It's been 15 years since the Sun Ra Arkestra last played San Francisco, and saxophonist Marshall Allen, the group's bandleader, isn't exactly sure how the group's upcoming performance will go. He's held rehearsals with core members of the Arkestra, and they've practiced their way through a vast repertory of songs, but surprise guests will surely turn up; they always do. And while Allen likes to prepare, he likes the element of surprise even better. Unpredictability and improvisation are key elements of the cosmic philosophy and "Afrofuturism" style of jazz that the Arkestra has been crafting, under various names and with different lineups, since Sun Ra created the group in the mid-1950s. "I can plan everything but it may not go quite right, it may not work," Allen, 89, says during a recent phone conversation. "We will feel the vibrations of the city, and whatever we feel is good for California, that's what we will do." Sun Ra played many forms of jazz — swing and bebop — but he and the Arkestra are best known for experiments in free jazz. Sun Ra died in 1993, and Allen has led the group since 1995. Expect to see roughly 15 performers onstage at the San Francisco show. "Sun Ra would say there's a thousand ways to play or attack a note," Allen says. "People know what they have to do with Sun Ra's band — it's not what you know or what you can do, but what you can do on the day based on what we're feeling."