Reactionless Propulsion

Metric Drive propulsion mode that employs the gravitational properties of NSET-summoned gravity sources and warp bubbles to accelerate a spacecraft without the use of rocket propellant. Reactionless propulsion typically takes over from a ship's Torch Drive once it is sufficiently distant from any sources of gravitational interference that could destabilize the sensitive metric, which largely bars the use of reactionless propulsion for fast travel in planetary or stellar gravity wells. NSET brane mechanics also slow down spacecraft entering the presence of large masses, preventing catastrophic collisions - one reason that the NSET brane is widely supposed to have been engineered by (unknown) Kardashevs.
If they are headed in the right direction, spacecraft under reactionless propulsion can then begin the journey to another star system by accelerating to near lightspeed, and lay a Krasnikov tube in their wake for the FTL return journey if none leading to the destination already exist.
Reactionless propulsion cannot be used to go FTL - that is, not "locally" FTL, with respect to immediate surroundings. Spacetime tunnels like Krasnikov tubes and wormholes provide global FTL capability, canceling out the travel time incurred by an STL journey, but at no point on the trip does any part of the metric travel at FTL speed in the immediate reference frame of any other. FTL travel in the manner of a "pure warp drive" is not possible, at least at the current state of transhuman engineering. There are several reasons for this:

1. Causal decoupling: The outside of a warp bubble reaching FTL speed would no longer be connected to the chain of cause and effect inside the bubble, where the ship and bubble are at rest (slower than light) relative to one another. The ship would become unable to control the bubble.
2. Requires tachyons: No known parties are capable of producing tachyons, which would be required to accelerate the bubble to FTL speeds in the first place.
3. Hawking radiation: At FTL speeds, Hawking radiation generated on the interior of the bubble becomes tremendous, even granting the protections offered by the NSET brane, and lethally irradiates anything inside.

Metric drives in interstellar reactionless propulsion mode also have a unique ability to avoid space debris that might otherwise impact with the bubble and cause a burst of blueshifted radiation across the interior: they can (and actually must, because it reduces the necessary negative stress-energy) become smaller on the outside than the inside, reaching ratios of interior to exterior volume that the Doctor could only dream of. Warp bubbles in operation are actually, generally, invisible - to the outside world, they appear as a mote of dust. They can still be detected, however, by the macroscopic optical effects associated with their immense gravitational curvature.
Reactionless propulsion, along with FTL, was always something of an open secret of relativity: Capitalist Age scientists often suppressed their enthusiasm for such concepts, lest their credibility come under threat, but there was nothing in special or general relativity that strictly forbade either of these things. Spacetime metrics (that is, configurations of the "shape of spacetime" or the gravity field) that permitted globally spacelike trajectories (moving faster in space than in time, opposite to slower-than-light motion which travels more in time than space) were known since Einstein's day that did not correspond to anything yet observed in reality, yet were perfectly valid solutions to the field equations governing general relativity. Alcubierre proposed an early warp bubble metric that allowed reactionless propulsion, which might be thought of as an ancestor to modern metric drive theory.