Stellar debut. Agnibaan, the rocket that defied the odds

First-time-right is not common with anything about space, certainly not for a newly-designed rocket, with the engine minted on a 3D printer. But if the Chennai-based, IIT Madras incubated start-up, Agnikul Cosmos, got the maiden flight of its Agnibaan rocket right, it was because of the mountain of preparation that preceded the launch.

“We did 20,000 computer simulations to study what could go wrong with the trajectory,” says a delighted Dr Satyanarayanan Chakravarthy, professor of aerospace and combustion engineering at IIT Madras, who mentored Agnikul to the successful launch on the morning of June 1. Each simulation checked the behaviour of the rocket against a certain variable, such as a gust of wind from a certain direction.

Alongside, they did about 40 static tests to see how the machine works under hot-fire conditions — especially the gimbal mechanism that ensures the stability of the rocket. In the end, the flight that took the rocket to a height of 6 km and let it fall into the sea 2 km from the launch pad was a success. “Very few rockets have had success in its maiden flight,” Chakravarthy told quantum.

True, Agnikul launch was aborted thrice — twice shortly before the planned lift-off, but those aborted launches only demonstrated that the Automated Launch System was robust and could catch any bugs.

The rocking rocket

A few points about the rocket must be kept in mind.

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First, it was India’s first rocket that was powered by liquid fuel in the core. All the ISRO rockets have solid fuel in the core, though the strap-on rockets (those little ones that cling to the sides of the main rocket at the bottom) were liquid-fuel fired. Incidentally, the Agnibaan is also designed to be fitted with strap-ons, when heavy payloads ask for it. Second, it was for the first time in India that a semi-cryogenic engine was used — ATF (fuel) at ambient temperature and liquid oxygen (oxidiser) in cryogenic condition. This meant that the fuel loading had to begin only 3 hours before the lift-off.

Third, what flew on June 1 was not the full 2-stage rocket, but the top half of it with a single engine. The full rocket, with a cluster of 4-7 engines in the lower stage, would be test-launched later. Fourth, it was also the first private rocket that was controlled during the entire flight — its velocity, attitude and position fully telemetered, and could be destroyed by a person on the ground if it went awry. The other rocket start-up, Skyroot Aerospace, test-flew its rocket in November 2022, but it was a sounding rocket that just went up and came down. (This is not to say that Skyroot’s vehicle is inferior to Agnikul’s or Skyroot lags in technology—different companies adopt different pathways for developing a vehicle.)

Extreme caution marked the making of the rocket. First, it was powered for a thrust of 1.1, which meant that the thrust was just a little more than its weight — as such, the rocket didn’t shoot-off into the skies but ascended slowly. More thrust would have meant burning more fuel at the launch pad. Also, for design simplicity, it was an un-throttled engine — no throttle to adjust the thrust by controlling the flow of the fuel and the oxidiser into the combustion chamber.

Second, the launch sequence was so designed that ignition would happen 7 seconds before lift-off and while it would take only two seconds for enough pressure to build up inside the combustion chamber, the vehicle waited for a full five seconds before leaving the ground.

The next steps for Agnikul is to build the full, two-stage vehicle and demonstrate stage-separation, on the ground. Chakravarthy believes the company would be able to come to this point in nine months.