Looking at the video (https://vine.co/v/euEpIVegiIx) I'm struck by the dramatic attitude changes. I wouldn't expect a problem with the throttle of the main engine to directly change the rocket's attitude and I don't see why the control system would respond to incorrect thrust by purposefully changing attitude. I'm therefore guessing that the malfunctioning valve is related to attitude control, i.e. either thrusters or the system that gimbals the main engine. But the thrusters are nitrogen gas (no biprop) and the main engine gimbaling is hydraulic (also not biprop) so what's the "biprop throttle valve"?
Elon tweeted "...Even w 1 lit, it can't hover, so always land at high g". I assume the "it can't hover" means there is not enough fuel left to hover. Or could it mean that a single engine can't throttle low enough to hover with the lightened first stage?
I envision a landing pad at sea or on land with numerous such mechanisms to secure the base of the rocket as its landing. Otherwise the rocket is too tall, and the legs with to small a diameter to assure landing in a majority of landing weather conditions, even with the valve issue solved.
The second one. A single Merlin, throttled down, produces more thrust then the weight of the empty rocket, by a pretty good margin.
cleaned that vine up a bit:
Don't you just hate "stiction" :-)
I was having trouble with the notion that there even is a "biprop throttle valve" anywhere on the F9 first stage. I had been thinking there were separate LOX and fuel valves feeding the pre-burner, right? But then google tells me:
More complicated than that. You'd need to compare telemetry/tracking with sims. Depends on where/when the deviation occurred. Remember that the stage bends - can act like a whip if you apply too much force at the wrong time. So even considerable increase isn't an instant answer.Once you find where the issue is (by matching a sim to actual), you rerun the sims with increased thrust til it begins to work, then you exhaustively simulate variations to prove you've adequately bounded the situation. Tedious.
It is unarguable that active guidance (the rocket doesn't need to communicate with the barge for that) is more precise, since by using absolute coordinates you add the errors in barge and rocket station navigation.
What pre-burner?Merlin is a GG cycle engine
Why can people understand that what is done on the barge will be the exact thing used on land?