# Another energy/thrust question:

sonhouse
Posers and Puzzles 30 Jan '10 21:07
1. sonhouse
Fast and Curious
30 Jan '10 21:074 edits
Do you gain anything if you have a VASIMIR kind of ion rocket where the thrust is directly related to the power input, how much power you have to accelerate the ions out the rear, or front if you want to decelerate, so you can see where this is somewhat related to the huge particle accelerators like in Cern or Brookhaven, the big machines that accelerate particles to say 99.999999 % of c.

So if you have ions that mass some number, whatever, 1 attogram per particle say, at rest, and you accelerate them to some non-relativistic energy, say 1000 Km/second, where you would get an incredible specific impulse number, whatever, say 10,000 as compared to 450 max from an O2/H2 chemical rocket, in that kind of ion thruster, you really conserve on fuel so the thing can accelerate for months on end and not run out of fuel if you have some kind of mega power source like a nuclear or fusion reactor or better yet an anti-matter reactor.

The question I have is lets suppose we use that 1 attogram per ion mass rating, totally a guess there but lets use that #, 1000 attograms=1 picogram, 1,000,000 attograms =1 nanogram, 1 billion attograms=1 micrograms and so forth, and you have an ion accelerator that accelerates those 1 attogram particles at the same velocity as the best of the big earthbound accelerators, and you get say, 99.9999999 % c then the mass of each particle goes way up. Now besides just the increase in velocity which by itself gets you an incredible increase in specific impulse rating from 450, chemical max, to say 20,000 or so which the best of the best of the ion thrusters can get, what do you gain by also getting relativistic change of mass where each particle now masses say 1 nanogram, 1000X its original mass simply because it is going that close to c.

Of course I am assuming some kind of magic power source, Terawatts? whatever, by whatever means you get it.

It looks to me like you would get 1000 times the actual thrust besides the increase in specific impulse rating which would be up in the millions I suppose.

It looks to me like if you could keep increasing the power to the accelerator portion of the ion rocket there would be some maximum in the specific impulse # because the ejecta can go no more than c but the actual thrust could go up because the mass of the ejecta goes up the closer you get to c.

Does this line of reasoning hold up?
2. wolfgang59
13 Feb '10 21:01
Originally posted by sonhouse
Do you gain anything if you have a VASIMIR kind of ion rocket where the thrust is directly related to the power input, how much power you have to accelerate the ions out the rear, or front if you want to decelerate, so you can see where this is somewhat related to the huge particle accelerators like in Cern or Brookhaven, the big machines that accelerate pa ...[text shortened]... the mass of the ejecta goes up the closer you get to c.

Does this line of reasoning hold up?
As I (partially) understand it as the velocity of the ejecta approachs c the inertial mass increases which means that an increasingly large force is required.

I dont think you are going to get ought for nought as they say in Yorkshire!

Also you seem to be arbitrarily mixing energy/power/force.
3. sonhouse
Fast and Curious
15 Feb '10 00:03
Originally posted by wolfgang59
As I (partially) understand it as the velocity of the ejecta approachs c the inertial mass increases which means that an increasingly large force is required.

I dont think you are going to get ought for nought as they say in Yorkshire!

Also you seem to be arbitrarily mixing energy/power/force.
Well supposing you have an energy source enough to accelerate the ions to within a hairsbreadth of c, then at least you would need less fuel I would think, if the ion is say, 1000X its rest mass, then you would need 1000X less of them for the same thrust, I think. It has been pointed out I have a fuzzy understanding of energy, force and such. I have to wrap my head around that more effectively.