When architect Dave Sellers invented the Mad River Rocket in 1987, his goal was to build a safer and more maneuverable sled so that kids in Warren could ride down nearby Prickly Mountain without crashing into the trees. He never imagined that one day they'd be using his sled for sailing off jumps, pulling back flips and barrel rolls, and even bombing down snow-covered rooftops. But as Sellers puts it, "The older I get, the more ideas I want to get out there and let them run."

They ran with it, all right. In the last few years the Mad River Rocket Company in Warren has launched an entirely new winter sport: freesledding. Their lightweight but durable sleds have opened up thousands of acres of Vermont mountain terrain that are too densely wooded to negotiate on a snowboard or skis. In the process, Mad River Rocket has become one of Vermont's fastest-growing winter recreation companies, with sales soaring into the stratosphere.

Surprisingly, the Rocket nearly aborted before leaving the ground. Shortly after Sellers invented the sled, he took one to the Seattle headquarters of REI, one of the nation's largest outdoor-gear retailers. When he finally met the person in charge of winter sporting goods, the guy seemed thoroughly unimpressed with his invention. So Sellers gave him an impromptu demonstration of the sled's capabilities -- by riding it down a winding, carpeted staircase.

"I'm sliding and zigzagging around the stairwell and the guy goes, 'Holy shit!'" Sellers recalls. "I walked away thinking, These guys will sell a million sleds. They never bought a single one."

That's likely to change soon. The Mad River Rocket has caught on among a wide range of outdoor winter enthusiasts who are drawn to the sled's versatility, comfort, durability and light weight -- it's only 47 inches long and weighs 6 pounds. Snow-shoers can carry them up the mountain on their backs and then sled down an ungroomed slope at the end of the day. Ice climbers can use them for dragging their gear to a climbing spot and then schuss home through the trees after an exhausting day on the ropes. The largest age group now using the Rockets aren't children or early teens but 17- to 30-year-olds.

Part of the Rocket's appeal is that it's easy to ride and control. Unlike a traditional toboggan or sled that you sit or lie on, the Rocket rider kneels on foam kneepads in a crouched position, similar to kneeboarding or C-1 canoeing; the company's slogan is "Get on your knees!" The Rocket has a hard, polyethylene hull and webbed, nylon straps that fasten across the back of the rider's legs, leaving both hands free for steering, balancing and braking. Riders can carve turns down a mountain like a snowboarder and use their hands to stop or pivot around trees.

Like a kayaker, a Rocket rider sits high enough on the sled to see what's coming up ahead but low enough to duck under branches, sap lines and other obstacles that would clothesline a skier or snowboarder. Technically speaking, you don't fall off a Rocket so much as roll over in one. And according to those who have ridden them, it only takes a few minutes to get the hang of it.

The Mad River Rocket Company has been around for more than a decade, but the business is just taking off. Sellers gives all the credit to the company's 25-year-old president, Whitney Phillips, an extreme-sports enthusiast who was hired three years ago. Phillips has transformed the company from a small, basement operation to a global enterprise with sales reps throughout the United States, Canada and Japan.

Through much of the 1990s, Mad River Rockets were vacuum-pressed and assembled in the cluttered basement of Sellers' architectural firm in Warren and sold to locals right out the back door. But the company was hemorrhaging money -- between $70,000 and $100,000 a year. Why? "Corporate liability insurance," Phillips explains. "You tell 'em you're putting a kid on it and sending him down a hill, and they're like, 'Whoa!'"

Another factor? Sellers' passion isn't sporting goods, but architecture. His innovative and unusual projects range from the renovation of the historic Pitcher Inn in Warren to the design of Patch Adams' new hospital in West Virginia. The Gesundheit Institute, as it's known, looks like something lifted from the pages of a Dr. Seuss book, complete with an enormous, eyeball-shaped examination room, a performance stage that resembles a smiling face, and the world's largest whoopee cushion.

But Sellers wasn't interested in letting the sled business die, either. So he brought Phillips aboard. "He's a natural athlete and knows the whole ski-industry business," says Sellers, "and, he loves every wild sport there is." Phillips, a former ski racer who attended Champlain College and the University of Vermont, realized that the company needed to change its infrastructure if it wanted to sell to large retailers. He invested in new pressure molds and hired a Dumpster-lid manufacturer in Dayton, Ohio, that could press out the Rocket sleds in record time.

"It used to take us eight weeks to make 500 sleds," Phillips recalls. "Now we can do 2500 units in 24 hours." In Phillips' first year, Mad River Rockets sold 280 sleds; the second year, 700. This year, Phillips says, the company is on track to sell 6000 sleds. The adult sled retails for about $85.

These days, the sleds practically market themselves. Several years ago, Phillips got a phone call from a couple of teenagers in Underhill who asked him: What are the best tricks anyone's doing on the Mad River Rocket? "I said, 'I don't know -- a back flip?" Phillips recalls. "They said, 'Can we send you a video?'"

The two-hour home movie, shot by Nathan Steinbauer and Isaac Fleming, featured the teens flying across their yard on the side of Mount Mansfield, pulling back flips, rail slides, 720s and other feats of aerial insanity. In one scene, a Rocket rider sleds from the upper roof of his house to the lower roof, then drops into a snow bank. In another, one of the teens launches off a cliff and lands in a cushion of powder. Steinbauer and Fleming even set up a website -- http://www.freesled ding.homestead.com -- that features their X-Games-like stunts, including instructions on how to do them.

"These guys are so into it," Phillips says. "Every year they go around to their neighbors and collect their leaves so they can make those big-ass jumps in their yard... Now they're getting 10 to 12 sleds from us a year." And, it's spawned a new generation of younger sledders who are getting into the act.

Despite all this local enthusiasm, Mad River Rockets aren't allowed on Vermont's ski lifts -- yet. But that, too, may soon change. Phillips says he's talking with a couple of local ski resorts about setting up a pilot project to allow the sleds on their slopes --a battle snowboarders fought not that long ago.

In the meantime, the number of places to freesled in Vermont is virtually unlimited. And Sellers, an alternative-energy buff, makes another good point: The only fuel you'll burn while riding the Rocket is whatever you ate before hiking up the mountain.