No F-15 without the XB-70 (via the MiG-25)

This is the second post in an occasional series on aviation, so far mostly military and looking back historically. Previously, we talked about bombers: why some will make it 100 years while the newest get retired. Today: how we see our opponents' actions affects what we're doing, and how we might not have the F-15 Eagle without the XB-70 Valkyrie.

XB-70 Valkyrie: USA Mach-3 intercontinental bomber


MiG-25 Foxbat: Soviet Mach-3 anti-bomber interceptor

F-15 Eagle: USA Mach-2.5 anti-fighter fighter

The bomber...

In the 1950s, the United States needed a new intercontinental bomber. In order to maintain deterrence with the Soviet Union, the US needed a bomber which could reliably deliver nuclear weapons across huge distances through extensive Soviet air defenses. If the Soviets thought that they could intercept all or most of the US bombers before they hit their targets, then they might think that a nuclear war was "winnable" and start one. The solution was a bomber so fast and flying so high that no Soviet aircraft could intercept it. In 1957 North American Aviation was awarded the contract for what became the XB-70, which would fly faster than three times the speed of sound and at an altitude of 77,000 feet.

...that scared the Soviets into an interceptor...

Unsurprisingly, the Soviets didn't like the idea of nuclear bombs dropping on their country without any means to stop them and accelerated research on advanced interceptor aircraft. That led to the Mikoyan-Gurevich (MiG) 25 "Foxbat".

Side note: NATO created "reporting names" for Soviet aircraft because they might not know the official model number. All aircraft identified as "fighters" had reporting names starting with "F", all bombers (e.g. Tu-95 Bear, Tu-16 Badger) started with "B", etc. An elegant system! In the case of the MiG-29 Fulcrum, Russian pilots liked the name enough that they started using it too.

First shown to the public in 1967, the MiG-25 was specifically designed to intercept US bombers with a nominal top speed above Mach 3.2 (top speed that wouldn't burn out the engines was Mach 2.8), had a powerful radar to find its targets, and a quartet of air-to-air missiles. US and Western intelligence agencies didn't think it was an interceptor but instead an air superiority fighter (i.e. one to shoot down other fighters).

The MiG had huge engines and huge wings. Given intelligence estimates of the weight of the aircraft, that should have given it high thrust and low wing loading (i.e. it had a bunch of "extra" lift). A fighter with those characteristics can make hard turns and then quickly recover the energy needed for its next maneuver. An aircraft with low thrust can't accelerate after a turn slows it down, and an aircraft with high wing loading loses either speed or altitude in the turn and needs lots of thrust to recover them. So according to the West's best estimates, the Soviets had a brand-new fighter which put their F-4 Phantom IIs at risk, especially since the F-4s had performed much more poorly than hoped over Vietnam.

...that terrified the Americans into a fighter

In 1967 when the MiG-25 went public, the US was looking to replace the F-4 Phantom II. The intended replacement was the TFX program (eventually the F-111) which was supposed to serve as the main fighter for both the Air Force and Navy to save on development costs (*cough*). The TFX was a vision of the future without dogfighting: it had a big radar, speed, and a large payload of missiles, but no internal gun and of limited maneuverability. Thus the TFX looked like easy prey to the MiG-25.

This gave leverage to factions within the Air Force who wanted a smaller fighter than the TFX with high thrust and low wing loading. And they got it: the F-15 had a maximum speed of Mach 2.5, huge thrust (thrust/weigh ratio was around 1:1, meaning that the engines had enough power to keep it flying without any help from the wings), and such large wings that it was nicknamed the "flying tennis court". First flying in 1972, the F-15 is still in production today as the F-15EX Eagle II. This ability to keep being upgraded is probably more evidence for my "dangers of optimizing" theory, as the F-15's replacement (the F-22 Raptor) has come and gone with retirement of the existing fleet planned before the F-15's.

But were they right to be afraid?

The XB-70 never went into production. Two working prototypes were built, but the mission went away. In 1962 the Titan II intercontinental ballistic missile (ICBM) entered service. It was the first US ICBM with "storable" propellants which meant that it could be kept fueled and ready for launch in a protected silo. ICBMs were also much more difficult to intercept. The Air Force realized that the deterrence mission intended for the B-70 could be fulfilled by the Titan II, and so solving the last problems of the XB-70 and paying for a new fleet to enter service weren't worth it. The MiG-25 could not intercept ICBMs, so it ended up without any bombers to intercept.

The MiG-25 was much heavier than Western analysts expected: it had an empty weight of 44,000 lbs. For comparison, that's the normal fully-loaded weight of an F-15C; the F-15 weighs only 29,000 lbs empty. In order to handle the heat from the speed of its flight, the MiG-25 had a stainless steel construction rather than titanium which made it heavier. In addition, the absolute G-limit (how hard the aircraft can turn) was only 4.5 Gs (2.2 with full fuel); the F-15's limit is 9+ Gs which is about as much as a person can stand without passing out. The MiG-25 was an able interceptor, but not a dogfighter and in some ways it was closer to the philosophy of the F-111 than the F-15: heavy, fast, big radar, missile-armed.

Conclusions

Organizations and people react to how they perceive their environments, although their perceptions may not match reality. All three aircraft named here are marvels of engineering in their own way; I certainly feel the romance of the XB-70 having seen one up-close. And if not for the XB-70, we might not have gotten one of the cooler fighter jets out there. After all, who else is going to escort President Harrison Ford's airplane?

If you have any comments, please reach out to me at blog@saprobst.com or this page is cross-posted at LinkedIn and you can leave a comment there

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