On the night of Aug. 30, if all goes well, Guion Steward Bluford II will confront the moment of maximum stress known to astronauts as "Max Q."

Roughly 50 seconds after liftoff and 50,000 feet up in the dark sky above Cape Canaveral, the space shuttle Challenger will bull its way through the pummeling jet streams of the stratosphere on its way toward orbit.

Caught briefly in the roiling air currents, with the Earth's gravity tugging back at three times its normal force, the buffeted spacecraft will rattle violently, as if Bluford's world and its atmosphere were struggling one last time to thwart this particular man's attempt to escape.

Twenty-two years after the United States first rocketed men into space, Bluford will prove that "the right stuff" comes in hues other than white, becoming his country's first black space traveler. It is a symbolic role he did not particularly want.

The 40-year-old Air Force lieutenant colonel was almost 35 before he decided to take his fling at becoming an astronaut. He faced a field of almost 9,000 applicants. When he survived the cut to 35, along with two other black men and a half dozen women including Sally K. Ride, he was asked if he felt driven to break the racial barrier and become the first black to look down on Earth.

His answer was no. He said it was partly because he was not ready to set another goal for himself, but mostly because he simply wanted to fly in space and return to the privacy he has cultivated so long without facing history's glare.

"It might be a bad thing to be first , if you stop and think about it," Bluford said, addressing the mission's impact on himself rather than the opportunity it provided to speak for his race. "It might be better to be second or third because then you can enjoy it and disappear--return to the society you came out of without someone always poking you in the side and saying you were first."

When Ride was told she would be America's first woman astronaut, she excitedly called her mother. Bluford, who was informed at the same time that he had been scheduled for the next shuttle flight, went back to his office and quietly finished some paperwork, not telling his wife and two teen-age sons until later that night. His two brothers heard the news on the radio the next day.

Some civil-rights activists, long critical of NASA for waiting so many years before putting a black American in space, complained that Ride would be first. But it did not bother Bluford. It delighted him.

"She can carry the spear and get the attention," Bluford said. "That relieves me."

Bluford, for that matter, never has seen his race as an obstacle. "I can't really say I had any obstacles," the quietest of astronauts said recently. "If I had any obstacles, they were self-made."

His life story is a study in contradictions: the story of a shy and reticent youth who will be known to history as a black pioneer, a youngster whose mother once thought him the least likely of her three sons to make a success of himself, a struggling student who persevered to earn a master's degree and a doctorate, a loner who says he has no best friends and no heroes but who is about to be seen as a hero himself, a self-described "average guy" who became far more than average by pressing on when things got tough and by setting each new goal only after the last had been achieved.

Looking at the trim and sterile space agency biography of Bluford, it would be easy to conclude that his career was as preordained as that of a John Glenn or a Neil Armstrong.

Bluford grew up in a racially mixed, middle-class, row-house neighborhood in Philadelphia. His father was a mechanical engineer, and one of young Guy's early interests was model airplanes.

His mother was a teacher who worked double duty to put her sons through college. His brother, Eugene, 37, is a Los Angeles computer programmer and has a doctoral degree. Another brother, Kenneth, 33, a teacher by profession, is finishing a doctorate in English.

After graduation from Philadelphia's Overbrook High, Guy Bluford took a degree in aerospace engineering from Penn State in 1964. He went from ROTC into the Air Force, got his wings in 1965, and went to Vietnam in 1967 as copilot of a high-performance F4C jet fighter.

He was stationed at Cam Rahn Bay as a member of the 557th Tactical Fighter Squadron, flying 133 combat missions, 65 over North Vietnam. He came back with an assortment of medals.

Bluford then earned a master of science degree at the Air Force Institute of Technology in 1974 and a doctorate of philosophy in aerospace engineering with a minor in laser physics in 1978, just before he was chosen to become an astronaut.

But the real Guy Bluford story lies between the lines of this bare-bones biography. He tells little of it himself, protecting his privacy, a trait ingrained in him by an extraordinary and insular family that fits no stereotype, black or white.

If you walk down West Philadelphia's Media Street toward the four-bedroom row house numbered 5522, you pass run-down houses, some of them vacant with walls splashed with ghetto graffiti. But 25 years ago, it was a proud place, with scrubbed walks and trimmed shrubs.

The houses were filled with equally prideful people, none more prideful than the Bluford family. It was a family of achievers, whose ancestors had achieved without joining or bucking the white world.

Lolita Bluford, Guy's mother, was a schoolteacher whose family included a sister, Carol Brice Carey, a well-known contralto and now a voice coach at the University of Oklahoma. Mrs. Bluford's father was John Brice, a Presbyterian minister and educator.

On Bluford's father's side, a sister, Lucille Bluford, was editor of the Kansas City Call. A cousin was Dr. F.D. Bluford, a former president of the North Carolina Agricultural and Technical College in Greensboro.

Guy Bluford--called "Bunny" by his family--had few close friends as a youth, preferring the solitary activities of model airplanes, crossword puzzles, "mind-twister" games and religion. The others in his family were much the same. They never entertained, and evenings were devoted to readings in Christian Science, a religion their mother adopted in the mid-1950s.

Bluford's father served in the Signal Corps in World War II and then worked as a mechanical engineer at Philadelphia's Frankfort Arsenal. He suffered from epilepsy and was laid off for health reasons in the mid-1950s. This changed the family's routine drastically, as his wife took a second job teaching at night.

"I think the decline of my father, and the decline of his self-respect as he got sicker, affected Bunny more than anything in his life," Kenneth said. "My father was the closest thing my brother ever had to a hero. Bunny was always quiet and serious. But he got more so after that."

Kenneth also remembered Guy's struggles in high school. "Bunny just had to work harder than the rest of us," his brother said. "He put in very long hours. He was always a little behind and trying to catch up. He was not like a kid who was unusually bright, with his mind darting all over the place, making discoveries here and there. In school, Bunny was always slugging it out."

Kenneth also remembered a time when Guy didn't slug it out. On Saturdays, Kenneth carried Guy's paper route, which passed the house of a neighborhood bully who regularly lay in wait for him.

"One day it got kind of rough and when I came home I wanted Guy to go out and 'big brother' it with the kid--you know, beat him up or scare him into leaving me alone or something," Kenneth said. "Instead, he went out and talked to the kid, then came back to me and said, 'You know, Kenneth, that boy had a point.' It wasn't exactly what I wanted him to do. But Guy avoided emotional situations. He had a way of defusing them."

One of the worst moments in the house on Media Street was the day the high school adviser brought news that Guy didn't seem equipped for college, no matter how hard he worked. Find him a trade, the adviser suggested.

A Bluford son not going to college? Heresy, said his parents. The subject never was brought up again.

A lot of subjects were not brought up, including the civil rights struggles of the 1950s and '60s.

"We just didn't think that way," Kenneth said. "It didn't come up in a personal way as an excuse for anything. If it came up in a political way, it was put down quickly. Mother just said, 'Why doesn't the government go out and chase those Commies instead of worrying about people who won't work?' And that was the end of that."

Bluford's parents were "Eisenhower Republicans" who voted for Richard M. Nixon over John F. Kennedy in 1960. Kenneth said the family practiced a "cultural conservatism" and "Bunny was that way, too, with a strong belief in the Protestant work ethic."

Kenneth, currently unemployed, described himself as "somewhere to the left of Mao Tse-tung"--a long distance from the "cultural conservatism" still practiced by his brother.

Noting that Guy turned 21 the day President Kennedy was assassinated, his brother said, ". . . when Kennedy was shot he really symbolized everything that should have affected my brother: civil rights, Vietnam, even the space program. But Bunny was so intense I don't think he had spent five minutes thinking about any of those things."

By that time, Guy was struggling through his senior year at Penn State, still the quiet, inward youth who had time only for his studies, Air Force ROTC, meetings of the off-campus Christian Science group and a quiet romance with Linda Tull, a student from Philadelphia whom he married in his senior year.

Prof. Barnes McCormick, now head of the aerospace engineering department, remembered Bluford the way everyone did, "as a quiet fellow and an average student, not the sort you would expect to be interviewed about 20 years later."

McCormick said he doubts he would have remembered Bluford at all had he not been the only black in the engineering school.

Three years after his college graduation, Guy was flying jets in Vietnam. Kenneth, then a high school senior, was protesting that war.

"I guess I acted like the little brother as squirt," Kenneth recalled. "I tried to talk to him about Vietnam. I said, 'Don't you realize you are bombing peasants and children?' He just blanked out on it. To Guy that just didn't compute. He was doing his duty, doing a job he was good at. He didn't get mad at me. He didn't do anything. So we just stopped talking about it.

"But that was the way he was and still is. He just does not allow emotional events to intrude in his life. In emotional situations, he would defuse things somehow, not let them happen--the way he did with the kid on the paper route."

Still, Kenneth said his brother "found himself" in Vietnam, and believes that was a good thing. "I think becoming a pilot galvanized him and Vietnam gave him a chance to be a real pilot. It was then that he changed--maybe overcame the hurt of our father's decline--and became an efficient, effective person."

Bluford's father died two months after Guy returned from Vietnam in 1968. His mother, who had worried so much about his chance for success in life, died two months before he was accepted as an astronaut. She never knew he had applied.

Aboard the Challenger on its scheduled Aug. 30 liftoff, the first at night, Bluford is to sit in the seat that was occupied by Ride, just behind pilot Richard H. Truly and copilot Daniel C. Brandenstein. Part of his job will be to help them shudder through Max Q on takeoff.

Later he will have primary responsibility for the in-orbit launch of a communications satellite for the Indian government. He also will assist in a scientific experiment to separate live pancreas cells as part of a long-range biomedical project NASA hopes will lead to development of a pharmaceutical industry in space.

After five days in orbit, Bluford and his fellow astronauts are to drop back to Earth at California's Edwards Air Force Base in the shuttle's first night landing.

A year ago, at an all-black high school in Camden, N.J., Bluford made one of his rare speeches. Afterward, a shy youngster held back from the crowd, then finally approached Bluford and told him he wanted to join the Air Force.

Bluford looked at the youth knowingly, perhaps seeing himself 20 years earlier. Then he patted him on the shoulder and left him with a message: "Go out there and buck it."