Able IV
World’s First Space Engine – 1959
The Able IV space engine is one of the very most significant surviving artifacts of the early days of the great “Space Race” initiated by the Russian Sputnik satellite, October 4, 1957.  The earliest space efforts by both USA and USSR were to launch a satellite into earth orbit, with more failures than successes.  However, within a year, both nations turned their focus to the moon, with national prestige, and potential military implications, in the balance.
The ill-starred Atlas-Able space program was central to America’s early lunar program.  Although these particular missions were ultimately unsuccessful (thanks to balky boosters), they were, at the time, the most technically advanced satellites ever made.  Achieving a successful, close in, lunar orbit required a means of “steering” the satellites en route to their planned destination.  To that end, and despite significant technical challenges, in 1959, two 1 identical Able IV space engines were designed and built under contract with NASA by the Space Technology Laboratories (STL) of TRW.    Able IV was the world’s first liquid propellant spacecraft engine designed for use in space that was actually flown.
Able IV on Ceiling
 This impressive machine is 54” long by 21” diameter.
The monopropellant hydrazine (N2H4) fueled Able IV engine was capable of multiple restarts in space allowing midcourse velocity correction and retro propulsion to adjust the spacecraft’s trajectory en route to the planned lunar orbit.  Previously, rocket engines burned continuously until their fuel was exhausted.
Hydrazine fuel had the advantage that it did not require an igniter or an oxidizer.  Therefore, the innovative engine concept was simple, and potentially, very reliable.  A catalyst will break down hydrazine into ammonia, nitrogen, and hydrogen in an exothermic process well suited to rocket propulsion. 
However, hydrazine decomposition requires preheating the catalyst above 500 degrees Fahrenheit.  STL utilized a technique called "bipropellant slug start" in its Able4/Able 5 engines.2  This approximately 200 millisecond hypergolic reaction generated enough heat to activate the catalyst for the hydrazine engine.  This history making design allowed up to 6 restarts of the engine in space.  Rocket thrust chambers were mounted on opposing ends of the spin axis.  The injection rocket for entering lunar orbit could fire twice.  The vernier rocket could be fired four times for mid course propulsion and maneuvers.3
Able IV on Ceiling
 Vernier Rocket for mid course correction
The space engine included a large central hydrazine tank and two smaller nitrogen tanks.  The STL Able IV space engine was mounted inside a 39” sphere (Pioneer P-3) which also contained the mission scientific and communication instruments and power supplies.  The space engine’s thrust exhaust and antennae extended from either end of the satellite sphere.  Mounted on this sphere, four solar panels were designed to extend in a paddle wheel configuration to charge the satellite’s batteries.4
Able IV on Ceiling
 Insertion Rocket to enter lunar orbit
The first of the two Able IV engines was launched on the Pioneer P-3 lunar mission on November 26, 1959.  Unfortunately, the payload shroud broke away only 45 seconds from launch causing the satellite to crash into the Atlantic Ocean.5  The other Able IV engine never got a chance to fly as its designated launch vehicle was destroyed September 24, 1959, in a pre-launch test firing without the payload and space engine in place (Pioneer P-1).6  The remaining Atlas-Able launch vehicles were already committed to other planned space missions.7  Therefore, the twin Able IV space engine was never launched.  This is the engine which still exists today.
In 1960, STL built two8 more hydrazine monopropellant space engines of the same design and designated Able V-A and Able V-B.  These engines were launched September 25, 1960 (Pioneer P-30) and December 15, 1960 (Pioneer P-31).  Unfortunately, both launches failed shortly after lift off.  However, on the 17 minute Pioneer P-30 mission, the Able V-A engine got to see action.  Despite a subpar second stage burn which prevented the P-30 satellite from reaching escape velocity, ground controllers were still able to fire the Able V-A engine.9   Therefore, STL’s Able IV/V space engine “was the first to successfully ignite and operate in space.” 10  
Descendants of the Able IV engine are still in use today.  Thanks in part to the pioneering efforts of STL, hydrazine engines can be found maneuvering the International Space Station, the Space Shuttle, communication satellites, and more.
TRW presented the remaining Able IV space engine to a retiring VP in 1970.
 TRW Presentation Plaque for Able IV
 History of STL Able IV/V Space Engine Launches
Date Vehicle Payload Comments
9/24/1959 Atlas-C Able Pioneer P-1
Able IV
Payload and space engine not present when pre-launch test exploded. Able IV space engine unharmed.
11/26/1959 Atlas-D Able Pioneer P-3
Able IV
Payload shroud broke away 45 sec after launch.  First launch of spacecraft engine designed for use in space.
9/25/1960 Atlas-D Able Pioneer P-30
Able V-A
Second stage failure. Mission lasted 17 minutes.  First time that an onboard engine was fired on a space vehicle.
12/15/1960 Atlas-D Able Pioneer P-31
Able V-B
Exploded 68 seconds after launch.

All launches from Cape Canaveral
Launch Source: NSSDC Master Catalog
Rev G, August 13, 2010
R Enichen
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[1]           “What does Able Mean?” TRW program file, April 11, 1966, Approved by R.H. Droz April 27, 1966
[2]            STL Sentinel, date uncertain
[5]           TRW/STL Space Log, Vol. 1, No. 2, Sept 1960, pgs 19-20.
[6]           Gunter’s Space page, Pioneer P-1, P-3, P-30, P-31
[7]           Time Magazine, “Space, We’re in trouble”, Dec 7, 1959 
[8]           “What does Able Mean?” TRW program file, April 11, 1966, Approved by R.H. Droz April 27, 1966
[9]           A Chronology of Deep Space and Planetary Probes 1958 – 2000, June 2002, A.A. Siddiqi, pg 26"
[10]         STL Sentinel, March 5 1965