TITUSVILLE, Fla. (AP) _ A founding father of the shuttle program says he thinks the ''blow torch'' theory is the most likely explanation of the explosion that destroyed the space shuttle Challenger.

''It's the most plausible thing I've seen,'' said Sam Beddingfield, referring to the theory that the right solid rocket booster began leaking hot flame and ignited the 526,000-gallon external fuel tank. ''It's obvious that the external tank came apart and a leaking SRB could be the thing that did it.'' he said.

''By the same token it could have been a thrown turbine blade, or a variety of things,'' added Beddingfield, the recently retired deputy director of shuttle projects management at the Kennedy Space Center.

He predicted that the National Aeronautics and Space Administration would know the cause within six months.

''That's about how long it took us to determine the cause of the Apollo 1 fire,'' said Beddingfield, who first came to Cape Canaveral 26 years ago to help launch the fledgling Mercury program that sent John Glenn into orbit.

Beddingfield, who first began working on the shuttle program in 1969, said that while NASA undoubtedly will find the cause, the reason behind it might forever remain a mystery.

''For example,'' he said, ''if the cause turns out to be a loose bolt they may never know why it came loose. Was it improperly installed? Was it metal fatigue? But, whatever the cause they'll be able to remedy it and see it doesn't happen again.''

He said the shuttle program could be delayed for as much as almost two years - as was Apollo - if the cause turns out to be a complex technical malfunction that requires requalification of parts and equipment.

''But,'' he added, ''if it's a simple fix they should get back in businss pretty quick. It probably won't be simple, though, The shuttle program's philosophy, you know, calls for an intact abort. That means a given system is designed to fail twice and still be operational.

''The third failure calls for aborting the mission with the orbiter intact because there's no bailing out of the Challenger. It's like an airliner in that respect.''

He said the crew chamber, even if still relatively intact, undoubtedly was ruptured by the explosion or the eight-mile plunge into the ocean.

''It's pressurized and it would be floating if there wasn't at least a crack in the cabin,'' Beddingfield said. ''And, obviously the crew chamber isn't floating or it would have been found by now.''