The Lowest-Flying Jet Engine

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 http://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."