The United States within the next year or two needs to begin developing a new space transportation system, which will have "staggering" costs but is essential to President Reagan's Strategic Defense Initiative, a senior SDI official said yesterday.

The U.S. military must also begin devising battle tactics for war in space, Air Force Col. George M. Hess Jr. said. Instead of thinking about defending a few satellites, the military must begin planning for battles involving 3,000 satellites arrayed against 5,000 attackers, he said.

"The Lord Nelsons and General Rommels of space have yet to emerge from the ranks," he said.

Hess is the director of survivability, lethality and key technologies for the Defense Department's SDI Organization, which is charged with giving shape to Reagan's controversial vision of a space-based defense against nuclear missiles. The colonel addressed his remarks to an industry seminar in Rosslyn sponsored by Pasha Publications, which publishes the newsletters Military Space and SDI Monitor.

For SDI to succeed, Hess said, Congress must support a new, unmanned, largely reusable space cargo system that will be more reliable than the shuttle. Although such a system would be cheaper to operate than the shuttle, initial investment would be more -- and would have to be paid while the National Aeronautics and Space Administration and the Air Force spend billions of dollars to recover from the recent shuttle and Titan rocket launch accidents.

"The costs are going to be staggering," Hess said. "When you add it all up, you're looking at an investment that OMB [the Office of Management and Budget] will find quite interesting."

A NASA-Defense Department study, which the White House commissioned last May, will recommend such a system in a report to the National Security Council next month, Hess said. He stressed, however, that the SDI office has not decided exactly what the new system should look like.

The new cargo spacecraft would be developed separately from the National Aerospace Plane, a multibillion-dollar initiative Reagan proposed earlier this year. That plane would take off like an airplane but fly in space like the shuttle, but SDI officials do not believe its deployment date -- the year 2000 -- is soon enough for their needs.

Lt. Col. Louis A. Kouts, deputy for Air Force space plans and policy, said at yesterday's seminar that the SDI Organization needs to achieve a tenfold reduction in launch costs. He said that, to meet current needs, the military would have to launch about 250,000 pounds of satellites each year; but if SDI is fully deployed, that figure would rise to 4.4 million pounds of space armaments each year.

The key to SDI, sometimes known as "Star Wars," will be to make the space-based weapons and sensors invulnerable, Hess said. SDI critics have said that the satellites, lasers, mirrors and other Star Wars gear will be easier to shoot down than the missiles SDI is targeting.

Hess said that Lt. Gen. James A. Abrahamson, SDI director, understands the significance of the issue. "We can get any amount of money out of General Abrahamson we want to invest in survivability for anybody who's got a good idea," he said.

Hess rejected one proposed solution to the survivability challenge: a "proliferation" of satellites so that the Soviet Union could not target enough of them. He said that will not work unless satellites become "as cheap as 25-inch televisions."

Instead, the former director of the Air Force antisatellite weapon program stressed "local battle tactics," the "disposition and maneuver of forces."

"There is nothing fundamentally different between what we're doing in space and what was done on the battlefields of World War II," he said.