 The first mid @-@ course correction came 11 hours into the flight . Testing on the ground had shown that the Service Propulsion System ( SPS ) engine had a small chance of exploding when burned for long periods unless its combustion chamber was " coated " first . Burning the engine for a short period would accomplish coating . This first correction burn was only 2 @.@ 4 seconds and added about 20 @.@ 4 ft / s ( 6 @.@ 2 m / s ) velocity prograde ( in the direction of travel ) . This change was less than the planned 24 @.@ 8 ft / s ( 7 @.@ 6 m / s ) , because of a bubble of helium in the oxidizer lines , which caused unexpectedly low propellant pressure . The crew had to use the small RCS thrusters to make up the shortfall . Two later planned mid @-@ course corrections were canceled because the Apollo 8 trajectory was found to be perfect . 
