The inertial drive, a relatively recent advance in aerospace technology, is a device that provides thrust without a corresponding reaction. There is a common misconception that the name derives from the fact that they produce thrust by creating inertia. While there is some truth to that, the word inertial actually refers to an inertial frame a frame of reference that is not acted upon by any accelerating or decelerating force. A rocket, for example, is acted on by a reaction force of escaping gases; it is not inertial. Neither is a magnetic drive that pushes against external magnetic objects. In other words, an inertial drive is totally self-contained. It does not rely on an action upon some outside mass.
At least locally, an inertial drive appears to generate an accelerating force upon itself with creating a corresponding balancing force on something else, in clear violation of Newton’s third law of motion and (actually equivalently) the conservation of momentum. Virtually all experts agree that on the cosmic scale momentum is still conserved, but there are two or three hundred competing theories as to how this actually takes place.
The Jupiter 2 has two main inertial drives for horizontal propulsion, together capable of sustaining an acceleration in excess of 3 gravities. There are four smaller units placed 90° around the rim for vertical lift, and four yet smaller ones oriented downward that are used in conjunction with the lift units for attitude control. Although inertial drive technology has been available for some time, it was not practical until quantum coupling made the fusion core possible to provide power for it.
Inertial Drive Internals
The earliest known inertial drive that actually worked was constructed in the 1950’s. It was entirely mechanical, yet it embodied the one crucial requirement of any inertial drive system — gyroscopic precession. The unit consisted of a number gyroscopes mounted on the end of arms that could swivel up and down. An upward force was achieved by forcing the gyroscopes downward along an arc. This is simple Newtonian physics: a force down upon the gyroscopes yields a force upward at the base. Naturally, precession created a rotating force as well, but this was inconsequential. Now, to get the gyroscopes back up, the apparatus would rotate the assembly in the other direction. Gyroscopic precession would return the spinning masses to their original position without a corresponding downward force on the base. For each cycle, we have an upward impulse without a corresponding downward one. Of course this mechanism was primitive by today’s standards and was unable to lift its own weight (only reduce it), but it showed that such a thing was possible and that we need to examine mechanical systems on a cosmic scale.
Today, spinning gyroscopes are replaced by spinning atoms, but the general principle remains the same. The atoms in question are of germanium locked in a gallium arsenide matrix, but they require the presence of silicon and tin atoms immediately adjacent. The arrangement must be precise, and how such a solid state device is constructed, especially on the scale needed for inertial drives, is one of the closely guarded secrets of the government. This object, some 10 cm thick and 220 cm in diameter for the main drives, looks roughly like a glass disk and is called an inertial gyrator.
Naturally, it takes more than a disk to achieve thrust. We spin the atoms (equivalent to trillions of gyroscopes) using an array of solid state microwave generators mounted directly behind the gyrator. The role of forcing the gyroscopes downward is accomplished by a rotating magnetic field passing laterally through the disk. The process of rotation is achieved, curiously, by spinning the disk. As you will recall, the original mechanical device was pulsed — it operated on a periodic cycle. Inertial drives still do. In fact, the early inertial gyrator models used a gigantic pulse motor to rotate the disk in about 10 degree increments at a pulse frequency of about 50 Hz. Needless to say, this was wasteful of power and vibrated atrociously. Later it was discovered that the disk could spin continuously, and the pulse effect achieved by specialized modulation of the magnetic field. This allowed much higher pulse rates, upwards of 10kHz and higher overall efficiency. Still, one can hear the whine of the engines as they run.
Practical inertial drives stack several gyrators in an array, using the same motor to spin them all, and a magnetic thrust bearing to transfer the propulsive force from the spindle to the housing.. Those aboard the Jupiter 2 use eight gyrators each and spin in opposite directions to keep the torque balanced.
There is an important caveat to note about the inertial drives, one that became relevant only with the Jupiter 2. Since they rely on gyroscopic precession, which is a mass effect, their performance is severely hampered by a subwarp field. The reduced mass does allow the system to operate at a higher speed and this compensates somewhat, but even so the maximum thrust attainable with the subwarp field energized to a typical cruising density is about 0.4 g’s. This is why the craft is to accelerate to terminal base velocity before the subwarp field is energized. The lift drives can provide enough thrust after that point to counter the quantum drag from the miniscule amount of matter in interstellar space.