Paul M. McElroy

System Concept For A Moderate Cost Large Deployable Reflector (LDR)

A study was carried out at the Jet Propulsion Laboratory during the first quarter of 1985 to develop a system concept for NASAs Large Deployable Reflector (LDR). This new system concept meets the primary scientific requirements and minimizes the cost and development time. The LDR requirements were investigated to determine whether or not the major cost drivers could be significantly relaxed without compromising the scientific utility of LDR. In particular, the telescope wavefront error is defined so as to maximize scientific return per dollar. Major features of the concept are a four-mirror, two-stage optical system; a lightweight structural composite segmented primary reflector; and a deployable truss backup structure with integral thermal shield. The two-stage optics uses active figure control at the quaternary reflector located at the primary reflector exit pupil, allowing the large primary to be passive. The lightweight composite reflector panels limit the short wavelength operation to approximately 30 pm but reduce the total primary reflector weight by a factor of 3 to 4 over competing technologies. System optical performance is calculated including aperture efficiency, Strehl ratio, and off-axis performance. On-orbit thermal analysis indicates a primary reflector equilibrium temperature of less than 200 K with a maximum gradient of =°C across the 20 m aperture. Weight and volume estimates are consistent with a single Shuttle launch and are based on Space Station assembly and checkout.

Light Curing Rigidizable Inflatable Wing

The objective of this study was to prove the feasibility of using light-curing resins to rigidize an inflatable wing for terrestrial and space applications. Current inflatable wings rely on the continuous presence of an inflation gas to maintain their shape. Rigidization of inflatable wings provides several potential advantages, including reducing the vulnerability to punctures, increasing stiffness and load-carrying capability, allowing a higher aspect ratio for high altitude efficiency and longer missions, and reducing weight by eliminating the make up pressurization supply. This study was a multifaceted approach that included defining operating environments for Mars survey craft and military UAVs; analyzing wing loads during deployment and rigidization as a function of internal pressure and leak rate to determine needed rigidization times; developing rapid cure resin formulations with long shelf lives; fabricating, deploying, and rigidizing a wing half-span; and testing and characterizing the rigidized wing. Results show that the wings must deploy and cure rapidly at low temperatures for some missions. The maximum time allowed for the resin to rigidize is the range in time that the inflated and unrigidized wing maintains structural integrity to fly and provide lift for the vehicle while the wing is undergoing rigidization. A series of epoxy acrylate-based resin formulations were developed that cure in 10 seconds or less at 0qC. These resins also exhibited greater than 10 year storage lifetimes in accelerated aging studies and showed mechanical properties close to thermally cured aerospace epoxies. A half-span demonstration Eppler 398 airfoil was fabricated from E-glass fabric/ATI-ROCTME37X1 resin and a polyurethane bladder. After fabrication, the wing was packed and deployed two times. The unrigidized prepreg material was very compliant and was able to be packed tightly. After the packing and deployment trials were completed, the wing was inflated to 7 psig and given a 30-minute solar cure. The rigidized wing exhibited the desired high stiffness without inflation pressure.

UV RIGIDIZABLE CARBON-REINFORCED ISOGRID INFLATABLE BOOMS

The objective of this study was to demonstrate sunlight cure (UV) of a carbon fiber-reinforced open isogrid tube for Gossamer-type spacecraft. An epoxybased resin was developed and characterized that cures in sunlight at low temperatures (10°C) on carbon and carbon/glass hybrid tows. 1.5-m-long open isogrid tubes were fabricated using wet filament winding techniques. The tubes were sunlight cured and tested for degree of cure and mechanical properties. The demonstration hardware had a 99 percent cure and showed peak buckling loads equivalent to thermally cured tubes. This technology will allow fabrication of large, lightweight and low cost inflatable Gossamer structures that have significantly improved compliant packing efficiency without degradation of deployed precision and mechanical performance.

UV rigidized carbon-reinforced isogrid boom for Gossamer applications

This work examined the feasibility of curing carbon fiber-reinforced open isogrid structures using sunlight. An orbital thermal analysis was conducted for these Gossamer structures with no insulation to determine the temperature profiles during the cure process. An epoxy-based resin was developed that showed near complete cure on carbon and hybrid carbon/glass tows and also cured at low temperatures. Demonstration hardware cured in sunlight and tested in compression to failure performed as well as similar thermally cured isogrid composites.

Resin and Manufacturing Development for Light Curing Inflatable Composite Booms

†† ‡‡ Inflatable structures that become rigid after reaching the required shape are a promising approach for fabricating large space structures. A need exists for a controlled, clean rigidization technology to harden inflatable spacecraft after they have achieved the required shape. This program is addressing that need through the development of a family of radiation (ultraviolet [UV] and visible light) curable resins for structural composite matrices termed Ridigization on Commandi (ROC). These resins are being formulated to cure in low-temperature conditions with varying kinetics at low power inputs and at various wavelengths. This program is investigating cure using internal light sources under a blanket of multi-layer insulation (MLI). A study of using visible light emitting diodes (LEDs) for the internal light sources is presented in this paper. Topics covered include selection of LEDs and resin sensitizers that are active at those wavelengths, modeling of resin cure kinetics, measurement of resin mechanical properties after curing with LEDs, modeling of LED placement in isogrid booms, and manufacturing of isogrid booms using internal LEDs. Results show that the use of internal cure with LEDs is a viable approach for rigidizing inflatable space structures with low power in cold conditions. When optimized, the ROC technology will provide a versatile rigidization technology for the inflatables community.

Technical Approach For The Development Of Structural Composite Mirrors

Light weight, high precision, low cost structural composite mirrors have tremendous potential for enabling affordable space telescope systems. The Large Deployable Reflector (LDR) is an example of such a system. It is a 20 meter diameter, earth orbiting submillimeter telescope. Its technology requirements are for panels that are from 1 to 2 meters in size with areal densities of 5 to 10 Kg/m2 and surface figure precision of a few microns. JPL and the Hexcel Corp. have entered into a joint technology activity, sponsored by the NASA Precision Segmented Reflector (PSR) Program, for the development of such mirrors. Highly specialized manufacturing and materials processing techniques have been developed by Hexcel for the production of high precision, light weight and low cost composite mirrors. JPL has developed an analytical simulation capability for composite mirrors that characterizes their mechanipal and thermal performance in terms of the materials properties and configurations. This capability is the basis of detail panel designs for thermal stability, test simulation, test/analysis correlation and projection of performance for specific applications. This combination of capabilities from both organizations has resulted in the development of graphite/epoxy mirrors up to 1.0 meter in size with surface precision of a few microns rms while weighing only 6 Kg/m2. This paper describes that development program. The PSR Panel Program, over a four year period is for mirrors up to 1.5 meters with surface precision and LDR orbital thermal stabilities on the order of one micron.

Open-Celled Rigid Foams for Self-Deploying Foam Antenna Structures

The overall goal of this program is the development of an open-celled urethane foam system for use in self-deploying antenna structures. Advantages of such a system relative to current inflatable or self-deploying systems include high volumetric efficiency of packing, inherent restoring force, low (or no) outgassing, low thermal conductivity, high dynamic damping, mechanical isotropy, infinite shelf life, and easy fabrication with methods amenable to construction of large structures (i.e., spraying). The performance of our rigid open-cell foam systems, their crush and recovery behavior, potential packing scenarios, RF performance, and modeling are discussed.

Fabrication and Testing of Self-Deploying Foam Antenna Structures

A copper-coated, open-cell foam antenna structure was fabricated and demonstrated. The coated foams produced could be compacted upon heating and subsequently redeployed to their original dimensions with good retention of surface characteristics. RF testing demonstrated that the foam-based antenna was capable of some beam focusing, although the gain loss was rather high in comparison with a standard metal reflector of the same geometry. These losses are believed to result primarily from high surface error (710 micron) relative to the standard surface. This surface error was likely a result of foam warpage caused by internal stresses present in the relatively thin specimens. The results of orbital thermal analysis and modeling of the associated distortions indicate that a simple change in reflector geometry from constant thickness to bi-concave could correct most of the surface error observed in the foam antenna system. Overall, the results continue to indicate the promise of these systems for use in lightweight, low stowage volume communications systems.