Why India's submarines have to come up for air, and the home grown machine meant to keep them hidden

A hunter that has to breathe

A submarine's whole value is that nobody knows where it is. Once it is found, it becomes the hunted.

India's Kalvari-class submarines are diesel-electric. Under water they run on batteries. When the batteries run low, they need their diesel engines, and diesel engines need air.¹

So the submarine has to come close to the surface to recharge. Think of a diver who keeps coming up to breathe. Every trip up is a chance to be seen.

What if it could breathe less?

That is the idea behind Air Independent Propulsion, or AIP: an extra source of power that does not need outside air. The Ministry of Defence says it boosts underwater endurance by reducing dependence on atmospheric oxygen.¹

India's version is built around fuel cells, which make electricity from hydrogen. The DRDO Chairman stressed keeping the exacting safety culture that hydrogen and fuel cell systems require. Hydrogen gives a lot of energy, and it is unforgiving if handled badly.¹²

The heart of the system, the energy module, was developed by DRDO's Naval Materials Research Laboratory.¹

Why is this so hard to get to sea?

A machine that works in a laboratory is not yet a machine that works in a submarine. This one has to pass through four sets of hands.¹

A DRDO laboratory designed it. L&T's AM Naik Heavy Engineering Complex at Hazira in Gujarat is bringing it to life. The Navy tests and validates it. Mazagon Dock Shipbuilders will fit it into a submarine.¹

Every handover is a place where time can be lost. And the question that matters most, how many more days a submarine can stay hidden, has no official answer yet. The ministry has not published that figure, and we have not used unofficial estimates.¹

24 October: switching it on

On 24 October 2026 the Setting to Work Phase 2 activity began at Hazira. Setting to work is the stage where a finished machine is switched on and brought up step by step, the way a new power plant is tested before it feeds the grid.¹

The Defence Secretary, who also chairs DRDO, attended virtually and called it productive reality on the shop floor. The Navy's Chief of Materiel was there in person.¹²

Next, the ministry says, the module will be energised to produce power and tested hard against the Navy's protocols. Then it goes to Mazagon Dock for fitting on a Kalvari-class submarine in early 2027.¹

What could still go wrong?

Setting to work is not proof at sea. The power tests and the Navy's acceptance trials are still ahead, and the early 2027 date depends on both.¹

The ministry has not named which submarine gets the system first. Until one dives with it, the diver still has to come up for air.¹

It was a busy month under the sea. The Navy also commissioned a shallow water submarine hunter and signed for more heavyweight torpedoes, stories we tell separately.⁴³