Cruise Control

The recent BrahMos test-firing has brought into focus the importance of cruise missiles

By Pravin Sawhney

 

The successful maiden test-firing of BrahMos supersonic cruise missile from an underwater pontoon on March 20 was more important than has been understood in the popular perception. Media focus, unfortunately, has been on the negatives: there is no submarine platform (Project 75I) to use this version of BrahMos missile; the missile has been test-fired from a stationary underwater pontoon and not a submarine; as Project 75I acquisition is a good decade away (the RFP has still not been issued), the underwater cruise missile technology would become obsolete, are some of the comments which betray both a lack of understanding of the subject and its operational utility and context.

It seems to have been forgotten that Pakistan successfully test-fired its long range 1,000km sub-sonic Babur cruise missile, in August 2005, which since has joined its army’s inventory, upsetting the operational balance between the two militaries. While bean counting of assets between India and Pakistan is unnecessary, a new weapon system joining one’s inventory has serious operational implications for both sides, since neither is expected to use nukes early in a war. Probably a step by step approach is required to understand the latest progress in the BrahMos phenomenon: how cruise missiles relate to war? The quality of cruise missile with India and Pakistan, and the road ahead, are questions that need to be pondered over. The issue assumes importance since cruise missiles, unlike ballistic missiles, will be used freely in a conventional war.

A cruise missile is a dispensable, pilot-less, guided, continuously-powered, endo-atmospheric (stays within the atmosphere) vehicle that is supported by wings and is powered by the same kind of jet engine as an aircraft. Unlike a ballistic missile, that is powered and usually guided for only the brief initial part of its flight till it leaves the atmosphere, a cruise missile requires continuous power and guidance, since both the velocity and the direction of its flight can be unpredictably altered by local weather conditions or changes in the performance of its propulsion system. For instance, a ballistic missile is guided for the first five of the 20 minutes it takes to travel 5,000km; a cruise missile, which usually flies at subsonic speed, would require close to six hours of continuous guided flight to cover the same distance. Hence, guidance errors that accumulate with time would be almost a hundred times larger for a cruise missile than for a ballistic missile with a comparable range. Accurate arrival of a cruise missile at a target is achieved with continuous inertial guidance only by correcting it from time to time with fresh information about the missile’s position.

In terms of cost, cruise missiles are less costly to design, develop, procure, maintain and operate than ballistic missiles. In operational terms, cruise missiles are better suited than ballistic missiles for use with conventional warheads as their accuracy is far better. The aerodynamic stability of the cruise missile permits the use of less-sophisticated and therefore, less costly guidance and control methods than in the case of ballistic missiles, which undergo the stresses of re-entry into the atmosphere with high speed. For example, cruise missiles can receive satellite navigation corrections all the way to the target from the US Global Positioning System (GPS) or Russian Global Navigation Satellite System (GLONASS) leading to 10 metres Circular Error Probability. CEP is a measure of accuracy, defined as the radius of a circle in which 50 per cent of missiles are successfully delivered.

Another appealing operational feature of cruise missiles is that they can be placed in canisters, which makes them easy to maintain and operate in harsh environment. Their relative compact size offers more flexible launch options, more mobility for ground-launched versions, and a smaller logistics burden, which reduces their battlefield vulnerability to detection — and thus improves their pre-launch survivability. Moreover, cruise missiles dictate no special launch pad stability requirements and can be launched from ships, submarines, aircraft and ground launchers with ease.

Most importantly, cruise missiles can fly low and hence pose severe detection challenges even for airborne radars due to ground clutter. Moreover, cruise missiles’ exhaust plumes are not generally detected by launch warning systems, and unlike ballistic missiles, their flight paths are unpredictable. Given the fact that reductions in radar cross-section are easier to accomplish in cruise missile designs than in manned aircraft, cruise missile pose a formidable challenge to modern air defence systems. In comparison, at least to a limited extent, defences against ballistic missiles are available with the US, Russia, Israel and China.

In more specific terms, the operational importance of cruise missiles owes to the advances in propulsion (engine), guidance and navigation technologies. The air breathing engines for propulsion are of two types: turbojet and turbofan. Turbofan engines consume much less fuel than turbojets of equivalent size; hence are more complex system and extremely expensive. Accordingly, turbofan engines are considered suitable for long-range cruise missiles with ranges between 600km to 2,000km. At present, only a few advanced countries have mastered the turbofan propulsion technology. Interestingly, China is amongst them. In 1994, the Clinton administration in the US approved a half-billion-dollar sale of turbofan engines by AlliedSignal to China for use in business aircraft. These engines were reversed engineered by the Chinese to upgrade their Silkworms Anti-Ship Cruise Missiles (ASCM) to 600km range. China publicly unveiled its WS500 turbofan engine (subsequently used in Babur Land Attack Cruise Missile) at the Zhuhai Air Show in late 2004. Developed by the Chinese Gas Turbine Establishment, the WS500 is claimed to produce around 1,125lbs of thrust. By comparison, the US Tomahawk engine produces 700lbs thrust.

The turbojet engine is more widely used in cruise missile with ranges up to 500km, referred to as tactical missile. The ramjet propulsion engine is a derivative of turbojet engine. Unlike in the case of turbojet propulsion that produces subsonic speeds, in ramjet, adequate pressure is built up within the engine to produce supersonic speeds of Mach 2 (Mach 1 is equivalent to the speed of sound which is 1,000km per hour) to Mach 4. The main disadvantage of the ramjet is that it requires to be boosted from static to a suitable high velocity, usually around Mach 2, to create a high enough pressure (called ram pressure) for the ramjet propulsion to work. However, a ramjet is much simpler than turbojet or turbofan propulsion.

Regarding the navigation and control of cruise missiles, it can be done by various methods that include simple mid-course correction by pre-programmed autopilot, and terminal guidance by passive radio frequency homing, radar, or passive Infra Red. The Inertial Navigation System (INS) that uses accelerometers and gyroscopes that detect motion and calculate changes in relative position are not very helpful with cruise missiles as given their slow motion and long range, adequate inaccuracies accumulate that make it unreliable for use in conventional missions. The answer is to integrate GPS with INS. The problem with the GPS is that the US defence department has intentionally added an inaccuracy in the system called Selective Availability (SA), so that only the US military gets the accurate signal codes for its use. Interestingly, US companies themselves have created a technique called the Digital Ground Precision System (DGPS) which has removed most of the inherent GPS’ SA inaccuracies. Commercial DGPS are available in the open market and India and Pakistan are amongst the many countries that have sought the DGPS.

Probably what makes cruise missiles an attractive weapon system for developing countries is the stronger consensus amongst the Missile Technology Control Regime (MTCR) members that restricting ballistic missiles is more important than cruise missiles and Unmanned Aerial Vehicle (UAV) systems. This has motivated many countries to upgrade ASCM and UAV to Land Attack Cruise Missile (LACM). Moreover, the MTCR threshold of 500kg payload and 300km range is more suited for ballistic rather than cruise missiles. From an engineering standpoint, it is relatively easy to scale-up the range of an existing cruise missile system than a ballistic missile. The technology required to produce a 600km range cruise missile is not fundamentally different from that needed for very short-range cruise missiles. Hence, UAV technologies falling clearly below the MTCR threshold can be exported and applied to the development of long-range cruise missiles. Moreover, the structures, propulsion, autopilot, and navigation systems used in manned aircraft are essentially interchangeable with those of cruise missiles. Against this backdrop, Pakistan’s Babur and India’s BrahMos cruise missiles need to be assessed to determine their operational capabilities. 

 

Babur

Babur’s two technology advantages are its turbofan propulsion, and its navigation and guidance system comprising the radar altimeter and a digital imaging infrared seeker. Unlike India that started with the low-end of technology by making BrahMos into an ASCM to be upgraded to LACM, Babur’s evolution appears to be the other way round. It has been developed as a long-range LACM and its naval version will be made in limited numbers to be carried by F-22P guided missile frigates.

The radar altimeter enables the cruise missile to fly as low as 20m over water, 50m over moderately hilly terrain, and 100m over mountains. (This capability makes the missile difficult to detect with ground-based radar). Fitted with turbofan propulsion, a cruise missile is capable of ranges up to 2,000km at low altitude and perhaps 50 per cent more if the first 1,500km are flown at higher altitude and the rest at tree top level.

As a thumb rule, more fuel is consumed if a cruise missile travels low, and inversely, less fuel is expended if a cruise missile travels at higher altitudes within the atmosphere. Given these technical parameters, Pakistan media reports claiming that the 10 August 2005 test-firing of Babur achieved a range of 500km at a low cruising altitude of 100m at a speed of Mach 0.7 sound plausible. Moreover, Babur has tremendous inherent potential to be made into an accurate long range LACM.

The introduction of Babur missile by Pakistan coincides well with the Chinese focus on cruise missiles with longer ranges since the Nineties. For example, China reportedly has a number of cruise missile programmes underway. These include the YJ-62 long-range ASCM, as well as air and ground launched derivatives. For missile guidance, China has acquired active radar guidance for terminal guidance in addition to electro-optical seeker for LACM.

It will be appropriate to assume that China, which cares little about international non-proliferation treaties and obligations, would have shared advanced cruise missile technology with Pakistan. In summation, Babur, with state-of-the-art turbofan technology procured from China, is expected to be used as sub-sonic LACM and on surface ships.

 

BrahMos

What makes BrahMos cruise missile extraordinary is that it is the only significant weapon system produced by the Defence Research and Development Organisation (DRDO) which has evolved in reasonable time and cost-lines and has been accepted by all the three defence services (navy, army and air force) readily without government pressure. The missile has two-stage propulsion — solid propellant rocket for initial acceleration and liquid fuelled ramjet for sustained supersonic cruise — achieving a speed of 2.8 Mach to deliver a 300kg payload. Russia has provided propulsion, while India is responsible for navigation based on inertial navigation system.

A brainchild of Dr APJ Abdul Kalam, BrahMos has matured through Dr Sivathanu Pillai’s efforts as a multi-role cruise missile. The inter-governmental joint venture agreement signed between India and Russia on 12 February 1998 formed the BrahMos Aerospace to build ASCM to be jointly designed, developed, produced and marketed. Fifteen years later, in February 2012 when BrahMos celebrated its landmark anniversary, it had achieved the unexpected. Starting with INS Rajput in 2005, BrahMos has been accepted by the navy to be its offensive weapon on all surface ships. The army has accepted three versions of BrahMos LACM. Three regiments of BrahMos Block I and II have been raised as part of artillery divisions. Block I is with radio frequency seekers, while Block II has indigenous software developed by BrahMos and DRDO for better accuracy against smaller targets.

In tandem with GPS, an accuracy of 10m has been achieved. Block III version is meant for mountains, where BrahMos’ steep-dive capability meant to cover targets behind mountain ridges (called dead ground in army parlance) was successfully demonstrated to the army on 5 September 2010.  Orders for a regiment of BrahMos LACM Block III version have since been placed. The BrahMos LACM version will be provided with an Infrared missile seeker with an inbuilt camera to provide simultaneous photographs of the target to minimise collateral damage.

Meanwhile, work has started on adopting BrahMos for the air and undersea versions. The air version will be lighter in weight and the government has cleared modifications on 42 numbers of Su-30MKI aircraft to have the supersonic missile. All checks and analysis by HAL and the Sukhoi design bureau have been completed and the field ‘drop test’ of BrahMos from the Su-30MKI is slated for the end of the year, with the production expected to commence in 2014.

Meanwhile, BrahMos created a record of sorts by its recent underwater launch from a pontoon platform. The navy is satisfied with the tests and preliminary acceptance to have eight vertically launched BrahMos supersonic cruise missiles on each P-75I submarine has been cleared. Consider a realistic naval firepower potential a decade from now: All Kilo-class submarines will be fitted with the Russian Klub 3E-14E LACM. With each submarine carrying 16 to 18 of these 300km range missiles, the navy will have devastating firepower to employ in various tactical scenarios. This is not all. With P-75I submarines getting inducted into service, they would be fitted with both Klub 3E-14E LACM and improved BrahMos missiles; then, even reduced submarine numbers will be made good by better capabilities.

Probably the biggest limitations of BrahMos are its 290km range and 300kg warhead, well under the MTCR range threshold of 300km and 500kg. Fitted with ramjet propulsion, BrahMos rises up to an altitude of nearly 10km to 12km, before the ramjet propulsion takes over to provide the missile with a speed of Mach 2.8 during the cruising phase. BrahMos’ advantage is that with its high speed, it is capable of travelling its maximum range in four minutes. Even as the missile during its initial phase will provide a sizeable signature for the enemy acquisition radar, there will be little time to take counter measures to stop the missile.

On balance, BrahMos has three distinctive features. One, except for the air version, it has a universal launcher for its naval and land versions. Two, the same missile, without any modification, can be employed against any ship or land targets. And three, no land forces in the world are equipped with supersonic cruise missiles. Specific to Pakistan, with kinetic energy nine times more than Babur, BrahMos is a formidable supersonic cruise missile.  

What India now urgently needs is a cruise missiles policy, whose long-term developmental focus should be three-pronged: to improve BrahMos to hypersonic speeds, to work on a long range subsonic cruise missile with indigenous turbofan propulsion, and Cruise Missile Defence (CMD). There will also be the need to decide various platforms for cruise missiles, as being the prime target of the enemy these will be vulnerable. For example, the LACM can be fired from mobile launchers, hardened silos and submarines; the ideal, of course, will be to have a mix of all three.

There is a need to exploit BrahMos, which is available in all three sea, land and air versions, optimally. At present, BrahMos uses the GPS. It is known that the Indian Space and Research Organisation (ISRO) is working on an indigenous GPS, which has been partially successful. Once accomplished, this will be a major breakthrough in providing secure and improved guidance to cruise missiles. Another cruise missile project should concentrate on scramjet propulsion, to take the missile to hypersonic speeds of Mach 10, something that BrahMos Aerospace has initiated. It may be recalled that conceptual work on hypersonic propulsion was started in 1993 by the then DRDO chief, Dr APJ Abdul Kalam. In addition to the engine itself, the need will be for composite materials that can withstand high temperatures for trans-atmospheric flight. Unfortunately, material management has never been India’s strength and outside help (Russia) would be needed.

Probably the most daunting challenge will be the Cruise Missile Defence (CMD), especially when futuristic missiles are expected to have hypersonic propulsion. The answer to this lies in directed energy weapons, which derive their destructive power from electromagnetic energy or subatomic particle beams aimed against an incoming warhead, and travel close to or at the speed of light. For example, laser light can be used as directed energy weapon. Laser guided weapons differ from the anti-missile weapon systems in three fundamental ways: One, in laser weapons destructive energy is transported to the target in the form of an intense beam of electromagnetic waves rather than in the form of an explosive charge carried inside a missile or shell. Two, this energy travels at the speed of light, that is, 3.10 (to power 8) metres per second, compared with 1,000 to 2,000m per second that a supersonic missile should be capable of. And three, the laser beam can damage a target only if it physically strikes it. It is axiomatic that laser weapons are best directed from space. While India is in no way near such advanced research and in principle opposes militarisation of space, it must start appreciating the non-military applications of space.

In a rare candid admission, a former DRDO chief, M. Natarajan has said: “The lack of success in developing indigenous propulsion systems for the country’s major programmes is a cause of concern. Affected are programmes such as the aero engines for fighter aircraft and unmanned aerial vehicles, engines for tanks and naval propulsion and ramjet and hypersonic propulsion for missiles.” Taking cue, the DRDO should abandon its penchant for hype and boasting, and indeed concentrate on essentials with transparency. The growth trajectory of BrahMos Aerospace has set the roadmap for future developments.