When the space shuttle Columbia returned to Earth after its maiden flight in April, 1981, it took NASA more than six months to get Columbia ready for its second flight.

The third flight of Columbia came in March, 1982, four months after the second flight. The fourth was in June, getting what the space agency calls its shuttle "turnaround time" down to three months. The fifth flight, a bit of a setback in NASA's quest for a quicker turnaround, didn't go up until November, 1982.

But the last three flights, all of them on a new spaceliner called Challenger, have convinced the space agency that it can shorten the time between flights each time it flies the shuttle into space.

Flight six came in April of this year, flight seven in June and flight eight was due to lift off from the Kennedy Space Center early this morning. If things continue to go the way they've been going, Columbia will return to space on the shuttle's ninth mission late in October.

The shuttle schedule for the next few years is optimistic. The 10th flight is targeted for January. Number 11 will go up in April, and the next three flights are scheduled to go up in May, June and July--a one-month "turnaround time."

Assuming that all goes well, NASA expects to put two flights up next August, one on the 5th and the second on the 30th, in a 25-day turnaround that the agency hopes will become standard.

"Our goal is to routinely have a shuttle flight every 25 days," Air Force Lt. Gen. James A. Abrahamson, associate administrator of NASA, said recently. "We don't think that's unrealistic."

The reason for Abrahamson's optimism is that the space agency will have a fleet of four spaceliners on duty sometime in 1984. The third space shuttle, named Discover, is to be delivered to the space agency on schedule next month.

The fourth, named Atlantis, is due to be delivered sometime next year. Not only are all shuttle deliveries on schedule, each one is being built lighter so it can carry more weight into space and each one is being equipped with more powerful engines than the one before it.

Challenger is 2,500 pounds lighter than Columbia, Discover will be at least 900 pounds lighter than Challenger and Atlantis almost 1,000 pounds lighter than Discover. Each shuttle costs about $1.2 billion, basically no change in cost for the entire four-ship fleet.

The next flight of the space shuttle Columbia will be the first to carry the $1 billion Spacelab built by the European Space Agency. Spacelab, which carries as many as 40 different experiments, fills up the entire cargo bay of the space shuttle and is operated by two astronauts who fly on the shuttle as extra members of the normal four- or five-man crew. One of the Spacelab astronauts on the next flight is a German, the first time a European will fly on an American spacecraft.

NASA officials expect that at least 20 percent of future shuttle flights will involve Spacelab. The goal is to fly Spacelab twice a year, in part to keep the European Space Agency and its sizable budget as involved as possible in the space shuttle program.