Armament and Weapons

F-106 Gunsight Development by John E. Mantei

I flew every mission for the initial F-106 Gunsight development during the summer of 1972 at Tyndall Air Force Base. The M-61 gun had been installed on one F-106 earlier, but no one could come up with a computing site that would fit in the cockpit.

The whole program was about to be shut down. But I convinced the squadron commander that it would be worth trying a new approach. He was given the authority to continue the program using my concept. The rest of the squadron was against me because "people a lot smarter than me had said my approach was not feasible". They overlooked the fact that I had access to some very smart people that were working in the area of software that would make it possible to efficiently calculate the ballistic solution very accurately. Previous gun sights, including the F-15, were analog devices in a large "black box" that had significant lag and therefore not very accurate in a dynamic dogfight. Even though range to the target is a large variable, only a crude estimate was provided.

The very smart people were instructors at the USAF Academy. They had triple master's degrees from MIT. One of them had written a paper on their concept for a digital computation that could generate a continuous solution showing the actual bullet flight path. It would use and compare relevant parameters from various electronic components already in the fire control system. This comparison technique (Kalman filtering) made the system very accurate and required no additional avionics. I called them and they said they and MIT instructors were working on the aiming system for the side-firing C-130 gunship at Eglin AFB. I met them there and we agreed the F-106 avionics could probably handle the ballistics calculations.

The digital computer on the F-106 was very old technology by 1972, but it was designed to calculate the ballistics of the large Genie rocket, the primary weapon on the F-106. The gun could easily be mounted in place of the Genie; and it was possible to add the parameters for the 20 mm ammunition to the software. Only target practice ball ammo was fired during the project.

My approach was to project a display from the existing radar scope to a combining glass mounted above. The squadron technical support group built a simplified radar scope projection housing for me. Airborne, one significant issue was obvious. The radar scope was backlit by a flood gun in the CRT. This light projected to the combining glass like a bright full moon that could obscure the target. Luckily the resident avionics company representative talked to just the right person at the factory who gave us a low-voltage way to turn off the flood gun. Other optical properties of the existing system were very good.

The Academy folks gave a contract to a small company owned by an ex-MIT instructor who hired MIT instructors to work with me at Tyndall during the summer. One part of the contract team developed the hardware off site, while the USAFA/MIT team developed the software. The hardware consisted primarily of a periscope that projected the radar scope face up to the combining glass. By the time we were ready to start the flight test, many of the faculty folks had to return to their schools. So I worked with one or two MIT people. The genius that wrote the software, Dr. Potter, frequently modified it during the flight test. He did not change the calculations; only the display and the switcholigy. He worked at night on the standard F-106 avionics maintenance work station.

We would meet first thing in the morning to explain the changes he had made. He would go to bed during the day while I would fly one or two test flights. Then we would meet in the evening to talk about the results and additional refinements. He never made an error that I could detect in the air. On-board instrumentation helped verify firing conditions like g's, angle-off, and range. The contractor was unable to provide an integrated heads-up and heads-down camera so we obtained bullet stream images from a high-speed camera hung on an F-101 flying in formation with me. I had to tell the F-101 pilot when I was about to shoot because the high speed camera would run through all the film in a hurry. We doubled the number of tracer rounds to help with the photo coverage.

We used a new target that was still in development for most of the flights called “Figat”. The F-15 test program at Edwards Flight Test Center also used it. It was shaped like the standard dart but was about three times as large. F-4's could carry it on a standard bomb rackbut Tyndall had no F-4's. So my supervisor, Walt Davis, and the special devices shop put steel skids on the target and we drug it off the runway with a cable behind an F-101. The Figat developer said it could not be shot down with ball ammunition. When word got out about a few spectacular landings, quite a crowd would show up for them. The Figat had a recovery parachute and a Doppler bullet counter. I also shot at two Firebee drones. The drones could pull more g's than the towed targets but were very small. Shooting at the drones was the equivalent of shooting at a target the size of an F-106 radome.

Tensions were high at Tyndall that summer. At our one-and-only test planning meeting, one of the good-ole-boys recently assigned to ADC Ops. said to me: we know this project is going to fail and we are going to make sure you get blamed. After the success on the first round of flights, we went to higher g levels. Soon I noticed a difference in the recorded data and what I was seeing in the cockpit. The data was about one g lower. Something was loading down the signal going to the computer. My favorite flight-test engineer, Capt. Gamble, agreed to check it out. Sure enough, he found an impedance mismatch between the instrumentation and the aircraft sensor signal. He redesigned the instrumentation card and we finished buttoning up my aircraft about two a. m. The effects of that change caught me by surprise on the flight later that morning. The display was much more dynamic and my scores dropped significantly. Ironically, the one star general from ADC headquarters, head of operational squadrons including the test squadron, was at Tyndall and wanted an update on my project. The final decision to go ahead with my project was made on the Plans side of ADC, despite opposition from Operations, after briefings by the Academy guys. I gave him an update on our progress and included the instrumentation problem and impact on the scores. Unfortunately the only question he asked was what time did I leave the flight line. Later I was told I could not be on the flight line after 2200. Fortunately my family had left to spend part of the summer with grandparents in Kansas.

Initially we scheduled two flights a day, seven days a week. Even though I shot down a few targets and a few didn't survive the landing, the squadron was able to provide targets. I shot the very first one down, it was said, because "golden bullets" hit the parachute housing and deployed it and another hit the clevis attaching the target to the tow cable and fracturing it. Initially we always got about the same score for all the firing passes.To get more discrimination, we had General Electric reps replace the barrel choke with one that tightened the pattern. After we tightened the bullet pattern, one good burst would destroy the Figat. As the supply of targets got tight, the test group commander asked me to try not to kill the target on the first pass and to get as many passes as I could because the two-star who approved the project said we needed to get 100 valid, scored passes to finish the initial tests. After our early success, the F-15 program added a digital gun sight as an option to the analog system carried forward from the F-4. The analog system was in a large "black box" that was eventually eliminated from the F-15 avionics.

To compensate for the extremely sensitive display, I flew bare-handed and held my breath when I fired. The display was very accurate but every disturbance was reflected in display movement. We fired at ranges from 500 to 1500 feet, always with radar ranging. I did not ask that damping be added to the calculations because I wanted statistically valid data with one set of software. To get the damping optimized would require many more flights under a wide range of g levels, angle off, etc. I did recommend damping be added during future flight test in my final test report. Stu Cranston and others did just that with the production version of the sight.

About halfway through the flights I was told that my next target would be the Firebee drone. No Firebee had ever been shot down by a gun. While I was coming in on the fourth pass I almost ran into the drone. On the previous pass bullets hit the gas tank and it ran out of gas. It was automatically slowing down prior to deploying the recovery parachute. Then late in the program I was given another drone for a target. This one had a modification that would allow it to pull higher g's. Part of the challenge was just keeping the drone in sight. We started at long range hoping to get in all seven passes. As I was pulling in for the last pass, everything looked perfectly aligned so I fired before I could tell the chase to turn on the cameras. I saw tracers going in the tailpipe. In a fraction of a second there was a big white fireball, the recovery chute fully inflated and everything disappeared. How was that for a storybook ending for my project!