The Roton C-9 , the first fully reusable, single stage low orbit commercial space craft experienced a serious vibration problem in forward flight. Measured accelerations were 15 to 18 time greater than the maximum set be military specifications and about 6 times greater than a human can tolerate for a 1 hour period. Navcon Engineering was asked to determine the root cause of the vibration and develop appropriate mitigation measures.
The Roton C-9 is pictured in forward flight during a test run in Mojave, CA. The rotor blades proved capable of lifting the craft off the ground and moving it sideways. Looking like a stripped down version of a conventional space capsule, the Roton was designed to get into space entirely on it’s own steam, and without the usual, wasteful discarding of various boosters and stages along the way. After re-entry, it slows its landing speed with helicopter-like rotor blades which provide lift and braking actions.
A frequency response survey was conducted using an impact excitation method. Measurements were acquired on the main structure and throughout the Crew Compartment. The frequency response measurements were compared with flight vibration spectra and rotor excitation profiles. It was determined that the "helicopter" blade wash was exciting a principal structural resonance causing the excessive vibration. The impact survey was conducted using a HP35670A analyzer and PCB instrumentation. The instrumented sledge hammer measures 3' long and weighs 12 lbs. And yes, he couldn't move his arms for more than a week after this test.