Infinity in 15 kilograms

While space is hard, there are also different kinds of hardness. For example, on April 15, ISRO issued a press release saying it had successfully tested nozzles made of a carbon-carbon composite that would replace those made of Columbium alloy in the PSLV rocket’s fourth stage and thus increase the rocket’s payload capacity by 15 kg. Just 15 kg!

The successful testing of the C-C nozzle divergent marked a major milestone for ISRO. On March 19, 2024, a 60-second hot test was conducted at the High-Altitude Test (HAT) facility in ISRO Propulsion Complex (IPRC), Mahendragiri, confirming the system’s performance and hardware integrity. Subsequent tests, including a 200-second hot test on April 2, 2024, further validated the nozzle’s capabilities, with temperatures reaching 1216K, matching predictions.

Granted, the PSLV’s cost of launching a single kilogram to low-earth orbit is more than 8 lakh rupees (a very conservative estimate, I reckon) – meaning an additional 15 kg means at least an additional Rs 1.2 crore per launch. But finances alone are not a useful way to evaluate this addition: more payload mass could mean, say, one additional instrument onboard an indigenous spacecraft instead of waiting for a larger rocket to become available or postponing that instrument’s launch to a future mission.

But equally fascinating, and pride- and notice-worthy, to me is the fact that ISRO’s scientists and engineers were able to fine-tune the PSLV to this extent. This isn’t to say I’m surprised they were able to do it at all; on the contrary, it means the feat is as much about the benefits accruing to the rocket, and the Indian space programme by extension, as about R&D advances on the materials science front. It speaks to the oft-underestimated importance of the foundations on which a space programme is built.

Vikram Sarabhai Space Centre … has leveraged advanced materials like Carbon-Carbon (C-C) Composites to create a nozzle divergent that offers exceptional properties. By utilizing processes such as carbonization of green composites, Chemical Vapor Infiltration, and High-Temperature Treatment, it has produced a nozzle with low density, high specific strength, and excellent stiffness, capable of retaining mechanical properties even at elevated temperatures.

A key feature of the C-C nozzle is its special anti-oxidation coating of Silicon Carbide, which extends its operational limits in oxidizing environments. This innovation not only reduces thermally induced stresses but also enhances corrosion resistance, allowing for extended operational temperature limits in hostile environments.

The advances here draw from insights into metallurgy, crystallography, ceramic engineering, composite materials, numerical methods, etc., which in turn stand on the shoulders of people trained well enough in these areas, the educational institutions (and their teachers) that did so, and the schooling system and socio-economic support structures that brought them there. A country needs a lot to go right for achievements like squeezing an extra 15 kg into the payload capacity of an already highly fine-tuned machine to be possible. It’s a bummer that such advances are currently largely vertically restricted, except in the case of the Indian space programme, rather than diffusing freely across sectors.

Other enterprises ought to have these particular advantages ISRO enjoys. Even should one or two rockets fail, a test not work out or a spacecraft go kaput sooner than designed, the PSLV’s new carbon-carbon-composite nozzles stand for the idea that we have everything we need to keep trying, including the opportunity to do better next time. They represent the idea of how advances in one field of research can lead to advances in another, such that each field is no longer held back by the limitations of its starting conditions.