# Novel methods for propelling a rocket

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

1. ### DaveC426913Valued Senior Member

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Not sure that's true. Wind moving forward across a slanted sail will push the boat forward. That's what iceaura is alluding to.

3. ### KittamaruNever cruel nor cowardly...Staff Member

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Aye, but the sailboat analogy to a rocket is flawed:

The rocket has nothing to function as a keel, to enable the "tacking" effect to occur.

5. ### DaveC426913Valued Senior Member

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Agree. The analogy breaks down.

7. ### exchemistValued Senior Member

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I think it has to be true. If you have a force with a forward component, there must be a concomitant rearward change of momentum in the air. Some wind must be deflected to take a more rearward course than it would have otherwise done, surely?

8. ### exchemistValued Senior Member

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But surely, all the keel or daggerboard does is isolate the forward component of force by resisting the lateral component? If you analyse the situation in terms of components, the analogy must hold, I think - see also my other post made before I saw this one.

9. ### DaveC426913Valued Senior Member

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It also fixes the direction of the wind relative to the craft and relative to the sail. If the direction of the wind didn't stay fixed, the force on the sails would die.

And it also establishes what we mean by "moving forward". If the boat were lifted into the air, so that it were free to be pushed in whatever direction the fan wanted, we could no longer claim it was moving forward due to the fan/sail; it would simply be fan pushing it in whatever direction the fan was oriented relative to the craft. And that would fail the initial criterion.

10. ### iceauraValued Senior Member

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That's what bouncing it off the angled sail does.
That's how sailboats sail upwind.
Agreeing that this entire sailboat discussion is off topic, nevertheless: a rocket does have a long axis that in the atmosphere provides resistance to side travel - there is a keel effect available, if anyone's interested. Although sideways blasting a rocket's fins with it's own propulsion gasses strikes me as even less likely to go well than nose mounting the engines - which is already something I would pay 50 cents to see tried on Youtube, as long as no living beings were involved.

11. ### James RJust this guy, you know?Staff Member

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Yes, but it can only push the boat forward if the air itself is directed backwards. Newton's 3rd law.

Exactly.

Yes. They sail upwind by altering the path that the air flowing over the sail takes when it leaves the sail to the rear of the boat.

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It looks like we're mostly agreed on this.

I just wanted to point out that a rocket in space doesn't have the benefit of an atmosphere (or anything else) to push against. So it has to literally throw stuff out the back in order to accelerate forwards.

In an atmosphere, a rocket's fins do have aerodynamic properties - just like the ailerons on an aeroplane.

12. ### iceauraValued Senior Member

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Agreed.

Although iirc there is even in this a complexity, in an article I read years ago and cannot find at the moment: by taking advantage of the nonlinearity of the relationship between momentum and velocity, it is apparently possible in theory to sort of inchworm one's way through space by extending and contracting along the desired travel path. The trick, iirc, was to extend and retract mass at different rates.

13. ### DaveC426913Valued Senior Member

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That would be an interesting read. It seems to violate Newton's Laws.

14. ### exchemistValued Senior Member

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That is presumably the point, I imagine.

The real momentum (as opposed to the Newtonian approximation) is γmv, where γ = √(1- v²/c²). So it is non-linear. A bit. If you are going sodding fast.

15. ### exchemistValued Senior Member

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The Hawker Harrier used that principle, admittedly with a fanjet engine, so a lot less hot.

16. ### Rocket PaulRegistered Member

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Hello guys been very busy off the internet and looks like I have created a lot to talk about, so here I go and ready to take the flack, I will probably walk away with egg on my face with so many clever people on this site but I guess its a good way to get the theory tested.

Okay I don't know how to put a picture up on this site so please follow the link to a temporary Blog I opened http://magjet.blogspot.co.uk/ please reply here and not the blog guess I am going to be busy for a while with some explanations.

17. ### BaldeeeValued Senior Member

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Okay, let's walk through the stages of the vehicle launch to after you switch off engines.

1. The engines fire up and the inner part moves up, the outer part doesn't, and the work the engine does initially is to compress the magnets closer together.
2. At some point the upward force on the upper magnet overcomes the weight of the outer hull, and the whole rocket lifts off the ground.
3. At this point, assuming the magnets compress no further, the rocket behaves exactly like a standard rocket and is no more or less stable than if the magnets were simply a lump of metal joining inner and outer parts.
4. At the point the rocket switches its engines off, the magnets will no longer be under the force that is compressing them and so they will repel each other.
In doing so the inner and outer will both move in opposite directions away from their combined centre of gravity.
If the inner section is restrained from sliding all the way out, the two sections will then bounce back and forth (like a spring) until their respective motion is dampened.

But in terms of stability, this is no more stable than a standard rocket.
You're also burning fuel at the start that gets stored in the magnetic resistance between inner and outer parts.
This stored energy is released once you switch the engine off, for example, at which time you might waste it through sound and heat, having to dampen the respective motions of the two parts.
Plus the storage is done when the overall mass of the combined rocket is heaviest, so you'll actually be less efficient for that as well.

So from what I can tell it's not a more stable configuration, it's not more efficient, and seems an unnecessarily complex set-up.

Other than that, though...

One last thing, you say it might enable us to leave earth's gravity at a slower speed...?
To break free of earth's gravity requires a certain velocity.
You can take as long as you want getting there but you do have to reach that velocity, otherwise earth's gravity will pull you back.
This is referred to as "escape velocity" and for the earth is about 11.2 km/s.
So no matter what the design, you can't escape the earth's gravity if you travel slower than this.

18. ### Rocket PaulRegistered Member

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Correct the inner rocket moves up the guide rails and makes the repelling field then once the whole mass has been overcome the whole craft moves skywards.
With a controlled amount of thrust from the engine (inner rocket) to lets say just enough to lift the rocket then let us not forget the energy from the repelling magnets, with force directed to front and back of the craft (outer skin) and magnets have levitation capabilities then the now more balanced rocket can move as slow as it likes this saving huge amounts of fuel, there can be no bouncing back and forth unless the magnets are more powerful than the engine, no need to switch the engine off or the whole system fails.

Last edited: Oct 19, 2017
19. ### BaldeeeValued Senior Member

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What do you think is providing the thrust to be able to lift the combined weight of the inner and outer section?
Hint: it is not the magnets.

???
Fundamentally the system you describe is no different to a standard rocket.
The thrust acts from the bottom, and must overcome the combined weight of the entire system to fly.

The magnets merely act as an energy storage device while in compression and have negligible (although I'm trying to think of how they might be non-zero) effect on the flight characteristics of the rocket.
As for going as "slow as it likes", why do you think this would save fuel?

If you have a rocket hovering off the ground then it is going at zero speed and burning as much fuel as it needs to overcome its own weight.
This is inefficient.
Travelling only slightly faster than this, slowly rising, is also inefficient.
Rockets need to get out of the atmosphere as soon as they can, although there is quite possibly a sweet-spot between speed and efficiency due to aerodynamics.

But your idea is fundamentally flawed due your notion that the magnets will have any effect on the flight characteristics of the rocket.
The thrust is still at the bottom.
The thrust will still still act through the centre of thrust (which one hopes is the centre of mass).
The stability will still be the same as a normal rocket.
What the magnets will do very slightly is alter the location of centre of mass as they compress, but that will be an almost negligible effect if the rocket maintains a constant thrust once they are as compressed as they can be.

So where do you think the efficiency is coming from?
Without the magnets you have the net thrust (thrusters up less weight of the inner and outer shell down) and in your design you have exactly the same, less small element of stored energy through the compression of the magnets.
But in flight the storage is maxed out, the net thrusts are then identical equations.
The magnets and their repulsive forces are not part of that equation.

Dywyddyr likes this.

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21. ### Gawdzilla SamaValued Senior Member

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You could take a bunch of detective mysteries up with you and throw them in the opposite direction from where you want to go.

That would be a novel approach to propulsion.

22. ### Rocket PaulRegistered Member

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I agree the rocket does work the same as a normal rocket but with the free moving engine we can now add a direct lift to the top of the rocket via the magnets or if you prefer a spring so we have force at the front and back making the rocket more balanced.
Rockets today have to use Olympic swimming pools of fuel to escape earths gravity but at a slower speed large amounts of fuel could be saved the same as if you traveled in your car 100 miles at 100 miles an hour or 100 miles at 20 miles an hour.

Last edited: Oct 19, 2017
23. ### BaldeeeValued Senior Member

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First, lift is an aerodynamic property, acting through the centre of pressure, whereas what you are referring to is thrust.
Second, there is no more propulsion at the front of your rocket than there is in a standard design.
All you have is the thrust of the engine being transferred through the magnets in the same way it is transferred through the rest of the vehicle, (albeit with some storage and release of energy as they compress and repel with changing engine thrust).
When the magnets compress it is because the weight of the upper magnet (plus weight of outer shell) is greater than the upward thrust of the lower magnet.
As the lower magnet rises, and compresses with the upper magnet, you get a repulsive force upward but that is matched equally and oppositely by a repulsive force downward.
The net is a zero repulsive force.
All you then end up with is the force from the engine thrust being applied to the entire mass.
First, as stated, we could not move slower: you need a certain speed to achieve orbit, and 11.2km/s to break free of earth's gravity.
Second, think of the magnets as a spring, that compresses and tries to repel, okay.
Well, physical structures are exactly the same.
Replace the spring, or the magnet, with a lump of steel and when the engine thrusts that steel will compress, and will want to repel.
So having replaced the magnet system with something that has the same process of compressing and repelling, but is what we consider a standard rocket, you are now saying that the magnet system provides some free energy from somewhere??
I admire you for trying to think outside the box, but not for your lack of understanding of the basic physics.
And I am decidedly not the best teacher.
So I won't try to explain things further lest I only confuse you.

But, needless to say, you have not developed anything new, just a more complex (and less efficient, due to the space required) way of doing what is already currently done.