There is a popular image of the extreme sports guy: roughly 19 years old, baggy jeans and frequent use of the words "dude" and "stoked" while flipping up and down on big ramps.

Bob Swartz doesn't fit it.

He's 46. Married with two kids, he lives on a wooded cul-de-sac in Waldorf. He is an engineering technician for the U.S. Naval Research Laboratory, where he helps build classified antenna and computer systems around the globe.

But his sport of choice is street luge. He lies on his back on an elongated skateboard two inches from the ground. Feet first, he flies down roads at more than 60 mph. He recently finished second at a race in Upstate New York. To stop, he uses his feet, generating so much smoke that he's had to glue strips of motorcycle tires to the bottom of the wrestling shoes he wears while racing.

Over the years, Swartz has swallowed whole the challenges of this small corner of extreme sports, where participants have no dedicated place to practice. Cars pull out of driveways. Residents sometimes call the police. Cats and squirrels become hazards. When Swartz zips down hills in Charles County subdivisions, his wife, Cathy, often sets up midway down to serve as a lookout. She holds up signal flags and talks with him by two-way radio.

Like other things he does, Swartz's plan to take the sport more mainstream begins to make sense only after he's had a long time to explain it. In this case, he recently bought a $5,000 jet engine -- one designed for unmanned military aircraft -- and attached it to the back of a luge. He gives exhibitions at major drag races, having thus far hit 77.76 mph. He aims to break the coveted 100 mph barrier, perhaps by advancing to a twin-engine design next year.

"What possessed you to do this?" Swartz was asked last weekend over a drag-strip public address system in Rockingham, N.C.

"To draw attention to gravity sports," he told the crowd, referring to how he races down hills without jet power. Then he shot down the track. As part of Rockingham's pre-Halloween nighttime races, Swartz donned a glow-in-the-dark skeleton suit over his thick, protective leather racing uniform. He also wears a motorcycle helmet.

Swartz grew up the son of a wallpaper hanger and a nurse in southern New Jersey. As a 7-year-old, he remembers, he fashioned a go-cart out of wood scraps and a set of small wheels his grandfather gave him. He rebuilt bikes and lawnmowers. His parents thought he'd be a scientist or a doctor.

He didn't like classrooms, though, and enrolled in electronics vocational school. Along the way, he rode dirt motorcycles. He and Cathy also went scuba diving. In 1994, Swartz was flipping through TV channels when footage of street luge riders in Seattle stopped his fingers. "Uh-oh," his wife said.

What followed were 10 years over which Swartz rode the crest of the sport and then wiped out along with it -- both professionally and personally.

Street luge can be breathtaking, especially when filmed by tiny onboard cameras. Riders steer by leaning left or right. They draft behind one another, like stock-car racers, which allows them to build up momentum to zip by the rider in the lead. In 2000, Diane Sawyer climbed aboard a street luge for a gentle spin through Manhattan's Riverside Park on "Good Morning America." Before she did, Swartz helped teach her.

But the sport couldn't sustain itself. To put on races, organizers had to convince local officials that it was a good idea to close off long sections of their roads and lay down hay bales so errant riders wouldn't fly into signs, trees, guardrails or spectators. Promoters fell away. The sport was dropped from ESPN's vaunted X Games in 2001.

Swartz kept at it. He held safety clinics for new riders, preaching the art of using the luge as a shield in the event of trouble by holding it and turning away from an oncoming object. "Riders young and old: Listen to Bob," racer Richard Hodkinson, a doctoral candidate at the University of Southern California, once posted on a street luge message board. "He's saved my limbs at least six times with his advice."

Yet Swartz pushed his own limits. In 2002, he broke his right shinbone during a New Hampshire race. At a subsequent Fourth of July barbecue at a friend's house, he used a small electric saw to remove the cast, affixing a smaller one so he could compete in a key race he'd qualified for in Kaunertal, Austria. "It was obvious to my family that I was just obsessed," he said. "I couldn't let it go."

After nearly 25 years of marriage, Swartz said, Cathy moved out. He halted luging. "My whole focus was getting her back," he remembered.

The two one-time Roman Catholics began attending a nearby Baptist church and eventually got back together.

But Swartz never lost his need for speed.

Last year, while tooling around the Internet, he found a jet engine that offered intense power at only five pounds. Putting it together fit his new priorities to spend more time at home, rather than drive off in search of hills in western Virginia or a race in South Africa. His plan, chronicled on www.jetluge.net, alarmed such friends as Darren Lott, author of the "Street Luge Survival Guide." Lott's concerns eventually were allayed when he learned of the luge's safety features. One of the two onboard computers automatically helps shut the vehicle down, for example, if overheating or other problems are detected. "It was the old Bob," Lott remembered thinking. "And Bob hadn't lost his mind."

For this, drag-racing fans can be thankful.

"Take a look at the starting line, folks. You're not going to believe this," Aaron Polburn, president of the International Hot Rod Association, announced on a recent Friday night at Maryland International Raceway in St. Mary's County. Swartz tore down the raceway. "I have now seen it all," Polburn said, cracking up.

He expects to hire Swartz for at least four national dragster shows next year, hoping to draw out Swartz's articulate nature with more interviews over the PA system. "When his mouth opens," Polburn said, "it is the complete opposite of what he does."

Swartz still pursues traditional gravity luge and hopes to well into his fifties -- even if he often has to do so on the relative flat terrain of Charles County. On a recent Sunday afternoon, a luge rider half his age, Justin Crenshaw, arrived at his home from Fairfax. Swartz has taken the former top-level soapbox derby rider under his wing.

The two climbed into the Swartzes' big blue truck. Cathy drove them to a nearby subdivision off Bensville Road. On the way, Swartz turned to the second seat inside the truck's cab, offering safety tips to Crenshaw. If you veer off the road, lift your feet. "They continue to try to brake," he said of other luge riders, "and that's what breaks feet."

Cathy dropped them off atop a hill and drove down to take a spotting position. "Wait, wait," she said into the two-way radio, sticking a warning flag out the window. "A guy's coming up the hill. . . . Okay, you're all right."

Moments later, the two whizzed by, smoothly negotiating a hairpin turn. Cathy hit the gas, pulling in behind them to guard against cars approaching from behind and to haul them back up for more practice. Swartz said he stays in his lane and follows the rules of the road, like a fast cyclist. And on this day, he hit only 35 mph. No residents complained, and one even offered to help.

"Car behind you, guys," Yvonne Clements said atop the hill, standing on a driveway.

"Thank you," Swartz said from inside his helmet. He waited for the car to pass. He and Crenshaw headed down.

Later that day, he and his wife were back home, working at adjacent desks in their study. Cathy worked on finances for their church. Swartz looked up Internet video of a man in Tennessee who had attached a cluster of rockets to the back of his luge, a man Swartz has been in touch with. "It would be a neat show," he said. "Rocket versus jet."

Bob Swartz waits to start a ride with his luge, which has a $5,000 jet engine attached to it, at the Maryland International Raceway.Swartz speeds through a turn ahead of Justin Crenshaw. Crenshaw, a former top-level soapbox derby rider, traveled from Fairfax to practice with Swartz in Charles County.