John Young, NASA's chief astronaut and commander of the first space shuttle flight, said in a memorandum released yesterday that the space agency has placed "launch schedule pressure" above safety and endangered the lives of astronauts.

In the most stinging rebuke of the National Aeronautics and Space Administration so far from one of its own, Young listed a series of potentially dangerous shuttle flights before the Challenger explosion, and he said the shuttle program cannot survive unless it is reorganized to place "Flight Safety First."

The March 4 memo was reported Saturday by The Houston Post. It was released yesterday by NASA with a brief statement from Rear Adm. Richard H. Truly, newly named associate administrator for space flight, 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."

Young noted that top NASA officials had testified before the presidential commission investigating the Jan. 28 Challenger disaster that they had acted properly in deciding to launch the shuttle, which exploded 73 seconds into the flight, killing all seven astronauts.

"While it is difficult to believe that any humans can have such complete and total confidence," he said, "it is even more difficult to understand a management system that allows us to fly a solid rocket booster single-seal design" that could lead to an explosion with such disastrous results.

"There is only one driving reason that such a potentially dangerous system would ever be allowed to fly -- launch schedule pressure."

Though couched in space agency jargon that may obscure some of its meaning to outsiders, Young's memo is likely to hit NASA officials hard, especially now that it has been made public.

Young says, in effect, that the astronaut corps has stood by silently as NASA officials cut corners and took gambles with their lives in the interest of meeting pressures to step up the launch schedule. Now that the odds have run out, with Challenger, Young is saying, it is time to speak out.

Young's memo contrasts sharply with the attitude that astronauts are thought to display as part of "the right stuff." Instead of the casual acceptance of danger, the memo indicates that the astronauts are not prepared to accept any longer the kinds of known dangers to which they believe they have been subjected.

"An Urgent Request," Young's memo said. "By whatever management method it takes, we must make Flight Safety First. People being responsible for making Flight Safety First when the launch schedule is First cannot possibly make Flight Safety First no matter what they say.

"The enclosure of problems with other flights shows that these goals have always been opposite ones. It also shows overall Flight Safety does not win in these cases. Flight Safety, to be safe, has to have real teeth in it. It will not be free. For starters, we should not allow any increase in the inherent risk of operating the space shuttle just to increase the launch rate, or reduce operating costs, or fly unsafe payloads.

"If we have to put tough risk assessment or hazards analysis on all of the real-time operational management decision-making process for the life of this program, then we need to do it. If we do not consider Flight Safety First all the time at all levels of NASA, this machinery and this program will NOT make it.

"If the management system is not continuously self-assessing with respect to Flight Safety of the inherently hazardous business that we are in, it will NOT last. If the management system is not big enough to STOP the space shuttle program whenever necessary to make Flight Safety corrections, it will NOT survive and neither will our three space shuttles or their flightcrews."

Although NASA had come to describe its shuttle as an operational vehicle, suggesting its flights were almost routine, Young contends that the spacecraft is still an experimental device that should be operated only under the strictest rules of safety.

He accused NASA of sacrificing its once-proud insistence on flight safety as the highest priority, and he pleaded that the Challenger disaster, which followed three years of official knowledge that the booster's O-ring seals could fail, would lead to restoration of safety first.

The Challenger disaster is generally thought to have resulted from a rupture of seals used in the joints between segments of Challenger's solid rocket booster. The escaping hot gases caused the booster to tear away from the shuttle's external tank, rupturing it, and spilling its highly explosive contents.

But Young listed a series of specific technical problems that could have resulted in disaster on other flights, including:

*Aug. 27, 1985: Discovery was launched through two cloud decks with light rain in the area. Young said "winds in storms plus tile damage drag might lose the vehicle and crew in an abort."

*Oct. 30, 1985: Challenger was launched without a backup regulator in a key maneuvering rocket system. In the worst-case scenerio, "loss of vehicle and crew would result."

*Jan. 12, 1986: Columbia was launched after a record number of delays. After one delay, engineers found that a launch pad fuel sensor had snapped off and lodged in a crucial valve. "Potential loss of vehicle and crew if each of three valves does not operate reliably at main engine cutoff," Young wrote.

"Our space shuttle machinery is not airline machinery," Young wrote. "As the launch rate increases, we will start having directly increasing numbers of various conditions and events . . . where things are not working normally and management will still want to go fly.

"We have already, as the enclosure shows in part, launched with less than certain full reliability and full redundancy of the systems, including the flightcrews, that we operate. We are under continuing pressure to launch without full-up avionics from computers to other sensors."