Non-relativistic rocket equations:
The quantity “ve” is the exhaust velocity, a measure of quality of a rocket determining the rocket’s “gas mileage”.
Relativistic rocket equation:
Where the quantity delta-r is called the rapidity and is additive.
The “Isp” is the specific impulse of the propulsion system, a measure of quality analogous to the exhaust velocity for a non-relativistic rocket.
For long journeys, the Isp, rather than the thrust, becomes the major factor in determining flight time. This is because high Isp allows for much longer burn times, and therefore much greater accumulation of velocity.
Also, by definition of the specific impulse:
Where the Isp is in seconds and g0 is the gravitational acceleration.