The U.S. space shuttle was never equipped for an unmanned test flight like the one planned tonight for the Soviets' look-alike orbiter. At the time, the program was behind schedule, money was tight and the technology was not well-developed.

NASA, however, says it had confidence that its design would work the first time when the shuttle Columbia made its debut more than seven years ago.Despite the striking resemblance of the two spacecraft, there are numerous differences between them.

The Soviet shuttle's main engines will be on the Energia rocket instead of on the reusable orbiter. The craft has small jet engines that give it greater maneuverability for landing. And it has liquid-fuel booster rockets instead of solid-fuel boosters like the one that destroyed Challenger in January 1986, grounding the American program for 32 months.

The Soviet Union announced Wednesday that its shuttle would be launched at 9:23 p.m. MDT Friday, or 6:23 a.m. Moscow time Saturday. The shuttle is named Buran, Russian for snowstorm.

Soviet officials said Buran's first flight would be pilotless to avoid deaths in case of an accident like the Challenger explosion.

The National Aeronautics and Space Administration said its decision to have two astronauts pilot the first U.S. shuttle reflected engineers' confidence that it would work the first time, but only if humans were aboard.

Spacecraft in the earlier Mercury, Gemini and Apollo programs underwent extensive unmanned testing before men flew them.

But the shuttle program was running three years behind schedule and financial pressure on NASA was rising. Flying the shuttle without astronauts would have required extensive and expensive changes to allow the automatic pilot to respond to radio commands.

Officials also noted that the American shuttle was built with the technology of the early 1970s, when it would have been more difficult to develop an automated landing system for so complex a craft. The Soviets, coming along a decade later, could take advantage of the latest developments.

As a precaution, the U.S. shuttle included ejection seats for the two pilots who flew early shakedown missions. But there was room for only two such devices, and they were removed when crews expanded.

The Soviet shuttle will be boosted into orbit by the new Energia rocket, which can lift a 220,000-pound payload. That rivals the power of America's huge Saturn 5 rocket, which was scrapped for economy reasons after taking six Apollo crews to the moon.

The liquid-hydrogen, liquid-oxygen booster has had only one test flight, in May 1987. The Soviets called the test a success, even though the payload failed to reach orbit.

The Soviet shuttle's main engines are attached to the Energia, meaning they will be jettisoned and cannot be reused. On the U.S. shuttle, the main motors are part of the orbiter and can be reflown up to 55 times.

By not carrying its engines, the Soviet shuttle can haul more cargo than its U.S. cousin, about 66,000 pounds compared with 55,000 pounds. The disadvantage is that new, expensive powerplants have to be built for each launch.

Assisting the Soviet craft into space are four strap-on rockets, also powered by liquid hydrogen and liquid oxygen. The U.S. vehicle has two solid-fuel strap-on motors.

After descending from orbit, the American shuttle becomes a powerless glider. If it were unable to reach its runway, or if high crosswinds or something else spoiled its final approach, the crew would have to bail out and leave the vehicle to crash.

The Soviet craft's two small jet engines would enable it to make its final approach at a safer, shallower angle and allow greater maneuverability, including the possibility of a second landing attempt.