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Full text of "NASA Technical Reports Server (NTRS) 20110020534: Flexible Shields for Protecting Spacecraft Against Debris"

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Books & Reports 

Flexible Shields for 
Protecting Spacecraft 
Against Debris 

A report presents the concept of 
Flexshield — a class of versatile, light- 
weight, flexible shields for protecting 
spacecraft against impacts by small mete- 
ors and orbiting debris. The Flexshield 
concept incorporates elements of, but 
goes beyond, prior spacecraft-shielding 
concepts, including those of Whipple 
shields and, more recently, multi-shock 
shields and multi-shock blankets. A 
shield of the Flexshield type includes 
multiple outer layers (called “bumpers” 
in the art) made, variously, of advanced 
ceramic and/or polymeric fibers spaced 
apart from each other by a lightweight 
foam. As in prior such shields, the 
bumpers serve to shock an impinging hy- 
pervelocity particle, causing it to disinte- 
grate, vaporize, and spread out over a 
larger area so that it can be stopped by 
an innermost layer (back sheet). The 
flexibility of the fabric layers and com- 
pressibility of the foam make it possible 
to compress and fold the shield for trans- 
port, then deploy the shield for use. The 
shield can be attached to a spacecraft by 
use of snaps, hook-and-pile patches, or 
other devices. The shield can also con- 
tain multilayer insulation material, so 
that it provides some thermal protection 
in addition to mechanical protection. 

This work was done by Eric L. Chris- 
tiansen and Jeanne Lee Crews of Johnson 
Space Center. 

This invention is owned by NASA, and a 
patent application has been filed. Inquiries 
concerning nonexclusive or exclusive license 
for its commercial development should be ad- 
dressed to the Patent Counsel, Johnson Space 
Center, (281) 483-0837. Refer to MSC- 

Part 2 of a Computational 
Study of a Drop-Laden 
Mixing Layer 

This second of three reports on a 
computational study of a mixing layer 
laden with evaporating liquid drops 
presents the evaluation of Large Eddy 
Simulation (LES) models. The LES 
models were evaluated on an existing 
database that had been generated 
using Direct Numerical Simulation 
(DNS) . The DNS method and the data- 
base are described in the first report of 

this series, “Part 1 of a Computational 
Study of a Drop-Laden Mixing Layer” 
(NPO-30719), NASA Tech Briefs, Vol. 28, 
No. 7 (July 2004), page 59. The LES 
equations, which are derived by apply- 
ing a spatial filter to the DNS set, gov- 
ern the evolution of the larger scales of 
the flow and can therefore be solved on 
a coarser grid. Consistent with the re- 
duction in grid points, the DNS drops 
would be represented by fewer drops, 
called “computational drops” in the 
LES context. The LES equations con- 
tain terms that cannot be directly com- 
puted on the coarser grid and that 
must instead be modeled. Two types of 
models are necessary: (1) those for the 
filtered source terms representing the 
effects of drops on the filtered flow 
field and (2) those for the sub-grid 
scale (SGS) fluxes arising from filter- 
ing the convective terms in the DNS 
equations. All of the filtered-source- 
term models that were developed were 
found to overestimate the filtered 
source terms. For modeling the SGS 
fluxes, constant-coefficient Smagorin- 
sky, gradient, and scale-similarity mod- 
els were assessed and calibrated on the 
DNS database. The Smagorinsky model 
correlated poorly with the SGS fluxes, 
whereas the gradient and scale-similar- 
ity models were well correlated with the 
SGS quantities that they represented. 

This work was done by Nora Okong'o and 
Josette Bellan of Caltech for NASA’s Jet 
Propulsion Laboratory. Further informa- 
tion is contained in a TSP (see page 1 ). 

© Controllable Curved 

Mirrors Made From Single- 
Layer EAR Films 

A document proposes that light- 
weight, deployable, large-aperture, con- 
trollable curved mirrors made of reflec- 
tively coated thin electroactive-polymer 
(EAP) films be developed for use in 
spaceborne microwave and optical sys- 
tems. In these mirrors, the EAP films 
would serve as both structures and ac- 
tuators. EAPs that are potentially suit- 
able for such use include piezoelectric, 
electrostrictive, ferroelectric, and di- 
electric polymers. These materials ex- 
hibit strains proportional to the squares 
of applied electric fields. Utilizing this 
phenomenon, a curved mirror accord- 
ing to the proposal could be made from 

a flat film, upon which a nonuniform 
electrostatic potential (decreasing from 
the center toward the edge) would be 
imposed to obtain a required curva- 
ture. The effect would be analogous to 
that of an old-fashioned metalworking 
practice in which a flat metal sheet is 
made into a bowl by hammering it re- 
peatedly, the frequency of hammer 
blows decreasing with distance from the 
center. In operation, the nonuniform 
electrostatic potential could be im- 
posed by use of an electron gun. Calcu- 
lations have shown that by use of a sin- 
gle-layer film made of a currently 
available EAP, it would be possible to 
control the focal length of a 2-m-diame- 
ter mirror from infinity to 1.25 m. 

This work was done by Xiaoqi Bao, Yoseph 
Bar-Cohen, and Stewart Sherrit of Caltech for 
NASA’s Jet Propulsion Laboratory. Fur- 
ther information is contained in a TSP (see 
page 1). 


& Demonstration of a Pyrotechnic 
Bolt-Retractor System 

A paper describes a demonstration of 
the X-38 bolt-retractor system (BRS) on 
a spacecraft-simulating apparatus, 
called the Large Mobility Base, in 
NASA’s Flight Robotics Laboratory 
(FRL) . The BRS design was proven safe 
by testing in NASA’s Pyrotechnic Shock 
Facility (PSF) before being demon- 
strated in the FRL. The paper describes 
the BRS, FRL, PSF, and interface hard- 
ware. Information on the bolt-retraction 
time and spacecraft-simulator accelera- 
tion, and an analysis of forces, are pre- 
sented. The purpose of the demonstra- 
tion was to show the capability of the 
FRL for testing of the use of pyrotech- 
nics to separate stages of a spacecraft. 
Although a formal test was not per- 
formed because of schedule and budget 
constraints, the data in the report show 
that the BRS is a successful design con- 
cept and the FRL is suitable for future 
separation tests. 

This work was done by Nick Johnston, 
Rafiq Ahmed, Craig Garrison, Joseph 
Gaines, and Jason Waggoner of Marshall 
Space Flight Center. To obtain a copy of 
the paper, ”X-38 Bolt Retractor Subsystem 
Separation Demonstration, ” NASA/TM- 
2002-212047, September 2002, access 
http:/ / trs. nis. nasa. gov/ archive/ 00000604. 

NASA Tech Briefs, September 2004