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#420
by
kevin-rf
on 04 Oct, 2013 11:40
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With the turbo pumps everyone seems to be forgetting the fact that you are constantly accelerating liquid inside of the pump, imparting spin to it. So even after startup you still have a constant torque from the fluid running through the pump being imparted on the system.
The turbine is not spinning inside a vacuum, if it was several parts would exit the housing at very high speed.
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#421
by
Jim
on 04 Oct, 2013 11:41
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no, the same thing will happen.
When he lets go of the cd player before he pushes it (1:09), it is starting to rotate.
The same thing happens with a gyroscopically stabilized binoculars.
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#422
by
Mike_1179
on 04 Oct, 2013 12:01
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Honestly, I thought this was high school physics.
A Helicopter has to constantly fight against rotation (with a tail rotor, for example) since the rotor is acting against the atmosphere, and so there is an external torque.
A spacecraft with a rotary component that's internal to itself is almost, but not entirely, absolutely unlike a helicopter. Torque is only applied when the spinning component changes rotational speed. Rev up your car, and if the engine is mounted lengthwise, you'll notice the car tilts. Momentarily. When the RPM changes.
The only way a rocket in steady state can gain spin is if there is a rotational component to the plume that is exiting the closed system. Or, if there's an interaction with the atmosphere. (Which might be affected by the presence of the plume)
If there is friction in the internal component then there is constant torque and transfer of momentum. The cage around a toy gyroscope ends up spinning with the gyro rotor.
I agree that's what the gyroscope does, but it's because the rotor is slowing down. If you add a motor, attached to the frame, that keeps the rotor at constant speed, then the cage will not rotate (after startup). There is indeed torque, in fact there are two torques, and they cancel. The rotor drag tries rotate the frame, but the motor applies an exactly opposite torque on the frame, while doing its work to keep the rotor at constant speed. Net result - no rotation of the cage.
You can't switch reference frames in the middle of a model.
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#423
by
LouScheffer
on 04 Oct, 2013 12:42
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With the turbo pumps everyone seems to be forgetting the fact that you are constantly accelerating liquid inside of the pump, imparting spin to it. So even after startup you still have a constant torque from the fluid running through the pump being imparted on the system.
This would be correct if the fluid was spraying out in all directions - that's exactly how a lawn sprinkler works. But in this case the casing of the pump collects the fluid and directs (hopefully towards the combustion chamber). The moving fluid exerts forces on the pump case, plumbing, and combustion chamber. If at the end the combustion products proceed straight out the axis of the rocket, with no net spin, then these forces exactly cancel, and there is no net force after startup. (And rocket designers try very hard to insure no net spin at the output, since it's inefficient (that energy could have gone into thrust) and it takes RCS effort to null out).
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#424
by
Chris Bergin
on 04 Oct, 2013 13:41
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Ok, so this has wandered into a splinter topic. I can't split it because it's raced away with itself.
So we're going to have to let this go.
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#425
by
cambrianera
on 04 Oct, 2013 14:17
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no, the same thing will happen.
When he lets go of the cd player before he pushes it (1:09), it is starting to rotate.
The same thing happens with a gyroscopically stabilized binoculars.
18s starting the CD player the case turn CCW (CD inside is accelerating CW)
24s doing first time the trick (hearphone cable attached!) the case rotates CW (clearly due to the CW movement of hearphones)
54s repeating the trick very small movement CW
1m09s removed the hearphone cable, case rotating already when leaving hand (before push) rotation CW
1m22s redoing again, rotation CCW
The case rotation is only due to the slightly off-axis release or push.
And this is the much blamed wikipedia:
The law of conservation of angular momentum states that when no external torque acts on an object or a closed system of objects, no change of angular momentum can occur.
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#426
by
mlindner
on 04 Oct, 2013 14:24
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Think of reaction wheels on a spacecraft. When you spin them at constant speed, the spacecraft does not endlessly accelerate, even though the motor has to constantly apply torque to overcome bearing friction.
Wrong, there are other torques on the spacecraft that cancel this out. Look up magnetorquer
(Sorry, ended up channeling Jim there for a moment...)
As someone who worked on designing a cubesat that was going to have reaction wheels...
Reaction wheels in steady state spin, unless poorly designed do not impart any spin on the spacecraft UNLESS they change speed. The wheels are used to spin the spacecraft by increasing their speed and then used to stop the spacecraft by slightly reducing the spin. Magnetorquors are only used for de-saturating the reaction wheels when they have been accelerated to a high rate of spin and cannot spin any further. This mainly happens only directly after spacecraft sep when an initially high spin is given to the spacecraft and slowly much later to burn away built up friction torque from the spinning wheels.
I'm surprised Jim got this wrong, but multiple reaction wheels are NOT used to cancel each other out, that defeats the point. Multiple are used for redundancy for the case when an axis looses a reaction wheel.
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#427
by
mlindner
on 04 Oct, 2013 14:25
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Liquid level.
Your profile section has "Done arguing with amateurs," but aren't you the amateur in all this? What's your so-called expert background that qualifies you to talk about torques exerted by rocket motors?
Jim is an actual expert, but he's also forgetting CoAM too right now. Today this forum is in an odd world....
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#428
by
Antares
on 04 Oct, 2013 15:27
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I read Blazotron's explanation about turbine drive gas, which sounds very accurate but does not envelope all of the flight events I know of. I'll have to ponder how to reconcile those. Engine transients can cause vehicle roll, and in my experience is far more plausible than aero moment causing a sustained roll (when not designed to do so) since turbulent flow is so random.
Go Buckeyes.
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#429
by
douglas100
on 04 Oct, 2013 15:35
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I've been following this discussion with some bemusement. I'm waiting to see if SpaceX is going to post the video of the descent. If it shows that the stage was spinning before the final single engine burn was started, then I'm not sure what the argument has been about.
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#430
by
LouScheffer
on 04 Oct, 2013 15:55
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I've been following this discussion with some bemusement. I'm waiting to see if SpaceX is going to post the video of the descent. If it shows that the stage was spinning before the final single engine burn was started, then I'm not sure what the argument has been about.
This is just a change in initial conditions. According to some of the arguments above, assuming the stage is already spinning, it will change spin rate (either up or down) once the pump is started (depending on the relation between the pump direction and the pre-existing spin). My bet, however, is that starting the pump results in no discernable change to the spin rate.
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#431
by
douglas100
on 04 Oct, 2013 16:31
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And I thought the argument was about what started the spin in the first place. If it was spinning before engine start then Musk is probably right, the main cause would be aerodynamic. That could be countered by beefing up the the thrusters, changing control algorithms, even adding extra aero surfaces as some have argued. If torque effects from starting and running the engine are significant then it would be easier to handle them if the vehicle was stable in the first place.
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#432
by
Robotbeat
on 04 Oct, 2013 17:06
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Jim has his helicopter physics correct. Source: first hand knowledge. Torque is always present so long as the rotor is being powered. I would expect the impeller and turbines in a rocket engine would also impart torque so long as they were being driven by combustion gasses.
Bingo
... and yet, the torque will be miniscule compared to that of the helicopter because the moment arm is tiny.
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#433
by
aero
on 04 Oct, 2013 17:27
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Torque imparted on the vehicle by the compressor blades or by the turbine stator blades? Or the compressor stator blades or the turbine power blades? Seems to me that somewhere in there you have a conservation of angular momentum situation. No, or very limited unbalanced torque within the compressor/power turbine system.
Of course the helicopter blades torque the helicopter body. The blades create lift by tilting to an angle of attack to the airflow. The total force vector is perpendicular to the angle of attack of the blades. The cosine part of the total force is lift, the sine part of the total force is induced drag on the blades. Induced drag on the blades is overcame by engine power providing torque on the rotor shaft but the engine power applies an equal and opposite torque the helicopter body. Hence a tail rotor is used to counter the torque on the body.
But if you look at the total helicopter system including the main rotor, engine and tail rotor, there isn't an unbalanced torque. Similarly, if you look at the total fuel pump system on the Merlin D, including the pump and the power turbine, there isn't an unbalanced torque.
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#434
by
Robotbeat
on 04 Oct, 2013 17:29
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Indeed, aero! Now we are getting somewhere.
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#435
by
Comga
on 04 Oct, 2013 19:59
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I think Chris asked us to give up this discussion of torque and rotation but, also being unable to resist, it is really simple.
In a steady state, with the rotating machinery at constant rotational speed and the flow of fluids established, regardless of path, if the exhaust coming out is not rotating, there is no rotary torque from the propulsion system other than from a roll-control nozzle. When the turbopump spins up or down, and when the speed at which the fluids are rotating are changing, there is torque. However, to the degree that I have heard the event described, the rotation occured after spin-up and ignition, and during the steady state operation where there was no internal torque generated. The spin-up torque had to come from somewhere else like aerodynamic forces.
Almost everyone here is correct, to some degree. A lot of the disagreement is we are not doing calculations or even drawing pictures. "Engineering is done with numbers. All else is opinion." or something like that.
I will now comply and drop the subject.
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#436
by
Robotbeat
on 04 Oct, 2013 20:10
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...
In a steady state, with the rotating machinery at constant rotational speed and the flow of fluids established, regardless of path, if the exhaust coming out is not rotating, there is no rotary torque from the propulsion system other than from a roll-control nozzle. ...
Precisely. I doubt any significant net steady-state torque from the turbopump survives passing through the plumbing and the injector(s).
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#437
by
dawei
on 05 Oct, 2013 02:54
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Personally, I blame the unicorns from the flame trench who were upset that their dance party was disrupted by the launch....Oh wait, wrong launch complex.... Scratch that.
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#438
by
Jason1701
on 05 Oct, 2013 06:28
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I'm glad we could resolve this before it made xkcd "What If."
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#439
by
bioelectromechanic
on 05 Oct, 2013 15:37
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Any idea why they couldn't try re-starting the second stage again, and instead opted to vent the tanks?