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arXiv:1506.00494physics.pop-ph

Reconciling a Reactionless Propulsive Drive with the First Law of Thermodynamics

Andrew J. Higgins

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.

arXiv comments
Journal reference
DOI
Subjects
physics.pop-phPopular Physics

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Discussed in comments5 threads

  1. NASA Funded Scientists Claim Breakthrough in Propellantless Propulsion 1 comment

    • 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…

      roywigginsView on HNParent context

  2. 'Impossible' EmDrive Space Thruster May Really Be Impossible 4 comments

    • Any reactionless drive better than a photon drive is probably a perpetual motion machine. https://arxiv.org/abs/1506.00494

      roywigginsView on HNParent context

    • Such a drive would necessarily be a perpetual motion machine. Perpetual motion machines are a dime a dozen and we don't test all of them rigorously because approximately none of them will ever work. https://arxiv.org/abs/1506.00494

      roywigginsView on HNParent context

    • 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…

      roywigginsView on HNParent context

    • I found this paper pretty convincing. Any "reactionless" drive more efficient than a photon drive is probably a perpetual motion machine: https://arxiv.org/abs/1506.00494 The claimed thrust of the EMDrive was incompatible with photon thrust- it was much too powerful.

      roywigginsView on HNParent context

  3. The SpaceDrive Project – First Results on EMDrive and Mach-Effect Thrusters 3 comments

    • And indeed, you can build a perpetual motion machine out of a reactionless drive. "any device with a thrust-to-power ratio greater than the photon rocket would be able to operate as a perpetual motion machine" https://arxiv.org/abs/1506.00494

      roywigginsView on HNParent context

    • 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…

      roywigginsView on HNParent context

    • Does this sidestep the perpetual motion problem? https://arxiv.org/abs/1506.00494

      roywigginsView on HNParent context

  4. NASA's Peer-Reviewed EM Drive Paper Has Finally Been Published 1 comment

    • 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…

      akvadrakoView on HNParent context

  5. EmDrive: China claims success with this 'reactionless' engine for space travel 1 comment

    • 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…

      beevai142View on HNParent context

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