India's solid-fuel ballistic missile "Agni"
 
Norbert Brügge, Germany
 

Agni-TD

The original Agni-TD was an amalgam of the Prithvi-I and the SLV-3 booster. The Agni-TD is a two-stage missile with the first stage using the first-stage solid-fuel booster motor of the SLV-3 satellite launch vehicle. This marked the first time that India had used directly a component of its civilian space research program for military purposes. The Agni-TD was a cheap test vehicle to prove re-entry and guidance technology for use on a more advanced platform. The missile used a solid booster that was improved but similar to S-1 stage. Instead of developing a new solid motor for the second stage, which would have involved significant delays, it used a shortened version of the liquid fuelled Prithvi-I motor.

The first Agni launch on 22 May 1989 used a shortened Prithvi-I stage as the second stage. The second Agni test used a second stage with more fuel and longer burn that was ignited before separation thus obviating the need of six-ullage motors used in the earlier launch. The RV used multi-directionally reinforced carbon-fiber preform (MRCP) technology. The last test of the basic Agni-TD on 19 February 1994 was a major technical breakthrough.

First Stage: The booster motor is one meter in diameter and ten meters in length. It has approximately 9 tons solid propellant. The stage features three segments of propellant grain, with an internal star configuration for increased thrust during the initial boost phase. The motor case is made of a high-strength 15CDV6 steel and is fabricated by conventional rolling and welding techniques. The propellant used in Agni-TD consists of the AP-Al-PBAN composite propellant. All later Agni variants use HTPB (hydroxyl-terminated polybutadiene). The propellant is of star configuration with a loading density of 78%. It is case bonded with a liner system between propellant and insulation. The motor's nozzle is built from 15CDV6 steel; a carbon-phenolic thermal protection system is used for the convergent throat, high-density graphite is used for the throat, and carbon and silica-phenolic lining is used in the fore end and aft end of the divergent.
Second Stage: Agni-TD used a Prithvi-I as its second stage, which has two small engines. The initial test flight used a Prithvi-I with lesser fuel of  TG-02/AK-20 (Xylidiene + Triethylamine/IRFNA+ NTO), later flights used full fuel configuration. The case material of the second stage is aluminium alloy.

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Agni I

First stage nozzle

               HSTDV launcher

Shot-test of vehicle in complete configuration

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Agni II

The Agni-II was first tested on 11 April 1999 from a converted rail carriage, with a carriage roof that slides open to allow the missile to be raised to the vertical for launch by two large hydraulic pistons. The missile was launched from Wheeler Island, Balasore. The Agni-II missile can also be launched from a road TEL vehicle, as demonstrated in the second test flight on 17 January 2001. This missile has a theoretical maximum range of some 3,000 km with a 1000 kg payload.

First Stage: The Agni-II's first stage is largely similar to that of Agni-TD's first stage. However the Agni-II booster is believed to employ more energetic fuel similar to that used on the PSLV's (Polar Satellite Launch Vehicle) booster stage - ISP of 269 (vacuum) and 237 (sea-level). It has a propellant mass of ~9 tons.
Second Stage: The Agni-II's second stage weighs ~4200 kg and uses solid fuel propellant. Its case is presumably made of the same material, high-strength 15CDV6 steel, as the booster stage for ease of manufacturing. This solid propellant stage has flex nozzles for thrust vectoring, for precise trajectory control. Unlike the Agni-TD, the solid fuel second stage does not require retro motors for proper stage separation. Evidently it uses a vented inter-stage.

The second stage of Agni-II
with 3.57 tons of propellant

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Agni IV (syn. Agni IIAT; Agni II+; Agni IIP)

The Agni-IIP (AT) is the result of a CIP (Continuous Improvement Program) with Agni-II. Different reports indicate India developing a more advanced technology (AT) version of Agni-II putting into use, state-of-the-art technologies to significantly improve the Agni-II performance as well as to adapt it to the newer and lighter nuclear payload. The new propulsion system could greatly increase the missile's range to 4000 km (1500 kg payload) and potentially to ICBM range for lighter payload.
The Agni-IIP is likely to incorporate the following changes:

• A larger diameter booster stage made of stronger 250-Marging steel, improving fuel loading and fuel mass-fraction.

• Lightweight carbon composite motor casing for the second stage, instead of steel casing, improving its mass-fraction.

• Lighter  RV, with all carbon composite re-entry heat-shield, multi directional carbon re-entry nose tip and control surfaces.

First Stage: A new 1.3 meter diameter booster case made of 250-Maraging steel with improved chamber pressure and fuel mass-fraction. Approximately 11 meter long and 20 tonne mass
Second Stage: Lightweight filament wound carbon composite motor casing for Stage-II with improved fuel mass-fraction.

Agni III (syn. Agni IIITD)

 
The successor to the Agni-II and the ultimate development of the Agni family is the Agni-III.  On 17 May 2006 DRDO publicly announced the existence of the missile and issued a short video clip. The Agni-3 D1 was first test fired on 9 July 2006, but it failed.

In the Agni tribe, Agni-III is a family of large diameter, solid fuel missiles. They are compact and small enough for easy mobility and can be easily packaged for deployment on variety of surface/sub-surface platforms. The 48 tonnes Agni-III uses solid fuel stages with an overall diameter of 2.0 meters. The Agni-III is a 2 stage 16 m long missile with a unary RV payload. The first stage is 7.7 meters long and the second stage 3.1 meters long with a 1.1 meter vented inter-stage.

The Agni-III has two solid fuelled stages of 2.0 meters diameter. This diameter is compatible with a recently tested Indian sub-surface launch system, that has a 2.4 meter diameter launch tube. Booster stage uses lightweight composite material case to achieve high fuel mass fraction that is necessary for a small but long range missile. the second stage case is made of maraging steel that will eventually use light weight composites. Thus initial Agni-3 version would see payload capability degraded by about 450 kg.
First Stage: The first stage is approximately 31 tonnes mass and length of 7.7 meters. This stage's specific impulse (ISP) of 237/269 is believed to be better than Agni-II booster and closer to large solid motor currently in use on Indian space launchers. The stage has 28 tonnes (approx.) of high -energy solid fuel and 75 seconds burn time. Composite material case reduces the dead weight increasing its propulsion efficiency. It employs flex nozzle for yaw, pitch and roll thus dispenses with air fins, enabling container stowage & launch. This is the first Indian use of flex nozzle solid fuel stage for booster stage allowing greater flight control during the ascent through dense atmosphere including the maximum-Q point approximately 30 seconds into the flight.
Second Stage: The 1.1 meter long vented interstage is light-weight and ensures better vehicle control and reliable second stage separation. The second stage mass is about 12 tonnes and length of 3.9 meters (including 0.8 m payload adapter). The stage currently employs maraging steel case that could be later upgraded to composites to maximize fuel mass fraction that is critical to realize long range, especially with lighter payload. The second stage also employs flex nozzles to provide necessary in-flight trajectory control. The high energy solid fuel of the stage burns slowly for about 105 seconds using high expansion ratio nozzle making it a very efficient engine.

    
Agni III TD

Agni III TD

Agni V TD

Agni V

The two stage Agni-III would eventually evolve to a full range ICBM by addition of a third stage. This 15 meter tall Agni-V would weight about 52 tonnes. India may soon test Agni-V. The Agni-V will employ three solid fuel stages. The first stage same or similar to that of Agni-III. The second stage is a composite fiber case construction that is otherwise similar to second stage of Agni-III.
The third stage is a new stage that is approximately 1 meter long, mass of about 3.3 tonnes and flex nozzle for trajectory control. Composite case construction to get high fuel mass Ratio.

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Agni-P (AIP)

The Agni-P, presented at the end of June 2021,  is an improved Agni-V

Agni Defence Vehicle (ADV)