ISRO’s indigenous thrust gets a fillip

On February 7, the Indian Space Research Organisation (ISRO) successfully tested the re-ignition of the made-in-India CE20 cryogenic engine in space-like vacuum conditions at the ISRO Propulsion Complex, Mahendragiri, Tamil Nadu. This is another remarkable milestone in ISRO’s saga of indigenous cryogenic technology.

Most of us turn off the motor while driving down a slope and let gravity accelerate us downward. We restart the engine and continue driving as we approach the foothill. Similarly, to place satellites in various orbits or for intricate interplanetary missions, we must turn on the cryogenic engine mid-flight, after a period of days or months. For example, a lander mission to Mars will require the engine to be turned on after roughly 10 months. Currently, ISRO has only one engine, the Liquid Apogee Motor, which can be re-ignited and powered by liquid fuel.

In the trial, the upgraded CE20-U engine worked as planned under fuel tank pressure conditions expected to prevail while restarting mid-flight. ISRO will make additional alterations and perform step-by-step trials before rolling out the CE20-U engine for the next-generation launch vehicle.

When released, pressurised air exits a balloon, thus propelling it in the opposite direction. This illustrates the basic rocket principle, known as Newton’s third law. Likewise, if hot gas can be generated and released from a nozzle, the rocket can be made to propel forward in the opposite direction. Combustion requires fuel and oxygen, which are referred to as propellants. Rocket propellants are of three types: solid, liquid and gaseous. Instead of a solid propellant, one might utilise kerosene and a suitable oxidiser as fuel. This is a liquid propellant rocket. Solids and liquids are less voluminous than gases. In normal temperature and pressure, the amount of water in its vapour stage will take up 16 times the space it would take in its liquid state. As a result, gaseous fuels must be cooled to liquefy them before they can be used as rocket fuel. That is cryogenic fuel.

The term ‘cryo’ means ice-cold or chilled. Cryogenics is the science and technology of substances at temperatures below -153°C, the boiling point of methane. The most common cryogenic propellants are hydrogen liquefied at

-253°C as fuel and oxygen liquefied at -183°C as oxidiser. When liquefied hydrogen and liquefied oxygen combine, they are highly reactive and produce steam.

The lighter the gas, the lesser the energy required to accelerate it. As hydrogen is the lightest of the atoms, we can use it as fuel and liquid oxygen as an oxidiser to achieve more thrust per kilogramme of fuel, making it ideal for space travel. Cryogenic rockets are critical for deep space missions such as lunar landings and interplanetary probes and for putting larger payloads into geostationary orbit 36,000 km above earth’s surface.

The US, Russia, Japan, India, France and China are the only countries operating cryogenic rocket engines. These engines are used only in the upper stage of the vehicle and operate beyond the earth’s atmosphere after the vehicle has entered space. According to international consensus, anything more than 100 km beyond the earth’s surface is considered space.

If cryogenics are more efficient, why not use them immediately at liftoff? The high efficiency of cryogenic engines is not suitable for the initial launch stages, where more raw power is required to overcome gravity.

Once the red light turns green, you try to go as fast as possible. You need fuel with good pickup. However, while driving long distances on a motorway, a fuel-efficient engine benefits your wallet and the environment. Petrol vehicles have a higher initial pickup than diesel cars, but the latter have more pulling power and mileage. Similarly, the launch vehicle must overcome inertia, climb into the sky, race against gravity’s gripping hands and then overcome atmospheric drag. Solid or liquid fuel with a powerful thrust is the best option. However, once there is no atmospheric resistance in space, the vehicle is already propelling, and a fuel-efficient engine provides superior mileage.

The Soviets and the Americans fought to create more efficient launch vehicles capable of putting heavier spacecraft into orbit, reaching the moon, and undertaking interplanetary travel. The RL10, which debuted in 1963, was the first cryogenic rocket engine built in the US. It was utilised in the Saturn 1 rocket during the early stages of the Apollo moon landing mission and still powers US launch vehicles.

The Soviets designed the RD-56, also known as the 11D56, at about the same time in 1964. They shifted their focus from fully cryogenic to semi-cryogenic, developing the powerful RD-180, which used liquid kerosene and liquefied oxygen as launch propellants. In parallel, the Soviets developed a better-designed cryogenic engine, the KVD-1, that was sold to India.

By the 1990s, ISRO was looking for cryogenic technology and had initially approached Japan and the US. The engines were far too pricey. Meanwhile, the Soviets were eager not only to sell their KVD-1 engines but also to transfer knowledge, allowing ISRO to manufacture and produce its own cryogenic engines at a significantly lower cost than that quoted by US corporations.

In the following months, the Soviet Union collapsed and the US exerted pressure on Russia not to export the technology, claiming that India might use it as a nuclear missile. The argument was flimsy because missiles must be ready for launch in seconds, while cryogenic engines require at least 24 hours of fuelling. It was a hypocritical irony because the US came forward only months ago to sell engines at significantly higher prices. The Russians honoured their word and delivered six engines, but they could not impart technology due to the American embargo.

Slapped with the ban, ISRO learnt from the six engines and created its own cryogenic engine, CE20. The Indian design is not a replica, as the Russian engines are staged combustion engines, but the made-in-India CE20 are gas-generator cycle engines. After years of hard labour, the CE20-powered LVM3 launch vehicle delivered the GSAT-19 into geostationary transfer orbit on June 5, 2017. The engine was fitted in the Chandrayaan 2 and 3 launch vehicles. The human-rated engine will assist Indian astronauts in reaching orbit during the Gaganyaan expedition. With re-ignition capability, the CE20 engine will power Indian spacecraft on ambitious interplanetary missions.