More from this 1998 India Today piece:

The real problem was not that India did not know how to make bombs. It was how to build them to a size that fitted into an aircraft bomb pod, ruggedise the device to take the stress of carting and incorporate safety mechanisms to ensure that they do not go off accidentally. The 1974 device was said to be so heavy that only a transport aircraft could deliver it-which would make it a sitting duck. So when Rajiv Gandhi gave the go-ahead, the DRDO, which initially provided only the implosion mechanism, began to get involved in a big way for the first time. K. Santhanam, who had been recently transferred from the Atomic Energy Department, among other things, began to coordinate the effort as technical adviser to the DRDO.

Finally, India’s two giant scientific establishments, which had operated in relative isolation, began to synergise their strengths. And the results began paying off. By 1989, India had refined its ability to drop nuclear bombs using combat aircraft. By the time the 1995 tests started, the DRDO and the atomic energy team had made major design changes in the bomb, reduced its weight considerably and increased its yield. Elaborate safety packages for delivery had been worked out. And missiles were also developed as delivery vehicles. To describe what he calls this “great partnership”, Kalam devised a simple equation: nuclear technology + defence technology = nuclear weapon technology and systems.