NASA hopes to resolve concern about high vibrations in its new Ares 1 rocket by using a shock absorber-like passive damper between the first and second stages and a computer-controlled, motor-driven system of spring-mounted weights at the base of the booster to actively cancel out unwanted up-and-down oscillations.

To mitigate the thrust oscillation potential on the Ares 1 rocket, engineers are looking to use a combination approach. They have proposed an active tuned mass absorber that would detect the frequency and amplitude of the thrust oscillation with accelerometers and internal pressure sensors, and use battery-powered motors to move spring mounted weights up and down to damp the vibration out. Engineers are also looking to use a passive "compliance structure" -- essentially a spring-loaded ring that would detune the stack by softening the interface between the first and upper stages while preserving lateral stability -- in the Ares 1 design concept. Credit: NASA

The Ares 1 rocket will use an extended five-segment space shuttle solid-fuel booster as its first stage and a hydrogen-fueled upper stage to boost Orion crew capsules into low-Earth orbit. The Ares-Orion spacecraft will replace the space shuttle after the winged orbiters are retired in 2010. First operational use of the new rocket is expected around 2015.

But shuttle boosters experience high vibration levels as they exhaust their loads of solid propellant. Engineers have been assessing a variety of techniques for reducing those vibrations to make sure they will not affect a crew's ability to perform.

"We're dealing with a vibration that takes place very late in first stage burn, it's a smooth ride up until around between 105 and 115 seconds," said Garry Lyles, associate director of technical management at NASA's Marshall Space Flight Center. "Most of the activity occurs around 115 seconds for a very few seconds."

The longitudinal vibration does not affect the structural integrity of the rocket, he said, but it could affect an astronaut's ability to read cockpit instruments and respond to emergencies.

"The effects that we were dealing with were primarily effects on the crew and we're primarily dealing with crew performance, that is, the ability of the crew to read the console displays and respond to what they see," Lyles said. "We've found that the crew health issue is relatively easy to mitigate, but we're dealing with crew performance, primarily. We set a guideline on ourselves to have the vibration on the crew down to around a quarter of a G in the longitudinal direction."

Without any mitigation, loads as high as five or six times the force of Earth's gravity - Gs - could be experienced. With a strictly passive system, engineers believe the loads would drop to around one G. While that would not pose a health concern, it still could impair performance, causing blurry vision.

During a teleconference today, program managers said they are recommending a system that uses a combination of passive and active vibration control to ensure Orion astronauts do not experience more than about a quarter of a G in unwanted longitudinal vibration.

The passive system is a "compliance structure" between the first and second stages that reduces the rigidity of the interface and "has the effect of detuning the vehicle frequencies away from the acoustic frequencies of the motor," Lyles said. "That's basically a spring and damper ring that will be designed into the structure between the first stage and the upper stage."

For the active system, 16 cylinders would be mounted around the interior circumference of the flared base of the first stage. Each cylinder would feature spring-mounted weights that can be moved on computer command to counteract vibrations sensed by a redundant control system.

"It will sense the response of the vehicle and sense the motor vibrations and respond to that by moving a mass and spring system to cancel the vibrations of the vehicle at that point," Lyles said. "We believe we have a system that will be extremely reliable when it's developed."

The system would use battery-powered motors to move the 16 100- to 150-pound weights as required. With a fully redundant control system, Lyles said, "we can lose multiple actuators and still effectively cancel the vibrations on the crew."

"In fact, it turns out we can design this system so that if we lose all active control, that is, our redundant controller goes out and we have no active control in the system, the whole system fails as a passive tuned-mass absorber and we'd still be well under the health limits on the crew," Lyles said.

The system will add weight and thus reduce the mass that can be delivered to the moon or low-Earth orbit by 1,200 to 1,400 pounds. It is not a one-to-one loss, however, because mass added to the first stage reduces the mass delivered to orbit by about one tenth.

In any case, managers already were holding about 8,000 pounds of performance margin in reserve to handle unexpected problems like the vibration issue.

Steve Cook, manager of the Ares Project at the Marshall Space Flight Center, said engineers have rigged up a chair at the Ames Research Center In California that can subject a subject to the expected vibration levels.

"I've had a chance to ride in the chair, I wanted to see what this felt like," he said. "The closest analogy I can come up with, if you're driving down the interstate and you have to pull off and your wheels go off on the shoulder a little bit and you feel the rumble that's vibrating to remind you to get back on the road, that's the closest thing that I felt.

"It doesn't specifically bother you, it's just kind of a high vibration rumble, if you will. All you notice is if you're looking at a screen, at a quarter G you can read everything fine as you get a little higher, things get a little blurrier to read."

Engineers are planning a series of tests on upcoming shuttle flights to measure vibration levels just before booster jettison and to better characterize the effects of that vibration on an astronaut's ability to read Orion-like cockpit panels. Sensors also will collect structural loads data to improve computer modeling of the Ares 1 rocket.

"We believe we have a system we can go forward with," Lyles said. "We're working toward a proof of concept in the near future."