The 5 kN (1,125 lbf) aerospike rocket thruster from our previous post may look simple from the outside. But a glimpse inside reveals the intricate structure of its cooling system. Liquid oxygen is routed through the central spike, where it makes a 180° turn before entering the spiraling channels that regeneratively cool the spike. The oxygen absorbs heat that would otherwise cause the spike to melt. This turns the oxygen gaseous and gives the pressure-fed engine a useful Isp boost. The kerosene follows a separate path. It enters through the ring-shaped fuel manifold at the engine outlet, then flows through another swirling pattern of regenerative-cooling channels along the hot side of the outer toroidal shroud. From there, it reaches the monolithically integrated injector head. The injector uses two opposing simplex swirlers to combine the gaseous oxygen with the liquid kerosene for optimal mixing.