Weaponized Exhaust: Ah, yes – the Kzinti lesson. What can we say about the Kzinti lesson.
Well, we can say that it is both entirely true and entirely useless (in actual space combat, although on the ground your mileage may differ).
It’s entirely true because, as Larry Niven said, “A reaction drive’s efficiency as a weapon is in direct proportion to its efficiency as a drive,” and obviously torch drives that let you tool around the system at consistent single-digit g accelerations are very efficient drives indeed. Unless you are using incredibly exotic, non-standard-molecular-matter materials, anything you park directly behind a torch drive will evaporate like a pat of butter under a blowtorch. No question.
It’s entirely useless because, as Atomic Rockets points out, “propulsion exhaust is poorly collimated, which means after a very short range it will have expanded and dissipated into harmlessness”. This is even perfectly deliberate as well as unintentional – it’s a basic safety feature for being able to use drives in the vicinity of anything else, ever. As such, unless you’re at point-blank range, you won’t be teaching this particular lesson – and if you let someone reach point-blank range in anything capable of being Kzinted, or make doing so part of your offensive plan, you fail space warfare tactics forever. Note that the inner engagement envelope begins at an entire light-second.
So, basically, if you want to weaponize your drive, stick to something that’s supposed to be collimated. Like launching lasers, or one of these.