Modifications to correct a potentially deadly vibration from the solid-fuel first-stage of the Ares I crew launch vehicle could range from tweaking the geometry of the propellant inside the rocket motor to unlocking seat shock absorbers in the Orion capsule so they protect astronauts on launch as well as landing.

Engineers at the Ares I project office here are working with experts from across the country to better understand the thrust oscillation issue in the first stage, a five-segment version of the four-segment reusable solid rocket motor (RSRM) that is fired in pairs to power the space shuttle stack off the launch pad.

“Conservative” calculations of the potential frequency and amplitude of a thrust oscillation that could occur in the first stage as it nears burnout, and of the way that vibration links to the rest of the vehicle, suggest that it could set up a resonance that would damage critical components and harm the crew (AW&ST Dec. 10, 2007, p. 60).

A thrust-oscillation “focus team,” convened in November 2007, has since calculated that the problem may not be as severe as it appeared earlier in the fall. But the work continues under a looming March deadline, set so designers on both the launch vehicle and Orion can start work in earnest on mitigating the effect, if necessary, before preliminary design review (PDR) at the end of the summer.

“That gives us a good view of the problem with what we see as how big the risk is, [along with] what are the right mitigation strategies for any residual risk left, so that going into PDR we have a good handle on it and we’re designing for it,” says Garry Lyles, an experienced launch vehicle engineer at Marshall who heads the focus team. “You’re not waiting downstream of the [PDR] to start designing your system to accommodate the oscillation.”

The work of Lyles’ team, which draws from across NASA and industry, is aimed at bounding both the forcing function that the oscillation creates on the stack, and how the whole stack—including the Orion capsule atop it—responds.

A small in-house “tiger team” reported last year that the forcing function from the oscillation might “couple with vehicle axial modes, localized components modes or other fluid/acoustic modes which could cause adverse effects on humans, lead to structural failures, and/or cause unacceptable controllability issues,” according to the team’s Oct. 30, 2007, presentation papers.

Of particular concern was the effect of the oscillation on the crew. Based on static ground tests of four-segment boosters conducted throughout the history of the shuttle program, and some very limited information extracted from data collected during shuttle flights for other purposes, the tiger team’s “best estimate” for the vibration’s average amplitude at the Orion command module would be 4.3g. But the Orion specifications set a crew health vibration limit of less than 0.6g rms (root mean square) in any axis over 1 min. during ascent.

“Vibrations beyond 0.6g rms for 1 min. are considered intolerable to humans,” state the “human system integration requirements” for the Constellation Program, which is developing Orion and Ares I. “It is expected that internal organs could be damaged if the level of vibration or the time period for these levels were increased. In studies, subjects [exposed to] such levels for 1 and 3 min. reported that they had to exert great effort to finish the test. Pain was reported primarily in the thorax, abdomen and skeletal musculature. Varying effects on blood pressure and respiratory rate were also observed.” Getting those figures into spec “could be a significant design challenge,” the tiger team reported. And there is also a concern that the vibrations will damage fragile hardware in the cryogenic Ares I upper stage, the avionics ring above it and the Orion service module that rides behind the crew capsule on top of the stack.

But the focus team has since calculated that the problem may not be as severe as originally feared. Nominally the oscillation frequency of a five-segment booster is 12 Hz. (compared with 15 Hz. for the four-segment version). But after that it gets complicated. Translating RSRM ground-test data into accurate forcing function figures and the stack’s response to that force is extremely difficult, particularly since the upper-stage and Orion designs remain immature and oscillation data are based on ground tests.