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Buckminster
Well-Known Member
Buckminster
Well-Known Member
Probably in some cases. Just a 1% risk is way too much. 1% likelihood of a tile falling off multiplied by 1% likelihood it ends in the death of 100 martians is maybe borderline for space travel?? Also, if it affects reusability and requires repair then that won't be good enough for Elon.
I think "most places" would imply "critical areas". Bursting a tank would mean the loss of structural integrity of the vehicle. I doubt there are many areas on the vehicle that could tolerate a tile-diameter plasma torch intruding into the interior.
But it looks like we're going to get another Everyday Astronaut interview/tour video. Maybe Tim can offer him a solution to the reentry problem
Entry generates lot of heat energy. Distributing this energy for longer time interval lowers temperature. So deceleration must be as slow as possible. For that aerodynamic lift is needed to maintain max possible altitude. At those speeds lift can be generated only by keeping nose up, perhaps at 45⁰. 90⁰ nose up only after heating is not a problem and ship needs to avoid overshooting landing area.
Ben W
Chess Grandmaster (Supervised)
Temperature is determined by relative speed. Decelerating more slowly may counterintuitively mean a higher temperature for a longer period, though counterbalanced by a lower pressure. Effective heat flux is maybe a better metric.
A secondary heatshield layer of e.g. honeycomb alumina underneath the tiles may provide enough protection to avoid catastrophe if a tile falls off, though it may require significant repair before the next reflight. I wonder if SpaceX will go back to transpirational cooling for any of this?
Ben W
Chess Grandmaster (Supervised)
Starship would need to have a nearly full tank (or at least 1/2 tank) to accomplish this, far in excess of its nominal payload capacity. This would require several extra Tanker flights and an orbital refueling stop per mission, but note that Tankers must also be able to reenter safely, without such a pre-reentry slowdown. Also it wouldn't be feasible for Mars entry, unless you're sending a fleet of expendable Depots along with every ship. Probably it would make much more sense to overbuild Crew Starship's heatshield at the expense of payload, and plan on losing a Tanker or non-Crew Starship (without redundant heat-shielding) every now and then, if the tile-only reliability can't be brought up to 100%.
Ooo! Ooo! Delta-v question!
Assume a 100 ton ship and 100 tons of cargo. It would need ~100 tons of propellant to reduce its velocity by 4166 m/s (15,000 km/h).
If we can assume an empty ship of 100 tons mass, then it needs ~70 tons of propellant.
Those final tons of propellant hit much harder than the initial ones.
The above scenarios don't account for landing propellant or a reserve. The landing propellant mass is apparently under 10 tons.
Note that a propellant depot on orbit would require V3 tankers. If a Starship can put only 100 tons of cargo in LEO, and needs at least 70 tons of propellant to brake like this, then the tanker would need almost all of its own cargo to reenter. If a V3 tanker can take 200 tons, then that would mean getting 130 tons to the depot, with 70 tons used for reentry. It's a pretty costly way to operate.
I wonder if they'll try gluing on all the tiles. Or ceramic velcro or something. three steel pins per tile must be a good chunk of the mass of the system.
Ben W
Chess Grandmaster (Supervised)
Check your calculations? With Raptor v3’s anticipated 382s vacuum ISP, and 200T final mass (including landing fuel + reserve), I get ~408T of propellant needed to decelerate by 4166m/s. So a bit less than half a tank, but still a lot.
Attaching the tiles directly to each other (via wires at their attachment points, say, not gluing edges together) might help a single loose tile stay in place. (If it coming off anyway would be catastrophic, there’s not much downside to this.) Fabricating the tiles as a “metamaterial” that gradually and continuously changes composition from the outside (PicaX) to the inside (e.g. steel, or at least something more securely fastenable to steel) might also help. Biological structures do this all the time.
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Weird. Not sure where I went wrong there. You're correct. If 100 tons of landing mass, it'll need 200 tons of propellant. If 200 tons of landing mass, it'll need 400 tons of propellant.
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.
Why would you want an ablative?
Ben W
Chess Grandmaster (Supervised)
Brain fart, I meant HRSI ceramic. My point was that if connecting the current tile material directly to steel seems unreliable (as evidenced by tiles continuing to break/fall off), then tapering the material from HRSI on the outside to something tougher on the inside (perhaps via 3D-printing) might be a workable approach.
However, having a thin ablative layer underneath the HRSI tiles might still provide just enough additional protection to survive reentry in the event a tile or two does fall off. I’ll be interested to see what final design they come up with.
This is untrue, at least as a sequitur.
Deceleration does not add any heat to the spacecraft. Stress, yes. Heat, no.
Slower deceleration will result in more time at high speed and so results in more heat, not less.
This was such a "cool" novel idea- a pity it didn't come to pass
Ben W
Chess Grandmaster (Supervised)
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?)
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.
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.
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