Much of NASA’s aerospace research overlaps various fields. For example, improving EMP tolerance of space-based systems involves studying plasma interactions in a high-voltage system operated in the ionosphere. But a related subject is establishing design practices that may have previously increased adverse plasma interactions and recommending means of eliminating or mitigating such reactions in future platforms.
Standards for lightning protection tests were developed in the 1950s, under FAA and Department of Defense (DOD) auspices. Those studies mainly addressed electrical bonding of aircraft components and protection of fuel systems. However, in the next decade, dramatic events such as the in-flight destruction of a Boeing 707 and the triggered responses of Apollo 12 clearly demonstrated the need for greater research. With advent of the Space Shuttle, NASA required further means of lightning protection, a process that began in the 1970s and continued well beyond the Shuttle’s inaugural flight, in 1981.
Greater interagency cooperation led to new research programs in the 1980s involving NASA, the Air Force, the FAA, and the government of France. The goal was to develop a lightning-protection design philosophy, which in turn required standards and guidelines for various aerospace vehicles.
NASA’s approach to lightning research has emphasized detection and avoidance, predicated on minimizing the risk of strikes, but then, if strikes occur nevertheless, ameliorating their damaging effects. Because early detection enhances avoidance, the two approaches work hand in glove. Translating those related philosophies into research and thence to design practices contains obvious benefits. The relationship between lightning research and protective design was noted by researchers for Lightning Technologies, Inc., in evaluating lightning protection for digital engine control systems. They emphasized, "The coordination between the airframe manufacturer and system supplies in this process is fundamental to adequate protection.” Because it is usually impractical to perform full-threat tests on fully configured aircraft, lightning protection depends upon accurate simulation using complete aircraft with full systems aboard. NASA, and other Federal agencies and military services, has undertaken such studies, dating to its work on the F-8 DFBW testbed of the early 1970s, as discussed subsequently.
In their Storm Hazards Research Program (SHRP) from 1980 to 1986, Langley researchers found that multiple lightning strikes inject random electric currents into an airframe, causing rapidly changing magnetic fields that can lead to erroneous responses, faulty commands, or other "upsets” in electronic systems. In 1987, the FAA (and other nations’ aviation authorities) required that aircraft electronic systems performing flight-critical functions be protected from multiple-burst lightning.
At least from the 1970s, NASA recognized that vacuum tube electronics were inherently more resistant to lightning-induced voltage surges than were solid-state avionics. (The same was true for EMP effects. When researchers in the late 1970s were able to examine the avionics of the Soviet MiG-25 Foxbat, after defection of a Foxbat pilot to Japan, they were surprised to discover that much of its avionics were tube-based, clearly with EMP considerations in mind.) While new microcircuitry obviously was more vulnerable to upset or damage, many new-generation aircraft would have critical electronic systems such as fly-by-wire control systems.
Therefore, lightning represented a serious potential hazard to safety of flight for aircraft employing first-generation electronic flight control architectures and systems. A partial solution was redundancy of flight controls and other airborne systems, but in 1978, there were few if any standards addressing indirect effects of lightning. That time, however, was one of intensive interest in electronic flight controls. New fly-by-wire aircraft such as the F-16 were on the verge of entering squadron service. Even more radical designs—notably highly unstable early stealth aircraft such as the Lockheed XST Have Blue testbed, the Northrop Tacit Blue, the Lockheed F-117, and the NASA-Rockwell Space Shuttle orbiter— were either already flying or well underway down the development path.
NASA’s digital fly-by-wire (DFBW) F-8C Crusader afforded a ready means of evaluating lightning-induced voltages, via ground simulation and evaluation of electrodynamic effects upon its flight control computer. Dryden’s subsequent research represented the first experimental investigation of lightning-induced effects on any FBW system, digital or analog.
A summary concluded:
Results are significant, both for this particular aircraft and for future generations of aircraft and other aerospace vehicles such as the Space Shuttle, which will employ digital FBW FCSs. Particular conclusions are: Equipment bays in a typical metallic airframe are poorly shielded and permit substantial voltages to be induced in unshielded electrical cabling. Lightning-induced voltages in a typical a/c cabling system pose a serious hazard to modern electronics, and positive steps must be taken to minimize the impact of these voltages on system operation. Induced voltages of similar magnitudes will appear simultaneously in all channels of a redundant system. A single-point ground does not eliminate lightning-induced voltages. It reduces the amount of diffusion-flux induced and structural IR voltage but permits significant aperture-flux induced voltages. Cable shielding, surge suppression, grounding and interface modifications offer means of protection, but successful design will require a coordinated sharing of responsibility among those who design the interconnecting cabling and those who design the electronics. A set of transient control levels for system cabling and transient design levels for electronics, separated by a margin of safety, should be established as design criteria.
The F-8 DFBW program is the subject of a companion study on electronic flight controls and so is not treated in greater detail here. In brief, a Navy Ling-Temco-Vought F-8 Crusader jet fighter was modified with a digital electronic flight control system and test-flown at the NASA Flight Research Center (later the NASA Dryden Flight Research Center). When the F-8 DFBW program ended in 1985, it had made 210 flights, with direct benefits to aircraft as varied as the F-16, the F/A-18, the Boeing 777, and the Space Shuttle. It constituted an excellent example of how NASA research can prove and refine design concepts, which are then translated into design practice.
The versatile F-106B program also yielded useful information on protection of digital computers and other airborne systems that translated into later design concepts. As NASA engineer-historian Joseph Chambers subsequently wrote: "These findings are now reflected in lightning environment and test standards used to verify adequacy of protection for electrical and avionics systems against lightning hazards. They are also used to demonstrate compliance with regulations issued by airworthiness certifying authorities worldwide that require lightning strikes not adversely affect the aircraft systems performing critical and essential functions.”
Similarly, NASA experience at lightning-prone Florida launch sites provided an obvious basis for identifying and implementing design practices for future use. A 1999 lessons-learned study identified design considerations for lightning-strike survivability. Seeking to avoid natural or triggered lightning in future launches, NASA sought improvements in electromagnetic compatibility (EMC) for launch sites used by the Shuttle and other launch systems. They included proper grounding of vehicle and ground-support equipment, bonding requirements, and circuit protection. Those aims were achieved mainly via wire shielding and transient limiters.
In conclusion, it is difficult to improve upon D. L. Johnson and W. W. Vaughn’s blunt assessment that "Lightning protection assessment and design consideration are critical functions in the design and development of an aerospace vehicle. The project’s engineer responsible for lightning must be involved in preliminary design and remain an integral member of the design and development team throughout vehicle construction and verification tests.” This lesson is applicable to many aerospace technical disciplines and reflects the decades of experience embedded within NASA and its predecessor, the NACA, involving high-technology (and often high-risk) research, testing, and evaluation. Lightning will continue to draw the interest of the Agency’s researchers, for there is still much that remains to be learned about this beautiful and inherently dangerous electrodynamic phenomenon and its interactions with those who fly.
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