Half a croissant, on a plate, with a sign in front of it saying '50c'
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This would work fine, except in terms of success.

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Reverse fusion ion drive

Make your mass.
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Ion drives are supernifty. But the problem is they still gotta shoot something. They need mass. Eventually a far-ranging vessel will run out of stuff to shoot and will have to stop to reload, or scoop up mass on the fly which means a net slowdown as vessel must collide with environmental particle in order to harvest it.

E = mc^2 which means m = E/c^2. A deep space vessel could accumulate energy using solar panels or by some other means, and then convert it to an ion to shoot. Reload could thus take place without slowing.

Obviously such an approach works best in the situation where energy is abundant and mass scarce; if we are purely scifi something like zeropoint energy could be used.

bungston, Jun 09 2010

Comet nucleus http://en.wikipedia.../wiki/Comet#Nucleus
Lots of useful stuff, for free... [8th of 7, Jun 09 2010]


       Solutions that have been suggested include harvesting interstellar hydrogen. Collecting starlight and converting it to hydrogen would be terribly inefficient.
ldischler, Jun 09 2010

       What [ldischler] said.   

       The Bussard Ramscoop as a design is [widely-known-to-exist].   

       Inverse-square law means that in interstellar space, the amount of energy that can be harvested with photovoltaics is derisory.   

       Aquiring more reaction mass can be achieved by bouncing a gas giant and scooping up some of the upper atmosphere on the way through, or grabbing comets and crunching them up.   


       (not our fishbone, by the way).
8th of 7, Jun 09 2010

       I'd have to check, but I think you'll find that if you're in a situation where you've got an energy source, but no reactive mass, you're much better off using a photon drive rather than making some reaction mass and putting it through an ion drive.   

       Of course, if you're going to use solar panels to gather the energy (inneficiently), then use that energy to run a photon drive, then why not use a solar sail?   

       I've read some discussions about craft using a solar sail with some of the surface covered in photovoltaics, which charge batteries for use in a photon drive for directional control/maneuvers. Or something like that.
Custardguts, Jun 09 2010

       Photon drive? Whazzat? A big flashlight?
RayfordSteele, Jun 10 2010

       Quite big, yes.
8th of 7, Jun 10 2010

       //Photon drive?//   

       Yep. You're better off using the momentum of those photons, as they will give you the maximum thrust for the energy.
ldischler, Jun 10 2010

       [+] "starlight drive": cool... wouldn't work but still cool.   

       wonder if Dr Bussard designed his ramjet to be regenerative.
FlyingToaster, Jun 11 2010

       I agree with Custardguts and ldischler.   

       E^2=(pc)^2 + (mc^2)^2   

       So you have two options:
set m to zero and get a photon with E/c momentum (for a photon drive); or set p to zero and get a mass of E/c^2 (for an ion drive) (which you'd then have to accelarate to some velocity* to get an equivalent amount of momentum).

       *the equation implies you'd need to accelarate it to c (the speed of light), but that's not taking into account relativistic effects. So it's probably less than c but pretty close.   

       Since all you really want for propulsion is momentum, photon drive is clearly the better option.
xaviergisz, Jun 11 2010


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