Efficient types of nozzles used in rockets

Question

Is there any other type of nozzle other than common convergent-divergent nozzle (see picture below) which is used in many rockets?

       

          ^{A convergent-divergent nozzle (Source: Spirax Sarco)}

Which is more efficient in increasing the velocity of exhaust gas?

Please attach pictures of nozzles cited into your answers.

Answer 1

Yes, there are several other nozzle types in use, being tested or patented. Let's first recap on the various nozzle types used in aeronautics:

• Low ratio nozzle is predominantly used on the civil aircrafts and also some low speed reconnaissance airplanes, and is convergent-divergent de Laval nozzle with an extremely low inlet-outlet area pressure ratio that prevents choking at low air speeds, reduces generated noise, and is as reliable as they come:

       

       ^{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}

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And now for the fun part - 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.}

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And then there are many other subtypes, combining several of same or different nozzle types into a single design:

• Expanding nozzle which is a type of rocket nozzle that, unlike traditional designs, maintains its efficiency at a wide range of altitudes. It is a member of the class of altitude compensating nozzles, a class that also includes the plug nozzle and aerospike. While the expanding nozzle is the least technically advanced and simplest to understand from a modeling point of view, it also appears to be the most difficult design to build.

• Stepped nozzle (dual-bell nozzle): a de Laval rocket nozzle which has altitude compensating properties.

• Dual-expander nozzle that is a composite cycle rocket engine having an inner engine disposed to discharge directly into the nozzle of an outer engine.

• Dual-throat nozzle (or dual fuel capability nozzle) which is a fuel nozzle and gas turbine combustor capable of operating on multiple fuels with reduced carbon build-up to the fuel nozzle and adjacent combustor components is disclosed. The fuel nozzle incorporates a reconfigured gas fuel assembly and mixing tube to eliminate known areas of recirculation. Furthermore, the liquid fuel assembly includes reconfigured spray characteristics to further reduce droplet interaction with the mixing tube.

Suggested additional reading: Kostas Makris' blog post on Nozzle Design

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^{It appears I've somewhat overstretched the capability of our contents parser to format hypertext, links, e.t.c., so I didn't add any photographs for the last group of nozzles. I've based my list somewhat loosely on Wikipedia, omitting some duplicated designs and adding a few more to those not described there. It should be noted however, that there isn't any official classification, short of their use in various industries (which might vary greatly), so I've adopted my own for this purpose.}

Answer 2

The explanation of nozzles working by decreasing the velocity of flow is not correct. Nozzles ALWAYS accelerate the flow passing through them. One must first understand what kind of flow is passing through the nozzle. If the nozzle inlet flow is moderately subsonic, the flow will accelerate at the nozzle throat (the minimum area) and impart the highest momentum to the gas. Since thrust is based largely on momentum, no expansion downstream of the throat is necessary (just look at commercial jet engine nozzles) and the gas is expelled to the atmosphere. It also holds that the gas pressure will be its lowest at the throat. Now if the nozzle inlet flow speed is high enough, the flow will accelerate to Mach 1 at the throat. In this case, the flow becomes supersonic and things happen in reverse. In supersonic flow, expanding area causes acceleration and pressure drop, opposite to that for subsonic flow. So after the flow reaches Mach 1 at the throat, an expanding area is needed downstream to further accelerate it. This is why nozzles expand and do so in such a way so as to achieve a desired exit velocity. As for the internal pressure distribution inside the supersonic nozzle, it continues to drop over the length of the nozzle downstream of the throat. Again, this is opposite to what happens for subsonic flow. The pressure forces provide some thrust, but not much. The pressure component of thrust is relatively insignificant compared to the momentum contribution from the accelerated flow. And incidentally, except for detonation-based engines, the nozzle inlet flow from any engine (jet or rocket) is always subsonic.

This statement about nozzle expansion, however, is correct enough for this audience:

In designing the nozzle of real rockets, there is a balance to be struck; at lower altitude, the atmospheric pressure is higher, so the best nozzle shape would widen less than a nozzle for use in space.