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What does Starship currently use for attitude control thrusters?

The attitude of ITF-3 Starship appeared out of control from engine shut-down, on through re-entry.

According to How does the Falcon 9 upper stage settle its propellants?

<Falcon 9 > uses the cold nitrogen gas RCS thrusters. … Elon Musk has talked about using the ullage gas vents as RCS thrusters and for propellant settling as well. … Super Heavy will not have separate RCS thrusters, it will use ullage gas instead. SpaceX might do the same with Starship.

I cannot find a description of the RCS thrusters on the current incarnation of Starship. I assume that "using ullage gas" means venting boil-off as cold gas. Or is it heated before discharge? Or even ignited?

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  • $\begingroup$ I'm not sure this is settled design. In 2021, Musk said the orbital flight tests would be using hot-gas methalox thrusters for attitude control, but then he backed down from that position, saying hot gas was an "unnecessary complication" for flight testing, so I have no idea whether the current bunch are using methalox or cold N2. $\endgroup$ Mar 15 at 19:58
  • $\begingroup$ I recall that a few years back when Tim Dodd did one of his Starbase tours with Elon, the idea of using ullage gas (instead of dedicated RCS thrusters) for attitude control came up. Elon seemed to think that it was a good idea that they had not considered before. $\endgroup$
    – phil1008
    Mar 15 at 23:51
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    $\begingroup$ @phil1008 - if that's the interview I'm thinking of, Elon was explaining that they planned to use ullage thrust for control on the booster. Tim Dodd then confirmed "only on the booster, right?", at which point Elon paused and thought for a moment and then said yes, but that's something we need to think about. Elon later said that it was Tim's question that led him to think about using it also on Starship which apparently up until then they had not been planning. EDIT - now I see that Jörg has put that in his answer. $\endgroup$ Mar 16 at 11:10

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The Starship prototypes up to and including SN15 used cold-gas nitrogen thrusters. There has never been an official or unofficial statement with more details, but I always assumed those were just the ones used on Falcon 9 because … why not‽ After all, they used Tesla Model Y batteries and motors for the flaps.

However, this was always just a stop-gap solution. One of the goals of the Starship system is to simplify the architecture, and having another propellant on the vehicle runs counter to that. The plan was to develop methalox hot-gas thrusters.

One dummy article was spotted around Starbase years ago, likely for fit checks and I believe at least one prototype had been spotted.

However, in a 2021 Starbase tour with Tim Dodd, the Everyday Astronaut, Elon Musk mentioned that they were using ullage gas thrusters on the Super Heavy booster and only using the methalox thrusters on Starship. When Tim Dodd asked why not also use the ullage gas thrusters on Starship, Elon Musk started to answer why it doesn't make sense but realized halfway through the answer that it actually does make sense. Since then, the methalox hot-gas thrusters were never seen again and never mentioned anywhere.

Shortly after this interview, it could be observed that the location and shape of the ullage vents on both Super Heavy and Starship changed significantly. If you look at the ullage vents now, they have a distinctly shaped nozzle and many of them are arranged in the tell-tale three-axis pattern. Some of them were also moved away from the center of gravity where there is a larger lever arm.

So, the best guess we have now is that both the Super Heavy booster and Starship use ullage gas thrusters. Note that this is not unheard of. In fact, Elon Musk mentioned that SpaceX actually uses the passivization vent of the Falcon 9 upper stage to help reorient it for disposal, by orienting the rocket in the right way before opening the valve.

You might have seen a couple of joke posts on 𝕏 during the last couple of hours, saying it is Tim Dodd's fault that Ship 28 tumbled out of control during re-entry: that interview is what they are referring to.

I assume that "using ullage gas" means venting boil-off as cold gas.

It is working like a cold-gas thruster in that there is no combustion and no chemical reaction. It's just opening a valve on a pressure vessel. However, the gas is not actually "cold". In fact, you want your ullage gas to be as hot as possible without compromising the structure of the tank wall.

So, the gas is hot, but it's not a hot-gas thruster (which essentially refers to a thruster with a combustion chamber), it operates like a cold-gas thruster (which essentially refers to the operating principle of a bottle rocket – a pressure vessel with a valve).

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    $\begingroup$ JörgWMittag - I will have to try and find it again, but I happened to watch part of Tim Dodd's recent Starship coverage where he recounted this story, and ironically he said that it was by no means his intention to suggest using ullage thrust on Starship, he said in fact he was actually skeptical of the idea at the time and was hoping that they wouldn't use it on the ship and was just confirming that this wasn't the case. $\endgroup$ Mar 16 at 11:19
  • $\begingroup$ Tim Dodd also asked during the interview, "Did you say it on Twitter that you're gonna eliminate the cold gas thrusters or hot gas thrusters on the B4, for the first orbital test?" So, there may be more information on Twitter. $\endgroup$
    – phil1008
    Mar 19 at 0:42

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