U.S. Navy's Great Green Fleet debuts in Pacific By Suzanne Roig HONOLULU (Reuters) - The U.S. Navy's "Great Green Fleet," a group of warships and fighter jets burning an expensive blend of biofuels and petroleum, made its operational debut on Wednesday as the Senate prepared for a political fight over the program's cost. Squadrons of F/A-18 Super Hornets powered by conventional jet fuel mixed with recycled cooking grease and algae oil screamed off the flight deck of the aircraft carrier USS Nimitz, during international military exercises in the central Pacific. Two destroyers and a guided-missile cruiser plied the ocean using a similar fuel mixture. Congressional critics, led by Republican U.S. Senator John McCain of Arizona, have argued biofuel is far too expensive for the military to help develop at a time when defense budgets face massive cuts. But Navy Secretary Ray Mabus said Wednesday's demonstration proved that the green-fuels blend, while about four times more costly per gallon than conventional fuels, was safe and effective in combat situations. Those aircraft are flying the way they always do. The ships steamed the way they always do. There was no difference with the fuel," he told sailors and reporters assembled in an aircraft hangar aboard the Nimitz, the carrier group's flagship. The Nimitz itself runs on nuclear power. The so-called Great Green Fleet is a key element of a Pentagon initiative to use the buying power of the U.S. military - the world's largest single oil consumer - to help foster a competitive biofuels industry. Mabus and other supporters of the program say curbing the military's reliance on fossil fuels and making alternative energy more commercially viable would ultimately lower costs while bolstering national security. The green-fuel operations of the Nimitz's strike group were conducted as part of this year's 22-nation Rim of the Pacific training exercises, the largest annual multinational warfare maneuvers on the high seas. The war games, staged 100 nautical miles north of Hawaii, began last Friday and will run for six weeks. The 450,000 gallons of biofuel the Navy purchased at $26 a gallon for the occasion and blended in equal parts with petroleum-based fuels to fill up three warships and 71 aircraft was just enough to last two to three days, Navy officials said on Wednesday. A year-old Defense Department report predicted the military will spend $2 billion more annually if it pursues its biofuels goals. And a more recent study by the RAND Corporation think tank said renewable fuels for U.S. military vessels and jets are likely to remain far more expensive than petroleum products absent a technological breakthrough. McCain and other opponents of the program have seized on the fact that the Navy paid more than $26 a gallon for the biofuels it purchased for this week's Green Fleet demonstration, compared with less than $4 a gallon for convention fuel. The Navy has noted that the 50-50 mix of biofuel and petroleum-based fuels, formulated as a "drop-in" blend requiring no modification to aircraft or ship engines, cost a combined $15 a gallon. Most of the biofuel half of the mixture, about 90 percent, was rendered from cooking oil waste. The other 10 percent was refined from algae. Still, critics have painted the green fuels initiative as a waste of funds at a time when the federal budget, including Pentagon spending, is severely strained and energy companies are finding large quantities of oil and gas in the United States. Congressional Republicans have denounced the military's green energy push as another attempt by the Obama administration to promote alternative fuels even when they make little economic sense, as in the case of the government-funded solar panel maker Solyndra, which went bankrupt last year. In Washington, the U.S. Senate is girding for a political battle over legislation backed primarily by Republican lawmakers to bar further military spending on biofuels that are more expensive than petroleum products. Mabus, however, argued that it was in the military's best interests to help establish a stable supply of competitively priced alternative fuels, saying the volatile nature of oil markets leaves the Defense Department vulnerable. Every dollar increase in the price of a barrel of oil costs the Navy alone $30 million, he said. Additional reporting by David Alexander in Washington; Writing by Steve Gorman; Editing by Andrew Heavens