Fly by Light

March 1, 1988

Fiber optic flight-control sys­tems are finally coming to the Air Force. After more than a decade of research—while the other ser­vices forged ahead in implementing this new technology—the Air Force is now ready to take fiber optics out of the laboratory and put it into two of its top aircraft programs for the 1990s—the Advanced Tactical Fighter (ATF) and the X-30 Nation­al Aerospace Plane (NASP) being developed in cooperation with NASA.

The Air Force originally took the lead in this technology. The Flight Dynamics Laboratory at Wright-Patterson AFB, Ohio, awarded a development contract to Honeywell in 1977 for the DIGITAC (digital tac­tical aircraft control) program to demonstrate the use of advanced computers and data buses in opera­tional aircraft. First flight of the A-7D test aircraft was on February 7, 1975, and on March 24, 1982, at Edwards AFB, Calif., it made the first totally fiber optic-controlled flight using a single-fiber flight-con­trol system.

DIGITAC and the parallel re­search programs of the other ser­vices have demonstrated three ma­jor advantages of fiber optics for airborne applications: reduced weight, greater data-handling capa­bility, and immunity to electromag­netic interference (EMI) and the electromagnetic pulse (EMP) caused by nuclear blasts. Conserva­tive aircraft designers were reluc­tant to gamble on this new technolo­gy, however, until there was suffi­cient flight experience.

Now, with the DIGITAC experi­ence behind them, the ATF and X-30 program managers at Aero­nautical Systems Division (ASD) say that they are confident enough about fiber optics to specify it for their programs, although not neces­sarily for the flight controls.

“Fiber optic technology has ma­tured to a point where use for both multiplex and point-to-point data paths is considered an acceptable risk in view of fiber’s advantages over older wire circuits,” Col. (Brig. Gen. selectee) James A. Fain, Jr., ATF system program director, told AIR FORCE Magazine. Dr. Robert R. Barthelemy, NASP program manager at ASD, added that the high data rates expected to be required for the X-30 (more than five times greater than those for the F- 15 and F-16) would require fiber optic data links and possibly also a new generation of optical computers using photons rather than electrons for data processing.

In both cases, the principal factor in favor of fiber optics is improved data handling. “The modular, inte­grated avionics architecture of the ATF relies on a high-speed data bus [HSDB] to interconnect avionics functions,” Colonel Fain said. “Data rates up to 50,000,000 bits per second are projected. Both of the contractor teams for the current ATF demonstration/validation phase [Lockheed teamed with Boe­ing and General Dynamics and Nor­throp teamed with McDonnell Douglas] have adopted a fiber optic HSDB in their designs.”

“Both the ATF contractor teams are investigating issues like the protocol and the use of active or passive couplers,” added an ASD spokesman. “This is not regarded as a significant technical risk area.”

Moving Ahead on the X-30

Dr. Barthelemy said that the X-30 program office was currently con­centrating on the propulsion system for the hypersonic aerospace vehi­cle. “We’re at the stage of the pro­gram where we’re just beginning to look at the controls and the commu­nications . . . but the good news is that the technology is moving an order of magnitude every five years,” he said.

With last fall’s selection of the major X-30 contractors (General Dynamics, McDonnell Douglas, and Rockwell International on the airframe and Rockwell/Rocketdyne and United Technologies/Pratt & Whitney on the propulsion system), Dr. Barthelemy expects a technolo­gy readiness review by 1990 and first flight in early 1993. That means that the technologies eventually em­ployed will have to be in hand by 1990. That’s going to be tougher for the propulsion and the airframe ma­terials than for the avionics, accord­ing to Dr. Barthelemy, but he’s not ready to call avionics “the short pole in the tent.”

The other two features of fiber optics—weight savings and EMI/ EMP immunity—will also be impor­tant for the X-30, Dr. Barthelemy contends. Aerospace vehicles de­rived from the X-30 are expected to be used as test-beds for the Strate­gic Defense Initiative, and that means they will have to reduce the cost of placing payloads in low earth orbit by a factor often, according to Dr. Barthelemy—from about $4,000 per pound for today’s Space Shuttle to $400 for the hypersonic vehicles of the future. Weight may be an even more critical factor for this applica­tion than for tactical aircraft.

The first operational use of fiber optics in an aircraft was achieved by the Marine Corps in its AV-8B ground support aircraft, although this was strictly for data handling and not for flight control. In-house studies by McDonnell Douglas in 1977, followed by flight tests in 1981, led in 1983 to the first produc­tion deliveries of aircraft equipped with fiber optics.

The purpose wasn’t to reduce weight or save money, according to Gus Weinstock, electronics tech­nology branch chief at McDonnell Douglas, but to prove this technolo­gy would work in aircraft. This first installation was only thirty-three feet long, connecting a digital map set used in the navigation system to a cockpit panel, and had a data rate of only 125,000 bits per second.

The most useful result of this effort, Mr. Weinstock recalls, is that it demonstrated that maintenance personnel could successfully work with the tiny fiber cables. The ca­bles were subjected to a worst-case environment—including gasoline, oil, and other contaminants—and they still worked after they were in­stalled.

Potential Pitfall

Mr. Weinstock tells a story on himself that illustrates a potential pitfall of using fiber optics. The fiber optic cable assemblies had to be shipped to AV-8B co-prime contrac­tor British Aerospace for installa­tion, and Mr. Weinstock had them cut an extra foot long to be sure they would fit into the aircraft. As it turned out, that extra foot wasn’t necessary and actually caused the cables to fail because the installers had to crimp the cables to jam them into the available space. All cables are now cut to the exact length.

McDonnell Douglas is consider­ing the use of fiber optics in the F/A-18 Navy tactical aircraft and a night attack version of the AV-8B. However, the next major airborne application of fiber optics, this time in a full flight-control mode, will be in the Navy’s new blimp, known as the Navy Airship Study Program (which also goes by the acronym NASP).

This program, for which a consor­tium of Westinghouse and the Brit­ish firm Airship Industries won de­velopment contracts totaling $169 million last summer, will produce a prototype model of a new-technolo­gy airship capable of housing along-range over-the-horizon radar to warn ships of incoming cruise mis­siles. The basic idea is to find a bet­ter way to detect these missiles be­fore it’s too late, and that means spotting them over the horizon at the time of launch.

“These new airships represent the cutting edge in airship technolo­gy,” says J. W. Phipps, President of Westinghouse-Airship Industries. “The difference between these new ships and the lumbering old blimps you are used to seeing at football games is like comparing a World War II airplane to modern jets now active with US forces.”

To improve performance and reduce vulnerability, the Wes­tinghouse-AI team chose fiber op­tics for both the flight controls and for the data bus to transmit data from the radar to the central processors. The purpose is to provide two critical capabilities: greater bandwidth than conventional cop­per cabling and virtual invisibility to enemy radars since there are no electromagnetic emissions.

NASP development is due to be completed within five years, and the first flight of the fiber optic-con­trolled airship is scheduled to take place before 1991. The contracts contain options for up to five addi­tional operational development model airships.

Beyond the Navy airships, the leading candidate for the next major application of fiber optic flight con­trols is the Army’s proposed new class of helicopters, the LHX (for Light Helicopter, Experimental, al­though the Army would like to re­name it the ATH, for Advanced Tac­tical Helicopter, in order to give it the same status as the Air Force’s ATF and the Navy’s Advanced Tac­tical Aircraft, or ATA). Flight dem­onstration tests have been under way since 1985, using a modified UH-60A Black Hawk helicopter, under the Army’s Advanced Digital/ Optical Control System (ADOCS) program. Boeing is the prime con­tractor.

ADOCS, if successfully imple­mented in LHX, would give a major boost to the idea of controlling an aircraft totally (that is, without a mechanical or electrical backup) by light. The problem isn’t with the technology, however, but with the whopping $66 billion price tag for the LHX program.

The Army insists it needs a new generation of helicopters to replace the more than 4,000 still in its in­ventory from the Vietnam era, but Congress has consistently cut off funds. LHX would advance tech­nology across a broad front, includ­ing an automated cockpit based on artificial intelligence concepts, but its future is uncertain. The program was recently cut in half and may be deleted altogether.

Reducing Cost and Complexity

Still, the Army is optimistic about fiber optic technology for future military aircraft. “I think fiber op­tics will give the protection we’re looking for in the future,” noted Joel L. Terry, Jr., team leader for flight control in the Army’s Aviation Ap­plied Technology Directorate, Fort Eustis, Va., “but first we’ve got to do a lot of work to get the cost and complexity of the transducers re­duced.

“It’s just about like working with fly-by-wire, except right now we’re being careful with the fibers. They’re a little more delicate,” he added.

Although the Air Force did ex­periment with optical data links in the YC-14 prototype short takeoff and landing (STOL) transport, first flight-tested in 1976, most of the ser­vice’s experience with fiber optics has been in nonflight applications. Such experience includes the in­stallation of a 147-kilometer fiber optic cable at the Missile Test Cen­ter at Vandenberg AFB, Calif., to serve as the primary communica­tions link for controlling ground and flight tests of the Peacekeeper, com­munications links for the ground-launched cruise missile (GLCM), and the AN/GRC-206 tactical radio for forward air controllers.

In the GLCM deployment in En­gland, each flight of missiles con­tains two control vans and four launchers—all interconnected via a redundant network of optical fiber cables. The launchers, located 300 meters away from the vans, receive checkout and firing commands via two six-channel fiber cables.

The GRC-206 radio uses two-channel fiber optic cables in one-kilometer lengths to link jeep-mounted radios to headquarters op­erations located behind the forward edge of the battle area (FEBA).

In a parallel development, Wes­tinghouse-designed fiber optic ca­bling systems were delivered to the air forces of Australia and Egypt to disperse elements of air defense ra­dar systems and thus reduce their vulnerability to homing antiradia­tion missiles.

Many of these initial applications were made possible by using the mature multimode technology of fiber optics, in which many rays of light are transmitted along the op­tical fiber, or waveguide. The Navy, for example, has used what is known as the large core fiber (LCF), in which a 100-micron fiber operat­ing at the standard 850-nanometer wavelength is enclosed in a 140-mi­cron cladding. This was the type of fiber used in the AV-8B.

This approach is particularly good for such short lengths as those needed for flight controls and other avionics applications, because the LCF is easier to connect and splice. Other military users have begun using a smaller, more efficient vari­ety of multimode fiber with a core diameter of fifty microns and clad­ding of 125 microns.

But the availability of single-mode optical fibers offers even greater advantages: reduced at­tenuation that makes possible lon­ger distances between repeaters; greater bandwidth, allowing further size and weight reductions and sys­tem upgradability; and improved ra­diation hardening.

Single-Mode Fibers

To understand the significance of single-mode fibers compared to the original multimode, consider the basic processes involved. Optical fibers are made of liquid silicon and germanium tetrachloride and then drawn into fine strands to achieve unprecedented levels of transpar­ency.

A pane of ordinary window glass an inch thick permits half the light to pass through it, and high-quality op­tical glass, such as that used for eye­glasses and microscopes, can be ten feet thick before half the light is dis­persed or absorbed. For optical fibers, the comparable figure is two and a half miles for multimode and twelve miles for single-mode.

Single-mode fibers, as the name implies, use a small optical core to carry a single ray of light, which greatly reduces signal distortion in digital and analog systems. Single-mode fibers operate in the regions of minimum signal loss, either 1,300 or 1,550 nanometers.

In addition to the lower attenua­tion and therefore greater distances between repeaters, single-mode fibers are also more radiation-re­sistant than the earlier multimode varieties. The reason is that single-mode technology requires less dop­ant to be added to the silica-core matrix, with the result that less color darkens the fiber. A radiation dose of 3,700 rads on a multimode fiber results in signal loss of about twelve dB some ten seconds after exposure, which effectively shuts down the system. For a single-mode fiber under the same conditions, the loss is less than three dB.

With regard to EMI, signals can be transmitted over fiber through electrically noisy areas with ex­tremely low bit error rates and with no possibility of electronic jam­ming. This is particularly important for aircraft and other weapons plat­forms. It also has the added advan­tage of enabling equipment to oper­ate during thunderstorms, around air bases, and even on the battle­field.

Optical fibers also have two big advantages in a nuclear environ­ment. The first is their EMP immu­nity, which allows signals to be transmitted following a nuclear event. Destructive high-energy voltage and current pulses do not couple into the receivers and trans­mitters, thus preserving their func­tionality. The second is optical fibers’ ability to recover within min­utes of exposure to high-radiation weapons bursts.

Fiber optics offer yet another useful property for military applica­tions. Because they use photons rather than electrons, they do not pose electrical shock or fire haz­ards. This safety factor enables them to be used near ammunition storage areas and fuel tanks.

Summing up the advantages of fiber optic flight-control systems, H. A. Rediess and E. C. Buckley of HR Textron, Inc., Irvine, Calif., in a study conducted for NASA’s Lang­ley Research Center, concluded: “The higher data rates afforded by fiber optics will enhance the system capabilities and design options, par­ticularly in the area of highly redun­dant, fault-tolerant architectures. Increased use of composite mate­rials in airframes would also moti­vate use of fiber optics because of the loss of shielding now provided by the metallic skin.”

What’s the Down Side

Any technology that sounds this good must have a down side, and fiber optics is no exception. The Langley-sponsored study identified four: high costs, specialized train­ing for repair and maintenance, low tensile strength of the fibers, and potential signal losses, particularly at the connectors.

As fiber optics usage grows—in both the military and commercial market sectors—costs should con­tinue to decline along the classical curve previously demonstrated by the semiconductor and other high-technology industries. This will en­able military program managers to use off-the-shelf commercial prod­ucts without the expense and delays of custom designs—and with the as­surance that alternate sources of supply will be available.

This is not happening yet—in fact, Kessler Marketing Intelligence of Newport, R. I., estimates that the military is paying a premium of fifty-six cents a meter for multi-mode fiber vs. an industry average of fifty-one cents—but it should happen once there is sufficient vol­ume to force standardization and thus open the way for off-the-shelf procurement. Kessler is predicting a drop to forty-eight cents a meter this year for multimode fiber.

Regarding training, Rediess and Buckley comment, “The training re­quired for repair and maintenance is temporal, and the special tools will become commonplace.” Also, ten­sile strength is steadily rising. The theoretical tensile limit for silica-clad fiber is more than 800,000 pounds per square inch. Fiber op­tic companies have begun supply­ing long lengths of fibers at 400,000 psi values. And signal loss is being reduced by shifting transmission to the higher wavelengths and using improved connectors.

Mark Landin, senior sales engi­neer in Corning Glass Works’s Gov­ernment and Military Advanced Fiber Products Department, warns of another potential problem: exces­sive heat. Fiber optics used in to­day’s operational systems meet the + 85° C. to – 60° C. temperature re­quirement, which is adequate for tactical aircraft and blimps but may not do the job for the temperature extremes that the X-30 and its deriv­atives are likely to encounter. One solution, according to program manager Barthelemy, is improved cooling within the vehicle by using the slush liquid hydrogen fuel. Mr. Landin reports that fibers capable of withstanding temperatures up to 200° C. have been produced by using Teflon coatings on the fibers.

But as with any new technology, time is on the side of the users. As the HR Textron analysts put it, “The disadvantages of using optics are partially temporal and will be mini­mized as the technology matures and experience is gained.”

John Rhea is a free-lance writer living in Woodstock, Va. He has written about technology issues for military publications in this country and overseas and is currently the editor of Space World. His most recent article for this magazine, “Sensors Across the Spectrum,” appeared in the November 87 issue.