Why does the F-111 have a capsule ejection system rather than a conventional system?

Question

Following on from the questions F-111 emergency landing with no wheel; what is done to reduce risks when loosing a wheel? and Are there any known incidents of ejections by capsule? What are the survival chances?

I wondered, what is the reason for the ejection capsule on the F111?

It doesn't fly particularly high or fast (compared to aircraft with conventional ejection seats). The only reason I can think of is that the pilots sit almost side-side and it would presumably be tricky to fire the first seat without seriously inconveniencing the remaining crew member.

Are there any side-side aircraft with ejection seats?

Answer 1

The reason the F-111 has an ejection capsule is pretty weird in itself- its there because USN wanted it.

In 1960, USAF asked for an aircraft capable of supersonic flight for 400 miles, and capable of operating from short/unprepared fields and capable of crossing Atlantic unrefuelled through Specific Operational Requirement (SOR) 183. At the same time, USN was trying to develop a fleet defender for its carrier groups which had high loiter time (> 6hrs) and can detect/launch missiles at incoming bombers and sea skimming missiles at over 100 miles. To this extent, the Douglas F6D Missileer was proposed, which got cancelled in 1961.

Unfortunately for USAF (and lot of others), US SecDef Robert McNamara decided that both these requirements could be met by the same aircraft- actually, the only things they agreed about were that the aircraft would have swing-wing, have two seats and have two engines.

The USAF wanted tandem seating, while the USN wanted a side-by-side seating for improving crew co-ordination sharing the large radar display to launch missiles (like the A-6 Intruder, which had side by side seating, but conventional ejection seats). The reason the ejection capsule was included was that USN believed that it would give better chances of survival in case of ejection over sea (where it was to function as a life raft) and in high speed ejection. The USAF hated this as it increased the weight by over 500 lb, affecting performance.

In fact, the Boeing proposal included conventional ejections seats, which was required to be changed into a capsule (and was selected before McNamara intervened and selected the other one). In the end, USN escaped from the project (ironically due to weight and performance issues), while the USAF and others got stuck with the ejection capsule, which they never wanted in the first place.

Answer 2

My response is that I was a design engineer on the F-111 project with responsibilities associated with the design and test of the module. Traditional ejection seat concept provided no survivable protection for escape at the upper (higher) speeds and altitudes of the operational envelope of the F-111. This is true for tandem or side-by-side seating configurations. The only significant design constraint imposed by the Navy was to be able to separate up to 30 feet under water. Interestingly, the control stick could be converted into a “bilge” pump handle in event of water landing. The first use of the escape module was from a test aircraft having lost hydraulics in the flight control system. This occurred over Bowie, Texas. The pilot scratched his finger climbing through a barbed wire fence! The only injury!

Answer 3

It has to do with the realities of a supersonic ejection at high altitudes, which is usually fatal for unprotected aircrews, both from bodily trauma from exposure to a supersonic slipstream and the extreme low temperatures and pressures which would be encountered at typical cruise altitudes.

The United States invested in increasingly faster supersonic bombers in the 1950s and 60s in order to thwart conventional defences and interceptors operated by the east bloc, so the realities of Mach 2 or Mach 3 flight demanded these kind of enclosed and pressurized escape systems in the event of an inflight emergency or enemy attack. But the realities of modern Warsaw Pact IADS using radar guided missiles made supersonic and easily detected bomber fleets an impractical investment for strategic nuclear delivery. With the return to the subsonic flight envelope and lower operational altitudes made conventional ejection seats a much more mature and realistic option for an escape system.

A good example of this was the Rockwell B-1A, which was to have used an large four man escape capsule similar to the F-111's prior to its cancellation by the Carter Administration. It's successor, the B-1B which was optimized for slower and low level penetration abandoned the escape capsule for the mature and established McDonnell Douglas ACES II ejection seats at each crew station.

Additionally, escape capsules are large, heavy, complex pieces of equipment, not to mention expensive. The F-111's escape capsule cost as much as a completed F-86 SaberJet. Complexity increases the number of failure modes and risk of failure during emergency use exponentially, requiring more maintenance, etc.

Curiously one high altitude, high speed cruiser, the Lockheed A-12/SR-71, used conventional ejection seats throughout its career albeit requiring the crew to suit up in full pressure suits during the flight. I do not believe that the seats were intended for use at supersonic speeds, leaving no means of escape for the crew during this regime of flight. This was also true of the North American X-15, a Mach 6 research aircraft where an ejection at cruise altitude and speeds would have certainly proved fatal.

Are there any side-side aircraft with ejection seats?

Yes. Multiple aircraft with this cockpit configuration used ejection seats including the B-1, B-52, B-2, A-6, OV-1, and T-37.