>>14269451Laser energy input plus propellant = high thrust good Isp. Pure laser propulsion is the most efficient form of propulsion but is extremely low thrust. If you are scooting around between asteroids in the main belt, where you may only need 200 m/s of delta V at a time but are trying to push around 5000 ton blocks of harvested material between extraction sites and industrial stations/habitats, then higher thrust matters more.
Besides, minmaxing efficiency only matters when your per-maneuver delta V requirements are very high, for example doing a fast transfer to the outer planets. In those cases, apart from either end of your transportation route, you can't just pick up more propellant anywhere, so you need the maximum possible delta V per unit of propellant that you can carry. If your per-maneuver delta V is low, then with ISRU you probably have an abundance of propellant anyway, so extreme reliability and other performance metrics beyond Isp become more useful.
I talked about something similar to this before on the topic of the best ways to use nuclear thermal engines, which in my opinion is on water-propelled orbital launch vehicles operating at the icy moons of the outer planets. These stages would only get an Isp in vacuum of around 350, maybe 380 if we can push the core temperatures really high with better materials technology, but as Raptor makes very obvious, an Isp of 350 to 380 is still extremely useful and very viable for space propulsion. Also, due to the higher mass flow rate due to using a denser propellant, the thrust to weight ratio of nuclear engines using water is significantly higher than the TWR when they are designed to use hydrogen, making them even more useful for low gravity launch vehicles.
IMO this laser heated thermal propulsion system, if it actually could achieve >2000 Isp, would be a great tool to unlock human transport to Jupiter and beyond. Of course this relies on some very good laser technology which we don't have yet
