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The sign accompanying the Atlas vernier start tank. It reads

Atlas Vernier Engine Start Tank

In addition to its large booster and sustainer engines, each Mercury-Atlas rocket had a pair of Rocketdyne YLR101-NA-13 vernier rockets for roll control. Igniting these small engines required a pair of start tanks like the one displayed here. Before launch, one tank held RP-1 fuel and the other liquid oxygen. The tanks held only enough propellants to fire the verniers for a few seconds. After the sustainer engine came up to speed, however, its turbopump took over the job of supplying propellant to the verniers.

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