The explosion yesterday of a Delta rocket over Cape Canaveral, coupled with the explosions of the space shuttle Challenger Jan. 28 and the Titan rocket April 18, leaves the United States with almost no means to boost commercial satellites or military payloads into space.

The Delta, the Titan and the shuttle have been the workhorses of the American space fleet, accounting for the vast majority of major orbital payloads, both commercial and military. Now, all three programs are suspended.

"Because nobody can launch on Delta, because nobody can launch on Titan, because nobody can launch on the shuttle, this virtually cripples America's easy access to space," former NASA assistant administrator David Williamson said last night.

Several officials, contacted last night, said the disasters have potentially serious national security implications. But White House chief of staff Donald T. Regan, speaking to reporters in Tokyo, said: "Just because we've had accidents, whether it's a baseball team or what have you, you go into a slump and you pull out of it. And I think this is a slump in our space program, and we'll pull out of it."

Regan said the Delta rocket had been reliable and this accident is not a threat to national security. He said the administration hopes to reactivate the program as quickly as possible.

The Delta has been a virtual pickup truck into space, the most reliable vehicle in the fleet, with 43 successful launches over nearly a decade before yesterday's failure, and 178 total launches.

Three Delta rockets remain, with six Titans and three shuttle craft, but all are unavailable pending investigations.

The United States, therefore, is left with two types of launch vehicles, both with severe limitations, according to Joseph Mahon, head of the expendable launch vehicle program for NASA:

*The Atlas-Centaur rocket can boost heavy payloads, but is in short supply. Only three Atlas-Centaurs remain, Mahon said, and all are booked to launch Navy navigation satellites, two later this year, the third in 1987. Production of the Atlas-Centaur vehicles was phased out in anticipation that the shuttle would carry the majority of military payloads, and it cannot be immediately restored. The next Atlas-Centaur launch is planned May 22 from Cape Canaveral.

*The relatively tiny Scout rocket is limited to lifting payloads of 450 pounds or less into low orbits. Twelve Scout vehicles are left, and 11 already are scheduled to launch Defense Department payloads between now and 1988. The 12th is scheduled to launch a scientific satellite.

NASA officials at Cape Canaveral said last night Deltas were scheduled to launch two satellites for Strategic Defense Initiative experiments this August and again in August 1987.

The destruction of the $57 million weather satellite also leaves forecasting of major storm systems headed for North America in a precarious position, according to William Bishop, head of the satellite program for the National Oceanic and Atmospheric Administration.

Only one similar weather satellite is in orbit. Normally there are two, to cover the United States and well into the Atlantic and Pacific. NOAA has one more satellite, to be ready for launch this summer. Bishop said those two satellites are all that remain until a fresh supply is ready in 1990.

"My fear is that if the next one fails on launch, or this one fails in orbit, we'll have no coverage until 1990," he said.

The accidents also call into question the inherent reliability of the American aerospace industry at a time when the Reagan administration has embarked on its complex and ambitious SDI, or "Star Wars" program, which assumes the ability to put satellites into space virtually at will.

Adm. Eugene Carroll of the Center for Defense Information said last night the temporary grounding of the U.S. space program calls into question the very basis of the SDI.