(FORTUNE Magazine) – ANYONE who has had a car rebuilt can relate to the U.S. Navy's overhaul program for aircraft carriers at the Philadelphia Naval Shipyard. The ships admittedly are bigger and more complex -- their generators make enough electricity for a city of 40,000 people -- but the concept is the same. Why buy a new car (average price: $12,000) or a new supercarrier (average price: $3.5 billion) when you can rebuild your old one just like new for less than a quarter of the cost? The $870-million renaissance of the 25-year-old U.S.S. Independence, pictured on these pages, is the third supercarrier overhaul since 1980 under the Service Life Extension Program. The Navy, dealing with seemingly constant challenges in the Persian Gulf and elsewhere, plans to rebuild its entire fleet of eight conventionally powered supercarriers by the year 2002 for a total price exceeding $7 billion. The brass are counting on at least 15 extra years of service from each rebuilt ship -- about half the expected life of a brand-new one. But shipyard bosses figure these carriers are as likely to go the full 30-year distance as anything with champagne spilled over its bow. Aside from some troublesome boiler leaks in the rebuilt U.S.S. Saratoga, ! the results so far have been uplifting. The Navy rates its supercarrier fleet every year, and last year the Saratoga and the rebuilt Forrestal won seven of the fleet's 13 battle-efficiency awards, including best overall ship. Renovating a 20-level structure with a four-acre flight deck, ten miles of electric cable, and thousands of pipes and valves -- not to mention the intricacies of advanced weapon systems -- is a mammoth logistical job. The make-over of the Independence took four years to plan and three more to complete. Engineers puzzled out new plans by superimposing proposed changes on computer screens showing the original plans as well as drawings of all previous repairs. During the ''rip-out'' phase, workers cut over 800 holes into the ship to pull out equipment and inspect and repaint the honeycomb of tanks in the hull. One of the trickiest jobs was lifting the four main engines and boilers through gaping openings carved through seven decks, taking the engines apart for rebuilding in shipyard shops, and then reinstalling them. Every part in the ship had to be numbered and color coded. The workers have replaced most of the parts and replated the holes they cut, grinding down the welds in the deck to remove the last visible scars of a massive operation. The crew is back on board and sometime next January a Navy admiral will come up from Washington for a test drive. If she passes, the Independence, which has already seen action off Vietnam, Lebanon, Israel, Iran, and Grenada, will then steam out for further adventure at sea.