Current Issue Vol 22.2 April-June 2007
Articles
Strategic Missiles
Arun S Vishwakarma, B.E, M.Tech
The Agni series of strategic missiles were developed as part of Integrated Guided Missile Programme (IGDMP). The IGDMP was launched in 1983 to achieve self-sufficiency in missile capability, with simultaneous development of a wide range of missile: Nag, Prithvi, Akash, Trishul & Agni-Technology Demonstrator. It envisaged common development and reuse of key technologies, and manufacturing facilities, ranging from propulsion system, aerodynamics, avionics, flight control, sensors and warhead.
The IGDMP missiles involved close end-user involvement in all phases, from weapon specifications to development. It is important to note that in 1983 military users had specific domain expertise in weapons systems related to their services, viz Air Force, Army and Navy. At that time Strategic Forces Command (SFC) or Nuclear Command Authority (NCA) did not exist to take on the role of end-user. That role as we know now was played by a joint team of strategic thinkers, military technologist and select military officers. One can notice that strategic missiles are a different class not only in this aspect but also in the technology & system development aspects.
Strategic missiles have to blend with other key elements that make a complete strategic weapon system viz:
- Strategic policy organisation
- Strategic nuclear warhead
- Strategic missiles
- Precision navigation
- Command and control
India’s strategic weapons programme started in early 70’s with Project Devil (SRBM) and Project Valiant (ICBM). Project Valiant was an attempt to build 8,000 Km range missile. Valiant missile had three liquid fuel stages. The first stage consisting of cluster of four engines, each of 30 tonnes thrust, second stage of two 30 tonnes thrust engines and last stage with a single 30 tonnes thrust1. The missile lift off weight was about 85 tonnes. The 30 tonnes thrust liquid engine was first test fired on 10 May 1974, but soon thereafter the project was cancelled because of insufficient progress, weak programme management and organisation structure2.
Re-entry Vehicle (RV)
Unlike most other military weapons, long range ballistic missiles are designed and customised around the Re-entry Vehicle. Like other long range ballistic missiles Agni RV design is driven by the following vital specifications:
- Payload3 weight and shape
- Range
- Launch platform
Impact of Payload Mass on Rocket Weight
For a given maximum range the MTOW (Maximum Take off Weight) of missile is almost directly proportional to the payload weight. Reducing payload mass involves realising compact and small weapon. Equally important is realisation of light weight RV and other gear that ride the RV.
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Figure1: Missile weight determined by fuel mass-fraction
Smaller payload mass result in other overheads that limits the lowest missile mass. On the other hand, heavy payload mass (say for MIRV configuration) incurs the penalty of providing for a payload nose shield that must be carried up to an altitude of 90 km before it can be jettisoned.
Effect of Fabrication Technology on Weight
For a given range and payload the MTOW (Maximum Take-Off Weight) of missile is largely dependent on rocket motor’s MF (Mass Fraction) and number of motor stages. The fuel’s ISP (Specific Impulse) is also important, but all modern long range missile use solid fuel thus they have similar ISP. Indian solid fuel chemistry choice, however, has better growth potential compared to US fuel standardised for Space-Shuttle SRB (Solid Rocket Booster)4. Motor’s mass fraction is the single most important factor in determining the weight of missile, especially those designed for 8000 to 18,000 Km range. A case to the point is the recently tested French M51 submarine launched missile5. The 56 tonne M51 has 3 stage and all three motor case are made of carbon-carbon fiber composite material to maximise the fuel mass fraction so the missile can fit into current launch tube of French nuclear submarines6 carrying the M41 missile, yet it carries 50% more payload over 50% more range. The M51 missile’s range is 9,000 km for 6 MIRV payload (1,400 kg including penetration aid). Yet the same missile delivers full ICBM range (20,000 km) for a configuration with fewer (2) MIRVs providing flexibility to France.
Impact of Payload Mass on Rocket Range
As can be expected the missile range increases for lighter payload. For lighter payload the range in particular is affected by the fuel Mass-Faction (MF) of the last stage. The range for a set MTOW and fabrication technology is dramatically improved if the missile is partitioned into 3 stages. Increased number of stages does impact cost and reliability but its long range performance is much superior for reduced throw-weight configuration..jpg)
Figure 2: Range vs payload: MF of last motor limits range for light payload
Agni Re-entry vehicle evolved over 3 phases:
- Concept evaluation
- RV for Agni-TD, Agni-II, Agni-I
- RV for Agni-III