4 ms·

Making it a little puzzling ask to why the engines didn't try harder to lower the vertical velocity on approach. Perhaps there are improvements to be made in th

Making it a little puzzling ask to why the engines didn't try harder to lower the vertical velocity on approach. Perhaps there are improvements to be made in the control systems.

fleitz 12y ago
I have a feeling their engineers may have considered this. This is after all rocket science.
- darkmighty 12y ago
  I also have this feeling, but from my limited experience with control systems you might want to try the simplest first. There is a tremendous amount of variables to consider, and the analysis of how to take each into account is non trivial -- if you don't measure some variable precisely enough it might make the whole system less stable, the solution might get worse due to increased computational load, the conditioning might degrade, etc. So with the number of controls they have and the predictive models available, I'm sure they're actually scrambling to refine their systems right now.
  - will_hughes 12y ago
    The thrust to weight ratio of a single engine firing is so high that they have no option but to fire at the last moment. Firing for longer would result in the rocket going back up, it can't hover.
- TeMPOraL 12y ago
  You'd be surprised what Bill lets Jeb fly at times... ( ;) ).
toomuchtodo 12y ago
When near fuel exhaustion, the Merlin engine has a thrust to weight ratio above 1. Its firing is timed in order to land at a distance above the landing surface that will allow for a reusable vehicle (close enough to not snap off the landing legs). It carries only enough fuel to do this while also using that same fuel for hydraulically actuating grid fins for aerodynamic control while in a controlled fall to the ground. Stopping a first stage rocket traveling back to an ocean going barge from supersonic speeds with substantial lateral velocity is hard, no matter what engine is performing its suicide burn while coordinating with RCS thrusters for attitude keeping.
- justin66 12y ago
  > When near fuel exhaustion, the Merlin engine has a thrust to weight ratio above 1. Wouldn't it have a thrust to weight ratio well above 1 all the time? The ratio will be highest just as it's about to run out of fuel. About 160 to 1?
  - jmgao 12y ago
    That's a thrust to weight ratio above 1 when only one of its 9 engines are firing, and running at minimum throttle to boot.
  - will_hughes 12y ago
    For the final stage of landing only a single Merlin engine is lit. They're also limited in how much they can control the throttle on it. Even at minimum throttle, it's not possible for it to hover.
    - justin66 12y ago
      Odd responses. I guess I shouldn't have phrased my response in the form of a question. Here's another try: > When near fuel exhaustion, the Merlin engine has a thrust to weight ratio above 1. The rocket will always have a thrust to weight ratio well above 1.
      - peeters 12y ago
        I still don't understand the point of your comment then. Yes, the rocket's (potential) TWR will be above 1 at all points of the flight. What does that have to do with the comment you're replying to? The Falcon 9 has 9 engines. The combined thrust of all of them has to be greater than the weight of the fully loaded rocket. That doesn't mean a single engine will always have more thrust than the weight of the entire rocket.
      - deleted 12y ago
        [deleted]
      - 12y ago
TeMPOraL 12y ago
Vertical was fine, it's the residual lateral velocity that tipped over and killed the rocket. Also, I can't find the tweet now (I think it was deleted) but initially Musk said they determined that 'static friction in bipropellant valves introduced a lag into system response that the control system wasn't prepared to handle'.

Making it a little puzzling ask to why the engines didn't try harder to lower the vertical velocity on approach. Perhaps there are improvements to be made in the control systems.

fleitz 12y ago

I have a feeling their engineers may have considered this. This is after all rocket science.

darkmighty 12y ago
I also have this feeling, but from my limited experience with control systems you might want to try the simplest first. There is a tremendous amount of variables to consider, and the analysis of how to take each into account is non trivial -- if you don't measure some variable precisely enough it might make the whole system less stable, the solution might get worse due to increased computational load, the conditioning might degrade, etc. So with the number of controls they have and the predictive models available, I'm sure they're actually scrambling to refine their systems right now.
- will_hughes 12y ago
  The thrust to weight ratio of a single engine firing is so high that they have no option but to fire at the last moment. Firing for longer would result in the rocket going back up, it can't hover.
TeMPOraL 12y ago
You'd be surprised what Bill lets Jeb fly at times... ( ;) ).

toomuchtodo 12y ago

When near fuel exhaustion, the Merlin engine has a thrust to weight ratio above 1. Its firing is timed in order to land at a distance above the landing surface that will allow for a reusable vehicle (close enough to not snap off the landing legs). It carries only enough fuel to do this while also using that same fuel for hydraulically actuating grid fins for aerodynamic control while in a controlled fall to the ground. Stopping a first stage rocket traveling back to an ocean going barge from supersonic speeds with substantial lateral velocity is hard, no matter what engine is performing its suicide burn while coordinating with RCS thrusters for attitude keeping.

justin66 12y ago
> When near fuel exhaustion, the Merlin engine has a thrust to weight ratio above 1. Wouldn't it have a thrust to weight ratio well above 1 all the time? The ratio will be highest just as it's about to run out of fuel. About 160 to 1?
- jmgao 12y ago
  That's a thrust to weight ratio above 1 when only one of its 9 engines are firing, and running at minimum throttle to boot.
- will_hughes 12y ago
  For the final stage of landing only a single Merlin engine is lit. They're also limited in how much they can control the throttle on it. Even at minimum throttle, it's not possible for it to hover.
  - justin66 12y ago
    Odd responses. I guess I shouldn't have phrased my response in the form of a question. Here's another try: > When near fuel exhaustion, the Merlin engine has a thrust to weight ratio above 1. The rocket will always have a thrust to weight ratio well above 1.
    - peeters 12y ago
      I still don't understand the point of your comment then. Yes, the rocket's (potential) TWR will be above 1 at all points of the flight. What does that have to do with the comment you're replying to? The Falcon 9 has 9 engines. The combined thrust of all of them has to be greater than the weight of the fully loaded rocket. That doesn't mean a single engine will always have more thrust than the weight of the entire rocket.
    - deleted 12y ago
      [deleted]
    - 12y ago

TeMPOraL 12y ago

Vertical was fine, it's the residual lateral velocity that tipped over and killed the rocket. Also, I can't find the tweet now (I think it was deleted) but initially Musk said they determined that 'static friction in bipropellant valves introduced a lag into system response that the control system wasn't prepared to handle'.