DRDO’s Project SFDR proves that solid propellants are here to stay
The Defence Research and Development Organisation (DRDO) is developing a technology in what could be the basis for future air to air, surface to air and air to ground missiles. Project SFDR, which stands for Solid Fuel Ducted Rocket or a solid booster cum Ramjet propulsion, is being worked upon by Defence Research and Development Laboratory (DRDL) lab for some years now and it had even called tenders for manufacturing the launcher way back in 2015. The SFDR will be attached with a solid booster to initially propel it to speeds at which it can start operating.
Most current missiles use a booster/sustainer configuration with solid or liquid propellants. The booster stage propels them to top speeds after which the sustainer sustains the missile velocity for some time before complete burnout after which they glide towards the target. While this is a simple and tested means, it has limitations of maximum engagement ranges especially against manoeuvring targets which can bleed the energy of conventional missiles.
To improve endgame manoeuvring missiles started incorporating multi-pulse rocket motors like the ones in the Astra MK1 and Rafael Stunner, the second pulse doesn’t fire simultaneously but rather at the terminal stages when the seeker has locked onto the target, greatly enhancing endgame manoeuvrability. The MBDA meteor missile broke the impasse by incorporating a Throttleable Ducted Rocket (TDR) version of the ramjet made by the famous propellant manufacturer Bayern-Chemie of Germany. The TDR acts like an extended sustainer with variable thrust in a solid propellant but can sustain thrust for far longer periods as it gets the oxidiser from the air, instead storing it as in solid fuelled missiles.
While the DRDO has some experience with liquid/solid fuelled ramjet through the BRAHMOS and the Akash project, making a light, compact throttleable solid fuel version is a considerable challenge. One of the expected applications would be the future versions based on the Astra missile which explains why the SFDR experiment is reported to be based on the Astra airframe. If achieved, the SFDR equipped Astra will be huge improvement over the current version in terms of engagement kinematics and provide long range interception capabilities.
However, SFDR comes with its own limitations. Raytheon, the manufacturer of the AIM-120 AMRAAM, had carried out studies to incorporate various propulsion systems to improve the AMRAAM but arrived at the conclusion that the ramjet motors are heavier and take time to reach top speed in the initial phases than conventional motor, and are thus less agile. This can affect the kill probability at short to medium ranges which is where a majority of the engagements are going to take place.
The DRDO, having expressed before that the Astra Mk1 will not be replaced by newer versions of it but will be complemented by them, also reinforces the fact that solid propellants are here to stay. The Rafales, to be inducted by the Indian Air Force (IAF), will use the shorter range MBDA Mica to complement the long range Meteor and the DRDO may as well be looking to do the same with Astra Mk1, MK2 and the SFDR project.