8
Lightning Hazards
In the course of fifty calendar years and countless man-years of collective experience, the Langmuir scientists and their colleagues have become well acquainted with the hazards posed by lightning. When a good stroke hits the metal roof of the lab cupola, it sounds like a bomb going off. When it hits a tree, the tree is usually charred; sometimes, if the current vaporizes enough sap and water, it explodes. Occasionally lightning strikes someone standing with upraised putter beneath a thunderstorm, and there is one less golfer in the world. Anecdotal experience with lightning goes back to the beginning of recorded history. But science demands more than anecdotes. Accordingly, recent years have seen a great increase in the systematic, quantitative analysis of what happens when lightning strikes an object or creature.
Myths abound concerning lightning strikes at ground level -that television sets attract lightning, for example--but it is in the skies that people are most afraid of lightning. Flying is, at best, a great expression of faith in technology; a good thunderstorm, with winds dangerous in their own right in addition to lightning, tends to erode that faith in air travelers who normally suppress their doubts. Looking out the window and seeing lightning in midair makes many people wonder what would happen if lightning were to hit the airplane.
And lightning does hit airplanes fairly often. Usually the strike goes unnoticed. Only the ground crews doing a routine inspection after the flight notice the damage, which is usually confined to small burn holes. (When a jet streaks through the path of a lightning flash, the successive components of the flash burn a neat line of pinholes in the skin of the plane.) Occasionally the damage is more spectacular. In the early sixties, for example, a 707 had part of its radome cracked by lightning. Other types of problems are possible. A pilot on final approach, for example, could be momentarily blinded by a lightning flash just ahead.
Although most plane crashes in thunderstorms are caused by wind, lightning occasionally plays a direct role. For example, lightning can burn through the metal skin of an aircraft; if it penetrates a nearly empty fuel tank that is full of fuel vapor and has not been charged with an inert gas, an explosion can result. Fortunately, such dramatic events are uncommon in metal-skinned planes. But two trends in the design of high-performance aircraft are increasing their lightning vulnerability.
The first is the use of plastic-like composites for the airframes. These materials are stronger and lighter than most common engineering metals. The catch is that they usually do not conduct electricity. Aircraft designers used to wrap their planes in metal and take it for granted that lightning would not get inside. Composites have forced them to think analytically about lightning and its behavior, because protection against lightning no longer automatically comes with the hull of the aircraft.
Second, the innards of the modern aircraft are more vulnerable to lightning. Avionics, or aviation electronics, has come a long way in a short time. The pilot of a state-of-the-art airliner sits before a computerized instrument panel that shows all the information a pilot needs on cathode-ray monitors. Sophisticated radar probes the skies ahead. A computer and a stack of radios allow precise navigation to every part of the globe. All of this equipment has to be hardened against lightning damage to a certain extent. But how can it be hardened? And to what extent? These are the questions that designers have to ask themselves, and man's knowledge of lightning is such that they often have to go to basic research for the answers.
Even the controls of the latest aircraft are electronic. An older airplane--a 707, say, or an F-4 Phantom, has control surfaces linked to the pilot's hands and feet by cables and hydraulic power boosters. Such linkages are virtually lightning-proof, but they are heavy and slow to respond. Enter the fly-by-wire aircraft, such as an F/A-18, maneuvered by servomotors, with a computer and a bundle of wires between the pilot and the control surfaces. Light. Lightning-fast. But also lightning-vulnerable.
The Grumman X-29, which became famous for its forward-swept wings, is the ultimate embodiment of these new trends. It is a totally fly-by-wire design, and is made mostly of composites. The X-29 will never be flown anywhere near a thunderstorm. It will probably never even be taxied on a wet runway. But with the advent of new technologies and materials, the government and aerospace companies are eager to find out exactly what lightning does to an airplane.
The theoretical side of that research brought Air Force Flight Dynamics Laboratory scientists to Langmuir three years in a row. Physicist Richard Richmond was one of the leaders in their effort to quantify the electromagnetic effects of a lightning strike on a fuselage. He designed LSO-1 and its successor, LSO-2, the instrument-stuffed Lightning Strike Objects. He and a few colleagues hauled these forty-foot aluminum cylinders from Wright-Patterson Air Force Base near Dayton, Ohio, to Langmuir Lab.
Once at Langmuir, the scientists suspended the cylinder inside a tripod of telephone poles. A battery inside the LSO provided power without the risk that lightning-induced surges would come down the power lines. To achieve even more isolation, the data from the instruments were optically encoded inside the cylinder and sent to a nearby trailer via fiber optics. Since fiber optics do not conduct electricity, there was no way for lightning to induce a surge on them. Richmond and his colleagues waited inside the trailer.
And waited. And waited. Their final year at the lab, 1982, was a disappointing thunderstorm year. On several occasions, however, the visiting French scientists managed to trigger lightning that struck the cylinder. Finally, late in the summer, a big thunderstorm rumbled up over the test site and lingered for an hour or more. Richmond, who was in Socorro that day, drove up during the storm and parked in a safe area: He watched in ecstasy as lightning struck his cylinder three time--twice after rocket fire and once all by itself. When the storm blew over, he rushed to the trailer, only to find that a power outage had disabled his data recorder.
The efforts were not a total loss, however. Richmond had good data from earlier storms at Langmuir, and also from storms at the Kennedy Space Center. He obtained the kind of data that would interest a physicist rather than an engineer. His goal was to develop a mathematical model of the electric and magnetic fields around a conductive cylinder, a shape chosen primarily for its mathematical simplicity. (Physicists choose spherical or cylindrical symmetry anywhere they can, because it makes their job so much easier.) The fact that the cylinder resembled an aircraft fuselage was secondary.
Other visitors to Langmuir have more pragmatic goals. The best way to test a supposedly lightning-hardened design is to trigger lightning onto it and see what happens. Although Sandia National Laboratories in Albuquerque, a major defense research lab, has learned how to trigger lightning, Langmuir is far more experienced in the field and has better facilities and a lot more thunderstorms. So a team of Sandia engineers came to Langmuir late in the summer of 1982 with a "package."
The package came up the mountain early in the morning. Along with it came a dozen or so security officers. Most of them were in uniform and carried assault rifles. Their boss, who wore a suit, left his smile at home, if indeed he owned one. He was big enough to make a good defensive end, so he did not look awkward packing his .44 Magnum in a shoulder holster. The group took possession of the peak of South Baldy and set up their package and a machine gun on the summit. All morning, the students at the lab speculated about the contents of the package. Guidance electronics for a cruise missile? Avionics for an F-14? The arming mechanism for a nuclear weapon?
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ISO-2. John Shortess photograph. |
Few people knew, and they weren't talking. Security is tight at Sandia Labs, located next to Kirtland Air Force Base. The scientists who came with the package wore their security clearance badges at Langmuir. They were not happy about the presence of the French scientists at the site. (The two visiting scientists from the People's Republic of China who were living at the lab had been sent down to Socorro for the occasion.) Lightning was triggered upon the package that afternoon--a fine stroke, measured at seventy-seven kiloamps of peak current--and the Sandians went home. Nobody ever said whether the package had passed its test.
As a rule, the military presence at Langmuir is not nearly that blatant, but lightning research has many military applications. A constant reminder is Kiva II, built next to Kiva I at the behest of the Air Force and the Summa Corporation, an Albuquerque firm. Dr. Carl Baum of Summa Corporation personally inspected the lightning-proofing of Kiva II, right down to specifying what kinds of transient arrestors should be built into the power supply. Baum is a brilliant electrical engineer according to most opinions, including his own. He and Moore enjoy their arguments. The general understanding is that Moore is the final authority atop South Baldy, but that Baum has ways of clamping the Air Force money hose.
Kiva Two is essentially a bigger version of Kiva 1. It is about twenty feet in diameter, round, with a steel roof flush with the surface of the peak. It has two walls of steel with a crawl space in between. The aluminum door is elaborately grounded. Above it, the top of South Baldy is covered with steel wire fencing, with grounding stakes every few feet. The fencing provides an electromagnetically smooth and highly conductive ground plane for the measurements made at the kivas; this becomes important at data-analysis time. The need for the elaborate grounding and transient-suppression schemes inside the kivas is more immediate. The kivas, which are manned throughout the storm experiments, are at the top of South Baldy. Kiva II has rocket launchers on its roof, and lightning is triggered onto a target next to the launchers.
Some of the work done at Langmuir has military applications aside from the obvious usefulness of triggered lightning in equipment testing. One of the effects of a nuclear explosion is a strong electromagnetic pulse, or EMP. Some scientists have speculated that a large hydrogen bomb exploded many miles above the center of the United States could disable non-hardened communications and the nation-s power grid through EMP damage. Lightning produces an EMP--a much weaker one than that of an H-bomb, but good enough for research purposes at short distances. By studying the electromagnetic effects of lightning, defense researchers can get a better idea of what EMP does and how to harden military equipment against it.
Lightning hazards can arise in the most unlikely areas. After the Comprehensive Test Ban Treaty was signed in 1963, outlawing nuclear weapons tests in the atmosphere, undersea, and in space, the United States needed a way to monitor Soviet compliance. An electro-optical system was developed to watch for the bright flash of an illegal nuclear explosion in space. Unfortunately, the system did not work properly, since it could not reliably distinguish a nuclear test from lightning. In 1964, Marx Brook was awarded a contract by the Defense Advanced Research Projects Agency to find a way to make the system more discriminating.
Brook started by studying the differences between a lightning flash and a bomb flash. Using the cameras and microwave instruments in the cupola at Langmuir, he investigated the spectrum of lighting and found that electrical discharges in the presence of water cause the hydrogen atoms in the water to emit light of a specific wavelength. It was also known that another unique wavelength is emitted by nuclear blasts, which provided a double-check. The lightning-filtering system Brook proposed after this research was tested in short order and was quite successful.
Hardening against the thermal and electromagnetic effects of lightning is also important to the explosives industry. Moore, along with Brook and physicist E. Philip Krider of the University of Arizona in Tucson, published A Study of Lightning Protection Systems in 1983. The report resulted from a multi-year study funded by the Defense Munitions Board. The object: to make recommendations for better lightning protection for ordnance plants.
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Triggered lightning strikes.
LSO-2.
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The possibility that an explosives plant might be blown off the map by a lightning stroke has long been recognized. In 1876, in a report to the British Association for the Advancement of Science, physicist James Clerk Maxwell wrote, "What we really wish is to prevent the possibility of an electric discharge taking place within a certain region, say, the inside of a gunpowder manufactory." Modern-day explosives manufacturers agree wholeheartedly. Brook, Moore, and Krider mention two American ordnance plants that blew up after lightning strikes in the 1970s.
The explosions took place under circumstances that kept the national news media away. When asked for more details, Moore, if he trusts his questioner, will give specific facts. Along with the facts he will give a brief lecture about earnest safety officers conscientiously applying what was then the state of the art in lightning protection.
The task facing these safety officers is formidable. Electric companies, for example, have to take measures to protect their equipment against lightning damage. But they use what is called "probabilistic protection." In other words, they take whatever steps they deem sufficient to keep most of the power on most of the time. In deciding, they balance the danger of consumer irritation against the high cost of lightning protection.
As Brook pointed out in the report, the foundation of probabilistic protection is indicated by the name: fuzzy estimates of the probability that lightning will strike the power grid in a certain region. When the stakes are fairly low--some equipment damage, a few individuals complaining because their freezers defrosted, and some time spent in turning the power back on--probabilistic protection is adequate. But in an ordnance plant, it is not good enough at all. If lightning gets into an explosives magazine, it is small comfort indeed to know that the odds were a million to one against it.
When the study began, the three researchers already knew one way to protect explosives plants from lightning. It is simple in theory: put the plant inside a Faraday cage, a completely enclosed and well-grounded metal structure. Maxwell recommended that approach in 1876. But putting a modern factory inside a perfect Faraday cage would be impossible; the electromagnetic pulse of a lightning stroke, for example, can induce a surge on power lines, giving lightning a path into an otherwise well-protected facility. It does not even take a direct hit.
Protection against such damage is the factory's responsibility. The equipment installed by the power company is there to protect the power company; it does very little for the consumer. Designers of lightning protection systems have to take a thousand such things into consideration.
In searching for ways to protect a factory for lightning, they start at the top. Lightning rods, invented by Benjamim Franklin, remain the basic instrument of protection from lightning damage. Franklin himself came up with two very different theories of how the rods might work.
Franklin invented the lightning rod in 1750 after investigating the point-discharge phenomenon, which enabled him to bleed away static charges with a sharp needle. Reasoning that the same principle should apply to charged thunderclouds, he put up sharp lightning rods to discharge the clouds slowly and safely. The idea is still current; a commercial lightning eliminator resembling a knight's mace was tested at Langmuir in 1981. The device was mined by a direct hit, discharging jokes about the lightning eliminator that was eliminated by lightning.
In light of what is now known about lightning, such devices seem ridiculous. Lightning is a large-scale phenomenon that originates many kilometers above the ground, whereas point discharge is strictly a small-scale effect. But this was unknown in 1750; Franklin can hardly be faulted for his erroneous explanation of a valuable invention. He made observations and found out that trying to discharge the clouds would not work. Then he proposed, correctly, that the rods were useful because they intercepted lightning strokes and carried them harmlessly to ground.
But what is the optimum design for a lightning rod? Sharp ones--the kind advocated by Franklin--and blunt ones are available. Both are supposed to intercept lightning, but evidence suggests that sharp rods often fail to do so. Sharp rods on the Apollo 12 gantry, on the Langmuir radar tower, and on one ill-fated explosives factory were bypassed by lightning in favor of lower objects supposedly within the rods' "cone of protection."
It seems that the sharp rods give off such a high point-discharge current that the electric field immediately above the tip is weakened, whereupon the lightning goes to something below that has a stronger electric field around it. The cone of protection supposedly extends downward and outward at a forty-five-degree angle from the tip of a lightning rod. Diagrams have been published showing the cone of protection below such things as radio antennas on boats. But, as Moore pointed out in the January 1983 issue of the Journal of the Franklin Institute, the cone of protection is not something to trust one's life to.
Besides the protection of buildings and aircraft, other, more esoteric problems appear in unexpected areas. For example, the spinning rotors of a helicopter can, under certain circumstances, build up enough electric charge to spell trouble for anyone who grabs the cargo hook. This has nothing to do with lightning, but it is an electrical discharge phenomenon, so Brook is currently investigating it.
Also of concern to the military is the chance that a ground-based particle-beam weapon, something that might be used if the Strategic Defense Initiative or "Star Wars" plan ever comes to fruition, could trigger lightning onto itself by creating an ionized channel to a cloudbase. High-voltage static electricity in all its forms is quirky enough--and dangerous enough--to give plenty of job security to those who would design ways of protecting against it.
Next: Chapter 9 -- Where Do They Go From Here?
Previous: Chapter 7 -- The Dream Thunderstorm