DLR Engineers Develop Compact Quartz Thrust Balance for Electric Space Propulsion Testing

Researchers at the German Aerospace Center in Göttingen have developed a compact thrust balance designed to evaluate electric space propulsion devices under vacuum conditions. Electric thrusters achieve high propellant economy and controllability compared to chemical propulsion, making them increasingly vital for satellite attitude control, station keeping, and orbital transfers from geostationary transfer orbits to geostationary orbits. However, these systems produce low absolute thrust forces ranging from millinewtons down to micronewtons, operating with thrust-to-mass ratios that frequently fall below one to one thousand. Because achieving required orbital momentum changes entails extended thruster firing durations, rigorous ground qualification inside vacuum test chambers is essential. To address the diagnostic challenges of measuring these subtle forces over long testing campaigns, engineers initiated development of the DLR Electric Propulsion Thrust Balance, known as DEPB, creating an instrument capable of resolving forces well below one millinewton.

Ground testing of electric propulsion thrusters requires facilities that simulate space environments while accommodating continuous propellant mass flows. DLR conducts these campaigns within the Simulationsanlage Treibstrahlen Göttingen for electric engines, or STG-ET, one of the largest electric propulsion testing installations globally. The facility centers on a vacuum chamber with an inner diameter of 5 meters and a length of 12.2 meters. Chamber evacuation employs a roughing pump stand containing a rotating vane pump and a Roots pump, alongside four Edwards turbomolecular pumps connected to a secondary roughing set to reach a standby pressure around 10^-6 millibar. High pumping capability under thruster gas loads is maintained by eighteen Oerlikon Leybold cryopumps, achieving standby pressures in the low 10^-7 millibar range. For room-temperature propellants, pumping speeds reach 276,000 liters per second for xenon, 360,000 liters per second for krypton, and 452,000 liters per second for argon.

Diagnostic instruments inside the STG-ET chamber support extensive plasma analysis and long-duration testing. Plume investigations utilize a permanently installed C-shaped scanner carrying fifteen Faraday cups to measure ion beam currents, alongside optical windows, vacuum feedthroughs, and a residual gas mass spectrometer. High-energy ion sputtering is mitigated by lining sections of the chamber walls with protective graphite sheets and installing a water-cooled graphite target at the rear wall. A removable front-end chamber lid permits direct access for mounting thrusters and diagnostic assemblies. The vacuum chamber can maintain continuous low pressures for extended testing windows lasting weeks or months. Measuring thrust inside this facility requires specialized thrust balances to resolve small forces while coping with environmental disturbances. Thrust balances must consistently withstand external vibrations, electromagnetic interference, and facility thermal shifts without losing measurement fidelity.

Developing thrust balances for electric propulsion involves structural trade-offs across common balance configurations, including vertical weigh cells, normal pendulums, torsional pendulums, inverted pendulums, inverted double pendulums, and counterbalanced inverted double pendulums. A fundamental complication is routing utility lines—including electrical power cabling, propellant feed tubes, and cooling water circuits—from the stationary facility onto the floating thruster platform. Shifting and mechanical stiffness within these lines introduce unknown restoring forces and position-dependent tensions that easily reach millinewton levels. Furthermore, electric thrusters release thermal losses during firing, raising internal hardware temperatures. In vacuum conditions where convective heat transfer cannot occur, balances must rely entirely on radiative dissipation or conductive paths like liquid cooling or electrical heaters. Using water cooling introduces fluid lines that exert temperature-dependent mechanical forces on the weighing platform, making thermal control and feedline isolation critical factors in preventing measurement errors.

Since 2013, primary thrust measurements at STG-ET have been conducted with a balance built in cooperation with Advanced Space Technologies, or AST. Designed as a counterbalanced inverted double pendulum, the AST instrument measures forces up to 250 millinewtons for thruster payloads weighing up to 40 kilograms. Heavy thruster assemblies on the top platform are counterbalanced by masses on a lower platform to prevent tipping into mechanical travel limits. The AST balance has served in test campaigns evaluating a Safran PPS Hall effect thruster, an Ariane Group RIT22 radiofrequency ion engine, and a smaller RIT10 ion engine generating 1 to 2 millinewtons at the bottom of the measurement range. Operating the AST balance requires fine-tuning control loop settings for each payload mass. Untuned operating conditions lead to mechanical oscillations and baseline shifts, while experimental runs demonstrated long-term thermal drift exceeding specified thresholds.

Operational experience with the AST balance prompted DLR to develop an alternative measurement system. Calibrating the AST stand relies on a motorized jig lifting three reference weights via a wire and pulley, producing calibration force plateaus of 93.9, 196.9, and 301.9 millinewtons. Pulley friction and bearing stick-slip introduce measurement errors exceeding tens of millinewtons, necessitating four back-and-forth movement cycles at each step to average out hysteresis. In addition, the AST software source code is unavailable to DLR, preventing internal modifications or custom control upgrades. The large physical bulk and heavy mass of the AST balance also restrict its operation exclusively to the primary STG-ET chamber, preventing testing inside smaller research chambers. These factors motivated the design of the DEPB balance, targeting a compact system fitting into a 1-meter diameter chamber, supporting 50-kilogram thrusters, measuring thrust up to 500 millinewtons, and utilizing in-house software with direct calibration control.

To suppress the thermal drift observed in the aluminum-heavy AST balance, the DEPB incorporates an inverted double pendulum structure where structural crossmembers and flexure bearings are merged into monolithic rods made of fused quartz. Fused quartz exhibits a thermal expansion coefficient of 5.1 x 10^-7 per Kelvin up to 100 degrees Celsius, more than an order of magnitude lower than that of aluminum. The upper thruster platform connects to the balance base plate through eight quartz rods, comprising four vertical rods that support payload weight and four crossed rods configured to inhibit lateral deflection. To damp mechanical vibrations without mechanical contact, an eddy current brake featuring a copper plate positioned between two permanent magnets is mounted within the assembly. This structural arrangement minimizes sensitivity to thermal expansion gradients that develop when thrusters operate for long periods in vacuum, maintaining geometric alignment without complex counterweights.

Force sensing in the DEPB is achieved using a commercial Sartorius WZA224 weigh cell. The WZA unit weighs approximately 2 kilograms, features an actively compensated mechanism with an optical position sensor, communicates via an RS232 computer interface, and provides a nominal vertical range of 220 grams with 0.1-milligram resolution. Because the sensor was designed for upright laboratory weighing, extensive qualification was conducted to confirm that its internal components and external spirit level could withstand vacuum conditions. Furthermore, the unit is mounted rotated by 90 degrees to measure horizontal thrust forces. In this sideways orientation, the sensor’s internal mechanical preload changes, reducing its measurable force capacity from approximately 2 newtons down to 0.8 newtons. Nevertheless, qualification tests confirmed that the WZA sensor operates reliably in vacuum and provides a repeatable force-dependent signal when subjected to horizontal thrust platform displacements.

Because tilting the WZA sensor invalidates its factory calibration and the balance mounting introduces mechanical restoring forces, a dedicated calibration method is required. DLR discarded wire-and-pulley weight systems due to complexity, friction, and wire slip, developing an in-situ calibration mechanism using a motorized linear motion stage. The actuator, a Newport TRA6PPV6 motion stage, presses a calibration spring directly against the thrust platform to apply known horizontal loads. Although initial tests evaluated standard cylindrical coil springs, they proved prone to lateral bending during compression. The team selected a straight spring steel wire with a 90-degree bend at its tip, which resists sideways deflection. The bent end features a smooth rounded contact point that contacts the thrust platform without punching into the surface. This motorized spring assembly delivers reproducible thrust simulation forces directly along the balance measuring axis, eliminating mechanical pulley errors.

Calibration of the spring stage was carried out by mounting the Newport actuator vertically above a second Sartorius WZA cell in its nominal upright orientation, establishing a traceable travel-versus-force function. Advancing the motion stage between 0 and 9.5 millimeters produced forces up to the specified 500-millinewton ceiling. The recorded data exhibited a linear response from 3 to 6 millimeters travel, followed by a reproducible slope kink between 6 and 9.5 millimeters attributed to spring mounting geometry. To model this response without introducing arbitrary split-fitting thresholds, researchers fitted the dataset with a fifth-order polynomial achieving a correlation coefficient below 0.9999. The polynomial was normalized to pass through zero at contact, determining a travel offset of 2.51378 millimeters. To avoid mathematical divergence caused by polynomial extrapolation outside the empirical data range, operational calibration travel in the DEPB is strictly limited to 7 millimeters past initial contact.

Software control and thermal housing design complete the DEPB system architecture. Built around the commercial load cell and motion actuator, custom software calculates tare offsets internally while offering user-selected recording intervals alongside a fast data acquisition mode reaching 20 Hertz. To stabilize internal temperatures, the balance frame incorporates water-cooled sidewalls. Cleanroom testing demonstrated that adding sidewalls improved measurement stability across daily ambient temperature swings by an order of magnitude, while active cooling water at 24 degrees Celsius added a further 20 percent stability gain. Preliminary dynamic tests utilized an electromagnetic relay coil and pushrod to generate repeatable 0.87-newton force pulses without thruster feedlines, showing a 10 percent to 90 percent rise time of 0.24 seconds and resolving pulses down to a 1 Hertz repetition rate. While the DEPB currently awaits future vacuum trials with live thrusters, these initial benchmarks confirm sub-millinewton resolution and robust calibration control.

Subject of Research: Development and calibration of a compact thrust balance for electric space propulsion testing at DLR

Article Title: Thrust measurement and thrust balance development at DLR’s electric propulsion test facility

Article References: Neumann, A., Simon, J., & Schmidt, J. (2021). Thrust measurement and thrust balance development at DLR’s electric propulsion test facility. EPJ Techniques and Instrumentation, 8(1), Article 17. https://doi.org/10.1140/epjti/s40485-021-00074-7

Image Credits: AI Generated

DOI: 10.1140/epjti/s40485-021-00074-7

Keywords: Electric propulsion, Thrust balance, German Aerospace Center, STG-ET facility, Quartz flexure bearings, Sartorius WZA224, Vacuum ground testing, Newport motion stage, Force calibration, Ion thruster diagnostics, Thermal drift mitigation, Water-cooled enclosure, Eddy current brake, Millinewton force measurement, Inverted double pendulum, Rise time analysis, Cryopump vacuum system