The Senate Appropriations Committee, while claiming to be looking for fat in President Reagan's defense budget, last week ordered the Pentagon to revamp a rocket production program so some of the work could go to a company in Louisiana.

The shift was made for political reasons, not military. The Army says the change could waste $100 million.

In the House, the chairman of the Appropriations subcommittee on defense is demanding that the Navy buy a radar from a contractor near his district.

The Navy says this would waste $2 billion.

The debate about the defense budget is mostly a lofty one. The arguments tend to be over such things as national security and fiscal restraint. But the defense budget also has an earthier side: defense projects, just like public works and all other federal projects, are a form of pork.

The politics sometimes are even international.

Northrop Corp. of California, whose chief executive is friendly with Reagan, is in desperate need of customers for its F5G Tigershark fighter plane. The company has spent more than $360 million of its own money on the plane so far, and still has no firm order in its book.

But the oil-rich sheikdom of Bahrain on the Persian Gulf has now conveniently decided that it wants to have a modern air force, and the Pentagon thinks it has just the plane. The administration announced last week that it intends to sell Bahrain four Northrop F5G Tigersharks.

This is even though the administration earlier this year had adjudged the F5G too advanced to be sold to another country, Taiwan, offering instead the older Northrop F5E fighter plane, a weapon less threatening in China's eyes.

Political judgments are pre-empting military ones in two of several log-rolling exercises within the Senate and House appropriations committees.

Exhibit A is Sen. J. Bennett Johnston (D-La.) against the Army in the matter of the Multiple Launch Rocket System, which would hurl 13-foot rockets against enemy strong points in much the manner of the "Stalin Organ" of World War II fame.

Johnston last week persuaded the Senate Appropriations Committee, of which he is a member, to require the Army to split up its rocket business rather than stick with the Vought Corp. to save money through high-volume production.

Johnston, in a letter to Chairman Ted Stevens (R-Alaska) of the Senate Appropriations subcommittee on defense, said the Longhorn Army Ammunition Plant and the Louisiana Army Ammunition Plant in his state should get the chance to get in on some of the $4 billion Multiple Launch Rocket System domestic production. The Thiokol Corp., which lost out early in the rocket competition, would make part of the rockets at those plants.

Johnston put the proposition to Stevens like this: "The Army should be directed to assure both long-term cost competition and a reliable source of supply by establishing second source production at the Army's existing facilities."

Objecting to Johnston's proposed derailment of the Army's plan to commit itself to several years' production from Vought if the price was right, Undersecretary James R. Ambrose fired off a letter to Stevens, saying:

"I do not support a directed second source as a sound business decision from the perspective of the Congress, the Army or the country."

In attached memos, the Army hit harder, saying the Johnston plan " . . . appears to request the Army to disregard real competition." It said the senator's letter to Stevens "is another attempt by the losing contractor in a previous head-to-head competition to politically reverse a well conceived and workable acquisition strategy in an environment advantageous to the Army and the U.S. taxpayer."

Another Army memo said it would cost between $93 million and $105 million to gear up the Louisiana plants to produce the Multiple Launch Rocket System.

Vought is producing the rockets in Camden, Ark. Arkansas' two Democratic Senators, Dale Bumpers and David H. Pryor, are fighting Johnston in an effort to keep all the domestic production in their state.

A Vought official said 700 company jobs are tied to the rocket contract, half in Camden, Ark., and the rest in the firm's headquarters in Dallas. Vought would build up to 30,000 of the rocket systems for about $4 billion if it gets the full domestic order.

Johnston persuaded the Senate defense subcommittee to add language to the Pentagon's fiscal 1983 appropriations bill directing the Army to reopen the rocket competition.

Bumpers failed on a 13-to-ll vote in the Senate Appropriations Committee to delete those marching orders from the money bill. Other attempts to do this will be made when the bill comes up for a vote on the Senate floor, probably this week.

Over in the House, Chairman Joseph P. Addabbo (D-N.Y.) of the House Appropriations subcommittee on defense has ordered the Navy to buy a radar made only by the Sperry Corp. The firm would build it in Great Neck, N.Y., boosting the Long Island area economy even though the plant is outside his district in Queens.

Sperry would get millions, perhaps billions, to improve the Mark 92 firing system on the Navy's FFG7 Oliver Perry class frigates. But Pentagon radar experts say the radar is not needed by those ships, that it would be virtually useless against Soviet radar-evading, Stealth anti-ship missiles and would cost "over $2.2 billion."

In one of many memos opposing the Addabbo-Sperry plan to upgrade the Mark 92, the Pentagon said the big threat to the light FFG7 frigates is Soviet submarines, not warships or aircraft. The FFG7s were given radar and related equipment to take on cruise missiles fired from submarines.

Part of the memo said: "It is characteristic of submarine-launched anti-ship missile attacks that the missiles do not arrive in large numbers and are not supported by jamming, since the submarine cannot jam without revealing its position, but do come in very low on the water and at very high speed. In order to defeat such attacks, the anti-air-warfare missile system must, above all, have very quick reactions. The Mark 92 firer control system with the Mark 13 gided missile launcher and the medium range Standard Missile 1 was selected for the Perry class with this in view."

The Pentagon said Sperry contends that its new radar system could be added to the fleet of FFG7s for $1.4 billion, but the real cost would end up "over $2.2 billion."

Although Addabbo has directed Navy Secretary John F. Lehman Jr. to go ahead with the Sperry radar, it will take the approval of his subcommittee and the parent House Appropriations Committee to fund the radar the Navy does not want.

Likewise, the jury is still out on the question of whether the Pentagon will enable Northrop to begin producing its new F5G Tigershark, now undergoing flight testing in California.

Northrop needs an order for many more than Bahrain's four F5Gs to go into production. Otherwise, it would have to build each plane virtually by hand and charge an uncompetitive price.

So Pentagon officials, who have told The Washington Post that they are under White House pressure to find buyers for Northrop's planes, have some distance to go in the political minefield that is military procurement.