Shoot 'Em Up
Sometimes you have to destroy the aircraft in order to save it.
- By Carl Hoffman
- Air & Space magazine, November 2002
(Page 3 of 4)
Long before the projectiles start flying, the engineers at China Lake review a STAR—system threat assessment report—which outlines the threats an aircraft is expected to face in combat. (Only threats that have a less-than-100-percent chance of a kill are tested.) Using computer models, engineers determine the paths of specific shots and which shot lines would “cause the aircraft to die,” as Tyson puts it. “We ask the modelers to figure out how to hit the component, and just what the probability [of that part’s failure] is if you’re flying at certain speeds, altitudes, and angles.” Then comes the delicate balancing act: Engineers must design ways to shoot bullets and missiles and explode fragments at aircraft operating under conditions that are life-like yet so precisely controlled that the tests don’t destroy the test article too soon.
Tyson began testing components of the Super Hornet in 1993, long before the first one was built. Live-fire tests on earlier F/A-18 versions had identified persistent fire problems, especially from shots to the fuel tanks along the airplane’s keel. To experiment repetitively with tests that might eat airplanes like a kid eats cookies, Tyson and a team of engineers at the lab built a full-size steel replica of the F/A-18 belly, using a design created by Northrop Grumman and imitating the Super Hornet’s fuel cells and dry bays (empty spaces adjacent to fuel tanks through which fuel lines pass). They mounted the replica in the lab’s giant high-velocity-airflow system, which uses four jet engines to mimic inflight airflows of up to 500 knots (575 mph) over various parts of the airplane, to study how fire spread in the vicinity of the tanks. Their eventual solution: a fire protection system in which a small rocket motor floods the bays with inert gases, a system similar to that which inflates car air bags. Today, the Super Hornet and V-22 Osprey are the first aircraft to have full dry-bay fire protection. In 1996 Tyson got a full-size wing, a year later he got an engine, in 1998 he got four F/A-18As to play with, and six years after starting he got his first genuine F/A-18E, a now-blackened boneyard hulk nicknamed Christine, after the indestructible vintage car in the Stephen King novel of the same name. But by that time all the development work had been done; Christine merely verified it.
“We did a series of seven tests on her,” Tyson says, leading me around the airplane, “and you can see different areas that have been impacted.” That’s an understatement. One wing’s leading edge has a hole wide enough to step through, more holes riddle the engine nacelles and intakes, and the belly is as blackened as the inside of a fireplace.
Tyson’s long series of tests—622 shots in seven years—identified not only the repercussions of bullet-ignited fires in the fuel tanks, engine nacelles, and dry bays but also a weakness on the horizontal stabilator’s attach points. All the components were redesigned, and Tyson shows me a video of the results. Christine is mounted on the test pad and air is flowing around her at several hundred miles an hour; bullets punch through the airplane; fires flare in the racing wind, then miraculously disappear. Cameras mounted inside the wings and fuel tanks show blackness, roaring fire, and then blackness again—all in half a second.
Between the Super Hornet and a Vietnam-era F-4 Phantom, there is no comparison: A Super Hornet has self-sealing polyurethane fuel tanks located away from ignition sources; short, self-sealing feed lines; redundant fuel pumps; wing tanks lined with open-cell foam; fire extinguishing systems in its dry bays; fire walls between the engine and the auxiliary power unit; redundant flight control computers with four separated electrical signal lines to actuators; and redundant, independent, and separated hydraulic power systems. Despite being 25 percent larger than the earlier F/A-18 Hornet, the Super Hornet’s vulnerable area is the same. Says Tyson: “The F/A-18E/F is the most thoroughly tested and aggressively protected tactical aircraft in the U.S. inventory.”
Yet only 34 of the 622 shots in the Super Hornet survivability test program were shot at a genuine Super Hornet, and even then, the aircraft was never loaded with the munitions it would carry to battle. This testing history highlights an important part of the legislation requiring live-fire testing: The 1986 Live Fire Test Law allows a waiver from realistic, full-up, systems-level testing if it would be “unreasonably expensive and impractical.” Had E/F hardware been used exclusively, according to live-fire test engineers, the tests would have cost several millions more than the $60-million-plus spent on the program, which, they say, met the ultimate goal—understanding the vulnerability of the aircraft’s various systems.
“In order to be granted a waiver,” notes Tim Horton, the head of the Survivability Division at China Lake, “the service, the defense department, and Congress must first approve a comprehensive alternative to a full-up test program that ensures the system will be tested adequately to meet both the spirit and intent of the law. In the case of the F/A-18E/F, a waiver was approved at all levels.”
But to Jim O’Bryon, the waiver process is a loophole, through which every aircraft program has been slipped since passage of the law. “What you want to learn in live fire is what you’d learn on the first day of combat,” says O’Bryon, “but the services hold that if they test all the pieces and use modeling and simulation, that means they’ve tested it all. But it’s not true. Not a single model based on physics exists today that can predict the effect of fire, the number-one killer. And you can’t do user casualty estimates from doing component testing. Can you predict how a car is going to react in a crash by testing the bumper alone? You have to test the whole thing.”