Skunk Works

At 80,000 feet, the outside air temperature was about minus 65 degrees F. As the inlet sucked in the air at Mach 3 through narrowed openings that compressed it, the air heated to 800 degrees. The bypass turbojet engines took the heated and high-pressure air (40 psi) and squeezed it further in a compressor, heating it to about 1,400 degrees F. At that point fuel was added to heat the air inside the burner to 2,300 degrees F. This supercharged air was then expanded through the turbine, before being fed into the roaring afterburners, superheating the combustible mix of gas and air to 3,400 degrees F, just 200 degrees below the maximum temperature for burning hydrocarbon fuels. The white-hot steel nozzle spit out its fiery plume in the form of diamond-shaped supersonic shock waves. Even in the frigid upper atmosphere, the air boiled at 200 degrees F for a thousand yards behind those booming engines. This unprecedented propulsive power sped the Blackbird at an unbelievable two-thirds of a mile a second.

About six months into our wind tunnel testing, I went to Kelly with joyful results: the inlets produced 64 percent of the airplane’s full-throttled power. The precise shaping of the inlets and our unique movable air throttle cones, or spikes, allowed us to achieve an astounding 84 percent propulsion efficiency at Mach 3, which was 20 percent more than that of any other supersonic propulsion system ever built.

Developing this air-inlet control system was the most exhausting, difficult, and nerve-racking work of my professional life. The design phase took more than a year. I borrowed a few people from the main plant, but my little team and I did most of the work. In fact the entire Skunk Works design group for the Blackbird totaled seventy-five, which was amazing. Nowadays, there would be more than twice that number just pushing papers around on any typical aerospace project.

Having today’s high-speed computers would have accelerated the design process and simplified much of our testing, but perfection was seldom a Skunk Works goal. If we were off in our calculations by a pound or a degree, it didn’t particularly concern us. We aimed to achieve a Chevrolet’s functional reliability rather than a Mercedes’s supposed perfection. Eighty percent efficiency would get the job done, so why strain resources and bust deadlines to achieve that extra 20 percent, which would cost as much as 50 percent more in overtime and delays and have little real impact on the overall performance of the aircraft itself?

As it happened, we achieved 70 percent efficiency within the first half year of our work, but to tweak it above that to our target of 80 percent took an additional fourteen months. Of primary concern was where to precisely locate the supersonic shock wave within the inlet walls. That was the key to achieving maximum efficiency, because the shock wave in the wrong place in the inlet would block incoming air, causing energy loss, drag, and in a worst-case scenario—stall.

I logged hundreds of hours testing inlet shapes and cone models at NASA’s Ames Research Center at Moffett Field in Northern California, a giant complex of high-speed wind tunnels. That became my second home, where I spent weeks at a time using their largest, most-powerful supersonic wind tunnel, a twenty-foot-long, ten-foot by ten-foot rectangular chamber powered by a gigantic compressor capable of driving an ocean liner, and a three-story cooling tower holding tens of thousands of gallons of water. Running Mach 3 pressures for several hours at a time drained so much electricity needed by local industry that we were forced to test only late at night, working usually until dawn. Wind tunnel tests cost us $10,000 to $15,000 an hour and we ran up a stupendous bill because our models were tested from every angle and on more than 250,000 separate measuring points, across a broad range of Mach numbers and pressures.

But Kelly preached that a precise model, even one like ours that was one-eighth the size of the real inlet, would provide precise measurements for the full-size model as well. So our wind tunnel testing was critical to the airplane’s success and usually ended at sunrise, when our exhausted little group of analysts finished computing the previous night’s test results. Nowadays, such calculations can be performed in a mini-second by supercomputers.

Kelly was now so desperate to save weight that he upped the ante to one hundred and fifty bucks to anyone who could save him a measly ten pounds. I suggested we inflate the Blackbird’s tires with helium and give each pilot a preflight enema. Kelly tried the helium idea, but helium bled right through the tires. The enema idea he left to me to try to promote among the pilots.

One bet easy to collect was that we would never have this airplane flying on time. By the end of 1960, we were over budget by 30 percent and Kelly was forced to concede we would be at least a year late in getting the Blackbird into the sky.