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Longitudinal oscillation of launch vehiclesDuring powered flight a vehicle may develop longitudinal self-excited oscillations, so-called oscillations, of its structure. The energy supplying the vibration is tapped from the thrust by the activity of the system itself; that is, oscillation of the structure causes oscillation of the propellant system, especially of the pumps. In this way an oscillating thrust can be created that, by a feedback loop, may sustain the structural oscillation under certain circumstances. Two special features of the system proved to be essential for creation of instability. One is the effect of the inherent time interval that the thrust oscillation is lagging behind the structural oscillation. The other is the decreased of system mass caused by the exhausting of gas. The latter feature may cause an initially stable system to become unstable. To examine the stability of the system, a single mass-spring model, which is the result of a one-term Galerkin approach to the equation of motion, has been considered. The Nyquist stability criterion leads to a stability graph that shows the stability conditions in terms of the system parameter and also demonstrates the significance of time lag, feedback magnitude, and loss of mass. An important conclusion can be drawn from the analysis: large relative displacements of the pump-engine masses favor instability. This is also confirmed by flight measurements.
Document ID
19730007197
Acquisition Source
Legacy CDMS
Document Type
Other - NASA Technical Note (TN)
Authors
Glaser, R. F. (NASA Marshall Space Flight Center Huntsville, AL, United States)
Date Acquired
September 2, 2013
Publication Date
January 1, 1973
Subject Category
Structural Mechanics
Report/Patent Number
M-371
NASA-TN-D-7091
Report Number: M-371
Report Number: NASA-TN-D-7091
Accession Number
73N15924
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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