NASA says spaceship's violent vibrations under control

• NewScientist.com news service
• Elise Kleeman

On the heels of a grim congressional report about problems with the development of the space shuttle's successor, NASA says it has everything under control with at least one potentially fatal issue – a violent shaking of the craft during lift-off.

The space shuttles are due to be retired in 2010. NASA is currently designing their replacement, 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.

The problem was made public in January, after the independent website NASA Watch and the Associated Press questioned NASA and filed Freedom of Information Act requests with the agency.

But at a media teleconference on Thursday, NASA officials said 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. "It is not a show stopper."

Swirling vortices

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.

"It's like pushing your kid on the swing," said Garry Lyles, associate director for technical management at Marshall Space Flight Center. "If you push your child at just the right frequency, you can really get them going high."

Adding weight

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.

Another option, Lyles said, 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.

Shock absorbers

NASA's goal is to lower the impact of the shaking to a force of 2 gs. Modifications to the Orion crew capsule, such as shock absorbers in the astronaut's seats, would further decrease the effect on the crew to between 0.14 and 0.30 gs.

"At those limits, they're able to read the console, to flip switches, to write, to do anything that they need to be doing," Lyles said.

Despite NASA officials' assurances that the vibration problem can and will be solved, though, the effort to replace the ageing shuttle fleet remains dogged by many other challenges, according to the Government Accountability Office, the investigative arm of the US Congress.

In a report released on Wednesday in advance of a congressional hearing on NASA's exploration initiative, the GAO highlighted many concerns that it says leave "considerable unknowns as to whether NASA's plans for these vehicles can be executed within schedule goals" and "pose risks to the successful outcome of the projects".

The GAO lists several concerns, including:

• A history of weight problems with the Ares I and Orion designs that could keep Orion from actually reaching orbit;

• Development of an essentially new engine for Ares I's upper stage;

• The lack of industry capability to produce the heat shields that Orion will need to re-enter Earth's atmosphere; and

• Insufficient test facilities for Ares I's new engine, for replicating the engine's vibration and acoustic environment, and for testing the thermal protection system for the Orion vehicle.

At this time, however, none of the problems are severe enough to foil NASA's next-generation spacecraft, Cristina Chaplain, the GAO's director of acquisition and sourcing management, said at the hearing.

There are no problems "that we have determined to be a fatal flaw", she said.

Reference :

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