March 30, 2009

I Mourn me the B747

A while back, a colleague and I were adamant that the U.S.'s 'Next generation bomber' could and should be a militarized 747 variant. Our rationale was that it had similar carrying capacity in terms of cargo weight, was a well-understood and readily-available airframe, and offered ample volume for military systems to be added. In addition, the military already operated several 747s.

We heard many arguments about why it couldn't be done, but none that we really 'bought' - they all sounded like 'because we don't want to fly civvy air!' to us. However, recently I was speaking with an Air Force colonel who is also a B-52 pilot, and he offered the best-explained reasons it won't work.

The 747, as a civilian airliner, is a hollow tube of a monocoque fuselage designed to be pressurized during flight. In addition, the cylindrical nature of that fuselage is what allows it to evenly distribute the forces generated by said pressure across the structural members. A bomb bay, however, must by definition be at ambient pressure. It should be at such before opening in order to avoid sudden pressure changes, and of course it will be once it's opened.

The issue is that if you were to section off a part of that cylinder and move it 'outside' the pressurized area, then the load on the bulkhead separating it from pressure will tend to concentrate on structural points rather than evenly distributing - and this will cause difficulty making the airframe strong enough to perform properly. Even if it can bear the strain, the cycles of pressure differential during normal operation will lead to increased metal fatigue.

Weight and Balance
The weight of the weapons a bomber intends to dispense must be placed as close to the center of gravity (or, at least, the fore/aft balance line) as possible. That way the sudden change in the aircraft's weight profile while dropping ordnance is balanced. On the BUFF and other 'high-wing' bombers, the mainspar passes through the fuselage high enough that the bomb bay can be placed very close to if not directly astride the midpoint of the wings. This means that when the ordnance is released, the airplane lightens but does pitch up or down at all.

On a 747, however, the mainspar is low - it passes through the lower part of the fuselage in order to maximize cabin space. As a result, the bomb bay cannot be placed directly below the wing balance line, but can only be placed fore or aft of the mainspar, with consequent disruption to the aircraft balance.

In addition, airliners are built (my advisor says) to travel sedately and predictably from one place to another. While their airframes are designed to handle external stresses from turbulence, they are not built with the intention of the aircraft suddenly shifting its internal structural load as it dispenses ordnance. Again, you'd end up with metal fatigue or failure without significant changes to the airframe.

I mentioned the Evergreen Aviation 747 water bomber. He agreed that the water bomber could carry a cargo weight equivalent to ordnance, but pointed out that the tankage for this cargo could be aligned overtop the mainspar and the water dispensed from valves, not large bays. I checked their website, and yep, he's right - not only that, the dispense system is done via pressurizing the tank, so once it's empty it can be sealed and remains pressurized to avoid pressure differentials weakening the airframe.

Finally, the water bomber is intended to drop its cargo low and slow - around 400 to 800 feet, at a speed of 140 knots, or just 30% above stall speed. Thus, even if the tanks were not pressurized, there would be a negligible difference between internal and external pressure.

I'm almost convinced. :-) Not that I doubt him, but I still think there's a role for a commercial heavy-lift airframe in the bombing missions the U.S. has seen. The USSR once threatened to treat all Boeing airframes as enemy targets if the U.S. built weapon-carrying versions - at least, I've heard that from various pilots, although I can't immediately dig up a source - and whether true or not, it points out a problem of militarizing the 747 airframe. KAL-007 was shot down despite being obviously a transport, and various persons associated with the shootdown maintained that it was 'easy to convert a 747 into a reconnaissance platform.'


Posted by jbz at March 30, 2009 11:33 PM | TrackBack

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