Chop a beach-front condominium, a mountain ski resort, or a hotel suite in San Francisco, Paris or New Orleans' French Quarter into separate ownership slices and you've got the hottest new concept in U.S. real estate.

"Time-sharing" is its name, and it is defying every economic and regulatory roadblock thrown in its way. Despite record-high interest rates and a real estate recession, consumers are buying $2,000 to $12,000 ownership shares as fast as developers can put them on the market.

More than 215,000 Americans have bought time-share "second home" interests in the past four yers, and total sale have doubled every year since 1977. This year a projected $1.5 billion in resort and urban time-shares may be sold -- up from about $800 million in 1979 -- a seeming contradiction in the midst of a sagging national economy.

The key to this phenomenon, according to time-share industry leaders who gathered here recently for a national conference, is that it is uniquely suited to an inflationary era.

Only about 2 percent of American families can afford the luxury of buying a new second home at the beach or in the mountins -- with typical cash outlays of $25,000 to $35,000 the first year alone. But the majority of Americans go on vacations every year, and a large percentage can afford to finance purchase of a one or two-week "share" of a second home.

The share may take the form of real estate -- complete with deed, title insurance and a mortgage -- or may take the form of a 10- to 40-yer "right- [words illegible] license, where a legal title remains with the project developer. Costs normally involve a one-time purchase price averaging $3,900 per week nationally and small annual maintenance charges starting at an average of about $120 for a week of time-sharing.

Beyond these expenses the unit costs nothing further for years into the future, enabling buyers to lock in vacation prices at 1980 levels. The time-share can be used by its owners every year, exchanged for someone else's in a resort thousands of miles away, rented out or sold for a profit.

Time-sharing of real estate began in Europe in the 1960s, but has found its most fertile market in the United States since the mid-1970s. Originally imported here to help rescue resort condo and hotel projects that were sitting empty after the 1974-75 recession, time-sharing has become a full-fledged real estate specialty in its own right.

Resorts containing time-shares now exist in 26 countries, and the concept is beginning to make in-roads in the corporate, non-recreational real estate market, where firms seek to control their recurring travel expenses by purchasing time-shares in San Francisco condos, London hotels and properties in Rio de Janeiro, Hong Kong, New York and other major urban centers.

Time-sharing is also making a dent in the U.S. and foreign sailboat market, with several companies offering fractional interests in 50- to 90-foot yachts. Some land-based developers offer time-shares that include both real estate and yacht ownership rights.

"Time-sharing has an almost unlimited future," says ebullient Keith Romeny of Salt Lake City, the attorney who heads the Resort Timesharing Council, the industry trade group.

He forsees time-shares being sold and resold by major Wall Street securities firms, neighborhood real estate brokers, travel agents and hotel chains -- in the hundreds of thousands.

Time-sharing does indeed appear to have the market ingredients for hugh growth, but also has significant potential pitfalls for consumers:

Time-share developments are by no means completely safe real estate ventures. Nearly 2,000 buyers of shares in a Colorado development known as the Stanley Hotel Vacation Club could lose $5 million as the result of a foreclosure against the developer. Other time-share resorts -- generally "right to use" projects rather than condominium projects -- have been mismanaged by inexperienced promoters and face financial shakeouts.

Sales misrepresentations of investment returns on time-share units, exchange possiblities, maintenance fees and future facilities are common. The Federal Trade Commission, as well as a number of state consumer protection offices, are investigating complaints against dozens of projects.

The quality of long-term management of many projects is a complete unknown, even to the developers most active in the field. Fractional ownership of properties can lead to poor maintenance in the early years, creating severe problems five to 10 years down the road.

Buyers should look hard at management plans before they sign to decide whether they're realistic. Otherwise they could get stuck with fractional interests in resort debts, rather than care free vacations.