Technologies for affordable SEC missions
The influence of technology in reducing spacecraft costs was evaluated by analyzing historical trend data for spacecraft subsystems.
spacecraft↗
Engineering topics
Dennehy, C. J.
Publications and source records attributed to Dennehy, C. J..
Design and Analysis of the ST7 Disturbance Reduction System (DRS) Spacecraft Controller
The Space Technology 7 experiment will perform an on-orbit system-level validation of two specific Disturbance Reduction System technologies: a gravitational reference sensor employing a free-floating test mass and a set of micronewton colloidal thrusters. The Disturbance Reduction System is designed to maintain a spacecraft's position with respect to the free-floating test mass to less than 10 nm/square root of Hz, over the frequency range 10(exp -3) Hz to 10(exp -2) Hz. This paper presents the design and analysis of the coupled drag-free and attitude control system that closes the loop between the gravitational reference sensor and the micronewton thrusters while incorporating star tracker data at low frequencies. The effects of actuation and measurement noise and disturbances on the spacecraft and test masses are evaluated in a seven-degree-of-freedom planar model incorporating two translational and one rotational degrees of freedom for the spacecraft and two translational degrees of freedom for each test mass.
Maghami, P. G.↗
Disturbance reduction system: testing technology for precision formation control
The Disturbance Reduction System (DRS) is a space technology demonstration within NASA's New Millenium Program.
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A technique for optimal temperature estimation for modeling sunrise/sunset thermal snap disturbance torque
A predictive temperature estimation technique which can be used to drive a model of the Sunrise/Sunset thermal 'snap' disturbance torque experienced by low Earth orbiting spacecraft is described. The twice per orbit impulsive disturbance torque is attributed to vehicle passage in and out of the Earth's shadow cone (umbra), during which large flexible appendages undergo rapidly changing thermal conditions. Flexible members, in particular solar arrays, experience rapid cooling during umbra entrance (Sunset) and rapid heating during exit (Sunrise). The thermal 'snap' phenomena has been observed during normal on-orbit operations of both the LANDSAT-4 satellite and the Communications Technology Satellite (CTS). Thermal 'snap' has also been predicted to be a dominant source of error for the TOPEX satellite. The fundamental equations used to model the Sunrise/Sunset thermal 'snap' disturbance torque for a typical solar array like structure will be described. For this derivation the array is assumed to be a thin, cantilevered beam. The time varying thermal gradient is shown to be the driving force behind predicting the thermal 'snap' disturbance torque and therefore motivates the need for accurate estimates of temperature. The development of a technique to optimally estimate appendage surface temperature is highlighted. The objective analysis method used is structured on the Gauss-Markov Theorem and provides an optimal temperature estimate at a prescribed location given data from a distributed thermal sensor network. The optimally estimated surface temperatures could then be used to compute the thermal gradient across the body. The estimation technique is demonstrated using a typical satellite solar array.
Zimbelman, D. F.↗
Sunrise/sunset thermal shock disturbance analysis and simulation for the TOPEX satellite
It is shown here that during normal on-orbit operations the TOPEX low-earth orbiting satellite is subjected to an impulsive disturbance torque caused by rapid heating of its solar array when entering and exiting the earth's shadow. Error budgets and simulation results are used to demonstrate that this sunrise/sunset torque disturbance is the dominant Normal Mission Mode (NMM) attitude error source. The detailed thermomechanical modeling, analysis, and simulation of this torque is described, and the predicted on-orbit performance of the NMM attitude control system in the face of the sunrise/sunset disturbance is presented. The disturbance results in temporary attitude perturbations that exceed NMM pointing requirements. However, they are below the maximum allowable pointing error which would cause the radar altimeter to break lock.
Dennehy, C. J.↗
An augmented MMS MACS for the Topex/Poseidon mission and beyond
The augmented Modular Attitude Control Subsystem (MACS) is described, with emphasis on the significant hardware modifications that have been incorporated into the Landsat MMS (Multimission Modular Spacecraft) MACS design to satisfy the Topex/Poseidon mission attitude control and determination requirements. Particular attention is given to a modification consisting in the addition of an earth pointing safe hold mode utilizing yaw coarse sun sensors to provide a yaw-slew capability for maintaining adequate illumination of the solar array. The design utility of this augmented MACS module for future spacecraft applications is pointed out.
Williams, R. J.↗
On-board attitude determination for the Topex satellite
This paper presents an overall technical description of the on-board attitude determination system for The Ocean Topography Experiment (Topex) satellite. The stellar-inertial attitude determination system being designed for the Topex satellite utilizes data from a three-axis NASA Standard DRIRU-II as well as data from an Advanced Star Tracer (ASTRA) and a Digital Fine Sun Sensor (DFSS). This system is a modified version of the baseline Multimission Modular Spacecraft (MMS) concept used on the Landsat missions. Extensive simulation and analysis of the MMS attitude determination approach was performed to verify suitability for the Topex application. The modifications to this baseline attitude determination scheme were identified to satisfy the unique Topex mission requirements.
Dennehy, C. J.↗
Design of the TOPEX Earth Pointing Safe Hold Mode
A technical description of the design and analysis of the Earth Pointing Safe Hold mode (EPSHM) for the Ocean Topography Experiment (TOPEX) satellite is presented. The EPSHM serves as the primary hardwired analog backup controller for safe-haven satellite operations in the event of on-orbit anomalies. The EPSHM is a modified version of the baseline Multimission Modular Satellite (MMS) EPSHM used on the Landsat-4 mission. Modifications to the MMS baseline EPSHM used on Landsat-4 were required to satisfy the unique TOPEX operational requirements. One such TOPEX operational requirement is that the EPSHM provide a spacecraft yaw-axis slewing capability to maintain adequate illumination of the solar array. The EPSHM architecture, constituent hardware components, performance requirements and predicted on-orbit performance are described.
Dennehy, C. J.↗
Attitude determination and control subsystem for the TOPEX satellite
The Ocean Topography Experiment satellite will carry a modular Attitude Determination and Control Subsystem (ADCS) which contains all equipment required for attitude determination, stabilization, and control, as well as hydrazine thruster firing control, during all mission phases. Attention is presently given to the ADCS's architecture, constituent hardware components, performance requirements, and predicted on-orbit performance compliance, with emphasis on the design and analysis of the Normal Mission Mode control algorithm furnishing the primary scientific data-acquisition operational mode. This mode's attitude determination and control of on-orbit performance is predicted to better than 43 arcsec.
Dennehy, C. J.↗