|Glowing means it goes fast!|
Below are a two new takes on reactionless thrusters, using "real" science (depending on how you define both "real" and "science"), as well as a new superscience reaction drive and two options for antimatter rockets.
RF Resonant Cavity Thruster (TL8^) [ANY!]
The RF resonant cavity thruster, known by a multitude of names such as the EmDrive, Cannae drive or Q-drive, is an emergent superscience engine that made the news last year when a test at NASA Eagleworks seemed to indicate that a crackpot invention actually worked. A more scientifically-rigorous and peer-reviewed follow-up study run by NASA's Jet Propulsion Laboratory was published in the Journal of Propulsion and Power from the American Institute of Aeronautics and Astronautics, tentatively verifying the results.
The mainstream scientific community is still extremely sceptical of the engine, for good reason, but it's passed enough tests that it can't be dismissed out of hand any more. Whether the drive is actually doing what any of it's inventors claim it is remains to be seen.
The supposed operating principle is microwaves beamed into an asymmetrical cavity, the shape causing an imbalance in radiation pressure on one side, generating thrust. While this explanation violates known physics, several other explanations have been proposed. The two most prominent use pilot wave theory (an unproven alternative interpretation of quantum mechanics), or posit that the engine "pushes off" the quantum vacuum plasma. Both are very far-fetched (but possible).
Each engine is a high-energy system that provides 0.001G acceleration at TL8, 0.01G at TL9, or 0.1G at TL10+. For all other purposes, including cost, they act like a rotary reactionless drive (GURPS Spaceships, p. 24).
Quantum Vacuum Plasma Thruster (TL12) [REAR!]
This drive uses the same principles as a magnetoplasmadynamic thruster, using the quantum vacuum plasma as reaction mass, removing the need to carry it's propellant on-board. This is a rare example of a reactionless drive that doesn't violate known physics, so the quantum vacuum plasma thruster is not superscience! (Though technically the engine is propellantless, not reactionless.)
Each engine is a high-energy system that provides 0.01G acceleration. For all other purposes, including cost, they act like a hot reactionless drive (GURPS Spaceships, p. 24).
Ion Torch (TL9^) [REAR!]
This is a high-perfomance, limited-superscience version of the ion drive (GURPS Spaceships, p. 22). Each engine provides 0.5G acceleration but is a high-energy system that requires one Power Point. Each fuel tank of ionisable reaction mass gives a deltaV of 9mps (TL9) or 12mps (TL10+). May be high-thrust (double acceleration, halve deltaV). Cost is doubled from a normal ion drive.
Option: Antilithium (TL11)
Normal antimatter drives use antiprotons, also known as antihydrogen, as the fuel source to add energy to the reaction mass. At higher TLs, heavier antimatter atoms can be produced, which liberate more energy but take much more energy to make. The practical limit of this is antilithium, the manufacture of which requires a multi-step process of bombarding antihydrogen with antineutrons, and forcing the results to undergo several nuclear fusion processes. This is extremely difficult but the reward is a fuel with a much higher energy, and higher deltaV for antimatter rockets.
Any antimatter engine may use antilithium: ×6 deltaV per tank, ×10 fuel cost. For example, an antimatter thermal rocket using antihydrogen has a deltaV of 1.8mps. Using antilithium, it has a deltaV of 1.8 × 6 = 10.8mps.
Option: TL-dependent Antimatter Fuel Cost
GURPS Spaceships gives a fixed cost for antimatter-catalysed and antimatter-boosted hydrogen or water, and for matter/antimatter fuel, while GURPS Ultra-Tech has the cost of antimatter dropping every TL after TL9. If you want the cost of antimatter fuel or reaction mass to obey the rules in GURPS Ultra-Tech, the cost should change based on TL. Consult the following table for the cost of the antimatter per ton of fuel. For antimatter-catalysed and antimatter-boosted, this cost is in addition to cost of the hydrogen, water, methane, or ammonia reaction mass. For matter/antimatter, the cost of the reaction mass is included.