Able and Able-Star Upper Stages

Despite the problems with the second stage of Vanguard, the air force used a modified version on its Thor-Able launch vehicle, showing the transfer of technology from the navy to the air service. The Able was more successful than the Vanguard prototype for two reasons. One was special cleaning and handling techniques for the propel­lant tanks that came into being after Vanguard had taken delivery of many of its tanks. Also, Thor-Able did not need to extract maximum performance from the second stage as Vanguard did, so it did not have to burn the very last dregs of propellant in the tanks. The residue that the air force did not need to burn contained more of the scale from the tanks than did the rest of the propellant. Consequently, the valves could close before most of the scale entered the fuel lines, the evident cause of many of Vanguard’s problems.30

Designated AJ10-40 (in contrast to the Vanguard second stage’s AJ10-37), the Able was a modified Vanguard stage that still used a regeneratively cooled combustion chamber with aluminum-tubular construction. Able kept the propellants (IWFNA and UDMH), tanks, helium pressurization system, and propellant valves from the Van­guard. The engine produced a thrust of about 7,500 pounds for roughly 120 seconds.31

First used in 1958, the Able upper stage remained in operation until January 1960, when Aerojet’s much more capable Able-Star replaced it. The newer stage resulted from an Advanced Research

Projects Agency directive of July 1, 1959. Aerojet could develop the Able-Star engine (AJ10-104) in a matter of months because it was derived from the Able engines and because it was simple. Directed to make the system rugged, with only those subsystems and compo­nents needed to meet requirements for restart, attitude control dur­ing coasting periods, and longer burning time than the Able could provide, Aerojet engineers sought “to achieve maximum flight ca­pability through limited redesign, overall simplification and opti­mum utilization of flight-proven components."32

Aerojet designed and built the combustion chamber to be “prac­tically identical" to the one used on the Able upper stages, so it re­mained an aluminum, regeneratively cooled, pressure-fed device. For unstated reasons that may have involved the air force’s desire to have the same propellants for Agena and Able-Star, the latter stage switched from the IWFNA used in Able to IRFNA (inhibited red fum­ing nitric acid) as the oxidizer, keeping UDMH as the fuel. Helium under pressure continued to feed the propellants to the combustion chamber, where the injector had concentric rings of orifices that mixed the hypergolic IRFNA and UDMH in an impinging-stream pattern. There were three helium containers, made of titanium, to supply the pressurizing gas. Experience had suggested no need for a nozzle closure diaphragm previously used to ensure high-altitude starts. Another change from Able was an optional nozzle extension 158 that allowed an expansion ratio of either 20:1 without it or 40:1 Chapter 4 with it. Rated thrust rose from 7,575 to 7,890 pounds with the noz­zle extended; rated specific impulse climbed proportionally.33

Although Aerojet’s design and development of Able-Star were quick, they were not problem-free. The virtually identical com­bustion chambers for the Able engines required test firings of only 115 seconds in duration. Able-Star’s had to undergo five static test firings of 300 seconds in duration because of the longer tanks and the increased burning time of the newer upper stage. With a new design for the injector manifold apparently resulting from the con­version to IRFNA as the oxidizer and coolant, during November 1959 Aerojet experienced a burnthrough of the injector plate and the cooling tubes in its vicinity. In a further piece of apparent cut – and-try engineering, Aerojet made appropriate (but unspecified) ad­justments to the designs, and two combustion chambers operated successfully for the full 300 seconds later that month.34

The Transit 1B launch by a Thor/Able-Star on April 13, 1960, marked the first programmed restart of a rocket engine in flight. For Transit 2A—launched on June 22, 1960—there was a problem with sloshing of the propellants in the stage-two tanks, which produced

roll forces. This resulted in an imperfect but usable orbit. To limit the sloshing, engineers added anti-slosh baffles to both Able-Star propellant tanks.35

In a successful launch of the Courier 1B satellite (on October 4, 1960), the anti-slosh baffles apparently had worked. The purpose of the satellite was to test the ability of a spacecraft to relieve crowded communications lines via delayed relay of information. The experi­ment was successful, with large amounts of data transmitted be­tween Puerto Rico and New Jersey. However, the delay of up to two hours before the message was repeated (after the satellite completed its orbit) was unsatisfactory for military purposes and telephone transmission, so the future lay with much higher, geosynchronous orbits. There, the satellite remained in a fixed position relative to a given location on the rotating Earth, allowing nearly instantaneous relay of messages. Overall, there were 20 Thor/Able-Star launches, of which 5 were failures, for a 75 percent success rate.36