4 ms·

If it works then we can just throw up a nuclear reactor into space and a small amount of nuclear fuel will create a continuous thrust which in space matters. Ca

If it works then we can just throw up a nuclear reactor into space and a small amount of nuclear fuel will create a continuous thrust which in space matters. Can lead to a spaceship that gets shuttled to which just ferries ships between planets.

ben_w 6y ago
If we could put a nuclear reactor in space large enough for this to be useful, we could use the reactor to boil water and get vastly more thrust. Or run an ion drive and get more thrust. Or just let the fission fragments out the back and get more thrust. What this would be useful for is (by no coincidence) also the exact reason why it is considered crank science: you could use it to build a perpetual motion machine because it violates conservation of energy/momentum laws.
- jiofih 6y ago
  The trick is not requiring propellant. You can’t carry 90 years worth of water or gas. The Dawn spacecraft launched to Ceres in 2007 used up 425kg of Xenon for only eleven days of thrust for its ion engine.
  - ben_w 6y ago
    If you let the fission fragments out the back of the reactor you get more thrust than this. (Even if the EmDrive worked).
    - jiofih 6y ago
      You still need tons of fuel for that, in the order of tens of tons per year travelled - unrealistic for travel outside our solar system. Thrust doesn’t really matter at that time scale.
      - ben_w 6y ago
        The fuel is the reactor fuel. Literally the fission products of the uranium/plutonium in your reactor, from where the power you want to use for the EmDrive comes from, would beat this. If the EmDrive actually worked, the correct use is to violate conservation of energy and momentum, not as a straightforward substitute for the engines we have now.
- doctor_eval 6y ago
  But isn’t it consuming energy? Where is that kilowatt going?
  - ben_w 6y ago
    What is the “it” here? EmDrive looks like an accounting error to me, not something that actually works. Any energy it expends turns into heat, radiating in a predictable but not necessarily isotropic way. This radiation carries momentum, but much less than the EmDrive claims to be capable of. A rocket or ion drive? The energy goes into the kinetic and gravitational potential of the exhaust. (Aside: kilowatts are power, not energy).
- Aeolun 6y ago
  If we can turn mass into energy, why can’t we turn energy into mass?
  - ben_w 6y ago
    You can, we’ve been doing that for ages. What’s the rest of your question?
bjo590 6y ago
Some napkin math. How long would it take a nuclear powered EmDrive to push a fully loaded semitrailer from 0m/s to the speed of the interntional space station? The maximum mass for a semi-truck fully loaded is 36000kg. The EmDrive generates 12millinewtons per kw. 1n = 1kg*(1m/s^2). Leo ~=7.66km/s^. A nuclear reactor produces ~1gw. We would need 275760000N to accelerate a semi truck to leo speeds. A nuclear powered EmDrive would take ~265 days to push a semi truck to low earth orbit speeds. The escape velocity of Earth's orbit is 11.19 km/s, and if you were going to take humans on the trip you'd need much more cargo than 36000kg. Granted, you wouldn't have to move the craft from 11.19km/s to 0km/s and then 0km/s to 11.19 km/s every time you entered and left orbit, but it would be a slow ride regardless.
- short_sells_poo 6y ago
  I think we can't ignore the weight of the nuclear reactor though - we are not lifting just the truck, the reactor has to carry it's own weight. And a 1GW reactor will dwarf the 36t payload I think.
- raducu 6y ago
  Please, stop with the 12millinewtons -- if the effect is real, the cavity can be made orders of magnitude more efficient. But it is 99.9999% not going to work at all, the emDrive would just be a cheat at space travel. Something that can transform energy into momentum without shooting stuff back is the Holy Grail of space travel -- after a warp drive.
imtringued 6y ago
Ion thrusters already do this though. Even with conventional rockets once you're in space finding new fuel shouldn't be a big deal. It's just a matter of building the infrastructure. Right now leaving earth is still the biggest problem.

ben_w 6y ago

If we could put a nuclear reactor in space large enough for this to be useful, we could use the reactor to boil water and get vastly more thrust. Or run an ion drive and get more thrust. Or just let the fission fragments out the back and get more thrust. What this would be useful for is (by no coincidence) also the exact reason why it is considered crank science: you could use it to build a perpetual motion machine because it violates conservation of energy/momentum laws.

jiofih 6y ago
The trick is not requiring propellant. You can’t carry 90 years worth of water or gas. The Dawn spacecraft launched to Ceres in 2007 used up 425kg of Xenon for only eleven days of thrust for its ion engine.
- ben_w 6y ago
  If you let the fission fragments out the back of the reactor you get more thrust than this. (Even if the EmDrive worked).
  - jiofih 6y ago
    You still need tons of fuel for that, in the order of tens of tons per year travelled - unrealistic for travel outside our solar system. Thrust doesn’t really matter at that time scale.
    - ben_w 6y ago
      The fuel is the reactor fuel. Literally the fission products of the uranium/plutonium in your reactor, from where the power you want to use for the EmDrive comes from, would beat this. If the EmDrive actually worked, the correct use is to violate conservation of energy and momentum, not as a straightforward substitute for the engines we have now.
doctor_eval 6y ago
But isn’t it consuming energy? Where is that kilowatt going?
- ben_w 6y ago
  What is the “it” here? EmDrive looks like an accounting error to me, not something that actually works. Any energy it expends turns into heat, radiating in a predictable but not necessarily isotropic way. This radiation carries momentum, but much less than the EmDrive claims to be capable of. A rocket or ion drive? The energy goes into the kinetic and gravitational potential of the exhaust. (Aside: kilowatts are power, not energy).
Aeolun 6y ago
If we can turn mass into energy, why can’t we turn energy into mass?
- ben_w 6y ago
  You can, we’ve been doing that for ages. What’s the rest of your question?

bjo590 6y ago

Some napkin math. How long would it take a nuclear powered EmDrive to push a fully loaded semitrailer from 0m/s to the speed of the interntional space station? The maximum mass for a semi-truck fully loaded is 36000kg. The EmDrive generates 12millinewtons per kw. 1n = 1kg*(1m/s^2). Leo ~=7.66km/s^. A nuclear reactor produces ~1gw. We would need 275760000N to accelerate a semi truck to leo speeds. A nuclear powered EmDrive would take ~265 days to push a semi truck to low earth orbit speeds. The escape velocity of Earth's orbit is 11.19 km/s, and if you were going to take humans on the trip you'd need much more cargo than 36000kg. Granted, you wouldn't have to move the craft from 11.19km/s to 0km/s and then 0km/s to 11.19 km/s every time you entered and left orbit, but it would be a slow ride regardless.

short_sells_poo 6y ago
I think we can't ignore the weight of the nuclear reactor though - we are not lifting just the truck, the reactor has to carry it's own weight. And a 1GW reactor will dwarf the 36t payload I think.
raducu 6y ago
Please, stop with the 12millinewtons -- if the effect is real, the cavity can be made orders of magnitude more efficient. But it is 99.9999% not going to work at all, the emDrive would just be a cheat at space travel. Something that can transform energy into momentum without shooting stuff back is the Holy Grail of space travel -- after a warp drive.

imtringued 6y ago

Ion thrusters already do this though. Even with conventional rockets once you're in space finding new fuel shouldn't be a big deal. It's just a matter of building the infrastructure. Right now leaving earth is still the biggest problem.