The current occurs because of the potential difference between the ends of the tether. Yes, this can be said to perform “work” on the Earth via interaction with the magnetic field, but the energy in this case is provided by the existing charge differential resulting in the pumping of charge driven by the motion of the spacecraft. If there were no charge differential (or the tether were non-conductive, as some other types of tethers are) the tether would develop no current and the Earth’s magnetic field wouldn’t give a flying fuck what it was doing.
Ohmic heating is a secondary effect which represents a net loss to the system, radiating lost energy (non-work, at least as far as the Earth is concerned) as heat, but this is a property of the electrical impedance of the tether and not interaction with the magnetic field per se (although the current is seeing resistance from motion through the magnetic field, so the result is proportional to the orbital speed and direction relative to magnetic field lines). The flow of charge isn’t propellant in the conventional sense of mass ejected from the system through the exit plane of the propulsion system, but it performs essentially the same function.
If momentum is locally conserved, the system isn’t “reactionless”; the vehicle will sense a reaction in proportion to the derivative of impulse with respect to time.
Exactly. The changing magnetic field will induce a potential difference whether or not there are charged particles in the vicinity. Electrons purely within the tether will swish back and forth as the field changes.
I imagine that the effect is greatly enhanced by collection and ejection of ambient particles, though.
I was thinking of the tether as a propellantless deorbiting system (we very briefly considered one for our cubesat, but went with a balloon instead), so ohmic heating would be the entire point and not exactly a “net loss”. Of course you’re right that it would be a loss if the goal were raising an orbit.
Actually, all of these drives violate thermodynamics OR relativity. The reason is that since you get a certain amount of thrust per a certain unit of power, and you expend no propellant, it would be possible to build an energy generator that breaks conservation of energy.
Either you have a robot spacecraft with one of these drives accelerate to a high velocity, so high that the gain in kinetic energy is greater than the energy supplied to the thruster, and you have the robot spacecraft fly in a circuit so you can collect this kinetic energy with electromagnetic decelerators. Or, you build a gigantic disk so the outer edge is spinning fast enough that the same principle applies.
If this drive gets less thrust as the spacecraft moves faster, this fixes the conservation of energy violation - and throws relativity out the window.
Now, some of the theorists researching these drives (there’s several flavors of these drives, all of the proponents do hold PhDs) claim they work because they are in some way interacting with the rest of the universe. Somehow the books are being balanced in that the universe is expanding faster or some other concrete, real world changes in momentum and energy are happening elsewhere to let us cheat momentum/energy conservation here.
In order for there to create a standing magnetic field the current flow has to be one direction, as it will when being driven by a charge differential between the ends. Again, the amount of loss due to ohmic heating is secondary to the drag created by current flow through the tether as it moves in orbit. If you wanted drag by ohmic heating alone, you wouldn’t need to deploy the tether end for end; you could just loop the tether and allow the magnetically-driven current to flow in a loop where the impedance of the tether material will provide drag.
I have no idea what point you are attempting to make with robot spacecraft or having them fly in a circuit, but you seem to miss the essential point that no claim is being made that resonant radio or microwave cavity propulsion is not some kind of over unity device; you have to input energy in the form of a radio frequency or microwave source, and in fact, the specific power efficiencies that are being claimed are actually quite poor (at least, this is true with the EmDrive; there isn’t enough content on the Cannae drive to make any kind of assessment). The “propellant”, so to speak, is the specific frequency of coherent electromagnetic radiation which is emitted directionally from the device. Now, whether the cavity can be “tuned” to such a degree to emit highly directional coherent radiation and produce any useful amount of thrust is in question, but even if it can there is still the question of how much resulting waste heat is produced by such a low efficiency that has to be rejected by other means, which is the problem with scaling up more conventional ion and magnetically accelerated plasma thrusters to useful levels. But there is no inherent violation of thermodynamic principles.
Stranger, are you an actual aerospace engineer? You should have seen this immediately. Let me explain this in smaller words.
Yes, the drive requires energy to operate. However, the drive gives more thrust than you would get from simply sending photons out the back.
Kinetic energy = 1/2 mv^2. The drive gives constant thrust for *constant *energy input, such that at any speed the spacecraft is traveling at, the amount of momentum gained per joule of energy expended is the same. It has to grant the same thrust regardless of speed of travel or relativity is wrong.
This means that there is a speed at which the total kinetic energy in the spacecraft is greater than all of the energy you put into the drive. I can prove this mathematically if you insist.
This means you could build an overunity device using this drive as the main component. It would essentially involve a spacecraft flying back and forth between 2 energy collectors that gather all the kinetic energy, which exceeds the input energy. I never said the drive by itself was overunity, but if the drive works, you can cheat conservation of energy.
As I said, I don’t doubt that the effect is stronger when you introduce charge collection/emission. I’m only saying that a conductive tether alone will still have some effect. A loop would be better yet but I don’t see how to keep it open. Tidal forces ensure that a tether stays moderately taut and “vertical”; the same forces would serve to elongate and close a loop.
Yeah, this is really a pretty damning point. Build a long tube with a little capsule that shuttles back and forth and magnetic decelerators at each end. The capsule accelerates using the thruster and the kinetic energy is collected at the ends. If the thruster really does have constant thrust for constant power, there will be a tube length at which the device becomes an over-unity device.
And as you say, you could have decreasing thrust with increasing velocity… but that violates relativity, since otherwise where’s your rest frame?
Surely so. Other types of electric rockets exist (ion thrusters, etc.) and they also provide constant thrust for constant power… but they don’t violate conservation of energy because the mass is constantly decreasing. Even in chemical rockets, the amount of energy imparted to the payload by the upper stage can exceed that contained in the propellants. That’s ok since it was the lower stages that provided the energy difference.
None of that applies to a true propellantless drive. As I mentioned, you could build a standalone over-unity device using the drive. You could even supply the energy externally (via laser, etc.).
How does one establish the necessary current without having a circuit of some kind? I mean I can see how you might get some kind of charge separation where the two ends of your tether have a different charge. But that is just electrostatics. How would you set up a current to do any work?
And as an aside, just how would this work in practice? Are we talking about a wire a couple of km long that we hang down from our satellites? Because that would be just like a cheese wire and cause all kinds of havoc. I know. I saw Gravity.
I don’t quite follow the argument, but I suspect you’re switching reference frames here . In one reference frame the KE of the spacecraft is zero. In another reference frame, the relationship between momentum and velocity and energy changes according to the velocity.
Maybe you should try proving it mathmatically, but make sure you’re always using the same reference frame.
One thing about the concerns over greater than unity. Somewhere in my limited reading about this in the past few days it was noted that the drive was speed/acceleration? limited. And this was a result of the physics/mathematical description of the device.
So, at least on paper, this type of drive is not breaking those particular laws of physics.
If the drive is speed limited, it would seem to break relativity. There’s no universal rest speed to the universe, so what would the speed limit be limited relative to?
If it’s acceleration-limited, surely you could just have two of them and get double the acceleration, unless, again, it’s interacting with some invisible, privileged-frame medium.
I’m just telling you guys what I remember. You’ll have to dig up the details yourself (but it can’t be buried too deep as I’ve only read a couple pages on the stuff recently.
Now that I think about it it, somewhere the over unity arguement was used to “prove” the physics as described must work. It went something like this:
Here’s the physics (fancy math/physics shown).
There are two ways to interpret the above. One means the drive works. The other means it wouldn’t work.
Ahhh, but there is problem. The second way leads to a violation of law X (I believe it was conservation of energy here).
Therefore the first interpretation must be correct and the thing works.
QED.
Well, that is the gist of what I remember anyways.
I’ll ignore the sniping personal insults and point out that nowhere did I say that the mass of the vehicle is constant or that the energy supply has to be onboard, which in your eagerness to “explain this in smaller words” you implicitly assumed. For this kind of system which produces such a tiny thrust, solar electric or external beamed power would likely be used. There is, again, no overunity condition, and in fact similar concepts like the EmDrive have very low power throughput efficiencies.
The “circuit” is between the high potential at the upper end of the satellite orbit and the “ground” potential at the lower terminus of the tether, relying on charge separation in the ionosphere. (The latter is itself complex, as there are currents running through the ionosphere, but for simplicity in this discussion we can ignore these.) The difference in charge creates current in the same way that the potential difference between the anode and cathode in a battery does. The stationary current by itself does nothing, but as this current-carrying tether is dragged across magnetic field lines it creates an opposing magnetic field that results in drag or alternatively, if current is driven the other direction, causes the magnetic field to push upwards or sideways depending on local field orientation.
Tethers up to several kilometers long have been deployed, but generally speaking this is only useful for relatively short tethers; most are a couple of hundred meters in length or shorter, and the proposed femtosat tethers are around 30 cm in length and are thinner than a human hair. As the tether gets longer gravity gradient and tidal forces become greater than the tensile strength of any extant material. Of course, you could deploy multiple tethers but they would have to be well spaced out in order to avoid both electrical and physical interactions, and because spacecraft tend to scale mass with volume there is a limit on just how much thrust/weight could be practically developed, hence why electrodynamic tethers are really only useful on spacecraft up to the size of ~6U Cubesats or similar sized small satellites. (Technically, the requirement for deorbiting is “25 years after end of operation” from which most smallsats in the LEO orbits to which they can be deployed will deorbit within but because of the large number of smallsats projected to be deployed in the next decade there has been considerable talk about requiring at least passive deorbit systems which enhance drag and ensure reentry within a few months of the end of operation.)
BTW, these should not be confused with momentum exchange tethers, which use rotational momentum to impart linear acceleration or provide stability, and on which electrodynamic effects are generally not desirable.
As far as hazard to other orbiting objects, yes, they have to be considered in the spacecraft envelope which is another reason why very long tethers are undesirable. In general, with smallsats, they’re in such low orbits that even should a catastrophic impact occur, the debris field will rapidly decay and reenter the atmosphere, and for the most part high value satellites are placed in much higher orbits and JSpOC ensures that any objects of strategic interest (like surveillance satellites and major telecom birds) are not threatened. As smallsats become more capable and want to occupy higher orbits to perform greater science objectives and commercial functions, however, this becomes a larger concern. The electrodynamic tether is desirable as a deorbit device because it can provide controlled deorbiting without any onboard propellant and the expense and complexity that comes with it, but as smallsats grow it is likely that above a certain orbit they’ll be required to carry onboard propulsion systems and this is an active area of interest, particularly in the 6U and larger CubeSat and similar sized smallsat forms, as it is for smallsat-based probes planned for use in lunar and interplanetary missions.
Stranger, you come on these forums and post walls of text that contain language indicating you think you know exactly what you’re talking about. The evidence is beginning to show otherwise. Let’s see what you say if I add the obvious assumptions you should have made in your analysis.
In this case, assume the drive is powered by a 100% efficient matter -> energy converter (not physically possible but does not break any laws) carried onboard the spacecraft with the drive. The drive does not eject any propellant, which was given in the original article
This theoretical matter->energy device eats a kilogram of matter and uses it to run the drive. If the spacecraft gains more kinetic energy than was contained in that kilogram of matter, and it started from rest relative to your starting point which contains the power generator, then you are exceeding unity gain.
If you attached a 100% efficient photon drive to this 100% matter->energy converter, you would gain exactly as much kinetic energy as the energy contained in those photons. Also, the thrust would be terrible unless you destroy an insane amount of matter.
Instead of continuing to follow me around and spew out veiled insults and invective, just go and create a Pit thread where I and others who aren’t interested in your mewling can conveniently ignore you. As for the rest of your arguement–such as it is–you persist in applying assumptions which are not part of my statement in order to question my qualifications and justify addressing me in “small words”.
This refutation of the Shawyer resonant cavity drive, which seems similar if not identical to the Cannae drive, was published by Greg Egan in 2007. This might give an idea of where the problems are, unless some new principle has emerged. http://gregegan.customer.netspace.net.au/SCIENCE/Cavity/Cavity.html
Habeed, you’ve been around long enough to know that insults are not permitted in General Questions. This is an official warning.
Colibri
General Questions Moderator
PS. I note there has been subsequent snark between you and Stranger. If either of you want to indulge in further personal attacks, take it to the Pit. Both of you drop the snark now.
In the general relativistic ‘swimmer’, whether the total force on the swimmer is zero depends on how you add up all the forces on the swimmer, however it’s only when certain symmetries exist can you find consistent ways of adding them.
