A propelling nozzle is the component of a jet engine that operates to constrict the flow, to form an exhaust jet and to maximise the velocity of propelling gases from the engine.
Propelling nozzles can be subsonic, sonic, or supersonic.Physically the nozzles can be convergent, or convergent-divergent. Convergent-divergent nozzles can give supersonic jet velocity within the divergent section, whereas in a convergent nozzle the exhaust fluid cannot exceed the speed of sound within the nozzle.
Propelling nozzles can be fixed geometry, or they can have variable geometry, to give different throat and exit diameters so as to deal with differences in ambient pressure, flow and engine pressure; thus permitting improvement of thrust and efficiency.
Boeing ecoDemonstrator tested on American Airlines airplane
- Ejector nozzle is the simpler of the variable exhaust nozzles, and is more commonly used on jet propelled aircrafts than the iris nozzles due to its simpler design of spring-loaded petals and are thus more reliable, but do produce more secondary airflow drag and are less efficient than some other, more advanced designs:
Variable Exhaust Nozzle, on the GE F404-400 low-bypass turbofan installed on a Boeing F/A-18 Hornet
- Iris nozzle is a variable exhaust nozzle commonly used on jet fighter airplanes and bombers and can adjust its contour by iris like petal design to maximize performance and avoid uneven pressure distribution (oblique shock). In some designs, they can also change the thrust vector (angle to aircraft), or add air brakes (e.g.: MiG-23 afterburner exhaust air brakes)
Iris nozzle afterburners on the F-15 Eagle fighter
The nozzle types used in astronautics, hypersonic experimental airplanes
- Bell nozzle is possibly the most commonly used nozzle type on rocket engines, for its simplicity, relatively low weight with advanced materials and in some designs even adjustability (see iris nozzle below) of the volume of its exhaust / expansion chamber: Rocket nozzle on V2 showing the classic shape
- Expansion-deflection nozzle (or Pintle Injector) is a type of propellant injection device for a rocket engine that was first used on a flight vehicle during the Apollo Program in the Lunar Excursion Module’s descent engine. Pintle injectors are currently used in SpaceX’s Merlin engines:
Patent application cross-section schematic diagram of the pintle injector
- Plug nozzle “Aerospike” (or Spike Nozzle) is an altitude compensating nozzle with the ideal contour a long, gradual pressure reducing ‘spike’, often with a wide (large volume) annular type combustion chamber at the base. This nozzle is self-compensating for atmospheric pressure, and the plug and the combustion chamber can vary in size for different applications (shorter convex shaped “spike plugs” are used also on civil aviation jet engines, and truncated/non-truncated or full-length concave spikes usually used for supersonic aircraft, rockets,…). Among main advantages is up to 30% reduction in propellant required at lower altitudes due to their self-compensating nature:
3D model of the components of the Aerospike engine with a slightly convex shaped “spike”
- Annular and Linear aerospike are variants on the truncated aerospike nozzle design, commonly with several turbine combustion exhausts placed linearly, or annularly over exhaust nozzle. Spike nozzle is truncated and allows for additional thrust with subsonic recirculating flow field forming at the truncated part, as the gases expand over the nozzle’s surface. Dynamics of a linear aerospike engine are explained in detail in this Linear Aerospike Engine video:
XRS-2200 linear aerospike engine for the X-33 program being tested
- SERN (Single Expansion Ramp Nozzle) is essentially a single side linear aerospike nozzle, but can be accompanied by more complex pitch and elevation control systems due to momentum transfer that can be angular to the aircraft / spacecraft due to throttling:
Many designs for space planes with scramjet 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’s Hyper-X programme, is a flying example.
Aurora Mach 5 (below) and SR-71 Mach 3 (above) reconnaissance aircraft flying in formation