“Necessity is the mother of all inventions”
This is indeed true in the case of Cryogenic rocket engines since they were invented when the world needed them the most. During the World War II the American, Soviet and German engineers independently found that their rocket engines require high mass flow rate for both the fuel and oxidizer used in the rocket engines to generate maximum thrust. Back then, the most common oxidizer and fuel combinations used were that of oxygen and hydrocarbons of low molecular weight respectively. Both these elements share the gaseous state of matter at the room temperature and pressure. When these are stored as gases in the fuel tanks of the rocket, they weigh down the rocket with both their size and mass thereby severely affecting the efficiency of rocket engine. To tackle this, the obvious way that was found by engineers was to cool both oxidizer and fuel to lower temperatures which are scientifically categorized as cryogenic temperatures. The range of temperature under this category is defined between -183 oC to -253 0C. At these cryogenic temperatures, the fuel and oxidizer enter the liquid state of matter and are thereafter stored as liquid in the fuel tanks. Because of this feature of these engines, they are also widely termed as either liquid propellant rocket engines or hybrid rocket engines.
The major components of a cryogenic rocket engine are same as that of a normal rocket engine with some additional components to facilitate the liquid state of fuel and oxidizer. The most vital component of any rocket engine is the combustion chamber, also termed as thrust chamber which takes in the fuel and oxidizer in a pre-set proportion and burns them to generate sufficient thrust for the rocket to take off. There are different methods of feeding the propellants (combination of fuel and oxidizer) to the chamber. For cryogenic rocket engines, most common methods used are either pressure fed or the pump fed. In a pressure fed scenario, there is usually a separate gas supply, normally helium which is used for pressurizing the tank that contains propellants and then force both fuel and oxidiser to the combustion chamber. It is important that the pressure of propellant tanks exceed the pressure in the combustion chamber to ensure that there is adequate flow of propellants to the latter. The pump-fed class of design is usually used in bigger rockets where high pressure by itself is not enough to ensure adequate flow of propellants to the combustion chamber.
Other components include pyrotechnic initiator (used for ignition), regulators, gas turbine, fuel injector, rocket engine nozzle and cryopumps for both fuel and oxidizer (in a pump fed class of design). These cryopumps are essentially turbopumps that are powered using fuel flow through the gas turbines.
Comparing Cryogenic Rocket Engines of different countries
Cryogenic rocket engines played a vital role in the success of America’s success in reaching the moon. By far, only six countries in the world have successfully deployed the cryogenic rocket engines. While USA, Russia, Japan, France have been using these engines for a long time now, People’s Republic of China and India have been the latest entry into this ivy league. The design specifics of engines of each country are different from the rest as shown in Figure 1.
Figure 1: Comparing design specifics of Cryogenic rockets from different countries
India’s entry into the cryogenic foray
In the year 2015, Indian Space Research Organisation (ISRO) successfully completed the endurance test of its indigenously designed and manufactured cryogenic engine. India had a deal with Russia for buying cryogenic engines without access to the technology. Several of Indian satellites were launched using GSLVs (Geo-Synchronous Satellite Launch Vehicle) which used these Russian made cryogenic engines. But after developing the indigenous cryogenic engine, GSLV MK-2 with this engine was successfully used to launch satellites in the January of 2017.
Some of the international news reports suspected this achievement of being a mere reverse engineering product from the Russian cryogenic engines. But there are some stark differences in the design of Russian and Indian engines to refute these claims. While the Russian class of design of was the staged combustion cycle, the Indian design uses gas generator cycle thus proving that it was indeed indigenous. With such technological advancements, India is poised to take on the world in terms of satellite launching facilities.
Credit to Science & Tech Writer Rachitha S.
Image and Video credit : ISRO