4 ms·

Hydraulic systems are usually closed, can anyone explain why hydraulic fluid is expendable in this case?

by duffyt 12y ago

fogleman 12y ago
An open hydraulic system is more lightweight.
- modeless 12y ago
  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.
  - pshc 12y ago
    I'm guessing you'd need more pistons, pumps, piping, etc. in a closed system. Probably need to repressurize the fluid.
  - TeMPOraL 12y ago
    Pumps and pipes. With an open system, you don't need to recompress the fluid and move it back, and from what I read, that hardware weights a lot.
    - bdonlan 12y ago
      Not to mention the power supply - what's going to run the pumps when the engines are shut off?
    - toomuchtodo 12y ago
      Correct! Instead of using compressor(s) and recirculation plumbing, you're using an accumulator [1]. Weight and complexity savings. [1] http://en.wikipedia.org/wiki/Hydraulic_accumulator http://en.wikipedia.org/wiki/Hydraulic_accumulator
  - rikf 12y ago
    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"
garindra 12y ago
From one of his tweets: > Hydraulics are usually closed, but that adds mass vs short acting open systems. F9 fins only work for 4 mins. We were ~10% off. https://twitter.com/elonmusk/status/554023312033341440 https://twitter.com/elonmusk/status/554023312033341440
- fit2rule 12y ago
  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.
  - marvin 12y ago
    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.
    - StavrosK 12y ago
      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?
      - fit2rule 12y ago
        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.
deleted 12y ago
[deleted]
hartror 12y ago
Detailed speculation on Reddit: https://www.reddit.com/r/spacex/comments/2s1lq9/my_guess_about_the_hydraulic_system/ https://www.reddit.com/r/spacex/comments/2s1lq9/my_guess_abo...
jccooper 12y ago
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.

Hydraulic systems are usually closed, can anyone explain why hydraulic fluid is expendable in this case?

by duffyt 12y ago

Hydraulic systems are usually closed, can anyone explain why hydraulic fluid is expendable in this case?

fogleman 12y ago

An open hydraulic system is more lightweight.

modeless 12y ago
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.
- pshc 12y ago
  I'm guessing you'd need more pistons, pumps, piping, etc. in a closed system. Probably need to repressurize the fluid.
- TeMPOraL 12y ago
  Pumps and pipes. With an open system, you don't need to recompress the fluid and move it back, and from what I read, that hardware weights a lot.
  - bdonlan 12y ago
    Not to mention the power supply - what's going to run the pumps when the engines are shut off?
  - toomuchtodo 12y ago
    Correct! Instead of using compressor(s) and recirculation plumbing, you're using an accumulator [1]. Weight and complexity savings. [1] http://en.wikipedia.org/wiki/Hydraulic_accumulator http://en.wikipedia.org/wiki/Hydraulic_accumulator
- rikf 12y ago
  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"

garindra 12y ago

From one of his tweets: > Hydraulics are usually closed, but that adds mass vs short acting open systems. F9 fins only work for 4 mins. We were ~10% off. https://twitter.com/elonmusk/status/554023312033341440 https://twitter.com/elonmusk/status/554023312033341440

fit2rule 12y ago
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.
- marvin 12y ago
  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.
  - StavrosK 12y ago
    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?
    - fit2rule 12y ago
      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.

deleted 12y ago

[deleted]

hartror 12y ago

Detailed speculation on Reddit: https://www.reddit.com/r/spacex/comments/2s1lq9/my_guess_about_the_hydraulic_system/ https://www.reddit.com/r/spacex/comments/2s1lq9/my_guess_abo...

jccooper 12y ago

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.