Thrust without reaction mass?

[Anders Troberg]

I once saw a TV program about a device to produce thrust without reaction mass, they even showed a working model on a canoe. Something tells me this is bullshit and that it will not work and I think I've heard claims that it is. The problem is that I can't see why it shouldn't work, which annoys me a lot. So, I put the question out to the talents here. Can someone explain what the problem is?

Here is how it supposedly works:

We have an axle, connected to a motor. On that axle is an excenterweight. The axle is mounted so that it points 90 degrees off the forward direction. If we look at how the spinning weight moves and think of a clock face, 12 will be forwards, 6 will be backwards and it's spinning clockwise. OK, enough setup, let's get it running.

When the weight reaches 9, a brake is applied. The weight is slowed down and the vehicle is propelled forward. The weight is then accelerated around 12, coasts by 3 and is once again accellerated around 6. Keep repeating this. The acceleration will cause some sideways force, but two such devices side by side, synchronized and counterrotating will cancel out the sideways forces.

It might not be the smothest ride, but once again, that can be fixed by more units running out of phase.

The device used in the TV show was more complicated, with mechanics to alter the radius depending on where the weight was, but I suspect that was more about making timing less critical than basic function. It might also be about obfuscation of basic principle, a common enough theme in "wondermechanics" such as perpetuum mobiles and similar devices. It might also be a way to be able to accelerate through all the cycle, except the braking part.

To me, this is basic physics and I can't see why it should not work, but I still can't shake the feeling that it can't work. Physics gurus, enlighten me!

[Washout661]

Conservation of momentum, ie like torque reaction in a helicopter, recoil from a rifle / shotgun. By stopping the wheel, you are unbalancing the system so the vehicle moves in order to conserve momentum.... but then I finished uni a good few years back now so I could be wrong.... only taken me 6 months to get around to posting something here though.



Steve

[larry cottrill]

Sounds like it's basically a projectile-less trebuchet with the addition of a braking system. Strictly speaking, the weight IS the reaction mass - it's just rigged to be continuously reusable.

There is no reason for this not to work. The mass is "recovered" to its starting point with less anti-thrust momentum than the thrust momentum delivered to the brake. This is astonishingly like a valveless pulsejet - the total mass injested in the anti-thrust direction is exactly the same as the total mass ejected (if you include the fuel), but at a totally dissimilar momentum value. So, you have net thrust after all is said and done.

This is nothing like a "perpetual motion machine", where no energy is added to the system to keep it going. Here, a motor is needed to re-position the weight for the next drop - just as in the pulsejet, fuel is needed to power the next blast.

L Cottrill

[Anders Troberg]

There is no reason for this not to work.

Then the question becomes: why is it not used?

It would be perfect for applications such as deep space probes, which have plenty of energy (often a nuclear reactor) but little room for reaction mass, not to mention that you don't have to spend fuel to accelerate unspent fuel. With a smart design, it could probably provide vectored thrust just by braking and acceleration on different parts of the cycle. Since such probes require very little thrust, the device could be made small and light.

It's a neat idea, even if it's probably not practical for most earth-bound applications.

This is nothing like a "perpetual motion machine", where no energy is added to the system to keep it going.

I didn't mean that it was, I just used it as the most common example of vastly exaggerated claims.

[Mike Everman]

I hold out hope that someday we will find a tricky way to do this (in space, a "free-free" environment, while outwardly observing all conservation laws. Surely someone in all of space and time has done this practically.

This type of device will work when it is in contact with something, best to be on the ground. So you can make "black boxes" that scoot themselves along the floor with all manner of whirly-gigs that accelerate a mass at a different rate forward than back. A sawtooth motion profile is good for this. Friction is less at higher velocity, so you get a net motion.

In a weightless environment, it is useful as you say, like we do on a child's swing, Anders, but only for changing the shape of an orbit, not the area of any particular swept angle. (I think that's the proper way to put it. Correct me if I'm wrong!

[mk]

Well, haven't seen your post Mike, as I wrote the following.

---

The comparison to a trebuchet (better think of those with wheels) was a good point. Makes for one of the easiest way understanding the principle.

The trebuchet should feature a fixed weight and arm length, in which we only shorten the shooting-arm, so that the weight could spin in a plane. Now being affected by a (homogenous) gravitational field -- being fulfilled properly enough for us on the earth's surface --, the weight shall have just enough of kinetic energy, so that it can overcome the potential in the upper most position. The weight will then spin with changing velocities and accelerations, an will show an oscillating behaviour of both, the weight and the frame because of conversation of momentum. Now implementing losses, some amount of kinetic energy would need to be added/replaced each cycle, of course. Doing this cleverly, e.g. timed, the trebuchet frame on wheels may now start to roll in one direction.

When drawing parallels to the device in question now, those effects might be to small, so that timed braking and accelerating becomes necessary to generate a "noticable" -- which means to our senses -- effect on trolley's state of motion.

Thus there does not seem to be anything misterious to it, and the device should work as described. At least under certain circumstances: (Continued below.)

Then the question becomes: why is it not used?

It would be perfect for applications such as deep space probes [...]

Trapped!
That's the single downside of conservation of (angular) momentum. Since there is no "fixed*" ground to work against in free space, you're probe would be heavily affected by counter-effects, i.e. it would be under the affect of changing angular momentums, such so that it would start to spin in one or the other direction easily.

Wait a minute! This means:

We, sluggishly, did not consider "momentum dissipation" to the earth via the frame in the trebuchet case. Hmmm, let's take a look at another example to get an idea of what I'am talking about:
When accelerating or braking a car, you will note it changing its horizontal line relatively to the flat surface. This is because of the counter acting angular momentum komponent excerted to the frame (conservation of momentum). And the frame now works against the ground, e.g. by excerting an extra force on the soil via the rear or front wheels.
Luckily our car was constructed in a way it cannot tip over.

To come to a conclusion: We see that it is not a perpetual device. Of course not. The trebuchet would demand a reservoir of potential energy, which has to be dissipated to the surrounding environment. Only dissipation makes the forward motion (increase in kinetic energy) possible. Now it may only remain in question how much of the amounts of excerted energy really go into increase of kinetic energy hand how much are eaten up by losses.

So, however, when an array of such devices should propulse any vehicle comfortably, I think we do have way easier methods to do it. Only think of regulating difficulties of applying brakes, accelerating etc. Well, but it might be a solution for certain niches, which I am by no means aware of.

Well, those were my thoughts. Please disagree if necessary, and correct the wrong points.
__________
*) Forces we are dealing with are just irrelevant when compared to other effects acting on the earth. Also the radius is large enough the surface locally different, so that we an savly assume the the ground as "fixed wall".

[leo]

The idea is not new
Go to this site and put som of the numbers in.

http://patft.uspto.gov/netahtml/PTO/srchnum.htm

2,350,248
2,886,976
3,683,707
3,807,244
3,492,881
3,584,515
4,770,063

strange that the don’t give patents on perpetual motion machines, without working proof, but give patents on these devices which I’m sure of the don’t work to.
May be the find Newtons laws less important than the thermodynamic laws.
By the way I think there are exceptions on every rules, but the wont be so simple.

[leo]

Here a paper I cut and paste from this NASA site.

http://www.grc.nasa.gov/WWW/bpp/overvie ... l_Inertial

Non-Viable Approaches:

Oscillation Thrusters & Gyroscopic Antigravity:
Mechanical devices are often claimed to produce net external thrust using just the motion of internal components. These devices fall into two categories, oscillation thrusters and gyroscopic devices. Their appearance of creating net thrust is attributable to misinterpretations of normal mechanical effects. The following short explanations were excerpted and edited from a NASA website about commonly submitted erroneous breakthroughs. [ 17]

Oscillation thrusters move a system of internal masses through a cycle where the motion in one direction is quicker than in the return direction. When the masses are accelerated quickly, the device has enough reaction force to overcome the friction of the floor and the device slides. When the internal masses return slowly in the other direction, the reaction forces are not sufficient to overcome the friction and the device does not move. The net effect is that the device moves in one direction across a frictional surface. In a frictionless environment the system's components would simply oscillate around their center of mass.

A gyroscopic thruster consists of a system of gyroscopes connected to a central body. When the central body is torqued, the gyros move in a way that appears to defy gravity. Actually the motion is due to gyroscopic precession and the forces are torques around the axes of the gyros' mounts. There is no net thrust created by the system.

To keep an open, yet rigorous, mind to the possibility that there has been some overlooked physical phenomena with such devices, it would be necessary to explicitly address all the conventional objections and pass at least a pendulum test. Any test results would have to be impartial and rigorously address all possible false-positive conclusions. There has not yet been any viable theory or experiment that reliably demonstrates that a genuine, external, net thrust can be obtained with one of these devices. If such tests are ever produced, and if a genuine new effect is found, then science will have to be revised, because it would then appear that such devices are violating conservation of momentum.

[mk]

Well put leo.

To express it another way round, we also quitely assumed systems under (large) friction affect. Ergo: No free-space application.

[Mike Everman]

Being very skeptical of it is a good thing. I have made versions of a claimed gyroscopic type (invented by Laithwaite), to prove to myself that the inventor was merely able to design something just a bit more complicated than he could analyze.

Yes, currently, there is no "black box" that can translate itself in free-free conditions, so let's just push it around with a pulse-jet!

I do believe, though, that we can solve this eventually, and hope like hell that it turns out to be easy, and uses a whirly-gig one can make in his home shop!

All we really need is a mass that can temporarily change it's inertial mass on command.

As a thought experiment, consider that a spinning wheel is heavier than a static wheel, owing to mass gain that comes from high velocity described by Special Relativity, but the amount is so, so small. But it would make a net force! So, IMHO, it IS POSSIBLE, and could be demonstrable, but a very tiny effect with the rpm's we are limited to, and the fact that you must spin up and down at the main cycle rate.

If you replace the mass in any of those "oscillation thrusters" gizmos, then you have the key to the stars, even if the mass to change by only a gram.

Let's make a machine that has a number of buckets that fill with virtual particles when moved rearward, which disappear when moved forward. There, let's build it!

[Anders Troberg]

Trapped!
That's the single downside of conservation of (angular) momentum. Since there is no "fixed*" ground to work against in free space, you're probe would be heavily affected by counter-effects, i.e. it would be under the affect of changing angular momentums, such so that it would start to spin in one or the other direction easily.

Two such devices mounted side by side, synchronized and counter-rotating would, as far as I can see, cancel out all forces except the ones in the direction of travel.

[leo]

All we really need is a mass that can temporarily change it's inertial mass on command.

As a thought experiment, consider that a spinning wheel is heavier than a static wheel, owing to mass gain that comes from high velocity described by Special Relativity, but the amount is so, so small. But it would make a net force!

That increasing mass would probably go slower and preserve angular momentum that way.
Again no net force, but if that’s a effect, maybe we can use that somehow.
Don’t get me wrong, I want these things to work, but so far I have never seen the real thing.
Since so long as I remember I think about perpetual motion and reaction less drives.
I always liked relativity and its effect and to think about these things, like length contraction and stuff I mean.

[mk]

Two such devices mounted side by side, synchronized and counter-rotating would, as far as I can see, cancel out all forces except the ones in the direction of travel.

Hmmm, seems like that.
Then it still would fulfill the case of a need "to work against something", when borrowing Mike's expression. E.g. such devices may then be used in pairs favourably.

However, practically it seems unrealistic, because each couple would have to consist of two (to a tiny, tiny bit almost) identically manufactured and operated devices. Failure in only one of the devices is what just must not happen.

More detailed: Though both devices of a pair would operate counterrotating to each other, energy addition and withdraw must not happen arbitraily. If the phase of one has been choosen, the counter part has to be tuned to exactly match it. Of course, speaking of two counter-rotating, or in this regard counter-oscillating weights, the (optimal) phase offset would appear to be 180°, when watched in or against the direction of motion.
Else the motion will again become somewhat freaky.

[Mike Everman]

I've always wondered, kinematics and net results not really sought... What happens if you forceably precess two flywheels against each other, such that there are no net outward effect, in a vacuum with magnetic bearings. The system designed such that energy goes in, but does not come out in any form whatsoever, acoustic, thermal, motion.

What happens, beside nothing, that is? As before, Special Relativity says it will be slightly heavier because its atoms are moving slightly faster than those around it. (This effect is indestinguishable from time moving slower for the device than for everything around it.) So, this is the tiny effect that should be there, but so tiny as to be unusable.

I am silly and not sophisticated enough to just know this, but what about all the energy you could dump in there that doesn't come out? Once the wheels are spun up, that's all the energy that gets merely stored. It just seems like there should be SOME effect that isn't obvious when energy goes in, and is not actually (or at least I'm trying to prevent it from being) converted to another form...

[Mike Everman]

Nevermind. I just realized that even though friction in the bearings was eliminated, the wheels will be stressed and strained, ultimately converting the input energy in to heat.
What we need then is a stasis field that disallows strain energy in a material, ha ha