ROCKET ENGINE

ROCKET ENGINE:

A rocket engine or simply rocket is a jet engine that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law. Since they need no external material to form their jet, rocket engines can be used for spacecraft propulsion as well as terrestrial uses, such as missiles. Most rocket engines are internal combustion engines, although non combusting forms also exist.

Rocket motor is a synonymous term that usually refers to solid rocket engines. Chemical rockets are rockets powered by exothermic chemical reactions of the propellant. Thermal rockets are rockets where the propellant is inert, but is heated by a power source such as solar or nuclear power.

PRINCIPLE OF OPERATION:

Most rocket engines produce thrust by the expulsion of a high-temperature, high-speed gaseous exhaust. This is typically created by high pressure (10-200 bar) combustion of solid or liquid propellants, consisting of fuel and oxidiser components, within a combustion chamber.

The fluid exhaust is then passed through a propelling nozzle which accelerates the exhaust to high speed, and the reaction pushes the engine in the opposite direction.

ROCKET NOZZLE:

The large bell or cone shaped expansion nozzle gives a rocket engine its characteristic shape.

In rockets the hot gas produced in the combustion chamber is permitted to escape from the combustion chamber through an opening (the "throat"), within a high expansion-ratio 'de Laval nozzle'.

Provided sufficient pressure is provided to the nozzle (about 2.5-3x above ambient pressure) the nozzle chokes and a supersonic jet is formed, dramatically accelerating the gas, converting most of the thermal energy into kinetic energy.

The exhaust speeds vary, depending on the expansion ratio the nozzle is designed to give, but exhaust speeds as high as ten times the speed of sound of sea level air are not uncommon.

THROTTLING:

Rockets can be throttled by controlling the propellant rate M (usually measured in kg/s or lb/s).

In principle rockets can be throttled down to an exit pressure of about one-third of ambient pressure (due to flow separation in nozzles) and up to a maximum limit determined only by the mechanical strength of the engine.

In practice, the degree to which rockets can be throttled varies greatly, but most rockets can be throttled by a factor of 2 without great difficulty; the typical limitation is combustion stability, as for example, injectors need a minimum pressure to avoid triggering damaging oscillations (chugging or combustion instabilities); but injectors can often be optimised and tested for wider ranges. Additionally, it is important that the exit pressure not be too far below ambient to avoid flow separation in the nozzle.

ENGERY EFFICENCY:

Rocket engine nozzles are surprisingly efficient heat engines for generating a high speed jet, as a consequence of the high combustion temperature and high compression ratio in accordance with the Carnot cycle, over 60% efficiency can be achieved with chemical rockets. For a vehicle employing a rocket engine the energetic efficiency is very good if the vehicle speed approaches or somewhat exceeds the exhaust velocity (relative to launch); but at low speeds the energy efficiency goes to 0% at zero speed

COOLING:

Most other jet engines have gas turbines in the hot exhaust. Due to their larger surface area, they are harder to cool and hence there is a need to run the combustion processes at much lower temperatures, losing efficiency. In addition duct engines use air as an oxidant, which contains 80% largely unreactive nitrogen, which dilutes the reaction and lowers the temperatures. Rockets have none of these inherent disadvantages.

Therefore in rockets temperatures employed are very often far higher than the melting point of the nozzle and combustion chamber materials, two exceptions are graphite and tungsten (~1200 K for copper), however both are subject to oxidation if not protected. Indeed many construction materials can make perfectly acceptable propellants in their own right. It is important that these materials be prevented from combusting, melting or vapourising to the point of failure. This is sometimes somewhat facetiously termed an 'engine rich exhaust'. Materials technology could potentially place an upper limit on the exhaust temperature of chemical rockets.
In rockets the coolant methods include:
1. Uncooled (used for short runs mainly during testing)
2. Ablative walls (walls are lined with a material that is continuously vapourised and carried away).
3. Radiative cooling (the chamber becomes almost white hot and radiates the heat away)
dump cooling (a propellant, usually hydrogen, is passed around the chamber and dumped)
4. Regenerative cooling (liquid rockets use the fuel, or occasionally the oxidiser, to cool the 5. Chamber via a cooling jacket before being injected)
6. Curtain cooling (propellant injection is arranged so the temperature of the gases is cooler at the walls)
7. Film cooling (surfaces are wetted with liquid propellant, which cools as it evaporates)

In all cases the cooling effect that prevents the wall from being destroyed is caused by a thin layer of insulating fluid (a boundary layer) that is in contact with the walls that is far cooler than the combustion temperature. Provided this boundary layer is intact the wall will not be damaged.