NASA Identifies Fix for Ares 1 Vibration Issue

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PASADENA, Calif. — NASA’s managers have identified a fix they say will protect astronauts from potentially dangerous levels of vibrations that could reach the planned Orion Crew Exploration Vehicle during its climb to orbit atop the Ares 1 rocket.

Dale Thomas, deputy program manager of NASA’s Constellation program, said officials reached a key decision in early September to pursue a so-called “dual-plane isolator” system to address the issue. The system consists of springs that will be inserted between the Ares 1 first stage and upper stage, and between the upper stage and the Orion crew capsule, to keep the violent shaking originating in the rocket’s main stage from reaching Orion and its crew.

The Constellation program encompasses the hardware NASA is developing to replace the space shuttle by 2015 and eventually return astronauts to the Moon.

“This is flight-proven technology and we’ve got the values of the springs set, we understand it, we’ve got it modeled, and this … takes care of roughly 98 percent of the [thrust oscillation problem] that our models predicted,” Thomas said Sept. 15 during the annual AIAA Space 2009 conference here. The conference was organized by the American Institute of Aeronautics and Astronautics (AIAA).

Thrust oscillation refers to the shaking that is expected to occur during first-stage flight of the Ares 1, which is derived from the space shuttle’s giant solid-rocket boosters. Although Ares 1 passed its preliminary design review in September 2008, engineers at NASA’s Marshall Space Flight Center in Huntsville, Ala., said at the time that they had not yet settled on a hardware fix for the problem.

Thomas said the program office is also eyeing a so-called LOX damper, which would use the mass of the Ares 1’s liquid-oxygen upper-stage propellant to dampen out vibrations. The program will evaluate the fix in December for potential integration into the Ares 1 design. This would enable NASA to get by with what Thomas referred to as a single-plane isolator.

“The LOX damper is maturing and our models predict it will put us well in excess of meeting the requirement,” Thomas said. “If that works, we can probably do away with the springs between the upper stage and Orion.” Such a single-plane isolator solution would cost about a third less than the dual-plane option, he said. “The message that’s coming across here is that we do have the thrust oscillation problem solved.”

Thomas said engineers are still analyzing test results from a Sept. 10 firing of the Ares 1 first stage in Promontory, Utah, but early indications are that the thrust oscillation is considerably less than predicted.

“Surprisingly, the motor was very quiet,” Thomas said. It was two to three times below what the model predicted in terms of the forcing function for thrust oscillation.”

Thrust oscillation is common to solid-rocket motors, the casings of which tend to resonate like organ pipes as their propellant burns from the inside out. Rocket designers commonly stiffen joints or make the vehicle heavier to prevent these up-and-down vibrations — or oscillations — from growing stronger as they travel up the length of the rocket.

For the past two years, NASA engineers have been looking at options for “de-tuning” Ares 1 to prevent oscillations originating in its solid-rocket main stage from synching up with the natural resonance of the rest of the vehicle, potentially exposing astronauts to unsafe levels of shaking by the time the vibrations reach the Orion capsule. NASA engineers expect Ares 1 to deliver a relatively smooth ride until about 115 seconds into the flight, when the first stage nears burnout and produces a thrust-oscillation problem that could impair the Orion crew’s ability to read displays and perform other duties.

Thomas’ comments come at a time of uncertainty for Orion and its Ares 1 launcher, which were scrutinized by a White House-appointed panel tasked in May with determining a range of options for NASA’s manned spaceflight program. The panel’s preliminary findings, shared with senior White House and NASA officials Aug. 14, indicate Orion and Ares 1 are unlikely to be ready to ferry astronauts to the international space station before 2017, two years beyond NASA’s long-promised March 2015 target.

“Any launch vehicle has challenges,” Thomas said. “We’ve solved this one. Now we’re working on some others. There’ll be still others that crop up, but we’ll get them nailed down.”

Thomas said the program is on track for the Ares-1X test flight scheduled for late October. But he said the test is likely to slip due to a crowded launch manifest at Cape Canaveral Air Force Station, Fla.