ISRO’s heaviest comms sat launch: All about LVM3, India’s biggest ever launch vehicle

So far, ISRO has contracted out the launch of the heavier satellites to private space agencies from other countries. The current launch is a milestone towards the growing capability of the launch vehicle, a modified version of which will also be used under the Gaganyaan mission to put humans in space.

Capability of LVM3

LVM3 — earlier referred to as Geosynchronous Launch Vehicle Mark 3 or GSLV Mk 3 — uses solid, liquid, as well as cryogenic fuel-based engines to put up to 8,000 kg in low Earth orbit and up to 4,000 kg in geosynchronous orbit.

The space agency’s initial plan was to use its workhorse PSLV for polar and low Earth orbit launches, usually much closer to the Earth’s surface, and use the GSLV II and GSLV Mk 3 to launch the satellites to the more distant geosynchronous orbits of around 36,000 km from the Earth’s surface.

The vehicle was later adapted in 2022 (amidst the Russia-Ukraine war) to launch 72 OneWeb satellites in two launches to low Earth orbit, prompting the change in name of the rocket. This came at a time when OneWeb was hard-pressed to find a launch provider — Russia had stopped flying the internet constellation citing data being provided to Ukraine, and one of the major European launchers, Ariane-5, was set to retire with its replacement, Ariane-6, not in commission. During these two OneWeb missions, the rocket carried a heavier payload of more than 5,700 kg, but to a low Earth orbit of around 450 km from the Earth’s surface.

As for India’s heavy satellites — and the country does have a few communication satellites weighing more than 4,000 kg — they were put in orbit by other private launchers. The 5,854 kg GSAT 11 and 4,181 kg GSAT-24 were launched by Arianespace. The space agency also relied on Elon Musk’s SpaceX last year to put the 4,700 kg GSAT-20 satellite into orbit.

For the current launch, the higher weight of the satellite — over its capacity of 4,000 kg to GTO — has been achieved by actually lowering the orbit slightly, with the highest point being somewhere around 29,970 km. The space agency, however, is working on various ways to increase the capacity of this launch vehicle.

Changes to be made

The space agency is working on a few ways to increase the carrying capacity of the launch vehicle, especially considering that it will be used for the country’s human spaceflight mission.

One of the ways is to increase the thrust produced by the third or the cryogenic upper stage of the rocket. This stage accounts for nearly 50% of the velocity needed to place the satellites in the geosynchronous transfer orbits. The C25 stage that is presently being used in the launch vehicle can carry only 28,000 kg of the propellant producing a thrust of 20 tonnes. The new C32 stage will be capable of carrying 32,000 kg of fuel and produce a 22-tonne thrust.

The space agency is also looking at using a semi-cryogenic engine instead of the liquid-propellant based second stage of the rocket. A cryogenic engine essentially uses liquefied gases at extremely low temperatures as the fuel — in this case liquid oxygen and hydrogen. A semi-cryogenic engine uses a liquefied gas and a liquid propellant. ISRO plans to use a refined kerosene and liquid oxygen based second stage. This will not only increase the capability of the launch vehicle, it might also be cheaper.

With the new engine, the vehicle is likely to become capable of carrying around 10,000 kg to the low Earth orbit instead of the current 8,000 kg. The heaviest payload it has carried to low Earth orbit is 5,800 kg for the OneWeb mission. The increased capability of the kerosene will be enough to carry the first and the lightest module of the Bharatiya Antariksh Station — India’s planned space station. A launch vehicle with a higher capability would be needed for the launch of the subsequent modules. The space agency is working on developing a new vehicle called Lunar Module Launch Vehicle (LMLV) that will be capable of carrying up to 80,000 kg in the low Earth orbit — mainly being designed to carry out missions sending humans to the moon.

Launches so far

ISRO’s heaviest launcher is also one of its most successful, with all seven of its flights putting satellites in the intended orbits. It is the launch vehicle that took both Chandrayaan-2 and Chandrayaan-3 to space, in addition to communication satellites GSAT-19 and GSAT-29.

To compare, four of the 18 launches undertaken by its slightly less powerful cousin, GSLV, have failed. When it comes to the space agency’s workhorse PSLV, three of the 63 missions undertaken have failed, with the most recent being in May this year, when the EOS-9 satellite could not be put in orbit after the third stage did not work as intended.

The launch vehicle, in its first flight in 2014, took a crew module for the country’s first-ever re-entry test to orbit. This is very important, especially for the human mission, where the space agency would need to ensure that the astronauts safely return to Earth, even through the massive heat generated due to friction in the atmosphere.