which would complete the launch with rockets. This would drastically reduce the amount of velocity needed to be provided by rockets to reach orbit. Well under a tenth of orbital velocity from a small rocket thruster is enough to raise perigee if a design prioritizes minimizing such, but hybrid proposals optionally reduce requirements for the mass driver itself by having a greater portion of delta-v by a rocket burn (or orbital momentum exchange tether). On Earth, a mass-driver design could possibly use well-tested maglev components. To launch a space vehicle with humans on board, a mass driver's track would need to be almost 1000 kilometres long if providing almost all the velocity to Low Earth Orbit, though a lesser length could provide major launch assist. Required length, if accelerating mainly at near a constant maximum acceptable g-force for passengers, is proportional to velocity squared. For instance, half of the velocity goal could correspond to a tunnel a quarter as long needing to be constructed, for the same acceleration. For rugged objects, much higher accelerations may suffice, allowing a far shorter track, potentially circular or helical (spiral). Another concept involves a large ring design whereby a space vehicle would circle the ring numerous times, gradually gaining speed, before being released into a launch corridor leading skyward. Mass drivers have been proposed for the disposal of nuclear waste in space: a projectile launched at much above Earth's escape velocity would escape the Solar System, with atmospheric passage at such speed calculated as survivable through an elongated projectile and a very substantial heatshield. Spacecraft-based mass drivers A spacecraft could carry a mass driver as its primary engine. With a suitable source of electrical power (probably a nuclear reactor) the spaceship could then use the mass driver to accelerate pieces of matter of almost any sort, boosting itself in the opposite direction. At the smallest scale of reaction mass, this type of drive is called an ion drive. No absolute theoretical limit is known for the size, acceleration or muzzle energy of linear motors. However, practical engineering constraints apply for such as the power-to-mass ratio, waste heat dissipation, and the energy intake able to be supplied and handled. Exhaust velocity is best neither too low nor too high. There is a mission-dependent limited optimal exhaust velocity and specific impulse for any thruster constrained by a limited amount of onboard spacecraft power. Thrust and momentum