 Of equal importance was the failure of three engines during flight . Researchers quickly determined that a leaking hydrogen fuel line ruptured when exposed to vacuum , causing a loss of fuel pressure in engine two . When an automatic shutoff attempted to close the liquid hydrogen valve and shut down engine two , it accidentally shut down engine three 's liquid oxygen due to a <unk> connection . As a result , engine three failed within one second of engine two 's shutdown . Further investigation revealed the same problem for the third @-@ stage engine — a faulty igniter line . The team modified the igniter lines and fuel conduits , hoping to avoid similar problems on future launches . 
