The objective is to design, develop and test an Engineering Model (EM) of a Xenon Flow Meter (XFM) with high accuracy across a wide flow range to provide a better prediction of the remaining propellant at mid-life than the current PVT method. The XFM shall have a measurement range of 0 – 25 mg/s with an accuracy of 0.5 %R over an operational pressure and temperature range of respectively 0 – 180 bar and -30 to +65 °C. A key issue is the severe physical property variation of the xenon over the operational pressure and temperature range, as the xenon can be in gas, liquid or supercritical phase.
The flow meter developed shall be compatible with conventional xenon feed systems, when located in the high and low pressure regions of the fluidic chain. The required electronics shall also be designed and manufactured in line with the target to have this unit integrated into existing electronic drive assemblies that support other Electric Propulsion (EP) functions. Different techniques to acquire the flow measurements shall be evaluated, with a goal to produce a lightweight, compact unit that may be incorporated into existing fluidic chain assemblies with no negative influence on performance, cleanliness or reliability.
A key issue for the XFM is the severe physical property variation of the xenon (e.g. density, viscosity, thermal capacity) over the operational pressure and temperature range, as the xenon can be in gas, liquid or supercritical phase. Typically, flow sensors are designed specifically for either gas or liquid, but not for all three phases. Another issue is the design of a test setup that is capable of calibrating the XFM with an accuracy of 0.5 %R.
With electric propulsion becoming more established in the domain of telecom applications, accurately predicting the quantities of propellant remaining will become increasingly important for precise planning for decommissioning. The XFM shall provide a better prediction of the remaining propellant at mid-life than the current PVT method. Due to the non-linear compressible behaviour of Xenon, the PVT method is least accuracy as the pressure and temperature approach the critical point (i.e. 58 bar), which typically is mid-life for a telecom platform. Combining both methods will also improve End-of-Life prediction of remaining propellant.
Additional benefits of the XFM are to provide closed-loop control of propellant management devices such as proportional valves and thermo-throttles. A further opportunity offered by implementing a dedicated flow meter into the EP system is to confirm correct integration of the propellant feed system through accurate on ground measurement of the achieved flow rates during spacecraft level Thermal Vacuum testing. The flow meter could also be beneficial for propellant gauging of other media, for example nitrogen in cold-gas propulsion systems.
The XFM shall have the following features:
The measurement signal shall be corrected for temperature and pressure influences using the XFM’s temperature sensor and the EP pressure sensor, which can be done on-ground.
The project plan is divided into two phases. Phase 1 consist of specifying the requirements, followed by a technology trade-off study. The requirement specification shall result from a detailed survey related to the use of flow meters within current and near European EP systems, including specific mission inducted requirements and system architectures. Based on the selected the technology, a Bread-Board Model (BBM), including drive-electronics, shall be designed, manufactured and tested. The BBM testing shall demonstrate critical components of the generic sensor design.
Phase 2 starts with the successful completion of the preliminary design review and shall contain the design, manufacturing and testing of EM XFM. For testing purposes, a unit test bench shall be constructed to facilitate unit-level test for stand-alone verification and characterization of the XFM.
The contract started on August 2009 with the first task to establish a generic requirement specification. Through a requirement survey, a requirement specification has been established that comprehends different XFM applications, current and future missions, and EP system architectures. The specification was reviewed by several Primes and EP manufacturers, and finalized in December 2009 by a successful Requirement Review.
Based upon the requirements, a market and technology survey has been performed to select the most potential technology for further development. After successful completion of the Conceptual Design Review end February 2010, a BBM sensor has been designed and manufactured. Currently, this sensor is undergoing a test campaign to demonstrate critical function and performance characteristics. The PDR and start of EM MAIT are scheduled for mid 2010.