Houston — The disappointing ''scrub'' of the space shuttle Columbia's launch Wednesday illustrates the importance of small details in spaceflight operations. Actually, there was nothing really wrong with the spacecraft when the ground control computer stopped the countdown clock 31 seconds before launch. But a change made in the computer instructions didn't work. A second attempt to launch was then scrubbed when the lubricant oil pressure in two of the shuttle's three auxiliary power units reached too high a level, indicating oil contamination.
As officials of the National Aeronautics and Space Administration (NASA) emphasized, the Space Transportation System (STS) being developed is much more than the Orbiter spacecraft such as Columbia. It is an interdependent complex of communications, ground facilities, and proce-dures for using them, as well as the spacecraft themselves. All of it has to blend into a working system.
''We're in a flight test program . . . we're going to be turning up little things we hadn't expected . . . before we go into an operational situation,'' said Neil Hutchinson, flight controller.
Columbia's first launch was postponed from April 10 to April 12 because of a computer timing error. This time a launch was scrubbed partly because of inadequate procedures.
The shuttle craft have tanks of supercold, high-pressure oxygen and nitrogen. These need to be at suitable pressures throughout the launching countdown.
At nine minutes before launch, the ground control computer found one oxygen tank below the 865 pounds per square inch (p.s.i.) guideline set for that point in the countdown. Controllers, who said nothing was really wrong, merely told the computer to ignore that guideline and proceed.
There is another such guideline of 800 p.s.i. at 31 seconds before launch. Three oxygen tanks were below that, but not dangerously so. Controllers were in the process of telling the computer to ignore the guidelines again when the computer locked itself up. It is not yet known why this happened, Mr. Hutchinson said. ''I'm sure they're going to find out before we go again.'' In the future, better procedures for overriding the guidelines will likely be developed.
Meanwhile, with the launch being held at 31 seconds, controllers had time to talk over a different problem - signs of blockage in the oil system of the units that power Columbia's hydraulic systems. Two of the systems had abnormally high pressure.
Hutchinson explained that the situation was not considered dangerous. If the computer had not stopped the count because of the oxygen pressure, the launch would have proceeded. But, Hutchinson said, the indications from the oil systems were unprecedented. Also, the weather was deteriorating. The delay already had lengthened the astronauts' work day to a point where some first-day mission activity probably would have been curtailed.
Taking all this into account, Hutchinson said, they decided to be conservative, scrub the launch, and inspect the auxiliary power system.
It is unlikely that any hardware fault is involved, according to Dwayne Weary , subsystems manager for the power unit. The most that is likely to be needed to correct the situation on future flights would be to tighten procedures for controlling oil purity. At this writing, it was not known when the launch will be rescheduled. It cannot take place before Friday, Nov. 6, in any case.