ISRO has been using cryogenic rocket engines for more than 20 years, including one foreign-made engine and two made in India. But for the first time ever, ISRO successfully restarted its indigenous C25 cryogenic stage in space during the LVM3 M5/CMOS-03 mission earlier this month. This is a major technological achievement.

Cryogenic engines are highly efficient and very difficult to build. They work using liquid hydrogen stored at –253°C and liquid oxygen stored at –183°C. Because these fuels must be kept extremely cold, storing and handling them is very challenging.

When the main mission was completed and the satellite was placed in orbit, ISRO tried something new. It reoriented the cryogenic stage, waited for 100 seconds, and then reignited the engine in the vacuum of space. ISRO said this was the first time India had restarted a cryogenic stage in deep space.

Why is restarting a cryogenic stage so difficult?

On Earth, gravity keeps the fuel settled at the bottom of the tank. But in microgravity, fuel floats around and forms bubbles. For the engine to restart, the fuel must be pushed to the bottom of the tank. To do this, special small motors called ullage motors or settling thrusters are used. If fuel does not settle properly, the engine may suck in gas instead of liquid, which can cause the turbopump to fail.

The fuel tanks also face extreme temperatures in space, ranging from 150°C to –150°C. If the tank gets too warm, the liquid hydrogen can start boiling, creating gas bubbles and changing pressure. Before restarting, the engine parts like turbopumps and injectors need to be cooled again to the right temperature. Enough fuel must be available at the correct pressure and temperature.

ISRO Chief Dr. V. Narayanan explained that even the ignitor must work in a vacuum. For safety, ISRO used three ignitors for this mission. ISRO has previously restarted the fourth stage of the PSLV rocket because it uses hypergolic fuels, which ignite on contact. But cryogenic engines do not work like that,  they require precise control and perfect conditions.

What does cryogenic restart allow?

A cryogenic restart means ISRO can now perform multi-orbit missions. This allows one rocket to drop satellites in different orbits, like a bus stopping at multiple points. It also helps ISRO safely de-orbit the rocket stage after the mission, reducing space debris and supporting space sustainability.

Dr. Narayanan said ISRO will conduct more such tests in future LVM3 missions to gather more data before using this technology for complex multi-orbit satellite deployments. This achievement shows a major step forward in India’s space capability.