Like some fire-belching Saturn rocket, last year's airfare bargains are fast disappearing into the stratosphere -- and by winter's end will likely be only a distant memory.

Fueled by the oil crunch, air fares have risen rapidly over the past year. In some cases, airlines report, their gas costs have soared by as much as 75 percent since the start of the year. Obviously the traveling public has to foot the bill.

Winter vacationers will find fares up 15 to 30 percent on most routes; and more increases are in the offing. The nation's largest carrier, United Airlines, says that by year's end, its fares will have risen 27.5 percent since Jan. 1 -- and that it expects to increase fares 22.5 percent next year. Add 'em up and you have a 50 percent boost over two years.

The transatlantic market has also seen a sharp increase, though not quite as severe as those on domestic runs. Even Laker's all-standby Skytrain, last summer's bargain at about $250 round trip, is now at about $299 -- a boost of almost 20 percent.

About the best advice to would-be travelers is to book and pay for your tickets as soon as possible. Any domestic tickets paid for in advance are now valid regardless of later fare boosts. Many international carriers offer a similar guarantee -- but many do not. Be sure you get it clear before purchase.

If you can get a guaranteed price in anything these days you'll wind up ahead. Even now, airlines are charting new rounds of increases to cover higher costs. Some went into effect recently, others are expected to go into effect at the start of the year. And with more more fuel increases around the corner, the spiral is likely to continue.

The rapid transformation from boom market to boom prices over a six-month period has created chaotic conditions at reservation counters and travel agencies everywhere. Agents find it increasingly difficult to keep up with constant changes. For many, the proliferation of discount fares last year was tough enough; now additional time must be spent in verifying and updating prices. Uncovering the cheapest fare has become a chore of monumental proportions. Not only can one find, for example, one set of fares to London and another set to Amsterdam, but the same fare may be offered by competing airlines at slightly different levels. How do you know which airlines fly to a destination? Your travel agent will have a copy of the Official Airline Guide (OAG), which tells all.

Logic would seem to dictate similar types of fares, but airline logic has a mystique of its own. Most -- not all -- of the airlines to London offer budget fares (you pick the week, the airline picks the date) and standby fares, as well as Apex and Super Apex fares for advance purchase. Several offer a 60-day excursion fare, one offers a 45-day excursion. Several offer GIT fares (group inclusive tours, requiring a package outlay), some do not.

But on the Amsterdam scene, there's no budget or standby fare. Three of the five airlines have an Apex fare -- sorry, no Super Apex. Four of the five offer excursion fares and a couple have non-affinity group fares. And not all fares are identical.

From there, of course, one must examine all the strings attached -- advance purchase requirements, package and payment restrictions, trip duration, stopover limitations, cancelation penalties, ad infinitum. If you're going with a family, don't overlook the effect of children's discounts; they may vary from airline to airline, and from fare to fare.

Finally, the competitive drive may alter any and all of these at any given moment. If one airline sees another's fare significantly undercutting its share of the business, that fare will soon be watched. And you're apt to get a fare quote one week quite different from one the week before.

Combine that with the uncertainties of the fuel-price pattern, and you can see why happy fare experts are a vanishing breed.

Where does that leave you? Probably in a quandary, with little recourse but to find a diligent agent or be prepared to check the alternatives yourself. If you do, approach airline clerks with skepticism; the word on changes often filters down slowly. For many travelers, the time invested in such probes may not be worth the savings. But if your destination is a distant one, and you're traveling with one or more persons, a half-hour's worth of phone time may pay off with a handsome dividend.

If you were to book an excursion fare to Amsterdam, for example, you'd pay $445 to KLM or Finnair, which boosted that round-trip rate from $340 on Nov. 21. The lowest fare is now Transamerica's $419, but that will rise to $449 in December.

Apart from finding such differences, are there any fares left of true bargain dimensions? A few, certainly. Fares to many parts of the Orient and Southeast Asia have actually come down, although to some areas -- like Australia -- you'll have to wait till March (when peak season ends there) to take full advantage of it. Then you'll be able to get a round-trip, $610 budget fare or $632 Apex from San Francisco.

And while charters have all but disappeared to many major markets, you'll still find a few good values to select spots, particularly to Las Vegas, Miami, the Caribbean and Mexico. (Note, though, that many travel agents remain unenthusiastic about charters and their higher incidence of problems.)

But the big cost-cutters of a year ago -- the domestic Super Saver and the overseas Apex -- are cut from more expensive cloth this year. The much-ballyhooed $108 cross-country fare, for instance, is now enshrined in the history books. While the name "Super Saver" still attaches, this winter's bootom-line, one-way cross-country fare is already at the $138 level -- and counting. However, Capitol International Airways begins Skysaver service on Dec. 1 from New York to Los Angeles for $260 round trip (one-way only after Dec. 15 will cost $149). Pan Am has its $89 cross-country standby fare, but the numbers who can take advantage of it are severely limited.

Moreover, the fuel squeeze and some dire economic predictions have begun to force some cutbacks in service. Here and there across the country flights have been eliminated and personnel cut back. In some cases, there are fewer night flights -- traditionally among the lowest-cost domestic fares. Since promotional fares are already limited in numbers, that may make it tougher still to travel on a budget.

Of course, everything is relative. Inflation has propelled everything skyward in recent years, although airline spokesmen are fond of pointing out that fares are still cheap in comparison with those of 30 or 40 years ago.

Having said all that, some good news:

Federal deregulation of the airline industry has opened more routes to more airlines, with some immediate results. New competition has sprung up on transatlantic routes, with National and TransAmerica now flying from New York to Amsterdam, and Capitol inaugurating New York-Brussels service. The latter offers the lowest fare to the Continent at $350 round trip.

On the domestic scene, more flights will be offered this winter than ever before on the busy Florida runs; Pan Am has begun daily service on this route from New York and TWA joins the crowd at Washington National on Dec. 15. That's welcome news to Florida-bound travelers uneasy over the gas situation. If dates are not a vital consideration, some hard-nosed shopping among the lines serving Washington (Delta, Eastern, National, Air Florida, USAir and TWA) could save a few bucks. Air Florida has the cheapest weekday rates for a confirmed seat. Note, too, that TWA is offering a $10 discount on round trips Jan. 1-Feb. 29 (except 13-17) if you book through a travel agent.

Naturally, if you're into holiday travel -- over Christmas or mid-February -- early booking is a must. Many of those flights have long since been booked, and even waitlists are short on openings.

At home, Super Savers or night coach fares, where offered, generally offer the best value. Overseas travelers should check for the advance-purchase Apex or Super Apex fares, which aren't available everywhere.

As always, shopping is the name of the fare game. Now more than ever, selective buying is the key.