The remains of Doohan, Mercury astronaut Gordon Cooper and some 200 others were loaded into the rocket Friday by Charles Chafer, chief executive of Celestis, a Texas company that contracts with rocket firms to send cremated remains into space.
"And we're ready to go," Chafer said after inserting the silver canister.
Jerry Larson, president of Connecticut-based UP Aerospace Inc., said the rocket will be launched April 28.
Families paid $495 to have a few grams of their loved one's ashes placed on the rocket.
Chafer said he's aware of the dedication of "Star Trek" fans.
"There's no doubt that we'll find a way to accommodate fans who travel here and want to be part of that experience," he said.
Doohan died in July 2005 at age 85.
The remains of "Star Trek" creator Gene Roddenberry were blasted into space in 1997.