The scene in the motel here today had all the trappings of a NASA briefing, with talk of payloads, countdowns, orbits and azimuths. It even had one of the original astronauts, Donald K. (Deke) Slayton.

But it will be free enterprise all the way next Wednesday when, on a remote island on the south Texas coast, a group of investors hopes to launch the first successful commercial space venture in U.S. history.

A year ago, Space Services Inc. of America, a Houston-based firm, tried a similar experiment. It ended when a valve froze and the liquid-fuel rocket exploded on the launch pad. A year wiser and with a full complement of expert subcontractors, Space Services Inc. is prepared to go again.

The chances of success are "99 and 44/100ths percent," said Slayton, who became SSI's vice chairman after retiring from NASA in March. "It's no longer the original amateur hour," said one company official.

Space Services' eventual goal is to provide quick and inexpensive transportation into space for private companies that want to put up their own satellites. If all goes well next week and with future test flights, the company hopes to be ready for paying customers by 1985. But all of that depends on a successful suborbital test. "What we're doing here is still a trial," cautioned David Hannah Jr., Space Services' chairman. "We still haven't launched a rocket yet."

Until that happens, corporate customers will continue to rely on the National Aeronautics and Space Administration to get their payloads into space or turn to foreign companies now in the business.

Space Services has come a long way since last summer's failure on Matagorda Island, and Hannah readily acknowledges his company is "a year wiser." Instead of using a liquid-fuel rocket built by a private company, SSI will send up a solid-fuel booster leased from NASA for $365,000. Solid-fuel rockets are considered more stable, and the rocket motor that SSI has purchased, a Minuteman I second stage, has an excellent record. Company officials said the motor has achieved 18 consecutive successful launches for the government.

Space Services also has brought in a host of expert subcontractors to help with the project, including Space Vector Corp., which assembled the rocket and is in charge of the launch. It has performed a similar job for various government agencies. "We didn't even know about them last year," Hannah said.

Another major change is the arrival of Slayton, the persevering astronaut who waited 16 years before flying in space. He is overseeing all aspects of the test flight.

"He's been a tough taskmaster," said Space Services spokesman Walter Pennino.

The launch is scheduled for no earlier than 10 a.m. Wednesday at a newly built launch pad on Matagorda Island, which is along the Gulf coast about 45 miles northeast of Corpus Christi and near the winter habitat of the whooping crane. The land is owned by a subsidiary of the American Liberty Oil Co. of Dallas, whose chairman, Toddie Lee Wynne, is one of Hannah's financial backers. The Federal Aviation Administration yesterday gave its approval for the launch.

The 39-foot rocket, dubbed Conestoga I after the covered wagons used by American pioneers in the 19th century, is expected to be launched on a 10-minute, 26-second flight and to reach an altitude of 167 nautical miles. A mock payload will be aboard, and the test calls for it to separate from the booster and continue 279 nautical miles downrange. The equipment is scheduled to land in international waters in the Gulf of Mexico.

Space Services officials estimate the launch will cost about $2.5 million. Last year's unsuccessful test cost about $1.2 million. With all the talk about making history in space, Hannah was asked today why he and his fellow investors -- nearly all of whom are Texans -- are attempting to become the first private U.S. space company.

"It's a hard business deal," he replied.