https://twitter.com/cygnusx112/status/1072541873724710913QuoteThe CBI truck backed up to the #Falcon9 and connected into the rear. They must be draining something before moving to the transporter. #SpaceX
The CBI truck backed up to the #Falcon9 and connected into the rear. They must be draining something before moving to the transporter. #SpaceX
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. The gas generator also contributes a small amount of thrust, and it is off center. So the combined thrust vector can actually have a slight effect on cancelling roll.
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.
Doing The Math On The Spinning Falcon 9 BoosterScott ManleyPublished on Dec 11, 2018How much does deploying the landing legs change the rotation?Let's run the numbers.
Quote from: Scylla on 12/11/2018 10:14 pmDoing The Math On The Spinning Falcon 9 BoosterScott ManleyPublished on Dec 11, 2018How much does deploying the landing legs change the rotation?Let's run the numbers.Shorter Scott Manley for those who don't watch the video:Leg deployment roughly doubles the roll moment of inertia.So leg deployment by itself will reduce roll rate by about half.(But increased MOI from leg deployment reduces effectiveness of RCS roll thrusters)Graphical analysis of video shows RCS thrusters are in fact reducing roll rate *before* leg deployment.Then leg deployment further reduces roll rate by increasing MOI.But roll rate is not fully nulled before legs hit water.Legs hitting water may have been the final factor in nulling the roll rate.Hope I got that more or less correct. Thanks, Scott.
>>> But increased MOI from leg deployment reduces effectiveness of RCS roll thrustersThis scratched my ears first time when I watched Scott's video and now it is on NASA forum.IMO, this is not correct.Remember momentum conservation law.The speed of rotation has slowed down 1/2x, because MOI doubled after legs were deployed, so for the body to keep the same momentum, the speed of rotation should be halved.RCS should fight against F9's momentum. It does not matter if legs are deployed or not. The momentum is the same (well provided we do not have any more forces adding to rotation, which in case of deployed legs is probabaly negative, that is the legs are slowing rotation down due to air resistance, and then due to water resistance).My point is: RCS effectiveness in roll elimination have not changed with legs deployment.
If the control system didn't take that into account, they'd get unexpected spin during engine operation.
Quote from: graywolf on 12/12/2018 01:53 pm>>> But increased MOI from leg deployment reduces effectiveness of RCS roll thrustersThis scratched my ears first time when I watched Scott's video and now it is on NASA forum.IMO, this is not correct.Remember momentum conservation law.The speed of rotation has slowed down 1/2x, because MOI doubled after legs were deployed, so for the body to keep the same momentum, the speed of rotation should be halved.RCS should fight against F9's momentum. It does not matter if legs are deployed or not. The momentum is the same (well provided we do not have any more forces adding to rotation, which in case of deployed legs is probabaly negative, that is the legs are slowing rotation down due to air resistance, and then due to water resistance).My point is: RCS effectiveness in roll elimination have not changed with legs deployment.Welcome to the forum! The mass is conserved but the distribution has changed and so has the angular velocity. The RCS will lose some effectiveness do to having to act from a shorter lever arm relative to the tip of the extended legs...https://www.grc.nasa.gov/www/k-12/airplane/angdva.html
Quote from: Rocket Science on 12/12/2018 02:36 pmQuote from: graywolf on 12/12/2018 01:53 pm>>> But increased MOI from leg deployment reduces effectiveness of RCS roll thrustersThis scratched my ears first time when I watched Scott's video and now it is on NASA forum.IMO, this is not correct.Remember momentum conservation law.The speed of rotation has slowed down 1/2x, because MOI doubled after legs were deployed, so for the body to keep the same momentum, the speed of rotation should be halved.RCS should fight against F9's momentum. It does not matter if legs are deployed or not. The momentum is the same (well provided we do not have any more forces adding to rotation, which in case of deployed legs is probabaly negative, that is the legs are slowing rotation down due to air resistance, and then due to water resistance).My point is: RCS effectiveness in roll elimination have not changed with legs deployment.Welcome to the forum! The mass is conserved but the distribution has changed and so has the angular velocity. The RCS will lose some effectiveness do to having to act from a shorter lever arm relative to the tip of the extended legs...https://www.grc.nasa.gov/www/k-12/airplane/angdva.htmlNo, graywolf is correct as far as he goes. Assuming a fixed quantity of stored momentum, it will take the same amount of RCS total angular impulse (torque x time) to zero out that angular momentum.The only difference between legs closed or legs extended, in the ideal case, is that with legs closed the stage is rolling faster with lower MOI, and with legs extended the stage rolls slower with higher MOI. But since total angular impulse remains the same (MOI x roll rate), in each case the RCS thruster will need a burn of the same duration to cancel out the spin.The RCS thrusters don't know nor care "where" the angular momentum is stored on the stage. But as long as that quantity remains fixed, a fixed torque applied for a fixed duration will null it out.
Quote from: graywolf on 12/12/2018 01:53 pm>>> But increased MOI from leg deployment reduces effectiveness of RCS roll thrustersThis scratched my ears first time when I watched Scott's video and now it is on NASA forum.IMO, this is not correct.Remember momentum conservation law.The speed of rotation has slowed down 1/2x, because MOI doubled after legs were deployed, so for the body to keep the same momentum, the speed of rotation should be halved.RCS should fight against F9's momentum. It does not matter if legs are deployed or not. The momentum is the same (well provided we do not have any more forces adding to rotation, which in case of deployed legs is probabaly negative, that is the legs are slowing rotation down due to air resistance, and then due to water resistance).My point is: RCS effectiveness in roll elimination have not changed with legs deployment.Leg touching water, that I obviously buy.Maybe that's why one leg was lost, too.Did they ever fish it out? I mean they have the exact surface coordinate, and a good estimate of surface currents...-----ABCD: Always Be Counting Down
Yes, I think we all understand high school physics. can we move on now?
