The Orion nuclear pulse drive combines a very high exhaust velocity, from 20,000 to 30,000 m/s, with meganewtons of thrust. Many spacecraft propulsion drives can achieve one of these or the other, but nuclear pulse rockets are the only proposed technology that could potentially deliver both (see spacecraft propulsion for more speculative systems). Specific impulse measures how much thrust can be derived from a given mass of fuel, and is the standard figure of merit for rocketry.
Unmanned Orion-style nuclear pulse rockets can tolerate very large accelerations. A human-crewed Orion, however, must use damped springs behind the pusher plate to smooth the instantaneous acceleration to a level that humans can withstand–typically about 1–3 g.
The high performance depends on the high exhaust velocity, in order to maximize the rocket's force for a given mass of propellant. The velocity of the plasma debris is proportional to the square root of the change in the temperature (Tc) of the nuclear fireball. Since fireballs routinely achieve ten million degrees Celsius or more in less than a millisecond, they create very high velocities. However, a practical design must also limit the destructive radius of the fireball. The diameter of the nuclear fireball is proportional to the square root of the bomb's explosive yield.
The shape of the bomb's reaction mass is critical to efficiency. The original project designed bombs with a reaction mass made of tungsten. The bomb's geometry and materials focused the x-rays and plasma from the core of nuclear explosive to hit the reaction mass. In effect each bomb would be a nuclear shaped charge.
A bomb with a cylinder of reaction mass expands into a flat, disk-shaped wave of plasma when it explodes. A bomb with a disk-shaped reaction mass expands into a far more efficient cigar-shaped wave of plasma debris. The cigar shape focuses much of the plasma to impinge onto the pusher-plate.
A 10 kiloton of TNT equivalent atomic explosion will achieve a plasma debris velocity of about 100,000 m/s, and the destructive plasma fireball is only about 100 meters in diameter. A 1 megaton of TNT explosion will have a plasma debris velocity of about 10,000,000 m/s but the diameter of the plasma fireball will be about 1000 m.
The maximum effective specific impulse, Isp, of an Orion nuclear pulse drive generally is equal to:
where C0 is the collimation factor (what fraction of the explosion plasma debris will actually hit the impulse absorber plate when a pulse unit explodes), Ve is the nuclear pulse unit plasma debris velocity, and gn is the standard acceleration of gravity (9.81 m/s²; this factor is not necessary if Isp is measured in N·s/kg or m/s). A collimation factor of nearly 0.5 can be achieved by matching the diameter of the pusher plate to the diameter of the nuclear fireball created by the explosion of a nuclear pulse unit.
