And there's a catch with increasing specific impulse. Specific impulse is proportional to exhaust velocity. The energy required to expel the exhaust is proport

And there's a catch with increasing specific impulse. Specific impulse is proportional to exhaust velocity. The energy required to expel the exhaust is proportional to the square of exhaust velocity. Because nothing is 100% efficient, high specific impulse means dissipating a lot of waste heat. Increase the thrust too much and you melt your vehicle.