The temperature of the gaseous products exiting from the nozzle is higher than the melting point of the nozzle material. So how do the nozzles survive without melting despite exposure to such temperatures? What is the feature which enables them to do so?
There is actually quite a lot of information on the subject in the Braeunig web site Basics of Space Flight. To give the salient points:
Liquid-fueled rocket engines
Both the nozzle and the combustion chamber itself need to be cooled. Although most of the thermal energy produced is ejected with the exhaust, some of it will indeed push hardware temperatures up if not checked. Techniques include:
- Regenerative cooling, where the propergols (both propergols, or just the fuel) are pumped through a jacket around the nozzle before going into the combustion chamber. This cools down the nozzle, and heats up the propergols that may be cryogenic in nature. This is what was used in the Saturn Vs' Rocketdyne F-1 engines, and the Space Shuttle main engines.
- Dump cooling, similar to the above but the fuel used to cool the nozzle is dumped overboard instead of being fed into the combustion chamber and being used as, well, fuel.
- Film and transpirative cooling, where a thin film of coolant or fuel is created near the chamber wall.
- Ablative cooling, where part of the combustion chamber or nozzle wall is sacrificed. As the material is melted or otherwise burned away, it absorbs some of the thermal energy, thus saving whatever is behind it.
- Radiation cooling, where nozzle design is such that heat has time to be evacuated through the part's exterior wall. This is used mainly in small engines, not main.
Solid-fueled rocket engines
No information available. Nothing. Zilch. Nada.
It would seem that the nozzle and combustion chamber materials just need to be strong enough to withstand the heat from the burning gasses. It is perhaps worthwhile to note that solid rocket engines are designed to burn just one single time, and for a relatively short period of time (even the Space Shuttle boosters were extensively overhauled before being put back into service, with any necessary parts changed). Rocket engines need to withstand working conditions for just minutes, instead of the thousands of hours an A/C jet engine turbine would be expected to work without failure.
It would also be reasonable to assume -though this is strictly something I am making up on the go, with much hand waving- that of the techniques described above for liquid-fueled rocket engines, at least two could be easily adapted to solid engines:
- Film cooling, with an extra tank of cooling liquid on board.
- Ablative cooling, with some refractive material used to coat the inner surface of the nozzle.
The environment of a solid rocket nozzle is indeed very harsh. In 1975 Nasa published SP-8115 specifically about solid rocket motor nozzles:
...design values a designer might encounter: throat diameters from about 1/2 inch to nearly 90 inches, motor pressures from under 400 pounds per square inch to 2000 pounds per square inch, expansion ratios from less than 4 to over 50, firing durations from less than one second to 200 seconds, thrust from a few hundred pounds to over 5 million pounds, flame temperatures from 5100° F to over 6000° F, and a wide variety of propellant compositions.
While advances have been made in material and propellant sciences, this serves to illustrate the difficulty in designing rocket nozzles to withstand these pressures and temperatures without melting.
At the time nozzles were made out of a single piece of molded polycrystalline graphite, supported by metal housing structures. They eroded (ablative) but were low cost.
Tungsten was used for a time, but it was heavy and often cracked.
Later advances led to high strength carbon fiber and carbon matrix, often referred to as a "carbon-carbon" material.
Also called reinforced carbon-carbon, it is used today in a wide variety of industries, from the leading wing edges and nose cones of spacecraft and ICBMs, to formula one race car brake rotors. In some forms it is well over $100,000 per square foot.
Not only does it withstand high temperatures well, it can withstand thermal shock and has a low coefficient of thermal expansion. This means a significantly reduced risk of cracking due to thermal stress.
It is a brittle ceramic, though, with reduced impact resistance compared to some high temperature materials. It was this material that broke on the Space Shuttle Columbia due to an insulation strike which ultimately led to the later shuttle destruction on re-entry.
In a rocket nozzle, impact resistance is not an issue - thermal shock and temperature is, and so this material is ideal for this use. Even so, it is still ablative, and is not considered re-usable. Re-usable rockets, however, are almost always of the liquid variety for re-fueling purposes, and those nozzles are liquid cooled from the cryogenic fuel sources.
In some cases the fuel itself will be cycled through tubes in the nozzle to keep the nozzle within a defined operating temperature range.
That's the case with the Rocketdyne F-1, for example.
I just noticed a comment referring to this practice. The F-1 is the actually the engine that was used on the Saturn V.
I remember watching this video from Youtube that provide most of the science and technologies needed to build a space shutter. And it also explained how to nozzle survive from the heat generated by the rocket.
Basically it passes the fuel to the engine through small pipes at the outer layer of the nozzles whereby the fuel is very cold.
Since the fuel is cold, the heat from flame of the rocket will be absorb by the fuel's pipe and avoiding direct heat contact with the nozzle (Which I assume this is what they called Thermal Equilibrium?). Hence avoiding the heat to melt the nozzle.
Answer by @ALAN WARD are far more accurate than mine, but I think the video can provide easier understanding.