Low priority work on solutions that seem non-favoured can result in the ability to rapidly pivot if things change.
Or you could go with Occams razor that they've accepted PICAX can't hack it for upper stage recovery without unacceptable loss of payload and are going with another solution.
PICA-X is really light weight. So unless they assume it would ablate too much for rapid reusability, it should be fine.
Over a 58m long 9m wide cylinder that's a bit over 9 tonnes for a 1 inch layer.
Quote from: john smith 19 on 05/10/2018 10:24 pmOver a 58m long 9m wide cylinder that's a bit over 9 tonnes for a 1 inch layer.It is only 48 meters long and tapered towards the front. In return we have a small amount of wing surface that also needs to be protected. PICA-X is a little lighter than the original PICA at 0.2 grams per cm3.I am assuming an average thickness of 4 cm. The whole ship wont need the full 4 cm of thickness. In return one can probably assume a thicker layer around the bottom half of the ship.With all this in mind, I am estimating a very conservative 9.5 tonnes for the whole spaceship covered in PICA-X with an average thickness of 4 cm. I think that is pretty decent at 12% of the dry mass. I normally would assume about 20% of the dry mass to go to TPS. So there is still some margin there.
Quote from: Elmar Moelzer on 05/11/2018 12:02 amQuote from: john smith 19 on 05/10/2018 10:24 pmOver a 58m long 9m wide cylinder that's a bit over 9 tonnes for a 1 inch layer.It is only 48 meters long and tapered towards the front. In return we have a small amount of wing surface that also needs to be protected. PICA-X is a little lighter than the original PICA at 0.2 grams per cm3.I am assuming an average thickness of 4 cm. The whole ship wont need the full 4 cm of thickness. In return one can probably assume a thicker layer around the bottom half of the ship.With all this in mind, I am estimating a very conservative 9.5 tonnes for the whole spaceship covered in PICA-X with an average thickness of 4 cm. I think that is pretty decent at 12% of the dry mass. I normally would assume about 20% of the dry mass to go to TPS. So there is still some margin there.So about 11.1% of the structural mass of the BFS. Again the question is wheather it's good enough to cope with dissipating a combined kinetic and potential energy per Kg of structure that's about 20x the F9 booster copes with.And AFAIK it can't.
Why are you comparing it with F9 booster at all? You should compare it with Dragon.
Quote from: Eerie on 05/12/2018 06:23 pmWhy are you comparing it with F9 booster at all? You should compare it with Dragon.Did you not know that the BFS is basically a stage with wings (whose nearest resemblance is to the space shuttle) while Dragon is basically a cone which enters base first?So yes the F9 US is the relevant item to compare BFS with in terms of shape and surface area. And both have a lot of surface area to protect.Which is where the mass per unit area (Kg//m^2) starts to become important. What's acceptable over 14m^2 may not be at 678 m^2.
Quote from: john smith 19 on 05/19/2018 07:01 amQuote from: Eerie on 05/12/2018 06:23 pmWhy are you comparing it with F9 booster at all? You should compare it with Dragon.Did you not know that the BFS is basically a stage with wings (whose nearest resemblance is to the space shuttle) while Dragon is basically a cone which enters base first?So yes the F9 US is the relevant item to compare BFS with in terms of shape and surface area. And both have a lot of surface area to protect.Which is where the mass per unit area (Kg//m^2) starts to become important. What's acceptable over 14m^2 may not be at 678 m^2.The difference is velocity. This alone makes them incomparable.
In fact BFS's closest match is either Shuttle or the X37b,with all the issues that implies.
Pica-X comes in tiles. I believe this is one major reason why Pica was was not selected for the Orion heat shield. The tiles are much larger though and should have less issues dropping off. Also with a mostly symmetric shape the number of different tiles will be much smaller than with the Shuttle.
Quote from: guckyfan on 05/26/2018 12:05 pmPica-X comes in tiles. I believe this is one major reason why Pica was was not selected for the Orion heat shield. The tiles are much larger though and should have less issues dropping off. Also with a mostly symmetric shape the number of different tiles will be much smaller than with the Shuttle.No, the tiles are not symmetric. Thats because the capsule enters at an angle and the center of influx air is not at the center of the capsule.Look at the Dragon heat shield:https://i.imgur.com/s7yORbX.jpghttps://forum.nasaspaceflight.com/index.php?topic=41016.msg1593168#msg1593168
Quote from: Semmel on 05/29/2018 10:59 amQuote from: guckyfan on 05/26/2018 12:05 pmPica-X comes in tiles. I believe this is one major reason why Pica was was not selected for the Orion heat shield. The tiles are much larger though and should have less issues dropping off. Also with a mostly symmetric shape the number of different tiles will be much smaller than with the Shuttle.No, the tiles are not symmetric. Thats because the capsule enters at an angle and the center of influx air is not at the center of the capsule.Look at the Dragon heat shield:https://i.imgur.com/s7yORbX.jpghttps://forum.nasaspaceflight.com/index.php?topic=41016.msg1593168#msg1593168I was talking about the BFS heatshield. Possibly your argumentapplies to that as well. But I would expect that there would be a large number of mostly identical tiles at least on the cylincrical part of the body.Edit: Maybe that was not quite clear because I mentioned the Orion heatshield.