Zhen He

Precision electromagnetic calibration technique for micro-Newton thrust stands

This paper introduces a new direct non-contact electromagnetic calibration technique for high precision measurements of micro-thrust and impulse. A ring-shaped electromagnet with an air gap is used in the calibration. The calibration force is produced by the interaction of a uniform magnetic field with a copper wire current in the air gap. This force depends linearly on this current as well as the steady angular displacement of the torsion arm of the thrust stand. The range of calibration force is very large and the calibration force is easy to generate and insensitive to the arm displacement. The calibration uncertainty for a 150-μN force is 4.17 μN. The more influential factor on the calibration uncertainty is the magnetization of the electromagnet core due to the copper wire current. In the impulse calibration, the exerted impulse is linearly dependent on the maximal angular displacement of the torsion arm. The uncertainty in the impulse calibration is determined by uncertainties in both the force calibration and the pulse time.

A thermal model for nanosecond pulsed laser ablation of aluminum

In order to simulate the nanosecond pulsed laser ablation of aluminum, a novel model was presented for the target ablation and plume expansion. The simulation of the target ablation was based on one-dimensional heat conduction, taking into account temperature dependent material properties, phase transition, dielectric transition and phase explosion. While the simulation of the plume expansion was based on one-dimensional gas-dynamical equation, taking into account ionization, plume absorption and shielding. By coupling the calculations of the target ablation and plume expansion, the characteristics of the target and plume were obtained. And the calculated results were in good agreement with the experimental data, in terms of ablation threshold and depth within the fluence range of the tested laser. Subsequently, investigations were carried out to analyze the mechanisms of nanosecond pulsed laser ablation. The calculated results showed that the maximum surface temperature remained at about 90% of the criti...

High precision micro-impulse measurements for micro-thrusters based on torsional pendulum and sympathetic resonance techniques.

A sympathetic resonance theory is analyzed and applied in a newly developed torsional pendulum to measure the micro-impulse produced by a μN s-class ablative pulsed plasma thruster. According to theoretical analysis on the dynamical behaviors of a torsional pendulum, the resonance amplification effect of micro-signals is presented. In addition, a new micro-impulse measurement method based on sympathetic resonance theory is proposed as an improvement of the original single pulse measurement method. In contrast with the single pulse measurement method, the advantages of sympathetic resonance method are significant. First, because of the magnification of vibration signals due to resonance processes, measurement precision for the sympathetic resonance method becomes higher especially in reducing reading error. With an increase in peak number, the relative errors induced by readout of voltage signals decrease to approximately ±1.9% for the sympathetic resonance mode, whereas the relative error in single pulse mode is estimated as ±13.4%. Besides, by using the resonance amplification effect the sympathetic resonance method makes it possible to measure an extremely low-impulse beyond the resolution of a thrust stand without redesigning or purchasing a new one. Moreover, because of the simple operational principle and structure the sympathetic resonance method is much more convenient and inexpensive to be implemented than other high-precision methods. Finally, the sympathetic resonance measurement method can also be applied in other thrust stands to improve further the ability to measure the low-impulse bits.

Numerical Simulation of Combustion for Freely Falling Gelled Fuel Droplets under Normal Gravity Conditions

Understanding the evaporation and combustion mechanisms of single droplets of gel propellant is the first stage to predict the burning characteristics in the combustion chamber. This paper, taking into account convection heat for freely falling gelled fuel droplets under normal gravity conditions, as well unsteady mass diffusion and thermal diffusion inside droplet, a theoretical model was developed to understand mass and heat transport mechanisms, and bubble growth within the gel droplet during processes of droplet combustion. The results show that at the first stage, shrinkage of the radius obeys the d2-law; steep temperature gradient and fuel mass concentration gradient appear within droplet, especially region near droplet surface. At the second stage, liquid fuel near the gellant layer within droplet starts to boiling, gellant layer formation resist the vaporizing fuel gas flow to extent; the vapor region appears between gellant layer and vaporizing surface within the droplet, and the droplet expands, swells, the layer thickness decreases until it ruptures.

Discharge Characteristics of a Laser-Electromagnetic Coupling Plasma Thruster for Spacecraft Propulsion

The main aim of this work was to construct and investigate an optical and electromagnetic coupling thruster for spacecraft propulsion. This thruster, called as laser-electromagnetic coupling plasma thruster (LEMPT), ablates solid propellants, ionizing the gaseous matter and forming a plasma jet to export impulse bit for the spacecraft. Firstly, tests were performed analyzing the influence of physical variables (initial voltage bias of capacitors, i.e., storage energy of capacitors and laser output repetition frequency) on the discharge characteristics which represents the performance of the thruster. Secondly, the stability quality of the thruster is tested and evaluated in experiment. Ethylene-vinyl acetate copolymer (known as EVA) is used as a propellant for the thruster in the experiments herein. The results have shown that this LEMPT can be used for spacecraft in the future.

Numerical Simulation of Gel Propellant Droplet Breakup in Airflow

The droplet breakup is of importance in variety of spay combustion field. In order to investigate gel propellant droplet breakup in a gaseous phase, the Navier-Stokes equations in their axi-symmetric form was solved using the finite volume technique, and the Volume of Fluid method (VOF) was employed for tracking the free liquid-gas interface during process of droplet breakup. The results were compared with available published experimental data for Newtonian droplet, and validated the numerical model. The results show that increasing Weber number (We) can accelerate drop breakup; under the low shear rate, increasing K leads to droplet more resistant to breakup, drop breakup is dominated by K value; under the high shear rate, reducing n value results in stronger the shear-thinning behavior, and a decrease in resistant to breakup, n value governs the droplet breakup.

Numerical Investigation on Laser Ablation Characteristics of PTFE in Advanced Propulsion Systems

The Polytetrafluoroethylene (PTFE or Teflon) based propellants may be used in Pulsed Plasma Thruster, laser ablation thruster and other advanced propulsion systems. Because of the complex behaviors and phenomena of PTFE in ablation process, the study on thrusters’ operation process becomes complicated. Thermal and mechanical events are investigated, including phase transition, thermo-chemical and optical property variations, and multi-pulses laser ablation of PTFE. Considering more details including internal absorption of radiation, reflectivity of material, surface emission, a one-dimensional ablation model is developed and implemented numerically using a non-uniform grid, and implicit finite-volume method to gain greater insight into the process of laser ablation. The model is validated against analytical solutions and is in accordance with previous experimental results. The parameters of optical transmittance, reflectance and absorption coefficients are measured in experiments and are used in the numerical simulation. The laser ablation characteristics of PTFE are investigated, including the effects of wavelength and multipulses. It’s indicated that the laser ablation processes are influenced intensively by changing the laser wavelength and the effects of multiple pulses are also significant. The above numerical simulation provides insight into physical mechanisms of laser ablation, and suggests potential ways of improving thruster’s efficiency

Influence of Electrode Flare Angle on the Performance of Pulsed Plasma Thruster

In order to study the influence of electrode flare angle on the performance of Ablation Pulsed Plasma Thruster. Discharge character, plasma velocity and performance over different electrode flare angles of the Pulsed Plasma Thruster at 13.5J initial energy are measured experimentally, the effect of electrode flare angle on the electrode inductance gradient, equivalent circuit parameters and energy transfer efficiency are analyzed. It can be seen that the circuit parameters and inductance distribution are changed with electrode flare angle, the impulse bit, specific impulse, thruster efficiency and mean exhaust velocity increase non-linearly with flare angle increasing from 0 degree to 27 degree and the Pulsed Plasma Thruster gets the maximum thruster performance at 27 degree flare angle. It shows that with the increase of electrode flare angle the fraction of ablated mass accelerated magnetically and the impulse bit created by Lorentz force decrease are the reasons inducing the change in thruster performance.

Effect of Deposited Angle on Structure and Surface Morphology of Fluorocarbon Films Deposited by Pulsed Plasma Thruster

A series of fluorocarbon films were deposited on glass substrates by Pulsed Plasma Thruster. The effects of deposited angle on chemical structure and surface morphology of these films were investigated using X-ray photoelectron spectroscopy (XPS) and scanning probe microscope (SPM). XPS data imply various carbon-related chemical components present on the surface of the deposited films are strongly impacted by the deposited angle. AFM measurements show that the increase in deposited angle can smoothen the surface morphology and decrease the RMS roughness. Changes in density of the neutral particle and flux of ions that reach the surface affect the formation of fluorocarbon clusters in the films and reduce surface roughness.