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What’s the Deal with Rocket Vibrations?

Vibrations produced during a rocket’s flight have a great impact on its systems.

In my coverage of NASA’s successor to the space shuttle, the Ares rocket, and the next crew capsule, Orion, I have come across a common theme: vibrations.

While it may seem like a simple concept, vibrations or shaking can have a very powerful effect on a rocket’s avionics, hardware, and any humans onboard. They come from such sources as the thrust of the solid rocket motors or boosters at lift off and even during flight, the burning of rocket propellant, and the sheer speed at which the rocket travels–over four times the speed of sound.

NASA’s Dan Kanigan does a great job of explaining vibrations and their effect on the Ares I-X test flight rocket in this blog post. In addition, he talks about NASA’s solution to the problem.

The vibration that is produced by the burning of the solid rocket propellant in the first stage booster is called thrust oscillation. These vibrations – or oscillations – come in the form of waves, which travel up and down the length of the rocket like a musical note through an organ pipe. One of the biggest challenges in any rocket design is developing avionics (aviation electronics) that can function in this vibrating environment.

Vibration is not just a rocket issue, though. All electronic hardware is tested for its ability to handle shock and vibration. An MP3 player, for example, has to be tested for its ability to handle the vibrations from someone walking or jogging while holding it, placing it on a countertop, or accidentally dropping it on the floor. However, compared to the workout that Ares I-X’s avionics receive, your MP3 player has got it easy. Imagine shaking that MP3 player inside an automatic paint can shaker for two minutes while continuing to play your favorite tunes. That’s kind of what the electronics of the I-X are up against.

Jon Cowart, the deputy project manager for Ares I-X at Kennedy Space Center, said in an interview earlier this year, that gathering data such as aerodynamic data, stresses, strains, pressures, temperatures, and vibrations is the most important part of the rocket’s mission, and it will be used to make the Ares I even better.

Yet, the Ares I-X is just one example. NASA has been fighting the vibration issue since Apollo. Then such longitudinal oscillations caused by the burn out of the first stage booster were referred to as the “Pogo effect” and were especially troublesome for Saturn V. During an unmanned Apollo 6 flight, a critical test before manned flights could be approved, vibrations actually caused the main engine to shut down.

The new launch abort systems being built for NASA’s Orion crew exploration vehicle is facing the same challenges. When the capsule and launch abort tower, which sits on top of the capsule, separate, a solid rocket motor called the abort motor fires, burning the propellant very rapidly to thrust the vehicle away from the rocket. The plume of hot gases emitted by the motor are the greatest source of noise and vibrations the vehicle will experience during flight, says Henri Fuhrmann, program manager of the new launch abort system at Orbital Sciences, an aerospace company that has partnered with NASA to design and develop the escape system. Thus the engineers have developed reverse-flow nozzles that turn the flow of the gases 155 degrees, away from the capsule. This will reduce the acoustics, vibrations, and loads on the capsule that could damage the electronics and hardware. This is the first time the technology has been developed at this scale, says Fuhrmann. The launch escape system on Apollo had direct flow motors and therefore had to add extra steel structures to increase the distance between the motor and the crew capsule. The Russian’s launch escape system on the Soyuz spacecraft also uses direct flow nozzles.

Kanigan says that the next challenge is to make sure you can control the rocket at such vibrations. For NASA’s new launch abort system engineers built a unique motor called the attitude control motor that steers and controls the capsule. It is also the first of its kind and can fly the vehicle forward and backward, and flip the capsule over to reorient it so the heat shield is facing forward.

Both Ares I-X and the Orion crew exploration vehicle are part of the Constellation Program, a plan to send humans to the moon and, then, to Mars. Norm Augustine, the chairman of a panel charged with reviewing this plan, said in a press briefing after the committee’s first meeting where it heard ideas on alternative designs to the Ares rockets that the most interesting thing was the talk of how vibrations affect rocket design.

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