The Vela series began with the launch of Vela 1/2 on October 17, 1963, a flight also marking the maiden voyage of the Atlas-Agena SLV-3 vehicle. The second pair of satellites launched on July 17, 1964, and the third on July 20, 1965. The last launch miscarried slightly when one Atlas vernier engine shut down at liftoff, while the other vernier operated at above-normal thrust levels. This resulted in a slightly lower than normal inclination for the satellites, however the mission was carried out successfully. The problem was traced to a malfunction of the vernier LOX poppet valve.