Small engines are everywhere
Most rocket engines ever built are small. Satellites carry thrusters of roughly one newton for attitude control, often running on a single propellant that decomposes over a catalyst. Bipropellant thrusters of ten to a few hundred newtons raise orbits and steer spacecraft and landers.
What changes with size
Thrust scales with the throat area, roughly the square of the engine’s size. The heat-loaded wall area scales differently, and the chamber needs a certain volume for the propellants to mix and burn, set by the characteristic length L*. As engines shrink, the wall becomes large compared with the propellant flow available to cool it, and cooling channels become fractions of a millimetre.
What 3D printing changes
Metal additive manufacturing makes fine internal channels and one-piece injectors possible without welding dozens of parts. That reopens design options for small engines, such as regenerative cooling at sizes where it used to be impractical.