Engineers will test deliberately flawed joints of a space shuttle booster rocket today at the Morton Thiokol test site in Utah, a company spokesman said. The test is the sixth in a series and the second to use full-size hardware, Morton Thiokol spokesman Rocky Raab said. Thiokol engineers will focus on defects allowing a leak path to the O-rings. It was a flaw in the seals on the shuttle Challenger, combined with unusually low temperatures at launch time, that allowed hot gases to leak from one of the two solid-fuel boosters and ignite the shuttle's fuel tank, causing the Jan. 28, 1986, explosion. "We have to demonstrate sooner or later that the primary O-ring will seat under pressure, or is already seated under pressure, and the only way to do that is to get pressure to it," Raab said.