Temperatures inside a rocket engine and nozzle can reach up to 3 300° Celsius or 6 000° Fahrenheit. This will melt the most metals, raising the question of how rocket engines manage to stay cool and not melt.
Rocket engines primarily stay cool by utilizing one or a combination of cooling techniques, including regenerative cooling, film cooling, and ablative cooling. Additionally, methods like radiative cooling and changing the fuel-to-oxidizer ratio are also deployed to prevent the engine from melting.
During any orbital rocket launch, the amount of heat generated by a launch vehicle’s engines is very apparent by simply looking at the flames, steam, and smoke it generates on the launchpad and below its nozzles as it powers into the atmosphere.
What one observes, though, is only a tiny fraction of the heat that is actually generated within the rocket engine itself. The hot gases created within a rocket engine typically reach temperatures that are more than half the surface temperature of the sun.
For example, the temperatures inside the F1 engine of a Saturn V rocket used during the Apollo missions reached close to 3300° Celsius (6 000° Fahrenheit). This is comparable to modern launch vehicles like the Space Shuttle and SpaceX’s workhorse, the Falcon 9 rocket.
These temperatures are hot enough to melt the vast majority of metals. Aluminum melts at 660° Celsius (1220° Fahrenheit), stainless steel at 1510° Celsius (2750° Fahrenheit), and even titanium will melt at 1670° Celsius (3040° Fahrenheit).
To keep the combustion chamber and nozzle walls relatively cool and prevent them from melting, rocket engine manufacturers utilize a number of cooling techniques, as the following section will illustrate.
How Rocket Engines Stay Cool
As was clearly illustrated during the introduction, the temperatures reached within the combustion chamber and nozzle of a rocket engine far exceed the melting point of most metals and need some form of active or passive cooling to keep it from melting.
Through the years, engineers managed to devise a number of cooling techniques to keep rocket engines cool and prevent the combustion chamber walls, nozzle, and other surface areas from melting.
(Apart from the potential melting of a rocket engine, overheating can result in the catastrophic failure of the entire launch vehicle. Learn more about why exactly rockets fail and explode in this article.)
Each cooling method can be used on its own, but in most cases, several cooling techniques are deployed simultaneously in the same engine to effectively cool it down. The primary and most commonly used cooling techniques in modern rockets are:
- Regenerative Cooling
- Film Cooling (Curtain Cooling)
- Ablative Cooling
- Dump Cooling
- Radiative Cooling
- Fuel/Oxidizer Ratio
One can get a better understanding of how each cooling method works, as well their benefits and drawbacks, by taking a closer look at each one individually.
Regenerative cooling is a form of active engine cooling where the rocket’s fuel is pumped through a series of channels inside the combustion chamber and nozzle walls to prevent them from melting.
It is the most commonly used and effective way of keeping a rocket engine’s walls cool. It was used as early as the Apollo missions of the 1960s and 70s and is still used today in modern launch vehicles like the Merlin engine used in SpaceX’s Falcon 9 rocket.
Essentially, this form of cooling consists of a series of channels situated inside the walls of the combustion chamber, throat, and upper section of the nozzle. The rocket’s fuel is then pumped under pressure through the channels to cool the walls down.
The relatively cold fuel is first circulated through these channels in the chamber & nozzle walls before being pumped back into the combustion chamber, where it is combined with the oxidizer and combusted to provide the thrust that propels the rocket forward.
Traditional chamber and nozzle walls consisted of hundreds of steel tubes that were shaped in the desired profile. They were combined with a binder metal and melted together through a process called brazing, which resulted in a single structure with hollow channels inside.
However, advancements in technology allow modern chamber & nozzle walls to be constructed using a liner and structural jacket. The liner typically consists of a conductive metal like copper alloy, with a strong, heat-resistant metal like Inconel forming the jacket.
First, the basic liner shape is forged using hot spin-forming, after which it is machined to form the precise shape of the combustion chamber and nozzle. It is then slotted (the process through which the channels are cut into the metal) on its outer surface.
(A metal like copper alloy is generally chosen because of its highly conductive properties, which means it can conduct the cooler temperatures of the coolant flowing through it to the inner walls and structural jacket on the outside with greater ease.)
A structural jacket is then fitted to the slotted outer wall of the liner, and the two metals are combined into a single piece, using advanced electro-plating, welding, and brazing to form a solid bond.
(A strong, heat-resistant metal like Inconel is often used as a jacket to withstand the high temperatures reached in a rocket engine, but in some cases, other heat-resistant metals like stainless steel are also utilized.)
As is the case with the more traditional tube-based chamber & nozzle walls, the fuel is pumped through the channels slotted in the liner to cool the walls down before it is pumped back to the combustion chamber.
Film Cooling (Curtain Cooling)
Film cooling is a form of active engine cooling where fuel in its liquid or gaseous form is injected along the inside walls of the combustion chamber and nozzle to create a protective boundary layer, partially insulating the walls of the rocket engine from the hot gases.
Alongside regenerative cooling, this form of cooling is also one of the most widely-used practices to keep rocket engine walls cool, and the two types of cooling are often used in conjunction in the same engine to optimize cooling.
Typically, extra fuel is pumped through additional holes in the outer perimeter of the fuel/oxidizer injector close to the chamber walls of the engine’s combustion chamber.
The fuel does not react with the oxidizer and is trapped between the inside walls and the combusted gases flowing through the chamber. This creates a thin film coating that flows along the inside walls of the chamber, throat, and upper nozzle section.
The thin coating of relatively cool fuel creates a protective layer that helps to keep the engine walls cool. The fuel continues to flow along the channel walls until it starts mixing with the oxidizer or the oxygen in the atmosphere as it exits the nozzle where it combusts.
Fuel can also be allowed to bleed through holes created in the chamber walls to provide additional film protection in regions where the heat buildup is more severe, like the throat of the nozzle where temperatures and pressures are at their peak.
Exhaust gases from the engine’s gas generator can also be used as a form of film cooling. The gas generator creates the hot gases that drive the turbopumps that feed fuel and oxidizer into the combustion chamber. Expended gases are often dumped overboard.
However, since the regenerative cooling of a rocket engine often ends at the nozzle extension, the exhaust gas can be rerouted via the gas generator’s exhaust manifold to exit through holes near the nozzle extension, creating a layer of gas to protect the nozzle walls.
Ablative cooling is a form of engine cooling where the inside walls of the combustion chamber and nozzle are covered with a layer of material designed to erode and burn away as it heats up, carrying the excess heat away as it exits the nozzle alongside the hot gases.
Regenerative and film cooling are very effective ways of keeping a rocket cool, but the number of components and processes that has to be in place to make them work efficiently add a lot of complexity and extra weight to the engine.
A simpler and more cost-effective means of negating the intense heat buildup is to add a layer of strong, heat-resistant materials like carbon composites, silica, or graphite to the inner walls of the combustion chamber and nozzle.
As the hot gases pass over the chamber walls, it heats the surface of the ablative materials to the point where it starts to erode and is blown out the back of the nozzle with the exhaust gases and, importantly, carrying the heat with it in the process.
This form of cooling has a downside, though. As the ablative material burns away, the inside of the combustion chamber and nozzle starts to expand in volume as the surface material continues to be eroded.
This will impact the engine efficiency since the combustion chamber & nozzle’s throat are designed to precise measurements to create the correct pressures and temperatures to function optimally. As more material burns away, they become increasingly less efficient.
This type of cooling can also not be used in engines that have to burn for long periods on end since there is a limited amount of ablative material that can be eroded away, and adding sufficient materials for extended burns will add too much weight to the engine.
It is also not ideal for engines that are intended to be reused at a later stage since refurbishing such an engine will require an extensive overhaul and rebuild that will be expensive and time-consuming, which will negate the benefits of reusing the engine.
Dump cooling is a form of engine cooling similar to regenerative cooling, but instead of relaying the fuel back to the combustion chamber after circulating through the channels in the chamber walls, it is simply dumped overboard at the aft end of the nozzle.
More than 80% of a rocket’s mass consists of fuel during launch. This is because the amount of energy necessary to leave the Earth’s atmosphere and reach orbit requires a staggering amount of fuel. As a result, fuel efficiency is of the utmost importance in rocket design.
It should come as no surprise that any unnecessary wastage of rocket propellant will be viewed as less than ideal. As a result, dump cooling is primarily a theoretical solution to cool rocket engines, with very little if any practical application.
Radiative cooling is a form of engine cooling typically used by vacuum optimized rocket engine nozzles to radiate heat away from the nozzle into space. A metal with a high melting point and good thermal conductivity is typically used to manufacture the nozzle extensions.
(A vacuum optimized engine nozzle is a nozzle optimized to work more efficiently in the vacuum of space. Learn more about the difference between vacuum and sea level optimized engine nozzles in this article.)
In modern times, most readers have the luxury of observing any major rocket launch from the comfort of their homes. Whenever one watches the deployment of a launch vehicle’s upper stage, the red glow of its engine nozzles is clearly visible.
The red glow comes from the heat generated by the hot exhaust gases that turn the nozzle red hot. In the vacuum of space where there is no air to conduct heat, the engine nozzle can make use of the environment by radiating the heat away from its surface into space.
A material like Niobium alloy that has a high melting point, conducts heat very well (has good thermal conductivity), and has a high resistance to thermal shocks is ideal to use in nozzle extensions that use this form of cooling.
The idea or proposal behind using the “heatsink” concept to keep a rocket engine’s chamber and nozzle walls cool and prevent them from melting is by using a thicker metal that will take much longer to heat up to the point of melting.
By using a thicker metal, a much larger area needs to be warmed up, which will take longer than utilizing a thinner material. The thicker wall will be able to absorb more heat before it reaches melting point, effectively acting as a heatsink.
However, there are at least two major drawbacks that make putting this method into practice an unfeasible option.
Firstly, the amount of weight that will need to be added to make this form of cooling even remotely effective will simply make a rocket engine too heavy to be used in any practical application. Weight saving remains a primary concern of any rocket design.
Secondly, even if weight was not such a significant factor, at temperatures of 6 000° Celsius (3 300° Fahrenheit), even the most heat resistant metals with added bulk will not be able to stay structurally sound for the duration of a typical burn period.
Changing a rocket engine’s fuel to oxidizer ratio is a sometimes overlooked form of engine cooling. By changing the mixing ratio of the propellent so that an engine runs either fuel or oxidizer rich, the temperature generated inside a rocket engine can be reduced.
The optimum fuel to oxidizer ratio, where maximum efficiency is achieved and all the fuel burned, also results in the most energy being released and the highest temperatures generated. These temperatures are often too high for the engine components to withstand.
As a result, many launch vehicles run a propellant mix in their combustion chambers that are either fuel or oxidizer rich, which lowers the temperatures generated by combustion.
This form of cooling is especially beneficial for use in the engine’s gas generator or preburner (used to create the hot gases that drive the engine’s turbopumps) since not many of the other cooling methods could be utilized to keep the spinning turbine cool.
Keeping a rocket engine generating exhaust gases at temperatures of up to 6 000° Celsius (3 300° Fahrenheit) is no small feat. It is only thanks to the ingenuity and perseverance of rocket engineers that modern engines can function at all.
(It has to be important to note that a rocket engine forms only part of a much larger & complex launch vehicle. Learn more about what exactly an orbital rocket is and how it functions in this article.)
As this article illustrated, several cooling techniques are deployed to keep the walls of a rocket engine’s combustion chamber and nozzle cool and prevent them from melting.
Sometimes, using one of these methods will be sufficient to keep the engine cool, but more often than not, several cooling techniques are used in conjunction with each other in the same engine to keep the extreme temperatures from destroying it.
This article was originally published on headedforspace.com. If it is now published on any other site, it was done without permission from the copyright owner.