First lets define an aerospike. In its simplest form it is basically an inverted bell nozzle. It uses a system of several combustion chambers and a divergent convergent nozzle. Controversial to most belief the Aerospike does not end in a point. This is because of the fact that to end in a point with maximum efficiency it would need to be infinitely long. This is not possible. Therefore they end with a small point that blows (typically) subsonic gases out of it.
@Arstotzka most people imagine them as a point. The name doesn’t help. But most games get them wrong.