Titan I also was the first true multi-stage (two or more stages) design. The Atlas missile had all three of its main rocket engines ignited at launch (two were jettisoned during flight) due to concerns about igniting rocket engines at high altitude and maintaining combustion stability. Martin, in part, was selected as the contractor because it had "recognized the 'magnitude of the altitude start problem' for the second stage and had a good suggestion for solving it." Titan I's second-stage engines were reliable enough to be ignited at altitude, after separation from the first stage booster. The first stage, besides including heavy fuel tanks and engines, also had launch interface equipment and the launch pad thrust ring with it. When the first stage had finished consuming its propellant, it dropped away, thereby decreasing the mass of the vehicle. Titan I's ability to jettison this mass prior to the ignition of the second stage meant that Titan I had a much greater total range (and a greater range per pound of second-stage fuel) than Atlas, even if the total fuel load of Atlas had been greater. As North American Aviation's Rocketdyne Division was the only manufacturer of large liquid propellent rocket engines the Air Force Western Development Division decided to develop a second source for them. Aerojet-General was selected to design and manufacture the engines for the Titan. Aerojet produced the excellent LR87-AJ-3 (booster) and LR91-AJ-3 (sustainer). George P. Sutton wrote "Aerojet's most successful set of large LPRE was that for the booster and sustainer stages of the versions of the Titan vehicle".