Wiki Super Heavy/Starship - General Development Discussion

[scaesare]

Part of the reason (I think) is that methane is really not the best liquid to use for transpiration. It pyrolizes in high temperatures, causing coking (similar to burning "fuel-rich") which could clog up the transpiration pores. LOX is not much better; pure oxygen is highly reactive. It's possible they could do the transpiration with a separate small tank of e.g. liquid nitrogen I suppose, although its heat of vaporization is less than half of methane. And maybe they'll do something like this for the most vulnerable, highest-heat areas, though probably not for the entire heat shield.

My guess is that they'll end up with an ablative layer underneath the tiles, as a backup in case a tile falls off. Starship's heat shield area is about 900 square meters. For comparison, Dragon uses an 8cm-thick PICA-X heat shield, about 2cm of which is "consumed" on each reentry. (1cm charred, 1cm pyrolized.) So suppose an intermediate 2cm PICA-X layer would be sufficient to keep Starship intact if a few tiles fall off. PICA-X has a density around 0.27g/cm^3, so a 2cm layer underneath Starship's heat shield would mass just under 5 metric tons. Not great, but not crazy, especially if it's eventually only used on crew Starship, or for landing high-value payloads. (Hubble retrieval anyone?)

Didn't Elon also say that it the liquid could (somewhat counter-intuitively) get "shock frozen" in the pores when the ship slams in to the atmosphere?

 

[scaesare]

I like it. I'm reminded of a wall of ivy. It either goes as a single sheet or not at all. And, in ivy fashion, do the velcro thing to keep it attached. If a tile detaches, the velcro could allow it to spontaneously reattach, even if only partially. Call Elon.

It would be interesting if they could develop something with the consistency of epoxy they could apply directly to a honey-comb like structure attached mechanically to the skin...

 

[JB47394]

It would be interesting if they could develop something with the consistency of epoxy they could apply directly to a honey-comb like structure attached mechanically to the skin...

Do you mean like kitchen tile sheets being secured to a wall with grout? I'm trying to understand the combination of epoxy, honeycomb and mechanical. Is the epoxy intended to work like Loctite on the mechanical attachment?

 

[scaesare]

Do you mean like kitchen tile sheets being secured to a wall with grout? I'm trying to understand the combination of epoxy, honeycomb and mechanical. Is the epoxy intended to work like Loctite on the mechanical attachment?

I was thinking of the heatshield itself to be applied in an epoxy-like fashion, in essence taking @Ben W 's idea of attaching the edges a step further and avoiding them all together (the best edge is no edge!).

The honeycomb structure it's applied over would be the mechanical fastening to the ship... anchored perhaps at the same locations as the pins currently:

 

[JB47394]

I was thinking of the heatshield itself to be applied in an epoxy-like fashion, in essence taking @Ben W 's idea of attaching the edges a step further and avoiding them all together (the best edge is no edge!).

The honeycomb structure it's applied over would be the mechanical fastening to the ship... anchored perhaps at the same locations as the pins currently:

What material like that won't ablate? If it's a ceramic paste, then it needs to be fired - which is why they have discrete tiles. It would be neat if the paste could be set strong enough to make it to orbit, then get fired properly during reentry.

What about repairs? Chip out the stuff that's damaged and apply new paste? That would make for a sight after a few rounds of repairs.

 

[scaesare]

What material like that won't ablate? If it's a ceramic paste, then it needs to be fired - which is why they have discrete tiles. It would be neat if the paste could be set strong enough to make it to orbit, then get fired properly during reentry.

What about repairs? Chip out the stuff that's damaged and apply new paste? That would make for a sight after a few rounds of repairs.

Yeah... I'm not a materials science expert by any means. Just that if there were a way to combine the advantages of a single "surface" like the ablative heat shields (for these purposes, I understand they are individually filled "cells"), along with the multi-use capability of a tile, that would be an interesting development.

Actually, the ablative "cells" has me wondering if a much smaller tiles would be advantageous in that the loss of one tile 10x smaller would represent a much smaller cross section of compromised surface... of course you might then lose 10x as many... not to mention attachment challenges...

But, while I'm wishing for pie-in-the-sky stuff, I'd like a tiger... they're cool.

 

[JB47394]

Actually, the ablative "cells" has me wondering if a much smaller tiles would be advantageous in that the loss of one tile 10x smaller would represent a much smaller cross section of compromised surface... of course you might then lose 10x as many... not to mention attachment challenges...

That takes me back to the tile sheet approach. Create a sheet of a bunch of small tiles that are meshed together with wires or something, then install it as a unit against that stainless steel velcro surface. If a tile is damaged, peel off the entire section and lay a new one. Though I'm not sure that this gains much over monolithic tiles of the same size.

I understand they are individually filled "cells"

That's how Apollo and Orion tackle their ablative heat shields. It is done like that because we don't have any other means of handling reentry heating when returning from the Moon. The only reusable systems that I've heard talk about are the ceramic tiles and liquid used as an ablator (transpiration cooling).

 

[Ben W]

I don't understand "relative speed" here. Isn't temperature tied to absolute speed (along with air density & duration)?

Relative speed of spacecraft and atmosphere. The atmosphere spins along with the planet, so technically that should be taken into account. (E.g. there would be more heating when reentering from a high-inclination or retrograde orbit for this reason.) But to a first approximation, “absolute speed” (in non-spinning Earth’s frame) is probably close enough. Splitting hairs.

 

[Ben W]

That's how Apollo and Orion tackle their ablative heat shields. It is done like that because we don't have any other means of handling reentry heating when returning from the Moon. The only reusable systems that I've heard talk about are the ceramic tiles and liquid used as an ablator (transpiration cooling).

Does this imply that Starship’s reusable tile approach won’t work for Mars and Mars return? Could they simply make the tiles thicker to compensate for escape-velocity reentry heating?

Re tiles falling off, is it known by what precise mechanism they tend to detach? Is it due to the brittleness of the tiles causing them to fracture, or the pins not holding them strongly enough, or an adhesive failure, or ? (The Shuttle just used adhesive, IIRC.)

Any of these might be solved (just theorizing) by 3D-printing a honeycomb “skeleton” (made of e.g. alumina or carbon-carbon) that’s thicker in back (solid at the very back) and thinner in front, filling it in with the raw ceramic tile material, then firing it in a kiln. The connection between Starship’s skin and the back of the skeleton could be made more robust than connecting to the ceramic directly.

 

[Yelobird]

Does this imply that Starship’s reusable tile approach won’t work for Mars and Mars return? Could they simply make the tiles thicker to compensate for escape-velocity reentry heating?

Re tiles falling off, is it known by what precise mechanism they tend to detach? Is it due to the brittleness of the tiles causing them to fracture, or the pins not holding them strongly enough, or an adhesive failure, or ? (The Shuttle just used adhesive, IIRC.)

Any of these might be solved (just theorizing) by 3D-printing a honeycomb “skeleton” (made of e.g. alumina or carbon-carbon) that’s thicker in back (solid at the very back) and thinner in front, filling it in with the raw ceramic tile material, then firing it in a kiln. The connection between Starship’s skin and the back of the skeleton could be made more robust than connecting to the ceramic directly.

Seems with the standard expansion/contraction known with metals along with the hostile use case adhesive will always be the weak point.

 

[Ben W]

Seems with the standard expansion/contraction known with metals along with the hostile use case adhesive will always be the weak point.

Agreed, which is why connecting the tiles to the ship via physical interlocking may be a more robust solution than adhesive. If the ceramic were physically interlocked with e.g. a carbon-carbon skeleton, and the skeleton were physically interlocked with the ship’s stainless frame (perhaps via rings instead of pins), then it would be much less dependent on adhesive, and/or more robust to adhesive failure. A loose tile would be better than a missing tile, presumably.

 

[JB47394]

Does this imply that Starship’s reusable tile approach won’t work for Mars and Mars return? Could they simply make the tiles thicker to compensate for escape-velocity reentry heating?

Here's a page on stackexchange about reentry speed from Mars. It observes that SpaceX believes that arrival at Mars would be at 7.5 km/s, but the poster goes on to say that the return speed is going to be around 12 km/s - at best. Some returns put the speed at twice that. LEO reentry is 6.9 km/s. SpaceX expects some ablation of the heat shield at Mars, so I would expect any attempt at Earth reentry at 12 km/s would do a lot more damage, perhaps requiring a wholesale replacement of the shield. I have my doubts that the vehicle would survive at the high end of the poster's range.

Here's a porkchop plot of reentry speed for various dates.

For reference, the Space Shuttle tiles could handle 1260 C (6.9 km/s), SpaceX's tiles can handle 1377 C (6.9 km/s), and Orion's ablative shield can handle 2760 (11.2 km/s). I guess return from Mars is on the to do list.

Re tiles falling off, is it known by what precise mechanism they tend to detach? Is it due to the brittleness of the tiles causing them to fracture, or the pins not holding them strongly enough, or an adhesive failure, or ? (The Shuttle just used adhesive, IIRC.)

Here's that shot of IFT-2 vs IFT-3 showing tile loss. The tiles that are missing mostly appear to be around seams between sections, and they also seem to be whole tiles. Based on that, I'd say that they had a problem with adhesives (I believe the tiles at the seams are glued on). That was apparently corrected for IFT-3, and they're applying extra effort to those tiles, such as on the nose, for IFT-4.

Agreed, which is why connecting the tiles to the ship via physical interlocking may be a more robust solution than adhesive. If the ceramic were physically interlocked with e.g. a carbon-carbon skeleton, and the skeleton were physically interlocked with the ship’s stainless frame (perhaps via rings instead of pins), then it would be much less dependent on adhesive, and/or more robust to adhesive failure. A loose tile would be better than a missing tile, presumably.

I still like the stainless steel velcro idea as a middle ground between adhesives and posts. Everything would move together. SpaceX has clearly addressed the problem of expansion of metal parts within the tiles, so a metal-to-metal connection seems to be solved.

 

[Ben W]

Here's a page on stackexchange about reentry speed from Mars. It observes that SpaceX believes that arrival at Mars would be at 7.5 km/s, but the poster goes on to say that the return speed is going to be around 12 km/s - at best. Some returns put the speed at twice that. LEO reentry is 6.9 km/s. SpaceX expects some ablation of the heat shield at Mars, so I would expect any attempt at Earth reentry at 12 km/s would do a lot more damage, perhaps requiring a wholesale replacement of the shield. I have my doubts that the vehicle would survive at the high end of the poster's range.

If there were e.g. a propellant depot on Deimos, a returning Starship could have enough extra delta-v to bleed off quite a bit of speed (~4km/s) approaching Earth entry, at the expense of transit time. But by the time this is happening regularly, ion propulsion may be a feasible and better option for reducing transit time. (A fast burn would still work better for reducing reentry speed.)

 

[Ben W]

It begs the question of what the loss rate would be for ships without the additional protection. I also wonder about the volume of debris being dumped on points west of the Cape. Thirty eight percent of Space Shuttle Columbia was recovered, and that vehicle wasn't made of stainless steel.

That's a really good point. Until they can prove extreme reliability, they'll probably have to line up their reentries such that catastrophic failures would at least happen over the Gulf instead of over populated areas. I wonder if it will require marine restrictions for the first several Cape landing attempts? And what's the risk that it makes it beyond the west coast of Florida and then catastrophically fails over land? Disney World would not appreciate that, I think.

I think this is a problem that needs solving. Fortunately, it's early days, where they have but a single attempt at reentry under their belts. They don't yet know the performance of the tiles on reentry.

Or more to the point, sporadic lack of tiles, though Elon implied that the prognosis for that wasn't good. I wonder if they have any way to detect missing tiles or hotspots in realtime, such as an IR camera inside the fuel tanks? The flap cameras can see some tiles, but not nearly all.

 

[mongo]

I wonder if they have any way to detect missing tiles or hotspots in realtime, such as an IR camera inside the fuel tanks? The flap cameras can see some tiles, but not nearly all.

Elon has mentioned previously that a normal camera inside the tank without lights would detect hot spots (once they get to glowing temps)

 

[Ben W]

Elon has mentioned previously that a normal camera inside the tank without lights would detect hot spots (once they get to glowing temps)

Oh Elon, Pure Vision to the end. I would think that an IR camera could detect the hotspots (or any such abnormalities, such as mild unexpected heating between tiles) far sooner than visible-light cameras could. But I suppose if anything happens where it could be diagnostically helpful, they can always add it to the next flight!

 

[ecarfan]

Here's that shot of IFT-2 vs IFT-3 showing tile loss. The tiles that are missing mostly appear to be around seams between sections, and they also seem to be whole tiles. Based on that, I'd say that they had a problem with adhesives (I believe the tiles at the seams are glued on).

As per Tim Dodd’s recent video (that you linked to in another thread) SpaceX appears to be trying to reduce the number of seams in recognition of the difficulty they are having with securing tiles in those areas.

 

[scaesare]

Does this imply that Starship’s reusable tile approach won’t work for Mars and Mars return? Could they simply make the tiles thicker to compensate for escape-velocity reentry heating?

Re tiles falling off, is it known by what precise mechanism they tend to detach? Is it due to the brittleness of the tiles causing them to fracture, or the pins not holding them strongly enough, or an adhesive failure, or ? (The Shuttle just used adhesive, IIRC.)

Any of these might be solved (just theorizing) by 3D-printing a honeycomb “skeleton” (made of e.g. alumina or carbon-carbon) that’s thicker in back (solid at the very back) and thinner in front, filling it in with the raw ceramic tile material, then firing it in a kiln. The connection between Starship’s skin and the back of the skeleton could be made more robust than connecting to the ceramic directly.

We had some discussion about this previously... a couple posts I made with pics/video... other folks chimed in too...

Are those tiles being attached with pins or glued on? They don't seem to be visible in that pic... Are they attached in individual sets for each tile and that's why they aren't visible?

We've seen pins attached ahead of tiles before:

View attachment 1020778

(responding to my own post)

Some interesting discussion/speculation about the tile fastening/pin problems:

Some of the recovered tiles on the beach that seem to have fallen off we've had close up pics of, and interestingly the mechanical " sockets" the pins seit in seemed to not be deformed...

 

[scaesare]

That takes me back to the tile sheet approach. Create a sheet of a bunch of small tiles that are meshed together with wires or something, then install it as a unit against that stainless steel velcro surface. If a tile is damaged, peel off the entire section and lay a new one. Though I'm not sure that this gains much over monolithic tiles of the same size.

That's how Apollo and Orion tackle their ablative heat shields. It is done like that because we don't have any other means of handling reentry heating when returning from the Moon. The only reusable systems that I've heard talk about are the ceramic tiles and liquid used as an ablator (transpiration cooling).

That's an interesting idea... could cure in an oven and yet still apply en mass...

 

[JB47394]

SpaceX is tackling putting tile mount pins right across the weld lines of barrel sections, eliminating the need to hand glue specially-shaped tiles in those areas. This is an image of a test article that came off the new welding jig in one of the tall bays. I assume the white underlayment will be included on production hardware.