The first commercially sponsored rocket in U.S. history blew up on its launch pad on a remote Texas island yesterday during its first engine test.

"We blew the baby up on the pad," said Walt Pennino, a spokesman for the two companies that had underwritten the project.

No one was injured, Pennino said, but the rocket was destroyed and the effort has been set back at least a year.

"They're in the real world now," Pennino said. "This is what happens in the rocketry field. But they knew their chances were at best 50-50."

Pennino said engineers on Matagorda Island, Tex., about 60 miles from Corpus Christi, told him a main valve on the liquid oxygen tank failed to open, causing pressure to build up. "It was supposed to be a three- to four-second test," Pennino said. "It blew up before it happended."

The explosion set off a brush fire in the cow pasture that was the launch site and it took engineers at least half an hour to put it out. The explosion occurred about 5 p.m.

The rocket was the work of two companies, Space Services Inc. of Houston and GCH Inc. of Sunnyvale, Calif. About $15 million had been spent developing the 55-foot rocket, designed as the workhorse vehicle to launch communications and other satellites into space.

The engine test had been delayed several times because of bad weather and technical difficulties. Yesterday's test was intended as a preliminary to a full-scale engine test with the rocket bolted to its launch stand. Sponsors of the project hoped to test-launch the vehicle, called Percheron, later this month.

SSI and GCH hoped to challenge NASA's monopoly in the space business by offering cheap transportation into outer space for corporations wishing to launch their own satellites. They planned to charge $3 million to $5 million to send up a satellite, a fraction of NASA's cost.

Pennino, reached in Delaware, said the companies would go back to work to develop another rocket. "They'll go right back to the drawing board and build another," he said.