Validation By Analysis
Today, Propellant Management Device (PMD) performance is validated only through detailed analysis. Ground tests cannot hope to verify that a PMD will meet the mission requirements. Drop tower testing, zero g flight (vomit comet) testing, or even Space Shuttle testing can be accomplished but none can completely validate a PMD and none are needed to successfully fly a PMD. In fact, testing can be more misleading than helpful and a PMD that passes a test may not function as intended in space. Early testing showed that the basic underlying physics are correct and can be used to fully validate a PMD. The shear number of PMDs flying that were validated by solely by analysis supports validation by analysis.
However, the analysis approach can affect the reliability of the PMD and the ability of the PMD to deal with off design conditions. The approach is often the differentiator between PMD designers.
Approach & Philosophy
PMD Technology's approach and philosophy is important because the reliability of the end product starts with the approach. PMD Technology uses a design and analysis approach which maximizes reliability and allows for reasonable off design operation. PMD Technology custom designs each PMD for the intended mission. By choosing appropriate PMD components for the given mission, the PMD performance can be tuned to maximize reliability. In addition, proper component choice will maximize available off design PMD capability given the specific mission constraints - thus maximizing mission flexibility. PMD Technology does not design for test and does not maximize safety factors - both could and often do compromise flight reliability.
For example, the gallery arm is the most capable PMD component and also the least reliable. A gallery arm can provide omnidurational and omnidirectional operation making it capable of meeting many missions (there are exceptions). Unfortunately, gallery arms do not provide the most reliable solution for three reasons. First, galleries typically use a large amount of screen in close proximity to the tank wall - which has occasionally failed in structural vibration testing (gallery arms without screen demonstrate poor transient capability). Second, because a gallery arm must typically extend around the tank, the hydrostatics are high and fine screen often required. The finer the screen, the smaller the wire diameter and the more fragile the screen. Third and perhaps most importantly, the transients associated with accelerating and decelerating the liquid inside the arms when demand changes can provide substantial loads on the porous elements - especially with small gallery arms and/or perforated sheet in lieu of screen. While gallery arms may be required for some missions and can be successfully designed and analyzed (there are several PMD Technology designed gallery arms on this website's PMD Photos page), using galleries in missions that do not require them reduces reliability for no valid reason.
PMD Technology's Approach Maximizes Reliability AND Mission Flexibility |
