By Geoff Fein
Over the past year, the Navy has been resolving challenges with its CH-53K development effort including issues with the center of gravity and weight reduction, as well as ensuring that no new requirements are added to the program, according to a Navy official.
Additionally, the Marine Corps and Navy are looking to buy an additional 44 CH-53Ks as the Marine Corps swells to 202,000, Capt. Rick Muldoon, CH-53 programs manager, told Defense Daily in an interview last week.
Back in the January ’08 time frame, program officials became aware there was an issue with the CH-53K’s center of gravity (CG). “The CG slipped away from us,” Muldoon said.
The problem occurred in specific flight regimes, he added, “[when it had] low fuel or payload in the back of the aircraft.”
The problem could have been a big deal for the Navy and Marine Corps had it not been caught in the early stages of the design process, Muldoon said. The CH-53K was still in the preliminary design phase, he added.
“[It] gave us the opportunity to look at what is the art of the possible, what are the range of things we can do to fix this CG problem,” Muldoon said.
One solution designed by prime contractor Sikorsky [UTX] was to change the main shift tilt coming out of the main gearbox. “We actually have a tilt on the 53E, [a] five degree forward tilt on the rotor,” Muldoon said.
“Analysis showed backing that off to four-and-a-quarter, just three-quarters of a degree change, grew our CG envelope substantially and allowed us to even bring down the hover attitude,” Muldoon explained.
The CH-53 tends to hover nose up, making it a challenge for pilots, Muldoon noted. Sikorsky’s design change brought the hover attitude down some and grew the center of gravity envelope, he added. “That took care of that near-term issue.”
Additionally, some avionics equipment was relocated from the aft of the center of gravity forward, giving the Navy some additional margin in the design.
“If you look at helicopter design, especially single rotor, the migration of CG over time, over design, and over time during operational life shifts aft…it just does,” Muldoon said.
While remedies were found for the center of gravity issue, program officials still had to grapple with weight on the 53K.
General Electric [GE] brought their GE-38 helicopter engine, which is being specifically designed for the CH-53K, to the table, Muldoon said.
The GE-38 brought additional horsepower that the Navy didn’t anticipate in the early design phases, Muldoon added. “Instead of having a 6,000 shaft horsepower motor, we got a 7,500 shaft horsepower motor,” he added.
The new engine coupled with new fourth generation rotor blades designed by Sikorsky gave the 53K team an opportunity to grow the aircraft’s gross weight. “We actually bumped max design gross weight to 88,000 pounds from 84,000. That’s hover with a load gross weight, not sitting on a ramp gross weight,” Muldoon pointed out.
Another critical technology development was the gearbox design. “Because standard gearbox designs can’t handle the kind of power that these motors are going to deliver, we have to use what we call a split torque design,” Muldoon said.
Sikorsky built a gearbox to test the core technology, he noted.
Testing has completed on the gearbox, and Muldoon said the program office is very happy with the design.
A third technology the team was exploring was the rotor damper assembly.
According to Muldoon, the CH-53E is one of the most expensive platforms in the Department of Defense to operate. “It was never designed with reliability and maintainability in mind. The [CH-53K] is being designed from the ground up with that kind of focus.”
“We looked at all the design elements in the E that were eating our lunch, relative to reliability, and said we have to come up with something way better than that. One of those things was the damper,” Muldoon added. “One of the very promising technologies was a rotary viscoelastic damper.”
Sikorsky had two companies build prototypes to flush out the challenges with that particular technology, he said.
The new rotary viscoelastic damper was anticipated to improve reliability four times over the existing damper, Muldoon said. “That’s why it was worth exploring.”
Unfortunately, prototype testing showed that to incorporate the rotary viscoelastic damper on the 53K would lead to a weight gain of 250 pounds. “That kind of broke our threshold of pain, from a design trade perspective. We collectively made the determination that we should take an off-ramp to a more mature technology.”
Muldoon noted that the rotary viscoelastic damper is a good technology and could possibly work on other platforms. “We just couldn’t make it work for these kinds of loads. What we are doing is taking a mature linear hydraulic damper technology that is being used on the S-92 and scaling it up to 53K damper loads.”
“That’s a mature technology. Instead of getting four times improvement in reliability we will get around two times plus,” he said. “It’s still an improvement. It’s a big enough improvement that we are still well within our targets, from a modeling perspective, on overall platform reliability.”
Although Muldoon does not see the likelihood of the rotary viscoelastic damper spiraling into future builds of the CH-53K, he does have a team looking at future spiral technologies for the aircraft.
“They work as a filtering system for us because obviously when you are in critical design like we are you have already flowed the requirements into your allocated baseline. The last thing you want to do is have creeping requirements,” he said. “I am being a real stickler on this–no requirements growth.”
When technologies look like a great opportunity, but are not currently part of the program’s baseline, the team “bends them into a kind of a pre-planned product improvement perspective,” Muldoon said.
“We are in the process of really laying out details on a technology road map, where platforms could or should go,” he said.
Muldoon added, the team understands there is a line they are not crossing, when it comes to the CH-53K’s design.
“We are locked in. I am not adding anything…if anything I’ll take stuff out, but I am not putting anything back in. It’s just not a cost efficient thing to do,” he said. “[John] Young [the Pentagon’s acquisition chief] is very adamant about that…and that’s our approach.”