The Aerojet Rocketdyne RS-68 (Rocket System 68) is a

liquid-fuel rocket A liquid-propellant rocket or liquid rocket utilizes a rocket engine that uses liquid propellants. Liquids are desirable because they have a reasonably high density and high specific impulse (''I''sp). This allows the volume of the propellant ta ...

engine that uses

liquid hydrogen Liquid hydrogen (LH2 or LH2) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form. To exist as a liquid, H2 must be cooled below its critical point of 33 K. However, for it to be in a fully l ...

(LH2) and

liquid oxygen Liquid oxygen—abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries—is the liquid form of molecular oxygen. It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an app ...

(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 Delta IV is a group of five expendable launch systems in the Delta rocket family introduced in the early 2000s. Originally designed by Boeing's Defense, Space and Security division for the Evolved Expendable Launch Vehicle (EELV) program, th ...

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 The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding ...

's (NASA)

Ares V The Ares V (formerly known as the Cargo Launch Vehicle or CaLV) was the planned cargo launch component of the cancelled NASA Constellation program, which was to have replaced the Space Shuttle after its retirement in 2011. Ares V was also pl ...

rocket before cancellation of the rocket and the Constellation Program altogether.

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 The Aerojet Rocketdyne RS-25, also known as the Space Shuttle Main Engine (SSME), is a liquid-fuel cryogenic rocket engine that was used on NASA's Space Shuttle and is currently used on the Space Launch System (SLS). Designed and manufactu ...

(SSME)."AIAA 2002-4324, Propulsion for the 21st Century—RS-68"
. AIAA, July 8–10, 2002. 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. The RS-68 was certified in December 2001 for use on Delta IV rockets. An RS-68 is part of each

Common Booster Core. The largest of the launch vehicles, the Delta IV Heavy, uses three CBCs mounted together. At its maximum 102%

thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...

, the engine produces in a vacuum and at sea level. The engine's mass is . With this thrust, the engine has a thrust-to-weight ratio of 51.2 and a specific impulse of in a vacuum and 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 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.

Proposed uses

In 2006, NASA announced an intention to use five RS-68 engines instead of SSMEs on the planned

. 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 The Constellation program (abbreviated CxP) was a crewed spaceflight program developed by NASA, the space agency of the United States, from 2005 to 2009. The major goals of the program were "completion of the International Space Station" and a ...

in 2010. NASA's current successor heavy-lift vehicle, the

Space Launch System The Space Launch System (SLS) is an American super heavy-lift expendable launch vehicle developed by NASA. As of 2022, SLS has the highest payload capacity of any rocket in operational service, as well as the greatest liftoff thrust of any ...

, will use four RS-25 engines instead.

Human-rating

In 2008, it was reported that the RS-68 needs over 200 changes to receive a

human-rating certification Human-rating certification, also known as man-rating or crew-rating, is the certification of a spacecraft or launch vehicle as capable of safely transporting humans. There is no one particular standard for human-rating a spacecraft or launch veh ...

. NASA has stated that those changes include health monitoring, removal of the fuel-rich environment at liftoff, and improving the robustness of its subsystems.

Variants

* RS-68 is the original version. It produces thrust at sea level. * RS-68A is an improved version. It produces thrust at sea level and 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 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.

References

External links