This probably depends on how rapidly you can make the propellant material expand (more force) and how big you can build the expansion chamber/nozzle (more volume).
Practical space propulsion is pretty much just throwing shit out the back of your vehicle as fast as possible.
These kinds of engines get about double the exhaust velocity of high efficiency conventional rockets. (A hydrolox engine like that used on the space shuttle or the centaur upper stage getting an exhaust velocity of about 4500 m/s and a nuclear thermal getting about 8000m/s)
This probably depends on how rapidly you can make the propellant material expand (more force) and how big you can build the expansion chamber/nozzle (more volume).
Practical space propulsion is pretty much just throwing shit out the back of your vehicle as fast as possible.
These kinds of engines get about double the exhaust velocity of high efficiency conventional rockets. (A hydrolox engine like that used on the space shuttle or the centaur upper stage getting an exhaust velocity of about 4500 m/s and a nuclear thermal getting about 8000m/s)