Capturing Sunlight

Jurassic jets work to improve space travel

December 1, 2005

"Feel that? I'll bet that's the trop."

"Yeah, that's probably it." Bill Rieke, chief of aircraft operations at NASA's Glenn Research Center, is hand-flying a Learjet 25 from the right seat and Kurt Blankenship, the center's senior pilot and safety officer, is flying left seat as we pass through 37,000 feet about 50 miles east of Detroit. We're flying a solar-cell-calibration mission to collect data on the cells' performance. The pilots are describing the unique light turbulence that comes in two little waves; the first comes at 37,000 feet, then there are another couple of bumps 500 feet higher.

We feel that gentle jostling only because of the very smooth air we're flying in since leaving Cleveland 15 minutes earlier. Those soft bumps mean that we most likely have transitioned from the troposphere — or "trop" — to the stratosphere, an observation that's confirmed a few minutes later by a slight temperature increase at 40,000 feet.

Today's flight is one of many flown by NASA to calibrate solar cells for future space vehicles. The data collected on these flights are extrapolated to measure solar cell performance in space. Ultimately this work will find its way to the moon and Mars as refined solar arrays that power satellites and rovers.

Our platform for this mission, though, is anything but Space Age. Built in 1965, it soldiers on into the rarefied air of the upper atmosphere with aplomb at a point in its life when many similar airplanes would have been retired: The Learjet is old in age but young in flight hours and high-value test modifications. Those modifications include altered engine nozzles designed to lessen the chance of a compressor stall and flameout; the addition of a movable containment vessel and hatch that allows the solar cells to be exposed safely to stratospheric sunlight; and multiple types of electrical power for this and other experiments.

But even with the modifications, it's still an early Learjet — built back when the name was two words — with all of the high-altitude handling warts that made this model famous, or infamous. With the original wing, the difference between Mach overspeed and the stall buffet at 45,000 feet is barely the width of the needle on the airspeed/mach meter. Nonetheless, the NASA flight crews put this old airplane through its paces, piloting some of the most precise flights imaginable — all in the name of science.

Solar cell calibration by aircraft requires flight above the troposphere (home to about 95 percent of atmospheric particulates and moisture) into the stratosphere, which is almost entirely clear of these light-scattering elements.

Rieke hand-flew the Learjet all the way from Cleveland. For these missions, the pilots complain about the autopilot, saying it is an unnecessary piece of equipment that only adds weight. I wasn't spared, either. On the ground they groused that my body weight, another seat, cameras, helmet, and oxygen equipment cost them about 600 feet in altitude.

Still, the airplane jumps into the sky. We taxi from NASA Glenn's ramp onto the taxiways at Cleveland-Hopkins International Airport. It's 32 degrees Fahrenheit and we're lightly loaded, so we're airborne with gear retracted less than halfway down the 9,000-foot-long runway. By the time departure control is contacted we are already at our intermediate level off at 6,000 feet and throttling back to stay under 250 knots indicated airspeed (KIAS). After climb clearance the thrust levers are set at maximum possible thrust and stay there until we start our descent. I'm crammed into a seat, surrounded by scientific equipment, computers, and the bare aluminum airframe.

Sun capture

When we hit the top of the trop, the rate-of-climb indicator is no longer pinned at 6,000 fpm as it had been when we were climbing through 10,000 feet. It hovers around a 1,000-fpm rate of climb. As we pass 45,000 feet our rate of climb drops to 400 fpm. The airspeed is now about 200 KIAS, much different than the 280 in our initial climb once our airspace-induced speed restriction ended. However, that 200 KIAS shows we're traveling at 76 percent of the speed of sound (Mach 0.76). Blankenship now asks Minneapolis Center for a block altitude of Flight Level 450 to 510. Minneapolis clears us and we're allowed to maneuver in that altitude range until we start a descent.

"That's as high as we'll get today," Rieke says as we plateau at 47,000 feet to keep the airspeed at 190 KIAS or above. This leaves about 50 knots for the zoom climb. Blankenship now notifies Minneapolis Center that in about four minutes we'll need a pilot's-discretion [PD] descent to 33,000 feet. Center replies, "NASA 616, you're cleared PD down to FL330." (That's a few thousand feet below the estimated top of the troposphere.)

With the cabin altitude at 26,000 feet, Blankenship flips a switch and the solar cell hatch on the left side of the fuselage retracts over an opening that was once a window. The unshuttered window exposes the collimating tube, which adjusts the line of sight with the sun so that only solar and not ambient light hits the cells. The tube limits the solar cells' field of view to the sun. Because of the flexibility required in mounting the tube to the fuselage, the cabin is run at reduced pressure to relieve the stress on the attachment point.

We're using a military oxygen system and masks so we can pressure-breathe in case there's a breach in the containment tube with subsequent cabin blowout. In fact, we've been breathing 100-percent oxygen since engine start some 50 minutes ago to displace the nitrogen in our blood — a precaution to prevent aeroembolism (the bends) during decompression.

Until now we've been climbing on a 360-degree heading. Rieke now starts a shallow turn to a westerly heading. The exact required heading is determined by the sun angle and the time we arrive over the starting point for the maneuver. He explains that the initial positioning prior to pull-up is a little "by guess, by golly" because the exact heading isn't known until the airplane is in the descent with the sun in the middle of the sighting device.

Fancy flying

Now Rieke says we're ready for the pull-up, and Blankenship turns on the engine ignitors as Rieke throttles back slightly to guard against flameout. The nose is pulled up about 20 degrees and the pushover begins as the airspeed slows to about 160 KIAS. The airspeed bottoms out at 140 KIAS as a white, hot dot the size of a BB flies from the bottom-right corner of the glareshield-mounted sight to its center and stays there. The altitude peaks at 49,000 feet at the top of the loft and the airplane starts to descend.

During this delicate maneuver, the trick is to acquire the sun dead center in the sun sight (what the pilots call the "solarcell" sight), keep it there, and establish a rate of descent at 1,500 fpm without changing angle of bank more than 2 degrees. All this while hand-flying a 20-series Lear, an airplane well known for its unique handling at altitude.

There's more. To accurately track the sun, the pilot can control his rate of descent only by varying the heading. Two degrees is the absolute limit, but it's a matter of pride among the three pilots who fly these missions to stay within 1 degree of bank. As if that weren't enough to worry about, the pilot must simultaneously reduce power to not exceed Mach 0.82, the Lear's limit, as the airplane descends.

"I've learned to keep focused on the solar cell sight, keeping the sun dot in the center. Then you learn to adjust trim, pitch, and bank, and throttles," Blankenship explains.

There's another element in the mix that is controlled by the "professors," the research pilots' term for the ground-bound Ph.D.s and scientists who run the science experiments.

It is up to the professors to set the tube and sun sight on the ground to the corresponding sun angle for the time, date, and geographic position. If the tube and sight are mis-set by a degree or so, "you wind up cross-controlling the plane to keep the sun in the sight. For small errors you can correct with cross-control. For a larger error, you may be unable to fly it," Blankenship says.

The actual sun tracking is about 10 minutes of intense concentration, and many times the pilot must hold rudder pressure if the sight is misaligned or "if you are late or early by more than a few minutes. At the same time the yaw damper is fighting to keep the airplane in balanced flight. It's very challenging to keep the (sun dot) dead center while you're descending at 1,500 fpm, coming from less dense air down to the 30,000-foot range. That affects inputs as well," he adds.

And how well they've managed to keep that BB in the middle of a circle one-inch in diameter comes the next day when David Snyder, the principal investigator for solar cell calibration, sends the data plot for the pilots to review. "The good ones are hung on the board with names and dates, the less-than-perfect plots somehow disappear," Rieke says.

A dicey business

Many pilots would prefer not to hand-fly a 20-series Lear above 35,000 feet, but for these pilots hand-flying the Lear at 49,000 feet is all in a day's work. During the pull-up, pushover, acquisition, and initial sun tracking the demands on the pilot are intense because you're hand-flying the Lear above its certified altitude or near stall during a pushover maneuver of less than 1 G. At 140 knots, the aircraft is very close to stall and the pilots are intensely focused on the sun sight. "It really doesn't matter what your attitude gyro indicates. The sun sight is the true reference. You do whatever's needed to keep the sun centered," Rieke explains.

Finessing the aircraft at high altitudes and low airspeed coming out of a 20- to 25-degree nose-high position to achieve the right speed and heading, achieve the correct rate of descent with the guidance system, and be in the position to have the sun dead center in the sight requires a certain amount of skill. "This maneuver definitely hones our skills for our other missions," Rieke says.

Solar cell research

The sun sight is mounted directly in front of the pilot and when the BB-size sun dot is centered, the sun shines directly on the solar cells at the base of the collimating tube mounted on the left side of the fuselage. When the sun is aligned perfectly with the tube, it shines directly on the solar cells, which are mounted at the tube's base.

With the sun centered, a heading slightly south of the desired heading means a high rate of descent. A heading slightly north of the desired heading means a too-slow, or no, descent. If that happens the pilot must push the nose down to avoid stall, but that means he loses the sun, Rieke explains. "You could screw up and lose the sun and take about 2,000 feet to re-establish the desired rate of descent with the sun centered." That means lost data points, which are captured automatically every 10 seconds.

Mission completed

Ten minutes after the loft and some 65 minutes after we leapt into Cleveland's murky, gray sky we land at Traverse City, Michigan. It's a beautiful clear day here, and everything so far has gone according to plan. Except for my experiment, that is. I was along for the ride with a small role to play in testing a new non-flight-qualified computer system at a cabin altitude of 26,000 feet for a potential upgrade of the data acquisition system. The computer I was testing couldn't handle the 26,000-foot cabin pressure and froze 20 minutes into the flight. While this modern piece of electronics can't stand the environment, the old Learjet seems to relish its high-tech role, providing bits and bytes of data that will make future space travel more feasible.


Leslie Sabbagh is a science journalist specializing in medicine, aerospace, and the military.  She is a student pilot.