating rate. Among other things this dictates skin temperature.

Ablative heat shields are primarily concerned with the first (1). But non-ablative ones are concerned with both (1 & 2), and it's easier to adjust them for 1 if a need arises (generally you improve the insulation layer). But adjusting for a higher peak heating (2) essentially means coming up with a different material which is not an easy change, and often means redoing your heatshield from scratch. And If you ramp peak heating just a tad too much we're simply out of known materials at all.

The thing about (2) is that it depends not just on velocity. It depends on deceleration as well. If you replace 2g braking with 4g braking you essentially doubled your peak heating.

At the same time you have likely reduced(!) heat pulse! This is counterintuitive, because kinetic energy to lose is the same regardless of how quickly you drop it. But the thing is, at certain point the more rarefied the ambient atmosphere the larger fraction of heat gets to the vehicle (and less is taken away by the ambient air). So when you pass entry interface heating initially grows much faster than deceleration. Just look at Shuttle reentry heating and deceleration profiles. Or some (SpaceX?) materials about Starship reentry. You're dropping speed slowly while heating is already high.

So if you have an ablative heatshield, you want to punch through the too thin air fast into high deceleration region. Total heat pulse is less that way, so you need less material to ablate. And you don't care that much about peak heating (within limits), as the temperature is primarily set by your ablative material's phase changes.

But if you have non-ablative heatshield, you prefer skidding on top of the atmosphere. The heat pulse is larger (often significantly), but you keep peak heating in check by keeping the whole kinetic energy drop rate in check. But if you have to generate more (negative) lift to hold onto a small low planet, you need to increase drag according to L:D ratio of your vehicle, which means higher deceleration which means higher kinetic energy drop rate and higher heating.

First, I never thanked you for this post.  It's a very clear problem statement, and I refer back to it quite often.

So how do things change if Starship intends to aerocapture instead of doing a direct EDL?  (This question applies for both Mars and Earth entry.)

I can think of a few of things:

1) You don't have to kill all the speed in a single direct EDL.  You only need enough to get you captured into an eccentric orbit.  That presumably makes the path through the atmosphere shorter, which reduces the heat pulse.

2) The Starship can cold-soak or barbecue roll to radiate away the heat pulse before beginning the actual EDL.

3) You also obviously get a chance to null out any guidance irregularities, allowing changes to both periapse altitude and inclination post-aerocapture.

4) The radius of curvature of the path requiring negative lift can be substantially greater, resulting in lower lift, which in turn reduces the amount of drag that's required.

It's #4 that I'm most curious about.  (See attachment.)  The deeper you can dip into the atmosphere before pulling out, the larger the radius of curvature of the path where substantial drag is produced.  But that means that the Starship has to endure a very brief period of high drag, and therefore high peak heating.  You obviously can't exceed a dynamic pressure that causes so high a peak heating that the tiles fail, but they're not going to soak at that temperature for very long.  This is the kinda-in-between case between pure peak heating and total heat pulse.

Is there a pony in here somewhere?  As always, the name of the game is the highest tolerable arrival v∞.


I wonder if since SS will be the first vehicle heading to mars with a reusable heat shield it will not do a capture burn, but instead only aerobrake.
IIRC all missions to mars have either 1) directly plunged in the atmosphere and EDLed, because they had ablative heat shields   2)did a capture burn aided by aerobraking

This maneuver would save a lot of dV, but the heating pulse and peak heating would need to be carefully studied. Also SS could aerobrake in a very  elliptical orbit, but that orbit might be month long, and so they would probably want to get deeper in the atmosphere to scrub more velocity.
That technique is known as 'aerocapture', and the major barrier is knowing atmosphere state at time of entry in order to properly plot a trajectory to scrub just enough velocity to enter the target orbit (too much = early entry at an uncontrolled location on an undesired trajectory, too little  = escape). Proposals to solve that problem include a swathe of probes sent ahead by a few hours to map multiple runs through the atmosphere to map density & temperature, and pre-placing Mars weather satellites to continuously measure the atmosphere in order to build and regularly update a volumetric atmospheric model.

Offline Alberto-Girardi

Re: Starship heat shield
« Reply #3456 on: 07/18/2023 01:31 pm »
I think for thermal you have 2 primary constraints, not 1:

1. Heat pulse, i.e. the one you mentioned. This is the amount of energy which gets to your ship.
2. Peak heating. This is the maximum heating rate. Among other things this dictates skin temperature.

Ablative heat shields are primarily concerned with the first (1). But non-ablative ones are concerned with both (1 & 2), and it's easier to adjust them for 1 if a need arises (generally you improve the insulation layer). But adjusting for a higher peak heating (2) essentially means coming up with a different material which is not an easy change, and often means redoing your heatshield from scratch. And If you ramp peak heating just a tad too much we're simply out of known materials at all.

The thing about (2) is that it depends not just on velocity. It depends on deceleration as well. If you replace 2g braking with 4g braking you essentially doubled your peak heating.

At the same time you have likely reduced(!) heat pulse! This is counterintuitive, because kinetic energy to lose is the same regardless of how quickly you drop it. But the thing is, at certain point the more rarefied the ambient atmosphere the larger fraction of heat gets to the vehicle (and less is taken away by the ambient air). So when you pass entry interface heating initially grows much faster than deceleration. Just look at Shuttle reentry heating and deceleration profiles. Or some (SpaceX?) materials about Starship reentry. You're dropping speed slowly while heating is already high.

So if you have an ablative heatshield, you want to punch through the too thin air fast into high deceleration region. Total heat pulse is less that way, so you need less material to ablate. And you don't care that much about peak heating (within limits), as the temperature is primarily set by your ablative material's phase changes.

But if you have non-ablative heatshield, you prefer skidding on top of the atmosphere. The heat pulse is larger (often significantly), but you keep peak heating in check by keeping the whole kinetic energy drop rate in check. But if you have to generate more (negative) lift to hold onto a small low planet, you need to increase drag according to L:D ratio of your vehicle, which means higher deceleration which means higher kinetic energy drop rate and higher heating.

First, I never thanked you for this post.  It's a very clear problem statement, and I refer back to it quite often.

So how do things change if Starship intends to aerocapture instead of doing a direct EDL?  (This question applies for both Mars and Earth entry.)

I can think of a few of things:

1) You don't have to kill all the speed in a single direct EDL.  You only need enough to get you captured into an eccentric orbit.  That presumably makes the path through the atmosphere shorter, which reduces the heat pulse.

2) The Starship can cold-soak or barbecue roll to radiate away the heat pulse before beginning the actual EDL.

3) You also obviously get a chance to null out any guidance irregularities, allowing changes to both periapse altitude and inclination post-aerocapture.

4) The radius of curvature of the path requiring negative lift can be substantially greater, resulting in lower lift, which in turn reduces the amount of drag that's required.

It's #4 that I'm most curious about.  (See attachment.)  The deeper you can dip into the atmosphere before pulling out, the larger the radius of curvature of the path where substantial drag is produced.  But that means that the Starship has to endure a very brief period of high drag, and therefore high peak heating.  You obviously can't exceed a dynamic pressure that causes so high a peak heating that the tiles fail, but they're not going to soak at that temperature for very long.  This is the kinda-in-between case between pure peak heating and total heat pulse.

Is there a pony in here somewhere?  As always, the name of the game is the highest tolerable arrival v∞.


I wonder if since SS will be the first vehicle heading to mars with a reusable heat shield it will not do a capture burn, but instead only aerobrake.
IIRC all missions to mars have either 1) directly plunged in the atmosphere and EDLed, because they had ablative heat shields   2)did a capture burn aided by aerobraking

This maneuver would save a lot of dV, but the heating pulse and peak heating would need to be carefully studied. Also SS could aerobrake in a very  elliptical orbit, but that orbit might be month long, and so they would probably want to get deeper in the atmosphere to scrub more velocity.
That technique is known as 'aerocapture', and the major barrier is knowing atmosphere state at time of entry in order to properly plot a trajectory to scrub just enough velocity to enter the target orbit (too much = early entry at an uncontrolled location on an undesired trajectory, too little  = escape). Proposals to solve that problem include a swathe of probes sent ahead by a few hours to map multiple runs through the atmosphere to map density & temperature, and pre-placing Mars weather satellites to continuously measure the atmosphere in order to build and regularly update a volumetric atmospheric model.

Thanks for the insight.
I wonder why such precise information on the state of the atmosphere is needed for aerocapture, that does not need to be extremely precise, but is not needed for standard EDL. The landing ellipse for Perseverance was about 7 miles, and that precision is not due to the sckycrane that only separates at 1.3 miles. Is the upper athmospere of mars so much turbolent and unpredictable and so an EDL is easier to perform accurately because it can go in the lower atmosphere?
Another example: the Orion capsule performs a skip reenty, on Earth. That meneuver is used to increase the accuracy of the landing, and is similar to aerocapture. Orion doesn't employ any probes sent in advance.
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Offline edzieba

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Re: Starship heat shield
« Reply #3457 on: 07/18/2023 02:18 pm »
I think for thermal you have 2 primary constraints, not 1:

1. Heat pulse, i.e. the one you mentioned. This is the amount of energy which gets to your ship.
2. Peak heating. This is the maximum heating rate. Among other things this dictates skin temperature.

Ablative heat shields are primarily concerned with the first (1). But non-ablative ones are concerned with both (1 & 2), and it's easier to adjust them for 1 if a need arises (generally you improve the insulation layer). But adjusting for a higher peak heating (2) essentially means coming up with a different material which is not an easy change, and often means redoing your heatshield from scratch. And If you ramp peak heating just a tad too much we're simply out of known materials at all.

The thing about (2) is that it depends not just on velocity. It depends on deceleration as well. If you replace 2g braking with 4g braking you essentially doubled your peak heating.

At the same time you have likely reduced(!) heat pulse! This is counterintuitive, because kinetic energy to lose is the same regardless of how quickly you drop it. But the thing is, at certain point the more rarefied the ambient atmosphere the larger fraction of heat gets to the vehicle (and less is taken away by the ambient air). So when you pass entry interface heating initially grows much faster than deceleration. Just look at Shuttle reentry heating and deceleration profiles. Or some (SpaceX?) materials about Starship reentry. You're dropping speed slowly while heating is already high.

So if you have an ablative heatshield, you want to punch through the too thin air fast into high deceleration region. Total heat pulse is less that way, so you need less material to ablate. And you don't care that much about peak heating (within limits), as the temperature is primarily set by your ablative material's phase changes.

But if you have non-ablative heatshield, you prefer skidding on top of the atmosphere. The heat pulse is larger (often significantly), but you keep peak heating in check by keeping the whole kinetic energy drop rate in check. But if you have to generate more (negative) lift to hold onto a small low planet, you need to increase drag according to L:D ratio of your vehicle, which means higher deceleration which means higher kinetic energy drop rate and higher heating.

First, I never thanked you for this post.  It's a very clear problem statement, and I refer back to it quite often.

So how do things change if Starship intends to aerocapture instead of doing a direct EDL?  (This question applies for both Mars and Earth entry.)

I can think of a few of things:

1) You don't have to kill all the speed in a single direct EDL.  You only need enough to get you captured into an eccentric orbit.  That presumably makes the path through the atmosphere shorter, which reduces the heat pulse.

2) The Starship can cold-soak or barbecue roll to radiate away the heat pulse before beginning the actual EDL.

3) You also obviously get a chance to null out any guidance irregularities, allowing changes to both periapse altitude and inclination post-aerocapture.

4) The radius of curvature of the path requiring negative lift can be substantially greater, resulting in lower lift, which in turn reduces the amount of drag that's required.

It's #4 that I'm most curious about.  (See attachment.)  The deeper you can dip into the atmosphere before pulling out, the larger the radius of curvature of the path where substantial drag is produced.  But that means that the Starship has to endure a very brief period of high drag, and therefore high peak heating.  You obviously can't exceed a dynamic pressure that causes so high a peak heating that the tiles fail, but they're not going to soak at that temperature for very long.  This is the kinda-in-between case between pure peak heating and total heat pulse.

Is there a pony in here somewhere?  As always, the name of the game is the highest tolerable arrival v∞.


I wonder if since SS will be the first vehicle heading to mars with a reusable heat shield it will not do a capture burn, but instead only aerobrake.
IIRC all missions to mars have either 1) directly plunged in the atmosphere and EDLed, because they had ablative heat shields   2)did a capture burn aided by aerobraking

This maneuver would save a lot of dV, but the heating pulse and peak heating would need to be carefully studied. Also SS could aerobrake in a very  elliptical orbit, but that orbit might be month long, and so they would probably want to get deeper in the atmosphere to scrub more velocity.
That technique is known as 'aerocapture', and the major barrier is knowing atmosphere state at time of entry in order to properly plot a trajectory to scrub just enough velocity to enter the target orbit (too much = early entry at an uncontrolled location on an undesired trajectory, too little  = escape). Proposals to solve that problem include a swathe of probes sent ahead by a few hours to map multiple runs through the atmosphere to map density & temperature, and pre-placing Mars weather satellites to continuously measure the atmosphere in order to build and regularly update a volumetric atmospheric model.

Thanks for the insight.
I wonder why such precise information on the state of the atmosphere is needed for aerocapture, that does not need to be extremely precise, but is not needed for standard EDL. The landing ellipse for Perseverance was about 7 miles, and that precision is not due to the sckycrane that only separates at 1.3 miles. Is the upper athmospere of mars so much turbolent and unpredictable and so an EDL is easier to perform accurately because it can go in the lower atmosphere?
Another example: the Orion capsule performs a skip reenty, on Earth. That meneuver is used to increase the accuracy of the landing, and is similar to aerocapture. Orion doesn't employ any probes sent in advance.
EDL spends a decent amount of time in the lower atmosphere, which is more 'stable' (e.g. for Mars a 1 Pa variance at 20km is a 1% change in pressure, a 1 Pa variance at 60km is a 100% variance in pressure), and for EDL a velocity residual has less of an effect on final trajectory than for aerocapture (an extra 100m/s may land you a few km down your landing ellipse, but an extra 100m/s after capture may be an escape trajectory or need 100m/s extra deltaV to correct).

On Earth, we have an extensive network of weather satellites measuring atmospheric properties at high fidelity and updating those measurements regularly, along with a vast network of ground stations to augment those satellites, and the ability to release balloons for even more measurements. Mars currently has none of that.

Offline drzerg

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Re: Starship heat shield
« Reply #3458 on: 07/18/2023 07:43 pm »
If starship could produce enough lift there will be margin of error that could be fixed by manuvering.

Offline OTV Booster

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Re: Starship heat shield
« Reply #3459 on: 07/19/2023 05:02 pm »
I think for thermal you have 2 primary constraints, not 1:

1. Heat pulse, i.e. the one you mentioned. This is the amount of energy which gets to your ship.
2. Peak heating. This is the maximum heating rate. Among other things this dictates skin temperature.

Ablative heat shields are primarily concerned with the first (1). But non-ablative ones are concerned with both (1 & 2), and it's easier to adjust them for 1 if a need arises (generally you improve the insulation layer). But adjusting for a higher peak heating (2) essentially means coming up with a different material which is not an easy change, and often means redoing your heatshield from scratch. And If you ramp peak heating just a tad too much we're simply out of known materials at all.

The thing about (2) is that it depends not just on velocity. It depends on deceleration as well. If you replace 2g braking with 4g braking you essentially doubled your peak heating.

At the same time you have likely reduced(!) heat pulse! This is counterintuitive, because kinetic energy to lose is the same regardless of how quickly you drop it. But the thing is, at certain point the more rarefied the ambient atmosphere the larger fraction of heat gets to the vehicle (and less is taken away by the ambient air). So when you pass entry interface heating initially grows much faster than deceleration. Just look at Shuttle reentry heating and deceleration profiles. Or some (SpaceX?) materials about Starship reentry. You're dropping speed slowly while heating is already high.

So if you have an ablative heatshield, you want to punch through the too thin air fast into high deceleration region. Total heat pulse is less that way, so you need less material to ablate. And you don't care that much about peak heating (within limits), as the temperature is primarily set by your ablative material's phase changes.

But if you have non-ablative heatshield, you prefer skidding on top of the atmosphere. The heat pulse is larger (often significantly), but you keep peak heating in check by keeping the whole kinetic energy drop rate in check. But if you have to generate more (negative) lift to hold onto a small low planet, you need to increase drag according to L:D ratio of your vehicle, which means higher deceleration which means higher kinetic energy drop rate and higher heating.

First, I never thanked you for this post.  It's a very clear problem statement, and I refer back to it quite often.

So how do things change if Starship intends to aerocapture instead of doing a direct EDL?  (This question applies for both Mars and Earth entry.)

I can think of a few of things:

1) You don't have to kill all the speed in a single direct EDL.  You only need enough to get you captured into an eccentric orbit.  That presumably makes the path through the atmosphere shorter, which reduces the heat pulse.

2) The Starship can cold-soak or barbecue roll to radiate away the heat pulse before beginning the actual EDL.

3) You also obviously get a chance to null out any guidance irregularities, allowing changes to both periapse altitude and inclination post-aerocapture.

4) The radius of curvature of the path requiring negative lift can be substantially greater, resulting in lower lift, which in turn reduces the amount of drag that's required.

It's #4 that I'm most curious about.  (See attachment.)  The deeper you can dip into the atmosphere before pulling out, the larger the radius of curvature of the path where substantial drag is produced.  But that means that the Starship has to endure a very brief period of high drag, and therefore high peak heating.  You obviously can't exceed a dynamic pressure that causes so high a peak heating that the tiles fail, but they're not going to soak at that temperature for very long.  This is the kinda-in-between case between pure peak heating and total heat pulse.

Is there a pony in here somewhere?  As always, the name of the game is the highest tolerable arrival v∞.


I wonder if since SS will be the first vehicle heading to mars with a reusable heat shield it will not do a capture burn, but instead only aerobrake.
IIRC all missions to mars have either 1) directly plunged in the atmosphere and EDLed, because they had ablative heat shields   2)did a capture burn aided by aerobraking

This maneuver would save a lot of dV, but the heating pulse and peak heating would need to be carefully studied. Also SS could aerobrake in a very  elliptical orbit, but that orbit might be month long, and so they would probably want to get deeper in the atmosphere to scrub more velocity.
That technique is known as 'aerocapture', and the major barrier is knowing atmosphere state at time of entry in order to properly plot a trajectory to scrub just enough velocity to enter the target orbit (too much = early entry at an uncontrolled location on an undesired trajectory, too little  = escape). Proposals to solve that problem include a swathe of probes sent ahead by a few hours to map multiple runs through the atmosphere to map density & temperature, and pre-placing Mars weather satellites to continuously measure the atmosphere in order to build and regularly update a volumetric atmospheric model.
Going a tad in the OT direction: if the first mission plans on long distance excursions a minimal StarLink constellation makes sense. If low enough, the sats get good data for a broad picture of the state of the atmosphere. Not sure if this is good enough for an aerobreaking map. It's in the right direction.
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