Rereading the interview posted by sahay, I noticed key design features that Dr Avinash Chander stated.
- Composite casings allowed higher pressure and thus greater thrust. Thrust is proportional to Area*Pressure
The lack of inter-stage and how design changes allowed weight savings.
In other words the first stage section has a composite shell inter-stage as part of the design.
And use of retro rockets improved the separation.
Separation is the most troublesome event in rocket flight.
This cannot be emphasized more.
A5 is a very efficient and highly reliable vehicle.
Having made the composite stages, we found that they were coming out better than the metallic stages, strength-wise and property-wise. So we could operate at a higher pressure. So you do not have losses due to gravity, and the losses are reduced. We then went through a total philosophy change. Up to Agni-III, we ignite the upper stage first, then separate the lower stage so that there is no problem of separation.
We decided to leave behind that culture of space vehicles. We now put big retro motors, which create a thrust of four tonnes each – totally 16 tonnes of thrust – just to separate the stages so that no dead weight is passed on to the upper stage.
Correspondingly, we decided to make the mission stronger so that there are no interfaces and the separation is clean. We studied and created extensive models to simulate them on the ground in all types of disturbed conditions in wind tunnels. With all that, we could remove the inter-stages altogether. The weight we had reduced by making the upper stages of composites was fed back into the third upper stage. The weight did not increase overall, but the total energy increased considerably. To reach the 3,000-km range, you need a velocity of five kilometres per second. To reach the 5,000-km range, the velocity has to be more than six kilometres a second.
That was our approach to the repackaging of our vehicle. We made major modifications in the upper stage. V.G. Sekaran, Director, Advanced Systems Laboratory [ASL], DRDO, played a primary role in showing us how to repackage the payload structures so that the weight comes down by 1,000 kg.