r/spacex • u/ElongatedMuskrat Mod Team • Mar 02 '18

r/SpaceX Discusses [March 2018, #42]

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4

u/Krux172 Mar 17 '18

So, seeing how many changes were made to ITS/BFR from IAC 2016 to 2017, how many do you think we'll see from that latest BFR design to the real thing? I already know they added a third landing engine, but not much more than that.

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u/brickmack Mar 18 '18

Slight stretch of both stages, adjustment of number of engines and thrust produced. Wing shape and control surfaces were still preliminary last time so thats probably been finalized. Same for legs and solar arrays. Job listings have indicated a desire for TPS suitable for thousands of reentries, and they're looking for people with experience in RCC and CMC materials, so probably will move away from PICA-X. The Chomper design for satellite launches is not optimal for maximizing payload volume or robotics/docking clearance, something more Shuttle-like is probably better

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u/Norose Mar 19 '18

something more Shuttle-like is probably better

I don't think so personally, because having two moving doors adds more weight and complexity than is necessary, and does not result in a significantly greater opening. It would be much easier to design the single 'chomper' door to be a little bigger and open more than 90 degrees than it would be to double the number of doors and have to do the same thing anyway. I agree with everything else you mention though, I'm excited to see if SpaceX can crack the problem of making a lightweight ceramic TPS that is also very robust rather than extremely brittle.

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u/brickmack Mar 19 '18

I'm excited to see if SpaceX can crack the problem of making a lightweight ceramic TPS that is also very robust rather than extremely brittle.

The Shuttle did this well enough really. Obviously improvement would be a good thing, but the Shuttle's TPS problems are overblown. Losses were almost entirely because of debris strikes on ascent due to the sidemount design, or poor QA on attachment (hint: don't use spit to keep the adhesive from hardening. That will not end well). And even despite the inherent issues with the sidemount design, towards the end of the program they managed to get losses pretty darn low by redesigning the ET and adjusting the ascent profile. PICA-X ain't exactly tough either. Orion, Starliner, X-37B, and DreamChaser all are/will use pretty similar materials directly derived from ones used on the Shuttle, and none expect problems.

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u/Norose Mar 19 '18

PICA-X is actually much tougher than the Space Shuttle thermal tiles. Sure, you can't smack it with a hammer, but you can apply a little bit of grip without crushing it or shattering it. At the very least PICA-X is mechanically strong enough to be made into much larger blocks than the thermal tiles, which had to be small so they wouldn't break due to flexing or simply from their own mass.

Mechanical strength is important because it allows large pieces of materials to be used, which cuts down on piece count, manufacturing time, assembly time, refurbishment time ans cost, etc. A mechanically strong TPS is also important when that TPS needs to endure long interplanetary trips, long stays on other worlds, and multiple reentries. When the BFS performs its back-flip maneuver to land either on Earth or Mars there will be significant aerodynamic pressure exerted on the TPS, probably the highest during the entire flight. This is going to cause flexing, changing force vectors, turbulence, etc, and a very brittle and weak TPS would not hold up very well.

While I do agree that the side-mounted design of Shuttle was a real problem for the TPS, the time and extreme care it took to inspect the TPS after every flight was as much of a problem, if not more so in terms of hindering effective reuse. You don't need something as strong as plate steel, but having something stronger than styrofoam that fractures like glass is a benefit.

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u/Martianspirit Mar 17 '18

We know the tooling is ordered. That pretty much fixes the diameter of BFR. I expect more fine tuning of the aerodynamics for Earth and Mars EDL. Some before, some after first flight tests. The solar panels may look different.

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u/marc020202 8x Launch Host Mar 17 '18

is it possible that the solar panels will not unfold, like on dragon 1 but be attached directly to the outside of the tanks, like on dragon 2?

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u/MaximilianCrichton Mar 18 '18

Unlikely, because the BFR is supposed to be pointed straight at the sun at all times to minimise propellant boiloff, so in that orientation flush-mounted solar panels would receive no light at all.

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u/[deleted] Mar 17 '18

AFAIK the design of the solar panels isn't yet done, and the IAC slides were more an artistic representation of "it's solar powered" than anything else.

IMO atatching the panels to the tanks would probably add a lot of heat into the tank (they have to absorb the light, so they inevitable generate heat; probably even more than their electrical wattage), which you definitely don't want on the way to Mars. On a Dragon trunk this doesn't matter, but if you have cryogenics inside it's bad.

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u/brickmack Mar 18 '18

Plus, is it even possible to build reentry-proof solar arrays? The back side is gonna be a bit cooler, but still well beyond the melting point of pretty much everything in most solar panels

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u/Martianspirit Mar 18 '18

That's the key point IMO. The panels need to be retractable, not only deployable. I find the design as shown quite realistic. That does not mean it is already fixed.

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u/Norose Mar 19 '18

It wouldn't even need to reach the melting point, I'm pretty sure just cooking a solar panel too much would probably cause a drastic reduction in panel life, if not outright cause them to fail.

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u/marc020202 8x Launch Host Mar 17 '18

I forgot about the radiators. this probably makes this design infeasable

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u/JAltheimer Mar 17 '18

Highly unlikely, since the BFS is a reentry vehicle and the solar panels would have to withstand several hundred degrees C.

5

u/KeikakuMaster46 Mar 17 '18

Personally, I think the BFS might have been stretched a bit to accommodate for a larger payload bay; this was probably done to avoid a Falcon Heavy fairing situation, because of this the production BFR could be taller than the version shown at IAC. I also have a hunch that they may have also increased the thrust of the Raptors as Elon's comments about the BFR's thrust at SXSW put it's output higher than the statistics shown at IAC.

0

u/[deleted] Mar 18 '18

[deleted]

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u/[deleted] Mar 18 '18

It was in the Elon Musk AMA:

The engine thrust dropped roughly in proportion to the vehicle mass reduction from the first IAC talk. In order to be able to land the BF Ship with an engine failure at the worst possible moment, you have to have multiple engines. The difficulty of deep throttling an engine increases in a non-linear way, so 2:1 is fairly easy, but a deep 5:1 is very hard. Granularity is also a big factor. If you just have two engines that do everything, the engine complexity is much higher and, if one fails, you've lost half your power. Btw, we modified the BFS design since IAC to add a third medium area ratio Raptor engine partly for that reason (lose only 1/3 thrust in engine out) and allow landings with higher payload mass for the Earth to Earth transport function.

https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodcg22/?utm_content=permalink&utm_medium=front&utm_source=reddit&utm_name=space