Whatever happened to....?

In the early days of jet engine development around the last years of WWII and the early post-war period, little difference was seen between jets and rockets. both, after all, operated by pushing a stream of hot gas at high speeds. But later on the bypass engine was developed. This engine uses the hot gases to turn a big fan, which produces a large-volume mass of air moving backward at lower speeds, most of which bypasses the engines combustion chamber. As it turns out, this is more efficient.

The early engines are now called turbojets, the bypass engines are turbofans.

A bigger fan meant a bigger volume of air, whereupon a third type, the turboprop engine was born. This is a propeller turned by a jet engine. It is very efficient, but it cannot produce the type of speeds found in regular passenger jets. So it's been confined to regional planes, plus a few uses in the military like the famous C-130 Hercules tactical cargo plane.

In the mid-late 80s Boeing and McDonnell Douglas played with a fourth variant called an unducted fan engine. Like the turboprop, the fan is outside the engine. Unlike the turboprop, the fan sits at the back of the engine. in fact it featured a pair of counter-rotating fans. These have a large number of blades, which are swept back due to the speed of the air they move.

These engines could produce regular jet type speeds (ie Mach 0.85 or so). In addition they were quieter than regular turbofans and, crucially, up to 40% or so more efficient. That's a HUGE reduction in the use of fuel.

While both Boeing and McD designed prototypes, none were built. The engines were tested on modified B-727s and DC-9s. Both projects quietly died in the midst of historic low oil prices.

Oil prices are fickle and volatile. When they rise, they do impact airline operating expenses significantly. Even when they're stable, or low, they are a major factor in operating costs. I have to wonder, then, why this type of highly efficient engine was allowed to fall by the wayside.

Oh, there were some challenges. The fans were so big that both companies designed planes with the engines on the tail. The common under the wing engine placement of most passenger jet types wouldn't be able to accommodate this type of engine at all, the fans would hit the floor. But I'm sure some clever aerospace engineers would have found some solution, be it high wings, engines trailing the wings, 4 engines on the tail, etc.

But even if confined to medium-sized planes like the 727 or DC-9, they'd been revolutionary. Today the dominant types are the B737 and A320 families, both of which are essentially what came to replace the larger version of the 727 and DC-9.

Boeing's prototype, the 7J7, envisioned something new as well. A twin-aisle narrow-body plane. I wonder what configuration would be used, perhaps 2-2-2. That would be genius, as all seats would be either aisle or window. no more dreaded middle seats at all!

My thinking is that the Unducted fan engines weren't really all that efficient. If they had been, someone would have put them on a plane by now. if not any of the major commercial plane manufacturers, then one of the business-types makers, or an independent designer of unconventional frame like Burt Ruttan.

Another paper airplane, one that never got off the drawing board, was the McDonell Douglas MD-12.

It was a proposed successor to the DC-10 (we got the MD-11 instead). It would have been a four engine wide body, otherwise similar to the DC-10 in layout. It would curiously also have been America's first, and sole, 4 engine single deck wide body, similar to the A-340.

In this case it's easy to see what the decision was: simpler to modify an existing design than to develop a completely new one.