Encyclopedia Astronautica
Delta 4H - 2

American space tug. Study 2001. Upper stage / space tug - in production. Delta 4 second stage with hydrogen tank increased to 5.1 m diameter.

The upgraded cryogenic second-stage Pratt & Whitney RL10B-2 engine was based on the 30-year heritage of the reliable RL10 engine. It incorporated an extendable exit cone for increased specific impulse (Isp) and payload capability. The basic engine and turbo pump were unchanged relative to the RL10. The engine gimbal system used electromechanical actuators that increased reliability while reducing both cost and weight. The propulsion system and attitude control system (ACS) utilized flight-proven off-the-shelf components. The second-stage propulsion system produced a thrust of 24,750 lb with a total propellant load of 37,090 lb, providing a total burn time of approximately 700 sec. Missions requiring more than one restart were accommodated by adding an extra helium bottle for the additional tank repressurization.

Gross mass: 30,710 kg (67,700 lb).
Unfuelled mass: 3,490 kg (7,690 lb).
Height: 12.00 m (39.00 ft).
Diameter: 2.44 m (8.00 ft).
Span: 5.00 m (16.40 ft).
Thrust: 110.05 kN (24,740 lbf).
Specific impulse: 462 s.

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Associated Countries
Associated Engines
  • RL-10B-2 Pratt and Whitney lox/lh2 rocket engine. 110 kN. In production. Isp=462s. Used on Delta 3 , Delta IV launch vehicles. First flight 1998. Extendable exit cone for increased specific impulse; electromechanical actuators replace hydraulic systems. More...

See also
Associated Propellants
  • Lox/LH2 Liquid oxygen was the earliest, cheapest, safest, and eventually the preferred oxidiser for large space launchers. Its main drawback is that it is moderately cryogenic, and therefore not suitable for military uses where storage of the fuelled missile and quick launch are required. Liquid hydrogen was identified by all the leading rocket visionaries as the theoretically ideal rocket fuel. It had big drawbacks, however - it was highly cryogenic, and it had a very low density, making for large tanks. The United States mastered hydrogen technology for the highly classified Lockheed CL-400 Suntan reconnaissance aircraft in the mid-1950's. The technology was transferred to the Centaur rocket stage program, and by the mid-1960's the United States was flying the Centaur and Saturn upper stages using the fuel. It was adopted for the core of the space shuttle, and Centaur stages still fly today. More...

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