At what point in the flight do the legs deploy on a Falcon 9R landing?
Originally they were described as opening fairly early, to help avoid spinning the stage, (cause of first landing failure on Cassiopia launch).
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Looking at the videos posted of the landing, in the last 10 seconds or so of flight the legs extend at least halfway, or at least until they are parellel to the engine exhaust region.
Only in the last second or three as it approaches the ground (or barge) do they fully extend. The extending of he legs can be shown in these two pictures, where you can see it change from partially open to fully open just before touch down:
Likely this is done to protect them from the hot exhaust exposure any longer than needed.
But it does seem like they need 4 valves (one per leg) to work twice instead of just once and adds a level of complexity and possible failures.
You can also see that in this sped up GIF of the landing.
Elon Musk tweeted that by the 21st launch or so, they hope to make more use of the legs as aero surfaces. This implies earlier opening in later missions.
There is a big mess about landing gear (legs). In an early statement Elon Musk, say that legs may be used as stabilizers (fins):
"This particular stage was not equipped with landing gear, which could have helped stabilize the stage like fins would on an aircraft"
The role of legs as airbrakes has been also conceptualized. However, air brakes in front of the vehicle will make it statically unstable. Opening the legs at higher velocity will possibly destroy the rocket. It is very unusual to place the air brakes in front of craft.
Although, the legs can’t contribute significantly to deceleration, they can contribute to loss of stability. Because of its specific deign the opening of legs is loosely controllable. A long pneumatic cylinders actuated by highly pressurized helium opened beautifully designed flaps used as landing gear by sliding a metal piston end down the groove in flaps. This arrangement makes opening speed to depend on drag of incident flow and friction in groove.
As result, asymmetrically opened legs may be observed on this video. Along with their unusually large cross section and negative dihedral, that symmetry may contribute to loss of stability.
Observed ‘double stack’ opening is caused most likely by specific kinematics of the opening mechanism.
Elon Musk tweeted that around Falcon flight 21 they will be changing to be more used as aero surfaces, implying early opening.