laser thruster.

[KerikBalm]

On 12/4/2019 at 4:47 PM, Flying dutchman said:

If we could somehow build a one tonne 1000mw reactor with a 1000mw laser a lightyear could be traversed in 112 years

Well, you just kicked up the reactor power (already likely way too high) by a factor of 1,000x

On top of that, you have to assume that your reactor can operate for 112 years on its onboard fuel supply. Modern ships require refueling every 20 years or so... https://en.wikipedia.org/wiki/A4W_reactor

That reactor is only 550 MW of thermal power (less once converted to electricity and photons).

So as you use up fissile fuel, what do you do, jettison it? That is wasted reaction mass. Its inefficient to send some energy/mass back at 1c, and send most of your energy/mass (only a small fraction of the rest mass of U-233/U-235/P-239 is converted to energy) at essentially 0 m/s.

If you want to get max dV from a fission reaction, you don't use light as your remass, you use fission fragments... like with the orion drive

Its most efficient to use all the released energy from the fuel to accelerate the fuel to high velocity. 

[Guest]

Orion drive doesn't use fission fragments, it uses vaporized tungsten. It's also not particularly efficient, the main advantages are thrust and lack of containment issues, it's basically a thermal engine that runs at a ludicrously huge temperature. For true fission fragment propulsion, you're looking for an FFRE. Specific impulse is awesome, thrust sucks. Technically complex, but there are several concepts.

[KerikBalm]

Yea, the standard orion concept has very little remass in fission fragment form.

As mentioned, ffre engine designs may have bad thrust levels... The thing with really high isp and good thrust is that you need really really high energy outputs... Like the power output of a detonating nuke.

So if we could imagine an ideal system where you have a fission device that has a 100% fission rate, and some way if using it ... a magnetic nozzle like the mini-mag Orion concept?

Since it would involve a nuclear detonation, I would put it as a orion drive variant... But yeah, it would be quite different from the standard shaped charge with tungsten, maybe sig amounts of U 238, and... I don't know, a 10% fission rate?

[Guest]

That'd be NSWR, possibly also some variant of flow-stabilized fusion drive. As a bonus, both of those provide steady acceleration, which can be problematic for crewed flights.

However, total energy requirements are always at the root of the problem. If you demand both high thrust and Isp for a sustained period of time, the question quickly becomes where to get all that energy from in first place.