The roar of rocket engines rumbled across the lagoons of Florida's Atlantic Coast today as the space shuttle Discovery lifted away on the 18th mission of the 100-ton spaceliners, this time carrying a Saudi prince, with tasks including a test of a "Star Wars" laser.

By 4:30 p.m. today the crew had successfully deployed a Mexican satellite, Morelos, named for a hero of Mexico's fight for independence from Spain, which headed for a permanent position 22,300 miles above the Equator west of the Galapagos Islands. The first of two identical satellites, the drum-shaped Morelos will hover over the western Pacific Ocean and beam television programs to the most remote regions of Mexico.

The shuttle crew of seven, including five Americans, flew into orbit at 7:33 a.m. today carrying communications satellites from Mexico, Saudi Arabia and U.S. giant, American Telephone & Telegraph Corp. In Discovery's cargo bay were also three West German experiments and the first satellite built to be launched from the shuttle and retrieved in the same mission.

Though he was the last crew member named to this mission, Saudi Prince Sultan Salman Al-Saud drew the most attention the first day even though he spoke only once during the first nine hours of the flight.

"It's just great, the view is fantastic from here," Prince Al-Saud said. "We wish you were with us."

Before going to sleep tonight, Al-Saud's assignment was to spot the new crescent moon from his vantage point, as the Islamic holy month of Ramadan was coming to a close.

Just before 7 p.m., Al-Saud was told by Mission Control that the new moon marking the end of Ramadan had been sighted in Saudi Arabia. "Congratulations from his majesty, the king. The whole world down here now knows that the new month is about to begin," the center said.

As Discovery was about to cross Central Africa toward Yemen, Oman and the United Arab Emirates, Al-Saud said, "That's great. I am just sitting here setting up my camera to photograph the new moon."

Besides the prince, the crew includes French Air Force Col. Patrick Baudry and U.S. Air Force Col. John M. Fabian, Navy Capt. Daniel C. Brandenstein, Navy Cdr. John O. Creighton, Air Force Lt. Col. Steven R. Nagel and scientist-astronaut Shannon W. Lucid, the sixth American woman to fly into space.

On Tuesday, the crew will deploy an Arabsat satellite for a 22-nation consortium. On Wednesday a Telstar-3 satellite, designed to relay as many as 24 simultaneous color television broadcasts, will be launched for AT&T.

Discovery's mission also kicks off the Reagan administration's expensive and controversial "Star Wars" program, known officially as the Strategic Defense Initiative.

As Discovery passes over the Hawaiian Islands on its 37th orbit of Earth, Nagel will place an 8-inch mirror in the shuttle's side hatch window while engineers at an Air Force installation on the island of Maui shoot a beam of green laser light at Discovery to see if it can be reflected back to Maui.

Behind the experiment is the idea that laser beams can be fired from Earth and bounced off orbiting mirrors to destroy an enemy satellite.

On Thursday Lucid will take over the cockpit's computer console and, using the shuttle's 50-foot-long robot arm, lift from the cargo bay and deploy a box-shaped satellite called Spartan.

The satellite will be allowed to drift about 120 miles ahead of the shuttle, where for two days it will take X-ray pictures of the superheated center of the Milky Way galaxy. The satellite and videotaped astronomical pictures will be retrieved by Lucid, again using the robot arm to pull the satellite back into the cargo bay.

The first flight of an Arab into space today triggered one of the largest private celebrations of a "first space flight" at Kennedy Space Center.

About 230 Arab "guests" of the National Aeronautics and Space Administration attended the liftoff of the space shuttle and the Saudi prince, 28, a nephew of Saudi King Fahd and a grandson of the late King Saud.

Watching the launch under scrutiny of Saudi security guards were 29 Saudi princes and one Saudi sheik. Four of the princes are brothers of Prince Al-Saud.