Starship heat shield

[edzieba]

A question: If we suppose that too many tiles fall off the ship and it does not survive re-entry, would adding a thin ablative layer under the tiles (replacing or in addition to the thermal blankets) be too heavy? Too expensive? Is it even possible?

Since you don't know a-priori which tile will fail, and at what point in the EDL sequence (e.g. worst case is the tile fails before entry starts), then you'd need to apply an ablative heatshield across the entire vehicle capable of surviving the entire EDL sequence. At that point, you've added the mass of an entire second TPS to the vehicle.

[john smith 19]

A question: If we suppose that too many tiles fall off the ship and it does not survive re-entry, would adding a thin ablative layer under the tiles (replacing or in addition to the thermal blankets) be too heavy? Too expensive? Is it even possible?

Well there is a flexible version of both SIRCA and PICA ablatives that can be cut with a (sharp) knife. This makes working around holes, hatches etc much quicker and easier.

So it's a possible option that's not necessaily even very labor intensive.

But how long's it going to last? How thick should it be? Just to give some perspective the amount of kinetic and potential energy you need to lose dropping from LEO is about 11.4x that you need to lose from about M6 (roughly the F9's S1 staging velocity). So if F9 S1 needs a 1mm thick layer SS will need 11.4mm. Of course you could then run probabilistic risk assessment, IE lots of monte carlo runs, to decide where the most likely/most serious tile losses would be and just put some under those areas.

The benefit of a few standard size tiles is their interchanagability. Logically you'd have sensors to monitor tile damage and replace those from a small(ish) stock carried on board.

But it's complicated.  Totally passive (but likely quite heavy) system of sub-tile ablative Vs monitoring and sensors and some kind of tile replacement process on orbit before you start entry?

Also while such ablatives exist how do you attach them to SS? AFAIK they are usually attached to aluminum structures, not steel. Usually (AFAIK) by some kind of adhesive. However a significant part of that surface is going to be at cryogenic temperatures, possibly after a minimum of at least a 3 month (but maybe up to 6?) cold soak. Maybe there's an OTS adhesive that can handle that combo already.

My instinct is they will ignore this issue and focus on just getting a vehicle to orbit with the standard TPS as designed.

Obviously that first vehicle when it goes up will a)Be completely automated b) Have lots of sensors. c) Assuming it lands safetly be studied in minute detail.

Until that happens planning a backup TPS, with no solid idea of how the baseline TPS will fail to begin with, seems a bit premature. 

[john smith 19]

Since you don't know a-priori which tile will fail, and at what point in the EDL sequence (e.g. worst case is the tile fails before entry starts), then you'd need to apply an ablative heatshield across the entire vehicle capable of surviving the entire EDL sequence. At that point, you've added the mass of an entire second TPS to the vehicle.

That would be the worst case scenario. 

I guess to get a baseline for this you'd need to find out how much ablative is burnt off the Dragon heat shield on reentry. It's the only one that runs the full range orbital velocity to zero.

Anyone have any numbers for this?

[edzieba]

Since you don't know a-priori which tile will fail, and at what point in the EDL sequence (e.g. worst case is the tile fails before entry starts), then you'd need to apply an ablative heatshield across the entire vehicle capable of surviving the entire EDL sequence. At that point, you've added the mass of an entire second TPS to the vehicle.

That would be the worst case scenario. 

I guess to get a baseline for this you'd need to find out how much ablative is burnt off the Dragon heat shield on reentry. It's the only one that runs the full range orbital velocity to zero.

Anyone have any numbers for this?

Dragon TPS would has a more benign re-entry environment than Starship sees.

For ablative TPS, entry trajectory is designed to reduce total thermal input, with high peak temperatures being acceptable. Too much total heat soaks through the TPS and the system fails, but higher peak heat for a short time means less total ablation and less heat soak. That's the sort of trajectory Dragon flies, which is limited more by crew (and structural) desired g tolerance than peak heating.

For a radiative TPS (like Starship's) the goal is to keep peak heating within acceptable limits, with total thermal input being much less of an issue. That's the sort of trajectory Starship flies. Flying PICA-X in the same environment is liable to burn through a much thicker layer whilst also heating up the backside to a higher temperature.

[RamsesBic]

Hmmm... So it is the tiles or nothing? No back-up possible.
I think the ship can survive losing a few tiles here and there, but we have to see if that is true or just what we hope for.

Btw, there are two types of lost tiles. Those lost on launch (could in theory be repaired on orbit) and those lost at some point during re-entry, which would just need some help to make it down.

Or will losing cargo ships now and then be acceptable and the cost of doing business?

[livingjw]

Hmmm... So it is the tiles or nothing? No back-up possible.
I think the ship can survive losing a few tiles here and there, but we have to see if that is true or just what we hope for.

Btw, there are two types of lost tiles. Those lost on launch (could in theory be repaired on orbit) and those lost at some point during re-entry, which would just need some help to make it down.

Or will losing cargo ships now and then be acceptable and the cost of doing business?

There are other TPS technologies. One that I have studied is C/Sic composite shells (one to two feet on a side) mechanically fixed, backed with Saffil or similar insulation. They come in at about the same weight. Advanced Carbon/carbon shells also would work. Both would be more expensive and lower TRL in this application. SpaceX's current approach is coming along, and it certainly is repairable. Durability yet to be fully demonstrated. I expect to see incremental improvements to their current TPS approach.

John

[john smith 19]

Dragon TPS would has a more benign re-entry environment than Starship sees.

For ablative TPS, entry trajectory is designed to reduce total thermal input, with high peak temperatures being acceptable. Too much total heat soaks through the TPS and the system fails, but higher peak heat for a short time means less total ablation and less heat soak. That's the sort of trajectory Dragon flies, which is limited more by crew (and structural) desired g tolerance than peak heating.

For a radiative TPS (like Starship's) the goal is to keep peak heating within acceptable limits, with total thermal input being much less of an issue. That's the sort of trajectory Starship flies. Flying PICA-X in the same environment is liable to burn through a much thicker layer whilst also heating up the backside to a higher temperature.

I'm aware of the difference between an ablative and a radiative TPS, as well as the difference in the form of the heat pulse they are designed to cope with.

But given we know the approximate density of PICAX with the thickness lost on a Dragon reentry we can start to get an idea of what sort of mass such a backup would add to the SS empty weight.

It won't be accurate, that's a given. But it is a place to start.

[john smith 19]

Hmmm... So it is the tiles or nothing? No back-up possible.

Not at all, but a)No previous vehicle has had a 2nd heatshield behind the first b)Any option adds weight, so the question is do you really need it?

I think the ship can survive losing a few tiles here and there, but we have to see if that is true or just what we hope for.

You're probably right. It comes down to 2 questions. Where is the tile(s) lost and whenin the trajectory. If it's a single tile from the top side (as it comes in side on) just as it's horizontal speed drops to near zero to an Earth landing then the answer is probably yes. Lose it from the lower side as it start to decellerate (IE at near orbital speed) and it probably won't.

Btw, there are two types of lost tiles. Those lost on launch (could in theory be repaired on orbit) and those lost at some point during re-entry, which would just need some help to make it down.

Or will losing cargo ships now and then be acceptable and the cost of doing business?

Actually tiles could be lost any time during the ascent trajectory, not just launch.

In practice with the amount of cargo Musk plans to send to mars (about 9 for every passenger carrier IIRC) the answer would have to be yes. 

Except since the same design is also the passenger carrier then if it does happen then it's going to have to be fixed and proved to be fixed, as they will not be astronaughts, they will be "spaceflight participants" as the FAA calls them. Lower levels of risk will be expected (by the FAA if no one else) and also demonstrated either by a lot of flights or a detailed mathematical analysis whose assumptions can be justified.

[RamsesBic]

Hmmm... So it is the tiles or nothing? No back-up possible.

Not at all, but a)No previous vehicle has had a 2nd heatshield behind the first b)Any option adds weight, so the question is do you really need it?

a) I am aware of that. Starships are already using thermal blankets and nets, that is more than one is it not?
b) It would add weight, but the question is what would be preferable? Not adding weight or losing a ship?

I think the ship can survive losing a few tiles here and there, but we have to see if that is true or just what we hope for.

You're probably right. It comes down to 2 questions. Where is the tile(s) lost and whenin the trajectory. If it's a single tile from the top side (as it comes in side on) just as it's horizontal speed drops to near zero to an Earth landing then the answer is probably yes. Lose it from the lower side as it start to decellerate (IE at near orbital speed) and it probably won't.

Btw, there are two types of lost tiles. Those lost on launch (could in theory be repaired on orbit) and those lost at some point during re-entry, which would just need some help to make it down.

Or will losing cargo ships now and then be acceptable and the cost of doing business?

Actually tiles could be lost any time during the ascent trajectory, not just launch.

In practice with the amount of cargo Musk plans to send to mars (about 9 for every passenger carrier IIRC) the answer would have to be yes. 

Except since the same design is also the passenger carrier then if it does happen then it's going to have to be fixed and proved to be fixed, as they will not be astronaughts, they will be "spaceflight participants" as the FAA calls them. Lower levels of risk will be expected (by the FAA if no one else) and also demonstrated either by a lot of flights or a detailed mathematical analysis whose assumptions can be justified.

Maybe I should have said "before re-entry" rather than launch.

I am assuming that the system will improve as the number of flights increases such that when humans are to board a ship the problem would be negligible.

[OTV Booster]

Re: loosing tiles. I can't put my finger on it but there has been a NASA paper linked on NSF showing a relationship between radius of curvature and shock stand-off.


Assume the very tippy tip tile on the nose were lost. The radius of curvature would normally be the nose radius. Remove the tile and the radius of curvature that is of interest would be the top edge of the next tile down on the centerline. This edge is sharp and the radius of curvature of the shock front would be correspondingly small. My guess is that there would be some risk of shock impingement where the tile is missing. If the shock does not impinge and the shock stand-off distance small, heat input might be mitigated by any cryo cooling the header tank would give. Then the problem becomes landing propellant loss.


The entire centerline would be a concern with the the (I'm at a loss for the correct word) point towards the bottom of the ogive that is the first point of atmospheric 'contact' being of special concern. Moving away from the centerline, the shock stand-off distance increases and the risk associated with tile loss goes down.


This looks like a map for a backup TPS system, if future experience warrants. All the fiddly shapes, including the fins, makes my brain hurt but there may be a similar map here too. Key point: a backup TPS need not be full sized or of constant thickness.

[DanClemmensen]

Re: loosing tiles. I can't put my finger on it but there has been a NASA paper linked on NSF showing a relationship between radius of curvature and shock stand-off.


Assume the very tippy tip tile on the nose were lost. The radius of curvature would normally be the nose radius. Remove the tile and the radius of curvature that is of interest would be the top edge of the next tile down on the centerline. This edge is sharp and the radius of curvature of the shock front would be correspondingly small. My guess is that there would be some risk of shock impingement where the tile is missing. If the shock does not impinge and the shock stand-off distance small, heat input might be mitigated by any cryo cooling the header tank would give. Then the problem becomes landing propellant loss.


The entire centerline would be a concern with the the (I'm at a loss for the correct word) point towards the bottom of the ogive that is the first point of atmospheric 'contact' being of special concern. Moving away from the centerline, the shock stand-off distance increases and the risk associated with tile loss goes down.


This looks like a map for a backup TPS system, if future experience warrants. All the fiddly shapes, including the fins, makes my brain hurt but there may be a similar map here too. Key point: a backup TPS need not be full sized or of constant thickness.

SS does not enter "nose" first. It enters belly first. All references to "nose radius" in the re-entry literature refer to the curvature of the entering body in its direction of travel. If the tippy-tip TPS falls off, I don't know how to calculate the "radius" of the result. The Starship nose is sticking out toward the side of the plasma flow.

[RamsesBic]

The nose cone has glued tiles, just as the Shuttle had. Those don't fall off, unless they were not glued properly. (Anyone watching Starbase live today would have seen how hard it is to remove glued tiles. Chisel and big hammer needed.)

The ones most in danger imho are in the middle of the ship, held by studs. If one is not flush enough to the ones around it that corner could break off. If a tiles is not installed the correct way it could fall off too.

What no one can know for sure, until at least the first orbital flight, is whether or not losing one tile causes the loss of others around it. I don't know what kind of forces the plasma can exert on a tile, besides the heat. Is it capable of getting under the next tile and flicking it out?

[Hog]

The nose cone has glued tiles, just as the Shuttle had. Those don't fall off, unless they were not glued properly. (Anyone watching Starbase live today would have seen how hard it is to remove glued tiles. Chisel and big hammer needed.)

The ones most in danger imho are in the middle of the ship, held by studs. If one is not flush enough to the ones around it that corner could break off. If a tiles is not installed the correct way it could fall off too.

What no one can know for sure, until at least the first orbital flight, is whether or not losing one tile causes the loss of others around it. I don't know what kind of forces the plasma can exert on a tile, besides the heat. Is it capable of getting under the next tile and flicking it out?

Emphasis mine.
During STS times this was known as the "zipper effect".

[edzieba]

Re: loosing tiles. I can't put my finger on it but there has been a NASA paper linked on NSF showing a relationship between radius of curvature and shock stand-off.


Assume the very tippy tip tile on the nose were lost. The radius of curvature would normally be the nose radius. Remove the tile and the radius of curvature that is of interest would be the top edge of the next tile down on the centerline. This edge is sharp and the radius of curvature of the shock front would be correspondingly small. My guess is that there would be some risk of shock impingement where the tile is missing. If the shock does not impinge and the shock stand-off distance small, heat input might be mitigated by any cryo cooling the header tank would give. Then the problem becomes landing propellant loss.


The entire centerline would be a concern with the the (I'm at a loss for the correct word) point towards the bottom of the ogive that is the first point of atmospheric 'contact' being of special concern. Moving away from the centerline, the shock stand-off distance increases and the risk associated with tile loss goes down.


This looks like a map for a backup TPS system, if future experience warrants. All the fiddly shapes, including the fins, makes my brain hurt but there may be a similar map here too. Key point: a backup TPS need not be full sized or of constant thickness.

SS does not enter "nose" first. It enters belly first. All references to "nose radius" in the re-entry literature refer to the curvature of the entering body in its direction of travel. If the tippy-tip TPS falls off, I don't know how to calculate the "radius" of the result. The Starship nose is sticking out toward the side of the plasma flow.

Starship entry AoA is ~70°. The nose still still be first, and since the nose itself has an angle of ~60°, the stagnation point will just be a little to the nadir of the very tippy top.

[john smith 19]

Starship entry AoA is ~70°. The nose still still be first, and since the nose itself has an angle of ~60°, the stagnation point will just be a little to the nadir of the very tippy top.

Indeed. SS's entry is quite Shuttle like.  I'm reminded that Reinforced Carbon Carbon was only used on two areas of the Shuttle TPS, the wing leading edges and the nose area.

Obviously Musk is expecting the heating levels will be low enough not to need such extreme materials. That said based on the table given a couple of pages back SS's ballistic coefficient is only about 8%[EDIT lower ] than that of Shuttle. I guess changing the angle from 46 to 70 degrees will make quite a difference to peak temperatures.

[woods170]

The nose cone has glued tiles, just as the Shuttle had. Those don't fall off, unless they were not glued properly. (Anyone watching Starbase live today would have seen how hard it is to remove glued tiles. Chisel and big hammer needed.)

The ones most in danger imho are in the middle of the ship, held by studs. If one is not flush enough to the ones around it that corner could break off. If a tiles is not installed the correct way it could fall off too.

What no one can know for sure, until at least the first orbital flight, is whether or not losing one tile causes the loss of others around it. I don't know what kind of forces the plasma can exert on a tile, besides the heat. Is it capable of getting under the next tile and flicking it out?

Emphasis mine.
During STS times this was known as the "zipper effect".

The "zipper effect" was a concern during shuttle development. But flight experience proved that the "zipper effect" was highly unlikely to be triggered by a lost tile. Increased turbulence and thermal loading to the exposed side of the next tile was observed, but was not strong enough to force the debonding of that next tile. STS-27 being the best observations of this lack of zipper effect.

[RamsesBic]

The nose cone has glued tiles, just as the Shuttle had. Those don't fall off, unless they were not glued properly. (Anyone watching Starbase live today would have seen how hard it is to remove glued tiles. Chisel and big hammer needed.)

The ones most in danger imho are in the middle of the ship, held by studs. If one is not flush enough to the ones around it that corner could break off. If a tiles is not installed the correct way it could fall off too.

What no one can know for sure, until at least the first orbital flight, is whether or not losing one tile causes the loss of others around it. I don't know what kind of forces the plasma can exert on a tile, besides the heat. Is it capable of getting under the next tile and flicking it out?

Emphasis mine.
During STS times this was known as the "zipper effect".

The "zipper effect" was a concern during shuttle development. But flight experience proved that the "zipper effect" was highly unlikely to be triggered by a lost tile. Increased turbulence and thermal loading to the exposed side of the next tile was observed, but was not strong enough to force the debonding of that next tile. STS-27 being the best observations of this lack of zipper effect.

Good to know. Thanks.
But Starship uses studs and the Shuttle used glue, so the result might not be the same.

[eriblo]

The nose cone has glued tiles, just as the Shuttle had. Those don't fall off, unless they were not glued properly. (Anyone watching Starbase live today would have seen how hard it is to remove glued tiles. Chisel and big hammer needed.)

The ones most in danger imho are in the middle of the ship, held by studs. If one is not flush enough to the ones around it that corner could break off. If a tiles is not installed the correct way it could fall off too.

What no one can know for sure, until at least the first orbital flight, is whether or not losing one tile causes the loss of others around it. I don't know what kind of forces the plasma can exert on a tile, besides the heat. Is it capable of getting under the next tile and flicking it out?

Emphasis mine.
During STS times this was known as the "zipper effect".

The "zipper effect" was a concern during shuttle development. But flight experience proved that the "zipper effect" was highly unlikely to be triggered by a lost tile. Increased turbulence and thermal loading to the exposed side of the next tile was observed, but was not strong enough to force the debonding of that next tile. STS-27 being the best observations of this lack of zipper effect.

Good to know. Thanks.
But Starship uses studs and the Shuttle used glue, so the result might not be the same.

That glue the part of the TPS with the lowest temperature limit (which was fine as the aluminum structure beneath was even worse). The studs will likely be fine from a temperature standpoint.

Dynamic pressure from a high velocity gas flow getting in behind the tiles can dislodge then as we saw with the cold gas thrusters but that was a Mach 2-3 (?) stream of higher than sea level density nitrogen. Hopefully they have done the numbers and tested the tiles for Max-Q and reentry conditions...

[Reynold]

Hmmm... So it is the tiles or nothing? No back-up possible.
I think the ship can survive losing a few tiles here and there, but we have to see if that is true or just what we hope for.

I think part of the idea of the SS construction is that IS the backup.  Note that before Columbia, STS-27 lost tiles and had the aluminum skin melt but fortunately there was a stainless steel mounting plate for the structure underneath and that saved it.

If you need still another backup heat shield in addition to the stainless steel and the thermal tiles, you probably want to improve the thermal tiles and fastening process in some way rather than add another redundancy.  Or stick to doing reentry on Dragons, and accept a 1 in 10 vehicle loss on cargo runs of Starship. 

[Hog]

Hmmm... So it is the tiles or nothing? No back-up possible.
I think the ship can survive losing a few tiles here and there, but we have to see if that is true or just what we hope for.

I think part of the idea of the SS construction is that IS the backup.  Note that before Columbia, STS-27 lost tiles and had the aluminum skin melt but fortunately there was a stainless steel mounting plate for the structure underneath and that saved it.

If you need still another backup heat shield in addition to the stainless steel and the thermal tiles, you probably want to improve the thermal tiles and fastening process in some way rather than add another redundancy.  Or stick to doing reentry on Dragons, and accept a 1 in 10 vehicle loss on cargo runs of Starship.

Added a picture of the "melty" portion of Orbiter Vehicle (OV-104) Discovery following the tile loss.
xxxxxxxxxxxxxxxx


That 1/9 LOC/V(Loss of Crew/Vehicle) was approx the same safety stats for STS-5 through STS-9(STS-5 and STS-6 saw crew size jump from 2=left seated Commander and right seated Pilot up to 4 with 2 mission specialists seated behind the Commander/pilot. Then for STS-7 and STS-8 saw the 4 crew on the flight deck with a 5th crewmember down in the mid-deck and for STS-9 saw 4 on the flight deck and 2 in the mid-deck for a crew of 6), the ejection seat system was called the SR-1 system and used the Shuttle Ejection Escape Suit(SEES) (S1030A) which was a derivation of the David Clark Company's USAF S1030 pressure suits used in the Mach 3+ Blackbird series of recon., interceptor and NASA research planes.  The SR-1 seat system and the SEES pressure suits used for flights STS-1 through STS-4 improved the LOC/V rates up to 1 loss in 12 attempts  My times have changed, of course these LOCV rates were NOT known at the time and were calculated once STS had retired.
With President Reagan pronouncing the Space Shuttle as "OPERATIONAL" for STS-5, the SR-1 system was disabled on OV-102 Columbia with the crew wearing those blue cotton flight suits along with basic lightweight helmets and masks which by design, would supply oxygen to crewmembers from their carry along PEAPs(personal Egress Air Packs). the masks and PEAPs were designed to supply breathable air to the astronauts to enable them to escape the orbiter vehicle during emergency escapes from the crew compartment down to the mid-deck, out along the crew access arm and to the slide baskets and finally to the bunkers to shelter in place, or to awaiting M-113 armoured tracked tanks to evacuate them in case of casualties that had a time constraint(Golden Hour).
Following her 2nd operational mission(STS-9) Columbia entered an 18 month period back at her birthplace of Palmdale California beginning January 1984 back in California,undergoing modifications that removed the Orbiter Flight Test hardware and updating her to similar specifications as those of its sister orbiters.  OV-102 Columbia had the remnants of the SR-1 ejection system removed and the program moved forward with a "shirt-sleeves" internal operational environment.

attachments
1) astronauts at the bottom of the slidewires escape system wearing the  Shuttle Ejection Escape Suit
2) John Young sporting a SEES
3) The "melty" portion of Atlantis following STS-27