C25 cryogenic engine (L) with the environment friendly white cladding resting on the launch pad

India's largest rocket LVM-3 is at the second launch pad of the Satish Dhawan Space Centre in Sriharikota, getting ready for lift-off at 9 AM, Sunday, March 26. Sunday's launch will mark the sixth flight of the LVM-3 rocket. However, there's one thing that makes this LVM3-M3 vehicle different from all the LVM-3 rockets that flew before it - this rocket has a changed colour scheme at the cryogenic stage, the upper central portion of the rocket (that towers above the two boosters on the side). Instead of the standard grey/black, this time around the C25 cryogenic stage sports white.

To understand the reason and the science behind the colour change,  Dr S. Unnikrishnan Nair, Director, Vikram Sarabhai Space Centre (VSCC), ISRO. Dr Nair heads VSSC, which is ISRO's lead facility for developing its operational rockets and future space transportation systems. A rocket specialist with significant contributions towards launch vehicles and manned vehicle systems, Dr Nair explained that the reason behind the colour change takes into account environmental-friendly manufacturing processes, better insulation properties and the use of lightweight materials.

The C25 stage is a cryogenic stage, which comprises a cryogenic engine and fuel tanks that hold tons of frozen fuel - liquid oxygen and liquid hydrogen, along with its related systems. Engineering a cryogenic stage has unique design challenges, as liquid Hydrogen is stored at -253 degree centigrade and liquid Oxygen at -195 degree centigrade. To store these cryogenic fluids and maintain them at supercooled temperatures, special multi-layer insulation is provided for the fuel tanks and other structures.

For the first five launches of the LVM-3 rocket, ISRO had been using a grey/black insulation material for the C25 stage and now that material has been replaced with a white coloured one.

"The earlier insulation material was based on Hydrochlorofluorocarbon (HCFC) and owing to environmental concerns, we have switched to greener alternatives based on chemicals such as Hydrofluoro-Olefine," said Nair.

HCFCs are globally regarded as unsafe as they contain chlorine and have the potential to deplete the ozone layer. Owing to this, there are certain manufacturing caps on this material and ISRO is avoiding its use.

Regarding the grey/black colour being replaced with white, he added that the earlier anti-static coating (meant to prevent a build-up of static electricity) has been replaced with a white one. He reasoned that this was owing to solar absorptivity, as dark colours like black attract a lot of heat and affect the temperature of the cryogenic fuels.

Simply put, the use of white is similar to how cricketers wear all-whites for test matches when they have to play all day. White is meant to reflect the sun's heat away, particularly when the fully-built rocket stands exposed to the sun for several days at the launchpad.

Nair also explained that it is not easy to add an extra coat of paint or extra layer of material to a rocket's upper body, as every kilogram of weight comes at a premium.

Every added component or layer of coating/paint adds to the rocket's weight and affects the vehicle's payload-carrying capacity. So, we have to keep in mind the science, the utility and also the environmental aspects. Hence, it takes time to develop new materials that are lightweight, suited for the purpose and at the same time cost-effective and environment-friendly.