Audacious times generate audacious efforts, especially when national pride and security are perceived to be at stake. Such was the case in the 1950s and 1960s, with the Space Race that started with a Russian sphere whizzing around the planet and ended with Neil Armstrong’s footprint on the Moon. But at the same time, other efforts were underway to answer big questions of national import, such as determining how durable the United States’ strategic assets were, and whether they could withstand the known effects of electromagnetic pulse (EMP), a high-intensity burst of electromagnetic energy that could potentially disable a plane in flight. Finding out just what an EMP could do to a plane would take big engineering and a large forest’s worth of trees.
Planes and Pulses
Like most Cold War projects, the Air Force Weapons Lab Transmission-line Aircraft Simulator, or ATLAS-1, was enormous in every way. As much a work of deterrence theatre as it was a serious project to measure hardening of strategic assets, ATLAS-1 had to be big. By the time it was conceived in the late 1960s, much was known about the effects of EMP on civilian and military hardware. Having been somewhat accidentally discovered in 1962 during the Starfish Prime atmospheric nuclear tests in the Marshall Islands, which produced an EMP strong enough to disrupt electrical systems in Hawaii, EMPs were instantly recognized as a threat to military hardware that had to be dealt with.
Testing on EMP hardening of military hardware, primarily aircraft, was conducted at a feverish pace throughout the 1960s, mainly at Kirtland Air Force Base outside of Albuquerque, New Mexico. The basics of testing were simple enough — build an electrical pulse generator big enough to simulate an EMP, aim it at an aircraft, and see what happens. At least 18 separate EMP test facilities were built, each designed to test different parameters as aircraft systems became more complicated with the introduction of avionics and flight computers.
But almost all of these test stands suffered from a basic flaw — the aircraft under test was parked on the ground. This subjected them to a rapid double pulse of EMP, one incident from the pulse generator and another an instant later as the primary pulse was reflected by the tarmac. It could be possible that the double whammy would look powerful enough to kill an aircraft, while the same pulse in free space would leave the plane unharmed.
Short of detonating a nuke in the atmosphere and watching what happens to planes, there are only two ways to test EMP on aircraft in flight: move the plane away from the ground, or move the ground away from the plane. Dr. Charles Baum, at the time a captain in the Air Force, was in charge of development of ATLAS-1, and he realized that the latter option was the more practical. He envisioned a large platform built entirely of dielectric material so as not to couple with incident EMP. The platform would be built to take advantage of natural features of the landscape so that the ground would be as far below the aircraft as possible, making it look electrically as if the plane were in flight.
As simple as it sounds, the engineering behind ATLAS-1 was challenging, to say the least. Considering that the primary goal of the project was to test strategic assets like the B-52 Stratofortress and the E-4 National Emergency Airborne Command Post (NEACP), the modified Boeing 747 that serves as the “Flying White House” in times of emergency, the platform would have to be enormous. Given the weight of aircraft like these, the structure would also have to be enormously strong. Add in the fact that the supporting structure could contain no pieces of metal more than a few inches long, and some creative thinking was going to be necessary.
While surveying potential sites at Kirtland for ATLAS-1, Dr. Baum’s team identified a natural arroyo near the end of one of the runways. The site reminded Dr. Baum of pictures of the old West, with railroads flying across sagebrush-covered valleys on sturdy trestles built of massive timbers. He reasoned that if a wooden trestle could be built to support a loaded train, certainly one could support a B-52. Thus was born the Trestle.
The Trestle would take years to design and build. Everything about it was enormous — 200 feet (61 m) square, 120 feet (37 m) off the floor of the excavated arroyo, with a ramp 50 feet (15 m) wide and 400 feet (122 m) long to wheel planes onto it. Built almost entirely of Douglas Fir and Southern Yellow Pine, the lumber used in the Trestle and associated structures totaled 6.5 million board feet (154,000 cubic meters), consuming a sizable fraction of the output of the sawmills of the Pacific Northwest and Georgia for many months.
The timbers specified by the structural engineers were often far too large to be milled from a single tree — some were as thick as 40 inches (1 m). These timbers were produced by glue-lamination, where thinner boards are glued up under heat and pressure to form massive elements. Some were so large that they took three railroad cars to transport and were placed with 20-ton cranes.
Assembling these massive timbers into a structure that would be essentially transparent to radio frequency waves required innovative joinery. We’re all used to seeing wooden roller coasters, with thousands of timbers bolted together with galvanized bolts that are inspected and tightened regularly. But the Trestle needed to have no parts that could reflect the EMP back toward the plane, so huge bolts were out. Instead, dielectric fasteners on wood and fiberglass were used, in addition to small steel split rings set into joints to reinforce them.
Even though the highly combustible structure was engineered to stand with a plane on it for four hours after catching fire, there was a suppression system laced throughout the timbers, which was, of course, entirely non-metallic. The result of all this engineering was a platform strong enough to park a B-52 on with its engines running, large enough to allow the plane to be positioned at multiple angles to the EMP, and still be electrically invisible.
Something in the Gigawatt Range
But the Trestle itself was not the only huge aspect of ATLAS-1. The whole point of the thing was to bathe aircraft in extremely high powered EMP, and the generators, or pulsers, used to accomplish that earned superlatives of their own. Housed in fiberglass structures filled with sulfur hexafluoride and sitting high above the end of the platform on their own wooden trestles, the dual EMP generators flanked a wedge-shaped steel structure that pointed directly at the platform.
The Wedge was the only conductive structure near the test area and served as a ground plane for the Marx generators, which used banks of capacitors to discharge up to 10 megavolts into transmission lines strung above and to the sides of the platform. The transmission lines were connected to yet another wooden structure at the far end of the ramp that housed an enormous resistive load with a 50-ohm nominal impedance. The termination was considerably larger than the dummy load a ham might use to tune an antenna, though; the generators were capable of producing 200-gigawatt pulses lasting as little as 100 nanoseconds, which was perfect for simulating the effects of a nuclear-initiated EMP.
Simulations Always Win
Testing on planes began in 1980. Loading the enormous warbirds onto the platform was the most dangerous part of testing, especially for the B-52; while it’s belly landing gear was narrow enough to fit on the ramp, the outrigger wheels near the ends of the 185 foot (56 m) wingspan dangled over the arroyo, giving no support to the plane if it was to suddenly roll. Despite the dangers, no serious injuries were ever reported during the life of ATLAS-1. The facility conducted hundreds of tests on dozens of aircraft, including the B1B Lancer, every jet fighter from the F-14 to the F/A-18, and most of the aircraft of the Presidential Airlift Group.
Progress would catch up with the Trestle, both in terms of politics and technology. By 1991, with the Cold War over and improvements in computers allowing better simulations, the need for destructive EMP testing was gone. The Trestle still stands in the arroyo to this day, baking in the high New Mexico desert. Efforts are afoot to preserve it as a national monument, which only seems fitting given how much engineering went into it.