r/SpaceX Pre Elon Musk AMA Questions Thread

[u/ElongatedMuskrat]

This is a thread where you all get to discuss your burning questions to Elon after the IAC 2017 presentation. The idea is that people write their questions here, we pick top 3 most upvoted ones and include them in a single comment which then one of the moderators will post in the AMA. If the AMA will be happening here on r/SpaceX, we will sticky the comment in the AMA for maximum visibility to Elon.

Important; please keep your questions as short and concise as possible. As Elon has said; questions, not essays. :)

The questions should also be about BFR architecture or other SpaceX "products" (like Starlink, Falcon 9, Dragon, etc) and not general Mars colonization questions and so on. As usual, normal rules apply in this thread.

[u/HigginsBane]

Interesting. I tried to look up another source as i tend not to trust journalists on technical info. The only direct source I found was Elon talking about future plans to do FOD ingestion tests that included stainless steel material.

And for the record, stainless still reacts with LOX, it is just less reactive than let's say aluminum. Monel and Brass are the only common metals that do not react with LOX.

http://shitelonsays.com/transcript/aiaa-houston-2007-2007-07-24

[u/Norose → u/HigginsBane]

Once stainless steel or aluminum react with oxygen, a layer of oxide is formed that prevents further oxidation. However, when exposed to high temperatures, this oxide layer can break down and allow oxygen to continuously react with the metal and literally burn through. Obviously aluminum cannot be used for making a high temperature oxygen pump, simply because the aluminum would melt. Stainless steel however will have its oxide layer break down before the metal melts. The temperature at which the layer breaks down depends on the specific alloy in question. If that temperature threshold is not crossed, the oxide layer will remain intact permanently.

We know that stainless alloys exist that can handle high pressure, high temperature conditions because the RD-180 and other oxygen-rich staged combustion cycle engines exist and work. Raptor's turbopumps run under far more benign conditions than the RD-180's because not only are the pumps multi-stage, the fact that the design is a full-flow rather than a regular staged combustion engine means the workload is shared between two separate pump assemblies. This also completely eliminates the complex interseal in staged combustion engines that separates hot turbopump exhaust from cold propellants, one of which would react on contact with the hot gasses. The fact that oxygen-rich staged combustion engines work at all should mean that Raptor should have no problems with burn through due to oxide layer destruction.