The chairman of the presidential commission investigating the Challenger disaster has decided to expand the probe to consider issues raised by chief NASA astronaut John W. Young's March 4 memo, which accuses the space agency of placing "launch schedule pressure" above safety, sources close to the commission said today.

Young has given chairman William P. Rogers a "detailed" private briefing on the memo, and his charges are now being pursued by the panel, the sources said.

Following release of Young's memo on Saturday, other astronauts have begun to speak out more forcefully on safety.

Two other senior astronauts said today that Young warned of safety problems with the space shuttle program "for at least two years" before the Jan. 28 Challenger explosion that killed its crew of seven, but the warnings mostly went unheeded.

Paul Weitz, Young's deputy, said the safety issues discussed in the March 4 memo were repeatedly raised long before the Challenger disaster in Young memos to his immediate superior, George W.S. Abbey, director of crew operations.

"This is not a Johnny-came-lately thing, it's not just since the accident," Weitz said. "John's been writing these memos for at least two years."

Brewster Shaw, commander of two shuttle flights in 1983 and 1985, said the safety issues raised by Young during the last two years are so serious that he will refuse to fly again "with the system we have now."

The warnings, in internal memos that Young sent to superiors at the Johnson Space Center in Houston, discuss a wide range of safety features, including the quality of key components on the shuttle's orbiter as well as basic launch and landing practices, the astronauts said.

Shaw, assigned to work on the staff of the presidential commission, said the National Aeronautics and Space Administration will have to give up its ambitious goal of launching 24 shuttles a year by 1988 if the system is to operate safely again.

"Would I fly again with the system we have now? No," said Shaw in a telephone interview from his home in Houston. "I will fly again when we fix it.

"But I personally think it would be very difficult to safely support the kind of flight rate they're talking about two or three years from now. I don't think we could get anywhere close to that, especially with three orbiters."

Young's memo cited an "awesome" list of potential safety hazards that he said must be addressed before the shuttle program resumes. Young, who holds the U.S. space record with six flights, could not be reached for comment.

The heart of Young's concern is a dramatically increased launch schedule, theoretically designed to make the shuttle economically self-supporting, a goal that has helped sell the program on Capitol Hill.

But the pressure to maintain that schedule has resulted in potentially "catastrophic" safety risks to crew members, he said in the memo.

Young said the solid rocket booster system -- suspected of rupturing at a seal and causing the explosion -- was of a hazardous design, and "there is only one driving reason that such a potentially dangerous system would ever be allowed to fly -- launch schedule pressure."

Rear Adm. Richard H. Truly, newly named associate administrator for space flight, released Young's memo with a brief statement saying, "I certainly concur with John's thrust -- that flight safety must be NASA's first consideration. We will not launch again until safety-related issues have been properly addressed throughout the total NASA system."

Abbey could not be reached for comment today about the memos. His superior, Cliff Charlesworth, director of space operations in Houston, said he recalled seeing a "variety of memos" written by Young in recent years, but none that specifically dealt with launch schedule pressure.

"We do put pressure on ourselves with the schedule," Charlesworth said. "That's part of the total program, that's how we set about to build the shuttle. We'll have manifests to carry payloads that we signed agreements for . . . . But that doesn't affect safety. We don't launch unless we're ready to."