Space Station Solar Array Research Articles

Preliminary Measurements of ESD Propagation on a Round Robin Coupon

The propagation dynamics of spacecraft electrostatic discharge (ESD) flashover plasmas have been a topic of increasing interest in the past few years. To investigate ESD propagation and possible contributions from facilities, methodology, and analysis, we performed inverted gradient ESD tests of an International Space Station solar array coupon as part of a U.S.-wide Round-Robin test campaign. Geosynchronous earth orbit charging environments were simulated and current transients were simultaneously measured on ten solar array strings. In addition, images of ESD events were collected by low- and high-speed cameras, and an array of eight Langmuir probes positioned above the coupon was monitored. Preliminary results will be discussed, with special attention given to the effect of different analysis techniques on derived propagation velocity. Comparisons will be made with the results from other labs, and implications for spacecraft ESD events will be discussed.

First Preliminary Results From U.S. Round-Robin Tests

The first preliminary results are reported from the U.S. Round-Robin Test on Plasma Expansion Speed. The tests were performed at the NASA Glenn Research Center on two coupons (six strings) of International Space Station (ISS) solar cell arrays, with a separate small array to obtain arcs (because it is so difficult to get ISS arrays to arc). ISS arrays were used because they have no exposed interconnects to act as bare current collectors and confuse the experimental results. The preconstructed ISS strings were laid out approximately parallel to the plasma expansion velocity to allow for the best test of the simple plasma expansion front current waveform model. Several Langmuir probes were arranged above and to the sides of the sample to allow for measurement of the plasma propagation speed. In the initial set of tests, primary arcs and the consequent current waveforms were measured in a Low Earth Orbit-type plasma. In a second set of tests, two electron guns with diffusers were used to provide an approximately uniform Geosynchronous Earth Orbit-type environment, and primary arcs and current waveforms were obtained. The objective of this and other round-robin tests is to characterize primary arc waveforms in terms of speed and degree of discharge of arc plasmas produced by primary arcs, and their dependences on environment, capacitance per unit area, arc voltage, temperature, and so on. The final goal is to allow engineering estimates of arc current peaks and half-widths to allow confident design and construction of space solar arrays, and to allow mitigation techniques to be evaluated. This is the first in an extended series of tests to be performed at six different U.S. facilities, and with participation from ten different U.S. organizations.

Mitigation of Thruster Plume Erosion of International Space Station Solar Array Coatings

Covers advancements in spacecraft and tactical and strategic missile systems, including subsystem design and application, mission design and analysis, materials and structures, developments in space sciences, space processing and manufacturing, space operations, and applications of space technologies to other fields.

Managing Space Station Solar-Array Electrical Hazards for Sequential Shunt Unit Replacement

The U.S. solar-array strings on the International Space Station are connected to a sequential shunt unit. The sequential shunt unit's job is to shunt excess current from the solar array, such that just enough current is provided downstream to maintain the bus voltage while meeting the power load demand. Should a unit fail, its removal and replacement would normally be done with the solar arrays retracted to reduce the voltages and currents at the sequential shunt unit to safe levels. However, an alternate approach was desired to avoid the inherent risks associated with array retraction and redeployment. This approach allowed the replacement to be conducted via astronaut space walk or extra vehicular activity with the solar array still deployed. Removing a sequential shunt unit with the solar array in sunlight could result in substantial hardware damage and/or safety risk to the astronaut due to the voltages that may be present. Replacing the sequential shunt unit during eclipse would seem optimal, except that the maximum eclipse period is only 36 min, which is insufficient time. To guide the assessment and ameliorate hazards, the athours analyzed array string current and voltage capability during the various operating conditions using System Power Analysis for Capability Evaluation (SPACE), an electrical power system modeling code. This paper discusses six sequential shunt unit remove and replacement options and the associated analysis to develop a workable sequential shunt unit replacement procedure via extravehicular activity.

Neural Network for Positioning Space Station Solar Arrays

As a Shuttle approaches the Space Station Freedom for a rendezvous, the Shuttle's reaction control jet firinga pose a risk of excessive plume impingement load of Freedom solar arrays. The current solution to this problem, in which the array. are locked in a feathered position prior to the approach, may be neither accurate nor robust, and is also expensive. An alternative solution it proposed here: the active control of Freedom's. beta gimbals during the approach, positioning the arrays dynamically in such a way that they remain feathered relative to the Shuttle jet molt likely to cause an impingement load. An artificial neural network is proposed as a means to determining the gimbal angles that would drive plume angle of attack to zero.Such a network would be both accurate and robust, and could be less expensive to implement than the current solution. A network was trained via backpropagation, and results, which compare favorably to the current solution as well as to some other alternative, are presented. Other training options are currently being evaluated.

Space Station Solar Array Beta Gimbal Noncolaeated Control Study

The Space Station Solar Array Beta Gimbal pointing control system design uses a PID controller with a colocated actuator/sensor pair at the gimbal joint. It satisfies the regulation capability and disturbance rejection requirements for the present mission. Should an upgraded mission in which further residual vibration is to be suppressed, noncolocated additional sensors may have to be employed. However, instability due to spillover may result. This paper proposes a method to improve system performance through noncolocated design while preventing spillovers using positivity. Through positivity, the closed-loop system is guaranteed stable regardless of parametric uncertainty. Dynamic embeddings are constructed to recover positivity followed by invoking Positivity Theorem to synthesizing the controller. By adding a sensor at the center of the solar panel, the pointing performance is improved without jeopardizing stability due to noncolocation.

Influence of mass moment of inertia on the normal modes of a preloaded solar array mast

Earth-orbiting spacecraft often contain solar arrays or antennas supported by a preloaded mast. Due to weight and cost considerations, the supporting structures of the spacecraft appendages are made extremely light and flexible. Therefore, it is essential to investigate the influence of all physical and structural parameters on the dynamic behavior of the overall structure. The governing equation of motion and its general solution for the preloaded mast are developed. Furthermore, the mass moment of inertia of the mast subjected to bending vibrations is included in the governing equation of motion to investigate its influence on determining the circular frequencies. To verify the developed formulations, a finite element technique was implemented. The accuracy and limitation of the technique on calculating the circular frequencies are discussed. Although the study described in this paper primarily focuses on the mast for the space station solar arrays, the developed formulations and techniques can be applied to any large and flexible beam in zero gravity.

Grounding of space structures

Space structures, such as the Space Station solar arrays, must be extremely light-weight, flexible structures. Accurate prediction of the natural frequencies and mode shapes is essential for determining the structural adequacy of components, and designing a controls system. The tension pre-load in the ‘blanket’ of photovoltaic solar collectors, and the free/free boundary conditions of a structure in space, causes serious reservations on the use of standard finite element techniques of solution. In particular, a phenomenon known as ‘grounding’, or false stiffening, of the stiffness matrix occurs during rigid body rotation. This paper examines the grounding phenomenon in detail. Numerous stiffness matrices developed by others are examined for rigid body rotation capability, and found lacking. A force imbalance inherent in the formulations examined is the likely cause of the grounding problem, suggesting the need for a directed force formulation.

A new pre-loaded beam geometric stiffness matrix with full rigid body capabilities

Space structures, such as the Space Station solar arrays, must be extremely light-weight, flexible structures. Accurate prediction of the natural frequencies and mode shapes is essential for determining the structural adequacy of components, and designing a controls system. The tension pre-load in the ‘blanket’ of photovoltaic solar collectors, and the free/free boundary conditions of a structure in space, causes serious reservations on the use of standard finite element techniques of solution. In particular, a phenomenon known as ‘grounding’, or false stiffening, of the stiffness matrix occurs during rigid body rotation. The authors have previously shown that the grounding phenomenon is caused by a lack of rigid body rotational capability, and is typical in beam geometric stiffness matrices formulated by others, including those which contain higher order effects. The cause of the problem was identified as the force imbalance inherent in the formulations. In this paper, the authors develop a beam geometric stiffness matrix for a directed force problem, and show that the resultant global stiffness matrix contains complete rigid body mode capabilities, and performs very well in the diagonalization methodology customarily used in dynamic analysis.

Oxidation and Protection of Fiberglass-Epoxy Composite Masts for Photovoltaic Arrays in the Low Earth Orbital Environment

ABSTRACTFiberglass-epoxy composites have been considered for use as structural members for the mast of the Space Station solar array panel. The low Earth orbital environment in which Space Station is to operate is composed mainly of atomic oxygen, which has been shown to cause erosion of many organic materials and some metals. Ground based testing in a plasma asher was performed in order to determine the extent of degradation of fiberglass-epoxy composites when exposed to a simulated atomic oxygen environment. During exposure, the epoxy at the surface of the composite was oxidized, exposing individual glass fibers which could easily be removed. Several methods of protecting the composite were evaluated in an atomic oxygen environment and with thermal cycling and flexing: The protection techniques evaluated to date include an aluminum braid covering, an indium-tin eutectic and a silicone based paint. The open aluminum braid offered little protection while the CV-1144 coating offered some initial protection against atomic oxygen. The In-Sn eutectic coating provided initial protection against atomic oxygen, but appears to develop cracks which accelerate degradation by atomic oxygen when flexed. Coatings such as the In-Sn eutectic may provide adequate protection by containing the glass fibers even though mass loss still occurs.