Photon Rocket vs Conventional Propellant Rocket

[sanman]

I want to ask about the differences between a photon rocket and a rocket which uses conventional propellant.

A conventional rocket shoots exhaust matter out the back end, resulting in impulse and acceleration.
A photon rocket shoots photons out the back end, resulting in impulse and acceleration.

The exhaust propellant of a conventional rocket transfers some of its momentum to the rocket, which results in the impulse and acceleration for the rocket. This leaves the exhaust propellant particles with lower velocity (energy) than they otherwise would have left with.
The photon exhaust of a photon rocket transfers some of its momentum to the rocket, which results in the impulse and acceleration for the rocket. This leaves the photon exhaust particles with lower frequency (energy) than they otherwise would have left with.

A conventional rocket can incorporate an expansion nozzle and bell to drain/transfer even more momentum from the propellant exhaust particles to the rocket than would have otherwise been possible without the nozzle and bell, thus resulting in a more efficient rocket. This happens because the expansion nozzle and bell provide more collision opportunities for the exhaust propellant particles to transfer momentum to the rocket.

What is the analogous counterpart to the nozzle and bell, for a photon rocket? Is there anything?

 

[Drakkith]

What is the analogous counterpart to the nozzle and bell, for a photon rocket? Is there anything?

Hmmm... perhaps a parabolic reflector or lens to focus the outgoing rays?

 

[sanman]

Hmmm... perhaps a parabolic reflector or lens to focus the outgoing rays?

Can a parabolic reflector or lens increase the opportunities for momentum transfer to the rocket from each photon?

 

[Drakkith]

Can a parabolic reflector or lens increase the opportunities for momentum transfer to the rocket from each photon?

That I don't know. I was thinking more along the lines of making each photon move opposite to the direction of the rocket to increase the amount of thrust provided in that direction instead of having it simply be ejected in a large cone. But I admit I'm not too familiar with the design of photon rockets.

Wiki has a small note on this: http://en.wikipedia.org/wiki/Photon_rocket

Photons could be generated by onboard generators, as in the nuclear photonic rocket. The standard textbook case of such a rocket is the ideal case where all of the fuel is converted to photons which are radiated in the same direction. In more realistic treatments, one takes into account that the beam of photons is not perfectly collimated, that not all of the fuel is converted to photons, and so on.

 

[sanman]

Using photons as propellant isn't quite the same as using matter as propellant, since photons behave differently than matter does.

For instance, the propellant matter for a conventional rocket doesn't disappear, but meanwhile photons can readily be converted into other forms of energy and seem to "disappear" from the ledger. You can do a massflow balance for a conventional rocket, but a mass flow balance for a photon rocket seems more tricky, because photons can do more things to turn their apparent mass into something else.