Designed and manufactured in the United States by Rocketdyne (later Pratt & Whitney Rocketdyne and Aerojet Rocketdyne). 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 the cancellation of the rocket and the Constellation Program in 2010.
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 RS-25 Space Shuttle Main Engine (SSME).[4] 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.[4]
An RS-68 was part of each Delta IV Common Booster Core. The largest of the launch vehicles, the Delta IV Heavy, used three CBCs mounted together.[8]
The engine produced 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 was 14,560 pounds (6,600 kg). With this thrust, the engine had 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.[9] The RS-68 was gimbaled hydraulically and was capable of throttling between 58% and 102% thrust.[10]
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).[11] The first launch on 29 June 2012, from the Cape Canaveral Air Force Station used three RS-68A engines mounted in a Delta IV Heavy rocket.[12]
The RS-68 was retired as of the last Delta IV Heavy flight in April 2024.
Proposed uses
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.[13] Later, the Ares V was changed to use six RS-68 engines, designated the RS-68B.[14] Ares V was dropped as part of the cancellation of the Constellation program in 2010.[15] NASA's current successor heavy-lift vehicle, the Space Launch System, uses four RS-25 engines instead.[16]
Human-rating
In 2008, it was reported that the RS-68 needed over 200 changes to receive a human-rating certification.[17] NASA has stated that those changes include health monitoring, removal of the fuel-rich environment at liftoff, and improving the robustness of its subsystems.[18][19]
Variants
RS-68 is the original version. It produces 663,000 pounds-force (2,950 kN) thrust at sea level.[20]
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.[21] Its specific impulse in a vacuum is 411.9 seconds (4.039 km/s).[22] Certification was completed in April 2011.[23]
RS-68B was a proposed upgrade to be used in the Ares V launch vehicle for NASA's Constellation program.[14] 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.[24]
^"United Launch Alliance First RS-68A Hot-Fire Engine Test a Success" (Press release). United Launch Alliance. 25 September 2008. Retrieved 10 April 2023. 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.
