Trade Studies

The best way to fully understand the mission specification choices is to have PMD Technology do a trade study - trading mission objectives/requirement with PMD complexity.

With a trade study in hand, the customer can decide how to specify the mission to achieve the propulsion system goals at minimal PMD cost and complexity.

For example, geosynchronous satellites often have lateral facing thrusters for stationkeeping burns. If the PMD specification does not limit the duration of these burns, complex galleries may be required to access the pool on the tank side wall. However, stationkeeping burns on these satellites are of limited duration and the maximum amount of propellant used per burn can be specified. This allows a simple sponge to be implemented in the PMD to meet the requirement.

A PMD trade study can identify all these choices.

PMD Specification

Mission requirements drive the Propellant Management Device (PMD) conceptual design. Simple PMDs often have simple missions and vice versa. To obtain an optimal PMD requires that the customer understand the implications of specified requirements. It is easy to over-specify the PMD requirements and end up with a complex, heavy, expensive, and most importantly, less reliable PMD. To obtain the simplest, lightest, least costly and most reliable PMD for your mission requires careful PMD requirement specification.

The mission characteristics that must be specified include: the acceleration environment, the flow rates, and the propellant quantities demanded. These must be specified throughout mission including ground ops, launch ops and on orbit ops. To accomplish this a spacecraft layout with thruster, tank, and CG locations identified is most useful. From the layout, the possible on orbit accelerations can be determined. In addition to the layout, a description of all maneuvers which includes propellant quantity used, flow rate and primary acceleration (magnitude and direction) should be specified. Spin rates should also be specified for all spin phases. For a detailed list of questions to guide the specification process, please see this white paper or contact us.

PMD Design Process

PMDs are custom designed for the mission. The process is straightforward and has been repeated successfully over 35 times. PMDs are designed and validated completely and thoroughly by analysis. For more information on PMD Validation click here.

With the mission defined, the first step is to identify the PMD concept which will meet the mission and then size the individual components. Typically, a PMD Concept Review is conducted 6 to 8 weeks after go-ahead to discuss the PMD direction with the customer. The PMD concept is presented as well as an operational description of each phase of the mission and how the PMD reacts. In addition, the advantages and disadvantages of the chosen concept are discussed so the customer is confident that the PMD design path will meet his/her mission objectives.

After the concept review, the detailed design begins with preliminary PMD performance analysis. The process is iterative, with design changes evaluated by the manufacturer/detailed designer and PMD Technology. Approximately 4 months after go-ahead, a Preliminary Design Review is held. At the PDR, the results of the most important analyses and a detailed layout of the PMD are presented. At this point, the PMD is sufficiently mature that few changes will occur after PDR.

After the PDR, the final analyses are completed based on the detailed layout and a Performance Analysis Report written documenting the PMD functionality and validation. A CDR is held once the detailed drawings are complete at approx. 7 months after go-ahead and a final review of the PMD is conducted. After CDR manufacturing begins.