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I had noticed this before, but when landing, the legs do not open at exactly the same time or speed.

For the CRS-13 mission, it was really obvious in this video around 50 seconds into the video.

CRS-13 first stage landing video

Clearly it does not matter, since this is the 20th successfully landed first stage, 16th in a row, so I am quibbling. But I wonder why the difference in speed?

Presumably the mechanism for all 4 legs is identical and operates the same. Failure to lock like in one of the ASDS landings would be terrible, but so long as the slowest leg opens in time it may not really matter.

enter image description here

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    $\begingroup$ Probably it is not only identical but the same - all legs driven by single pressure source? And the one farthest away or the stickiest one needs more time? $\endgroup$
    – jkavalik
    Commented Dec 17, 2017 at 1:43
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    $\begingroup$ Are you talking about the right-hand leg in the video? $\endgroup$
    – Russell Borogove
    Commented Dec 17, 2017 at 4:34
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    $\begingroup$ Pressure source is pneumatic (high-pressure helium), see space.stackexchange.com/questions/13591/… IDK if there's one common source or 1 bottle per leg $\endgroup$
    – Hobbes
    Commented Dec 17, 2017 at 11:06
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    $\begingroup$ At a root level (without explaining the mechanism of why), the real reason I suspect is that there isn't a need for them all to deploy synchronously, so it's just not a process control. $\endgroup$
    – Tristan
    Commented Dec 17, 2017 at 19:00
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    $\begingroup$ Every morning when I have coffee I come back to this question and admire my GIF (joke, it's only once a week). Today I noticed that it's one leg opening faster than the others, not one leg slower. What is it they say? Moi? a stickler? $\endgroup$
    – uhoh
    Commented Feb 3, 2018 at 0:34

3 Answers 3

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The most likely explanation seems to be that there is no need for the legs to open simultaneously. The only requirement is that they open and lock in place reliably prior to touchdown. Therefor no effort was expended ensuring precise synchronization of the leg movements, and they are allowed some fuzziness in timing.

Drag, minute differences in pipe length and resistance or imperfections in cylinder sizing will all potentially produce different deployment rates.

A system feeding four leg cylinders from a common gas supply would tend to extend the leg with the least resistance most rapidly, and as each leg reaches maximum extension and locks in place the pressure available for the remaining legs increases, ensuring that any leg that "sticks" at partial deployment will receive additional force to ensure it catches up to the others by landing.

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The Falcon 9 landing legs are actuated by pneumatic cylinders powered by tank(s) of high-pressure helium. My guess is that very slight pressure deviations in the cylinders cause the legs to fully deploy at different times.

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  • $\begingroup$ Isn’t a “guess” more suited for a comment? Do you have any sources to bolster this claim? $\endgroup$
    – marked-down
    Commented Jan 22, 2018 at 21:21
  • $\begingroup$ Sorry, I'm new to this website. However, I can't think of any other reason that the legs would open like that. :) $\endgroup$
    – user22645
    Commented Jan 22, 2018 at 21:30
  • $\begingroup$ (apart from the comments submitted prior to mine) $\endgroup$
    – user22645
    Commented Jan 22, 2018 at 21:37
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Supposedly Musk has stated that the legs are actuated by nested, telescoping pistons using high pressure helium, given that the system needs to be ultra light.

Gas actuation requires mechanical latching of each segment of the deployed telescopic cylinder to be rigid, but is lighter than the same volume of hydraulic fluids would when fully extended. Falcon however uses hydraulics (probably driven by pressurized gas) to actuate the Grid Fins, which need to be precise and lock firm at any position.

Gas drive is logical for leg actuation, but why helium? Cold gas thrusters on board already use compressed nitrogen. And helium is “the most difficult gas to seal” due to its very small atom size.

Like others I also think that the deployment speeds of legs are not so critical to worth the hassle to control. However Musk has also said that earlier concepts considered using legs as active aerodynamic surfaces. Grid fins are probably much better for that purpose (faster and need lower actuating force). The speed of legs opening is determined by the force imbalance on them.

The driving force from a pressurized gas tank is a function of displacement. Actuating losses and external aerodynamic loads act against the driving force and are likely to be different on all three legs.

I wonder if one of the first stages that tipped over after landing on the drone ship when one of the legs were collapsing was actually caused by a slow leg opening when the cylinder-stroke-latching cycle hadn't finished yet.

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