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Ed Kyle

Thor to Thorad:  Prolific Agena D Boosters
Part Four of a Series Reviewing Thor Family History
by Ed Kyle, Updated 7/14/2016


tadc59s.jpg (7082 bytes)Thor-Agena D

Thor-Agena D used the "standardized" Agena D upper stage, a stage designed to fly atop Thor, Atlas, and Titan with minimal changes. 

An improved Bell XLR81-BA-9 restartable engine, still producing 7.26 tonnes thrust but now with a more-efficient 300 second specific impulse rating, powered Agena D.  The stage was the same size as Agena B, but weighed less at 6.821 tonnes loaded and 0.673 tonnes empty.  The improved engine efficiency provided up to 265 seconds of burn time, a 25 second increase from Agena B.   The two stage rocket could lift 1.15 tonnes to polar orbit.

Radio command guidance, through a Bell Telephone Laboratories (BTL) 600 system mounted in the Agena, provided position and velocity correction during the last minute or so of the Thor first stage burn and, on some flights, during the first Agena burn.  Both Thor and Agena were equipped with three-gyro autopilots that responded to the guidance correction signals, but both could also fly independent of radio command in "open loop" mode.  Radio command guidance improved insertion accuracy compared to "open loop" guidance. 

Agena's gyros were "uncaged" when the Thor verniers shut down, initiating the upper stage's inertial reference.  Agena restarts were usually triggered by ground command.

Agena D was built around a 1.524 meter diameter cylindrical tank section.   The aluminum section included a forward fuel tank and an aft oxidizer tank separated by a common bulkhead.  The tank section was 2.452 meters long.  A fuel duct ran from the fuel tank through the oxidizer tank to reach the engine.  Two fairings carried cables and pressurization lines around the outside of the tank section. 

agenanros.jpg (7545 bytes)Lockheed Agena Assembly Line

A 0.281 meter tall cylindrical forward section, positioned atop the fuel tank, housed Agena guidance and control systems.  It consisted of a tubular welded aluminum frame covered by beryllium panels.  The forward section housed Agena guidance, communication, and control equipment.  These included the radio guidance system, three-gyro inertial reference package, horizon sensor, velocity meter, sequence timer, command receivers and decoder, telemetry systems and transmitters, and an optional orbital programmer. 

Agena's aft section included the Bell engine and a "rack" composed of a magnesium structure and skin augmented by tubular aluminum braces.   This was part of what was termed the "Agena spaceframe system".  The approximately 2 meter long aft rack held one or more pressurized nitrogen tanks, two sets of triple cold-gas thruster packs for roll control and on-orbit maneuvering, and up to 450 kg of other, optional, equipment or payloads.  Solar panels were mounted to the aft rack on some missions, for example. 

Since Agena began flying before digital computers were small and light enough to fly on-board, basic ascent-phase "programming" was initially provided by a motor-driven mechanical 6,000 second timer - think of a more complicated version of a washing machine timer.  It had 72 switches in 12 banks that could be configured to control 24 separate discrete (on/off) events.  Additional timers, "programmed" by plugging jumpers into a patch panel, could be added for longer-duration missions.  

An optional "orbital programmer" was used to control Agena when out of ground station range.  It consisted of a punch-tape system that could accept radio commands, store them, and, when run, actuate up to 52 on/off events per orbit.  Four tapes could store 512 orbits worth of commands, enough for about a month's worth of unique low earth orbit command sets.   Later-model orbital programmers used magnetic core memories.     

Thor-Agena D flew 21 times from 1962 to 1967 from Vandenberg AFB pads 75-1-1, 75-1-2, and 75-3-5, carrying Keyhole 4 and 5 film return spysats, DMSP 3A military weather satellites, and a variety of electronic intelligence and experimental U.S.Navy satellites. 

tatads.jpg (11103 bytes)Thrust Augmented Thor Agena B/D

Thor-Agena was beefed up with the addition of three Thiokol Castor 1 solid rocket motors beginning in 1963.  These "Thrust Augmented Thor" (or TAT) boosters lifted two Agena B, and 61 Agena D, stages with payloads toward orbit from 1963 until 1968.  The Castor 1 motors combined to produce 69.4 tonnes of liftoff thrust for about 28 seconds, with tailoff ending at about 40 seconds.  The boosters  augmented the upgraded MB-3 Block 3 Thor first stage engine, which itself produced 78 tonnes of liftoff thrust for 150 seconds. 

TAT-Agena D, the most-oft flown U.S. Air Force Thor space launch vehicle, weighed 67.82 tonnes at liftoff and could lift roughly 1.5 tonnes to polar orbit.  The vehicle's busiest year was 1964, when 20 launches occurred from Vandenberg. 

Five pads, 75-1-1, 75-1-2, 75-3-4, 75-3-5, and PA-1-1, a former Atlas Agena pad on the U.S. Navy test facility at Point Arguello (incorporated into Vandenberg as South Vandenberg after 1964), handled TAT Agena D launches.  The launch vehicle required about 25 working days on its pad before launch, all but the last few days in horizontal position within the slide hanger.  The Agena stage was mated horizontally with Thor about 18 working days before launch.  The mated vehicle was erected to vertical only three days before launch.  Strap on motors were installed only two days before launch. 

For the first time, Keyhole 4A imaging satellites equipped with two film return "buckets" were flown.  TAT-Agena D also orbited electronic intelligence satellites and three NASA payloads (OGO 2 and 4 and PAGEOS).   

thoradsert2ws.jpg (23361 bytes)Thorad Agena D

The final U.S. Air Force Thor Agena was the Long Tank Thrust Augmented Thor (LTTAT)-Agena D, also known as Thorad Agena D.  Long Tank Thor was stretched to 21.4 meters in length, with the tapered kerosene tank replaced by constant 2.44 meter diameter tankage.  The stage now weighed 70.354 tonnes loaded, but only 3.715 tonnes empty.  The extra propellant extended the MB-3 Block 3 burn time to 215 seconds. 

The rocket depended on three strap-on Castor 2 boosters to kick it off the pad.  The Castor 2 boosters combined to produce 87.56 tonnes of thrust for 37 seconds.  Thorad Agena D weighed 91.625 tonnes at liftoff, stood about 34 meters tall, and hauled about 2 tonnes to polar orbit.

There were 43 Thorad Agena D launches from Vandenberg AFB between 1966 and 1972.  Launches took place from SLC 1W, 1E, 2E, and 3W, the former 75-3-4, 75-3-5, 75-1-1, and PA-1-1.  Payloads included double bucket Keyhole 4A and 4B satellites, signals intelligence satellites, and Nimbus weather satellites. 

The Nimbus satellites were powered by SNAP-19 RTGs.  On May 18, 1968, the launch of Nimbus B failed when the Thorad Agena D guidance system malfunctioned.   Unlike earlier SNAP RTGs, SNAP-19 was designed to survive launch vehicle failure.   As a result, the SNAP-19 RTG was salvaged from the Pacific.  After recovery, the RTG was actually refurbished and later flown again on Nimbus 3.

Program Finale

On May 25, 1972, the final Thorad-Agena D lifted off from SLC 3W, carrying Keyhole 4B-17.  The flight, using Thor booster number 571, was the last purpose-built U.S. Air Force Thor-based orbital launch vehicle, although space launches of converted Thor IRBM missiles would continue for several more years.  Thorad Agena was supplanted by more capable Titan-based launchers, and the Corona film return satellites were eventually replaced by satellites that transmit high resolution images by radio signal. 

By the time the last Thorad-Agena D flew, Thor stage production for NASA's Delta launch vehicle had moved from the old Santa Monica Douglas plant to the newer McDonnell Douglas facility at Huntington Beach, California.  Meanwhile, three of the five Vandenberg AFB Thor-Agena pads that had hosted so many launches were finally silenced.  SLC 2W continued to be used by Delta and SLC 3W was converted for use by Atlas.

The Thor-Agena family came nearer than any other U.S. launch vehicle to the mass-produced ideal.  From 1962 through 1964, Thor-Agenas flew about twice per month. They flew from relatively spartan launch sites, employing horizontal assembly and processing.  Launch processing, and launch pad facilities, grew slightly more complex with the addition of strap-on boosters and with the switch to Long Tank Thor.     

After 185 launches, and 160 successes during a 14 year span, Thor-Agena finally ended, but the Thor family was  anything but finished. 

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