4 ms·

Any propellant-less thruster that uses a constant amount of power and generates constant acceleration (beyond that of a "photon rocket", https://en.wikipedia.or

Any propellant-less thruster that uses a constant amount of power and generates constant acceleration (beyond that of a "photon rocket", https://en.wikipedia.org/wiki/Photon_rocket https://en.wikipedia.org/wiki/Photon_rocket) would violate the second law of thermodynamics. And if it does "work", then it becomes an infinite energy source. E.g. Let say the thruster uses 100W (100 J/s) and generated a thrust of 1N and the whole thing weighs 1kg. Look at how the kinetic energy (KE) changes with time. Time 0: v = 0 m/s, KE = 0J Time 1: v = 1 m/s, KE = 1J (delta = 1J) Time 2: v = 2 m/s, KE = 4J (delta = 3J) Time 3: v = 3 m/s, KE = 9J (delta = 6J) ... Time 50: v = 50 m/s, KE = 2500 J Time 51: v = 51 m/s, KE = 2601 J (delta = 101 J) <-- This is more than the 100 J that was put in Time 52: v = 52 m/s, KE = 2704 J (delta = 103 J) <-- Also more than the 100 J that was put in etc. Depending on the mass and thrust level, the "time to free energy" varies. So, any such thruster rigged up to an electric generator will eventually generate infinite power. Color me skeptical.

hyperhello 2y ago
I agree with your objection, but if it were in all unlikelihood true, maybe the universe would compensate for the energy imbalance in some new way (exploding or melting the source, refusing to go faster than input power dictates, just polarizing the electrons in place, etc) that itself could be useful somehow.
caseyy 2y ago
Well, a solar sail is propellant-less and has constant acceleration under technically a constant amount of power. The power source is external. Also magsails, electric charge sails, gravity assists (which could be seen as a gravity well sail) can have constant acceleration for a period of time. There are these more exotic methods where a drive interacts with its surrounding medium and the system is open, not self-contained. I don’t know if it’s the case that some method of this kind has been discovered. But in theory, you can push yourself off of anything that repels, pull yourself towards anything that attracts, or receive momentum from anything that has it in an open system. Every electric and magnetic charge you come across in space, you could use in theory, if you were yourself appropriately charged in that moment. This could use a lot of power and therefore a lot of work over time, and may not even be efficient. But it would still meet the propellantless criterium. It’s a bit sci-fi, of course. But probably not physically impossible. We humans propel ourselves without a propellant every day when we walk because we operate in an open system where there is a surrounding medium to grasp and pull or push against. Em drive and some other similar stuff were suggesting that they can make a force in a closed system, not through an interaction with a surrounding medium within which the truster exists, nor through expelling own propellant. It suggested a reactionless drive, which is not considered possible. But propellentless is very possible.
- DarmokJalad1701 2y ago
  > Well, a solar sail is propellant-less and has constant acceleration under technically a constant amount of power I guess I should have said reaction-less and not propellant-less. A solar sail is not a reaction-less drive. The momentum/energy is conserved in the system of solar sails + photons. There is no free lunch thre.
DarmokJalad1701 2y ago
Mild correction: I forgot a factor of 2 in the KE calculation. But the point still stands. Also it should have said "reaction-less" instead of propellant-less.

hyperhello 2y ago

I agree with your objection, but if it were in all unlikelihood true, maybe the universe would compensate for the energy imbalance in some new way (exploding or melting the source, refusing to go faster than input power dictates, just polarizing the electrons in place, etc) that itself could be useful somehow.

caseyy 2y ago

Well, a solar sail is propellant-less and has constant acceleration under technically a constant amount of power. The power source is external. Also magsails, electric charge sails, gravity assists (which could be seen as a gravity well sail) can have constant acceleration for a period of time. There are these more exotic methods where a drive interacts with its surrounding medium and the system is open, not self-contained. I don’t know if it’s the case that some method of this kind has been discovered. But in theory, you can push yourself off of anything that repels, pull yourself towards anything that attracts, or receive momentum from anything that has it in an open system. Every electric and magnetic charge you come across in space, you could use in theory, if you were yourself appropriately charged in that moment. This could use a lot of power and therefore a lot of work over time, and may not even be efficient. But it would still meet the propellantless criterium. It’s a bit sci-fi, of course. But probably not physically impossible. We humans propel ourselves without a propellant every day when we walk because we operate in an open system where there is a surrounding medium to grasp and pull or push against. Em drive and some other similar stuff were suggesting that they can make a force in a closed system, not through an interaction with a surrounding medium within which the truster exists, nor through expelling own propellant. It suggested a reactionless drive, which is not considered possible. But propellentless is very possible.

DarmokJalad1701 2y ago
> Well, a solar sail is propellant-less and has constant acceleration under technically a constant amount of power I guess I should have said reaction-less and not propellant-less. A solar sail is not a reaction-less drive. The momentum/energy is conserved in the system of solar sails + photons. There is no free lunch thre.

DarmokJalad1701 2y ago

Mild correction: I forgot a factor of 2 in the KE calculation. But the point still stands. Also it should have said "reaction-less" instead of propellant-less.