Tuesday, December 2, 2014

Could the F-106 have been turned into a multirole aircraft?

The F-106 Delta Dart, the 'Ultimate Interceptor' was one of the highest performing military aircraft of the 1950s, and might have been the best bomber killer at the time it was introduced (only the English Electric Lightning was comparable). However, compared to designs such as the F-4 Phantom, it was highly specialized, with armament and systems highly optimized for the bomber interception role. Other aircraft, such as the Phantom, would be used for other tasks during the 1960s and 1970s.

However, what if (for whatever reason), the USAF had not decided to procure the F-4? Could the F-106 have been turned into a reasonably effective air superiority fighter and/or attack aircraft? Let's find out.

The first question is the basic suitability of the airframe. According to the excellent Design for Air Combat manual, the following characteristics are required of a fighter aircraft;

Air-combat fighter

This type approaches its target either under ground or airborne direction, using its own radar, or by chance, at low to medium altitudes, and armed with a gun and highly manoeuvrable short/medium-range missiles. While the highest supersonic speeds are not necessary, the best possible instantaneous turning performance is essential. Airfield performance is important, since the fighter will be operating from first-line-of-defence bases. Its aerodynamic design will be governed by the need for good lift/drag at high g, high usable lift, low drag at all speeds, and high control power. It will require high thrust/weight with and without afterburning, and low combat fuel consumption.

Compare these to the requirements for an interceptor;

Interceptor fighter

Usually directed towards a non-manoeuvring target by ground or airborne control radar. It requires high speed and longitudinal acceleration, together with good high-altitude performance and long range. Its weapon load is characterised by long-range all-altitude missiles and associated radar. Since neither turning performance nor capability in low-speed combat are important, the primary aerodynamic design is largely governed by low profile and wave drag, and high cruise lift/drag. Low fuel consumption is also significant.


The F-106 has good supersonic speed (capable of Mach 2.3), and low drag. The thrust to weight ratio is also fairly good, roughly comparable to the F-4 at MTOW. What about the instantaneous turn performance? That's primarily a function of wing loading. Here's how the Phantom and Delta Dart compare;

F-106 wing loading at MTOW: ~309 kg/m^2
F-4 wing loading at MTOW: ~569 kg/m^2

As you can see, the F-106 has significantly lower wingloading that the Phantom. This means that its instantaneous turn performance should be far superior. This is supported by anecodotal accounts. According to the F-106 data sheet , the aircraft's structure could sustain a loading of 7.0 g at combat weight, not quite as good as something like the F-16, but not too shabby.

It's not all perfect, though. While the F-106's large delta wing will give it good instantaneous turn performance, it does mean that it's going to bleed energy quite quickly in a sustained turning engagement (as I recall, the MiG-21 had similar issues). This would put it a disadvantage in a low speed engagement against an aircraft such as the MiG-17. It should be noted that the low aspect ratio of the F-106's wing would help it do better in a turn fight at supersonic speeds.

The main issue with using the bog-standard F-106 as an air superiority fighter is its weaponry. The performance of the AIM-4 Falcon against maneuvering targets could charitably be described as shit. During the Vietnam conflict, it performed so poorly  that the US Air Force hurriedly adapted the US Navy's AIM-9 Sidewinder. Clearly, if we want to turn the F-106 into a multirole aircraft, we're going to have to drop the Falcon. The most logical replacement is the AIM-9. However, there is a problem. The AIM-9 is significantly longer than the AIM-4. Here's a diagram of the F-106, showing the internal weapons bay with AIM-4s;

Based on this diagram, and me doing some eyeballing, it looks like you might not able to fit two AIM-9s in the weapons bay lengthwise. Fortunately, the AIM-9's fin span is narrower than the AIM-4s, so you could still fit in, at least 2 horizontally, possibly 3. I would also note that the AIM-4 is over half the length of the radar guided AIM-7, so you should be able to fit at least two inside.

Of course, you can't just stick AIM-9s onto the launch rails and expect them to work. The avionics are going to have to be adapted somehow. This could potentially add weight and complexity. However, the F-106, in the interceptor role, was fitted with electronics needed to work with the Semi-Automatic Ground Environment (SAGE), a highly advanced computer system capable of autonomously flying the F-106 to intercept Soviet bombers. A variant used primarily as an air superiority aircraft would not need these electronics, and they could be removed. (As an added bonus, the necessary adaptations to carry the nuclear-tipped AIR-2 Genie rocket could also be removed.) As a result, I believe conversion of the F-106 to carry Sidewinders should be at least theoretically possible.

In order to give the F-106 true multirole capability, it must be able to use air to ground ordnance. The F-106, to my knowledge, never dropped bombs in real life. However, it did have underwing hardpoints (used to carry 370 gallon drop tanks). Photographic evidence indicates that they were at least theoretically capable of carrying air to ground ordnance;

I'm not sure whether the F-106 ever conducted test flights with this loadout, but I'm going to assume it did (why would you stick bombs on an aircraft in a forward deployed location if it can't even fly with them). Assuming those are Mk 82s on that rack, this gives the F-106 a bombload of 1500 kilograms (6x250kg). Not anywhere close to the Phantom, but nothing terrible. (In a scenario where the USAF doesn't get the F-4, the F-105 is going to be even more important in the strike role). Additionally, the underwing hardpoints could be used to carry additional air to air armaments. Likely options would be 2 AIM-7s, and 2 or 4 AIM-9s (four could be carried if a double rack such as was used on the F-8 was feasible). Four AIM-9s and two AIM-7s would be a respectable air-to-air loadout, not as much as the F-4 could carry, but then again, the F-106 is a smaller aircraft.

Based on this, I think it's clear that even without extreme modifications, the F-106 could be made into a capable multirole aircraft. It wouldn't be perfect, by any means, but perfect solutions rarely exist. A multirole F-106 could have been developed for relatively little cost and effort, and still been highly capable.

Not that better results might not be obtainable with more radical modifications...