Cryogenic Engine

A cryogenic engine is a type of rocket engine that uses propellants stored at extremely low temperatures. The term cryogenic is derived from the Greek words kryos meaning ‘cold’ and genic meaning ‘producing’. Such engines are designed to handle propellants that remain in liquid form only at very low temperatures, typically below –150°C. Cryogenic technology is widely employed in space exploration due to its high efficiency and superior performance compared with conventional chemical propulsion systems.

Background and Development

The concept of cryogenic propulsion emerged during the mid-20th century when scientists sought to develop more efficient rocket engines capable of achieving higher thrust-to-weight ratios. The key innovation was the use of liquid hydrogen (LH₂) as the fuel and liquid oxygen (LOX) as the oxidiser. These propellants, though difficult to store and handle, deliver exceptional specific impulse—a measure of engine efficiency.
The first successful use of cryogenic technology occurred in the United States during the 1960s with the Saturn V rocket developed by NASA for the Apollo missions. The Saturn V’s second and third stages used cryogenic engines to propel astronauts to the Moon. Subsequently, nations such as Russia, Japan, China, and India developed indigenous cryogenic propulsion systems for their space programmes.

Principle of Operation

A cryogenic engine operates on the principle of liquid propellant combustion. The process involves several key stages:

1. Storage of Propellants: Liquid hydrogen and liquid oxygen are stored in insulated tanks to prevent evaporation.
2. Pumping and Mixing: Turbopumps drive the cryogenic fluids into the combustion chamber under high pressure.
3. Ignition: The fuel and oxidiser mix and ignite, producing high-temperature gases.
4. Thrust Generation: The gases expand rapidly and are expelled through a nozzle at supersonic speed, creating thrust in the opposite direction (as per Newton’s third law).

The combustion temperature in a cryogenic engine can reach nearly 3,000°C, despite the propellants being stored at extremely low temperatures.

Components of a Cryogenic Engine

A typical cryogenic engine consists of:

• Fuel tank: Stores liquid hydrogen at about –253°C.
• Oxidiser tank: Stores liquid oxygen at about –183°C.
• Turbopumps: Pressurise and deliver propellants to the combustion chamber.
• Combustion chamber: The site where fuel and oxidiser combine to produce thrust.
• Injector: Ensures uniform mixing of fuel and oxidiser for stable combustion.
• Nozzle: Converts high-pressure gases into high-velocity exhaust to produce thrust.
• Cooling system: Utilises the cryogenic fuel to cool the nozzle and chamber walls to prevent overheating.

Advantages of Cryogenic Engines

Cryogenic engines are preferred in modern space missions due to several technical advantages:

• High efficiency: They provide higher specific impulse compared with solid or semi-cryogenic engines.
• Cleaner combustion: The main exhaust product is water vapour, making them environmentally friendly.
• Greater thrust: Capable of producing powerful thrust, suitable for heavy-lift and deep-space missions.
• Cost efficiency in payload delivery: They enable larger payloads to be carried into orbit with less fuel consumption.

Limitations and Challenges

Despite their advantages, cryogenic engines pose significant engineering and operational challenges:

• Storage difficulties: Maintaining hydrogen and oxygen in liquid form requires extremely low temperatures and special insulation.
• Complex design: The engine involves intricate systems of turbopumps, valves, and injectors that must operate with high precision.
• Delayed readiness: Cryogenic engines require extensive pre-cooling before launch, increasing preparation time.
• High development cost: The materials and technologies involved are expensive, demanding advanced research and testing facilities.

Cryogenic Technology in India

India’s journey with cryogenic technology began in the 1980s when the Indian Space Research Organisation (ISRO) sought to develop indigenous cryogenic upper stages for its Geosynchronous Satellite Launch Vehicle (GSLV). Initially, India procured limited cryogenic technology from Russia; however, after restrictions imposed by the Missile Technology Control Regime (MTCR), ISRO embarked on an indigenous development programme.
In 2010, ISRO successfully tested its first home-grown Cryogenic Upper Stage (CUS). The breakthrough came in 2014 with the GSLV-D5 mission, which placed the GSAT-14 satellite into orbit using an indigenous cryogenic engine. Since then, India’s cryogenic engines—particularly the CE-7.5 and CE-20—have powered numerous GSLV Mk II and Mk III missions. The CE-20 engine, used in the LVM3 (Launch Vehicle Mark-3), represents India’s most powerful cryogenic stage and plays a crucial role in deep-space and lunar missions, including Chandrayaan-2 and Chandrayaan-3.

Applications of Cryogenic Engines

Cryogenic engines have a broad range of applications in modern space programmes:

• Launching heavy payloads: Used in upper stages of rockets to place large satellites into geostationary orbits.
• Interplanetary missions: Provide the necessary propulsion for deep-space exploration missions.
• Reusable launch systems: Their efficiency makes them suitable for reusable rocket stages under development by agencies such as SpaceX and ISRO.
• Scientific research: Employed in missions requiring high precision and long-duration thrust.