Outside of science fiction, photon rockets are positioned as low-thrust, low-acceleration, space-use-only engines. But... do they have to be?
For purposes of this question, a "photon rocket" is anything which emits photons preferentially in one direction, with thrust being a function of total photon power output and combination. If a given photon source can produce a unidirectional beam, awesome; if, like a standard incandescent filament, it can't, then any lenses or reflector structures used to redirect the output and increase would count against the weight budget for calculating thrust-to-weight ratio.
So, ignoring fuel systems or power sources, and considering only the actual device that makes photons and shoots them out the back, how much thrust per weight could an optimized photon rocket really get, without invoking any magic?
- Assume "weight" as measured on Earth's surface--i.e., mass, multiplied by 9.8m/s^2
- This question is not asking about a "ship". It's asking about an engine.
- "Thrust to weight ratio only makes sense for a ship!" No, thrust-to-weight ratio is commonly calculated for isolated engines, independent of any larger vehicle that they might be integrated into. SpaceX engines: "The engine's 150:1 thrust-to-weight ratio is the highest ever achieved for a rocket engine."; Rocketdyne F1 engines: "Thrust-to-weight ratio 94.1" TWR is a critical statistic to know for a engine, because it constrains the types of ships and missions that you might choose to build around that engine or select that engine for.
- The maximum TWR for a ship designed around a given enginer ship is the TWR of the engine, in the limit as you design for tinier payloads attached to honkin' huge engines. That's why engine TWR is a relevant design factor--your ship's TWR can't be better than that.
- "Dissipating heat is not negligible because heat is dissipated as photons." Yes, that is exactly correct! But if you want to use those photons as part of your thrust, you need some structure to reflect them non-isotropically, which becomes part of your mass budget. If at affects the engine thrust, it is part of the engine!
- "There are so many variables, you can just pick a number for your story!" Sure, but some numbers are physically possible and some aren't. It's blatantly obvious that you can't push arbitrarily large quantities of power through an arbitrarily small device without destroying it, so what actually is the physical limit?
- If the amount of power you shove through device causes it to destroy itself, you don't have a photon rocket--maybe you did, at a lower power level, but you have turned it into a very poor electric thermal rocket. The TWR and max power rating of a Merlin rocket engine isn't measured by blowing it up, and those of an edge-of-the-envelope photon rocket shouldn't be measured that way either.