Encyclopedia Astronautica
AJ10


Aerojet Nitric acid/UDMH rocket engine family, originally designed for the Vanguard, then developed into versions for the Able and Delta upper stages.

The minor modification of the Vanguard aluminum tube thrust chamber to meet the Able requirements was accomplished in the record time of only three months. The major effort during this time was the testing of six aluminum tube thrust chambers for durations longer than the full burn time. This was done to develop confidence that the expected burn-through failure in the throat would occur at least 30% beyond the nominal duration, that it would be repeatable, and that the total impulse would be within specification limits. This was accomplished, and it provided the first opportunity for Aerojet's aluminum tube bundle engine to perform successfully in space.

Engine for Vanguard was AJ10-37; for later Able models AJ10-41 and AJ10-42. Total of 21 stages built and delivered by Aerojet.

In October 1957, Paul Degarabedian, an Associate Manager at Space Technology Laboratories (later TRW), proposed an American lunar / interplanetary launch vehicle combining the Air Force Thor IRBM; with the second and third stages of the Navy's Vanguard satellite launcher. This idea didn't make it past his management, but a month later he proposed a two stage Thor Vanguard, which he called the Thor A or Able, for test of subscale models of ICBM warheads. This was of interest, and within a month an order was placed with Aerojet for several Vanguard second stages. The STL team used the Thor's own guidance system, and built the payload interface compartment and ejection actuators in-house. It was also necessary to add 84 kg of ballast to prevent the two-stage missile's payload from overshooting its south Atlantic impact area and hitting Africa. STL Electronic Laboratory's George Mueller, later a key NASA manager in the Apollo program, was made project manager of the $1.55 million classified program. The first tests of the Aerojet Able stage began on February 21, 1958. The first Thor stage arrived on March 4. It was decided that the nosecones would carry an instrumented mouse to see if it could survive boost, sustained zero-G, and re-entry. Three weeks later the integrated tests were completed and the booster was declared ready for flight - only four months after it was conceived!

The first launch attempt on April 23 ended at T+146 when the Thor exploded due to a turbopump main bearing failure, and mouse Mia (inauspiciously named 'Missing In Action') perished. The same problem had occurred on other Thors and it took a while to identify the cause, take corrective action, and fix the turbopumps in the field. The second launch on July 9, with a ballistic nosecone and the Mouse Mia II aboard, was successful, although the nosecone could not be located by the recovery ship before it sank.

On the third launch, on July 23, telemetry indicated that mouse Wickie survived the flight into space through splashdown. But again the nosecone was not located. This completed the nosecone tests. In February it had been suggested that the configuration could be converted into a low-cost ICBM, dubbed Thoric (Thor Inter-Continental). This was an obvious threat to the Atlas program and got nowhere.

Thor B or Thor Baker was to use the three-stage combination originally proposed in October 1957 to allow the Air Force to send the first space probe to the moon. It was renamed Thor Able-I and the first launch attempt on August 17 was thwarted when the first stage failed at T+70 seconds. The next month NASA took over the program, and the Thor Able-I's payloads became the first of NASA's long series of Pioneer deep-space probes. The USAF used an upgraded version of the Aerojet Able stage for another series of reentry vehicle tests in 1959, while NASA and DARPA continued to use developed versions of the rocket for launch of Explorer, Transit, and Tiros satellites. Finally the addition of solid rocket boosters allowed larger upper stages to be carried and payload to be increased in the Delta series of rockets. The evolutionary descendant of the Thor Able, the Delta 7000, was still flying in the 21st Century. Payload had increased from 100 kg to over 5 tonnes, and the Delta was the most reliable and economical launch vehicle ever produced in the United States.

The initial Vanguard engine was not a derivative of the Aerobee thrust chamber. This had been Aerojet's original proposal, but had not been accepted by the Navy. The tubular (spaghetti)-walled chamber configuration was mostly influenced by the design experience from the Titan thrust chamber. The Vanguard thrust chamber had both an aluminum and backup stainless steel version. The latter was essentially mandated by Ed Elko (then head of the Thrust Chamber Section) and other thrust chamber elder statesmen within the company, because Aerojet had no experience with aluminum tubular thrust chambers. However, the weight advantage of the aluminum chamber was so great that the first fabrication order included 15 of the aluminum version and only 3 of the stainless steel. The two versions were virtually identical in external configuration, but there were minor differences relating to physical properties of the materials, and welding details.

The main proponent and original designer of the aluminum thrust chamber and injector was Harry Meyers, who had been hired from Bell Aircraft and had been a major contributor to that company's Rascal and Agena engines. A key element of the Vanguard engine was the injector. With a chamber internal diameter of about 8 inches, it consisted of a single 5.5 in. diameter circle of 1-on-l orifices (fuel on oxidizer, with the oxidizer aimed inward). Numerous iterations were required to obtain the desired balance of atomization and mixing necessary to achieve the target performance and stability. What finally did the trick was the addition of a single ring of fuel orifices near the center of the injector, to modify the gas circulation pattern. This was suggested by Sid Rumbold, based on his earlier work at M W Kellogg Company. Considerable analytical and tutorial efforts in the fields of heat transfer and fluid flow were conducted by Ed Elko and John Beerboom in trying to solve the problems of the early chamber wall burnouts and hot spots. This greatly improved the understanding of the situation, and the solution eventually came about by a combination of the injector spray pattern correction with the improved Titanium Carbide (TiC) wall coatings.

By the Fall of 1957, with the concurrence of the customer, the development work on the aluminum thrust chamber was suspended. The last block of second stage propulsion systems had to be released, and the continuing burnout problems with the tungsten carbide coated aluminum tubular chamber made it unacceptable to replace the stainless steel version. However the Air Force selected the aluminum chamber for a subsequent version of the engine used on their Thor-Able program.

The contractually required performance was met with the steel chamber and a respectable profit of more than 5% was realized, despite the early program problems.

Able was the first of many engine and application programs that flowed from the Vanguard experience base. These included Able, Ablestar, Delta, Fat Delta, the Japanese N II, and applications or offshoots such as Hydra, Saint (Satellite Intercept), and other classified programs. Included in all this were numerous upratings and incremental changes in the thrust chambers, tanks, and complete systems. Derivative programs included Transtage and Apollo SPS, and ultimately, the Shuttle OME. Delta thrust chamber assemblies of a considerably advanced configuration were still being produced by Aerojet well into the 21st Century - a total of over 50 years of continuous activity in this family.

The associated large number of different missions, vehicles, stages, and thrust chamber assemblies, and modifications thereof, has led to a nomenclature problem, and considerable confusion as to program details, relationships, and relative timing. A major example of this is that in the early years the Air Force called the vehicles that they procured "Thor-Able" or "Thor-Ablestar," but, NASA called all their Thor-based vehicles "Delta." No matter what they were called, they were all really Vanguard second stages, either with the original or larger diameter tanks. In those days Able or Ablestar meant Air Force, and Delta meant NASA. However, several years later, the name Delta was also applied to Aerojet's ablative thrust chambers and stages, even though some were procured by the Air Force.

Continuing development of the Vanguard aluminum thrust chamber assembly resulted in selection of this system by the Air Force for use with a Thor booster that was to be used to demonstrate the Atlas guidance system, and to explore nose cone reentry problems. This was called the Able program, and it began in November 1957. Thor was basically a single stage IRBM built by Douglas Aircraft that used essentially the same thrust chamber assembly as Atlas, and reached flight status before Atlas. Space Technology Laboratories (STL), and later the Aerospace Corporation (which was formed from part of STL in 1960), acted as system manager for the Thor-Able program and its Air Force successors. The Able system included the thrust chamber assembly, valves, tanks, pressurizing system, and any additional components to make up a complete second stage. The oxidizer was changed from the WFNA used in Vanguard, to RFNA. The first few Thor-Ables were delivered before the formation of NASA.

The minor modification of the Vanguard aluminum tube thrust chamber to meet the Able requirements was accomplished in the record time of only three months. The major effort during this time was the testing of six aluminum tube thrust chambers for durations longer than the full burn time. This was done to develop confidence that the expected burn-through failure in the throat would occur at least 30% beyond the nominal duration, that it would be repeatable, and that the total impulse would be within specification limits. This was accomplished, and it provided the first opportunity for Aerojet's aluminum tube bundle engine to perform successfully in space.

Able was the first of many engine and application programs that flowed from the Vanguard experience base. These included Able, Ablestar, Delta, Fat Delta, the Japanese N II, and applications or offshoots such as Hydra, Saint (Satellite Intercept), and other classified programs. Included in all this were numerous upratings and incremental changes in the thrust chambers, tanks, and complete systems. Derivative programs included Transtage and Apollo SPS, and ultimately, the Shuttle OME. Delta thrust chamber assemblies of a considerably advanced configuration were still being produced by Aerojet well into the 21st Century - a total of over 50 years of continuous activity in this family.

The associated large number of different missions, vehicles, stages, and thrust chamber assemblies, and modifications thereof, has led to a nomenclature problem, and considerable confusion as to program details, relationships, and relative timing. A major example of this is that in the early years the Air Force called the vehicles that they procured "Thor-Able" or "Thor-Ablestar," but, NASA called all their Thor-based vehicles "Delta." No matter what they were called, they were all really Vanguard second stages, either with the original or larger diameter tanks. In those days Able or Ablestar meant Air Force, and Delta meant NASA. However, several years later, the name Delta was also applied to Aerojet's ablative thrust chambers and stages, even though some were procured by the Air Force.

Continuing development of the Vanguard aluminum thrust chamber assembly resulted in selection of this system by the Air Force for use with a Thor booster that was to be used to demonstrate the Atlas guidance system, and to explore nose cone reentry problems. This was called the Able program, and it began in November 1957. Thor was basically a single stage IRBM built by Douglas Aircraft that used essentially the same thrust chamber assembly as Atlas, and reached flight status before Atlas. Space Technology Laboratories (STL), and later the Aerospace Corporation (which was formed from part of STL in 1960), acted as system manager for the Thor-Able program and its Air Force successors. The Able system included the thrust chamber assembly, valves, tanks, pressurizing system, and any additional components to make up a complete second stage. The oxidizer was changed from the WFNA used in Vanguard, to RFNA. The first few Thor-Ables were delivered before the formation of NASA.

The minor modification of the Vanguard aluminum tube thrust chamber to meet the Able requirements was accomplished in the record time of only three months. The major effort during this time was the testing of six aluminum tube thrust chambers for durations longer than the full burn time. This was done to develop confidence that the expected burn-through failure in the throat would occur at least 30% beyond the nominal duration, that it would be repeatable, and that the total impulse would be within specification limits. This was accomplished, and it provided the first opportunity for Aerojet's aluminum tube bundle engine to perform successfully in space.

As is almost always the case in such programs, the Air Force requested increases in the propulsion system capabilities in an effort to meet their ever-expanding mission requirements. As a result, the stainless steel version of the basic Able engine was selected, and it was uprated to increase thrust 34.7 kN to 37.0 kN and to increase the duration 2-1/2 times (easily done with the stainless steel thrust chamber) - and this configuration was called Ablestar. The Ablestar also included modifications that allowed in-space restarting - a first in the industry. The time required for developing and qualifying the Ablestar propulsion system was eight months, most of which was needed for the design, development and qualification of the much larger propellant tanks and titanium helium spheres. These remarkably short development times was a result of the basic simplicity of the Able design - mainly the low chamber pressure, hypergolic propellants, and gas pressurized propellant tanks. This simplicity also resulted in a number of additional very desirable features:

  • The ability to achieve rapid, relatively low cost modifications, and high reliability for a variety of missions
  • The ability to shift back to the aluminum thrust chamber and injector which provided an extremely good thrust to weight ratio (180, based on 3500 kgf thrust and a weight of 19.5 kg)
  • Very light weight tankage based on heat treated 410 stainless steel
  • Easily adjustable run time (in the stainless steel version) based on simply varying the length of the cylindrical section of the tanks.
In addition, the basic philosophy of pressure fed, low chamber pressure and ablative (rather than regeneratively cooled) thrust chambers for upper stage engines produced outstanding reliability and scalability. In a vacuum engine, a low chamber pressure still provides a reasonable expansion ratio, and thus reasonable performance. Secondly, low chamber pressure allows use of a very simple, pressure fed propellant system with relatively light and inexpensive tanks. Thirdly, the low chamber pressure results in lower heat transfer rates, thus making ablative chambers more practical - and they are inherently less expensive, and much mere reliable. And finally ablative chambers greatly simplify restarts in a vacuum environment because there are essentially no problems with cooling jacket and manifold fill times or coking in the coolant system.

Engine originally developed for the Vanguard launch vehicle, and subsequently developed for use on the Able and Delta upper stages and as the Apollo Service module engine.

The initial Vanguard engine was not a derivative of the Aerobee thrust chamber. This had been Aerojet's original proposal, but had not been accepted by the Navy. The tubular (spaghetti)-walled chamber configuration was mostly influenced by the design experience from the Titan thrust chamber. The Vanguard thrust chamber had both an aluminum and backup stainless steel version. The latter was essentially mandated by Ed Elko (then head of the Thrust Chamber Section) and other thrust chamber elder statesmen within the company, because Aerojet had no experience with aluminum tubular thrust chambers. However, the weight advantage of the aluminum chamber was so great that the first fabrication order included 15 of the aluminum version and only 3 of the stainless steel. The two versions were virtually identical in external configuration, but there were minor differences relating to physical properties of the materials, and welding details.

The main proponent and original designer of the aluminum thrust chamber and injector was Harry Meyers, who had been hired from Bell Aircraft and had been a major contributor to that company's Rascal and Agena engines. A key element of the Vanguard engine was the injector. With a chamber internal diameter of about 8 inches, it consisted of a single 5.5 in. diameter circle of 1-on-l orifices (fuel on oxidizer, with the oxidizer aimed inward). Numerous iterations were required to obtain the desired balance of atomization and mixing necessary to achieve the target performance and stability. What finally did the trick was the addition of a single ring of fuel orifices near the center of the injector, to modify the gas circulation pattern. This was suggested by Sid Rumbold, based on his earlier work at M W Kellogg Company. Considerable analytical and tutorial efforts in the fields of heat transfer and fluid flow were conducted by Ed Elko and John Beerboom in trying to solve the problems of the early chamber wall burnouts and hot spots. This greatly improved the understanding of the situation, and the solution eventually came about by a combination of the injector spray pattern correction with the improved Titanium Carbide (TiC) wall coatings.

By the Fall of 1957, with the concurrence of the customer, the development work on the aluminum thrust chamber was suspended. The last block of second stage propulsion systems had to be released, and the continuing burnout problems with the tungsten carbide coated aluminum tubular chamber made it unacceptable to replace the stainless steel version. However the Air Force selected the aluminum chamber for a subsequent version of the engine used on their Thor-Able program.

The contractually required performance was met with the steel chamber and a respectable profit of more than 5% was realized, despite the early program problems.

Able was the first of many engine and application programs that flowed from the Vanguard experience base. These included Able, Ablestar, Delta, Fat Delta, the Japanese N II, and applications or offshoots such as Hydra, Saint (Satellite Intercept), and other classified programs. Included in all this were numerous upratings and incremental changes in the thrust chambers, tanks, and complete systems. Derivative programs included Transtage and Apollo SPS, and ultimately, the Shuttle OME. Delta thrust chamber assemblies of a considerably advanced configuration were still being produced by Aerojet well into the 21st Century - a total of over 50 years of continuous activity in this family.

The associated large number of different missions, vehicles, stages, and thrust chamber assemblies, and modifications thereof, has led to a nomenclature problem, and considerable confusion as to program details, relationships, and relative timing. A major example of this is that in the early years the Air Force called the vehicles that they procured "Thor-Able" or "Thor-Ablestar," but, NASA called all their Thor-based vehicles "Delta." No matter what they were called, they were all really Vanguard second stages, either with the original or larger diameter tanks. In those days Able or Ablestar meant Air Force, and Delta meant NASA. However, several years later, the name Delta was also applied to Aerojet's ablative thrust chambers and stages, even though some were procured by the Air Force.

Continuing development of the Vanguard aluminum thrust chamber assembly resulted in selection of this system by the Air Force for use with a Thor booster that was to be used to demonstrate the Atlas guidance system, and to explore nose cone reentry problems. This was called the Able program, and it began in November 1957. Thor was basically a single stage IRBM built by Douglas Aircraft that used essentially the same thrust chamber assembly as Atlas, and reached flight status before Atlas. Space Technology Laboratories (STL), and later the Aerospace Corporation (which was formed from part of STL in 1960), acted as system manager for the Thor-Able program and its Air Force successors. The Able system included the thrust chamber assembly, valves, tanks, pressurizing system, and any additional components to make up a complete second stage. The oxidizer was changed from the WFNA used in Vanguard, to RFNA. The first few Thor-Ables were delivered before the formation of NASA.

The minor modification of the Vanguard aluminum tube thrust chamber to meet the Able requirements was accomplished in the record time of only three months. The major effort during this time was the testing of six aluminum tube thrust chambers for durations longer than the full burn time. This was done to develop confidence that the expected burn-through failure in the throat would occur at least 30% beyond the nominal duration, that it would be repeatable, and that the total impulse would be within specification limits. This was accomplished, and it provided the first opportunity for Aerojet's aluminum tube bundle engine to perform successfully in space.

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