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The things I can't understand from what Elon tweeted are:

"engine was low on thrust due (probably) to partial helium ingestion"

There are only tanks for liquid oxygen and CH4 - where did the helium come from?

"If autogenous pressurization had been used, CH4 bubbles would most likely have reverted to liquid"

Also I can't comprehend this:

"the helium ingestion was likely the result of a pressurization system that had been added to the CH4 header tank to correct an error that had occurred previously with the SN8"

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  • $\begingroup$ You're not the only one. Fuel and oxidizer are fed into the combustion chamber with turbopumps, not with pressure. As with a common sink, the drain must be at the (gravitational) "bottom". Aerodynamic acceleration forces can move the "bottom" away from the drain, resulting in gas ingestion. A stabilized vertical approach, rather than flipping with engines lit, may be helpful. $\endgroup$ Mar 14 '21 at 14:19
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    $\begingroup$ I know you are looking at a deeper understanding, but when I saw "explain the SN10 landing failure in laymans' terms," I felt the most accurate description was it bounced! $\endgroup$
    – Cort Ammon
    Mar 14 '21 at 18:26
  • $\begingroup$ @RobertDiGiovanni Turbopumps can produce a lot of pressure, but only as much underpressure (suction) as the pressure in the tank they pull from - can't decrease pressure below 0 bar. That limits the amount of fuel they can pump, due to creating cavitation on the intake side (pumping vacuum instead of fuel). To prevent that, the tanks must be pressurized. $\endgroup$
    – SF.
    Mar 15 '21 at 23:14
  • $\begingroup$ @SF agree there, but the pressurizing gas need only replace the volume of liquid drawn. (Real interesting stuff out there on the Atlas "balloon" tanks too). Real key is to get stable flow for full power landing. A little worried about fuel "pogo", as increased Gs will also alter fuel flow (in an unstable fashion, the more brake Gs, the more fuel flow). Getting it stable upright with a 'chute isn't glamorous, but it may be the most practical way to get a consistent burn. $\endgroup$ Mar 15 '21 at 23:48
  • $\begingroup$ @RobertDiGiovanni Don't underestimate the flow requirements of space engines. Saturn V's F1 used 20 tons of fuel per second. Think how wide your pipelines must be to draw 20 tons (so about 20 cubic meters) per second without causing cavitation and with just enough gas to replace the liquid - let's say at ~1 bar. This simply becomes unsustainable really fast, you just need extra pressure in the tank to provide the "push" on top of the "pull" of the turbopump to keep the fuel supply sufficient. $\endgroup$
    – SF.
    Mar 22 '21 at 11:53
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SpaceX plans to use autogenous pressurization for Starship. Autogenous pressurization means using the same consumable that is in the tank in its gaseous state to pressurize the tank. So, for the liquid methane tanks, you use gaseous methane for pressurization, etc.

The landing failure of Starship SN8 was caused by low pressure in the methane header tank, according to Elon Musk:

Fuel header tank pressure was low during landing burn, causing touchdown velocity to be high & RUD, but we got all the data we needed! Congrats SpaceX team hell yeah!!

As a quick fix, SpaceX switched from autogenous pressurization to using Helium as the pressurant:

SN9 will press CH4 header tank with helium. Long-term solution is under debate. Not clear what is lightest/simplest.

Helium pressurization is used by many rockets, including Falcon 9. For Starship, however, SpaceX is trying to

  • aggressively reduce the number of different consumables required and
  • only choose consumables that can easily be replenished on Mars by in-situ resource utilization.

For comparison, Falcon 9 with Dragon uses RP-1 as fuel, liquid oxygen as oxidizer, Helium as pressurant, TEA-TEB as igniter, some form of hydraulic fluid for thrust vectoring and grid fin control, and nitrogen for the cold-gas attitude control thrusters for Falcon 9, and Monomethylhydrazine as fuel and Dinitrogen tetroxide as oxidizer for the Dragon. That's eight different consumables of which only oxygen can be easily produced on Mars. Starship only uses methane as fuel and liquid oxygen as oxidizer. Pressurization is intended to be autogenous, ignition is done via spark ignition, the body flaps are actuated with electric motors (from a Tesla, actually), and the attitude control thrusters (which are yet to be developed) will be hot-gas thrusters derived from the Raptor, thus also using methane and oxygen.

At least, that's the plan. At the moment, pressurization is done with Helium and the attitude control thrusters haven't been developed yet, so they are currently using nitrogen cold-gas thrusters. (Probably the ones from Falcon 9, it wouldn't make sense to use anything else.)

Unfortunately, it appears that it was exactly the quick fix of using Helium pressurization which then caused the landing failure of SN10:

SN10 engine was low on thrust due (probably) to partial helium ingestion from fuel header tank. Impact of 10m/s crushed legs & part of skirt. Multiple fixes in work for SN11.

Chris Bergin from NASASpaceflight.com pointed out that the Helium pressurization system which caused the failure of SN10 was added to fix the failure of SN8:

This is a tricky one given that I believe said helium pressurization was added to the CH4 header tank to mitigate what happened with SN8.

To which Elon Musk replied that if methane had been used for pressurization, the bubbles would probably not have caused a problem:

Fair point. If autogenous pressurization had been used, CH4 bubbles would most likely have reverted to liquid.

He further explained that the specific problem the Helium pressurization system was meant to fix was to prevent "ullage collapse from slosh":

Helium in header was used to prevent ullage collapse from slosh, which happened in prior flight.

Elon Musk also took responsibility for the decision:

My fault for approving. Sounded good at the time.

Tim Dodd the Everyday Astronaut asked whether future Starships will have slosh baffles:

Are there baffles in future designs to prevent slosh?

Elon Musk replied there were already slosh baffles at least in SN10 but that one of the slosh baffles may actually have caused the problem:

There were baffles, but one may have acted like a straw to suck bubbles in from above liquid/gas level.

Elon Musk further remarked that SpaceX had actually seen something similar with Falcon 1:

Something similar happened on an early Falcon 1 flight, resulting in unexpectedly high liquid oxygen residuals at main engine cutoff.

Interestingly, shortly after the test of SN10, Elon Musk remarked that SpaceX had never seen the behavior before:

Thrust was low despite being commanded high for reasons unknown at present, hence hard touchdown. We’ve never seen this before.

It is quite interesting that after a longer investigation, it turned out that they had in fact seen something similar before in one of their very first tests ever.

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    $\begingroup$ Starship really is a fascinating engineering job, thanks for the all the details. $\endgroup$
    – DrMcCleod
    Mar 14 '21 at 12:01
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    $\begingroup$ Thank you so much...Now it's started to make sense $\endgroup$
    – Maharshi
    Mar 14 '21 at 15:45
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    $\begingroup$ Jorg, slight problem getting gas and liquid of the same substance in the same tank unless it is right at its boiling point (or under some pressure, in that case its warmer). Seems helium is better to accommodate colder LNG. I just wonder if it could be the same turbopump drawing from a narrower header pipe = lower thrust $\endgroup$ Mar 14 '21 at 17:49
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    $\begingroup$ If I look at the Wikipedia article on Falcon-1, it appears Musk is talking about the third launch, which had the first stage unexpectedly reignite during separation, ending the mission. $\endgroup$
    – Nzall
    Mar 15 '21 at 11:56
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Jorg W Mittag gave an excellent answer but missed a term: ullage collapse.

In normal human experience fuel sits on the bottom of its tank, you can draw fuel from the bottom of the tank without difficulty. However, in space flight things aren't so friendly. If you're not in atmosphere and not burning a rocket your fuel floats. You try to pump from the bottom of the tank and you probably get nothing.

The standard approach is to have some sort of rocket that isn't vulnerable to this problem that can be used to settle the fuel before lighting the main rocket—this is called ullage (and the burn is an ullage burn.) For spacecraft this is typically done by using the attitude control rockets which are normally designed not to have gas in the tank at all—not hard to do with small tanks, a big problem with the massive tanks of a booster. With the Saturn rockets it was done by having "small" solid rocket boosters (small only in comparison, they were large compared to what hobbyists normally send up) that were lit first before lighting the second and third stages. I haven't paid attention to how the Falcon 9 upper stage handles this.

Note that the Starship comes down on its side, while it's in atmosphere there's no issue of the fuel floating there is the issue of the fuel flowing away from the pipe that draws it in so they still have a problem with getting the fuel to flow as it lights and turns.

What he's saying is that during that burn the engine sucked in a bit of gas from the tank rather than the liquid it was supposed to be sucking. The problem was related to the fuel sloshing around in the tank from the turn.

If you suck up a few bubbles along with the drink in the bottom of the cup it's no big deal. When your engine sucks up some bubbles with the fuel it's supposed to be drinking it doesn't put out enough power.

I don't know why it made any difference whether it sucked helium or methane, whether it can burn or not doesn't mean you suck enough of it to land your rocket.

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    $\begingroup$ I believe what Elon Musk is saying is that when suck in a bubble of gaseous methane, which obviously has the same condensation point as the surrounding liquid (because it's also methane), then the bubble would condense while/before being sucked in. Not sure how that works from a physics standpoint, but that's how I interpret Elon Musk's statement. $\endgroup$ Mar 14 '21 at 9:12
  • $\begingroup$ @JörgWMittag I can't see it collapsing that fast. $\endgroup$ Mar 14 '21 at 21:56
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    $\begingroup$ A methane bubble would definitely collapse when it's compressed by the turbopump. However, that might be too late as ingesting a methane bubble into the turbopump means that too little methane is ingested. As such, I'd guess that Elon Musk means that the methane bubble would collapse before coming even close to the tank outlet. This would be due to the pressure and low temperature of the subcooled liquid surrounding the bubble. The gaseous methane would condense very quickly at the bubble surface, and the gas inside the bubble is not replenished by anything. $\endgroup$ Mar 15 '21 at 9:55
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    $\begingroup$ Actually, fuel floating in the tank has nothing to do with the atmosphere (or lack of it) and everything to do with weightlessness. Try throwing a half-full (half-empty will do if you prefer) water bottle up in the air, the contents will float away from the bottom as it reaches the top of the trajectory, just like it does for any rocket. $\endgroup$
    – TooTea
    Mar 15 '21 at 15:24
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    $\begingroup$ @TooTea the atmosphere provides resistance, and thus a force on the falling object, in contrast to weightlessness. $\endgroup$ Mar 15 '21 at 16:35
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The crash of Starship SN8 on landing was blamed on lack of pressure to the fuel (methane) from the header tanks (smaller tanks used in landing). SpaceX addressed this by adding a system to pressurize the header tanks with helium (Falcon 9 also uses helium to pressurize its fuel tanks, see Why does the Falcon 9 require a helium pressurization system?). This was intended to be a stop-gap measure for Starship before a more sophisticated autogenous pressurization system was developed (since the intention is to make Starship re-fuelable on Mars, where helium will be hard to come by). "Autogenous" means that the gas used to pressurize the fuel is derived from the fuel itself. Musk is saying that some of the helium used for pressurization got into the fuel lines; since helium is inert this reduced combustion in the engine and therefore thrust. If methane gas were used to pressurize the liquid methane in an autogenous system, it wouldn't have this effect on combustion.

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  • $\begingroup$ Autogenous: self-created. $\endgroup$
    – RonJohn
    Mar 16 '21 at 15:26
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Remember this is an early prototype. SpaceX are still learning how best to deal with the design challenges and are still moving fast and braking things to allow them to quickly figure out the best solution. These prototypes are just tests and experiments and they learn a lot from the video and telemetry data received. Although SpaceX do calculations and make models they are no replacement for real world tests.

SN10 had gaseous helium tanks added to the nose cone as a temporary measure while they were trying to gain a better understanding and more control of the landing. With hindsight it might have been better to stick with autogeneous pressurization, but they did not know that at the time.

It’s often a good idea when initially learning how things work to have a simplistic model. In this case imagine a small model of Starship made of clear plastic. It’s easy to imagine what would happen to water inside the model and how it might flow from the main tank to the header tank and down into the engines if it was turned from horizontal to vertical. This conveys the general idea but utterly fails at the detail level for many reasons.

Starship is many orders of magnitude bigger than a plastic model and mass, volume and the motions of liquids don’t scale as you might expect.

Starship contains two cryogenic liquids rather than water. Both at higher temperature relative to their boiling points than room temperature water is compared to its boiling point.

Starship is pressurized to several bar pressure

During the final few seconds of landing Starship is far removed from equilibrium in temperature and pressure and it’s having a variable angular acceleration applied to it.

To make the simplistic model above more realistic it’s necessary to add some complexity (and yes I know the description below is not that realistic but hopefully it conveys some vague idea of the difficulties). Imagine the plastic rocket has been filled with hot pressurized mineral water. Now throw it in the air and when it comes down suddenly twist it vertical with the neck down. Then just before it hits the ground apply more pressure and quickly take the cap off. Question do any gas bubbles come out in the liquid stream or is pure liquid propelled out by the gas pressure now above it?

Now try this with something the height of Sydney Harbor Bridge that weighs more than 100 tones.

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