How does a single engine counter roll? This was a single-engine landing, not a three-engine landing, unless I'm mistaken.
It's amazing watching the entire structure flex and bend from the torques being put on the grid finds and then the whole structure twist when it impacts the water but holds together! That's some incredible engineering!
Latest Elon tweet update-"Pump is single string. Some landing systems are not redundant, as landing is considered ground safety critical, but not mission critical. Given this event, we will likely add a backup pump & lines."
Quote from: ellindsey on 12/05/2018 06:38 pmHow does a single engine counter roll? This was a single-engine landing, not a three-engine landing, unless I'm mistaken.Single-engine landing burns allow for more control authority
Quote from: ZachS09 on 12/05/2018 06:42 pmQuote from: ellindsey on 12/05/2018 06:38 pmHow does a single engine counter roll? This was a single-engine landing, not a three-engine landing, unless I'm mistaken.Single-engine landing burns allow for more control authorityA single M1d engine provides no roll control authority.
I wonder if the control software could be modified so that when a fin freezes the fin on the opposite side can be placed in a position to dampen the action of the first one and then let the two other grids fins do all the work? There wouldn't be as much control but if it can avoid a roll then there may be enough to get the booster down.
Quote from: dlapine on 12/05/2018 05:51 pmLatest Elon tweet update-"Pump is single string. Some landing systems are not redundant, as landing is considered ground safety critical, but not mission critical. Given this event, we will likely add a backup pump & lines."Block 5.1
Does Falcon still have independent gas-generator nozzle actuation for the centre Merlin, or was that dumped in the move to the octaweb?
Quote from: ellindsey on 12/05/2018 06:38 pmHow does a single engine counter roll? This was a single-engine landing, not a three-engine landing, unless I'm mistaken.Air drag will slow it down to some extent and also the moment the legs pop out the moment of inertia suddenly increases (like a skater sticking their arms out in a spin) causing an instant de-spin effect.
Quote from: tonya on 12/05/2018 06:21 pmQuote from: sghill on 12/05/2018 05:52 pmQuote from: tonya on 12/05/2018 05:49 pmQuote from: EnigmaSCADA on 12/05/2018 05:43 pmEveryday Astronaut played a video of it a few minutes ago. Looked wild and then much calmer as landing burn started.It looked like the leg deploy reduced the roll quite swiftly, much like an ice skater but aerodynamic drag may have slowed it more than the shifting weight.As it slowed down, the stuck grid fins would have less authority (because there is not as much air flowing through them) and the RCS would have more authority. Makes sense that it looked more stable right as it neared a dead stop.That makes sense, and the legs are inline with the fins. As soon as the legs start to deploy, what little airflow there is over the fins will be turbulent. I couldn't see it in the video, but likely to be RCS that killed the roll.Transfer of momentum. When the legs deploy they transfer momentum slowing the roll. When that occurred the control system had enough authority to finish the roll dampening just before surface contact. It might have even survived a land landing.
Quote from: sghill on 12/05/2018 05:52 pmQuote from: tonya on 12/05/2018 05:49 pmQuote from: EnigmaSCADA on 12/05/2018 05:43 pmEveryday Astronaut played a video of it a few minutes ago. Looked wild and then much calmer as landing burn started.It looked like the leg deploy reduced the roll quite swiftly, much like an ice skater but aerodynamic drag may have slowed it more than the shifting weight.As it slowed down, the stuck grid fins would have less authority (because there is not as much air flowing through them) and the RCS would have more authority. Makes sense that it looked more stable right as it neared a dead stop.That makes sense, and the legs are inline with the fins. As soon as the legs start to deploy, what little airflow there is over the fins will be turbulent. I couldn't see it in the video, but likely to be RCS that killed the roll.
Quote from: tonya on 12/05/2018 05:49 pmQuote from: EnigmaSCADA on 12/05/2018 05:43 pmEveryday Astronaut played a video of it a few minutes ago. Looked wild and then much calmer as landing burn started.It looked like the leg deploy reduced the roll quite swiftly, much like an ice skater but aerodynamic drag may have slowed it more than the shifting weight.As it slowed down, the stuck grid fins would have less authority (because there is not as much air flowing through them) and the RCS would have more authority. Makes sense that it looked more stable right as it neared a dead stop.
Quote from: EnigmaSCADA on 12/05/2018 05:43 pmEveryday Astronaut played a video of it a few minutes ago. Looked wild and then much calmer as landing burn started.It looked like the leg deploy reduced the roll quite swiftly, much like an ice skater but aerodynamic drag may have slowed it more than the shifting weight.
Everyday Astronaut played a video of it a few minutes ago. Looked wild and then much calmer as landing burn started.
Quote from: StuffOfInterest on 12/05/2018 06:33 pmI wonder if the control software could be modified so that when a fin freezes the fin on the opposite side can be placed in a position to dampen the action of the first one and then let the two other grids fins do all the work? There wouldn't be as much control but if it can avoid a roll then there may be enough to get the booster down.That won't work. A hardover fin will not just cause a spin but also a pitching moment. If the opposite side fin tries to counter the spin, it will worsen the pitch. if it tries to counter the pitch, it will worsen the spin. Since pitching over while spinning makes things worse, (causing a tailspin like motion which probably stalls the gridfins and causes complete loss of control or breakup) it would have to counter the pitching moment, which worsens the spin, and then have the other two perpendicular fins try to counter that and spin the other direction.Issue with that is, then you have 4 fins in hardover and no control authority whatsoever. Its better to "life" with a bit of spin and have some control authority at least in the second set of perpendicular fins, so you can control the trajectoryremarkably, that seems to be what the stage did, as it never really went off course (if it had, AFTS would have needed to intervene) aside from the preprogrammed contingency-landing location just off the coast, which SpaceX mentioned in the past.
The only impact from NASA HSF Safety is that multiple flights with the new hardware fly demonstrating no interaction of the changes with any launch systems before DM-2.
Quote from: mlindner on 12/05/2018 06:40 pmIt's amazing watching the entire structure flex and bend from the torques being put on the grid finds and then the whole structure twist when it impacts the water but holds together! That's some incredible engineering!I think most of the bending is actually the camera not being firmly enough attached to the stage. (due to G-forces)
