Richard Strauss had a glorious operatic subject in Helen of Troy. But partly because of a wildly improbable book and chiefly because of temporary sterility of ideas, his "Egyptian Helen" is most of the time a bomb.

As Antal Dorati and the Detroit Symphony laid out the long, two-act music drama yesterday in the Kennedy Center, its costiveness became more oppressive as it continued.

Hugo von Hofmannsthal, usually able to provide Strauss texts that could inspire him sufficiently to override a tendency toward the diffuse and the fantastic, gave him, for his "Helen," a wild mixture of phantoms, sorceresses, and frequent magical potions.

The orchestral writing is opulent, luxuriating in the customary percussion, plus organ. Yet it adds up to little but empty wind much of the time. There are, of course, golden patches where suddenly a small ensemble of women's voices turn radiant. Or from Helen, Menelaus, the ravishing Aithra, Altair, or Da-ud come phrases of familiar beauty. But they are rare, and far too infrequent to save the whole.

Dorati led the work with superb awareness of its need to be kept moving. Among his principals, all of whom will join him Tuesday in Detroit to record the opera, the light, flexible soprano of Barbara Hendricks was brilliant in sound and style. Birgit Finnila, too, as the deep-toned Sea-Shell was a pleasure when Strauss' orchestration permitted her to be heard.

A solid lyric tenor with the clear makings of a heldentenor was ideally projected by Matti Kastu. Unfortunately, most of the time he sang Menelaus with his eyes riveted to the printed score. Willard White was a valuable Altair and Curtis Rayam a sensitive Da-ud.

Gwyneth Jones sang the title role in her usual manner. She controls her huge voice some of the time, but at least as often drives it completely out of tune. The minor singers and Jewell Chorale were excellent.