The Pentagon’s plan to end its reliance on Russian rocket engines through speedy development of a new, all-U.S. booster is too risky and likely will prove overly expensive, outside acquisition experts privately warned last year.

The independent panel, according to people familiar with the details, ranked various strategies to resolve one of the Air Force’s most vexing problems: the best way to phase out the military’s workhorse Atlas V rocket and its Russian-built RD-18O engine. The experts evaluated the options for overall...