Combat testing The Cannon-armed Sabre

by John Henderson in collaboration with Lon Walter

It didn't take long for the pilots in Korea to realize the MiG-15 had more firepower than the F-86. In January 1951, Lt.Col. Bruce Hinton, 336th FIS Commander, briefed the 4th FM staff on the 21 days of combat recently flown out of Kimpo. His briefing included the admonishment THAT THE FIREPOWER OF THE F-86 IS NOT SUFFECIENTLY DESTRUCTIVE, AND SHOULD BE MODIFIED WITH A CALIBER HEAVY ENOUGH TO INSURE STRUCTURAL DAMAGE WITH A MINIMUM NUMBER OF HITS.

By September 1951, experienced Sabre pilots who had returned to the US brought with them the news that the F-86 needed heavier firepower. These included America's first three jet aces - Jim Jabara, Dick Becker, and Hoot Gibson; as well as WW2 aces John Meyer, Ralph Taylor, Billy Hovde, Glenn Eagleston, Bob Rankin, Jim Brooks, and Ben Emmert. Their reports also included other improvements that were needed, such as a more powerful engine and better gunsight - both of which were already being developed.

At Headquarters USAF, Col. Benjamin O. Davis, Chief of the Fighter Branch, Directorate of Operations, was given the overall job of solving F-86 gunnery system problems. This would eventually lead to the replacement of the Al CM gunsight with the type A-4; and the M3 .50 caliber machine gun with a more potent weapon. The latter program was to become Project GUN-VAL.

Under Lt.Col. John England, Chief of the Fighter Gunnery Section, a Gun Evaluation (GUN-VAL) Committee was formed - which included many 4th FIG combat veterans. Among the evidence they reviewed was a paper written by Maj. Martin Johansen, which was supported by his gun camera film showing a MiG-15 absorbing multiple hits from .50 caliber bullets without any evidence of mortal damage to the pilot, airframe, or engine. A decision was made to proceed immediately with a test of two 20mm cannon capable of high rates of fire. These were to be installed in specially modified F-86s. When available, they were to be combat tested if the Korean War was still on-going. The Los Angeles Division of North American Aviation was contracted to modify 12 undelivered F-86s with rebuilt gun bays and four 20mm guns. These were to be tested by NM pilots before delivery to the USAF. 10 of the Sabres were to receive Mauser guns designed in Germany during WW2 but never installed in combat aircraft. Much later, the remaining 2 aircraft got Swiss-designed Oerlikon guns that had been successfully used in WW2. The Oerlikon installation eventually proved unsuitable; and after testing at Eglin AFB, that portion of GUN-VAL was terminated.

The Mauser cannon design had been captured from the Germans during the waning days of WW2, finding its way to the Springfield Armory, then to Ford Motor Company (FMC) for further development. For the GUNVAL Sabres, FMC provided NM with models designated as T-160 20mm cannon; a gas operated, electrically fired, belt fed, revolving cylinder gun with a cyclic rate of 1400 rounds per minute. The ten F-86s with T160 guns were re-designated F-86F-2, while the Oerlikon-equipped models received later, became F-86F-3.

Major structural modifications were required to enlarge the Sabre's gun bays to accomodate the 170 lb., 6 foot long cannon. The prototype guns received by NM from Ford required improvements to the gun feed and firing mechanisms before they could be used in the F-86. The NM Armament Department, headed by Paul Peterson (who designed the GUN-VAL installation) and engineer Jim Robertson, identified and corrected these problems.

As planned, NM test pilot George Welch flew the inflight firing tests. He pronounced the guns to be functionally reliable. But there were no tests of gun gas purging or ingestion at high altitude, a problem that would surface in Korea. All 10 F-86F-2s were delivered in 1952; 8 were destined for Korea, and 2 went to the Armament Center (AFAC) at Eglin AFB for engineering tests. Three USAF pilots flew firing tests at Edwards AFB in the Korea-bound aircraft, before the Sabres were flown to McClellan AFB for processing and shipment to the Far East.

The three pilots, Lt.Col. Don Rodewald (Air Research & Development Command), Maj. Ray Evans (APGC Detachment Commander), and Capt Lonnie Moore (APGC), were joined in Korea by Lt.Col. George Jones (Air Training Command, Nellis AFB) and Lt.Col. Clay Peterson (Tactical Air Command). Under the Aegis of APGC, these five pilots comprised the "dedicated" GUN-VAL pilot team in Korea. Additionally, pilots from the 4th FIG would also fly the GUN-VAL Sabres in Korea, including Lt.Col. Vermont Garrison, Maj. Bob Moore, Capt. Murray Winslow, l Lt Dan Druen, and l Lt Jerrold Bradley.

Once in Korea, the GUN-VAL detachment, including the aircraft, 5 pilots, a 12 man armament specialist team from Eglin, and 4 civilians, were hosted at K-14 by the 335th FIS, commanded by Lt.Col. Garrison. The 335th provided maintenance support and a secure flight line for the GUNVAL aircraft.

The combat program kicked off in January 1953 and ran for 16 weeks, ending in April. GUN-VAL airplanes were often flown in mixed flights with .50 caliber Sabres so as not to `advertise' the presence of the cannon-equipped F-86s. At other times, a flight of four cannon-armed Sabres was part of the overall mission schedule. It has been learned that Russian pilots who flew against the F-86 in Korea often derided the standard .50 caliber armament as "pea-shooters". But once the 20mm cannons were fired, a MiG pilot quickly realized that he was not up against a normal Sabre.

Although 8 GUN-VAL airplanes arrived in the Far East, one was held at Tsuiki AB REMCO center, and remained there until late January. At that time, one of the F-86F-2s was lost. Gun gas ingestion caused a compressor stall and flameout, resulting in Capt. Murray Winslow's ejection when he was unable to get an airstart. Winslow was rescued from the Yellow Sea. The Tsuiki bird replaced this first loss.

While the lethality of the T160/F-86 combination was proven early on, the problem of compressor stalls and flameouts caused by gun gas ingestion, particularily at high altitudes, was a serious problem. Combining data obtained from combat missions and 55 non-combat test flights over Inchon Bay, it was finally determined that the gun gas `envelope' that the airplane was flying through, was the culprit. Gun gas secondary explosions were occurring around and ahead of the aircraft nose. The resulting air-gas mixture, when ingested into the J47 burner cans created a compressor stall.

To reduce the size and effect of the gun gas envelope, an interim fix (unpopular with the pilots) was devised. It consisted of installing a switch in the cockpit that limited the number of guns which could be fired simultaineously. Pilots were instructed to fire all four guns ONLY below 35,000 ft., and two guns between 35 and 40,000. There was to be no firing above 40,000 ft.. Although the project aircraft were never grounded, there was a hiatus on air-to-air combat while the non-combat tests were flown. During this hiatus, six air-to-ground missions were flown to test the effectiveness of the 20mm guns.

Armed with the findings from Korea, NM's Paul Peterson returned to NM to `find a fix' that would allow firing all four guns above 30,000 ft. without fear of a flameout. Working in NM's Armament Lab, he found that a means of deflecting the gasses away from the critical intake area was needed. This was accomplished by installing a small horseshoe-shaped deflector in the gun troughs. This forced the gasses away from the aircraft, yet allowed the projectiles to exit without deflection. To assist Peterson, the Air Force sent one of the AFAC GUN-VAL airplanes to NAA to finalize the design of the deflectors. Test flights on this aircraft confirmed the effectiveness of the `fix'. There were no high altitude firing-induced engine problems.

As soon as possible, Peterson hand-carried deflector `kits' to Korea for installation on combat Sabres. There were no flameouts in the final month of GUN-VAL flights in Korea. Although a few secondary gun gas explosions were observed in front of the aircraft, there were no compressor stalls. By the end of April 1953, the Korea GUN-VAL test was completed and the remaining 6 aircraft were returned to the US. (A second GUN-VAL Sabre was lost in April when the J47 turbine wheel failed. Capt. Lonnie Moore ejected and was rescued.)

The final engineering to cure the gun gas ingestion problem continued long after GUN-VAL ended as a project. Eventually, a satisfactory design was patented that would allow the deflector to be adapted as required for other aircraft. Project GUN-VAL had a lasting and beneficial effect on the safety and effectiveness of future Air Force fighter aircraft. In production, the prototype T160 became the M39 cannon, and served many years in the F-86H, F-100, and F-101 fighters.

Project GUN-VAL Scoreboard
    total flights   ---------------------------------------368
    combat missions -----------------------------------307
    MiG-15s: fired upon -------------------------------41
                hit -----------------------------------------22
pilot                    destroyed         probable         damaged
Maj. Evans               1                     1                    1
Lt.Col.Jones             2.5                   1                   1
Lt.Col. Peterson                               1                   1
Lt.Col. Rodewald                                                   1
Capt. Moore            1.5                  1
Lt.Col. Garrison         1                    1
unknown                                                                 8-13
totals                          6                    4                     3-18

No portion of this article may be used or reprinted without permission from the President of the F-86 Sabre Pilots Association or the editor of Sabre Jet Classics magazine.

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