INTRODUCTION: AGNI-TD/TTB

India's long-range missile program first started in around 1972 as Project Valiant, a three stage (liquid engine) ICBM missile. The project envisaged the use of three 30 tonne thrust liquid propulsion engines[5], developed in 1974, for the first stage booster and the second stage using one engine. The project was subsequently brought to a close and a more integrated missile development program was launched under the aegis of the IGMDP.
The Agni-TD/TTB (Technology Demonstrator / Technology Test Bed) project objectives were to test and validate:
1. Re-entry test vehicle to evaluate structure, guidance and control during re-entry into earth's atmosphere at hypersonic velocity. The RV used multi-directionally reinforced carbon-fiber preform (MRCP) technology.
2. Inertial Navigation System.
3. Rocket Staging.
Compared to the Prithvi, the Agni is a much larger system with a range of 2,500 km and a payload of 1,000 kg[6].The original Agni-TD/TTB was an amalgam of the Prithvi and the SLV-3 booster. The Agni-TD/TTB was a cheap test vehicle to prove re-entry and guidance technology for use on a more advanced platform[7]. 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 motor.

The first Agni launch on 22 May 1989 used a shortened Prithvi 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[8]. The RV used multi-directionally reinforced carbon-fiber preform (MRCP) technology[9]. The last test of the basic 'Agni-TD/TTB' on 19 February 1994 was a major technical breakthrough for India. The system tested, included a manoeuvrable re-entry vehicle for increased accuracy with terminal guidance[10]. This terminal guidance system reportedly consists of a scanning correlation optical system based on a scanning focal plane homing head in the infrared and millimeter wavelengths of the electromagnetic spectrum[11]. Considerable un-informed comments exist regarding the fact that Agni was only tested to a range of 1,450 km. No missile actually needs to be tested to a full range. It is possible to lift or depress the trajectory of the missile to simulate a longer range.
Dr. APJ Abdul Kalam stated that the missile could be fully deployed within two years[12]. Dr. Kalam also asserted that the Agni was ready for serial production while some simultaneous development flights aimed at achieving a much greater performance are undertaken[13]. Dr. Kalam claimed that no further test flights are necessary for the basic Agni system and that it is ready for production. In April 1995, Prime Minister PV Narasimha Rao (correctly) denied allegation[14] that under US and G7 pressure, India temporarily paused the Agni program after completion of the TD phase and after three test flights. In fact during the time, Prime Minister Rao secretly sanctioned the development of an augmented version of the Agni[15]as well as speed up efforts to build more advanced nuclear weapons[16] and set up a nuclear command and control system for the safe custody, deployment, and employment of such weapons, distributed over the country to ensure survivability and safety but totally under civilian control[17]. The Agni-TD program ran its course with the development and proving of crucial technologies for full-fledged, multi-staged, long-range ballistic missiles, including re-entry and navigation avionics. This missile reached engineering status and it is believed that none were released to the military, although during the Kargil imbroglio few units were made ready as nuclear deterrence[18]. This model is not believed to exist anymore, having being superseded by the Agni-II that has been put on line production and operationalized[19].
Description
Generally liquid fuelled missiles are more accurate because the navigation & control system can precisely control the impulse from the engine by controlling or limiting its thrust. Solid fuelled rockets can't turn off thrust on demand and further due to subtle manufacturing variances and actual operating conditions the exact impulse from a solid motor isn't known beforehand. All this makes control & aiming more challenging and the missile accuracy suffers unless mitigated by other means. The solid fuelled Agni's trajectory has a shallow re-entry angle and manoeuvring RV (MRV) use body-lift aerodynamics to correct trajectory error, as well as reduce the thermal stress of re-entry. The Agni's RV has a velocity correction package to correct launch trajectory variances. Some Agni RV versions use a set of solid fuelled thruster cartridges[20] of predetermined impulse, allowing the onboard guidance controller to trim velocity, using discrete combination of impulse quantum along desired spatial orientation.
Re-Entry Vehicle: Agni RV-Mk.1
The Agni's re-entry vehicle is designed to ensure that the temperature inside the vehicle does not exceed 60° Celsius, a condition necessary to protect the warhead and electronic systems placed inside. The high beta RV uses blunt nose tip made of carbon-carbon-composite, to generate a separating shockwave that takes away most of the heated plasma and a carbon-phenolic RV cover protects the body from high ambient temperature and pressure. During tests, the re-entry vehicle technology was fully demonstrated when the nose-cone withstood temperatures of 3,000° Celsius while the inside temperature was only 30° Celsius. The Agni RV-MRV Mk.1 nose tip is made of a multi-directionally woven, reinforced carbon-carbon fiber composite material[21]. The 0.8 meter diameter and ~4 meter long, re-entry vehicle consists of five sections. Each of these sections is made up of a two-layer composite construction. The inner layer is made up of carbon/epoxy filament mould constructed on a CNC winding machine and is designed to bear structural loads. The outer layer is made up of carbon/phenolic filament wound construction and cured in an autoclave at 7 bar pressure[22]. The outer ablative layer ensures high thermal robustness for shock and temperature extremes.
The RV appears to house an integrated High Altitude Motor (HAM), instead of a separate Post Boost Vehicle and classic purely passive ballistic warhead seen in western missiles, that is liquid fuelled and is used to correct impulse variance of solid fuelled stages and subtle launch trajectory variance. The 1980-vintage RV was reportedly designed to be able to carry a BARC-developed, boosted nuclear weapon of 200 KT yield weighing 1000 kg, also of 1980 vintage design. After making room for new and lighter Indian thermonuclear weapon payload, of 1995 vintage design, the MRV has room for about 200 kg (estimated) liquid fuel in pressurized vessels. Although for velocity correction, approximately 50 to 80 kg is estimated to be sufficient. There are indications that the MRV is intended to enter a gliding trajectory[23] when it enters atmosphere at an altitude of 100 km[24]. This has following implications:
1. The missile's range is extended.2. Less acute re-entry thermal stress.3. The manoeuvring makes it difficult for ABM defenses to intercept the missile.
Interestingly in 1987 IGMDP/ASL first envisaged developing a re-entry vehicle "designed for 100-250 Kg payload at speed of 7-8 km/sec"[25], clearly corresponding to a just a light weight fission-weapon & ICBM range[26]. But strategic requirement clearly also required high yield weapons that imposes bigger space and weight envelop. After some serious thinking, reviews and debates, the RV was to be designed for bigger payloads to match BARC's high yield weapon. The RV envelop was driven by weapon size and weight parameters corresponding to 200 Kt yield. In the 1980's BARC came up with boosted fission weapon design for the purpose with 1000Kg mass resulting in the Agni RV-MK.1. As evident from Agni-III RV design this first RV-MK.1 design was designed for the ultimate long range.
It is useful to note that Agni's advanced blunt nose high beta RV design using carbon-carbon nose tip and separating shockwave and is only now being incorporated in US ICBM replacing their mainstay RVs that hitherto used high beta RV using heat-sink concept employing sharp graphite nose tip and ablative cover sheath, with blunt nosed MK4A RV. Strangely it also looks very similar to Indian Agni-RV Mk-1 [26A].

Propulsion
First Stage: The booster motor is one meter in diameter and ten meters in length. It has approximately 9 tons solid propellant and a mass fraction of 0.865 (estimated). The stage features three segments of propellant grain, with an internal star configuration[27] 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 and later Agni variants use HTPB [hydroxyl-terminated polybutadiene]. The propellant is of star configuration[28] 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[29].
Second Stage: Agni-TD used a reduced Prithvi stage as its second stage. The initial test flight used a shortened version of Prithvi with lesser fuel, later flights used full fuel configuration. The liquid fuelled second stage of Agni-TD required in addition, two types of 'Ullage and Retro Motors' to maintain positive G-forces during stage separation and liquid engine startup. Both the ullage and retro motors are made of HE-15 aluminum alloy and use a double-base propellant. The motors are lined inside with high silica glass-phenolic ablative liners. Defense Research & Development Laboratory (DRDL) has expertise in the design, production, inspection, qualification, static-testing, and flight-testing of propulsion systems[30].