Guns and Slats: F-4E Peculiarities – Phantom Phamiliarisation (2024)

Video

Introduction

Besides differences in the avionics and upgrades, the two main features that make the F-4E stand out are the M61A1 20mm Vulcan 6-barrel rotary cannon and the installation of leading-edge slats. Other improvements materialised along the F-4E, but are less visible. For example, the older J79-GE-15 was replaced by the more powerful J79-GE-17, and they represent an evolution, rather than a design and perspective change.

The Gun

Let’s start with the gun. Very long story, very very short, with the advent of missiles, the big brains thought that the era of the gun was over, as threats were better dealt with by the explosive warheads of missiles. For this reason, the gun was deemed obsolete and was not installed in the Phantom. This comes across as a-la Giulio Douhet approach. Douhet was an Italian general who studied and theorised the usage of air power after the first world war. His ideas were drastic and presented a neat cut with most of the older theories. However, the majority of his ideas were proven very, very wrong by the upcoming second world war. In a somewhat similar fashion, the theory that called for the end of the aerial fights and the obsolescence of the gun was proven wrong on multiple levels.

I: Missile failures

In primis, the issues with the missiles that should have brought forward the rocket revolution, especially the AIM-4 Falcon and the early AIM-7 Sparrow. Accounts of Phantom crews capable of manoeuvring in an advantageous position just to be let down by the missiles are not hard to find. A gunpod could be installed under the belly of the Phantom, but this means adding drag. The SUU-23 was used by the F-4D and by the British Phantoms, who commonly carried it in Germany and in the Falklands.

II: Friends or Foes?

The second level is technological, but it is something that can affect present day engagements as well: the ability to discern Friends from Foes. Although the AIM-7 is potentially a BVR-capable missile, tight ROE and VID required, made it a de facto a WVR weapon. The Combat Tree IFF mentioned in the previous video, and the AN/ASX-1 TISEO, acronym for Television Infrared Detection and Ranging, tried to bypass this problem but, at least in the Vietnam War, most of the greatest successes came using ad hoc traps and tactics to lure the MiGs in.

III: A frigid Cold War

The third level is historical, as the Cold War never developed into a full-scale war, and the hordes of Soviet bombers never became the threat they could have been. Most of the conflicts of the period remain on a more limited scale, with several encounters between fighters, rather than strategic bombers.

The Cost of the Gun

In the Phantom-E, the gun came at a price. With the demise of the planned CORDS or Coherent-On-Receive Doppler System, an advanced system that never materialised, and with the adoption of the long slim nose necessary to fit the gun, the F-4E could not mount the new AWG-10 radar the Navy was using. Eventually, Westinghouse proposed the AN/APQ-120 radar, which resulted in a compromise between radar capabilities and gun. Another less expected benefit is the addition of another fuel tank, used as a counterweight for the gun itself.
The sketches below show the position of the M61A1 and the AN/APQ-120. As you can see, the room is very limited.

Gun: A useful tool?

So, was the gun worth it? In air-to-ground missions, a gun can be used for strafing and suppressing a target, I suppose an internal gun is better in this regard as it does not take an external hardpoint? I haven’t found enough accounts to comment on the usage of the gun during strike missions.

Anyway, It is in the air-to-air scenario that things become interesting, and the answer is: well, it is complicated. The Navy went fine without it and had greater success in terms of Kill to Death ratio after operations Linebacker 1 and 2. An impressive 6:1 for the Navy, against a meagre 2:1 for the Air Force, but these numbers do not tell the whole story of how poor the Air Force approach to training and rotations was, at least in the beginning. Certainly, they do not tell much about the usage of the gun.
Changing approach, if we consider only the Air Force gun kills, the F-105 Thunderchief scored the highest number. Next, the F-4D downed 6 aircraft using the SUU-23 gunpod, and the F-4E downed 5 aircraft using the internal gun.
These numbers unfortunately provide only a fragment of the full picture, but I suppose that the fact that internal guns are diffused and used in modern jets, endorses the direction the F-4E took.

The Wing

The second major difference between the F-4E and the Navy Phantoms of the similar era, is the wing. The Boundary-layer Control based Navy wings were apt carrier operations designs.
Quoting Anthony Thornborough and his book “USAF Phantoms”,

“They comprised of leading- and trailing-edge high-lift flaps, with air piped from the seventeenth stage of the engine compressors and blown out over the flaps to increase the lift properties of the wings.”

So, flaps come down, and the additional lift helps during landing and departure operations from the short deck of a carrier. As the speed increases, the flaps come up and form a clean low-drag wing, optimal for high-speed intercepts. However, this approach did not work very well with the Air Force, as piloting a heavy Phantom, rid with bombs, at mid-to-low altitude whilst avoiding triple-As and SAMs could cause buffeting and a potentially irrecoverable flat spin. The solution arrived in the form of the Agile Eagle program, where a new wing, featuring leading-edge slats, incredibly improved the stability and manoeuvrability at low speeds. The slats were hydraulically operated, and were extended at high angles of attack, thus preventing the airflow separation near the leading edge, resulting in more lift and stability. The cost, is a slightly reduced top speed.
The slats, in fact, retract above 600kts or 10.5 units of AoA or less, thus improving the wing profile, so to speak. The results are however less efficient than the previous Boundary-Layer control solution, but the slight increase in drag is a cost worth paying.

Closing Thoughts

In a sense, the two aspects discussed today almost go hand-in-hand, as the slats provide more stability at high AoA, a situation that often happens in dogfights, where the gun has more chances to be used.
Fun fact: the Navy, eventually, decided to upgrade their Phantoms as well, completing the refurbishment in the mid-to-late 70s and the F-4S. This version added smokeless engines, strengthening the airframe and adding the leading-edge slats. Apparently, the S in F-4S stands for Slats.

What about DCS? I do not think there is much to say here. In the game, having the gun is a no-brainer, as engagements protract themselves to the last litre of fuel, and the last bullet. The leading-edge slats will also help the Phantom during those fights and, in general, they should make the Phantom more forgiving, as witnessed by several pilots that flew both the Phantom-E and previous, pre leading-edge slats, versions.