London, April 5 : NASA has announced that vibrations on its new rocket Ares I during lift-off, which was predicted by a design review as being fatal, is milder than first thought and a number of possible mitigation measures has been developed.
According to a report in New Scientist, NASA is currently designing a replacement for its space shuttles - a rocket called Ares I and a crew capsule called Orion, which should begin flights in 2015.
But during a design review for Ares I in October 2007, engineers calculated that the rocket might vibrate so severely in the first few minutes of launch that the shaking could actually kill the crew and destroy the spacecraft.
But at a recent media teleconference, NASA officials said that a panel of experts had since found that the shaking was milder than first thought, and had developed a number of possible mitigation measures.
"What we have found as we go along the way is that this is a very manageable issue," said Steve Cook, manager of Ares Projects for NASA's Marshall Space Flight Center in Huntsville, Alabama, US.
At the root of the problem is vibration common to all solid rocket motors, including those used to launch the space shuttle.
As the first stage of the rocket fires, disturbances in the flow of fuel create swirling vortices of gas within the rocket chamber similar to the wake behind a speeding boat.
Unlike with the shuttle, however, the solid motor vibrations of Ares I would nearly match the natural frequency of the entire launch craft, which includes the crew-bearing Orion capsule.
The result is a wrenching, amplified rattling that would make the spacecraft expand and contract like a bouncing Pogo stick.
A team of in-house and industry experts assembled by NASA in the months since the discovery of the design flaw has found that the vibrations are not as violent as once thought.
Still, the shaking remains two to three times stronger than NASA hopes to achieve for the safety of crew and craft.
NASA has proposed several mitigation measures. The leading candidate would counteract vibrations by using "tuned mass dampers" - masses on springs that cancel out the spacecraft's oscillations by moving in the opposite direction.
According to Garry Lyles, associate director for technical management at Marshall Space Flight Center, another option is to alter the natural frequency of the launch vehicle until it no longer matches that of the solid motor during lift-off. The launch vehicle's vibration frequency could be decreased by adding weight to the craft or by making it less stiff.