Project Orion (nuclear Propulsion) - Basic Principles

Basic Principles

The Orion nuclear pulse drive combines a very high exhaust velocity, from 12 to 19 mi/s (19 to 31 km/s) in typical interplanetary designs, 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 meet the extreme power requirements to deliver both at once (see spacecraft propulsion for more speculative systems).

Specific impulse (Isp) measures how much thrust can be derived from a given mass of fuel, and is a standard figure of merit for rocketry. For any rocket propulsion, since the kinetic energy of exhaust goes up with velocity squared (kinetic energy = ½ mv2), whereas the momentum and thrust goes up with velocity linearly (momentum = mv), obtaining a particular level of thrust (as in a number of g acceleration) requires far more power each time that exhaust velocity and specific impulse (Isp) is much increased in a design goal. (For instance, the most fundamental reason that current and proposed electric propulsion systems of high Isp tend to be low thrust is due to their limits on available power. Their thrust is actually inversely proportional to Isp if power going into exhaust is constant or at its limit from heat dissipation needs or other engineering constraints). The Orion concept detonates nuclear explosions externally at a rate of power release which is beyond what nuclear reactors could survive internally with known materials and design.

Since weight is no limitation, an Orion craft can be extremely robust. An unmanned craft could tolerate very large accelerations, perhaps 100 g. A human-crewed Orion, however, must use some sort of damping system behind the pusher plate to smooth the instantaneous acceleration to a level that humans can comfortably withstand – typically about 2 to 4 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 (T_c) 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.

The maximum effective specific impulse, I_sp, 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), V_e is the nuclear pulse unit plasma debris velocity, and g_n is the standard acceleration of gravity (9.81 m/s2; this factor is not necessary if I_sp 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.

The smaller the bomb, the smaller each impulse will be, so the higher the rate of impulses and more than will be needed to achieve orbit. Smaller impulses also mean less g shock on the pusher plate and less need for damping to smooth out the acceleration.

The optimal Orion drive bomblet yield (for the human crewed 4,000 ton reference design) was calculated to be in the region of 0.15 KT, with approx 800 bombs needed to orbit and a bomb rate of approx 1 per second.