cryogenic engines are the most prestigious rocket engine technology due to their design and operational complexity only a few countries have actually been able to develop this intriguing engine let's start a cryogenic engine design journey starting with the fundamentals a simple rocket propels itself high into the sky using newton's third law in other words when the rocket engine ejects a high amount of mass at high speed the rocket gains an equal amount of momentum in the opposite direction obviously to eject a high amount of fuel at high speed rocket engines have to burn highly combustible fuel
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 eventually thrust using different types of valves obviously a rocket has to carry oxygen also with it the fuel and oxidizer together are called a propellant the first design challenge is to select the right fuel when selecting fuel for a rocket engine specific impulse is the most crucial term specific impulse is the amount of push the rocket gets by burning per unit propellant so rockets clearly need high specific impulse fuels a rocket derives
thrust due to the rate its momentum changes the more the speed of its exhaust the greater the momentum loss exhaust gas speed is proportional to exhaust gas temperature which is why the calorific value of the fuel is crucial thus high calorific value fuel with the least molecular weight tends to have high specific impulse based on these criteria the obvious choice is hydrogen as it has a very low molecular weight and a high calorific value resulting in a high specific impulse in addition to these factors hydrogen does not corrode engine parts and is not toxic to
the atmosphere when burned with oxygen however the main challenge with hydrogen is that at room temperature it takes a gaseous form therefore carrying hydrogen gas in huge tanks would make the space rocket bulky the only solution is to liquefy the hydrogen which is why cryogenics come into the picture the liquid hydrogen is looking so cool right liquefication of hydrogen results in a compact tank size to achieve this cool looking liquid hydrogen it has to undergo a long sequence of processes you can see how compressors condensers and throttling devices work together to bring the temperature down
to negative 253 degrees celsius gaseous hydrogen can only turn into liquid form at this extreme low temperature this liquid propellant is then transported in huge tanks and stored near the launch station where it is transferred into the rocket fuel tanks right before the launch oxygen gas also undergoes the same process there you go we have just produced the cryogenic propellants these two tanks are covered by a bigger outer tank which is made up of a highly durable aluminum lithium alloy do you see yellowish material shrouding the outer tank this material is in fact a 25
millimeter thick layer of thermally insulating polyurethane that has been applied with a spray foam technique its purpose is to protect the outer tank which has to face extreme heat while passing through 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 automatically flow to the engine which can cause combustion however the thrust generated wouldn't be sufficient for a successful takeoff to
propel the exhaust from the nozzle at high speeds a pump is needed to send the fuel and oxidizer to the combustion chamber what about an electric pump an electric pump would require a lot of energy storage for this task which would add more weight to the rocket a clever solution to pump the liquid hydrogen is to run the pump using a turbine which operates on expanded hydrogen moreover for efficient burning liquid hydrogen has to be converted into gaseous form how can we accomplish this to get the expanded hydrogen we simply need to circulate the liquid
hydrogen around the hot nozzle and combustion chamber this hot gas is then sent into the combustion chamber this configuration is called the expander cycle engine and the turbine pump arrangement is called a turbo pump the same technique is used to send liquid oxygen to the combustion chamber however liquid hydrogen cannot be pumped at a great speed using this method now a logical question would be why not burn a part of this fuel and use its exhaust to drive the turbine to answer that question let's introduce an additional small combustion chamber a small portion of liquid
propellants are burned and its high speed exhaust gases are used to run the turbine this engine cycle is known as the gas generator cycle and these kinds of cost-effective engines are used to propel spacex's falcon rockets however this configuration is not very efficient as some exhaust energy is totally lost the efficiency can be increased by diverting this exhaust from the turbine to the combustion chamber here a very small percentage of oxygen is used to burn the hydrogen this partial burning of hydrogen increases its temperature and pressure and then this fuel-rich mixture is completely burned into
the combustion chamber this arrangement is called the staged combustion cycle and this engine type gives the highest thrust and specific impulse however the pressure inside the combustion chamber is very high therefore it needs very powerful and expensive parts engine cycles are chosen for a particular rocket according to each mission's requirements a device called an injector plate is used to mix hydrogen and oxygen thoroughly in the combustion chamber here the propellants get atomized after the atomization the propellant is burned efficiently using a pyrotechnic igniter the temperature inside the cryogenic engine combustion chamber can reach as high
as 3000 degrees celsius which can cause material damage however the circulating liquid hydrogen around the combustion chamber helps to maintain the material temperature within the allowable limit hitting the proverbial two birds with one stone the high pressure gases which are expelled from the combustion chamber are accelerated to higher velocities via a converging diverging nozzle now let's see why development of a successful cryogenic engine is such a big challenge for a cryogenic engine the oxygen to hydrogen ratio is of the utmost importance the turbo pump does this crucial job and is therefore known as the heart
of a cryogenic engine the trick with turbo pump design is that the pumps that control the propellant flow rate are controlled by a turbine which is in turn controlled by propellant combustion a problem within the problem right this tricky pump speed control makes controlling propellant ratio extremely difficult some turbo pumps even use a gearbox to run the pumps at different speeds than the turbine another big design challenge in cryogenic engines is that of thermal insulation observe the thermal image of this rocket technology did you notice anything peculiar there is a very high temperature gradient in
many parts of the rocket strong thermal barriers should be designed to prevent the heat flow this kind of high temperature gradient is not common in other rocket engines making design of the thermal insulation unique to cryogenic rocket technology the third major challenge is the diffusion of liquid hydrogen inside metal structure cracks this process affects the strength of the metals drastically and special metal alloys had to be developed to overcome this issue cryogenic engines are used mostly in the second and third stages of a rocket with all these design complexities getting everything right requires immense work
this design complexity is the reason why very few countries have been able to master the cryogenic engine design further developments in cryogenic engines include the deep throttling engines for use across many mission phases thank you for listening would you like to be a part of our team please click the support button thank you
