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Reconciling a Reactionless Propulsive Drive with the First Law of Thermodynamics

Andrew J. Higgins

2015 · arXiv

Physics
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From the abstract

A "space drive" is a hypothetical device that generates a propulsive force in free space using an input of power without the need for a reaction mass. Any device that generates photons (e.g., a laser) would qualify as a propellantless "photon rocket," but the force generated by emitting photons per power input (3.33 $μ$N/kW) is too small to be a practical propulsion device. The ability to generate greater force per power input would be highly desirable, but, as demonstrated in this paper, such a device would be able to operate as a perpetual motion machine of the first kind. Since applying a constant force results in a constant acceleration, the kinetic energy of a mass driven by such a device increases quadratically with time, while the energy input increases only linearly with time. Thus, at some point, the kinetic energy of the device-driven mass exceeds the energy input, and if this energy is collected via decelerating the mass (via regenerative electromagnetic braking, for example), then there would be a net gain in energy. For devices with thrust-to-power ratios on the order of 1 N/kW that have been discussed recently in connection with the so-called EM drive, this breakeven occurs at velocities low enough to be feasible with current technology, clearly demonstrating the absurdity of such a device. When relativistic effects are taken into account, it is shown that the photon rocket can only reach energy breakeven as the accelerated mass asymptotically approaches the speed of light. Thus, any device with a thrust-to-power ratio greater than the photon rocket would be able to operate as a perpetual motion machine of the first kind, and thus should be excluded by the First Law of Thermodynamics.

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8 selected excerpts · 8 total mentions

roywiggins
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It's worth noting that if a propellentless drive works, it will probably generate free energy. This may adjust your expectations on how likely this idea is. How likely is a free energy device (ie, one that reliably gives you more energy out than you put in)? Well, this thruster can't be any more likely, because it would produce free energy. https://arxiv.org/abs/1506.00494 > the kinetic energy of a mass with a constant force applied will increase with the square of time, while the electrical energy input only increases linearly with time. A mass with such a drive attached would eventually have a kinetic energy greater than the energy input. ... > For example, an evacuated…
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track 10km long and supplied with 1 GW of power that is fed via induction along the track to a 100 kg mass equipped with an EM drive would result in the mass experiencing an acceleration of 4000 m/s2(i.e., 407 g’s) and having a velocity of 8.94 km/s at the end of the track, at which point the mass would have a kinetic energy of 4 GJ. The time to accelerate the mass is only 2.23 seconds,however, so the energy input required is only 2.23 GJ. At the end of the track, the mass could be decelerated via regenerative braking, generating more energy out (4 GJ) than was input (2.23 GJ), for a net gain of 1.76 GJ free energy.
Original comment NASA Funded Scientists Claim Breakthrough in Propellantless Propulsion
akvadrako
Neutral mention · rule
Basically, it's because as the drive keeps accelerating without losing mass, the velocity is proportional to the energy expended. However, it's kinetic energy increases with the velocity squared. Eventually, there is a cross-over point. You can create infinite energy by placing a bunch of drives on the spokes of a huge flywheel. See this paper for a more detailed answer: https://arxiv.org/abs/1506.00494
Original comment NASA's Peer-Reviewed EM Drive Paper Has Finally Been Published
beevai142
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Photon rockets are not actually reactionless, because the emitted photons carry momentum and energy out of the system. This makes conservation laws to hold for such systems, cf. https://arxiv.org/abs/1506.00494 Now, the emdrive people are apparently claiming that it is truly reactionless. If so, you can build a perpetual motion machine of the first kind (produces energy from nothing) out of it --- which should demonstrate the degree of plausibility of such claims. For photon rockets, there's apparently a hard upper bound on the efficiency, which limits the practical usability.
Original comment EmDrive: China claims success with this 'reactionless' engine for space travel
roywiggins
Neutral mention · rule
It would have been a perpetual motion machine of the first kind (you could get an unlimited amount of energy out of it): https://arxiv.org/abs/1506.00494 The evidence for it was no better than the evidence for any other perpetual motion machine, there was nothing that made it special other than not claiming to be a perpetual motion machine (despite being one).
Original comment 'Impossible' EmDrive Space Thruster May Really Be Impossible
roywiggins
Neutral mention · rule
Conservation of momentum is almost as important as conservation of energy, and indeed, if you have a reactionless drive you can extract unlimited energy: "applying a constant force results in a constant acceleration, the kinetic energy of a mass driven by such a device increases quadratically with time, while the energy input increases only linearly with time. Thus, at some point, the kinetic energy of the device-driven mass exceeds the energy input, and if this energy is collected via decelerating the mass (via regenerative electromagnetic braking, for example), then there would be a net gain in energy." https://arxiv.org/abs/1506.00494
Original comment The SpaceDrive Project – First Results on EMDrive and Mach-Effect Thrusters

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