What was the RS-68A, and Why Was It Important?

Key Takeaways

• The RS-68A generated 705,000 lbs of thrust at sea level, the most of any hydrogen engine flown
• Designed with 80% fewer parts than the Space Shuttle Main Engine to cut production costs
• Retired in April 2024 after powering the final Delta IV Heavy flight carrying NROL-70

The Engine That Redefined American Propulsion

By the mid-1990s, the United States faced an uncomfortable reality: its stockpile of rocket engine expertise had gone largely untested for a quarter century. The last truly large liquid-fueled engine developed in America had been the Space Shuttle Main Engine, which was certified in the late 1970s. A generation of engineers had passed through the industry, and the country’s ability to design, build, and certify a new heavy-lift propulsion system from scratch had never really been put to the test since Apollo.

The Evolved Expendable Launch Vehicle (EELV) program, launched by the U.S. Air Force in the early 1990s, changed that. It called for a new family of rockets that would be cheaper, more reliable, and more adaptable than anything then in operation. Boeing responded with the Delta IV concept, and to power it, Rocketdyne in Canoga Park, California, set out to design a new engine from scratch. The result was the RS-68, and its refined successor, the RS-68A, went on to become the most powerful hydrogen-burning rocket engine ever to fly.

That engine’s story spans almost three decades, from a blank sheet of paper in 1995 to a final, fire-and-thunder liftoff from Florida’s Space Coast on April 9, 2024. It’s a story about American industrial ambition, the tradeoffs between performance and cost, and the quiet, unheralded role that propulsion plays in putting the nation’s most sensitive satellites into orbit.

Why America Needed a New Engine

The context matters here. The Cold War had just ended, and the U.S. military was reassessing how it used space. Spy satellites were becoming larger, heavier, and more capable. The existing rocket fleet, including Delta II and Atlas II vehicles powered by older engines, wasn’t going to cut it for the next generation of national reconnaissance payloads. The Department of Defense needed heavier lift and wanted it at a lower per-launch price.

From that requirement came the EELV program. The Air Force invited competing proposals, and Boeing’s Delta IV concept relied on a new engine class entirely. Rocketdyne’s engineers chose liquid hydrogen and liquid oxygen as propellants, the same combination used by the Space Shuttle Main Engine. But the philosophy behind the RS-68’s design was almost opposite to the SSME’s.

The Space Shuttle Main Engine is an engineering marvel, a staged-combustion engine that squeezes every bit of efficiency out of its propellants. It runs at extraordinarily high chamber pressure and achieves a vacuum specific impulse of around 452 seconds. It’s also enormously complex, with hundreds of thousands of individual components and a production process that made each engine expensive and time-consuming to build. The SSME made sense for the Space Shuttle because it was reused up to 55 times. For a rocket that would fly once and be thrown away, that level of complexity was financial suicide.

The RS-68 was designed to be affordable for a single flight. Rocketdyne’s team, working from a design philosophy sometimes called “Cost as Independent Variable,” stripped the engine of everything that didn’t absolutely need to be there. Boost pumps were eliminated. The turbopumps used a single-shaft configuration with machined bladed disks, known as blisks, rather than the more complex assemblies in high-performance engines. The nozzle was made from an ablative material rather than being regeneratively cooled. The entire design prioritized simplicity and producibility.

The outcome was striking. The RS-68 had approximately 80% fewer parts than the Space Shuttle Main Engine despite being physically larger. Development cost was a fraction of what the SSME had required. And the engine, though less efficient per unit of propellant, could be produced and certified quickly. Starting active development in 1995, Rocketdyne completed the first successful test firing at the Air Force Research Laboratory at Edwards Air Force Base in California on September 11, 1998, and the engine was certified for flight use in December 2001.

That was a remarkable pace. Getting from blank-paper design to flight certification in just over five years for a completely new heavy-lift engine was genuinely unprecedented in the modern era of American rocketry.

From RS-68 to RS-68A

The baseline RS-68 first flew on November 20, 2002, aboard the inaugural Delta IV mission launched from Cape Canaveral. It produced approximately 650,000 pounds of thrust at sea level, enough to power the Delta IV’s Common Booster Core. The engine’s performance held up well, and the vehicle proved its worth across a range of national security and government missions.

But the Delta IV Heavy, which strapped three Common Booster Cores together, was running into payload capacity constraints. A particularly demanding payload for the National Reconnaissance Office required more lift than the existing engines could provide. Rather than redesign the entire rocket, United Launch Alliance and Rocketdyne, by then operating as Pratt & Whitney Rocketdyne, pursued an upgraded engine variant.

The RS-68A program, sometimes referred to as the Heavy Upgrade Program, began in earnest in the mid-2000s. The engineering focus was on improving turbopump performance to increase propellant mass flow through the combustion chamber, which directly raises thrust output. The team also worked to improve specific impulse, a measure of fuel efficiency, squeezing more energy from the same propellant load. Engineers accumulated over 2,900 seconds of test firing time across multiple engines during development and certification, confirming the upgrade’s reliability.

The first hot-fire test of the RS-68A occurred on September 25, 2008, on the B-1 test stand at NASA’s Stennis Space Center in Hancock County, Mississippi. Stennis had already become the primary test site for the RS-68 family after the initial development work at Edwards. Certification testing was completed in November 2010, with full flight certification granted in April 2011.

The performance gains were meaningful. Where the original RS-68 produced approximately 650,000 pounds of thrust at sea level, the RS-68A pushed that figure to 705,000 pounds. In vacuum, the engine generated approximately 800,000 pounds of thrust. Vacuum specific impulse climbed to about 412 seconds. At the vehicle level, the upgrade increased Delta IV Heavy’s payload capacity by roughly 8 to 13 percent depending on target orbit, which is the kind of margin that makes the difference between a mission being feasible or not.

The RS-68A made its first flight on June 29, 2012, when three engines powered a Delta IV Heavy carrying the NROL-15 classified reconnaissance payload from Cape Canaveral Air Force Station. The combined liftoff thrust from the three engines came to approximately 2.1 million pounds, about six percent more than what three original RS-68s had provided. From that point, the RS-68A gradually replaced the older variant across the Delta IV fleet, and by July 2015, when the Wideband Global Satcom 7 satellite launched on a Delta IV, all vehicles in the Delta IV family had fully transitioned to the RS-68A.

How the Engine Actually Works

For anyone who hasn’t spent time around rocket propulsion, the scale of the RS-68A can be hard to grasp. The engine is 17.1 feet tall and 8 feet in diameter. It weighs 14,876 pounds. When it runs, it burns liquid hydrogen and liquid oxygen, both stored at cryogenic temperatures hundreds of degrees below zero, in a combustion chamber operating at a pressure of around 1,400 pounds per square inch.

The RS-68A uses what’s known as a gas-generator cycle, one of the simpler and more widely used approaches in liquid rocket engine design. A small portion of the propellants, perhaps two to three percent, is routed to a separate gas generator, where they combust to produce hot gas. That gas then flows through a turbine, spinning it at extremely high speed. The turbine in turn drives the pumps that pull the main propellant loads out of the vehicle’s tanks and push them into the combustion chamber at the required pressure and flow rate.

The turbine exhaust doesn’t re-enter the main combustion chamber. It’s vented overboard, which is why the gas-generator cycle is sometimes called an open cycle. That’s the key efficiency tradeoff versus a staged-combustion engine like the SSME: a small amount of propellant energy is dumped, lowering the engine’s theoretical maximum specific impulse. For a reusable engine where efficiency is everything, that’s a meaningful sacrifice. For an expendable booster engine designed to be cost-effective and easy to produce, it’s an acceptable one.

The turbopumps on the RS-68A use a single-shaft design, meaning the fuel turbopump and the oxidizer turbopump both sit on the same rotating shaft rather than being driven by separate turbines. This reduces parts count and mechanical complexity. No boost pumps are needed, which further simplifies the design. The turbopumps are, of course, running at extreme speeds and handling propellants that would solidify any atmospheric moisture on contact. Their reliability is essential.

The combustion chamber is regeneratively cooled, meaning that the hydrogen propellant circulates through channels in the chamber wall before being injected for combustion, carrying away heat that would otherwise destroy the hardware. The nozzle is not regeneratively cooled. It uses an ablative material that slowly chars and erodes during the burn, absorbing and carrying away heat in the process. An ablative nozzle is simpler and cheaper to manufacture than a regeneratively cooled one, though it’s inherently single-use. Since the RS-68A was always an expendable engine, this was never a concern.

The engine is throttleable between 58 and 102 percent of its rated thrust level. That throttleability matters operationally. On a Delta IV Heavy launch, all three engines ignite and run at full thrust for the first seconds of flight. Around 50 seconds after liftoff, the central core throttles down to 58 percent while the two outboard strap-on cores continue at full power. This throttle-down conserves propellant in the center core, allowing it to continue burning after the strap-ons separate and extend its total burn duration. When the strap-ons separate, the center core throttles back up to 102 percent before eventually throttling down again just before its own cutoff.

The engine also provided roll control capability for the Delta IV through vectoring of turbine exhaust gases, adding yet another function to an already busy propulsion system.

Performance in Numbers

The RS-68A’s specifications are worth presenting clearly in one place, because the numbers tell a story of what the engine was and what it was meant to do.

Three RS-68A engines flying together on the Delta IV Heavy generated a combined sea-level thrust of approximately 2.1 million pounds at liftoff. To put that in perspective, each individual engine was producing the power equivalent of roughly 17 million horsepower, comparable to the output of about 11 Hoover Dams operating at full capacity. The entire vehicle at launch weighed approximately 1,616,000 pounds, and the rocket stood 235 feet tall.

The specific impulse of 412 seconds in vacuum is lower than the SSME’s 452 seconds, which reflects the efficiency penalty of the gas-generator cycle and the ablative nozzle. In practical mission terms, though, the RS-68A’s far lower cost per engine more than compensated for that efficiency gap when the vehicle was being designed to fly once. The original RS-68 cost approximately $20 million per engine. The RS-68A’s price was competitive with that figure, representing a fraction of what SSME production costs had historically been.

Manufacturing and Testing

The Aerojet Rocketdyne facility in Canoga Park, California, handled design and engineering for the RS-68 family throughout the engine’s operational life. Assembly work later shifted to the Engine Assembly Facility at NASA Stennis Space Center, a 100,000 square-foot factory capable of producing up to 40 RS-68 engines annually at peak capacity. The fact that the engine was both assembled and tested at Stennis was a milestone for the facility, making it the first engine with that distinction there.

Testing took place on the B-1 test stand at Stennis, which became synonymous with RS-68 and RS-68A certification over the years. Test firings at Stennis were full-duration hot-fire runs, pushing the engine through conditions representative of actual flight. The partnership between Aerojet Rocketdyne and NASA Stennis on the RS-68 program spanned more than two decades.

One aspect of RS-68A testing that deserves attention is the sheer accumulated test time. Across all certification and acceptance test firing campaigns, the engine logged thousands of seconds of cumulative hot-fire time before any engine flew in space. For the RS-68A certification effort alone, engineers accumulated over 2,900 seconds of test time. For an engine intended to fly once and never be reused, this level of pre-flight testing was a deliberate investment in verified reliability.

The final RS-68A acceptance test at Stennis took place on April 12, 2021, completing the last hot-fire run ever conducted on the engine. By that date, 65 RS-68A engines had flown with a perfect record.

The Delta IV Heavy

The RS-68A’s primary vehicle was the Delta IV Heavy, and the two are difficult to separate in any discussion of the engine’s significance. The Delta IV Heavy first flew on December 21, 2004, from Cape Canaveral, and at the time of its retirement it ranked as the third highest-capacity launch vehicle in active operation anywhere in the world.

The vehicle’s configuration was unconventional. Three Common Booster Cores, each a full-scale liquid hydrogen and liquid oxygen stage in its own right, were mounted side by side. Each carried a single RS-68A at its base. At liftoff, all three ignited together, and the center core throttled down partway through ascent to conserve propellant for an extended burn after the side cores separated. This asymmetric throttle strategy was one of the engineering solutions that made the three-core configuration work without requiring an entirely different propulsion system for the center vehicle.

The vehicle was all-cryogenic, which contributed to one of its most visually striking characteristics. Because liquid hydrogen is lighter than air, it vaporizes and rises after being purged from the engines during the pre-ignition sequence. The hydrogen-rich cloud that clings to the base of the rocket and along its flanks typically ignites into a brief fireball when the engines light. Every Delta IV Heavy launch produced this spectacular wall of flame that engulfed the lower portion of the vehicle for the first few seconds of flight. This did not damage the rocket. The thermal protection blankets on the lower section of each CBC were designed for exactly this scenario. But it was an arresting sight, and it earned the vehicle the informal nickname “the most metal of all rockets” from ULA CEO Tory Bruno.

The Delta IV Heavy was capable of delivering up to 63,470 pounds of payload to low Earth orbit and up to 14,880 pounds to geostationary orbit. Following the retirement of the Space Shuttle in 2011, it was briefly the most capable operational launch vehicle in the United States until SpaceX’s Falcon Heavy made its debut in February 2018.

Delta IV Heavy hardware production ended in May 2023. All 16 Delta IV Heavy missions were completed as planned.

Missions That Defined the RS-68A

The RS-68A flew on missions that ranged from classified national security payloads to some of the most scientifically consequential launches of the 21st century.

The engine’s most prominent public mission came on December 5, 2014, when a Delta IV Heavy carried the first Orion Multi-Purpose Crew Vehicle on its inaugural flight test, designated EFT-1 (Exploration Flight Test 1). The uncrewed Orion capsule, built by Lockheed Martin, lifted off from Space Launch Complex 37 at Cape Canaveral at 7:05 a.m. EST. The total liftoff mass of the rocket and spacecraft combination was approximately 1,630,000 pounds. The mission sent Orion into an elliptical orbit with a high point of around 3,600 miles above Earth, far enough to subject the capsule’s heat shield to entry velocities approaching those of a return from the Moon. Three RS-68A engines provided the primary propulsion for that ascent.

The EFT-1 mission was the only time the RS-68A directly supported a human-rated spacecraft, though no crew was aboard. It was a test of the capsule hardware and entry systems that would eventually carry astronauts on Artemis missions.

A second landmark RS-68A mission occurred on August 12, 2018, when a Delta IV Heavy launched the Parker Solar Probe from Cape Canaveral. The probe was destined to travel closer to the Sun than any spacecraft in history, studying the solar corona and the solar wind at distances as close as 3.8 million miles from the solar surface. The mission required the spacecraft to be accelerated to extraordinarily high velocities, making the performance of the RS-68A-powered first stage especially important. The Parker Solar Probe was named after physicist Eugene Parker, who attended the launch at age 91 and died in March 2022 at 94.

The majority of RS-68A missions involved classified national security payloads for the National Reconnaissance Office. The NRO operates the United States’ fleet of reconnaissance satellites, including optical imaging, radar imaging, and signals intelligence systems. These satellites are often among the heaviest and most complex spacecraft ever built, and the Delta IV Heavy’s ability to deliver large payloads to challenging orbits, including direct insertion to geostationary orbit, made it indispensable. Out of 16 total Delta IV Heavy flights, 12 carried NRO payloads.

The Road Not Taken: RS-68B and Ares V

No account of the RS-68A would be complete without acknowledging the version that was planned but never built, the RS-68B.

In 2006, NASA announced that it intended to use the RS-68 engine on the planned Ares V heavy-lift rocket, which was part of the Constellation Program intended to return humans to the Moon. The Ares V was eventually redesigned to use six RS-68B engines on a 33-foot-diameter core stage, along with two 5.5-segment solid rocket boosters derived from Shuttle heritage.

The RS-68B would have required modifications to the baseline design, including a different ablative nozzle configured for a longer burn duration, hardware changes to manage free hydrogen at ignition, and adjustments to reduce helium consumption during countdown. The multi-engine cluster environment of the Ares V also raised concerns about base heating. Studies determined that the ablative nozzle design was somewhat poorly suited to operating in close proximity to other burning engines, which would have required engineering solutions to manage the thermal environment at the base of the vehicle.

None of those engineering challenges were ever fully resolved in hardware. When the Obama administration cancelled the Constellation Program in 2010, the Ares V was cancelled along with it, and the RS-68B existed only in design studies. The Space Launch System that eventually emerged from the post-Constellation program planning used the RS-25, the updated Space Shuttle Main Engine, rather than any RS-68 variant.

There’s a reasonable argument that the RS-68B’s cancellation actually simplified the development path for what became SLS. The RS-25 was a known quantity with extensive flight heritage. An RS-68B would have required a substantial development and certification program. Whether the less expensive RS-68B approach would have produced a better or worse outcome than the RS-25-based SLS architecture isn’t knowable at this point, but the trade space was genuinely contested at the time. The cost argument for the RS-68 approach was never trivial, and the decision to go with the SSME-derived engines for SLS was made partly for reasons that had more to do with existing inventory and workforce continuity than with pure engineering merit.

Setting Itself on Fire

One of the more technically interesting aspects of the RS-68A’s operational life is the hydrogen ignition behavior that gave the Delta IV Heavy its visual identity.

Liquid hydrogen is colorless, odorless, and extraordinarily light. When it’s purged from the engine system during the final seconds of countdown, it rises and diffuses rapidly. The Radial Outward Firing Igniters (ROFIs) positioned at the base of each Common Booster Core were specifically designed to burn off this hydrogen before engine ignition, preventing the buildup of a flammable cloud that could cause a detonation at startup. These igniters fired like sparklers, consuming the hydrogen in a controlled burn rather than an uncontrolled explosion.

Even with the ROFIs, some residual hydrogen-rich gas typically ignited when the RS-68A engines lit. The resulting fireball would wash up the sides of the CBCs, burning off the thermal protection foam on the vehicle’s aft sections. Launch teams knew this was normal. The TPS foam was designed to handle it. But from a distance, the sight of a rocket apparently engulfed in flames while still sitting on the pad was genuinely alarming to first-time spectators.

The staggered ignition sequence for the three RS-68A engines on a Delta IV Heavy was also deliberate. The starboard CBC ignited at approximately T-7 seconds, followed two seconds later by the port and center cores. This sequencing was intended to further manage the hydrogen cloud dynamics and reduce peak transient loads on the pad structure at the moment of full ignition.

The Final Flight

The last time RS-68A engines fired in a launch mission was April 9, 2024, when a Delta IV Heavy carrying the NROL-70 classified payload lifted off from Space Launch Complex 37 at Cape Canaveral Space Force Station at 12:53 p.m. EDT.

The launch had been delayed. An initial attempt on March 28, 2024, was scrubbed due to high winds and technical issues. A second attempt was halted because of problems with a gaseous nitrogen pipeline serving multiple launch pads across the Kennedy Space Center and adjacent facilities. It took nearly two weeks of troubleshooting before the system was repaired and the launch team was confident enough to proceed.

On the day of the launch, weather was 90 percent favorable. NRO Mission Director Colonel Eric Zarybnisky gave the final “go” for launch, and the three RS-68A engines lit in their characteristic staggered sequence at T-7 seconds. The Delta IV Heavy lifted off on time, clearing the tower in the familiar blaze of hydrogen-fueled fire along its flanks.

The center core continued burning after the strap-on CBCs separated at approximately four minutes and 15 seconds into the flight. It shut down at 5.5 minutes. After that, the Delta Cryogenic Second Stage took over and burned its RL10 engine to deliver the payload to geostationary orbit. The second-stage burn, being a classified national security mission, was not broadcast publicly.

With that flight, the Delta IV Heavy’s 20-year career ended. And with it, the RS-68A’s operational life as a flying rocket engine came to a close. From the NROL-15 maiden flight in June 2012 to the NROL-70 retirement mission in April 2024, 65 RS-68A engines flew on operational missions without a single engine failure.

What Made the RS-68A Different

It’s worth stepping back and considering what the RS-68A represented in the context of American propulsion history. The engine was developed entirely with private company funds. Rocketdyne, Boeing, and their corporate successors did not receive government funding to design and build the RS-68. The Air Force’s EELV program provided launch contracts, and those contracts gave the manufacturer confidence to invest, but the engine’s development costs were not borne by the taxpayer the way the F-1, J-2, and SSME development programs had been.

That model was a significant departure from the Cold War era of American rocketry. It reflected a belief, which proved correct, that private industry could design and certify a major new propulsion system if given sufficient commercial incentive. The RS-68’s development costs were a fraction of what the SSME had required, despite producing an engine of comparable thrust class.

The engine also proved that the tradeoff between efficiency and cost could be managed intelligently. The RS-68A’s specific impulse of 412 seconds in vacuum compared unfavorably to the SSME’s 452 seconds on paper. But the missions it flew, and flew reliably, made that efficiency gap largely irrelevant in practice. A rocket engine’s value isn’t measured only in specific impulse. It’s measured in whether it delivers its payload on time, reliably, and within an acceptable cost envelope. By all three of those measures, the RS-68A performed.

The Transition to Vulcan and What Comes Next

The RS-68A’s retirement was a planned, orderly transition rather than a competitive defeat. ULA made the strategic decision to retire both the Delta IV and Atlas V families and consolidate its product line around the new Vulcan Centaurrocket. Vulcan uses two BE-4 engines built by Blue Origin, burning liquefied natural gas and liquid oxygen, with up to six solid rocket boosters for additional liftoff thrust.

Vulcan made its inaugural flight on January 8, 2024, three months before the Delta IV Heavy’s retirement. The transition was deliberate, with the final Delta IV Heavy missions serving as a bridge period while Vulcan established its flight record. It’s a competitive market: SpaceX’s Falcon Heavy has established itself as a lower-cost alternative for heavy payloads, and its reusability gives it an economic structure that an expendable vehicle simply can’t match.

There’s something that hasn’t received much public attention but is worth considering. The RS-68A’s retirement ends a specific lineage of American hydrogen-fueled booster engines that has no immediate successor. The RS-25 continues to fly on the Space Launch System, but that’s a very different application, serving as a core-stage engine on a super-heavy government rocket rather than a commercially developed booster. No private company in the United States is currently developing a large hydrogen-fueled expendable booster engine to succeed the RS-68A in that particular niche. The market has moved toward methane and kerosene-fueled engines for commercial heavy-lift. Whether hydrogen’s superior specific impulse eventually draws investment back to that propellant combination for large boosters is genuinely an open question. The answer will probably depend on the economics of liquid hydrogen production and handling as much as on any engineering consideration.

Aerojet Rocketdyne, now a subsidiary of L3Harris Technologies, continues work on the RS-25 for SLS and on other propulsion programs. The institutional knowledge that produced the RS-68 and RS-68A has not simply disappeared. But the specific expertise in designing and certifying a large gas-generator hydrogen booster from scratch is a capability that becomes harder to maintain the longer it goes unused.

Summary

The RS-68A was the largest hydrogen-burning rocket engine ever to fly, and it spent 12 years doing exactly what it was designed to do: reliably boosting some of America’s most critical payloads into orbit without a single engine failure across 65 operational flights. It was born from a deliberate decision to prioritize affordability and producibility over raw efficiency, a philosophy that proved well-suited to the expendable launch market of the late 1990s and early 2000s.

The engine’s legacy is more than just its thrust numbers or its specific impulse. It demonstrated that a private company could develop a world-class heavy propulsion system from blank paper to certified hardware in five years, using company funds, if the commercial incentive was sufficient. That model influenced how subsequent propulsion programs, including SpaceX’s own engine development efforts, approached the business of building rocket engines.

The RS-68A powered the Delta IV Heavy that launched Orion on its first flight, sent Parker Solar Probe on its journey to the Sun, and delivered a series of classified national security payloads during the final years of the Delta IV era. When the final RS-68A shut down at 5.5 minutes into the NROL-70 mission on April 9, 2024, it ended a chapter in American propulsion history that deserves more attention than it typically receives. The new chapter, written in methane and kerosene, has already begun, but the hydrogen-powered era it closes out made much of what follows possible.

Appendix: Top 10 Questions Answered in This Article

What is the RS-68A rocket engine?

The RS-68A is an upgraded variant of the Rocketdyne RS-68, a liquid hydrogen and liquid oxygen booster engine developed to power the Delta IV family of launch vehicles. It was built by Aerojet Rocketdyne and held the distinction of being the largest hydrogen-fueled rocket engine ever to fly operationally. The engine was certified for flight in April 2011 and retired following the final Delta IV Heavy mission in April 2024.

How much thrust does the RS-68A produce?

The RS-68A generates 705,000 pounds of thrust at sea level and approximately 800,000 pounds in vacuum. When three RS-68A engines were clustered together on a Delta IV Heavy, they produced a combined liftoff thrust of approximately 2.1 million pounds. This made the Delta IV Heavy, at the time of its retirement, the third highest-capacity active rocket in the world.

What propellants does the RS-68A use?

The RS-68A burns liquid hydrogen (LH2) as fuel and liquid oxygen (LOX) as oxidizer, both stored at cryogenic temperatures well below zero. This propellant combination produces clean, high-energy combustion and is among the most efficient available for rocket engines. The engine operates on a gas-generator cycle, where a small portion of propellant powers the turbines that drive the main propellant pumps.

How does the RS-68A compare to the Space Shuttle Main Engine?

The RS-68A produces more raw thrust than the Space Shuttle Main Engine (SSME) but achieves a lower vacuum specific impulse, around 412 seconds versus the SSME’s 452 seconds. The RS-68A was designed with roughly 80 percent fewer parts than the SSME, prioritizing low production cost for a single-use expendable application. The SSME was designed for reuse and maximum efficiency, making it more complex and expensive to produce.

When did the RS-68A first fly?

The RS-68A made its inaugural flight on June 29, 2012, when three engines powered a Delta IV Heavy carrying the NROL-15 classified reconnaissance payload from Cape Canaveral Air Force Station, Florida. The three engines produced a combined liftoff thrust of approximately 2.1 million pounds, about six percent more than the original RS-68 engines they replaced.

What was the RS-68A’s most famous non-military mission?

Among its most publicly visible missions, the RS-68A powered the Delta IV Heavy that launched NASA’s first Orion spacecraft on the EFT-1 test flight on December 5, 2014, and the Parker Solar Probe on August 12, 2018. The Parker Solar Probe mission sent a spacecraft closer to the Sun than any previous mission, and the RS-68A-powered first stage provided the initial energy needed for that trajectory.

Why was the RS-68A retired in 2024?

United Launch Alliance made a strategic decision to retire both the Delta IV and Atlas V rocket families and consolidate its operations around the new Vulcan Centaur rocket, which is designed to serve both national security and commercial customers at lower cost. The final Delta IV Heavy mission, NROL-70, launched on April 9, 2024, carrying a classified National Reconnaissance Office payload and ending the Delta family’s 64-year operational history.

What was the RS-68B and why was it never built?

The RS-68B was a proposed variant of the RS-68 engine intended for NASA’s Ares V rocket under the Constellation Program. The Ares V was to use six RS-68B engines on its core stage for lunar missions. When the Obama administration cancelled the Constellation Program in 2010, the Ares V and the RS-68B were cancelled with it, with the RS-68B never advancing beyond design studies.

Who manufactured the RS-68A?

The RS-68A was designed and manufactured by Rocketdyne, which operated under successive corporate names including Pratt & Whitney Rocketdyne and Aerojet Rocketdyne during the engine’s production life. Engine development was carried out at the Canoga Park, California facility, with assembly and acceptance testing taking place at NASA’s Stennis Space Center in Mississippi. Aerojet Rocketdyne is now a subsidiary of L3Harris Technologies.

How reliable was the RS-68A during its operational life?

The RS-68A completed 65 operational flights without a single engine failure, representing a perfect operational record from its first flight in June 2012 through its final flight in April 2024. The engine’s design philosophy, which prioritized simplicity and reliability over maximum efficiency, contributed to this performance. The acceptance testing program, which included over 2,900 seconds of cumulative test firing time during RS-68A certification alone, ensured each engine was thoroughly verified before flight.