Wednesday, September 9, 2015

Air Superiority F-106 - Pulling Out All the Stops

Months ago, I wrote a piece on the feasibility of the F-106 Delta Dart as an air superiority fighter. I ultimately came to the conclusion that the F-106 could, with slight modifications, serve as a decent multirole aircraft. But what if we made more than slight modifications?

Suppose that, at sometime around 1980 a country using the F-106 wanted to modify them such that they could be used in the air superiority role for at least another decade. How would they go about doing this?

Those of you who are more reasonable might ask why the country in question doesn't just buy a bunch of upgraded F-4s or MiG-23s (or even some F-15s or 16s) and use them instead. Well, they could, but that would be a quite boring end to this post.

I'm going to use the development of the Atlas Cheetah as a model for our F-106 improvement program. The Cheetah was developed by apartheid-era South Africa from their existing Mirage III airframes, and involved a quite extensive rework of the aircraft, including a complete rebuild of the airframe, fitting of an upgraded engine (in the Cheetah C model), as well as new electronics and weaponry. The Mirage III dates from approximately the same time period as the F-106, and is broadly comparable in performance, so such a project should be (theoretically) feasible.

Calling our new aircraft the F-106M, we can start by looking at what we don't need. The fairly extensive electronics fit allowing the F-106A to interface with the SemiAutomatic Ground Environment (SAGE) is redundant, so it can go. That should save us a couple hundred kilos, and free up some space. Equipment needed for the carriage of nuclear weapons (such as the AIR-2) can also get removed. In its place an autocannon such as the M61 can be permanently fitted.

Though the F-106 was originally designed as an interceptor, it actually has decent turning ability, at least at high speeds. However, at low speeds, the aircraft tends to lose energy. This tends to be an issue inherent with dynamically stable pure deltas, such as the F-106. One option to improve lift at low speeds and high angles of attack would be to fit canards, such as on the AJ37. This would require extensive modification to the forward fuselage and air intakes of the aircraft. Another option would be to fit leading edge extensions to the wings, as are found on the F-18. These leading edge extensions generate vortices, which increase lift at high angles of attack. Additionally, they would shift the center of lift forward, reducing the static margin of the aircraft and improving performance. (Here's a brief explanation of why instability is good in an aircraft.) Another benefit would be reduction of takeoff distance, i

The second major change is in the powerplant. The J75 is not a bad engine by any means, but by 1980 it's getting quite long in the tooth. If possible, we should re-engine the F-106M with a newer powerplant. One of the best candidates is the Pratt & Whitney F100. Though the F100 is about a meter longer than the J75, it is only 9 cm larger in diameter. Given that the structure of our improved F-106M will be undergoing a substantial refurbishment, increasing the diameter of the engine bay and lengthening the fuselage slightly should not be an insurmountable obstacle. In fact, the largest challenge would probably be redesigning the intakes to accommodate the F100. Obviously, some sort of variable geometry intake should be retained, to take advantage of the F-106's exceptional speed (variable geometry intakes are virtually essential for speeds about Mach 2).

In exchange for all this trouble, the F100 gives us significantly improved fuel consumption compared to the J75. Additionally, the F100 has a thrust to weight ratio about double that of the J75, so it ends up weighing about half as much. With the weight of the F-106M reduced by up to 1,000 kilograms compared to the base model, range will be improved, as will the maneuverability of the aircraft (thanks to reduced wing loading).

Most of the other changes are internal. An improved radar would be essential, as the F-106s aging 1950s era fire control systems were designed to target large, slow targets at extreme range in conjunction with ground control, rather than air superiority operations. One candidate would be the AN/APG-63, which was fitted to early model F-15s. Whether this or another radar is used, it is likely that a redesign of the nose section would be required (as was planned for the F-106C, which would have been fitted with the radar from the aborted XF-108 program). Avionics for ground attack would also be essential, in order to give the F-106M true multirole capability. These could be fitted in place of the deleted SAGE equipment found on the F-106A. Not only would this include fitting a modern bomb sight, but compatibility with guided munitions such as the GBU-12. In order to allow the F-106M to utilize laser guided munitions, it could be possible to give the aircraft a built-in laser designator. However, a more likely option would be the fitting of a targeting pod to one of the underwing hardpoints on appropriate missions.

Additionally, compatibility with modern weapons, such as all aspect variants of the AIM-9 and AIM-7. A helmet mounted sight (as developed by the South Africans in the 1970s), when coupled with an all-aspect IR missile would result in a massive increase in dog fighting capability. This would keep the F-106M viable in a close in fight, despite its inferiority in this arena against designs such as the MiG-29 or F-16.

Numerous other, smaller changes could be made, such as replacing the old ejection seats with an improved model, and fitting a new RWR. While completely rebuilding an F-106 into a 4th generation fighter was never likely to happen, it's interesting to look at whether such an undertaking might have been possible at all.

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