13 thoughts on “What Happened To SN11”

    1. I have no idea if the FTS was activated, or even if the FTS can do anything more than cause an engine shutdown.

      If the avionics are only partially gone but the fight control computers are still running, you could have some dead inputs and outputs (the input sensors are essentially random), leading the flight computer to go a bit crazy with the outputs.

      For example, say the tank pressure sensor inputs died, causing the controller to open the tank pressurization valves, leading to a catastrophic tank overpressure event.

      Another potential possibility is whether something like the TVC system can rip things apart if the position sensors fail and the overtravel sensors fail. On a regular piece of machinery, the overtravel sensors would wired as normally closed contacts in series with the output to the motor starter, such that an erroneous output wouldn’t result in physical damage to the equipment. But since they’re using a Tesla motor to run the hydraulics, it wouldn’t surprise me if they relied purely on software for everything, especially since hardwired safeties still wouldn’t prevent the engine bells from crashing into each other.

      It takes a slightly different mindset to design the electrical system on the assumption that the programmers are nuts enough to try to destroy the vehicle, but sometimes it protects against a whole lot of unforeseen situations. Sometimes you can take that down to the mechanical level, such as using pressure regulators, vents, and spring actuated valves to ensure that a tank can’t be over-pressured no matter what the flight software tries to do, similar to the way they added a burst disk to the tanks to protect against a catastrophic rupture even if the entire electrical system dies.

    2. No, by “avionics system” failure they mean sensor failure (fried by heat) led to software not understanding that an engine was burning up and should have been shut down permanently rather than shut down and restarted.

  1. I wonder if either Musk or Bezos gave any consideration to anhydrous ammonia before choosing methane and LNG respectively. As far as I recall, the only ammonia/lox engine ever to fly was the XLR-99 on the X-15, and it could run up to an hour between cleanings due to no coking. That’d be 40 or 50 flights of a first stage (Merlin 1D has to be cleaned between every flight). A lot harder to make ammonia on Mars, but not impossible.

    1. I doubt they would even be allowed to use anhydrous ammonia because they’re crashing the vehicle in what is essentially a residential neighborhood. When there’s an industrial ammonia leak the evacuation zone is very large.

      1. I used to work with pressurized chlorine gas. We had space suits. Anhydrous ammonia runs about $500 a ton right now, which is another issue. But as far as evacuations go, which is worse, liberating 6,000 tons of methane and liquid oxygen or 6,000 tons of ammonia and liquid oxygen? The US produces 6.5 million tons of anhydrous ammonia every year, so it’s not a big problem to work with.

        I remember I was driving along the highway with a dumb but sexy girlfriend I had years ago. She noticed we were passing a tanker truck with a lot of danger stickers on it, and wondered what was inside. “Hydrogen.” She actually screamed. It turned out she didn’t know there was a difference between a hydrogen bomb and just any old hydrogen gas.

  2. I read a book a while ago by a rocket fuels guy who worked on the XLR-99. They had lots of trouble with it with rough combustion.
    The guy was once on an airplane around this time andstruck up a conversation with his fellow passenger in the next seat and told him how the damn thing was going to kill someone. The guy then introduced himself as Scott Crossfield.

    1. Ammonia only burns under certain conditions, it was an early rocket engine, and the first and only flight model ammonia/lox (ammolox?) engine. Raptor is the first FFSC flight engine ever, and also having teething issues. It’d be interesting to know how an FFSC ammonia engine would perform.

      I think it’s time orbital launches of reusable orbital rockets went inland.

      1. I ran some rough numbers on an ethanol/LOX FFSC engine, using a higher grade ethanol than the V-2. It should have a higher ISP than the gas-generator cycle Merlin, and on a significantly cheaper fuel (based on bulk prices). On Mars, ethanol would be far easier to store than liquid methane.

        Interesting Nature paper on direct photocatalysis of methane into ethanol using copper on carbon nitride, instead of the high-pressure, high-temperature route.

        Ethanol would also raise fewer environmental and storage issues, and detanking after landing would be easier because you could just use a spigot, punch bowls, and college kids.

  3. Was it Rand himself here who once said “nobody in their right mind will ever build another ammonia/LOx rocket” ?

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