Andrew D. Ketsdever

Gas dynamic calibration of a nano-Newton thrust stand

The ability to measure extremely low thrust levels with unusual precision is becoming more critical as attempts are made to characterize the performance of emerging micropropulsion systems. Many new attitude control concepts for nanospacecraft involve the production of thrust below 1 μN. A simple, but uniquely successful thrust stand has been developed and used to measure thrust levels as low as 86.2 nano-Newtons (nN) with an estimated accuracy of ±11%. Thrust levels in the range of 712 (nN) to 1 μN have been measured with an estimated accuracy of ±2%. Thrust is measured from an underexpanded orifice operating in the free molecule flow regime with helium, argon, and nitrogen propellants. The thrust stand is calibrated using results from direct simulation Monte Carlo numerical models and analytical solutions for free molecule orifice flow. The accuracy of the gas dynamic calibration technique, using free molecule orifice flow, has also been investigated. It is shown that thrust stand calibration using high...

Comparison of force balance calibration techniques for the nano-Newton range

With the rapid progress of micro- and nanoscale fabrication technology, devices are continually being created which produce extremely small forces. This creates a distinct need for a measurement instrument and adequate calibration techniques which can resolve forces below 1 μN. Two calibration methods for force balance measurements in the nano-Newton range are presented. These methods are based on a free molecule gas dynamic expansion through a thin-walled orifice and the electrostatic actuation of a miniature comb drive. Due to the advantages and disadvantages of every calibration technique, multiple techniques are often required to validate performance results for microscale devices. Because these calibration techniques typically rely on completely different physical processes and can be described by different sets of analytical equations, the comparison of one technique to another is necessary when high accuracy is required. The gas dynamic and electrostatic force calibration techniques have been compa...

Experimental and numerical determination of micropropulsion device efficiencies at low Reynolds numbers

Abstract : The need for low thrust propulsion systems for maneuvers on micro- and nano-spacecraft is growing. Low thrust characteristics generally lead to low Reynolds number flows from propulsive devices that utilize nozzle expansions. Low Reynolds number flows of helium and nitrogen through a small conical nozzle and a thin-walled orifice have been investigated both numerically, using the Direct Simulation Monte Carlo technique, and experimentally, using a nano-Newton thrust stand. For throat Reynolds number less than 100, the nozzle to orifice thrust ratio is less than unity; however, the corresponding ratio of specific impulse remains greater than one for the Reynolds number range from 0.02 to 200. Once the Direct Simulation Monte Carlo model results were verified using experimental thrust and mass flow data, the model was used to investigate the effects of geometrical variations on the conical nozzles performance. At low Reynolds numbers, improvements to the specific impulse on the order of 4 to 8% were achieved through a combination of decreasing the nozzle length and increasing the nozzle expansion angle relative to the nominal experimental geometry.

Performance testing of a microfabricated propulsion system for nanosatellite applications

Abstract : There is a growing interest in the use of micro and nanosatellites within the aerospace community. Constellations of small satellites may eventually replace much larger, single function spacecraft as a cheaper, more flexible alternative. Micro-technologies will be required to enable small satellite missions including efficient, low-cost propulsion systems for maneuvering. A MEMS fabricated propulsion system has been developed for maneuvers on an upcoming University nanosatellite mission. The Free Molecule Micro-Resistojet (FMMR) is an electrothermal propulsion system designed for on-orbit maneuvers of nanosatellites, which are defined as spacecraft with an initial mass less than 10 kg. The FMMR has been tested using a torsion force balance to assess its performance using a variety of propellants including helium, argon, nitrogen and carbon dioxide. The experimental performance results compare favorably with results obtained from gas kinetic theory, which were used in the design phase to estimate the thrusters performance. The measured performance of the FMMR in this study has proven to be adequate to perform attitude control maneuvers for the University nanosatellite mission.

Measurements and computations of mass flow and momentum flux through short tubes in rarefied gases

Gas flows through orifices and short tubes have been extensively studied from the 1960s through the 1980s for both fundamental and practical reasons. These flows are a basic and often important element of various modern gas driven instruments. Recent advances in micro- and nanoscale technologies have paved the way for a generation of miniaturized devices in various application areas, from clinical analyses to biochemical detection to aerospace propulsion. The latter is the main area of interest of this study, where rarefied gas flow into a vacuum through short tubes with thickness-to-diameter ratios varying from 0.015 to 1.2 is investigated both experimentally and numerically with kinetic and continuum approaches. Helium and nitrogen gases are used in the range of Reynolds numbers from 0.02 to 770 (based on the tube diameter), corresponding to Knudsen numbers from 40 down to about 0.001. Propulsion properties of relatively thin and thick tubes are examined. Good agreement between experimental and numerica...

Investigation of Time-Dependent Forces on a Nano-Newton-Second Impulse Balance

A torsional impulse balance has been developed as a new diagnostic tool to study fundamental physical processes in micropropulsion systems and laser–surface interactions. The impulse balance has been designed and tested with a robust calibration system to measure impulsive forces with resolution as low as several nano-Newton-seconds. The behavior of the impulse balance was thoroughly studied and characterized. A simple analytical model of the balance’s motion was developed from the general equation of motion of an underdamped, harmonically oscillating system. Also, two distinct methods of analyzing the experimental data from the nano-impulse balance have been investigated. The first method resolves the total impulse as a function of the balance’s maximum deflection. The second method enables the determination of the impulse and/or force applied as a function of time from the balance’s time-resolved motion. A calibration scheme employing electrostatic actuation techniques is used to experimentally validate...

Measurements and simulation of orifice flow for micropropulsion testing

Numerical and experimental results for a rarefied gas expansion through a thin circular orifice are presented. The orifice flow was used as a calibration test for a torsional thrust stand designed to measure force levels from 10 - 6 to 10 - 3 N. Molecular nitrogen, argon, and helium at room temperature are used as test gases. The mass flux and thrust measurements are compared with the direct simulation Monte Carlo results for Knudsen numbers from 40 to 0.01 and plenum to facility background pressure ratios of 10 3 - 10 7 . Factors that affect the total propulsive force, such as jet backflow and facility background gas penetrating the jet, are analyzed. The measured and calculated mass flux and total propulsive force were found to agree well for Knudsen numbers less than 1.

Numerical and experimental investigation of microchannel flows with rough surfaces

A conical surface roughness model applicable to particle simulations has been developed. The model has been experimentally validated for channel flows using helium and nitrogen gases at Reynolds numbers from 0.01 to 10 based on inlet conditions. To efficiently simulate gas-surface interaction, molecular collisions with the actual rough surface are simulated by collisions with a randomly positioned conical hole having a fixed opening angle. This model requires only one surface parameter, average surface roughness angle. This model has also been linked to the Cercignani-Lampis scattering kernel as a required reference for use in deterministic kinetic solvers. Experiments were conducted on transitional flows through a 150μm tall, 1cm wide, 1.5cm long microchannel where the mean free path is on the order of the roughness size. The channel walls were made of silicon with: (i) polished smooth surfaces, (ii) regular triangular roughness, and (iii) regular square roughness with characteristic roughness scales of ...

Origins of radiometric forces on a circular vane with a temperature gradient

Radiometric force on a 0.12 m circular vane is studied experimentally and numerically over a wide range of pressures that cover the flow regimes from near free molecular to near continuum. In the experiment, the vane is resistively heated to about 419 K on one side and 394 K on the other side, and immersed in a rarefied argon gas. The radiometric force is then measured on a nano-Newton thrust stand in a 3 m vacuum chamber and compared with the present numerical predictions and analytical predictions proposed by various authors. The computational modelling is conducted with a kinetic approach based on the solution of ellipsoidal statistical Bhatnagar–Gross–Krook (ES-BGK) equation. Numerical modelling showed the importance of regions with elevated pressure observed near the edges of the vane for the radiometric force production. A simple empirical expression is proposed for the radiometric force as a function of pressure that is found to be in good agreement with the experimental data. The shear force on the lateral side of the vane was found to decrease the total radiometric force.

Thrust Stand Micromass Balance for the Direct Measurement of Specific Impulse

A technique has been developed to directly measure the specific impulse from pulsed thruster systems. The technique is especially useful for propulsion devices that use solid propellants, for which a direct measurement of the propellant mass flow is extremely difficult. A torsion balance is used with a horizontal axis of rotation. A thruster is placed on the balance such that the impulse of the thruster firing and the change in mass due to the expelled propellant act in the same direction. A combined impulse and steady-state force measurement (due to propellant mass loss) can then be decoupled to assess the ratio of the impulse to the weight of propellant expended, or the specific impulse. A model has been developed to show the utility of the technique for pulsed systems with a firing time less than the natural period of the balance. An experimental proof of principle study was also undertaken using the laser ablation of engineering-grade Buna, Viton, and Teflon propellants. Specific-impulse measurements on the order of 200 s have been demonstrated with this laser ablation thruster.