Thruster Failure Research Articles

Attitude control schemes for the first recovery mission of India

This paper describes the attitude control schemes for the various phases such as acquisition, on-orbit, orbit maneuver, de-boost maneuvers and coast phases of the India's first recovery mission Space Capsule Recovery Experiment-I (SRE-1). During the on-orbit phase, the SRE was configured to point the negative roll axis to Sun. The attitude referencing of SRE-1 was based on dry tuned gyros with updates from the attitude determined using on-board Sun sensors and magnetometer. For attitude acquisition, attitude maneuvers and for providing the velocity corrections for de-orbiting operations; a set of eight thrusters grouped in functionally redundant blocks were used. The control scheme with thrusters was based on proportional derivative controller with a modulator. In order to ensure micro-gravity environment during the on-orbit payload operations a linear quadratic regulator (LQR) based control scheme was designed to drive an orthogonal configuration of magnetic torquers which in turn produced three-axis control torque with the interaction of Earth's magnetic field. Proportional derivative control scheme with modulator was designed to track the steering commands during the velocity reduction as well as during the coasting phase of the de-orbiting operations. A novel thruster failure detection, isolation and reconfiguration scheme implemented on-board for the de-orbiting phase is also discussed in this paper.

Modeling of Total Thruster Performance for NASA's Evolutionary Xenon Thruster Ion Optics

An axisymmetric ion-optics model is applied to NASA’s Evolutionary Xenon Thruster (NEXT) ion engine. The model is used to simulate the performance of the entire thruster by modeling several apertures at varying radii on the thruster face and integrating the results. The integrated results are compared to experimentally measured data for the NEXT thruster, showing good agreement in most areas. The primary area of discrepancy is in the accelerator grid current, although erosion results suggest that the measured current is unaccountably high. The model is also used to estimate the life of the thruster before the onset of electron backstreaming. Three separate methods are applied, and each predicts thruster failure after approximately 40,000 hours of operation or 845 kg of xenon throughput.

Controlling Electron Backstreaming Phenomena Through the Use of a Transverse Magnetic Field

DEEP-SPACE mission propulsion requirements can be satisfied by the use of high specific impulse systems such as ion thrusters. For such missions. however. the ion thruster will be required to provide thrust for long periods of time. To meet the long operation time and high-propellant throughput requirements, thruster lifetime must be increased. In general, potential ion thruster failure mechanisms associated with long-duration thrusting can be grouped into four areas: (1) ion optics failure; (2) discharge cathode failure; (3) neutralizer failure; and (4) electron backstreaming caused by accelerator grid aperture enlargement brought on by accelerator grid erosion. The work presented here focuses on electron backstreaming. which occurs when the potential at the center of an accelerator grid aperture is insufficient to prevent the backflow of electrons into the ion thruster. The likelihood of this occurring depends on ion source operation time. plasma density, and grid voltages, as accelerator grid apertures enlarge as a result of erosion. Electrons that enter the gap between the high-voltage screen and accelerator grids are accelerated to the energies approximately equal to the beam voltage. This energetic electron beam (typically higher than 1 kV) can damage not only the ion source discharge cathode assembly. but also any of the discharge surfaces upstream of the ion acceleration optics that the electrons happen to impact. Indeed. past backstreaming studies have shown that near the backstreaming limit, which corresponds to the absolute value of the accelerator grid voltage below which electrons can backflow into the thruster, there is a rather sharp rise in temperature at structures such as the cathode keeper electrode. In this respect operation at accelerator grid voltages near the backstreaming limit is avoided. Generally speaking, electron backstreaming is prevented by operating the accelerator grid at a sufficiently negative voltage to ensure a sufficiently negative aperture center potential. This approach can provide the necessary margin assuming an expected aperture enlargement. Operation at very negative accelerator grid voltages, however, enhances ion charge-exchange and direct impingement erosion of the accelerator grid. The focus of the work presented here is the mitigation of electron backstreaming by the use of a magnetic field. The presence of a magnetic field oriented perpendicular to the thruster axis can significantly decrease the magnitude of the backflowing electron current by significantly reducing the electron diffusion coefficient. Negative ion sources utilize this principle to reduce the fraction of electrons in the negative ion beam. The focus of these efforts has been on the attenuation of electron current diffusing from the discharge plasma into the negative ion extraction optics by placing the transverse magnetic field upstream of the extraction electrodes. In contrast. in the case of positive ion sources such as ion thrusters, the approach taken in the work presented here is to apply the transverse field downstream of the ion extraction system so as to prevent electrons from flowing back into the source. It was found in the work presented here that the magnetic field also reduces the absolute value of the electron backstreaming limit voltage. In this respect. the applied transverse magnetic field provides two mechanisms for electron backstreaming mitigation: (1) electron current attenuation and (2) backstreaming limit voltage shift. Such a shift to less negative voltages can lead to reduced accelerator grid erosion rates.

Attitude Control of Underactuated Spacecraft

In this paper we consider the problem of maneuvering an axisymmetric spacecraft in the case that no torques about the symmetry axis can be exerted, for example due to a thruster failure. Explicit control laws are derived which steer such a spacecraft between states of pure spin between prescribed attitudes. These laws are found by minimizing a cost functional measuring the suitably weighted overall angular velocities during the maneuver. In arriving at the solution, differential geometric techniques in the theory of optimal control are used.

Fault-tolerant system design of an autonomous underwater vehicle ODIN: An experimental study

This paper describes the design and implementation of a fault-tolerant system for Omni-Directional Intelligent Navigator (ODIN), a six-degree-of-freedom autonomous underwater vehicle (AUV) designed at the University of Hawaii. A fault-tolerant system consists of three areas: fault detection, fault isolation and fault accommodation. Each area is described, and methods discussed in the literaturearebriefly reviewed. The presented design focuses on the thruster (or actuator) and sensor system failures in an AUV. Experimental results show that the presented approach is practical and effective when tolerable failures in actuators and sensors occur. This methodology and concept can be extended to fault-tolerant system design for various AUVs with hardware redundancy.