Quote from: Jakusb on 12/11/2018 09:24 amI understand how just 1 engine is not able to counter a spin as it can only target one direction at a time, but having it gimble in certain way (counter clockwise?) could it not help counter some spin? And what is the effect of the jet hitting the water? Would it not feedback in some way that might also kill any movement relative to the water? Or does the jet only give upward energy and no counter spin energy as feedback?How could a center engine gimbal counterclockwise?
I understand how just 1 engine is not able to counter a spin as it can only target one direction at a time, but having it gimble in certain way (counter clockwise?) could it not help counter some spin? And what is the effect of the jet hitting the water? Would it not feedback in some way that might also kill any movement relative to the water? Or does the jet only give upward energy and no counter spin energy as feedback?
Quote from: friendly3 on 12/11/2018 09:37 amQuote from: Jakusb on 12/11/2018 09:24 amI understand how just 1 engine is not able to counter a spin as it can only target one direction at a time, but having it gimble in certain way (counter clockwise?) could it not help counter some spin? And what is the effect of the jet hitting the water? Would it not feedback in some way that might also kill any movement relative to the water? Or does the jet only give upward energy and no counter spin energy as feedback?How could a center engine gimbal counterclockwise?Huh? The engine can gimbal in any direction. If you gimbal from center to north and then north east, then east, then southeast, then south- you are in effect gimballing counterclockwise, no?
That is indeed what I mean.. Also that energy should counter spin, would it not?
Quote from: Jakusb on 12/11/2018 09:50 amThat is indeed what I mean.. Also that energy should counter spin, would it not?No.Take a screwdriver, place it in a screw, and move it in a cone around the screw. It does not tighten or loosen the screw.
https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=46944.0;attach=1532170;sess=47262https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=46944.0;attach=1532172;sess=47262
He's actually right. You could use both the engine bell and the exhaust volume inside it as an impromptu "reaction wheel" by constantly actuating in a circular matter. And since the gas volume is being continuously expelled it even acts a bit like a yoyo despin-weight in fluid form, so it would be able to give a continuous rotational force without saturating. You rotate a gas volume relative to the core, then get rid of it. (The resulting gas jet would have spiral shape and expand away from the core)The effect is going to be miniscule and orders of magnitude below the aerodynamic effect of the grid fins or the effects achievable if the rotation axis goes NOT through the gimbal bearing. But if you were in vacuum and this is the only effect you can use, it might actually work.It has no practical application to this particular core landing, but I don't think you can completely neglect it in vacuum.
Quote from: CorvusCorax on 12/11/2018 11:10 amHe's actually right. You could use both the engine bell and the exhaust volume inside it as an impromptu "reaction wheel" by constantly actuating in a circular matter. And since the gas volume is being continuously expelled it even acts a bit like a yoyo despin-weight in fluid form, so it would be able to give a continuous rotational force without saturating. You rotate a gas volume relative to the core, then get rid of it. (The resulting gas jet would have spiral shape and expand away from the core)The effect is going to be miniscule and orders of magnitude below the aerodynamic effect of the grid fins or the effects achievable if the rotation axis goes NOT through the gimbal bearing. But if you were in vacuum and this is the only effect you can use, it might actually work.It has no practical application to this particular core landing, but I don't think you can completely neglect it in vacuum. What about the effect of the jet hitting (not rotating, thus counter moving) water?Would it move the water in rotating motion, or would it have no effect at all?If it would move the water, it would also feedback to the core, would it not?Or would the rotating energy of the jet hitting the water have no (significant) effect at all?
Yes, agreed on the observations.I can't find a good estimate on the force of the thrusters. It's v*dm/dt, and v is probably 500-600, but I can't find dm/dt. Given how quickly the thrusters flip a still-loaded stage around, they're not chump change, and 12 seconds is a lot of time. From this, I draw the same conclusion you did - that during most of the time, the fins overwhelm the thrusters.But the fins lose effectiveness towards the end (probably very non-linearly), and then both things happen - the legs deploy and cut the rotation speed as I calculated above, and the thrusters finally get their way.As for the main engine - as explained by several people above - when there are external forces on the booster (such as gravity, aerodynamic loads, etc) a center engine can absolutely affect axial spin. I think during that time, the fins are powerful enough that only something like the main engine can really fight them.As an aside - I'm surprised by how much spare capacity the RCS thrusters had.
I would be REAL respectful of a tank that was at 1K PSI. Just saying...
For a rocket in vacuum, a central engine can't affect spin.When there are other non axial forces, like gravity and fin forces, the central engine creates force pairs with all of then and so creates torques in all directions and affects axial spin.If the control system didn't take that into account, they'd get unexpected spin during engine operation.-----ABCD: Always Be Counting Down
If we're heading down the road of thruster thrust be mindful that its proportional (or at least related) to tank pressure and whatever pressure was available during the flip maneuver is likely to be significantly reduced by the time it gets close to landing in a normal landing and in this case where there was an ongoing battle between grid fins and thrusters I'd expect the pressure to be lower than what the designers had hoped for in their lowest pressure scenario. Or not, maybe the tanks are vastly larger than necessary.
Quote from: The Vorlon on 12/11/2018 02:57 pmI would be REAL respectful of a tank that was at 1K PSI. Just saying...Scuba tanks run 3 times higher than that and they get clanked around on docks and boat decks all over the world every minute of every day. Nothing special about 1,000 PSI per se. Only how robust the tank and fittings are (or aren’t) really matters.
Yesterday evening,There was an explosion of a dive tank on the Utila Cays,causing Mr Newton Diamond to lose his leg and Keily Pineda whom loss her life.Mr Newton suffered a massive amount of blood loss and needed to be taken to La Ceiba immediately. Utila is a small island and news travels very fast,and being our airport doesn’t have lights,everyone with a vehicle got together and went up on the airport to shine the way for the airplane to takeoff.He made it to La Ceiba safe where he was interned in the hospital for surgery. We’re happy to announce that he’s doing good,he will have a long and slow recovery but we have faith that he will be good.Keily was only 16 years old and she loss her life in this tragic accident. Her family is from the mainland but she’s born and raised on the Utila Cays and her body will be laid to rest here.We’d like to thank the entire community for coming together when called upon to help in time of need and also a special thanks to Captain Clint Gerner for your rapid response and generous contribution to saving Mr Newton’s life. We will forever be grateful for what you did for our community.Thank you all.-Utila 911 team
So if if we assume that Tesla thrusters would run at the same "ultra high pressure" as F9 thrusters, 1,000 lbf sounds like a reasonable minimum...