IR Signature Suppression of Modern Naval Ships

Most modern naval ships include some form of infrared signature suppression (IRSS) to reduce the ship susceptibility to IR guided anti-ship missiles. In some cases the IRSS may be very basic while in other ships great care has been taken in the ship design process to achieve a very low signature. The trend in recent years with new ship programs is towards a more systematic and comprehensive approach to IRSS. In previous years it was considered enough for a ship IR signature specification to consist of a single statement: the ship IR signature shall be minimized. This is no longer good enough. New ship design programs are including detailed IR signature management studies that include IR suppression tradeoff studies, detailed susceptibility analysis and cost benefit analysis. These studies consider the operating environment, the ship layout, and the anticipated threats. More and more of these studies are including detailed three dimensional computer modeling of ship IR images in realistic operating environments. These models are allowing the analysts to study important effects such as solar heating or reflection, sea surface clutter, flare decoy deployment and other complex processes that could not be considered in any detail until recently. All of these mean that new ships are being designed with lower signatures and improved survivability. This paper discusses the main contributions to ship IR signatures and what means are available to reduce or eliminate these signatures. Examples are given to illustrate the benefits and costs of IRSS under different operating conditions. SHIP IR SIGNATURE OVERVIEW: A ship signature is made up from two main components: Internally generated, and Externally generated. Internally generated signature sources include rejected heat from engines and other equipment, exhaust products from engines, waste air from ventilation systems and heat losses from heated internal spaces. The primary internal IR source results from the main machinery onboard any vessel, in particular drive engines and electrical generators. The magnitude of signatures produced by other sources such as heated windows, weapon systems, and deck mounted machinery is insignificant in comparison if main machinery is not suppressed. Externally generated sources result from the surfaces of a ship absorbing and/or reflecting radiation r eceived from its surroundings. The primary sources of background radiation are: the sun, sky radiance, and sea radiance. Effective IR suppression of a ship must consider both sources. Some argue that there is no point to suppressing the internally generated sources (plumes, uptakes, hot spots) because it is not possible to suppress the external sources. This ignores the fact that there is no solar heating at night or when the sky is overcast. It also ignores the fact that the sun also generates clutter. With some active measures such as water wash, the external sources can be taken care of to some degree. MAIN SHIP MACHINERY: Of all internally generated sources of IR, waste heat and combustion products from a vessel’s main machinery is the most significant. Figure 1 illustrates the ways in which the heat from a ship’s machinery can manifest itself in the form of IR emissions. Five types of IR sources, or “hot-spots” can be identified in Figure 1. First are the warm sections of hull, indicating the location of engine compartments on the other side. Heat 1 Presented at ASNE 21 st Century Combatant Technology Symposium, 27-30 January 1998. -24 4.1 4.2 4.3 4.4 4.5 4.6 wavelength ( μm) 0 2 4 6 8 10 12 C O N T R A S T R A D I A N T I N T E N S I T Y ( k W / μ m s r ) Legend 10 km 4100 m 1800 m 600 m 10 m 0 Figure 1 Infrared Image of a Typical Unsuppressed Ship radiating from running machinery heats the air in these compartments, that in turn convects to the uninsulated hull of the ship. Next are the funnel spaces, heated by engine room ventilation air and hot exhaust uptakes running through them. With no insulation installed on funnel walls (as in this case), the funnel exterior is heated much like the sections of ship’s hull. At the top of the two funnels can be seen the extremely hot (300-400(C typically) exhaust uptake metal. The relatively large area (2-5 m), high temperature, and location high on the ship make visible uptake metal the largest single contributor to internally generated signatures. Adding to the uptake metal hot-spot are the emissions from hot exhaust gases. A soot-free exhaust plume contains mostly hot CO 2 gas and water vapor, that radiates in a narrow spectral band from 4.1 to 4.6 μm. Much of the radiation in this waveband is quickly absorbed by the atmosphere, but some of this radiation can penetrate through many km of atmosphere. Even at ranges of 10 km or more, the plume can still be a very significant contributor to ship IR signature. Figure 2 presents the spectral emission from a typical GE LM2500 plume at a number of ranges. Figure 2 Spectral Emission of 75 kg/s @ 500 (C Plume The final hot-spot shown in Figure 1 that is a result of ship main machinery is the hot communications mast. The mast and other parts of the ship can be heated by hot plume impingement. The mast hot-spot is a concern in instances when a tailwind causes the plume to impinge upon a ship’s mast. Unsuppressed plumes can heat masts and mounted electronics to 100-200 (C, resulting in a very large hot-spot high above the water. The high impingement temperatures may also result in failure of the sens itive electronics mounted on the mast. To eliminate, or at the very least minimize the severity of the warm hull sections and funnel sides requires application of basic thermal design. Compartments should be ventilated to a sufficient extent as to keep compartment temperatures at below 50(C. Any compartments or funnel spaces that are heated to above ambient should have ther mal insulation applied to all external bulkheads. Applic ation of even 25 mm (1") of glass wool insulation can reduce outer skin temperatures to an acceptable contrast temperature. As a guideline, hull surfaces heated from within should not xceed a contrast temperature of ±5 (C. Other compartments around the ship that may have temperatures different from ambient should also be considered. Negatively contrasted surfaces resulting from cool, air conditioned electronics bays for example are to be avoided as well. The remaining hot-spots (ie. hot uptake metal, plume, plume impinged mast) are most effectively treated by suppressing their source, the hot exhaust gases from the main machinery. Simple suppression devices provide an optical block, or film cooling of hot uptake metal, ignoring the importance of hot plume emissions. Plume cooling is also required, so as to reduce direct IR emissions from the plume, and reduce mast temperatures under impingement situations. Figure 3 illustrates four popular IRSS devices in use today. Figure 3 Popular Engine Exhaust IRSS Devices