Boeing Rocketdyne RS-68


The Boeing RS-68 is a liquid hydrogen and liquid oxygen powered rocket engine that is used in the Delta IV launch vehicle family.

RS-68 Rocket Engine


Development began in 1995. Rocketdyne’s RS-68 was the first large liquid fueled rocket engine to be developed in the U.S. in 25 years. It was designed to propel Boeing’s Delta IV evolved expendable launch vehicle (EELV), it is a liquid hydrogen/liquid oxygen booster engine capable of producing 650,000 lb. of sea level thrust.

Rocketdyne engineers have put the RS-68 through a series of rigorous tests taking place simultaneously at Edwards Air Force Base in Canoga Park, California and NASA’s Stennis Space Center in Hancock County, Mississippi. Also in place at Stennis is the recently dedicated SSC Engine Assembly Facility, a 100,000 square foot factory capable of producing as many as 40 RS-68 engines per year.

The first launch of the Delta IV is scheduled in 2001.

The RS-68 has demonstrated it can run at 100 percent thrust.

The RS-68 rocket engine has gimbaling capability.

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he primary goal for the RS-68 was to reduce cost versus the SSME. Some sacrifice in chamber pressure and specific impulse was made, hurting efficiency; however, development time, part count, total cost, and assembly labor were reduced to a fraction of the SSME, despite the RS-68’s significantly larger size. Typically, the RS-68 runs at 102% rated thrust for the first few minutes of flight, and then throttles down to 58% rated thrust before main engine cutoff. On the Heavy variant, the main CBC’s engine throttles down to 58% rated thrust around 50 seconds after liftoff, while the strap-on CBCs remain at 102%. This allows the main CBC to conserve propellant and burn after booster separation. After the strap-on CBCs separate, the main CBC’s engine throttles back up to 102% before throttling back down to 58% prior to main engine cutoff.

RS-68A made its first test firing in 2008 and completed its certification test in 2010 with full certification of the engine in 2011. Delta IV Heavy made its first launch with the RS-68A in 2012 lofting the classified NROL-15 satellite.

Heavy Upgrade Program

Delta 360 Launch

An NRO triggered program to upgrade the RS-68 engine to an “RS-68A” variant, called “Heavy Upgrade Program”, or “HUG”, resulted in the first hot fire test of a prototype engine at Stennis Space Center in September 2008.  Two certification engines completed their testing programs during February and November of 2010, respectively.  The first three production engines, Nos. 30003-30005, were delivered from Stennis to Decatur during April 2011 after completing their hot fire acceptance tests.  

The three engines performed the first RS-68A launch on June 29, 2012, when Delta 360, a Delta 4-Heavy Upgrade, orbited the NROL-15 payload from Cape Canaveral.  The engines produced a combined liftoff thrust of nearly 955.28 tonnes (2.1 million pounds), a roughly 6 percent increase from the previous RS-68 engine thrust.  A post-launch press release noted that Pratt & Whitney Rocketdyne (PWR) had developed RS-68A specifically to be able to lift the NROL-15 payload.  The new engine produced 318.43 tonnes of liftoff thrust and 361.52 tonnes of thrust in vacuum.  RS-68A vacuum specific impulse was targeted for 414 seconds, about 6.5 seconds more than the RS-68 value.  

The improved performance was expected to increase Delta 4 Heavy’s performance by 8 to 13 percent, depending on orbit. RS-68A will be phased into the entire Delta 4 fleet starting in 2015.  The new engine will allow all of the Medium configurations to use a standard Medium+(5,4) core, rather than cores tailor made for each type of strap on solid motor set up.  As a result, most the Medium variants would see slight performance losses compared to their original versions, but the move would achieve program cost savings.

Another change will involve the second stage engine.   Excess RL10B-2 Delta 4 engines are being converted to RL10C-1 variants for use on Atlas 5.  As part of this effort, some changes will be made to RL10B-2 engines to improve commonality between Delta 4 and Atlas 5.  The resulting RL10C-2 engines for Delta 4 will look much like RL10B-2 but will have have improved ignition systems, an updated propellant valve design, and other changes. 

Main Engine

Design simplicity, demonstrated capability and cost-efficiency define the RS-68A, the main engine for the Delta IV. Designed and manufactured by Pratt & Whitney Rocketdyne, the throttleable RS-68A engine is the largest existing hydrogen-burning engine. Conceived using a simplified design approach, the resulting engine requires 80 percent fewer parts than the Space Shuttle main engine, is lower risk, has reduced development and production costs and has inherently reliable operation.

Nominal Thrust (sea level): 702,000 lbs

Specific Impulse (sea level): 362 seconds
Length: 204 in
Weight: 14,876 lbs
Fuel/Oxidizer: Liquid Hydrogen/Liquid Oxygen

Q: You have two test facilities—Stand 1A at Edwards AFB and stand B1 at Stennis. How is the work being divided up between the two?

There are two test facilities—Stand 1A at Edwards AFB and stand B1 at Stennis. Edwards Air Force Research Lab was used for end-of-mission high-inlet pressure testing. Stennis was used for long duration testing and have the capability during the tests of pumping propellant off of barges into the tank. These were useful for testing the 340 seconds of a heavy core profile. The B1 test stand at Stennis is has the capability to gimbal.

Major Commercial Partners on RS-68?

Two major subcontractors are:

1. Thiokol build the nozzle. The nozzle is ablative and has a similar design to the solid rocket booster nozzle on the shuttle.

2. Mitsubishi Heavy Industries in Japan make the propellant valves, some of other valves, and the heat exchanger which heats up liquid oxygen to make gaseous oxygen to pressurize the LOX tank on the Delta IV launch vehicle.

The rest of the suppliers are U.S.-based and there are about 15 main ones. In comparison to other programs Rocketdyne has been able to reduce the supplier base, which in past engines usually ran on the order of two to four hundred suppliers. In the RS-68 program suppliers has been reduced to just over 100, of which 15 of those provide 80 percent of the parts.

Development of RS-68 rocket engine compared to past development programs?

Every engine in the past was driven by performance. The RS-68 engine was driven by cost. This has made the design process very different than past development programs. All of the design trades that were based on cost. As a result, the RS-68 engines looks bigger and maybe a little heavier than prior engines because performance has been traded away and weight has crept in order to keep the cost down.

The RD-68 design has a totally different philosophy compared to the space shuttle main engine (SSME) which was designed to be performance driven. The SSME doesn’t have an extra ounce of weight on it. With this RD-68 rocket engine, there is alot of performance margin because of the use of oxygen/hydrogen propellants and that gives a lot of extra performance that can then trade away for simple manufacturing techniques.

State of the art design techniques with the ability of CAD databases in the 1990’s have allowed to design and analyze the RS-68 all in one database and in three dimensions. It has made analysis alot faster and easier. All parts of the RS-68 has fit together correctly from the beginning and Rocketdyne have had no misalignments and the other kinds of things you normally get the first time you try to put a new design together.

One objective of the development of the RS-68 was to satisfy a goal of the Air Force’s participation in this evolved launch vehicle program and that is to reach a point where parts can be built repetitively with absolutely no defects. Rocketdyne have achieved that on some of the parts.

Did you know?

There was a 25-year gap in the development of large liquid fueled engines in the U.S.  The last one developed was the space shuttle main engine (SSME). Rocketdyne developed the SSME.

Once the space shuttle was flying, the national policy was going to be that the shuttle would ultimately carry just about everything. As a result there was no need to develop a new engine for another booster. It was only after Challenger that that policy came under question. Once Challenger occurred, it was determined that expendable vehicles still played a part in the military and the government’s future plans. So the expendable vehicles continued, but of course they already had engines on board, so there was no new development.

The US Air Force began in 1995 talking about an evolved expendable launch vehicle and companies like Boeing and Lockheed realized that in order to meet those future needs in expendables, they were going to need new vehicles with new engines. And so Boeing of course chose to get started on the RS-68.

Q: Is Rocketdyne currently working on any other large engines like RS-68?

A: Right now the biggest booster engine we’re working on today is this engine. As far as what’s coming up in the future, certainly we’re always working with the government or anybody else who wants to look at new things. One of the things that NASA has been looking at is the possibility of a new big carbon-fueled-based engine, or kerosene-based engine to replace the solid rocket boosters on the shuttle. They’ve looked at that in a couple of configurations—liquid strap-ons, all the way up to a thing they call the liquid flyback booster, which would actually be a reusable liquid booster to go on the side of the shuttle. That’s really what that engine is for, because kerosene is very dense propellant, so it gives you a lot of propellant in a small space, which is something you need if you’re going to strap-on liquid boosters.

We’ve been talking to NASA about that. It’s in about the same thrust range as the RS-68 is, roughly. But that’s a study right now, not an ongoing program. That’s probably the biggest thing that we’ve been looking at.


Launchspace Magazine, May 17, 2000