Current Issue Vol 22.2 April-June 2007
Articles
Strategic Missiles: Part 2
Concept Evaluation of RV for Strategic Ballistic Missile
Arun S Vishwakarma, B.E, M.Tech
In mid-1980’s world’s contemporary strategic weapons development centered around weapon design of between 150-400 Kt yield, instead of Megaton yield that was the norm in 1960’s and 70’s. The Megaton yield of earlier nuclear age was necessary to account for and compensate for poor missile accuracy. Over period of time missile accuracy improved and the same effectiveness could be achieved with lighter and lower yield (150-300Kt) weapons. Lighter weapons also allowed many weapons to be launched on a missile (MIRV). MIRV evolved from multiple satellite launch programme of civilian space agency7. MIRV greatly improved effective use of nuclear weapons in both counter-force as well as counter-value strike role. For identical net yield, the damage from many smaller yield weapons that are well spread out is many times more compared to a megaton yield weapon8. The era of 1970’s and early 80’s was that of MAD (Mutually Assured Destruction), but technological improvement in 80’s enabled nuclear war-fighting doctrines, i.e. counter-force nuclear weapon application, culminating in development of smaller yield battlefield nuclear weapons including enhanced radiation weapon.
India tested a pure fission nuclear explosive on 18 May, 1974. In 1975 BARC started developing a miniaturised design using fusion boosted fission, and military grade triggers9. By early 1980s BARC had confidence to make robust 200 Kt yield weapon using boosted fission design.
In 1987 IGDMP first envisaged developing a re-entry vehicle that was “designed for 100-250 Kg payload at speed of 7-8 km/sec”10 clearly corresponding to a light weight medium yield fission-weapon & ICBM range11. But strategic requirement also required high yield weapons (about 200 Kt yield) that impose bigger space and weight requirements. After more reviews and debates, the RV was designed for bigger payloads to match BARC’s high yield weapon. BARC’s contemporary 200 Kt boosted fission weapon designed for the purpose weighed about 1000 Kg, that defined REX12 (RV-MK.1) on Agni-TD and later used on Agni-II and Agni-I. IGDMP Director Dr APJ Kalam later said that they evaluated 180 different Agni configurations before settling on the final choice._small.jpg)
Figure 3: Notional old and new Indian strategic weapon shape & size
The 1000 kg payload mass was also compatible with conventional weapon payload making the missile useful in non-strategic role.
Effect of Range and Payload Mass on RV
The range of a missile determines the minimum speed that the rocket imparts on the RV. Longer range requires greater velocity. A launch velocity of 7.7 km/sec (relative to earth) is enough to insert the payload into low earth orbit, yet a full range ICBM that can reach furthest corner of earth (range 20,000 km) requires velocity of 7.5 km/sec..jpg)
Figure 4: Missile Velocity vs Maximum Range
The kinetic energy builds up as square of the velocity, thus when payload re-enters the atmosphere at hypersonic velocity the RV encounters shockwave and supersonic drag that varies along the altitude. It encounters extreme temperature régime (about 3,000ºC) followed by extreme deceleration régime; corresponding to energy dissipation in excess of 100 Megawatt that can easily destroy anything but the toughest re-entry vehicle. Understandably ICBM RV undergoes the worst environment.
.jpg)
Figure 5: Dynamic pressure on RV at Mach-8. Source: NAL CFTFDD13 Bangalore
During re-entry the missile encounters extremely high temperature as the atmosphere tries to retard the speed of RV traveling at hypersonic speed. The high temperature can vaporise all known material and the only way to reduce the temperature is to spread the heat flux across a larger RV area and using an ablative material to form a blunt leading edge (typically a semi-hemispherical shaped body made of carbon-carbon reinforced composites).
As the missile descends further the atmosphere density increases and the drag rapidly rises, also a very strong shockwave builds at the leading edge increasing the local tip temperature well beyond 3,000º C. Tremendous dynamic pressure builds along the RV body creating drag that dissipates kinetic energy at the rate of 30 - 220 megawatt. Eventually the RV enters ‘MaxQ’ point where the dynamic pressure is maximum. This combination of extremely high temperature and dynamic pressure can easily wreck anything but a most carefully engineered system using a combination of exotic material and manufacturing process.
.jpg)
Figure 6: Typical flight profile (Altitude, Velocity, Drag and Heat)
Agni-TD (Technology Demonstrator)
The Agni-TD’s RV-Mk.1 nose tip is made of a multi-directionally woven, reinforced carbon-carbon fiber composite material. The 0.8 metre diameter (1.0 m flare skirt) and ~4 metre long, reentry 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 withstand structural loads. The outer layer is ablative and made of carbon/phenolic filament wound construction.
This 1980-circa RV (Figure 8-C) was designed to carry BARC-developed 1980 circa, boosted nuclear weapon of 200 kT yield weighing ~1,000 kg (Figure 3, Figure 8-A). The 80 cm body diameter is determined by the diameter of the boosted fission weapon. RV’s blunt nose tip of 30 cm spherical diameter and 14.3º cone angle indicates use of moderately high beta (²)14 design for ICBM class high reentry velocity yet modest temperature stress (3,000°C). The 1,000 kg RV sees peak power dissipation in excess of 120 megawatts (Figure 7: Effect of ‘²’ on RV stress)..jpg)
Figure 7: Effect of ‘²’ on RV stress
Agni-TD was first launched on 22 May, 1989 to prove the RV-Mk.1. The re-entry vehicle was 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. During tests, the re-entry vehicle technology was fully proved when the nose-cone withstood temperatures of 3,000°C while the inside temperature was only 30°C15. It is important to note that re-entry temperature peak is independent of the payload weight. Thus the Agni-TD with the RV fitted with a high altitude motor could achieve necessary test velocity, to fully qualify the re-entry regime.
_small.jpg)
Figure 8: Re-entry vehicle comparison (Bigger Image)
The RV housed an integrated High Altitude Motor (HAM). The liquid fuelled HAM is used to correct impulse variance of solid fuelled stages and subtle launch trajectory variance; approximately 50 to 80 kg fuel is estimated to be sufficient. There are indications that the RV is intended to enter a gliding trajectory when it enters atmosphere at an altitude of 100 km, this further reduces thermal stress.