Aerospace startup Polaris Raumflugzeuge has been awarded a contract by the German government's Bundesamt für Ausrüstung, Informationstechnik und Nutzung der Bundeswehr (BAAINBw) (Federal Office of Bundeswehr Equipment, Information Technology and In-Service Support) to develop and fly a revolutionary Linear AeroSpike (LAS) engine that replaces the conventional rocket nozzle with a curving wall-shaped plug.
Conventional rockets can be easily spotted by their bell-shaped nozzles that accelerate the hot gases shooting out of the combustion chamber. It's a remarkably successful design that has changed little over a century, but it still leaves a lot to be desired – especially because chemical rockets already operate close to their theoretical limit.
As an alternative, engineers have been looking at LASs since the 1950s. Developed from old plug-nozzle engines, the idea behind the aerospike engine is that it takes the conventional rocket bell nozzle and cuts away one side of it.
A rocket bell nozzle works by speeding up the propellant gases by controlling their expansion. It does this by the geometry of the bell, which is designed for a particular ambient air pressure. This causes a problem because air pressure changes with altitude, which means that a rocket designed for sea level loses its efficiency as it ascends.
This is one reason for multi-stage rockets. At higher altitudes, the booster needs a different rocket engine. Even if it's the same engine as used for liftoff, the upper stage engine needs a different bell configuration to handle the difference in pressure.