Chair Force Engineer

Wednesday, August 20, 2008

Skirting the Issue

After months of intense study, NASA's engineering teams seem to have settled on three solutions for the thrust-oscillation issue which threatened the safety of crews flying on Ares I:

1) Use of sixteen "tuned oscillator arrays," a passive damping system, inside the SRB aft skirt
2) A "SoftRide" Isolation System between Stage 2 and the Interstage
3) Possible mounting of crew seats on springs

The aft-skirt dampers shouldn't add too much mass to the system, and should work pretty reliably (unlike the active thruster concepts that have been studied in the past.) It should be noted that thrust oscillation should not be a problem on Ares V because the twin boosters cancel each other's vibrations during flight. It would make sense to replace the skirts with non-damping versions for the Ares V SRB's.

For years, various versions of SoftRide have been used to isolate fragile spacecraft from the vibration environment of their launchers. It's only natural to apply the SoftRide concept to Ares I. But there's two big differences between the Ares I SoftRide and all those which have flown before. First, the Ares I upper stage and Orion Spacecraft are far more massive than the microsatellites which have previously flown on SoftRide. Second, the Ares I upper stage and its SoftRide are subjected to aerodynamic loads in addition to the vibrational environment. This has not been an issue with previous SoftRide missions because the spacecraft and its isolators are protected inside the launcher's fairing.

Still unclear is whether NASA will give the SoftRide contract to CSA Engineering, award it to another vendor, or perform the work in-house. The CSA team has done a thorough and professional job in everything I have seen, and they deserve a shot at this challenging task.

The ideas presented have much promise for making Ares I a workable design. It's a shame that they will not be tested on Ares I-X in 2009. In the past, I have been critical of the marginal risk reduction that will be accomplished by Ares I-X. But it's the perfect way to get an early assessment of whether SoftRide can handle the second stage mass simulator and the aero loads. NASA could slip the Ares I-X schedule, work the thrust-oscillation solutions into this test flight, and retire much of the thrust oscillation risk that faces the real Ares I.