Smaller Bombs Offer Stealthy Aircraft Greater Firepower
The Air Force has been sponsoring development of miniature guided bombs, on the theory that conventionally sized munitions cannot fit the internal bays of stealthy aircraft such as the new F-22 Raptor. The argument in favor of such weapons, which offer only a fraction of the explosive payload of conventional guided bombs, is that in many cases higher precision can make up for much smaller yield. For example, it took more than a ton of explosive to destroy about half of the federal building in Oklahoma City, but a few pounds, properly placed, took the rest down. The Air Force argues that existing weapons already show much better precision than earlier "smart" bombs, and that they can be modified to hit within a few feet or even a few inches of the aim point. A typical claim is that small bombs can replace 80% of more massive ones. Advantages will include much easier shipping, which will make expeditionary Air Force units more effective. More compact weapons also will greatly magnify carrier striking power, since so many more will fit in the same magazine volume, and they also will simplify underway replenishment. Skeptics will point out that in only a very few cases will intelligence be so good that weapons really can be placed at the most critical places in a target building. Plans will not be available, particularly if the building is very important. It also may be difficult to obtain proper GPS coordinates. The typical procedure is to photograph the target area and then deduce coordinates by a process called mensuration. That introduces errors of its own. The more urgent the target, the less likely that its surroundings have been surveyed well enough to make mensuration very precise.
Recently, an Air Force officer suggested an alternative reason to develop the new small bombs: a new generation of explosives is coming that will be at least an order of magnitude more powerful than its predecessors. Such weapons have been rumored for some years. The mechanism most often cited is to place molecules in a metastable state, then release them back to their normal state. That is, a molecule normally occupies its lowest-energy state. Higher energy states generally are unstable. If the chemical is pushed into one of them, for example by being heated, it spontaneously jumps back down, releasing the energy difference as a photon (a flash of light). In a chemical explosion, chemical bonds within the molecule are broken, and their energy released. In some cases the molecule has another, higher, energy state that is stable unless considerable energy is added. It is like a ball in a shallow bowl high up a staircase. As long as the ball is in the bowl, it cannot roll down the stairs. The very small act of shoving the ball over the edge of the bowl allows it to roll all the way down. In energy terms, metastable may mean that the kind of energy available from heating does not suffice to push the molecule out of that state, but a somewhat larger dose of energy might suffice. A small shove might release very considerable energy (much depends on just how energetic the metastable state is). Of course, it may not be easy to create metastable molecules on a production basis. These molecules, moreover, might prove unusually sensitive to electromagnetic interference.
There also must be some question of how well detonation spreads through the metastable explosive. In a normal chemical explosive, each molecule that explodes provides the kick needed by the next, so there is a chain reaction. That is probably not enough to set off a metastable explosive. The metastable concept recalls the way lasers work: the population of atoms is pumped into a metastable state, one decays, and the others all decay in lock-step, to produce coherent radiation. In this case, the key is that the photon produced by the decay of one molecule or atom triggers the decay of another. Perhaps metastable explosives would be driven in a similar manner, coherently, like a massive laser. One key question must be whether the detonator will be so complex or so massive that, although the warhead will be quite small, the weapon as a whole will not be.
If such issues can be resolved, the claim is that a 100-pound weapon can be equivalent to a 2,000-pound bomb. Modern forms of guidance allow each weapon to be targeted independently as long as it is released within range of the target. New folding wings allow a bomb released at high altitude to glide as far as 60 miles. A large airplane, such as a B-2, might carry 200 such weapons, so in theory it could strike 200 targets in a single sortie. For that matter, a fighter-bomber presumably could carry dozens of miniature bombs, and thus can spread them among numerous targets.
There is one other twist to the bomb story. The small bombs would seem to be the ideal armament for a new generation of unmanned combat air vehicles (UCAYs) descended from UAYs. The catch is that, under treaties signed with the Soviets, the range of any land-based cruise missile is strictly limited. This applied to a planned U.S. version of Harpy, the Israeli loitering antiradar UAY. The principle of Harpy is that as long as the radar is turned off, the weapon does not fire. As soon as the radar tums on, Harpy hits it. Although in fact Harpy flies only a relatively few miles, in loitering it covers hundreds more than the treaty allows. In its case, the solution seems to be to place Harpy on board ship, where it is not subject to the treaty. This experience suggests that, should UCAYs come to pass at all, they will be on board ship.
Much will depend on just how expensive the new explosives are, but some very important naval applications depend on size or volume. For years, NATO has had a program to develop a very lightweight antisubmarine weapon that can be carried in large quantities and used fairly freely to see whether something is or is not a submarine. One current contender is the Italian A200, a minitorpedo the size of a large sonobuoy. A200 can excite the crew of a submarine, causing it to flee (and thus demonstrate that it really is a submarine), but it is unlikely to be fatal. If it had 10 or 20 times the explosive power, it would be a very different proposition, more than equivalent to current lightweight torpedoes.
Similarly, the lightweight torpedoes standard in the world's navies can sink submarines, but they are far too small to sink most surface ships; about 100 pounds of explosive just cannot suffice. The largest torpedoes have charges of less than 1,000 pounds, however, so a lightweight armed with the new explosive would be devastating. Some of its warhead weight presumably could be traded for greater speed and range. Submarines are quite limited in the total volume that they can devote to torpedoes or mines. If the standard torpedo were the size of a lightweight version, their situation would change drastically. Much the same can be said of mines. In the case of a torpedo, the current full diameter might well be retained, but the weapon could be made much shorter, hence far more efficient hydrodynamically.
New Waterjet Propulsors Are Efficient and Quiet
At Bourget Naval 2000, in October, Bird-John~ on, a U.S. company affiliated with Rolls-Royce, announced its new generation of highly efficient waterjet propulsors. For years Bird-Johnson has been responsible for the controllable reversible pitch propellers of U.S. gas-turbine warships. About five years ago it received a Maritech (Maritime Administration) contract to develop a high-powered (35,000 shaft horsepower [SHP]) waterjet. Foreign companies already made such waterjets, but there was no U.S. source. The technology then was about a decade old. Applications included large ships and very fast yachts. At least in theory, a pump-jet can handle much more power than a conventional propeller shaft because it does not suffer from the same sort of cavitation. For high speeds, pump-jets generally simply suck water out of the bottom of a craft and exhaust it through the transom. Bird-Johnson took a different approach, in which both entry and exhaust are underwater. It should be noted that none of this technology is directly related to submarine or torpedo pump-jets.
It turned out that a much more efficient unit could be developed, using design tools created at MIT. Current research suggests that a destroyer-scale pump-jet (50,000 SHP, as in an Arleigh Burke [DDG-51]-class guided missile destroyer) offers a combination of excellent acoustics and much greater efficiency, for substantial fuel savings (estimated at about 8%). Because the pump-jet, located on the underside of the hull, discharges well below the surface, the ship has no visible (white water) wake, although presumably nothing can eliminate the turbulent hydrodynamic wake created by the hull itself. Cavitation, whose noise limits sonar performance, occurs at a considerably higher speed, because the pump works against higher water pressure than a conventional propeller. Overall, the acoustic performance of the pump-jet is far better than that of most propellers, but it costs much less than exotic low-noise propellers. Maneuverability improves because the flow out of the pump-jet itself can be vectored; the ship does not need a conventional rudder. With an electric motor, the pump-jet can be podded beneath the hull, working through a single hull penetration, like a rudder. A ship so equipped can turn within its own length. Alternatively, the pump-jet can be buried in the hull, with an inlet and an outlet. Because the rotor runs at more than twice the speed of a conventional propeller, it can use a much smaller electric motor.
Tests with a model of an existing frigate showed that the pump-jet was about as efficient as the standard propeller at 25 knots, but more efficient both above and below that speed. The pump-jet also was strikingly quieter than a propeller.