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NASA crafts shock absorbers for Ares I rockets |
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Written by Derek Kessler on
Friday, 22 August 2008
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 It wasn’t exactly something that NASA had predicted early one when they chose the design for the Ares I rocket, but computer simulations of launches have shown that the shuttle booster-based vehicle would suffer from severe vibrations during launch that NASA engineers described as a “jackhammer effect.” While the shaking would not be debilitating or damaging to astronauts in the Orion CEV atop the rocket, the movement would be severe enough to prevent them from clearly reading instrument panels or pressing buttons and switches in the capsule. NASA’s solution: shock absorbers.
The unique dual shock absorber design would mount a spring and damper system between the Ares I’s first and second stages, and a set of sixteen spring-mounted weights near the bottom of the first stage. The weights and springs would isolate and cancel out the majority of the vibrations traveling up through the structure, much as the shock absorbers in your car do.
Without the shock absorbers, NASA’s simulations showed that the astronauts would experience shaking forces of up to 5 or 6 Gs, which is nearly twice what shuttle astronauts are subjected to on launch. With the upgrade, however, vibrations at the top of the Ares I will be reduced to an extremely palatable 1/4 G. Imagine driving your car across the rumble strips at the edge of a highway. Peak vibrations would only last a few seconds around the two minute mark after lift-off.
The vibrations had proven to be one of the top engineering issues for the Ares I, threatening to derail the entire project. It all stemmed from thrust oscillation in the Ares I’s first stage rocket motor. The Ares I was derived from the solid fuel rocket boosters used to help launch the space shuttles (they are the white columns on either side of the big orange external fuel tank). Thrust oscillation is a common occurrence with solid propellant rockets, as they burn from the inside out and the superheated gases swirl around inside to the point of creating resonance in the entire structure.
The addition of the shock absorbers will add about 625 kg (1400 lbs) to the weight of the Ares I rocket, thus reducing its payload capacity by the same amount. Currently, the first stage rocket has excess capacity of about 3600 kg (8000 lbs), so the addition of the shock absorbers should have little impact on the rocket’s final capability. For larger payloads, NASA is also developing the heavy-lifter Ares V rocket system.
The upper stage of the Ares I uses liquid propellant to boost the Orion CEV the rest of the way into orbit. External aerospace engineers have criticized NASA’s decision to use the solid-fuel rockets as primary propellant. Unlike the liquid fuel used by the Saturn V, Delta II, and Soyuz rockets, and the hybrid fuel system used by Scaled Composites’ SpaceShipOne, once ignited, a solid propellant system can not be turned off - it simply burns until all fuel is exhausted. A clear example of this is the Challenger disaster: after the shuttle and fuel tank exploded 14.6 km (48,000 ft) over the ocean just 73 seconds after launch, the solid rocket boosters continued into the sky for another 37 seconds until NASA mission control transmitted a booster self-destruct command.
NASA, however, defends their decision to use the solid fuel boosters on a financial basis. The solid propellant both cheaper and more powerful than comparable liquid propellant. Just one of the shuttle’s solid rocket boosters produces 1.8 times the thrust of the F-1 rocket engine. The F-1s are the most powerful liquid-fuel rocket motors ever built, and it took five of them to push the Saturn V rockets off the launch pad.
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"Physician, heal thyself."
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