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Ed Kyle

Thor, Able, and Star:  Thunder God to Orbit
Part Two of a Series Reviewing Thor Family History
by Ed Kyle, Updated 6/18/2009

thor130.jpg (27518 bytes)Thor-Able

It is an iconic photograph of the early space age.  A starkly white, futuristically streamlined rocket stands on its Florida launch pad, illuminated in the humid evening darkness by brilliant spotlights, sheathed in wisps of liquid oxygen boiloff, surrounded by hardhatted technicians - a rocket waiting to fly toward the Moon.   The year was 1958.  The rocket, poised to boost its satellite payload further from the Earth than any man-made object had previously flown, was named "Thor-Able". 

Before the Moon shot, Thor's first non-IRBM job was to test ICBM reentry vehicles and heat shields.  For this purpose Thor-Able was originally developed to fly long-range suborbital flights.  Such was the importance of this effort that Thor-Able began flying even before the Thor missile had completed its research and development flight tests. 

Thor-Able was a two-stage rocket created by stacking the Vanguard space launch vehicle's second stage atop Thor.  The second stage, dubbed "Able" for this application, was a pressure-fed design built by Space Technology Laboratories.  It burned nitric acid and unsymmetrical dimethyl hydrazine (UDMH) in a single 3.54 tonne thrust Aerojet AJ10 engine.  Able was 5.67 meters long, 0.81 meters in diameter, and weighed about 1.88 tonnes, a substantial increase from Thor's standard one tonne warhead/reentry vehicle mass.

To convert Thor from a single-stage IRBM into the first stage of a multistage vehicle, the missile's inertial guidance system was removed, replaced by a guidance system located in the Able stage.  The warhead was replaced by an interstage structure. 

thor128.jpg (26619 bytes)First PGRTV Flight (1/23/59)

Two-stage Thor-Able flew nine times from Cape Canaveral in 1958-59.  The first three flights were part of the Advanced ICBM Reentry Test Vehicle program to test ablative heat shield materials for an improved General Electric reentry vehicle for Atlas.  The first of these attempts, with Thor 116 on April 24, 1958, failed when the Thor main engine suffered a turbopump failure at T+146 seconds.  The second attempt, launched by Thor 118 on July 10, 1958, succeeded, flinging the reentry test vehicle more than 9,700 km downrange and 1,600 km into space.  The RTV successfully reentered and parachuted to the Atlantic.  It was the first time that a second stage had separated, ignited, and flown for the U.S. Air Force.  The success was repeated by Thor 119 with an Able stage that lofted its payload more than 10,100 km across the Atlantic two weeks later.

The final six two-stage Thor-Able flights were part of the Precision Guided Reentry Test Vehicle  program.  These flights used a "Thor-Able 2" configuration that tested the Titan ICBM radio-inertial guidance system.  The first of these missions failed on January 23, 1959 when an electrical failure at Thor 128's main engine cutoff prevented ignition and staging of the Able vehicle.   The next five flights, performed monthly from February to June, 1959, were all successful.  The April 8, 1959 flight was notable because the U.S. Navy successfully recovered the reentry vehicle.

thor134.jpg (12914 bytes)Thor-Able Orbits Explorer 6

Seven three-stage Thor-Able orbital launch attempts were also performed during 1958-1960.  These flights added an Allegheny Ballistics Laborary X-248 "Altair" solid rocket motor as a third stage to create the "Thor-Able 1" variant.  The spin-stabilized motor weighed 238 kg and was 1.83 meters long and 0.46 meters in diameter.  It provided 1.27 tonnes of thrust for about 38 seconds.  

The first orbital attempts were part of ARPA's "Operation Mona", better known as "Pioneer", the name assigned by NASA when it assumed control from ARPA after the first launch.  Pioneer was the first U.S. attempt to reach the Moon.  Pioneer only weighed 38 kg but still maxed out Thor-Able's payload capacity.  The spacecraft carried a simple imager, a radiation detector, a magnetometer and a microphone to detect micrometeorite hits.  Pioneer also was fitted with a small solid motor intended to perform a lunar orbit insertion burn.   

The first Pioneer lifted off on August 17, 1958, but was lost when Thor 127's LR79 experience a turbopump failure 77 seconds after liftoff.   The flight is usually identified as "Pioneer 0". 

Newly formed NASA took over the program before the second Pioneer attempt, which took place on October 11, 1958.  The Air Force actually ran the launch.  This time Thor (No. 130) performed well, but the Able stage underperformed, leaving Pioneer 1 on a 43 hour, 17 minute long suborbital trajectory with a 113,854 km apogee.  The failure is attributed by various sources as having been a programming error, a failed accelerometer, a minor pitch deviation, or an improper third stage separation.  Although Pioneer 1 didn't reach the Moon, it performed unprecedented measurements of the Earth's radiation belts. 

Pioneer-2 lifted off on November 8, 1958, but the third stage failed to ignite and the vehicle only reached 1,600 km, reentering over Africa 45 minutes after launch.
thor148.jpg (26804 bytes)Last Thor-Able Launches Tiros 1

The final four Thor-Able flights, performed for NASA from Cape Canaveral between July 1959 and January 1960, orbited Explorer 6, Pioneer 5, and Tiros 1, the first weather satellite.  The Transit 1A navigation satellite launch failed to reach orbit when the third stage failed.

Explorer 6 was boosted into a highly elliptical Earth orbit to study the radiation belts.  It also transmitted the first photographs taken of Earth from orbit.  Pioneer 5 flew into solar orbit. 

Tiros 1, a 120 kg weather satellite launched April 1, 1960, took tens of thousands of weather photos during several weeks of service.  It convincingly demonstrated the utility of the orbital weather satellite concept.

By April, 1960, the Thor-Vanguard (Thor-Able) combination had proved its relative reliability and utility to NASA.  Among NASA's early, interim stable of orbital space launchers, which included Juno I, Juno II, Vanguard, and Atlas Able, only Thor-Able had succeeded more times than it had failed.  NASA now planned to extend the success by fitting a never-flown, but already designed, Vanguard second stage gas-jet attitude control system to the Able stage, giving it the ability to coast and restart.   The new rocket, intended to fly for only a short time as an "interim" system while NASA waited for more powerful Atlas Centaur, would be called "Thor-Delta". 

thor283a.jpg (40440 bytes)Thor-Able-Star

In 1960, ARPA and the U.S. Air Force began flying Thor-Able-Star, originally named "Thor-Epsilon" which used the Aerojet General "Able-Star" second stage.

Able-Star, the world’s first restartable stage, was a "wide-body" compared to the Able stage that carried more than twice as much propellant.  Able-Star was 1.4 meters in diameter and 4.52 meters tall.  It weighed 4.497 tonnes loaded and 0.599 tonnes empty.  It was a helium pressure fed stage that used the same propellants as Able.  It also used a similar engine, the AJ10-104, which produced 3.674 tonnes of thrust.  The engine could burn for up to 262 seconds.     

A programmed autopilot controlled the Thor first stage, replacing the original Thor guidance system.  The tapered forbody section that had housed Thor's guidance system was replaced by a stepped interstage adapter, giving Thor-Able-Star a less-sleek, more business-like appearance than Thor-Able.  A lightweight guidance system on the second stage controlled the Able-Star portion of flight.  Nitrogen jets provided three-axis flight control of the Able-Star stage during coast periods.

Thor-Able-Star flew 19 times during 1960-65, including 11 launches from the Cape and 8 from Vandenberg AFB.  Using its second stage restart capabilty, it could lift more than 230 kg to a 1,000 km x 28.5 deg circular orbit.  Able-Star performed the first in-space restart on April 13, 1960.  After the stage completed its initial 258 second burn, it and its Transit 1B payload coasted for 19 minutes before the stage performed a second, 13 second long burn to finalize the 373 x 748 km x 51.28 deg orbit.

thor283.jpg (15305 bytes)Thor Able-Star Begins Failed Transit 3A Launch from Cape Canaveral LC 17B

Early launches from the Cape orbited the U.S. Navy's initial Transit (navigation) and U.S. Army's Courier (communications) satellites, along with Solrad/GRAB electronic intelligence radar signal "spy" satellites flown piggyback with Transit.  The final two of six Cape-launched Transit satellites were powered by SNAP 3B nuclear power sources (radio-isotope thermoelectric generators or "RTGs") - the first time that RTGs were launched into orbit.  Thor-Able-Star also orbited Anna 1B (Army, Navy, NASA, Air Force) a satellite that carried beacons for use in ground surveying.   

In 1963, Thor-Able-Star made its first flight from Vandenberg AFB.  The first launch took place from Complex 75-1-1, a former Thor IRBM test pad.   Thor-Able-Star would fly from nearby Complex 75-1-2 in 1964.  The sites were later renamed Space Launch Complex (SLC ) 2 East and 2 West, respectively. 

All eight Vandenberg launches carried U.S. Navy Transit navigation satellites, aimed toward near-polar orbits.  The first three Vandenberg-launched Transits used SNAP 9A RTG nuclear power sources, which were loaded with more Plutonium 238 metal fuel than the SNAP 3B RTGs.  The SNAP 9A design was discontinued after the Transit 5-BN-3 flight failed to reach orbit due to an Able-Star stage failure.  The RTG "disintegrated" in the atmosphere, as it had been designed to do, releasing about 1 kg of Plutonium 238, reportedly more than 20 times as much Plutonium 238 released during the Chernobyl incident years later, and nearly twice as much as all of the atmospheric nuclear tests performed since World War II [1].  Larger amounts of radioactive elements besides Plutonium 238 were, of course, released during those events.  After Transit 5-BN-3, SNAP RTGs were redesigned to contain nuclear fuel in the event of a launch failure.    

The final five Transit-O ("Oscar")  missions, all successful, used solar powered satellites.

Thor Able-Star's early flight record was spotty, with four launch vehicle failures, two by Thor and two by Able-Star, during the first 10 flights.  But only one of the final nine Thor Able-Stars, the one that carried the last nuclear powered Transit, failed, and the Thor first stage itself flew successfully during the final 14 flights.  

Although August 13, 1965 saw the last flight of Thor-Able-Star, parts of the Able-Star upper stage would migrate to NASA's ever-improving Delta launch vehicle. 


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