The Senate Appropriations Committee handed President Reagan his second major victory on defense in two days yesterday by voting 21 to 7 to build the B1 bomber.

On Monday, the House Appropriations Committee approved the land-based MX missile.

Reagan proposed both weapons systems in October as part of his plan to beef up strategic firepower. Both proposals have been controversial, with approval by Congress much in doubt.

Both Sen. Ernest F. Hollings (D-S.C.), who tried to kill the B1, and Rep. Joseph P. Addabbo (D-N.Y.), who sought to delete funding for the MX, vowed to renew their fights when the defense appropriations bills are debated in the two chambers later this week.

The House committee has approved $196.5 billion in spending authority for the Pentagon for fiscal 1982, about $4 billion less than Reagan sought, while the Senate unit, by voice vote yesterday, approved $203.8 billion. Congressional leaders hope to have a compromise version through both houses and ready to send to the president by Friday.

The MX was not voted on in the Senate committee yesterday. Chairman Mark O. Hatfield (R-Ore.) said he would wait until the bill reaches the floor before seeking to delete money for the MX and for chemical warfare. This left the Hollings B1 amendment as yesterday's key test of how well Reagan has managed to sell his defense strategy to the Senate.

Hollings said the B1 would not buy enough bang for the buck, and urged that the $2.4 billion earmarked for the bomber be spent instead on upgrading military readiness. He quoted testimony by Defense Secretary Caspar W. Weinberger that the B1, due to be ready in 1986, would not be able to penetrate Soviet air defenses after 1990.

Weinberger told the Senate Armed Services Committee Nov. 5 that, after the period of "probably 1988 to 1989," with the B1 "you lose the ability to penetrate unless someone wants to direct suicide missions, and that is not anything I am going to do."

At another point in the same hearing, Weinberger said: "The simple fact is that the information I have, the department has, on which the decision was based was that it would be no longer safe to utilize the B1 as a penetrating bomber after approximately that year"--1989 to 1990--"if the Soviet rate of development does continue at the rate we think it will...

"I think there is no question whatever," Weinberger continued, "that we will not be able to use the B1 as a penetrator after 1990. If it goes to 1992, that will be a little extra dividend."

In the last two weeks, Pentagon civilians, Air Force generals and Central Intelligence Agency Director William J. Casey have had to go to great lengths to undo the damage this testimony did to the B1 proposal in Congress.

On Nov. 10, Weinberger and Casey jointly signed a letter to the Senate defense appropriations subcommittee "intended to clarify capabilities" of the B1 and other U.S. bombers. In contrast to Weinberger's testimony, the letter said the new B1 "would have the capability to penetrate anticipated Soviet air defenses well into the 1990s in a multitude of employment modes and to perform effectively as a cruise missile carrier and as a conventional bomber into the next century."

Shortly before voting on the Hollings amendment yesterday, committee members received a secret briefing on the radar-evading Stealth bomber under development. The test versions of that plane were small, about the size of the Navy's A4 Skyhawk, and experienced crashes partly attributable to the unique shapes and other radar-foiling techniques employed. The Pentagon's argument is that Stealth cannot yet be relied on as the bomber of the future, so the B1 should be built until the advanced plane is proved out.

The briefing yesterday, and other ones like it in recent days, were credited by Pentagon officials with winning over several senators to the B1. Sen. John C. Stennis (Miss.), ranking Democrat on the Appropriations Committee, also was influential in yesterday's voting, according to committee sources.

Voting with Hollings against buying the B1 were Senate Minority Leader Robert C. Byrd (W.Va.) and four other Democrats: William Proxmire (Wis.), Thomas F. Eagleton (Mo.), Patrick J. Leahy (Vt.) and Dale Bumpers (Ark.). Hatfield was the lone Republican on the short side of the lopsided vote.