By

Arun choudhary,

08r21a2164.

MLRIT.

Flow velocity increasing from green to red in the direction of flow

A rocket engine uses a nozzle to accelerate hot exhaust to produce thrust as described by newton’s 3rd law of motion.

The amount of thrust produced depends mainly on the mass flow rate through the engine.

A nozzle is a relatively simple device, just a specially shaped tube through which hot gases flow. Rockets typically use a fixed convergent section followed by a fixed divergent section for the design of the nozzle. This nozzle configuration is called a convergent-divergent, or CD, nozzle.

Expansion Area Ratio

Underexpanded. Ambient. Overexpanded. Grossly overexpanded.

Loss of efficiency occurs in under and overexpanded nozzles.

Grossly overexpanded has improved efficiency but the exhaust jet is unstable.

A rocket nozzle includes three major sections 1.convegent duct.2.throat section3.divergent duct.

The performance of a rocket nozzle depends upon the ratio of cross sectionalexit area to the cross sectional throat area. This is called the expansion area ratio.

Types : conical and contoured nozzles are the most common types.Conical type nozzles are straight sided from throat to the exhaust.the constant angle does not force the exhaust gas to turn.The constant angle also makes this type of nozzle easy to build and design.

Whereasa contoured nozzle or bell shaped nozzles turn the flow which reducesefficiency and thrust due to shock waves resulting from the turn.more advanced designs such as annular or expansion-deflection nozzles,maintain their efficiency over larger pressure ranges. These are more complex and expensive to produce.

Contoured nozzle

A number of more sophisticated designs have been proposed and they can be Categorised by with which they achieve altitude compensation.

Nozzles with atmospheric boundary include:1.the expansion deflection nozzle.2.the plug nozzle.3.the aerospike.

Each of these allows the supersonic flow to adapt the ambient pressure by Expanding or contracting, thereby changing the exit ratio so that it is at(or near)Optimal exit pressure for corresponding altitude.The plug and aerospike nozzles are very similar in that they are radial in flow Designs but plug nozzles feature a solid centre body and aerospikes have a“ base bleed “of gases to Simulate a solid centrebody.

controlled flow separation nozzles include:

the expanding nozzle.bell nozzles with a removable insert andstepped nozzles or dual bell nozzles.

SERN:single expansion ramp nozzle.it’s a type of linear expansion nozzle where thegas

pressure transfers work only on one side . linear nozzlesAre not axi-symmetric but consist of a configuration of twoexpansion ramps. A SERN could alsobe seen as a single sided aerospike engine.

Bell nozzles

Stepped nozzles(dual bell)

Plug nozzle

Aerospike nozzle

Many designs for space planes with scramjetengines make use of SERNs because of the weight reductionat large expansion ratios, or the additional lift at under-expansion. The X-43, a test vehicle in NASA'sHyper-X programme, is a flying example.One of the practical problems with the use of SERNsis the fact that they produce a pitching moment dependingon the throttling of the engine, thereby requiring more control authority of the elevators and morecomplex control systems etc.

CONICAL NOZZLE BELL NOZZLE

The divergence angle from throat to exit remains constant.

Oblique shocks cannot be minimized.

Near the throat the nozzle diverges at larger angle but degree of divergence tapers off further downstream.

Near the nozzle exit the divergence angle is small.

Avoids oblique shocks and eases performance.

Superior performance: annular nozzles i.enozzles which account for altitude compensation may result in greater installed performance.

Low risk and failure : aerospike engines uses a simple gas generator cycle with a lower chamber pressure than typical rocket engines reducing the risk of catastropic expansion.

High expansion ratio accounts for deficient performance.

Lower vehicle drag: the aerospike nozzle fills the base position of the vehicle thereby reducing a type of drag called base drag.

Manufacturing : aerospike nozzle is most complex to manufacture and design.

Flight experience: no aerospike engine has ever flown in a rocket application . As a result of which little flight experience is gained.

Queries………………?