Cryogenic engines are the most prominent rocket engine technology, and only a few countries have been able to develop them due to their design and operational complexity.
Let’s start a Cryogenic Engine design journey starting with the fundamentals
A simple rocket uses Newton’s third law to propel itself high into the sky, which states that when a rocket engine ejects a large amount of mass at a high speed, the rocket gains an equal amount of momentum in the opposite direction. Obviously, to eject a large amount of fuel at high speeds, rocket engines must burn highly combustible fuel.
Liquid fuel-based rocket engines
Liquid fuel-based rocket engines are the most versatile engines for space propulsion. With these rockets, it is possible to efficiently control fuel injection and ultimately thrust using various types of valves. Of course, a rocket must also carry oxygen; the fuel and oxidizer are referred to as a propellant.
When choosing a fuel for a rocket engine, the most important term to remember is specific impulse. Specific impulse is the amount of thrust a rocket gets per unit propellant burned, so rockets clearly require high specific impulse fuels. A rocket derives thrust from the rate at which its momentum changes; the higher the speed of its exhaust, the greater the momentum loss. The calorific value of the fuel is important because exhaust gas speed is proportional to exhaust gas temperature.
Since a result, high calorific value fuel with the smallest molecular weight has a high specific impulse. Based on these characteristics, hydrogen is the obvious choice, as it has a very low molecular weight and a very high calorific value, resulting in a high specific impulse.
In addition to these benefits, when hydrogen is burned with oxygen, it does not damage engine parts and is not dangerous to the environment. However, the biggest disadvantage of hydrogen is that it is a gas at room temperature.
As a result, carrying hydrogen gas in large tanks would make the space rocket bulky. The only answer is to liquefy the hydrogen, which is where cryogenics comes in, Liquid hydrogen looks very interesting, right? Liquification of hydrogen results in a small tank size.
To create this cool-looking liquid hydrogen, it must go through a long series of procedures. You can see how compressors, condensers, and throttling devices work together to reduce the temperature to minus 253 degrees Celsius. At this extremely low temperature, gaseous hydrogen can only convert into liquid form.
This liquid propellant is then brought in large tanks and kept near the launch station, where it is transferred into the rocket fuel tanks just before the launch. Oxygen gas is also transported in large tanks and stored near the launch station.
There you have it, we’ve just finished producing cryogenic propellants. These two tanks are encased in a larger outer tank constructed of an extremely robust aluminum-lithium alloy. Do you notice the yellowish material shrouding the outer tank?
The exterior tank is protected by a 25-millimeter thick layer of thermally insulating polyurethane that was placed using a spray foam technique. Its function is to insulate the outer tank from severe heat when traveling through the earth’s atmosphere.
Now we have stored the cryogenic propellants lh2 and lox safely next let’s get into the mechanical design of the cryogenic engine.
What happens if we directly supply fuel from the fuel tank to the combustion chamber?
The liquid hydrogen and liquid oxygen will flow to the engine automatically, causing combustion. However, the thrust generated will not be enough for a successful takeoff. To propel the exhaust from the nozzle at high speeds, a pump will be required to send the fuel and oxidizer to the combustion chamber.
What about an electric pump?
This would require a lot of energy storage, which would add weight to the rocket. A clever solution to pump the liquid hydrogen is to use a turbine that runs on expanded hydrogen. Furthermore, liquid hydrogen must be converted into gaseous form for efficient burning. How can we do this?
We simply circulate liquid hydrogen around the heated nozzle and combustion chamber to obtain expanded hydrogen, which is then fed into the combustion chamber. This configuration is known as an expander cycle engine, and the turbine pump arrangement is known as a turbopump.
The same mechanism is used to deliver liquid oxygen to the combustion chamber, but liquid hydrogen cannot be pumped at high speeds using this way. To answer that point, why not burn a portion of this fuel and utilize the exhaust to power the turbine?
Let’s introduce a second small combustion chamber, where a small amount of liquid propellants is burned and the high-speed exhaust gases are used to power the turbine. This engine cycle is known as the gas generator cycle, and these types of cost-effective engines are used to power SpaceX’s Falcon rockets.
However, because some exhaust energy is completely lost in this configuration, the efficiency can be improved by diverting this exhaust from the turbine to the combustion chamber, where a very small percentage of oxygen is used to partially burn the hydrogen, increasing its temperature and pressure before the fuel-rich mixture is completely burned into the combustion chamber.
This configuration is known as the staged combustion cycle, and it produces the highest thrust and specific impulse. However, the pressure inside the combustion chamber is extremely high, necessitating the use of extremely powerful and expensive parts. Engine cycles are selected for each rocket based on the mission’s requirements.
The temperature inside the cryogenic engine combustion chamber can reach as high as 3000 degrees Celsius, which can cause material damage. An injector plate is used to thoroughly mix hydrogen and oxygen in the combustion chamber where the propellants are atomized. After atomization, the propellants are burned efficiently using a pyrotechnic igniter.
The circulating liquid hydrogen around the combustion chamber, on the other hand, helps to keep the material temperature within the allowable range, killing two birds with one stone. High-pressure gases exiting the combustion chamber are accelerated to higher velocities via a converging-diverging nozzle. Let’s look at why developing an effective cryogenic engine is such a difficult task.
The oxygen to hydrogen ratio is critical for a cryogenic engine, and the turbopump performs this vital function, earning it the moniker “heart of a cryogenic engine.”
The trick with turbopump design is that the pumps that control propellant flow rate are controlled by a turbine, which is controlled by propellant combustion. This tricky pump speed control makes controlling the propellant ratio extremely difficult, and some turbopumps even use a gearbox to run the pumps at different speeds than the turbine.
Another major design challenge in cryogenic engines is thermal insulation. Did you notice anything unusual about the thermal image of this rocket technology? There is a very high-temperature gradient in many parts of the rocket. Strong thermal barriers should be designed to prevent heat flow. This kind of high-temperature gradient is not common in other rocket engines, making the design more difficult.
The diffusion of liquid hydrogen into metal structure defects is the third main difficulty. This process has a significant impact on the strength of metals, necessitating the development of unique metal alloys to address this issue.
Cryogenic engines are mostly used in the second and third stages of a rocket, and with all of these design complexities, getting everything right takes a lot of work. This design complexity is one of the reasons why only a few countries have mastered cryogenic engine design. Other cryogenic engine developments include deep throttling engines for use across multiple mission phases.
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