A SERN, which stands for single expansion ramp nozzle, is a type of linear expansion nozzle

Nozzle

A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....

 where the gas pressure transfers work only on one side. Traditional nozzles are axially symmetric, and therefore surround the expanding gas. Linear nozzles are not axially symmetric, but consist of a 2D configuration of two expansion ramps. A SERN could also be seen as a single sided aerospike engine

Aerospike engine

The aerospike engine is a type of rocket engine that maintains its aerodynamic efficiency across a wide range of altitudes through the use of an aerospike nozzle. It is a member of the class of altitude compensating nozzle engines. A vehicle with an aerospike engine uses 25–30% less fuel at low...

.

Many designs for space planes with scramjet

Scramjet

A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow...

 engines make use of SERNs because of the weight reduction at large expansion ratios, or the additional lift at under-expansion. The X-43, a test vehicle in NASA

NASA

The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's Hyper-X programme, is a flying example.

One of the practical problems with the use of SERNs is the fact that they produce a pitching moment depending on the throttling of the engine, thereby requiring more control authority of the elevator

Elevator (aircraft)

Elevators are flight control surfaces, usually at the rear of an aircraft, which control the aircraft's orientation by changing the pitch of the aircraft, and so also the angle of attack of the wing. In simplified terms, they make the aircraft nose-up or nose-down...

s and more complex control systems etc.