British Tank Improves Mobility
Vickers, the British tank maker, is developing a new lightweight tank, which may be code-named Tracker. The all-electric vehicle will be powered by a fuel cell; each wheel will have its own motor. The conventional gun will be replaced by an electric rail gun (with twice the muzzle velocity of a conventional gun), a type periodically considered by the U.S. Navy (such a gun is reportedly being tested in Scotland). Some of the weight saved by the lighter electric gun might translate into an anti-helicopter defensive system.
The gun would be used mainly against other tanks; Vickers is proposing an accompanying robot vehicle to fire missiles armed with high explosives. The designers hope that careful design, the use of new compound (largely non-metallic) armor, and a crew of two will cut weight to about 15 tons without sacrificing much of the capability of conventional tanks weighing four or more times as much.
Naturally, Vickers also claims that its futuristic tank would cost far less than a conventional vehicle. It is only fair to point out that Vickers, which enjoys only a small share of the world tank market, would benefit enormously if it led a world tank revolution, overthrowing the current market leaders, the United States, Russia, and other former Soviet republics. Largely plastic armor, carefully faceted, would reduce radar detectability. Apparently it is difficult to make a plastic armor that can handle a wide variety of threats; Vickers proposes interchangeable panels, which might cause supply problems. On the other hand, plastic might even be made to change color to match the tank’s background.
The relatively cool-running fuel cell would reduce the tank's heat signature sharply. Its acoustic signature—used by the U.S. BAT submunition to home in on tanks—also would be reduced. Tanks already are extremely difficult to distinguish from surrounding clutter, particularly if that clutter includes running vehicles. During the 1980s, for example, an attempt to distinguish tracked from wheeled vehicles by radar (probably based on the modulation of the returning pulse by moving elements of the vehicle) failed dramatically. That is why heat and acoustic signatures are now favored. Critics of autonomously homing antitank munitions charge that many of them work only in high-contrast areas such as deserts, and that they would probably fail in built-up areas like Europe. If the most likely wars of the future are relatively small ones in just such areas—Bosnia, perhaps?—then tank signature reduction becomes a very interesting proposition.
Light weight may be even more attractive. Should the British project lead to a viable tank in a decade or so, it might have some interesting naval implications. An amphibious force establishing itself ashore is likely to face counterattacks by enemy main battle tanks. The best antidote is its own tanks, which now must come across the beach in LCACs or in conventional beaching craft. In either case, mines pose a major obstacle to bringing armored vehicles ashore. Since the vehicles are badly needed early in the assault, mine clearance becomes quite urgent.
A very light tank would be a different proposition. If it could be air-delivered over the beach, then early in the amphibious phase the attacking force might be able to forego quick mine clearance, although clearing a path across the beach still would be vital for supplying the force. Alternatively, it might be possible to deliver tanks by helicopter or V-22 well inland from the beach.
Amphibious warfare gains much of its efficacy from surprise. In the best situation, the enemy cannot predict which of many beaches the assault force may use. hence cannot concentrate troops to oppose it. This is the basis for the Marines’ current over-the- horizon assault strategy, and it also made the LCAC extremely attractive. Unfortunately, many Third World countries have such short shorelines that the choice of beach is fairly obvious, as in the Persian Gulf. Once the choice is known, the beach can be mined and defended. If, however, a viable force can land well inland, then the element of surprise would seem to have returned. For that to happen, the air-delivered force ought to include armor, the alternative, which the Marines are now investigating, is to rely on shipboard weapons called up by the landed force, but one might object that communications failures would ruin such a tactic.
Very light battle tanks would, of course, have other implications. Many more could be preloaded on Maritime Prepositioning Squadron ships, increasing Marine Corps mobility. For that matter, sealift loads would generally be reduced somewhat. It might also be argued that such tanks would make airlift far more effective than in the past.
The concept is nothing new; the U.S. Army fielded the M551 Sheridan for its airborne divisions, for example. What is new is the claim that evolving technology can make such a vehicle the equal of heavy tanks. The three key elements are the gun, the power-plant, and the armor.
The British concept links the gun and power-plant, to some extent, offsetting greater weight per horsepower against much less weight in the gun system; there is no propellant, only small- diameter penetrators are needed, and the rail gun’s very high muzzle velocity may offer other savings. The hope is that the separate motors in the wheels will be relatively light. They also promise reduced vulnerability to mines, since the loss of one wheel or even of one track is unlikely to stop the vehicle. Making it easier to apply different power levels to different wheels might make the tank far more maneuverable.
The British announcement suggests that some dramatic advance in fuel cell technology is expected within the next decade. Today, fuel cells demand considerable weight per horsepower. The most important current naval application is in the German Type 212 submarine, in which a fuel cell provides air-independent propulsion—about eight knots—using oxygen and hydrogen stored in tanks outside the pressure hull.
The cell itself weighs several tons, and it probably produces something like 600 to 800 horsepower (on the basis that half speed should require about half power). A tank might accommodate the weight (but not the volume) of the German cell, and stowing liquid or compressed gas might be rather less attractive. The Vickers tank apparently will stow liquid hydrogen but will take its oxygen from the air, presumably expending some of its electric power to do so.
One of the advantages, analogous to that Vickers is investigating, is decreased vulnerability: the ship no longer needs long propeller shafts susceptible to shock (motors can be quite close to the propellers they drive). Another may be that the gun of the future is largely electric (as in a rail gun or an electro-thermal gun, or even a directed-energy weapon). The entire ship power-plant could periodically be directed to charge up the capacitors for the gun. In an all-electric ship, a fuel cell might be a very useful emergency power source, if it could be made compact enough.
Stealth Emerges at Farnborough . . .
An inadvertent experiment at the Farnborough Air Show in September suggests some of the limits of current stealth technology. As the U.S. B-2 bomber flew in, a British infrared sensor attached to a Rapier antiaircraft missile system detected and then tracked it. Since Rapier is command guided, it seems to follow that the B-2 might easily have been shot down.
Stealth generally means antiradar measures, although accounts of stealthy aircraft generally include at least a bow to problems of infrared detection. For example, many stealthy aircraft bury their engines in the fuselage to reduce signatures and mix their hot exhausts with cold outside air. It also is possible to dope a jet or rocket exhaust to shift much of its emission from the infrared windows in the atmosphere to wavelengths that are more quickly absorbed. Even so, there is general agreement that infrared stealth is much more easily claimed than realized.
Part of the problem is that an infrared signature is much more than a few hot spots associated with engine exhaust. Even at low speeds, air friction heats the leading edges of wings and fuselage. Heating may be quite limited, but imaging infrared detectors easily pick up temperature differences of hundredths of a degree. To such detectors, a wing literally glows. That glow, moreover, probably cannot be eliminated. Worse, the faster the airplane, the brighter the glow.
In the past, it was assumed that missiles would simply fly towards the brightest infrared source, the engine tail pipe. Thus, when the engines of TWA Flight 800 were recovered relatively undamaged, it was widely reported that any theory that it had been shot down by a missile was clearly false. Then some unkind soul pointed out that some modern heat-seeking missiles can see more diffuse heat sources, particularly against the cold night sky. Too, many heat seekers are biased to hit forward of engine hot spots, in hopes that they will kill pilots—and also as a counter to flares ejected from the airplane. Either explanation accounts for a wing-root hit rather than in an engine.
New heat-seekers like the AIM-9X Sidewinder successor candidates generally use imaging seekers, which are entirely capable of detecting extended low-temperature sources, but then must cope with sunlit clouds—also extended sources.
It is also likely that even careful shaping does not offer immunity against long-range detection by low-frequency radars that are not influenced by shaping because their signals resonate with the overall size of the target. U.S. Navy experience suggests as much. After World War II, hoping to get better track precision, the U.S. Navy adopted L-band air search radars such as the SPS-6 to replace the long-wave (1.5 meter) wartime series. Then jet aircraft appeared. Although they had not been designed with stealth in mind, their blended forms drastically reduced radar cross section at L-band frequencies. Newer radars easily detected them only when they were carrying bombs and drop tanks, which provided numerous comer reflectors.
Older wartime sets, however, still detected them effectively so the Navy switched back to metric sets such as the SPS-29 and SPS-43 during the 1950s and 1960s; eventually, signal processing solved the L-band problem and led to the SPS-49.
One can, then, imagine a defense against stealthy aircraft such as the B-2. using current technology. They could be detected and tracked with limited precision at long range by ultra-high frequency (UHF) radar. Indeed, the Russians recently advertised what they claim is an anti-stealth modification to their standard export metric-wave air search set (skeptics will point to the absence of tests involving U.S. stealth aircraft).
In a conventional air defense system, the next step would be handover to a shorter-wave (higher precision) tracking radar, which would guide fighters or missiles. The careful shaping of a stealthy airplane eliminates this possibility. The Farnborough demonstration, however, suggests that an imaging infared camera, properly cued, might well be an effective stand-in. It might measure range using a laser, whose beam would surely be reflected by the airplane. Too, since stealth often seems to preclude violent maneuverability, even a bearings-only system might be able to measure range by the target motion analysis techniques common on board submarines (the simplest such techniques assume that the target is moving along a straight line).
Similarly, for the end game, a pilot with night vision or infrared goggles might prove quite effective. The B-2, for example, is both slow and sluggish. It has no problems until it is detected and tracked; after that it is, in effect, a sitting duck.
These considerations raise a wider question, one of particular relevance to the Navy. Really stealthy aircraft design entails considerable performance compromise (as in the B-2 or F-l 17). A major problem in the design of the abortive A-12 was the substitution of thrust vectoring for conventional control surfaces, again in hopes of reducing radar cross section.
So how much stealth is worth while? It may be fairest to say that, beyond a point, stealth comes at the cost of survivability once detected. Surely the B-2 exemplifies such a purchase, with its low speed and poor maneuverability. Perhaps the anti-stealth system outlined above will not be built, but the demonstration at Farnborough suggests that the era of true invisibility may turn out to be shorter than many imagine.