Aerospace startup Polaris Raumflugzeuge has been awarded a contract by the German government Federal Office for Equipment, Information Technology and Use of the German Armed Forces (BAAINBw) (Bundeswehr Office for Equipment, Information Technology and In-Service Support) has developed and piloted the revolutionary Linear AeroSpike (LAS) engine, which replaces the conventional rocket nozzle with a curved wall plug.
Conventional rockets are easily spotted by their bell-shaped nozzles, which accelerate hot gases ejected from the combustion chamber. It’s a wildly successful design that has changed little in over a century, but it still leaves a lot to be desired — not least because chemical rockets are already operating close to their theoretical limits.
As an alternative, engineers have been studying LAS since the 1950s. Developed from the old plug nozzle engine, the idea behind the air plug engine is that it takes the traditional rocket bell nozzle and cuts off one side of it.
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Rocket bell nozzles work by accelerating the propellant gas by controlling the expansion of the gas. It does this through the geometry of the bell, which is designed for a specific ambient air pressure. This causes a problem because air pressure changes with altitude, meaning rockets designed for sea level lose efficiency as they ascend.
This is one of the reasons for multi-stage rockets. At higher altitudes, the booster requires a different rocket motor. Even with the same engine used for liftoff, the upper stage requires a different bell structure to handle the pressure differential.
A gas nozzle goes around it, with one side having the same cross-section as the bell nozzle, the other side being open, while the top is a series of combustion chambers. As the hot gas exits the chamber, the spike contains one side, while the air pressure on the other side replaces the missing bell-shaped cross-section.
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If configured correctly, the Spike will be optimally set to work at sea level. As it rises and the air pressure drops, the virtual bell inflates, maintaining the engine’s efficiency. This gives engineers the opportunity to create an engine that is as efficient as conventional engines that can work from the ground to space, but is simpler, smaller and lighter. That means more space and weight to use more fuel, and the ability to lift larger payloads. It also means that the aircraft has a higher altitude ceiling, range and Mach number acceleration.
The problem is that it’s one thing to tackle a pneumatic piston engine in theory, and quite another to build one in practice. The biggest hurdle is that they generate a lot of heat, requiring new materials and cooling systems to cope, and 3D printing to manufacture them.
However, that didn’t stop NASA from developing a version for its space shuttle successor, the X-33/VentureStar, and testing the cold flow demonstrator on the back of the SR-71 Blackbird.
Under the new contract, Polaris is tasked with developing and flying a LAS engine that can be integrated into a scaled-up demonstration space shuttle that is larger and heavier than the three vehicles the company has previously built. If successful, it would mark the first time an aircraft engine has been fired in flight.
source: north star
