An RS-68 engine undergoing hot-fire testing at NASA's Stennis Space Center during its developmental phase.
|Country of origin||United States|
Pratt & Whitney Rocketdyne
|Application||First stage engine for the Delta IV rocket|
|Propellant||Liquid oxygen / Liquid hydrogen|
|Thrust (SL)||RS-68: 660,000 lbf (2,950 kN)|
RS-68A: 705,000 lbf (3,137 kN)
|Thrust-to-weight ratio||RS-68: 45.3|
|Chamber pressure||1,488 psi (10.26 MPa)|
|Isp (vac.)||RS-68: 410 s (4.0 km/s)|
RS-68A: 412 s (4.04 km/s)
|Length||17.1 ft (5.20 m)|
|Diameter||8 ft 0 in (2.43 m)|
|Dry weight||RS-68: 14,560 lb (6,600 kg)|
RS-68A: 14,870 lb (6,740 kg)
The Aerojet Rocketdyne RS-68 (Rocket System 68) is a liquid-fuel rocket engine that uses liquid hydrogen (LH2) and liquid oxygen (LOX) as propellants in a gas-generator power cycle. It is the largest hydrogen-fueled rocket engine ever flown.
Its development started in the 1990s with the goal of producing a simpler, less-costly, heavy-lift engine for the Delta IV launch system. Two versions of the engine have been produced: the original RS-68 and the improved RS-68A. A third version, the RS-68B, was planned for the National Aeronautics and Space Administration's (NASA) Ares V rocket before program cancellation.
Design and development
One of the main goals of the RS-68 program was to produce a simple engine that would be cost-effective when used for a single launch. To achieve this, the RS-68 has 80% fewer parts than the multi-launch Space Shuttle Main Engine (SSME). The adverse consequences of this simplicity were a significantly lower thrust-to-weight ratio and a 10% lower specific impulse compared to the SSME. The benefit of this simplicity is the RS-68's reduced construction cost.
At its maximum 102% thrust, the engine produces 758,000 pounds-force (3,370 kN) in a vacuum and 663,000 pounds-force (2,950 kN) at sea level. The engine's mass is 14,560 pounds (6,600 kg). With this thrust, the engine has a thrust-to-weight ratio of 51.2 and a specific impulse of 410 seconds (4.0 km/s) in a vacuum and 365 seconds (3.58 km/s) at sea level. The RS-68 is gimbaled hydraulically and is capable of throttling between 58% and 102% thrust.
The RS-68A is an updated version of the RS-68, with increased specific impulse and thrust (to over 700,000 pounds-force (3,100 kN) at sea level). The first launch on June 29, 2012, from the Cape Canaveral Air Force Station used three RS-68A engines mounted in a Delta IV Heavy rocket.
In 2006, NASA announced an intention to use five RS-68 engines instead of SSMEs on the planned Ares V. NASA chose the RS-68 because of its lower cost, about $20 million per engine including the cost of NASA's upgrades. The upgrades included a different ablative nozzle to accommodate a longer burn, a shorter start sequence, hardware changes to limit free hydrogen at ignition, and a reduction in the amount of helium used during countdown and flight. Thrust and specific impulse increases would occur under a separate upgrade program for the Delta IV rocket. Later, the Ares V was changed to use six RS-68 engines, designated the RS-68B. Ares V was dropped as part of the cancellation of the Constellation program in 2010. NASA's current successor heavy-lift vehicle, the Space Launch System, will use four RS-25 engines instead.
In 2008, it was reported that the RS-68 needs over 200 changes to receive a human-rating certification. NASA has stated that those changes include health monitoring, removal of the fuel-rich environment at liftoff, and improving the robustness of its subsystems.
- RS-68 is the original version. It produces 663,000 pounds-force (2,950 kN) thrust at sea level.
- RS-68A is an improved version. It produces 705,000 lbf (3,140 kN) thrust at sea level and 800,000 lbf (3,560 kN) thrust in a vacuum. 
- RS-68B was a proposed upgrade to be used in the Ares V launch vehicle for NASA's Constellation program. The Ares V was to use six RS-68B engines on a 10 metres (33 ft) diameter core stage, along with two 5.5-segment solid rocket boosters. It was later determined that the ablative nozzle of the RS-68 was poorly suited to this multi-engine environment, causing reduced engine efficiency and extreme heating at the base of the vehicle.
- Comparison of orbital rocket engines
- M-1 (rocket engine)
- National Launch System
- Rocketdyne J-2
- Space Launch Initiative
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Currently, the RS-68 engine can deliver more than 660,000 pounds of sea level thrust and the upgraded RS-68A will increase this to more than 700,000 pounds. The RS-68A also improves on the specific impulse, or fuel efficiency, of the RS-68.[permanent dead link]
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