Encyclopedia Astronautica

Nitric acid/UDMH propellant rocket stage. Loaded/empty mass 4,497/599 kg. Thrust 36.02 kN. Vacuum specific impulse 280 seconds. The Air Force requested increases in the propulsion system capabilities of the original Able upper stage design 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.

Cost $ : 5.000 million.

Status: Out of production.
Gross mass: 4,497 kg (9,914 lb).
Unfuelled mass: 599 kg (1,320 lb).
Height: 4.52 m (14.82 ft).
Diameter: 1.40 m (4.50 ft).
Thrust: 36.02 kN (8,099 lbf).
Specific impulse: 280 s.
Specific impulse sea level: 210 s.
Burn time: 296 s.
Number: 19 .

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Associated Countries
Associated Engines
  • AJ10-104 Aerojet Nitric acid/UDMH rocket engine. 35.1 kN. Isp=278s. Stainless steel version of the basic Able engine, uprated to increase thrust 34.7 kN to 37.0 kN and to increase the duration 2-1/2 times First flight 1960. More...

Associated Launch Vehicles
  • Thor Able-Star American orbital launch vehicle. As Thor Able but with enlarged Ablestar second stage with 2 1/2 x greater burn time. More...
  • Thor Ablestar 2 American orbital launch vehicle. Two stage vehicle consisting of 1 x Thor DSV-2A + 1 x Able-Star/AJ10-104D More...

Associated Manufacturers and Agencies
  • Aerojet American manufacturer of rockets, spacecraft, and rocket engines. Aerojet, Sacramento, CA, USA. More...

Associated Propellants
  • Nitric acid/UDMH Drawing on the German World War II Wasserfall rocket, nitric acid (HNO3) became the early storable oxidiser of choice for missiles and upper stages of the 1950's. To overcome various problems with its use, it was necessary to combine the nitric acid with N2O4 and passivation compounds. These formulae were considered extremely secret at the time. By the late 1950's it was apparent that N2O4 by itself was a better oxidiser. Therefore nitric acid was almost entirely replaced by pure N2O4 in storable liquid fuel rocket engines developed after 1960. Unsymmetrical Dimethylhydrazine ((CH3)2NNH2) became the storable liquid fuel of choice by the mid-1950's. Development of UDMH in the Soviet Union began in 1949. It is used in virtually all storable liquid rocket engines except for some orbital manoeuvring engines in the United States, where MMH has been preferred due to a slightly higher density and performance. More...

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