Simon Ramo in 2014 (Mel Melcon/Los Angeles Times)

Not long after leaving Howard Hughes’s fledgling Hughes Aircraft and starting an aerospace firm in Los Angeles in 1953, Simon Ramo received a call from President Dwight D. Eisenhower.

The president told the young scientist that the country needed to develop a rocket capable of delivering a nuclear warhead to the other side of the globe in less than an hour. Within three years, Dr. Ramo oversaw work on the program and successfully launched a Thor missile that flew 1,300 miles downrange from Cape Canaveral, Fla.

Dr. Ramo, the chief architect of America’s intercontinental ballistic missile system and an aerospace pioneer who helped shape Southern California into the nation’s center for high-tech weapons research, died June 27 at his home in Santa Monica, Calif. He was 103.

His son, Jim Ramo, confirmed the death but did not provide a cause.

Dr. Ramo was a California Institute of Technology whiz kid who co-founded aerospace giant TRW, and in his late 80s brokered one of the biggest mergers in the defense industry.

Aluminum foil balloons, one 12 feet in diameter, and a model of the rocket nose section designed to carry them into outer space in collapsed form, are shown in 1958 amid witnesses at the House Space Committee in Washington. Left to right: Dr. Hugh L. Dryden, director, National Advisory Committee for Aeronautics; Dr. Arthur Kantrowitz, director, Avco Everett Research Laboratory; Dr. Simon Ramo, Ramo-Woolridge; Krafft A. Ehricke, Convair expert; and Allen F. Donovan, Ramo-Woolridge. (Harvey Georges/AP)

Often called the “R” in TRW or just “Si” to friends and family, Dr. Ramo helped transform the region’s aerospace industry from metal-bending aircraft manufacturing to the world’s epicenter for sophisticated weapons research.

Although he officially retired from the aerospace industry in 1978, he continued to help lead major space and weapons developments and remained an active consultant to aerospace executives and an adviser to presidents, Cabinet members and Congress.

“There are a lot of names you can remember from the last 50 years or so in history, but few individuals have had more impact on American security and technology prowess than Ramo,” said Loren Thompson, a defense policy analyst at the Lexington Institute in Arlington, Va.

Dr. Ramo was best known for leading the development of the weapon that escalated the Cold War into a potentially apocalyptic struggle. It was a rocket that could deliver a nuclear warhead to a target 6,000 miles away in 30 minutes and destroy a city, undeterred by any defensive system.

The former Soviet Union and the United States built so many of the missiles that at one point, scientists estimated that the world could be destroyed 10 times over.

As a result, it fundamentally altered war planning and the worldview of two generations, who learned to live with Cold War brinkmanship and the deeply troubling concept known as “mutually assured destruction.” As long as there were enough nuclear missiles to destroy one another, it was considered irrational for one side to launch an attack, or at least that was the thinking behind it.

Dr. Ramo was born in Salt Lake City on May 7, 1913, to Lithuanian immigrant parents who owned a clothing store. He was an aspiring concert violinist until age 12, when he heard legendary violinist Jascha Heifetz and decided he would be better off pursuing a career in science.

He was a 1933 graduate of the University of Utah and earned a doctorate in electrical engineering and physics from Caltech at age 23. In 1936, he began working for General Electric, where he helped develop the electron microscope. His work on military-related programs kept him out of active-duty service in World War II.

After the war, he moved to Hughes Aircraft, then Howard Hughes’s airplane workshop in Culver City, Calif., to launch a division devoted to military electronics.

He said he went to work for Hughes because he knew that one of the richest men in the world at the time was an absentee owner who rarely came around. When he did show up, Dr. Ramo recalled in a Los Angeles Times interview, Hughes would “toss off” detailed directions about what kind of seat covers to buy for company-owned Chevrolets.

“He was a nut,” Dr. Ramo said.

He left Hughes in 1953 and formed what became the predecessor for TRW after the Defense Department grew wary about contracting sensitive military work to the eccentric Hughes. That same year, the Eisenhower administration bypassed big defense contractors and asked Dr. Ramo and his Caltech classmate Dean Everett Wooldridge, the “W” in TRW, to lead the development of the intercontinental ballistic missile.

The enormous task of overseeing the development of the ICBM, a feat Eisenhower considered more complex than building the atomic bomb, had fallen to two young scientists who had been on their own for less than a year and were working out of a former barbershop in the Westchester neighborhood of Los Angeles.

Eisenhower told them that developing the ICBM was “a research program of the highest national priority, second to no other,” as Dr. Ramo remembered the president’s words. In doing so, Eisenhower had bypassed corporate giants such as IBM and AT&T to award the nation’s largest military technology program to two relative unknowns.

It was a controversial move, but inside the Pentagon, Dr. Ramo and Wooldridge were seen as the top candidates. At Hughes Aircraft, they had developed an electronic fire-control system, used to direct weapons, that became a standard for Air Force fighter aircraft. They had an established record working at the leading edge of the new field of electronic warfare and guidance systems.

“Only the president and a few members of Congress knew about it,” Dr. Ramo recalled. “The idea was to do things as fast as possible, so we had to bypass bureaucracy.”

About the same time, the Pentagon became alarmed by intelligence reports that the Soviets were developing ballistic missiles that could reach the United States. For years, planners had based their strategy on defending against a possible attack by Soviet jets carrying nuclear bombs.

Operating in secrecy, Dr. Ramo and Wooldridge moved the ICBM operations to a former Catholic church in Inglewood, Calif., where the pair had to pull out the pews and the urinals in the bathrooms to make room for their research.

The celebration would be short-lived, however. The space race began in 1957 as the Soviets launched Sputnik, the first satellite to orbit the Earth.

That same year, Time magazine placed Dr. Ramo and Wooldridge on its cover, declaring them the “face of a new age,” while also calling them an unlikely pair. Dr. Ramo was described as flamboyant, mercurial and prone to speak impulsively, letting his thoughts bounce around, while Wooldridge, wearing gold-rimmed glasses, looked and acted like a professor, calm and introspective. Dr. Ramo relaxed by taking mambo lessons, Wooldridge by playing the organ, the article said.

“When we started, we thought that maybe, if we were wildly successful, we might eventually have a staff of 150 people,” Dr. Ramo said. Before it was acquired by Northrop Grumman in 2002, TRW had grown to about 100,000 workers.

In 1958, Ramo-Wooldridge merged with its financial backer, Thompson Products, and the company was eventually renamed TRW. Wooldridge was named president with Dr. Ramo as an executive director who was focused on technology development.

But four years later, at 49, Wooldridge retired and divorced himself from aerospace while Dr. Ramo continued leading development of military weapons, some of which to this day remain classified.

Dr. Ramo remained influential even as he neared 90 and became the unlikely matchmaker for one of the biggest mergers in the defense industry. In 2002, Northrop Grummanacquired TRW for $7.8 billion after Dr. Ramo prodded Northrop’s top executives, who had previously worked at TRW, to buy their former employer.

His awards included the Presidential Medal of Freedom, the nation’s highest civilian honor.

His wife, the former Virginia Smith, whom he married in 1937, died in 2009. Survivors include two sons, Jim and Alan.

During the ICBM’s development in the late 1950s, Dr. Ramo he became legendary for capsulizing complex ideas into off-the-cuff witticisms.

When the United States’ first ballistic missile rose about 6 inches above the launch pad before toppling over and exploding, he turned to an Air Force general and said: “Well, Benny, now that we know the thing can fly, all we have to do is improve its range a bit.”

He wrote and co-wrote 62 books on diverse subjects, including a guide to playing tennis that applied Machiavelli’s treatise to beating an opponent and a textbook on electromagnetic fields, which has sold more than a million copies and is used by 100 universities. In 2012, he wrote a book about drones and other battle bots titled “Let the Robots Do the Dying.” One of his other titles included “Meetings, Meetings and More Meetings: Getting Things Done When People Are Involved,” published in 2005.

— Los Angeles Times