That would have been interesting to simulate. Would that result in a gas bubble forming at the top of the downcomer, to be compressed by the methane when it violently reseats? Is compressed gas in the downcomer any harsher than pressurized liquid? Obviously, having such a gas bubble reach the engines would be catastrophic, but is it any worse while in the downcomer?
I think the cavitation would occur toward the bottom where there is the most mass of fluid pulling upward. Should revert rapidly. Downcomer could suck gas from tank during the event which wouldn't get to the engines until some time later...
Yeah, I was thinking about the latter. The flow would create a void at the entrance to the downcomer, then the reseated methane might trap a bubble of gas. It would have to travel the length of the downcomer, and may very well be broken up into lots of small bubbles, but it wouldn't be good when it arrived at the engines.
Ironically, boosters and second stages regularly "cross their streams". Beautiful effect.
+1.
I’m guessing SpaceX’s priority is the opposite order of your list (not that you listed them in priority). For this first hot staging attempt they probably were over cautious about not hitting Starship. Now that they have real data they can start to improve the maneuver to meet all three goals.
Good analysis. Another way to look at it: SpaceX's simulations for the hot staging must have had Starship initially separating from Booster at some particular rate, say 0.6 g. "All" that's needed (in quotes because it's an enormously difficult problem) is to adjust the thrust profiles to more closely match that trajectory. The problem isn't that the separation would then happen more slowly, because they would be adjusting it to happen at the same rate as initially modeled. (In other words, the separation in IFT-2 almost certainly happened _more quickly_ than anticipated, so slowing it back down to the originally modeled rate is probably a good thing, not a bad thing.) Also, to the extent this slowing down could be accomplished by reducing Starship's initial thrust rather than increasing Booster's thrust, that would be less stress on the hot staging ring than initially modeled, not more. The whole problem seems vaguely similar somehow to rapidly shifting gears with a stick-shift. Easy to get wrong at first with an unfamiliar car, but once it's dialed in it should become second nature.
You seem to be suggesting a cooler staging technique. Do you believe that's preferable or are you just pointing out that they could back off Starship's thrust to solve the problem? The goal of hot staging is to retain velocity, so I would think that SpaceX wants to have vehicle thrust levels as high as can be managed.
I haven't heard anything official. We saw the ship venting something, and LOX levels were dropping fast. So it's assumed it was venting LOX, and the big question is why that happened. They didn't lose any engines at the time of LOX venting, so something else caused it.
Or quick disconnect leak that got worse.
Just pointing out that they could. To the extent it allows them to make the hot staging ring less massive, it may turn out to be a performance win. But mostly just observing that there are multiple ways for SpaceX to mitigate the hot staging failure without requiring a hardware redesign. As soon as the separation occurs, the more of Booster's fuel that can be reserved for boostback/landing, the better. So if Booster uses one ton less fuel during hot staging, that's one more ton of fuel it could apply pre-staging to accelerate Starship, and that's a win too. (Just before separation, ~3/4 of the mass of the stack is Starship.)
I watched the footage again. The venting started on the left and was sustained. Later, there's a puff on the right, and then all the venting stops. LOX and CH4 use seems to stabilize, but then they shut down the engines and trigger the FTS shortly afterwards. Note that the ship was accelerating right until engine shutdown. I didn't do any numbers to see if the acceleration was dropping off. I'm assuming that the engines were nominal.
| Mission Time | Oxygen loss/burn rate | |
| 5:32 | 2.72 tons/second | Stable flight |
| 7:07 | 2.61 tons/second | Throttled down, venting starts |
| 7:40 | 3.06 tons/second | Puff on right, but still venting |
| 7:55 | 3.06 tons/second | Venting stops |
JB - appreciate all your analysis and participation.I watched the footage again. The venting started on the left and was sustained. Later, there's a puff on the right, and then all the venting stops. LOX and CH4 use seems to stabilize, but then they shut down the engines and trigger the FTS shortly afterwards. Note that the ship was accelerating right until engine shutdown. I didn't do any numbers to see if the acceleration was dropping off. I'm assuming that the engines were nominal.
The part that bothers me is that it appears to vent from opposite sides. That makes no sense because the QD and vents are on one side.
Come to think of it, it would take some pretty catastrophic autogenous pressurization failure to dump that much LOX.
As I have too much time on my hands, I tried to calculate the oxygen loss rate from the video. Here's the timeline I came up with
Timestamp Oxygen loss/burn rate 5:32 2.72 tons/second Stable flight 7:07 2.61 tons/second Throttled down, venting starts 7:40 3.06 tons/second Puff on right, but still venting 7:55 3.06 tons/second Venting stops
If correct, then it was venting nearly a half ton of oxygen per second. Note that the second line in the table shows a reduction in oxygen flow due to having had the engines already throttled down for a while. In other words, the venting didn't start when the engines were throttled down.
Here's a chart from NASASpaceFlight's forums for the big picture, but without the raw numbers. It shows that Starship was accelerating right to the end. I don't see a reduction in engine performance, except as a result of throttling down.
I guess I have to add that the chart shows throttle-down happening after the initial venting, while I have a reduced flow rate before venting. shrug
I'm sure those guys want to live at work. It's what I did as an idealistic young software engineer. I'd certainly be doing it for SpaceX if I was young enough and good enough.
If Ship is generating less thrust, that's less thrust booster has to counteract while in close proximity to stay acceleration positive.
Hmmm I originally thought Starship engine lights were staggered, RVacs first followed by the SL Raptors, but re-watching the HUD on the webcast, they all show lighting at once...
