Linear Fuel Lines Launch System
Just like gravity assist, but "fuel-assist" - fuel positioned in the rocket path to be ingested and used.
Interstellar flight is really hard. The approach, where rockets have to carry their own fuel, is rather limited and wasteful (why accelerate the fuel that you don't need to transport).
So, the idea of positioning the fuel in the rocket's path to be ingested and used seem like an obvious solution.
For example, we could imagine fuel lines in space positioned in advance in the rocket's path, that would start, spraying liquid oxygen and methane from separate directions to intercept with the rocket's receptacles, so that the rocket could collect fuel in space as it accelerates.
Surely, at some point, the rocket would reach such a velocity, that the droplets of fuel would collide with significant momentum and force, such that the receptacles wouldn't be able to hold, so, I think, at that point, instead of droplets, we would ideally switch to gaseous forms (think - directed vapors of oxygen and methane, with increasing rarity, as we further accelerate in speed).
Btw., this makes me think that, sufficiently advanced civilizations that live on super-Earths (which would make normal rocketry unfeasible) would probably be able to still escape their gravity with similar approach, by positioning fuel in the path of rocket, through use of some in-atmosphere engieering, based on Archimedes principle (buoyancy), to make sure that the rocket has fuel in its path.