Aviation Week & Space Technology, 08/18/2008 , page 42
Frank Morring, Jr., Washington
Orion/Ares I pushed back as engineers home in on vibration remedy
Engineers intent on solving a troublesome thrust oscillation problem besetting the next U.S. human launch system are set to brief bosses on ways to fix it, although tight funding and a better understanding of overall technical hurdles already has forced NASA to slide the target date of the vehicle’s first flight back a year.
Initial operational capability (IOC) of the planned Orion crew exploration vehicle and its Ares I crew launch vehicle will slip by at least a year, top officials say. The U.S. space agency is holding the March 2015 IOC "commitment date" for the new vehicles, but it has moved its "aggressive" internal target for the first piloted flight of the space shuttle follow-on to September 2014 from September 2013.
Under the probabilistic risk assessment techniques NASA is using to estimate the "gap" between the last space shuttle flight in 2010 and the first flight of Orion/Ares, there is a 65% chance of meeting the 2015 IOC, and only about a 50% chance of achieving the new earlier target, according to Jeff Hanley, manager of the Constellation Program that is developing the next generation of U.S. human spacecraft.
The new internal target is based on NASA’s recent funding history and its requested budget for future spending; it does not take into account any continuing funding resolution Congress may enact this fall that would hold agency spending in Fiscal 2009 to Fiscal 2008 levels, or otherwise change the requested appropriation.
Hanley says he was no longer confident that the old date of September 2013 would allow sufficient time to meet the goal. "September 2014 is much more in the realm of executability," he says.
The date change will require renegotiation of NASA’s Ares I and Orion development contracts, according to Doug Cooke, deputy associate administrator for exploration systems. It also will delay some of the planned testing leading to the March 2015 first flight of an Orion crew.
While that first flight date remains the official goal, a trial of the Orion pad abort system at White Sands Missile Range, N.M., already had slipped from the end of this year into 2009, Hanley says. The Ares I-X suborbital test of a largely boiler-plate version of the whole Orion/Ares I stack also was considered unlikely to meet its planned Apr. 15, 2009, target date even before the internal date was moved (AW&ST May 26, p. 22).
"Ares I-X and Pad Abort One are on a schedule that will be dictated by the progress the team makes to getting those to the finish line," Hanley says. "They’re not really affected by this schedule realignment. Our subsequent test program, Ascent Abort One and the ascent-abort test firings that follow probably will move to the right somewhat."
The Ares I-X test will carry instrumentation designed to help the Ares I project get better data on the thrust oscillation problem that cropped up last year. "Conservative" analysis predicted potentially dangerous vibrations in the Orion cabin triggered by thrust oscillation in the final few seconds of burning in the solid-fuel first stage (AW&ST Jan. 28, p. 400; Feb. 25, p. 36).
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NASA has slipped its internal target for the first flight of the Ares I rocket with a crewed Orion capsule by a year, but engineers have settled on a solution for a tricky thrust oscillation in the stack.Credit: NASA MARSHALL SPACE FLIGHT CENTER
Ares I engineers are to present their final recommendation on fixing the problem at NASA headquarters this month. After considering mounting the first-stage recovery parachutes on springs to "detune" the stack, they have settled on an approach designed to minimize changes to the Orion vehicle by handling almost all of the vibration in the first stage.
For a worst-case scenario, driven by data from Ares I-X and other testing, the Ares I project and its first-stage contractor-ATK-would develop an active tune mass absorber that would detect the frequency and amplitude of the thrust oscillation with accelerometers, and use battery-powered motors to move weights up and down to damp it out. The concept calls for mounting 16 - 20 of the devices on the aft skirt of the Ares I first stage.
Garry Lyles, an experienced launch vehicle engineer at Marshall Space Flight Center who heads up the effort to fix the thrust oscillation problem, says the approach will be able to reduce vibration loads on the crew in Orion to 0.25g, which is considered low enough for astronauts to be able to read displays and react to changing conditions effectively. It also would be able to handle variations in the vibrations produced by a given motor with "a lot of capability to tune in on the frequencies that were being generated."
Steve Cook, exploration launch vehicles project manager at Marshall, says the upcoming testing and analysis, including centrifuge tests at Ames Research Center that may update the Project Gemini-vintage human-loads guidelines, could eliminate the need for an active-damping concept. Lyles’ team has also studied passive damping in the aft skirt, which may be sufficient for handling the actual loads. A passive "compliance structure"-essentially a spring-loaded ring that would detune the stack by softening the interface between the first and upper stages-also is included in the design concept.
The active aft-skirt tune mass absorber would weigh about 6,500 lb., and the compliance structure would add another 6,000 lb. Both would drop away with the first stage, Cook says, meaning the maximum hit to overall vehicle performance in terms of payload to orbit would be 1,200-1,400 lb. That is within the performance margin maintained at this point in vehicle development, Cook says, and could drop if testing and analysis reveal the fixes are more than is needed.
"First, we’re going to look to see if we can leave off the compliance ring," Cook says. "Then we’d like to be able to go from an active system down to a passive system, and then the last thing we would do is to take the whole thing off as we go forward. So we’ve got a series of off-ramps with this design."
Cook says the thrust oscillation issue did not play into NASA’s decision to push back the internal IOC. The Ares I is in the final stages of preliminary design review (PDR), with a PDR board tentatively set for Sept. 10. But the thrust oscillation concern will be addressed in a "delta PDR" next spring, and developing the mitigation designs should fit within the overall schedule.
"The solution we’ve got works," Cook says. "It pounds it flat, gives us a lot of flexibility and we can handle it from a performance perspective."
While the Ares I/ Orion stack retains enough weight margin to accommodate the thrust oscillation fix, the project’s approach to weight reduction has come under fire from the Aerospace Safety Advisory Panel (ASAP), which warned that the "zero-base" approach used could make it difficult for necessary safety hardware to "earn its way in" to the vehicle design.
"When safety elements have to ‘earn their way’ onto a design that has already begun to take shape, objectivity and consistency in the decision-making could be compromised," the ASAP 2007 annual report states. But NASA believes its approach will produce safe spacecraft.
"Every system, whether it be safety-related or not, has undergone a great deal of scrutiny, so that we can understand exactly what it contributes to the spacecraft’s design, as well as the spacecraft’s reliability," Hanley says. "You start with a minimum functional design, and then you improve it in the areas where it gives you the greatest return. . . . We’re not done yet."
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