# Novel methods for propelling a rocket

Discussion in 'Physics & Math' started by Rocket Paul, Sep 23, 2017.

1. ### Rocket PaulRegistered Member

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yes its easy to debunk things but you still have not said if you agree there is two platforms, the answer to your earlier question about holding 2 magnets sat in your chair is fasten one to your chest and another one to a rocket both repelling run and hold some guide rails for the rocket to run on and ignite the rocket you and the chair will move, two separate platforms, just like what you are failing to see in the concept.

Last edited: Oct 26, 2017

3. ### exchemistValued Senior Member

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7,810
Yes but having a pair of repelling magnets to transfer the accelerating force from the engine to the body of the spacecraft is no different from having a rigid strut, or a rubber cushion, connecting the two.

So it is hard to see the point of "two platforms" when, from the viewpoint of the motor, it is just the total mass of both "platforms" that has to be accelerated.

What is the point of using magnets?

5. ### Rocket PaulRegistered Member

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56
Like you just quoted sit in a chair hold two magnets and you don't move, put a second platform and force and you will move, once we can get over this I will come to the point of the magnets, without a second platform then there is no change to rockets we use today all the force will be from the bottom.

7. ### exchemistValued Senior Member

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OK let's look at it.

You have one magnet on your chest and another attached to your rocket motor. You have not yet ignited the rocket motor. What will happen is the chair and rocket motor will move apart due to the repulsion of the magnets until they are far enough apart for the repulsive force to be too weak to move them further. Yes?

Then you ignite the motor. The gap between the magnets starts to close, until the repulsive force increases enough to set the chair in motion. After a bit a balance point will be reached at which the repulsive force between the magnets is exactly equal to the thrust from the motor. The distance between the magnets will thenceforth remain constant, and the chair and motor will accelerate as one body. Would you agree?

You could achieve the same effect by a rubber cushion, or a spring, between the chair and the motor, as this would also compress until the force resisting further compression became equal to the thrust from the rocket motor.

Now, tell me the point of the magnets.

8. ### Rocket PaulRegistered Member

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56
Nearly but you missed a vital point hold the guide rails but not the rocket, so although you are holding the rails you are not holding the free moving rocket two bodies, if the magnets are a problem then replace them with a spring and the same motion will happen.

9. ### exchemistValued Senior Member

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Sorry we have a person in a chair with a magnet on his chest, don't we? And a motor with another magnet on it, repelling the first magnet.

Where are the guide rails and what are they guiding? Do they simply make sure the motor only moves towards the chair rather than veering off laterally? If so then I don't see that making any difference. Or do they do something else?

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11. ### exchemistValued Senior Member

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7,810
Yes I know but it got in such a muddle I could not decipher what the scenario was. But I understand your man in the chair I think. So let's equip him or his motor with guide rails and you can explain their function. Do you still want to use magnets or should we use a spring that gets compressed instead? It seems to me the two are entirely equivalent.

12. ### Rocket PaulRegistered Member

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56
I thought I explained that, okay you say a man sat in a chair on wheels holding two repelling magnets is not going to go anywhere I say no he wont equal and opposite reaction to one platform, but if he fastens one magnet to his chest and attaches the other to the front of a rocket guided on a set of guide rails both magnets facing each other, ignites the rocket so it now moves freely down the rails causing the magnets to repel then you will move, two platforms one is you and the chair the other the free moving rocket.

13. ### exchemistValued Senior Member

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7,810
OK, so these rails: are they attached to the chair, to guide the rocket motor towards it and stop it veering off?

If so then the behaviour of the system should be as I described. The rocket ceases to be "free-moving" once the distance between the magnets has reduced enough that the repulsion between them is equal to its own thrust. Just like compressing a rubber cushion or a spring. From then on, the distance between rocket and chair will be constant and in effect they have become one system.

Agreed?

14. ### Rocket PaulRegistered Member

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yes one system but two platforms.

15. ### exchemistValued Senior Member

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Well if you like, but once the "slack" between these magnets has been taken up there is no point making the distinction, because it moves like a single rigid body.

So what is the point of the magnets?

16. ### KittamaruAshes to ashes, dust to dust. Adieu, Sciforums.Valued Senior Member

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Okay.

I don't think you are understanding the difference between something being held by anything at all, and something that is free falling/floating under its own thrust.

17. ### exchemistValued Senior Member

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I think I may have worked out what he is trying to do and why it won't work. He thinks that applying the thrust to the top of the rocket via these magnets confers stability, as if the engine were at the top like an Apollo escape tower.

The flaw in this idea is the guide rails. If the motor at the bottom has any tendency to veer to one side, it exerts a lateral force on the guide rails and thus transfers this lateral force to the body of the rocket to which they are attached, just as if the whole thing were a single rigid construction.

18. ### Rocket PaulRegistered Member

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56
I think you are missing the whole point of a second platform, the second platform takes all the reaction, because the platform is free moving its not giving any reaction to the craft in theory we have a re-actionless craft if that means anything having a re-actionless craft, the craft itself is only experiencing action the free moving engine takes all the reaction, without the second platform all force would be from the bottom and no force to the top.

19. ### Rocket PaulRegistered Member

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56
Now we are clutching at straws, really......okay there might be a small amount of drag but most force will be as I said forward.

20. ### exchemistValued Senior Member

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7,810
Hang on a minute. In the chair and motor scenario which "platform" is the second one?

And why do you say one is "free-moving" when I have explained that it is not, once the slack in the magnetic repulsion has been taken up? Don't forget, once the slack is taken up there is a force in the direction of motion equal to the thrust, so it is held rigidly in place relative to the chair along the direction of motion. And in the directions perpendicular to the direction of motion, the guide rails hold it rigidly in relation to the chair. That is why I say it becomes one rigid body: it has no degrees of freedom, in any direction, relative to the chair. It and the chair are one.

21. ### exchemistValued Senior Member

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7,810
As it is on a conventional rocket. The fact that you have to put guide rails on to keep the motor aligned means you have exactly the same tendency for the whole thing to topple as if the motor were rigidly attached at the bottom.

The reason Goddard's rocket was self-righting is that if the rocket veered away from vertical the mass of the body would be pulled by gravity back underneath the motor, i.e. the mass of the body could freely swing, beneath the point of application of the lifting force.

In your construction the guide rails prevent that from happening. So it will topple. And you will need to fit gimballed thrust and dynamic guidance, just as a conventional rocket.

22. ### Rocket PaulRegistered Member

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I say free moving because the motor is not fixed to the craft once the motor rises up the guide rails it can compress the distance between the magnets yep one system but two platforms, if we are making a repelling force between the magnets then we are creating a a force to the craft because one of them is fixed to the craft, the second (engine) platform remains independent to the craft unless the magnetic repelling field was stronger than the engine and pushed it back to the bottom then you would have cancellation equal and opposite reaction to a now one platform.

23. ### Rocket PaulRegistered Member

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56
the very reason for this theory is to stop the veering by lifting from the top and the bottom and the guide rails are to do just that Guide.