Shock Absorber Plan Set for NASA's New Rocket

NASA engineers have come up with a dual shock absorber solution to the problem of excessive vibrations with the new Ares I rocket that will launch the agency?s Orion space shuttle replacement.

Two different systems at the top and bottom of the new rocket?s first stage should reduce the booster?s peak vibrations during flight to what engineers described as a few-seconds-long ?jackhammer effect.?

The recommended shock absorber system includes a spring and damper system between the Ares I booster?s first and second stages, as well as a set of 16 spring-mounted weights in the aft skirt at the bottom of the first stage.

?It?s a lot like the shock absorbers on your car,? said Steve Cook, Ares project manager at NASA?s Marshall Space Flight Center in Huntsville, Ala., in a Tuesday teleconference. ?It isolates the vibrations just traveling through the structure, all the way up to the seat.?

Without the upgrade, an Ares I rocket and its astronaut crew would be subjected to shaking forces of up to five or six times Earth?s gravity (5 to 6 Gs), or about twice the force experienced by shuttle astronauts during launch, according to NASA?s early analysis. But with the shock absorbers in place, vibrations in the Ares 1 rocket should be limited to about 0.25 Gs, or one-fourth the force of Earth?s gravity, NASA engineers said.

The peak shaking should last just a few seconds near the 115-second mark just after liftoff, said Cook, who sat in a chair-based simulation of the vibration in a NASA test. He compared it to driving a car on the bumpy shoulder of a highway.

?It really doesn?t physically bother you,? Cook said. ?It?s just kind of a high vibration.?

A shaky hurdle

The vibration issue, one of the top engineering hurdles for NASA?s Ares I rocket, stemmed from a thrust oscillation in the launch vehicle?s solid rocket booster-derived first stage. Thrust oscillation is a common phenomenon in solid propellant rockets, which burn from the inside out allowing gases to swirl in the interior until they resonate the entire structure like an organ pipe.

The main concern centered on astronaut performance during an Ares I launch, said Garry Lyles, NASA?s associate director for technical management at the Marshall Space Flight Center in Huntsville, Ala. The higher vibrations were not a crew health concern, but could prevent astronauts from reading instrument panels or flipping switches precisely due to blurry vision.

?What we want to make sure of is that we don?t get into a situation that would leave aftereffects with the crew,? Lyles added. ?We want them to feel good after [spacecraft] separation.?

The planned shock absorbing system?s passive spring and damper component is designed to sit at the top of the first stage and reduce vibrations from thrust oscillations from a peak 6 Gs to about 1 G. The addition of 16 tuned mass absorbers, cylindrical shock absorbers that use motors to sense vibrations and nullify them using spring-mounted weights, would further limit the shaking to about 0.25 Gs, engineers said.

Altogether, the added equipment would reduce the lift capacity of the Ares I rocket?s first stage by up to 1,400 pounds (625 kg), though the booster segment currently has a margin of about 8,000 pounds (3,628 kg) to work with, Cook said.

The first stage of NASA?s Ares I rocket will use a five-segment solid rocket booster, one segment longer that the twin four-segment versions used on U.S. space shuttles today. The second stage relies on liquid propellant to boost the Orion capsule into orbit.

Engineers will attach sensors to the solid rocket boosters and astronaut seats on upcoming shuttle missions to better understand the amount of vibration spaceflyers are currently subjected to on the way up into space, Lyles said.

Meanwhile, Ares I rocket engineers will turn their attention to other details with the project.

?There?s nothing on our risk list that is what I would call a showstopper or a major issue that we can?t deal with,? Cook said.

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