Gravitational Impeller

A gravitational impeller, also known as a graviton rocket, gravitational rocket, or simply impeller, is an extreme-high-performance spacecraft propulsion system which generates thrust by expelling a focused beam of gravitational waves.

Physics

An impeller drive can be thought of as a photon rocket for gravitational radiation, and indeed has the same theoretical efficiency (one Newton of thrust requring 300 megawatts of gravitational waves). The prime difference, however, is that unlike conversion drives' exhaust—deca-petawatts of collimated gamma rays able to scour a trail of devastation across the face of a planet—the exhaust of an impeller is a beam of gravitational waves, which even at equally strong intensity can sleet straight through matter with no effect, being only barely detectable with sensitive instruments. (A gravitational wave strong enough to harm a person would need a power level on the same scale as the total energy produced by a star.)

At the heart of every impeller engine is some means of generating useful quantities of gravitational waves. In modern Starweb space this is usually a conversion metric salvaged from the ruins of Hexamite space or a warp battery charged with several megatons (if not giga- or teratons) of energy. Gravitational radiation emitted by this source is reflected and focused by a shell or nozzle of digravitic material like cavorite into a tight beam, the expulsion of which generates a reaction force to move the ship.

A conversion metric-powered impeller must be fed mass at the rate of several milligrams to kilograms per second, this is usually some convient substance like water or asteroidal rock which can be acquired cheaply and packed at high density into fuel tanks. Impellers powered by warp batteries must be pre-charged with all the energy they will need before the voyage commences, making this hardware setup rarely used.

Power can be tapped from a running impeller by intercepting a portion of the exhaust beam to turn cavorite windmills connected to a generator. Most modern impellers are designed with a static power mode which expels equal beams of gravitational radiation from both ends, allowing power to be extracted while producing no net thrust on the ship.

Parent Technologies

• Gravitational Conversion Metric
• Warp Battery

Children Technologies

• Impeller Warp

Operation

An impeller drive has a near-reactionless quality to it; a ship equipped with one will move with no apparent expenditure of reaction mass, draining its fuel tanks at a mere trickle, a far cry from the great torchships of the Second Interstellar Period, whose drive flares could be seen clear across a solar system and whose fuel loads reguarly made up 75% of their total mass.

Performance

As a conversion engine, efficiency is extreme: an impeller-drive ship which is 25% fuel by mass (mass ratio of 0.25) will have a delta-v capacity of approximately one-quarter lightspeed. (Versus even high-performance torchships, which are lucky to have a mass ratio less than 2—meaning two kilograms of fuel or propellant for every kilogram of payload—and primitive chemical rockets, for which fuel comprised over 90% of their gross weight.) An impeller enables fast, continual-acceleration interplanetary travel aboard a ship which only needs a small percent of its volume dedicated to tankage for conversion fuel. Specialized ships with more fuel and enhanced debris shielding can achieve interstellar voyages at mid-relativistic speeds, useful for explorers investigating wide-set binary suns or other targets some distance from a stargate.

The limit of an impeller's acceleration is given by the strength of the cavorite structures reflecting its gravitational waves: while the metric may be able to produce ever more power, eventually these frames will blow apart from the strain. Since cavorite is heavy, and support structures to connect it to a ship heavy as well, impeller drives usually top out at a lower acceleration than a high-thrust fusion torch or even a chemical-fueled missile, as they cannot take advantage of a "low-gear" mode to inject additional reaction mass into their exhaust streams. Maximum acceleration for impellers is usually around three to five gees for all but the smallest vessels or those fitted with large engine clusters. A missile or torchship can therefore usually chase down an impeller ship provided it starts out close enough. Over a long distance, the impeller vessel will escape pursuit as it is much more efficient than a high-thrust pursuer, which will burn through its propellant rapidly.

Hazards

Despite their harmless exhaust, gravitational impellers are nonetheless marked as Class II Controlled Technology by the Stellar Compact, requiring strict oversight and careful vetting of operators. The reason is simple: Kinetic energy.

An average impeller-driven ship, with a load of 25% fuel by weight, and given enough room, could easily attain a speed of about 25% that of light before its tanks ran dry. If it massed 10,000 metric tons and was aimed at a planet, it would strike with about 7 teratons of energy, far more than the combined arsenals of ancient Terra's nuclear stand-offs. And it is concievable a motivated terrorist could increase the fuel load of his kamikaze vessel, and attain an even-higher speed. Such an attack is hard to defend against, since an impeller ship can be rigged for cold running and produces no visible exhaust flare. Attempts at interception, if they occur too late, can still see the target world get hit by pieces of relativistic debris.

The Stellar Compact, at the Existential Risks Directorate's behest, thus requires its member states to hold impeller-drive ships to careful regulation and account for every engine manufactured or lost, lest any fall into evil hands. Captains are subject to intense psychological screening, and ships are required to file flightplans and operate transponders when traveling in all but the most remote Frontier regions. A few organizations, most notably the Free Traders, used impeller drives before the Compact's foundation and were grandfathered in under more lax rules, provided they held their membership to strict standards.

History

The first gravitational impellers were constructed in the Second Interstellar Period, but for decades were extremely sluggish due to the high mass of their cavorite reflectors, but gained performance as technology advanced. Nevertheless, it is only recently, in the post-Consolidation period of the Stellar Compact era that they've become competitive with conventional reaction motors or beam power. For ordinary civilian voyages, between planets and stargates, older propulsion systems are still far more cost-efficient.