Is there a chance someone could explain this - how does not being able to move the fins (with hydraulics) translate into a hard landing - were they just off the target landing spot because the fins couldn't be used to fine-tune the descent to within a meter of the target landing zone, and thus they 'missed' the barge by a few meters, or is something else at play?
EDIT: never mind - I've learned that they have a steering function, as well as drag inducing function, and they ran out of hydraulic fluid required to induce more drag, which would thus slow the rocket down. Leaving my comment here in case anyone else is curious.
People on /r/spacex have theorized (with the aid of comments from Musk) that running out of hydraulic fluid caused the grid fins to get stuck in an extreme position, which forced the rocket off course. Cold gas thrusters and gimbaling of the engine tried to correct this, but the end result was that the rocket had insufficient control authority to achieve a controlled landing.
How is the rocket going to achieve a controlled landing with no thrust? It seems to me that it would crash into the pad anyway. Or was it that they did have thrust, they were just unable to steer because of the lost pressure?
Yes I think you've got it right - they had thrust from the rocket motors, but they ran out of hydraulic fluid required to use the fans to induce more drag, so the rocket came down hard.
Basically, because they lost steering it came in too steep, and ended up falling onto the deck on its side because the engines couldn't compensate for that position.
If the rocket's trajectory is correct, all that is required is a final hard burn to negate all downward velocity a short distance above the barge deck.
You can do away with the pump, pipes, and power supply.
Since the main propulsion isn't going, closed hydraulics would have to have an independent power supply. Probably batteries. Instead, they replace all that with some compressed air. They can run out of compressed air, but can also run out of whatever powers the pumps in a closed system.
The hydraulic fluid is probably RP-1 and is probably dumped into the fuel tank, where it's available for later burning. Another 2-for-1.
Where do the weight savings come from? Hydraulic fluid is heavy; it seems like you could save quite a bit of weight by carrying less of it if it wasn't expendable. Not that I don't believe it, just curious.
Seems like they actually use RP-1 (rocket fuel) as the hydraulic fluid so that it is burned as fuel after it has been used. This means that the hydraulic fluid isnt "dead mass"
