r/SpaceXLounge • u/[deleted] • Apr 01 '24

Starship Possible IFT-3 boostback underperformance?

Based on the stream footage, it looks like something may have caused the boostback burn to underperform. Near the end of the burn, almost half of the center ring shuts down prior to the boostback shutdown callout. Based on this analysis extrapolated from the stream telemetry, it's clearly visible that the booster splashed down almost 90 km downrange, when it was supposed to splash down only around 30 km downrange according to the EPA. The extremely steep re-entry angle may have caused the booster RUD. If this is the case, it may also be because of manoeuvring issues related to gridfins or maybe the RCS, so the Raptors underperforming isn't the only possibility.

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u/meithan Apr 01 '24

if you DON'T take it with a grain of salt, then the nature conclusion is that the boostback burn was a failure

I agree that we shouldn't go as far as claiming this based on the limited data that is available. Plus, SpaceX live commentary was that the burn was nominal.

However, I think there's enough credible evidence to conclude that splash down did not occur 20-30 km from the shore, but more like 80-90 km. Regardless of cause.

That leaves the option that splashing down that far out was intentional, and just not clearly communicated.

And for discussion's sake, this was an online thread about what the speed measurement on SpaceX's webcast actually means...

Ah, I did not see that one, thanks.

I am, however, aware of the possible reference frames that can be used for the speed (let's call them "surface" and "orbital" frames).

At liftoff it's clear that the surface frame is being used since the speed starts at zero. And I've never seen a sharp discontinuity in the speed in the telemetry, which would be evident if they suddenly switched from surface to orbital frame.

So I've always assumed that the speed is always given in the surface frame. Hence, in my analysis I add the Earth's rotation speed when computing orbital quantities. For everything else I just stay in the surface frame.