As science-fiction rattletraps go, "Damnation Alley" represents a slight improvement on last week's "Starship Invasions," but it's also one of those movies that leave you wondering where they mislaid the rest of the story.

In retrospect, the film looks as if 20th Century-Fox might have designed it specifically to play the bottom half of double-bills with "Star Wars," but for all one knows "Damnation" may have been considered the safer and more commerical project at one time.

The movie gets off to a fairly promising start by witnessing a global nuclear war in the abstract. George Penpared and Jan-Micheal Vincent portray a team of Air Force missile technicians on duty at an ICBM base in the Mojave Desert when Doom Hour tolls. It becomes their responsibility to dispatch a group of missiles in the opposite direction after scanners confirm a massive attack on the United States.

A subtle mood of horro and shame is created by contemplating a nuclear holocaust from the secure isolation of a strategic military outpost. The path of the missiles is tracked by computer animation on an operations board, and as the hits are ticked off - Boston, New York, Trenton, Washington, etc - one's natural susceptibility is enhanced by the detached hermetic point of veiw. These men watiching the outside world being pulverized in the form of electronic signals are destined to be survivors, and the thought of how they'll cope with that survival is always an interesting subject for fictional speculation.

Unfortunately, "Damnation Alley" begins hitting the skids right after it blows civilization to atoms. Armageddon is a hard act to follos, it would seem. At any rate what follows in teh aftermath of the catastrophic downbeat in this picture is a succession of stale situation, beginning with a silly looking sci-fi monster menace: Vincent, a young daredevil, runs a gauntlet of giant, mutant spiders on his motorcycle.

Perhaps hoping to make things easier for themselves by sidestepping the most compelling aspects of their own premise, the filmmakes proceed to massacre the surcivors. Someone absentmindedly drops a cigarette butt on an open Playboy centerfold, precipitating explosions that destroy everyone remaining at the base except Peppard, Vincent, Paul Winfield and Kip Niven. They depart in a pair of supertanks dubbed Land Masters, seeking to make contact with a regular radio signal originating from Albany. N.

Niven is knocked off presently, and Winfield's amiabitlity doesn't prevent his character from being expendable soon after the party picks up a female stirvivor, Dominque Sanda, who has Las Vegas all to herself. Since the script never bothers to generate a romantic conflict between Peppard and Vincent over Sanda, it seems doubly pointless to eliminate Winfiled. There's no hint of cohabitation in the movie, let alone miscegenation.

Winfield runs afoul of maneating cockroaches in the ruins of Salt Lake City. This foolish stopover leads to the funniest line in the show, Perppard's command to vamoose because "This whole town is infested with killer cockroaches!" This sounds like a guaranteed audience-killer in any town, but it would almost be worth a trip to hear Salt Lake City moviegoers react to it.

But unlike "Planet of the Apes," which concealed a kicker at the fadeout, "Damnation Alley" dithers away in absurd affimation. Will people in Albany get a bigger laugh out of seeing their town depicted as the New Eden than people in Salt Lake City will at seeing their home disparaged as the future haven of demon roaches?