This is the video of the Falcon 9 first stage attempting a landing on SpaceX's barge. Why is the rocket moving so fast at impact?
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1$\begingroup$ That video shows the same set of frames played back at differing rates. Do we know that the first playback is at 1:1 with the rate at which the frames were captured? $\endgroup$– Russell BorogoveJan 18, 2015 at 3:20
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$\begingroup$ I assume it, it's looks like the first is the normal speed. $\endgroup$– user1685367Jan 19, 2015 at 12:54
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$\begingroup$ The first seems sped up from what I recall of the official Instagram video (this is some Russia Today edit) $\endgroup$– Nick TJan 28, 2016 at 5:18
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3 Answers
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It is actually moving quite slow vertically. It's speed comes from the horizontal component -- which is a result of the vehicle trying (unsuccessfully) to correct its horizontal position. The vehicle was unable to accomplish this with the aerodynamic control surfaces as intended. The control surfaces that were responsible for this ran out of hydraulic fluid.
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The Falcon 9 can't hover--even one engine at minimum is too much thrust. Thus it must do what KSP calls a suicide burn.
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1$\begingroup$ The term 'suicide burn' has existed long before KSP was popular. $\endgroup$ Jan 17, 2015 at 20:35
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2$\begingroup$ @Antilogical Google isn't giving me any non-KSP uses other than stray hits from "...suicide, burn..." $\endgroup$ Jan 18, 2015 at 0:31
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$\begingroup$ Hover Slam immediately followed by an RUD! :-) $\endgroup$– Rory Alsop ♦Jan 19, 2015 at 16:32
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The video from the first and second barge landing attempts make it pretty clear that the stage comes in pretty fast.
Because the Merlin 1D engine can only throttle down to 70% and they land on a single (center) engine, so 70% of 165,000 lbs of thrust is about 115.5 klbs. From SpaceFlight101, it lists the mass of an empty first stage at around 23-25,000kg (lets call that 55,000 lbs).
So they can only go down as low as 115,000 lbs of thrust, and they mass on the order of 55,000 lbs, so they have way to much thrust at landing time.
They need to reach 0 m/s speed at 0 meters of altitude, in what SpaceX has called a "Hover Slam". They practiced it on the Grasshopper test vehicle, which they apparently had ballasted to allow for hovering in initial tests of the control systems.
Thus it really is coming in pretty fast. It is also decelerating pretty darn quick. Which is by design.
This is why most speculative BFR projections assume 5-9 Raptor engines, as you want to be able to land close to T/W of 1 if possible. So you want a nice symmetric single engine to land on, and 5, 7, 9, and 13 seem to work geometrically for that.
