An RS-68 engine is test fired. Photo: Boeing

Powering the Delta 4 is Rocketdyne's RS-68 powerplant, the first large all-American liquid-fueled rocket engine built since the space shuttle main engine a quarter-century ago.

Developed over the past five years, the RS-68 is the world's largest hydrogen engine, yet relatively simple and cheap to produce.

"What we do is very, very difficult. But when we do it right, it's just a great feeling," said Dan Collins, Boeing's vice president of Delta programs.

"Nobody has ever developed an engine like this and done it in the amount time that Rocketdyne did. This is a great success and really the heart and soul of Delta 4 and what we are going to build our future on as far as our propulsion system."

Guzzling nearly a ton of propellants per second, the liquid hydrogen/liquid oxygen-fed RS-68 engine will produce 656,000 pounds of sea-level rated thrust.

"You have developed a system that is using 17 million horsepower and (the equivalent of) 11 Hoover Dams to go defeat gravity. Yeah, it's cool!"

During the Delta 4 launch, the engine will fire for the first four minutes of flight. It will be running at a throttle setting of 101 percent until the final seconds when a throttle-down to 58 percent is started in preparation for shutdown.

RS-68 built upon lessons of past
Although not directly derived from the space shuttle main engine (SSME), the RS-68 is comparable to that reusable, hydrogen-fueled engine developed by Rocketdyne in the 1970s.

"We've taken a lot of the good parts of the SSME," Collins said. "Then we've learned some of the lessons of operating that engine over the past two-and-a-half decades and really sat down with a clean sheet of paper and said 'if you were looking to take all of that experience and build something today, how would you go do it?' And that's what we came up with."

Driving the design of the RS-68 is cost. Keeping the expenses of producing the engine as low as possible will reduce the overall ticket to fly a satellite atop the Delta 4.

"The RS-68 is the first rocket engine designed specifically for low cost (to provide) affordable space-lift and worldwide commercial competitiveness," said Bryon Wood, the vice president and general manager of Rocketdyne.

Compared to the SSME, development time for the RS-68 was cut in half, the number of parts was reduced by 80 percent, the hand-touched labor reduced by 92 percent and non-recurring costs were cut by a factor of five.

"We've been able to get a lot of hand-work out of the RS-68 and replaced it with numerically-controlled machines. So instead of having a thrust chamber built up of a lot of tubes, we've machined this thing out of a solid piece of metal, which increases reliability," Collins said.

"A lot of those advancements help make the engine a more economical engine to build. So I really think it is a good balance of higher reliability, lower life-cycle cost and having the ability to all the development data and really have known this thing since inception gives us a whole lot of insight into the engine and what's going on with it."

The pieces and parts
The engine has 11 major components, including the combustion chamber, single oxygen and single hydrogen turbopumps, gimbal bearing, injector, gas generator, heat exchanger and fuel exhaust duct. It stands 17 feet tall, has a bell-shaped nozzle with an 8-foot diameter and features a quadrapod thrust frame that mates the engine to the Common Booster Core first stage.

"It has a regeneratively-cooled main chamber, which provides for operation of the engine in a gas generator cycle which means it has a small combustion chamber that drives the turbines. We made efficient use of the gases to do that by providing roll-control for the gases that exit the hydrogen turbopump," Wood explained.

All of the engine parts are delivered to NASA's Stennis Space Center in Mississippi where Rocketdyne has set up shop to assemble RS-68s, check them out and then conduct test firings on the B-1 stand there. Once the acceptance testing is completed, the engines are shipped to Boeing's factory in Decatur, Alabama, for installation into the Delta 4 rockets.

Put to the test
Over the past four years, RS-68 engines have logged more than 20,000 seconds of firing time during development, including tests with a thrust level of 105 percent, four percent higher than planned for launches. Three specific engines have been fired over 20 times for nearly 4,000 seconds each.

Flight engines are designed for eight starts and 1,200 seconds of burn time, well in excess of what is required. But the extra margin allows for acceptance testing and on-pad aborts without having to replace the engine.

The RS-68 was certified for use on Delta 4 in late 2001.

"It was a very, very good program," Collins said of the development. "It was a program where we learned a lot. We had our ups and downs at times, but when the team got this engine right where they wanted it to be, the amount of seconds and amount of testing we were able to put on it really escalated. We got through a lot of good testing in 2001, really gave us high confidence on the RS-68.

The particular engine for the first Delta 4 launch, number 20003, has undergone about 565 seconds of firing time during five tests, including the brief Flight Readiness Firing on launch pad 37B last month.

Collins said the engine passed the FRF with flying colors.

"We are checking the engine constantly all through the countdown. But during the last 8 and a half seconds, we are sampling engine data at about a thousand times per second. We have predictions and if we fall outside of those we will abort the launch.

"We fired that thing up and all the predictions were right down the middle of the corridor. It looked really clean."

The Delta 4 rocket undergoes the Flight Readiness Firing on October 14. Photo: Thom Baur/Boeing