India’s space program has long stood as a symbol of self-reliance, innovation, and cost-efficiency. But when it comes to rocket materials—especially those used in the most heat-intensive parts—dependence on foreign imports has often remained a necessity. A shining example is the nozzle divergent of the PSLV’s fourth stage, which has, until recently, relied on imported Columbium (C103), a rare and expensive material.

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That is now changing. In a major move toward Atmanirbhar Bharat, ISRO scientists have successfully tested a homegrown alternative—Stellite (KC20WN)—to replace Columbium. This cobalt-based alloy, enriched with elements like Chromium, Nickel, Tungsten, and Iron, has passed all its tests, including a final long-duration hot test on April 8, 2025, at ISRO’s Propulsion Complex in Mahendragiri. The result? A 665-second success and a potential 90% cost reduction.

What is the Nozzle Divergent—and Why Does It Matter?

In simple terms, the nozzle divergent is the flared end of a rocket engine where exhaust gases are expelled. It plays a key role in controlling the direction and speed of the rocket’s thrust. But it also bears the brunt of extreme heat—making the choice of material absolutely critical.

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Materials used in this part must not only survive but perform reliably at scorching temperatures that can go beyond 1100°C. Until now, the go-to material for ISRO’s PSLV fourth stage was Columbium (C103), a heat-resistant metal that is not available in India and must be imported—making it both costly and geopolitically risky.

Why Stellite? And What Makes It Special?

Stellite (KC20WN) isn’t just another substitute—it’s a strategic upgrade. This cobalt-based alloy is known for retaining strength even at very high temperatures, making it ideal for demanding aerospace use. It also offers good corrosion resistance, wear resistance, and long-term durability—key features when you’re dealing with rocket engines that have no room for error.

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Most importantly, it’s available and manufacturable within India. That means greater control over quality, reduced costs, and better supply security—all without compromising on performance.

ISRO’s Path to Qualification

Replacing such a critical component is never a plug-and-play task. It demands months of design tweaks, simulations, and real-world tests. ISRO conducted a series of hot tests on this new nozzle hardware. Two units were tested in three trials during the first phase. Then, on April 8, the final long-duration hot test was completed—lasting over 11 minutes.

This final test was the deal-sealer. The new nozzle withstood the intense thermal and mechanical stress, proving it could perform just as well—or better—than its imported counterpart.

A Big Step for India’s Aerospace Independence

What makes this achievement more than just a technical milestone is its deeper meaning. India is not only reducing its dependence on expensive foreign materials, but also building a stronger domestic supply chain. This not only boosts national confidence, but also ensures that future space missions are more sustainable and less vulnerable to global supply disruptions.

From launch vehicles to lunar missions, materials matter. And when these materials are sourced, tested, and perfected in India, the ripple effect touches everything—from reduced launch costs to greater export potential.

Conclusion: When Heat Meets Ingenuity

The successful switch from imported Columbium to indigenous Stellite alloy in PSLV’s fourth stage shows what Indian science and engineering can achieve with vision and perseverance. ISRO’s move isn’t just a technical change—it’s a quiet revolution in materials science, one that reflects India’s growing capability to fuel its space dreams from within.

And with 90% cost savings, it’s not just smart—it’s transformative.