Jet engine

The Basic Jet Engine is an air-breathing engine which can use liquid fuel from the C7 fuselages or FL-T fuel tanks.

Usage

Unlike rocket engines, a jet draws oxidiser from the atmosphere rather than taking it from an onboard tank. This is represented in-game by a much lower rate of fuel consumption. This engine cannot function effectively at a height of much more than 5km, but it is much more efficient to use jet engines in the lower atmosphere. Jets control a little differently to the rocket engines. The fan in front takes time to change speed, and if the throttle is too low the engine will stall. By contrast, rocket engines give instant response to your control input.

The basic jet engine works at full power on the ground, whereas the Turbojet Engine won't get up to full power until you're over a kilometer up. However, the turbojet is so much stronger that this advantage doesn't add up to much, and when you consider that the turbojet works at more than twice the altitude there really isn't any reason not to take it instead, as of KSP v0.15.

Currently, jets do not require you to fit air intakes to your spacecraft and function independently of whether you use a fan intake or not.