5 ms·

> using 4 rockets Looks like 8 to me, 4 groups of 2.

by mbell 12y ago

Mithaldu 12y ago
You're right, each group has a pair and i was going by what i saw in the animation. :)
mikeash 12y ago
Paired for redundancy. If one fails, the other one can take up the slack.
- peeters 12y ago
  So when Musk said it can recover from the loss of two engines, is that any two or only if the two are not in the same cluster?
  - mhandley 12y ago
    Watching the video again, I see that the SuperDracos are mounted pretty high up in the sides. Given the fuel tanks, Draco manouvering thrusters, life support system and so on are mounted lower down or in the base, plus the weight of the heat shield, I suspect the SuperDraco's are mounted above the centre of mass of the Dragon. This means it would be statically stable on just two opposite clusters. Four SuperDracos have 64,000lbs of thrust, which should be plenty. So, yes, I think it could cope with losing two engines in the same cluster.
    - mikeash 12y ago
      The height of the engines compared to the center of mass doesn't actually influence stability. The thrust vector turns with the vessel, so it's neutrally stable whether the engines are above or below. This is called the pendulum rocket fallacy: http://en.wikipedia.org/wiki/Pendulum_rocket_fallacy http://en.wikipedia.org/wiki/Pendulum_rocket_fallacy But don't feel bad, as you're in good company: Robert Goddard made the same mistake.
  - mikeash 12y ago
    Excellent question. I was wondering the same thing. It seems like it must be two engines that aren't in the same pair. If two engines in the same pair went, you'd have to cut off the opposite two to maintain symmetry, and then you're down four. If it could handle losing four engines, you'd think he would have said that.... On the other hand, maybe they can lose four engines as long as their opposite each other, and he didn't say that because it's not any four. It's not completely implausible. If each engine runs at 50% normally, then increasing the remaining four to 100% would give you the same thrust. Attitude control would be the big challenge, but perhaps smaller thrusters are able to handle it. Edit: I just realized that the landing thrusters double as the launch escape system. The LES needs really high thrust to get the fragile meaty cargo as far away from the exploding rocket as quickly as possible. The Soyuz LES produces accelerations of well over 10 gees, for example. If these engines are capable of that, then they must have huge margins, as far as thrust goes, for landing. Looking over reports, I must have missed it in the stream, but Musk said that each one generates 16,000lbs of thrust. While the capsule as a whole weighs well under that... and it has eight engines. It also looks like the engine pairs are angled apart somewhat, which should allow for some control even on only two opposite pairs. I'm going to go ahead and guess that it can tolerate losing any two engines, and up to four if it's lucky about which ones it loses.
    - jonny2112 12y ago
      I am pretty sure it was 1600 not 16,000. Still impressive given their size.
      - mikeash 12y ago
        It would have to be 16,000 to achieve the necessary thrust for a workable LES with 8 engines, and this SpaceX filing confirms it as 16,400lbs: http://www.faa.gov/about/office_org/headquarters_offices/ast/media/20140513_DragonFly_DraftEA_Appendices%28reduced%29.pdf http://www.faa.gov/about/office_org/headquarters_offices/ast...
    - InclinedPlane 12y ago
      Yup, same engines. The escape system is an integral part of the vehicle (rather than a bolt on system as with tractor abort) and is regularly used and tested. And because the escape system is always there it means they can abort at any point during launch all the way up to orbit, which is fairly unprecedented. I believe they can generate 8 gees of thrust with the escape system which would allow them to escape even if the rocket doesn't get shut down during an abort.
  - jccooper 12y ago
    When rocket scientists say "any two" they usually mean it. It can probably tolerate more, if they fail just right, but you can't assume that. And it seems plausible. The layout of the SuperDraco clusters is not equidistant; it's more of an "X". If you lose one cluster, the other nearby one can provide the makeup thrust without being too unbalanced, and there's still X and Y control authority from the other pair.
  - deleted 12y ago
    [deleted]

> using 4 rockets Looks like 8 to me, 4 groups of 2.

by mbell 12y ago

> using 4 rockets Looks like 8 to me, 4 groups of 2.

Mithaldu 12y ago

You're right, each group has a pair and i was going by what i saw in the animation. :)

mikeash 12y ago

Paired for redundancy. If one fails, the other one can take up the slack.

peeters 12y ago
So when Musk said it can recover from the loss of two engines, is that any two or only if the two are not in the same cluster?
- mhandley 12y ago
  Watching the video again, I see that the SuperDracos are mounted pretty high up in the sides. Given the fuel tanks, Draco manouvering thrusters, life support system and so on are mounted lower down or in the base, plus the weight of the heat shield, I suspect the SuperDraco's are mounted above the centre of mass of the Dragon. This means it would be statically stable on just two opposite clusters. Four SuperDracos have 64,000lbs of thrust, which should be plenty. So, yes, I think it could cope with losing two engines in the same cluster.
  - mikeash 12y ago
    The height of the engines compared to the center of mass doesn't actually influence stability. The thrust vector turns with the vessel, so it's neutrally stable whether the engines are above or below. This is called the pendulum rocket fallacy: http://en.wikipedia.org/wiki/Pendulum_rocket_fallacy http://en.wikipedia.org/wiki/Pendulum_rocket_fallacy But don't feel bad, as you're in good company: Robert Goddard made the same mistake.
- mikeash 12y ago
  Excellent question. I was wondering the same thing. It seems like it must be two engines that aren't in the same pair. If two engines in the same pair went, you'd have to cut off the opposite two to maintain symmetry, and then you're down four. If it could handle losing four engines, you'd think he would have said that.... On the other hand, maybe they can lose four engines as long as their opposite each other, and he didn't say that because it's not any four. It's not completely implausible. If each engine runs at 50% normally, then increasing the remaining four to 100% would give you the same thrust. Attitude control would be the big challenge, but perhaps smaller thrusters are able to handle it. Edit: I just realized that the landing thrusters double as the launch escape system. The LES needs really high thrust to get the fragile meaty cargo as far away from the exploding rocket as quickly as possible. The Soyuz LES produces accelerations of well over 10 gees, for example. If these engines are capable of that, then they must have huge margins, as far as thrust goes, for landing. Looking over reports, I must have missed it in the stream, but Musk said that each one generates 16,000lbs of thrust. While the capsule as a whole weighs well under that... and it has eight engines. It also looks like the engine pairs are angled apart somewhat, which should allow for some control even on only two opposite pairs. I'm going to go ahead and guess that it can tolerate losing any two engines, and up to four if it's lucky about which ones it loses.
  - jonny2112 12y ago
    I am pretty sure it was 1600 not 16,000. Still impressive given their size.
    - mikeash 12y ago
      It would have to be 16,000 to achieve the necessary thrust for a workable LES with 8 engines, and this SpaceX filing confirms it as 16,400lbs: http://www.faa.gov/about/office_org/headquarters_offices/ast/media/20140513_DragonFly_DraftEA_Appendices%28reduced%29.pdf http://www.faa.gov/about/office_org/headquarters_offices/ast...
  - InclinedPlane 12y ago
    Yup, same engines. The escape system is an integral part of the vehicle (rather than a bolt on system as with tractor abort) and is regularly used and tested. And because the escape system is always there it means they can abort at any point during launch all the way up to orbit, which is fairly unprecedented. I believe they can generate 8 gees of thrust with the escape system which would allow them to escape even if the rocket doesn't get shut down during an abort.
- jccooper 12y ago
  When rocket scientists say "any two" they usually mean it. It can probably tolerate more, if they fail just right, but you can't assume that. And it seems plausible. The layout of the SuperDraco clusters is not equidistant; it's more of an "X". If you lose one cluster, the other nearby one can provide the makeup thrust without being too unbalanced, and there's still X and Y control authority from the other pair.
- deleted 12y ago
  [deleted]