7 ms·

Actually it's not as much useful as an engine as it may seem, at least at current parameters. Ion thrusters are pretty good as low-power engines. And it's not c

Actually it's not as much useful as an engine as it may seem, at least at current parameters. Ion thrusters are pretty good as low-power engines. And it's not clear that emdrive is better than that even if it works. Maybe with powerful space-based nuclear reactors

deleted 10y ago
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jack9 10y ago
> Actually it's not as much useful as an engine as it may seem, It's worth more than any existing or previous theoretical engine. Put this rotating around a center point. Now put 1000s of them. Now we have power generation for outposts in space. The implications of a working EM drive are staggering.
- pugworthy 10y ago
  What makes the power to power them?
  - binarycoffee 10y ago
    easy: 1000000's of them rotating around a fixed point!
- krylon 10y ago
  Naive questions from someone who does not understand a lot about physics (to put it mildly): Did you just invent the perpetuum mobile, or did I misunderstand what you said? Or do I just not understand how this thing is supposed to work?
  - Filligree 10y ago
    He did, yeah. If it works as described, then you'd expect it to break conservation of energy along with conservation of momentum. That's another good reason to think it doesn't work.
  - gus_massa 10y ago
    Actually, yes! By special relativity you can't break the momentum conservation principle without breaking the energy conservation principle. Somehow most people feel more comfortable dismissing the momentum conservation principle, but they are intertwined. A carousel with Emdrives like the described in the previous comment should be a perpetual mobile and create energy if it's spinning in the right direction. (It will destroy energy in the reverse direction, so be careful.) [Anyway, I think that the emdrive doesn't work and the conservation of energy and momentum are safe.]
    - MPSimmons 10y ago
      The Emdrive takes energy, though, doesn't it? It's not like you can plug it in and have it run, unless I misunderstand a lot of claims.
      - sethrin 10y ago
        Put two emdrives on the ends of a rotating beam. Use the violation of conservation of energy/momentum to drive a generator. Infinite speed and energy are yours to command!
        MPSimmons 10y ago
        Driving the generator is going to rob the system of energy. That should be obvious, right? Nothing in the matter/energy equation is changing. This drive would just take electrical energy and convert it into kinetic energy.
      - Tuna-Fish 10y ago
        Yes. However, any true reactionless thrust is equivalent to a perpetual motion machine, because reactionless thrust drives produce the same acceleration regardless of speed, but the kinetic energy produced by the same acceleration goes up as the speed goes up. If you accelerate one kg from 0m/s to 1m/s, you impart 0.5joules of energy on it. If you accelerate it from 1m/s to 2m/s, you impart 1.5joules of energy on it. If you accelerate it from 2m/s to 3m/s, you impart 2.5 joules of energy on it, and so on and so on. Each added m/s costs more in energy. A reactionless drive working in a system with no preferred frame would add the same amount of acceleration for the same amount of cost, regardless of how much energy there already is. This would mean that eventually it would be going fast enough that the added kinetic energy would be more than however much energy it draws in. Then you can build a gigantic carousel that is spinned on the rim and takes energy from the middle and feeds some back in to move it, and now you have perpetual motion. This is why no real physicist actually thinks that this will be reactionless thrust. However, that does not necessarily mean it's useless. If it, for example, allows you to push against the earth's magnetic field more weight-efficiently than current magnetic propulsion systems, it would be a major win for satellite stationkeeping.
  - CydeWeys 10y ago
    He did not. The emdrive requires a very large amount of power to operate, way more per Newton than even an ion drive.
  - jack9 10y ago
    > Did you just invent the perpetuum mobile, That's the implication of the EM Drive. I didn't invent it, it's the very first thing that you think of once you violate the momentum conservation principle. This is why it's unlikely to be true, but at the same time an alchemists dream scenario.
- fooker 10y ago
  This takes a huge amount of electricity to work.
- CydeWeys 10y ago
  The emdrive (at least the current version) requires way more power per Newton of force than even ion engines, which are themselves famously power hungry. You definitely could not get free energy out of an emdrive because the amount of power required to run it is way more than you'd get out. The advantage of the emdrive is that it does not require reaction mass, at the cost of incredibly high power consumption. Unfortunately that makes it not really viable for anything; ion drives are limited by power, not reaction mass, so the emdrive is actually worse because it uses way more in power than it saves in not needing reaction mass.
  - wcoenen 10y ago
    The drive being power hungry would not change the fact that kinetic energy goes up quadratically with speed (E=(m*v^2)/2) while this drive supposedly can keep accelerating with only constant power input. So kinetic energy would go up quadratically with time while expended energy only goes up linearly. You'd get free energy very soon. Since violation of conservation of energy is very unlikely, I'd say that this is a sign that the drive doesn't actually work. Or at least that there is some gotcha that we haven't understood yet.
    - CydeWeys 10y ago
      Anything keeps accelerating with only constant power input. That's literally how all rockets in a vacuum work. Nothing special with an emdrive there. It doesn't mean that perpetual motion is possible, it just means that you're wrong about the ramifications of kinetic energy vis a vis power generation.
      - deleted 10y ago
        [deleted]
      - wcoenen 10y ago
        A rocket using chemical energy at a constant rate does not have this problem, because the excess energy it seems to gain from kinetic energy going up quadratically is balanced by a decrease in the kinetic energy that the expelled reaction mass is left with.
ribble 10y ago
did you not read the article? to mars in 70 days, i believe it says..
- photogrammetry 10y ago
  We can get to Mars in 90 days with chemical propulsion, today.
  - ribble 10y ago
    and this refutes how?
CamperBob2 10y ago
The difference is that eventually you run out of ions to thrust.
- CydeWeys 10y ago
  Practically speaking though the power generation is the limiting factor. An emdrive requires way more power, which means way more generating capabilities, which ends up taking up way more mass on your spacecraft than the ion engine reaction mass.
  - darkmarmot 10y ago
    If you have a nuclear reactor, the power vs mass loss could end up working in favor of the emdrive, especially for long range interstellar flights. None of this is, of course, practical anytime soon...
    - CydeWeys 10y ago
      Nuclear reactors are incredibly massive though, especially with all of the required shielding. We've never launched anything even as close to as massive as a space-borne nuclear reactor would be. I agree with you that it won't be practical anytime soon (or at least not politically), so as long as we're positing some future level of tech, why not nuclear fusion?
      - CamperBob2 10y ago
        I don't really see a huge difference between a spaceborne reactor and the kind that have been in constant use for decades in submarines. We just need a new Admiral Hyman Rickover to make it happen safely and effectively.
        CydeWeys 10y ago
        Two big problems: 1. Details on submarine nuclear reactors are classified, but five minutes' Googling shows that a reasonable guess for their total mass (including shielding) is 1,000 tons. Meanwhile, the entire payload to LEO of the largest rocket ever successfully launched, the Saturn V, is only 155 tons -- and that's for the entire top stage. 2. Submarine nuclear reactors use water for cooling. Water that is not available in space. Cooling would be a huge problem. So, existing submarine designs are not practical. You'd need to design something from the ground up that is much lighter, and the cooling system would be entirely different and likely more massive, since you don't have all of that free water available to dump heat into. Instead you're talking massive radiators.

deleted 10y ago

[deleted]

jack9 10y ago

> Actually it's not as much useful as an engine as it may seem, It's worth more than any existing or previous theoretical engine. Put this rotating around a center point. Now put 1000s of them. Now we have power generation for outposts in space. The implications of a working EM drive are staggering.

pugworthy 10y ago
What makes the power to power them?
- binarycoffee 10y ago
  easy: 1000000's of them rotating around a fixed point!
krylon 10y ago
Naive questions from someone who does not understand a lot about physics (to put it mildly): Did you just invent the perpetuum mobile, or did I misunderstand what you said? Or do I just not understand how this thing is supposed to work?
- Filligree 10y ago
  He did, yeah. If it works as described, then you'd expect it to break conservation of energy along with conservation of momentum. That's another good reason to think it doesn't work.
- gus_massa 10y ago
  Actually, yes! By special relativity you can't break the momentum conservation principle without breaking the energy conservation principle. Somehow most people feel more comfortable dismissing the momentum conservation principle, but they are intertwined. A carousel with Emdrives like the described in the previous comment should be a perpetual mobile and create energy if it's spinning in the right direction. (It will destroy energy in the reverse direction, so be careful.) [Anyway, I think that the emdrive doesn't work and the conservation of energy and momentum are safe.]
  - MPSimmons 10y ago
    The Emdrive takes energy, though, doesn't it? It's not like you can plug it in and have it run, unless I misunderstand a lot of claims.
    - sethrin 10y ago
      Put two emdrives on the ends of a rotating beam. Use the violation of conservation of energy/momentum to drive a generator. Infinite speed and energy are yours to command!
      - MPSimmons 10y ago
        Driving the generator is going to rob the system of energy. That should be obvious, right? Nothing in the matter/energy equation is changing. This drive would just take electrical energy and convert it into kinetic energy.
    - Tuna-Fish 10y ago
      Yes. However, any true reactionless thrust is equivalent to a perpetual motion machine, because reactionless thrust drives produce the same acceleration regardless of speed, but the kinetic energy produced by the same acceleration goes up as the speed goes up. If you accelerate one kg from 0m/s to 1m/s, you impart 0.5joules of energy on it. If you accelerate it from 1m/s to 2m/s, you impart 1.5joules of energy on it. If you accelerate it from 2m/s to 3m/s, you impart 2.5 joules of energy on it, and so on and so on. Each added m/s costs more in energy. A reactionless drive working in a system with no preferred frame would add the same amount of acceleration for the same amount of cost, regardless of how much energy there already is. This would mean that eventually it would be going fast enough that the added kinetic energy would be more than however much energy it draws in. Then you can build a gigantic carousel that is spinned on the rim and takes energy from the middle and feeds some back in to move it, and now you have perpetual motion. This is why no real physicist actually thinks that this will be reactionless thrust. However, that does not necessarily mean it's useless. If it, for example, allows you to push against the earth's magnetic field more weight-efficiently than current magnetic propulsion systems, it would be a major win for satellite stationkeeping.
- CydeWeys 10y ago
  He did not. The emdrive requires a very large amount of power to operate, way more per Newton than even an ion drive.
- jack9 10y ago
  > Did you just invent the perpetuum mobile, That's the implication of the EM Drive. I didn't invent it, it's the very first thing that you think of once you violate the momentum conservation principle. This is why it's unlikely to be true, but at the same time an alchemists dream scenario.
fooker 10y ago
This takes a huge amount of electricity to work.
CydeWeys 10y ago
The emdrive (at least the current version) requires way more power per Newton of force than even ion engines, which are themselves famously power hungry. You definitely could not get free energy out of an emdrive because the amount of power required to run it is way more than you'd get out. The advantage of the emdrive is that it does not require reaction mass, at the cost of incredibly high power consumption. Unfortunately that makes it not really viable for anything; ion drives are limited by power, not reaction mass, so the emdrive is actually worse because it uses way more in power than it saves in not needing reaction mass.
- wcoenen 10y ago
  The drive being power hungry would not change the fact that kinetic energy goes up quadratically with speed (E=(m*v^2)/2) while this drive supposedly can keep accelerating with only constant power input. So kinetic energy would go up quadratically with time while expended energy only goes up linearly. You'd get free energy very soon. Since violation of conservation of energy is very unlikely, I'd say that this is a sign that the drive doesn't actually work. Or at least that there is some gotcha that we haven't understood yet.
  - CydeWeys 10y ago
    Anything keeps accelerating with only constant power input. That's literally how all rockets in a vacuum work. Nothing special with an emdrive there. It doesn't mean that perpetual motion is possible, it just means that you're wrong about the ramifications of kinetic energy vis a vis power generation.
    - deleted 10y ago
      [deleted]
    - wcoenen 10y ago
      A rocket using chemical energy at a constant rate does not have this problem, because the excess energy it seems to gain from kinetic energy going up quadratically is balanced by a decrease in the kinetic energy that the expelled reaction mass is left with.

ribble 10y ago

did you not read the article? to mars in 70 days, i believe it says..

photogrammetry 10y ago
We can get to Mars in 90 days with chemical propulsion, today.
- ribble 10y ago
  and this refutes how?

CamperBob2 10y ago

The difference is that eventually you run out of ions to thrust.

CydeWeys 10y ago
Practically speaking though the power generation is the limiting factor. An emdrive requires way more power, which means way more generating capabilities, which ends up taking up way more mass on your spacecraft than the ion engine reaction mass.
- darkmarmot 10y ago
  If you have a nuclear reactor, the power vs mass loss could end up working in favor of the emdrive, especially for long range interstellar flights. None of this is, of course, practical anytime soon...
  - CydeWeys 10y ago
    Nuclear reactors are incredibly massive though, especially with all of the required shielding. We've never launched anything even as close to as massive as a space-borne nuclear reactor would be. I agree with you that it won't be practical anytime soon (or at least not politically), so as long as we're positing some future level of tech, why not nuclear fusion?
    - CamperBob2 10y ago
      I don't really see a huge difference between a spaceborne reactor and the kind that have been in constant use for decades in submarines. We just need a new Admiral Hyman Rickover to make it happen safely and effectively.
      - CydeWeys 10y ago
        Two big problems: 1. Details on submarine nuclear reactors are classified, but five minutes' Googling shows that a reasonable guess for their total mass (including shielding) is 1,000 tons. Meanwhile, the entire payload to LEO of the largest rocket ever successfully launched, the Saturn V, is only 155 tons -- and that's for the entire top stage. 2. Submarine nuclear reactors use water for cooling. Water that is not available in space. Cooling would be a huge problem. So, existing submarine designs are not practical. You'd need to design something from the ground up that is much lighter, and the cooling system would be entirely different and likely more massive, since you don't have all of that free water available to dump heat into. Instead you're talking massive radiators.