2011 IR&D Annual Report

A Novel Approach for Improved Axial Vibration Suppression of Multi-Stage Launch Vehicles, 18-R8108

Principal Investigators
David Ransom
Steve Green
Benjamin White
Harold Doiron

Inclusive Dates:  10/01/09 – 04/01/11

Background — A record number of launch vehicles (rockets) are in development at this time. This includes the National Aeronautics and Space Administration (NASA) yet to be named Heavy Lift Vehicle, as well as the privately funded Antares, Falcon 1 and 9, and the New Shepard vehicles being developed by Orbital Sciences Corporation (OSC), Space-X and Blue Origin, respectively. Each vehicle is in a different state of development. The NASA vehicle is progressing at a very slow pace as NASA efforts are focused on new engine systems at this time and not on the vehicle itself. OSC progress on Antares continues to make the news with the most recent development being the successful test firing of several AJ 26 engines, which will be used to power the first stage. Space-X is breaking records with successful launches of both Falcon 1 and 9 in the past year, most recently demonstrating re-entry capability with the recovery of the Dragon spacecraft from orbit. Blue Origin continues in silence, releasing no new information to the press in the past several years.

At the same time, expertise in some of the more specialized areas of vehicle development is lacking in the industry as engineers from the previous development programs leave the workforce. In the area of coupled propulsion and structural dynamics, very few experts outside of major aerospace companies are available to serve the needs of the growing commercial launch vehicle industry. This provides SwRI a unique opportunity to expand involvement in the launch vehicle business by re-developing the skills and institutional knowledge required to provide the expert guidance and support each of these development programs will need in the coming years.

Approach — The focus of this research project is the application of a relatively common propulsion system component (pogo accumulator) to a new vibration issue. In typical use, pogo accumulators are designed to improve the frequency separation between structural and propulsion system modes so that pogo-type vibrations are prevented. In the case of a launch vehicle with a solid first stage and a liquid second stage, only the second stage would have such a device. However, as predicted to occur on Ares I (part of the cancelled NASA Constellation Program), significant longitudinal vibration problems can occur during first-stage flight, which is propelled by a Solid Rocket Motor (SRM) adopted from the Space Shuttle program. This research project was established to focus on this problem through the adaptation of the conventional pogo accumulator for both first- and second-stage vibration control. Although the Ares I program has been suspended, the work is still relevant, since other solid-fueled rockets could benefit from such a technology in the future.

Properly designing such a device requires precise control of the three main dynamic characteristics of an accumulator: inertance, compliance and resistance. While inertance and compliance are well understood and accurately represented by linear models, resistance is a highly non-linear parameter and remains a poorly understood parameter in the design of pogo accumulators. It is common to under-estimate the damping of a hole pattern, which can lead to an ineffective accumulator in the worst-case scenario. The premise of this research project is that the mismatch between theory and experiment is tied largely to the presence of high turbulence in the mean flow passing by the accumulator. This highly turbulent flow generates flow disturbances at various frequencies, which pass into and out of the accumulators. This unaccounted-for dynamic flow occupies some of the available flow area through the resistance, effectively reducing the size of the flow area available to the system mode of interest. Therefore, as an initial step in developing this novel first-stage damper concept, this report also includes developing a validated flow resistance model that provides a much improved match between predicted and measured damping for typical pogo accumulator scenarios.

Accomplishments — The project approach resulted in multiple accomplishments. First, a validated flow resistance model for accumulator hole patterns in cross-flow now exists and can be used to design a specified flow resistance into any liquid feedline accumulator. This new capability is already being applied to a commercial launch vehicle program for one of SwRI's clients. Second, two potential design concepts for damping the SRM-induced oscillations for the Ares I launch vehicle were developed. Both designs result in a significant reduction in vibration, with one of the designs being much easier to implement in a real vehicle system. Although Ares I is no longer under development, the design continues as a viable concept for future solid-fueled rockets with liquid stages.

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