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 that was later canceled.
Design and development
A leading goal 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 percent fewer parts than the multi-launch Space Shuttle Main Engine (SSME). The adverse consequences of this simplicity were the RS-68's having a significantly lower thrust-to-weight ratio and a 10 percent lower specific impulse compared to the SSME. The benefit of this simplicity is the RS-68's reduced construction cost.
The RS-68 is part of the Common Booster Core (CBC) used to create the five variants of the Delta IV family of launch vehicles. The largest of the launch vehicles, the "Heavy", uses three CBCs mounted together.
At its maximum 102 percent 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 percent 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 that five RS-68 engines would be used 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.
It has been 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) 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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