If you're wondering why it's impossible to keep defense costs down in this country, perhaps I can help you.

The U.S. Navy wanted a new plane and, after budget considerations, opted for the F-14 made by Grumman Aerospace Corp., in Bethpage, Long Island.

The F-18 designed by McDonnell-Douglas was scrubbed because it was too expensive and too controversial. This might have been the end of it except that the engines for the F-18 would be made by General Electric in Lynn, Mass. Strong rumor has it that House Majority Leader Tip O'Neill of Massachusetts and Sens. Ted Kennedy and Ed Brooke leaned on the White House and the Pentagon to keep the F-18 alive, and after several telephone calls Defense Secretary Harold Brown decided the F-18 might not be a bad plane after all.

Four hundred and nineteen million dollars has been set aside for "Research and Development" for the F'18, but a spokesman for the Secretary said the decision was made "on its own merits" and had nothing to do with O'Neill, Kennedy and Brooke's interest in the project.

I believe it.

The reason is that I talked to an expert who advises O'Neill and Kennedy and Brooke on military affairs, and he advised me the last thing any of the three legislators would want to do is waste money on an airplane the Navy said it didn't want.

"The Navy needs this plane," he told me. "In the worst way. They just don't know they need it. But once it's sitting on their flight decks they'll be grateful to Tip and Teddy and Ed for persuading them to buy it."

"What makes the F-18 so great?" I asked.

"The General Electric engines which are produced in Lynn, Mass., by the finest technicians and engineers in the country."

"How do O'Neill, Kennedy and Brooke know that?"

"They all have GE appliances in their homes and they're aware of what a great product GE makes."

"Some people have said that if the engines for the plane were not made in Massachusetts, none of the three would have gone to bat for F-18."

"That's ridiculous. Tip has always wanted a multi-mission fighter that could outfly anything the Soviets have. Teddy has dreamed of this country someday having an aircraft with an increased wing area and a wider and longer fuselage to provide greater internal fuel capacity. And I've heard Ed Brooke say many times, 'What this country needs more than anything else is a fighter plane with a radar dish on its nose.'

"When it comes to national security these three men would never let politics stand in the way of the Navy getting the best plane that money can buy."

"Even if the Navy doesn't want it?"

"What does the Navy know about fighter planes? Tip and Teddy and Ed have been making airplane models since they were 6 years old."

"But if the Navy already ordered the F-14 from Grumman what are they going to do with the F-18?"

"They can sell their F-14s to the Shah of Iran. He'll buy anything that flies."

"I never thought of that," I admitted. "The whole thing makes sense when you explain it. The Navy gets a plane it doesn't want, GE's Massachusetts plant gets to build the engine, McDonnell-Douglas gets to build the plane, and the security of the country will be safe for years to come."

"Tip and Teddy and Ed couldn't have put it any better."

"What will it cost the taxpayer?"

"If you have to ask what it costs to have a new fighter plane you shouldn't own one."