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Barnstormers for a New Age

Published by Sigurd De Keyser on Mon Mar 7, 2005 10:03 pm
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For the engineers behind the revolutionary SpaceShipOne, the project was as much about fun as victory. And no one had more fun than aerodynamicist Jim Tighe.

Paul E. Teague Design News Here’s the first thing you need to know about the engineering behind Scaled Composite’s Ansari X Prize-winning SpaceShipOne (SS1): The team was in it for the fun.

Okay, the prize was certainly an incentive, as was the team’s shared dream of civilian flights to space. But basically, says Scaled Composites President and SS1 leader Burt Rutan, the goal was to have fun, as it is with all the company’s revolutionary design projects.

And perhaps no one had more fun during the SS1 design effort than Chief Aerodynamicist Jim Tighe, voted by readers as the Design News/ Timken Engineer of the Year. Serious beyond the norm for his 29 years of life, yet possessing an innocent and subtle sense of humor that often left other members of the team rolling with laughter, Tighe had the main responsibility for achieving performance, stability, control, and handling-quality goals. They were challenging, to say the least.

To beat the 25 other teams competing for the prize, SS1 had to fly up to 382,000 ft (62 miles) carrying a live pilot and the equivalent weight of two other people twice in a 14-day period. SS1 accomplished the task in seven days rather than 14, flying 13,000 ft higher than the X-15, the government-funded rocket plane that last flew in 1968. Tighe’s engineering and computer prowess, say his teammates, was a principal reason for the SS1’s success. “He is a phenomenal guy, brilliant yet practical,” says Mike Melvill, the test pilot who flew most of the SS1 trials.

Unlike the X-15 or Shuttle, there was no government financial support behind the project. The team worked with an approximately $20-million donation from Microsoft co-founder Paul Allen as their only funds. But they turned the lack of resources into an advantage by concentrating on only a few things. And speed was one of the most important.

Light—and Fast—as A Feather

Indeed, if there is one big reason for the success of SS1—besides the passion and fun-loving, daring nature of the engineering team, it’s this: The plane just doesn’t go very fast.

It’s that simple. The faster an aircraft goes, the more forces it contends with. Double the speed and you quadruple the forces. It’s a matter of maximizing lift over drag going “upstairs,” as the pilots call the ascent, and getting higher drag coming down. Work out the balance and you win.

The devil is in the details. For starters, there are a few important details about speed.

“There are different types of speed,” Tighe says. Actually, SS1 has a very fast “true” air speed, meaning the vehicle speed. It can exceed Mach 3, which is faster than an M16 rifle bullet. But its “indicated” airspeed—that measured by sensors that compare impact pressure to ambient pressure—is very low.

And that lower speed means the engineering team could design a lighter vehicle, critical given the fact that they were working on a relatively limited budget. When it launches, SS1 weighs 7,000 lbs, half of which is propellant—a mixture of nitrous oxide (laughing gas) and rubber. By contrast, the X15 weighed about 31,275 lbs.

Another of the little details behind the successful SS1 flights is the concept of “feathering.” With the help of pneumatics, the plane basically folds in half at the wings when it reaches apogee, coasts up a little more, then gently falls back toward the earth like a shuttlecock in a badminton game, with its entire weight turned toward the atmosphere. The 60-degree angle of attack (most airplanes fly with less than a 10-degree angle of attack) dramatically increases the drag and slows the descent.

Feathering allows re-entry deceleration to occur at a higher altitude, and enables the vehicle to align itself belly-up to the atmosphere automatically, with no pilot input.


Like A Model Airplane

The feathering concept is the brainchild of Rutan, who was the Design News Engineer of the Year in 1988. The magazine Aviation Week And Space Technology calls the concept a variation of an old free-flight model-airplane trick where you pop the entire tail up. In Rutan’s design, the aft part of the wing pops up with the tail booms.

The feather is a simple pneumatic system. There are two levers to the pilot’s left that he uses to control it.

Feathering may be common in model airplanes, but no one ever attempted to do it in a manned aircraft before. “We are the only people crazy enough to try it,” says Tighe, who admits even he thought it was crazy when he learned of the plan after joining Scaled three years ago.

But being crazy is one of the other secrets behind the SS1 success. Rutan’s conviction is that you have to believe in nonsense to achieve anything remarkable. “Our motto here,” says Kevin Mickey, Scaled’s chief of program management, “is ‘Hurry up and screw it up so we can fix it.’” That’s an attitude that wouldn’t cut it at NASA with all its rules, procedures, and adversity to risk, says Rutan, who likes to pronounce the space agency’s name as “Nay Say.”

Still, there was a major risk that feathering would not work.

Faced with that uncertainty, NASA would have scheduled a long series of sophisticated wind tunnel tests before even trying to implement so radical a design. Not Scaled. Instead, to save time and money, Tighe relied on computer analysis with Fluent computational fluid dynamics (CFD) software. “We ran hundreds of iterations for every mach number and every angle of attack so we could predict the forces on the structure,” he says.

Actually, the Fluent code was a major tool for Tighe in many aspects of his aerodynamics work. He used it for performance predictions, estimates of structural loads (helpful in sizing the vehicle), and modifications he would later make during flight testing.

Even with the CFD, Tighe took some calculated risks. The meshes—or elements—he designed for his models were coarse, not the kind of fine elements that most engineers say they need for near-perfectly reliable analyses. But then, in keeping with the barnstorming spirit at Scaled, he wasn’t aiming for absolute perfection. “We figured 80 percent was good enough because there is lots of margin in the plane,” he says. “We don’t design so that things just barely make it.”


Flying on the Ground

With his CFD plots in hand, Tighe then wrote his own computer program for use in a special SS1 simulator where the test pilots would train before flying the vehicle for real. Actually, he had to write the program because no commercial program existed.

The simulator is another of the secrets behind the SS1 success. Designed by Scaled Composites engineer and test pilot Pete Siebold, the six-degrees-of-freedom unit has a cockpit identical to the one in SS1, with an accurate pilot interface and an uncanny resemblance to a video game. As Scaled engineers tell it, the interface drives a flight model that determines the vehicle’s position and inputs expected by the avionics. Using Tighe’s CFD results, updated by flight-data analysis, the simulator computes position and altitude, displays appropriate scenery (borrowed from the video game X Plane), and enables practice of landings, takeoffs, feathering, and flight-control maneuvers.

Tighe’s experience with the simulator became critical during the first test flight. There was severe rolling caused by a malfunction in the diehedral effect, where pushing on the right rudder pedal causes the nose to move to the right and the left wing to produce more lift and move to the right. In this case, that natural reaction went awry. Melvill was in the cockpit trying to bring the plane under control, guided by Tighe who was at a computer in Scaled’s Mission Control. “Having him there was very reassuring,” Melvill says.

Later, armed with more Fluent CFD runs, Tighe reprogrammed the simulator to replicate the roll and then devised pilot techniques to prevent a recurrence of the problem. They worked.

Roadside Wind Tunnel

Tighe’s computer skills (he has a B.S. in computer science and aerospace engineering from the University of Colorado) were critical in solving that particular rolling problem, but his engineering instincts and flair for the unconventional came in handy on numerous other occasions. Early in the test-flight program, the tail stalled before the wing, resulting in a 15,000-ft altitude loss, bad for the pilot’s stomach as well as the vehicle. Tighe had predicted that the tail would stall prematurely. When it happened, he already had potential fixes in mind, and an unconventional way to test them.

Tail stall prediction is a matter of a detailed analysis of what airplane people call trim. If a vehicle is in trim, all of the forces and moments are in balance. The vehicle lift equals its weight, and it does not want to pitch up or down. Of particular importance with regard to tail stall are the moments. SS1’s horizontal tail must generate a force that will keep the airplane in balance. For some particular conditions, which Tighe and the Scaled team are reluctant to specify, the horizontal tail was unable to generate sufficient force to keep the airplane in trim, and that’s what led to the tail stall.

The fixes involved modifications to the tail and wing, the specifics of which he won’t disclose. But he needed a way to test the adjustments. Wind tunnel testing would have been the method of choice, but there was no time and not enough money. So, Tighe strapped various configurations of a boom tail on top of a Ford F-250 pickup truck and drove around the perimeter of the Mojave airport to get the necessary flow field and find which configuration provided the best horizontal tail lift and tail lift slope characteristics.

“The premature tail stall was the most serious safety issue we had aerodynamically, and he predicted it where I didn’t, though we didn’t think it would be as severe as it turned out to be,” Rutan says. “Without Jim’s insights, we wouldn’t have planned our tests so well.”

With lavish praise for his teammates, Tighe says that after winning the X Prize, he almost feels like retiring. Indeed, he is consistant in his thanks to everyone at Scaled, from other engineers to pilots to administrators for their role in the project.

But don’t look for him to cash in his 401K just yet. Rutan sees him as one of the team members most likely to take over his creative duties in the future instead; a perfect role for engineer of the year— and a person with a long career ahead of him.

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