For any given specific power, with a large which is desirable to save propellant mass, it turns out that the maximum acceleration is inversely proportional to .

Bill Tee's propellant mass is pure, distilled water shipped up to orbit from Ganymede.

In a dual-thrust solid fuel rocket motor, the propellant mass is composed of two different types or densities of fuel.

According to the Tsiolkovsky rocket equation, a rocket with higher exhaust velocity needs less propellant mass to achieve a given change of speed.

The theoretical exhaust velocity of a given propellant chemistry is a function of the energy released per unit of propellant mass (specific energy).

In other words, the propellant mass fraction is the ratio between the propellant mass and the initial mass of the vehicle.

However, for reaching orbital speed, hydrogen is a better fuel, since the high exhaust velocity and hence lower propellant mass reduces the take off weight.

The hydrogen is heated by the fusion plasma debris to increase thrust, at a cost of reduced exhaust velocity (348-463 km/s) and hence increased propellant mass.

This lack of a performance history in space meant that despite the potential savings in propellant mass, the technology was considered too experimental to be used for high-cost missions.

A closely related (but different) concept to energy efficiency is the rate of consumption of propellant mass.