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NASA Devises Solution For Ares Rocket Thrust Oscillation Problem

By | August 25, 2008

      NASA leaders announced a solution for an intense vibration, or thrust oscillation, in the Ares I rocket that will loft the next-generation U.S. space capsule Orion into orbit.

      The thrust oscillation will begin around 105 to 115 seconds after liftoff, and will last "for a very few seconds," according to Gary Lyles, associate NASA director for technical management.

      He and Jeff Hanley, manager of the Constellation Program that is developing Orion, Ares and the Altair lunar lander, along with Steve Cook, the Ares project manager, briefed journalists in a telephone conference.

      While extreme thrust oscillation could pose a health hazard for the crew, the solution would ensure they face no harm.

      There was concern that the oscillation might be severe enough that it would impair the ability of astronauts to read console displays.

      So the goal was to damp down the vibration to no more than 1/4 g, or one-fourth the force of Earth gravity, and the solution that NASA is moving to adopt will do that, successfully. Doing nothing would pound the crew with a force of 5 to 6 gs.

      There will be no adverse impact on the crew, or on vehicle structures, Lyles said. Crew vibration levels will be well below health-concern levels, he indicated. Astronauts were involved in devising the oscillation solutions, and there were no "vetoes" from the astronaut office.

      That "tune-able, flexible, modular" solution involves a spring and damper system, in the skirt in the first stage system.

      Because the Orion-Ares system still is in the early stages of development, NASA leaders wanted to ensure that any solution to the thrust oscillation problem would work regardless of how motor design modifications may be made.

      While this solution will involve an active vibration damping system, even a passive portion of the system went far toward solving the problem, cutting the impact on crew members to less than 1 g. Even if the active system were to fail, it still would be well under health limits for the crew. Officials are concerned about possible effects of vibration on astronauts’ eyes and ability to speak, both during and after the few seconds of intense thrust oscillation problems. Those riding in a chair with artificial vibration, at the Ames Research Center, found that as the vibration increases, writing becomes blurry and difficult to read.

      Now we get to the tricky part.

      This damping system will involve adding a hefty amount of weight, the enemy of any space launch capability.

      However, it is only weight added to the first stage, rather than to the second stage or the space capsule.

      Therefore, the weight doesn’t have to be carried all the way to space. So, there is a relatively low impact in the mass-to-orbit weight calculation, amounting to just 1,200 to 1,400 pounds even though there would be perhaps 16 or more spring-and-damper units weighing 100 or more pounds each. There also would be electric motors powered by batteries.

      Each thrust oscillation control unit would involve a cylinder with a mass in the middle of it and springs on either side of it, which briefers likened to the suspension on a car.

      The active element of the system senses an approaching vibration headed up the stack, and the motor system moves weight to counter the vibration.

      In the math on this, the 1,200 to 1,400 pounds-to-orbit that the thrust oscillation solution system will add to the first stage still leaves Orion-Ares developers with a cushion of more than 6,000 pounds to meet unexpected weight gain in the design.

      In contrast, adding some kind of vibration-damping system in crew seats in Orion would mean adding weight that would be a drag not just in the first stage of the Ares rocket, but would be there in the second stage, transit to the moon, and return to Earth.

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