Hi Lee

Simply, a rocket has to push itself and the fuel it will need for the trip. You very rapidly run up a huge mass budget. Also a photon rocket needs an extreme energy source for miniscule thrust and its power output has to be insanely high. A thrust of one newton is enough to accelerate a kilogram by a metre per second per second. A photon rocket needs a jet power of 300 million joules for a measly newton - a nuclear reactor with that output would mass ~ 60 - 30 tonnes. And it would need a lot of nuclear fuel and heat radiators massing hundreds of tons. More powerful reactors don't improve things much as the reactor/radiator mass keeps rising with the power output.

Nuclear fuel at most produces ~ 83 terajoules per kilogram when fully burnt up, but converting that to electric power, say, for making photons is only ~ 35 - 60% efficient. Making photons with lasers is inefficient as well, turning ~ 50% of the power input into photons, losing the rest as heat. So about 17.5% - 35% of the fuel's energy becomes photons. Hence there's a lot of fuel wasted with a photon rocket. And you have to push all of it when carrying it on a rocket.

Far, far easier to make bombs with the stuff and toss it out the back to go "bang!", riding the plasma burst with a magnetic field... and even easier to leave it all at home and shot it at the magnetic pusher plate.

Which is where we began this discussion.

Adam