While a hovering drone a few tens of feet away is an easy target, David Hambling explains (in Swarm Troopers), one approaching at a hundred miles an hour is virtually impossible to hit:
Hunters have difficulty hitting flying geese at more than about eighty yards, even with the spread of shot from a shotgun. Hitting one with a rifle is harder and putting a bullet through the Kevlar wing of a drone may only make it wobble. Unlike a goose or an airplane with “wet wings” containing fuel, a drone can only be seriously damaged by hitting a vital part.
A lethal drone like Switchblade will cover that last eighty yards to the target in around two seconds and its body presents a target four inches across. It can fly at low altitude, putting it below the horizon and making it difficult to see against a cluttered background. It can attack in complete darkness, and as it was seen in the section on swarming hunters, drones will come in from several directions at once. Some may even come from vertically above the target.
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Before [World War 2], it was estimated that [anti-aircraft] guns would score one hit for every two hundred rounds. In reality it took closer to twenty thousand. A shell takes ten seconds or more to reach its target at high altitude, in which time a WWII bomber will have travelled about fifty times its own length. The slightest mis-estimation of range or speed means the shell has no chance of hitting. Anti-aircraft batteries fired a curtain of shells into the path of oncoming bomber formations rather than aiming individually. The mass of shell bursts did at least act as a deterrent.
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Air defenses rarely shot down attacking aircraft. Shells did not hit planes, but sprayed them with high-velocity shrapnel fragments. The shrapnel generally caused minor damage or injured crew members, but this could force an aircraft to abort its mission and send it limping home. It took a lucky hit, or the cumulative damage from several near-misses, to down a plane.
Air defenses rarely shot down attacking aircraft. Shells did not hit planes, but sprayed them with high-velocity shrapnel fragments. The shrapnel generally caused minor damage or injured crew members, but this could force an aircraft to abort its mission and send it limping home. It took a lucky hit, or the cumulative damage from several near-misses, to down a plane.
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One approach was to make every fourth bullet from a machine gun a phosphorus tracer round that leaves a glowing trail. This showed the path of the bullets so the gunner could adjust his aim, directing the visible stream of bullets towards the target. Like the wall of shell bursts from larger guns, the stream of tracer was also a deterrent: it takes a steely nerve to deliberately fly into a hail of bullets.
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Unlike other aircraft, the kamikazes were not deterred by slight damage. Machines guns and 20mm and 40mm cannon consistently failed to prevent a kamikaze from hitting his target. Only the big five-inch naval guns could destroy a plane with one hit.
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One analyst calculates that, because they scored so many hits compared to the casualties suffered, kamikaze attacks cost the Japanese fewer planes per hit than other types of attack.
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Admiral Halsey’s solution to the kamikazes was an intensive program of air strikes on their airfields. Navy carrier air wings and Army Air Force B-29s destroyed large numbers of kamikazes on the ground, ending a threat that could not be stopped by anti-aircraft guns.
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The guided missile was the air-defense equivalent of the smart bomb. Instead of firing thousands of rounds and hoping for a lucky hit, a single projectile homed in on the target and guaranteed a shoot-down. Heat-seeking missiles were effective at close range, while bigger and heavier missiles with radar guidance took over at longer ranges.
In the 1960s, the US foot soldier had his personal air defense in the form of the Redeye missile. This was a portable heat-seeking missile that could take out a fast jet two miles away, an almost impossible feat even for a quadruple heavy-machine gun that had to be carried on a truck. The main problem with early versions of the Redeye was that it was purely a “revenge weapon” – it could only lock on to a jet’s exhaust from behind, so you couldn’t shoot down a plane until it had already flown over and bombed you.
In the same period, protection from heavy bombers was provided by the Nike Hercules. This missile stood forty feet high and flew at Mach 3 and had a range of eighty miles. While the Redeye carried two pounds of explosive, Nike Hercules was armed with a twenty-kiloton atomic warhead capable of bringing down a whole formation of bombers in one go.
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The plan was to take the existing M48 Patton tank and fit it with a new turret armed with a pair of WWII-era 40mm guns. Manual aiming was not enough; it would be guided by the radar from an F-16 aircraft with a new computerized fire-control system. On paper, the Sergeant York looked like a sound proposition.
The result was a billion-dollar fiasco. The Patton tank chassis were worn out, giving up after three hundred miles of road tests instead of the four thousand planned. The 40mm guns had been stored badly and were in poor shape. The biggest defect was the radar; designed for air-to-air combat in the open sky, it could not deal with all the clutter at ground level. It was easily confused by things like waving trees, which it mistook for helicopters.
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The modern Stinger looks a lot like the 1960s Redeye, and the Patriot missile looks like a smaller version of the old Nike. Rather than being bigger and more powerful, they are smarter and more agile. As with bombs, intelligence trumps brute force.
Modern missiles can spot targets faster and shrug off the clutter that confused Sergeant York. They are highly resistant to jamming and deception. They are harder to avoid in the dance of death known as the “terminal engagement phase,” when planes maneuver wildly in a desperate attempt to get away as the missile closes in.
Air defense has become a duel between radar operators and “defense suppression” aircraft equipped with electronic warfare pods, decoys, and missiles that home in on radar emissions. The attackers attempt to blind, confuse, or evade the defenses and get close enough to launch their missiles. A radar signal is like a searchlight on a dark night, advertising its position over a wide area. Radar operators respond by only turning their radar on at intervals, and by moving position when possible. It is a duel whose outcome is largely determined by who has the best technology.
The current refinement of the Patriot missile is state of the art. This is several generations on from the missile that was hailed (inaccurately) as the Scud-buster of the 1991 Gulf War. The fifteen hundred pound missile travels at almost a mile per second and can destroy an aircraft anywhere from treetop height to eighty thousand feet, at a range of a hundred miles away. Costing somewhere over a million dollars per shot, the Patriot is an effective weapon against a whole range of targets. A battery of Patriots can defend against attack helicopters like the Hind, strike aircraft, heavy bombers, and is now effective against Scuds and other ballistic missiles.
The recent focus has been on tweaking Patriot for missile defense because shooting down aircraft simply is not an issue. US air superiority in recent conflicts means that nobody has been in a position to bomb US forces. According to the USAF’s 2014 Posture Statement:
“Since April of 1953, roughly seven million American service members have deployed to combat and contingency operations all over the world. Thousands of them have died as they fought. Not a single one was killed by an enemy aircraft. We intend to keep it that way.”
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The sharp end of a Patriot missile battery comprises four launch vehicles, each with four missiles ready to fire. In principle, a Patriot battery can take on sixteen aircraft at a time (of four times that number with new, miniature PAC-3 missiles). While two or more missiles may sometimes be launched on different trajectories at a difficult target, the battery might take out sixteen Reapers in a matter of seconds.
Whether Patriot could even hit small drones is another question entirely.
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It is hard to image a three-quarter ton missile engaging a four-pound drone. And even if every missile worked perfectly, the seventeenth drone would get through — along with all those following.
Patriot missile batteries rely on radar, which is vulnerable; one hit could put the whole battery out of action. The drones might target the launch vehicles and personnel. Systems like the Patriot are not armored against attack, and the M983 trucks that transport the Patriot are as vulnerable as any other truck. Missiles are explosive targets full of flammable rocket fuel.
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Nor can the problem be solved by issuing Stingers to every soldier; at over $38,000 a shot, they are too expensive to be bought in such volumes. Worse, missiles like the Stinger are heat-seekers that depend on the target having a hot engine. A small drone with an electric motor is invisible.
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The USAF’s F-22A Raptor is arguably the best fighter in the world, but its six radar-guided AMRAAM missiles and two infrared Sidewinders will not dent a swarm, even if they were able to lock on. The Raptor’s 20mm cannon makes little difference. The rotary cannon has a high rate of fire to ensure a good chance of a hit, and the entire magazine is expended by six one-second bursts.
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Against most opponents, air supremacy means destroying enemy air fields so their aircraft cannot take off or land. This was the answer to the kamikaze threat.
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Small drones do not need a runway, air base, or hangars.
My (amateur) understanding is that a multi-megajoule railgun projectile is inefficient because it dumps too many megajoules into the atmosphere, producing a huge shock wave and setting the air on fire. Perhaps such inefficient dumping of energy into air could knock drones out of the air.
“Shells did not hit planes, but sprayed them with high-velocity shrapnel fragments.”
Even if railguns are not the answer, conventional flak cannons might work. A cannon or rocket could deliver a “flak” warhead to the general vicinity of the target drone. Because it is not necessary to score a direct hit, an explosive warhead could be detonated some distance away from the drone, but the shockwave might be sufficient to kill the drone. Perhaps the problem is that the artillerists might have to fire 20,000 flak shells to down one drone.