Combat Tree | The system that made the F-4 the deadliest fighter

In the early 1960s, the McDonnell Douglas F-4 Phantom arrived on the scene. Revolutionary for its time, the Phantom was fast, technologically advanced and multirole capable. From day one of the Vietnam war, the aircraft was employed with great efficiency, finding favour with the Air Force, Marines, and Navy.

However, the Phantom was by no means a perfect design, and the Vietnam War pushed the aircraft to its limits relatively quickly. As the airspace became increasingly hostile, the Phantom needed an edge in air-to-air combat. The answer was ‘Combat Tree’, a system that made the Phantom the deadliest aircraft in the skies over Vietnam.

BACKGROUND

From the outset of the Vietnam war, regular supplies from the Soviets kept northern forces outfitted with modern ground radars. This system improved over time, to such an extent that by the late 1960s, North Vietnamese radar coverage extended over practically the entire country.

US forces in the south attempted to adopt similar ground radar coverage, but even when paired with the introduction of airborne early warning aircraft, their air coverage was no better than the enemy’s.

The adoption of new tech – known as Combat Tree – sought to gain the upper hand by giving Phantom WSOs a distinct long-distance advantage over their adversaries. And the system worked!

APX-81 Combat Tree

Combat Tree would be made possible thanks to a suite of Identification Friend or Foe (or IFF) interrogation systems, the details of which remain unclear. The Wizzo could use this to scan the sky for IFF signals from enemy aircraft transponders.

According to technicians who worked on these aircraft, the interrogator system was interlinked with the radar scope, showing IFF returns as an overlay on the B-sweep azimuth. Crucially it could still be used with the radar on silent.

As with almost anything relating to IFF and encryption, the specifics of the Combat Tree system remain classified. Encryption related technologies are arguably the most classified military aircraft systems, so we can only work with publicly available information.

We know that combat tree was built around what was called the APX-81 by engineers, however pilots and cockpit labels had it as APX-80, which some technicians believe was a false name to confuse or obfuscate if the aircraft was ever captured.

This, however, was not the first system of its kind. In 1967, certain EC-121 Warning Stars (which were early warning aircraft) were equipped with a highly secretive interrogator known as the QRC-248, which allowed for identification of targets at long range and low altitude. However, Combat Tree would be the first widely proven interrogator on a fighter aircraft.

At the time, VPAF MiGs used IFF systems to avoid friendly fire incidents, and this opened up an opportunity to be exploited by phantom pilots.

With radar on silent, the APX-81 would pick up IFF signals from the Russian SRO-2 unit installed on north Vietnamese MiGs, then provide the challenge. The automatic return would confirm if the aircraft was friendly or hostile. From there, the APX-81 would somehow provide data regarding location. It remains a mystery as to how this system forced the automatic IFF response, since this would usually require the pilot to know the code used on that specific day by north Vietnamese pilots. Perhaps intelligence intercepted these, although more likely a system was developed which could somehow overcome the code lock, hence why even older encryption related tech remains classified.

Either way, this overcame a major problem; up until this point, crews had been held back by visual identification rules of engagement. This was in the name of safety, given how cluttered the sky was with friendly and enemy traffic over Vietnam, but this was specifically detrimental for the Phantom. Having to move in to gain visual ID, pilots would often then have to merge with North Vietnamese aircraft. Phantoms had several technological advantages over the MiG-21, such as better radar and better long-range weapons. However, having to push up close, these advantages were lost. In fact, once engaged in a dogfight, the Phantom held no major advantage over the more agile MiG.

Combat Tree would allow crews to safely ID targets and then fire at targets far beyond visual rangeBVR (Beyond Visual Range): In aerial warfare, engagements or combat operations that occur at distances where opposing aircraft cannot visually identif... More. While beyond visual rangeBVR (Beyond Visual Range): In aerial warfare, engagements or combat operations that occur at distances where opposing aircraft cannot visually identif... More combat had technically been possible for over a decade, Combat Tree marked the first time pilots could reliably and continually engage targets from afar.

According to pilots, there were some other advantages. Firstly, IFF signals could travel further than the F-4s radar could reliably scan for MiG-sized targets, essentially extending the F-4s ability to find targets. It also allowed pilots to fly higher and still locate targets. Prior to this, MiGs were often directed by ground controllers at low altitudes around the radar beams of F-4 flights, and even when within beam, when against ground clutter in mountainous regions it was near impossible for the Phantom Wizzos to get a solid lock.

Northern pilots had devised a tactic in which low flying groups would pass under or around MiG-Combat-Air-Patrols (known as MIGCAP flights), with radars off, listening to directions from the ground controller, then once behind the enemy flight, pull up into a high altitude climb to near 30,000 feet. From there they could then dive down on the Phantoms from a slight angle behind, fire infrared missiles which gave no radar warning, and then pass by at high speed in the dive, diverting at a slight angle back out of sight.

ARRIVAL

The hope was that Combat Tree would change this dynamic, and Phantoms would avoid such aerial ambushes.

Beginning in late 1971, a batch of F-4Ds would receive the APX-81 system, with the so called APX-80 control head inside the cockpit. Only a small number of aircraft were equipped with the system, likely a security precaution, as the Air Force remained vigilant to protect what they rightly identified as a serious game changer.

These aircraft would be attached to the 432nd Tactical Recon Wing at Udon Air Base.

According to testimony, only a few aircraft at a time would be in the air with the combat tree system, usually the flight lead, and sometimes the element lead. The rest of the flight would usually be F-4Es. Presumably, the combat tree aircraft would audibly relay their findings to the others, since only one would need to confirm IFF status and point out targets.

After about a year, some E models would also be enhanced to Combat Tree status. In November 1972, under the river haste program, new and improved E models were built – Block 48s – with new guns, slats, ergonomical improvements, and the TISEO system. Twenty were built for deployment. On November 20, the first 6 arrived at Udon Air Base, none had Combat Tree. However, a second batch of 6 arrived on December 18, all equipped with the APX-80 control head, and on January 13, 1973, the final batch of 8 would arrive, with 4 being Combat Tree capable.

Given the advanced nature of the system, Combat Tree aircraft kept a low profile. There was no outwardly visible difference to the airframe. For the most part they kept the same shape, same contextual camouflages, and so on. Supposedly some of the aircraft could only be differentiated by a small sticker warning of onboard explosives, which referred to an automatic detonator system which would destroy the APX if the pilots ejected.

The effectiveness of the program would soon be proven. Most notably, Combat Tree was the system that enabled Charles DeBellevue and Steve Ritchie to achieve ace status, landing them five kills in their aircraft.

This was exemplified during an Oyster flight on May 10^th, 1972 (an event we have discussed last year in our Vietnam aces video). Heading out in a four ship MIGCAP flight – three of which had Combat Tree – the group engaged an equally sized formation of MIGs. DeBellevue and Ritchie used Tree to lock up some of these MiGs beyond visual rangeBVR (Beyond Visual Range): In aerial warfare, engagements or combat operations that occur at distances where opposing aircraft cannot visually identif... More, while another jet – that of Roger Locher and Robert Lodge – also fired off missiles using the system. During the initial engagement three of the four MiGs in the first element were destroyed.

The engagement would turn deadly for the Phantoms, when they were then jumped by MiG-19s, destroying one aircraft which forced Locher to eject, and killed Lodge. However, the engagement itself did prove that the IFF system could be employed again and again to achieve lock.

North Vietnamese pilots eventually realised what was happening thanks to radar comms, and from that moment onward, orders were given to only switch on IFF transponders when absolutely necessary. While this technically overcame the combat tree system directly, it indirectly led to more confusion in the sky, resulting in SAMSurface-to-Air Missile: A ground-based or ship-launched guided missile system designed to detect, track, and destroy enemy aircraft, helicopters, crui... More sites having to hold back firing until certain of their target, and ground controllers had to pay more attention to differentiating friendly and enemy aircraft on their scopes.

As good as the technology was, Combat Tree was not infallible. According to one testimony, while flying with a Target Identification System Electro-Optical (TISEO) equipped F-4E, a Tree equipped F-4D made what it believed to be a MiG lock. However, upon electro-optical inspection, it was revealed to be another F-4.

The advantage of combat tree made the F-4 – for a period of time – the most capable fighter aircraft in the sky, unmatched by any other jet in the world in its ability to safely identify, lock, and engage enemies at beyond visual rangeBVR (Beyond Visual Range): In aerial warfare, engagements or combat operations that occur at distances where opposing aircraft cannot visually identif... More, even without ground radars or AWACSAWACS (Airborne Warning and Control System) – A mobile, airborne radar and command and control center mounted on aircraft platforms designed to detect... More confirmation.

Combat Tree most likely played a role in the development of similar but more advanced IFF interrogators, which began appearing in the F-14 and F-15, and the continual evolution in modern fourth and fifth gen fighters.