Next Friday and Saturday, the National Symphony Orchestra offers its big summer extravaganza--a concert performance of Wagner's "Das Rheingold" marking the 100th anniversary of the composer's death. Even without a chorus, it is a massive undertaking, involving 14 solo singers, eight horns, six harps, four Wagner tubas (which are different from ordinary tubas), a bass trumpet and 18 anvils tuned to F in three octaves. Since the NSO doesn't have quite enough percussionists to handle this much hardware, half the anvils (which come in small, medium and giant sizes) will be entrusted to string and brass players who will be paid extra (under the union contract) for their blacksmith work. The NSO believes its anvils, custom-made at Drums Unlimited, may be the only complete, tuned set in the country.

This lavish display is part of a programming experiment this summer in which the NSO is trying out a variety of material not available in its winter season. A Wagner extravaganza (automatically a deficit operation) is one extreme; a variety of pop programming (including next week's session featuring Up With People) is the other. One of the most ingenious ideas dreamed up by Henry Fogel, the orchestra's executive director, is a series of programs using only part of the orchestra--for example, a weekend of music for wind ensemble Sept. 2 and 3 with Frederick Fennell conducting. Last winter, when the orchestra played Shostakovich's 14th Symphony, which uses only 25 players, the NSO was able to give most of its players (including all the brass and woodwinds) a week's vacation while still selling tickets. The wind ensemble program will provide vacation time for those who performed in the Shostakovich. "We're paying salaries every week," says Fogel. "We might as well be earning money every week."

In last weekend's program of the complete Brandenburg Concertos, the problem was different: overtime. The Brandenburgs don't need a large orchestra, but the performance takes 2 1/2 hours. The solution was to split the NSO into two orchestras, one performing before intermission and the other one afterward. Conductor Vittorio Negri managed to last through the whole show.