The USAF Rome Laboratory and NASA Marshall Space Flight Center, through contract to Grumman Corporation, have developed a space-qualifiable test article for the Strategic Defense Initiative Organization to demonstrate the critical structural and mechanical elements of single-axis roll-out membrane deployment for Space Based Radar (SBR) applications. The Lens Antenna Deployment Demonstration (LADD) test article, originally designed as a shuttle-attached flight experiment, is a large precision space structure which is representative of operational designs for space-fed lens antennas. Although the flight experiment was cancelled due to funding constraints and major revisions in the Strategic Defense System (SDS) architecture, development of this test article was completed in June 1989. To take full advantage of the existence of this unique structure, a series of ground tests are proposed which include static, dynamic, and thermal measurements in a simulated space environment. An equally important objective of these tests is the verification of the analytical tools used to design and develop large precision space structures

Denton, Robert J., Jr.

Pugh, Mark L.

Strange, Timothy J.

English

NASA Technical Reports Server

N93-
THE DEVELOPMENT AND TESTING OF THE
LENS ANTENNA DEPLOYMENT DEMONSTRATION (LADD)
TEST ARTICLE
188 D
Mark L. Pugh and Robert J. Denton Jr.
Rome Laboratory/OCDS
Griffiss AFB, NY 13441-5700
Timothy J. Strange
OL-AC Phillips Laboratory/VTSS
Edwards AFB, CA 93523-5000
ABSTRACT
The USAF Rome Laboratory and NASA Marshall Space Flight Center, through contract
to Grumman Corporation, have developed a space-qualifiable test article for the Strategic
Defense Initiative Organization to demonstrate the critical structural and mechanical elements
of single-axis roll-out membrane deployment for Space Based Radar (SBR) application. The
Lens Antenna Deployment Demonstration (LADD) test article, originally designed as a
shuttle-attached flight experiment, is a large precision space structure which is representative
of operational designs for space-fed lens antennas. Although the flight experiment was
cancelled due to funding constraints and major revisions in the Strategic Defense System
(SDS) architecture, development of this test article was completed in June 1989. To take full
advantage of the existence of this unique structure, a series of ground tests are proposed
which include static, dynamic, and thermal measurements in a simulated space environment.
An equally important objective of these tests is the verification of the analytical tools used to
design and develop large precision space structures.
INTRODUCTION
The development of affordable multi-mission Space-Based Radar (SBR) concepts is
strongly dependent upon large space-deployable antenna technologies. For this application
two types of antenna concepts are being considered, the corporate-fed array and the space-fed
array. Due to satellite prime power limitations and the significant ranges that are involved,
large antennas are needed to provide detection and tracking of small targets in the presence
of clutter and jamming. These large antennas, however, must satisfy strict mass and stowed-
volume constraints imposed by the launch vehicle. The vacuum of space and the zero
gravity environment in which these systems will operate enables the deployment of very
large, low-mass antennas. The ability of these large low-mass space structures to properly
deploy and maintain antenna shape on-orbit is critical to radar performance. Even small
deformations in the shape of the antenna can cause path length errors which can result in the
loss of antenna gain, increased sidelobe levels, and beam pointing error.
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According to J. Ruze t, distortion by an amount A in the surface of a corporate-fed
array scanned to an angle 0 creates a path length error _ defined by the equation
e=AcosO
In contrast, a similar distortion in the surface of a space-fed array creates a path
length error
e =A[eos0 1
_1 +(_f)2
where f is the focal-length-to-diameter ratio (F/D); and r is the radial coordinate from the
center Of the aperture normalized to unity at the rim. It can be seen in thqs equation that the
second term reduces the path length error. This error compensation significantly decreases
the effect of out-of-plane distortions on radar performance for space-fed arrays, The result is
that for small scan angles the space-fed array is about one order of magnitude l_s_ sensitive
to out-of-plane distortion than the corporate-fed array. This result is significant for large
antennas, especially considering that performance specifications for typical corporate-fed
planar arrays require that flatness be maintained to within 1/10 wavelength (30 mm at L-
band, 3 mm at X-band) across the entire array. Figure 1 provides a graphical interpretation
of these equations with varying scan angle.
0 DEG SCAN
 jiiiiiiill
O FOR SMALL SCAN ANGLES x,xc_il
sPAcE_ o,sA.ou.I.oAs /_F .s_GSCA.SE.S,.,VE.OO,STO.T,ON /_y
.:;ii-i._-_-_.:;.::!::-
_ _ii"' ...=======================45PEGseA.
O DEG SCAN
DISTORTION
Figure 1. Path length error sensitivity to aperture distortion
154
The toleranceof the space-fedarray to out-of-planedistortion may allow the designof
SBRconceptsthat do not require electrical or mechanicalcompensationfor path length errors
to meetperformancespecifications. Reducedsensitivityto out-of-planedistortion also
suggeststhat lessrigid and correspondingly less massive support structures can be utilized.
Additionally, the elimination of a constrained feed structure enables concepts to be developed
which require fewer connections, allow for simpler deployment schemes, and are more
manufacturable.
Figure 2 illustrates a space-fed phased array lens SBR concept deployed on-orbit.
The concept depicted employs a single-axis roll-out, or "window shade _, deployment scheme
developed by Grumman Space Systems Division, Bethpage, NY. Primary components of the
system are the RF feed at the end of a continuous-longeron deployable mast and the aperture
which is deployed in a plane from the storage drum. Pretension forces in the long direction
provide out-of-plane stiffness to the membrane as it is suspended within its space frame. The
lightweight membrane supports transceiver modules that are powered by regulated dc through
a power distribution system which is an integral part of the membrane structure. Antenna
elements are attached to these modules. They receive both beam steering commands and
radar signals from the remote feed. The module introduces the necessary phase shifting to
steer the beam to the commanded direction and amplifies the radar signal. These in turn are
passed to the target side antenna elements for radiation toward the target. The reflected
signal is transmitted to the feed in an analogous manner.
Figure 2. Space-fed phased array lens SBR concept
155
BACKGROUND
The potential benefits of space-fed arrays for SBR application prompted DOD
development and testing of phased array membrane segments to validate concepts and support
configuration development. Membrane test articles have been developed and tested in a fixed
deployed condition for structural and RF evaluations. Since structures/mechanism validation
of membrane deployment and retraction is critical to future system designs, recent efforts
have focused on design and analysis of a deployment test article for ground and flight test
demonstrations.
In Oct 1985 Rome Laboratory and NASA Marshall Space Flight Center, through contract
to Grumrnan Corporation, initiated the design and fabrication of the LADD test article to
perform structural/mechanical tests on-orbit as a shuttle-attached experiment for the Strategic
Defense Initiative Organization (SDIO)I Planned tests inciud_ functional, static flatness,
dynamic, and thermodynamic measurements. Following the successful completion of these
tests, an active X-band array could be installed in the article for a second flight to
demonstrate imaging/discrimination for SDIO mission needs. Although major revisions to
the Strategl_6D_fense System (SDS) architecture eliminated Space:based radar from the n_ ,_
term system--an_=therein caused the LADD eXperiment's shu(t]e manifest to be canceled, the
test article was completed in Jun 89, essentially with a full flight pedigree.
The objective of this effort was to produce a space-fed lens antenna test article that could
be used for ground, and shuttle testing in a subsequent phase, in order to demonstrate the
structural feasibility and reliabiiity of the single-axis roll-out deployable SBR approach. Now
that the construCti0n Of the LADD test article is completed, analytical tools for predicting
behavior can be verified and an initial indication of structural damping can be determined.
Thus, it is now possible to obtain information on various aspects of the mechanical distortion
of the membrane. From this measured data the RF performance of the single-axis roll-out
deployable SBR can be predicted.
LADD DESCRIPTION
The LADD test article, shown in its stowed configuration in Figure 3 and its deployed
configuration in Figure 4, consists of a 2.44 m x 6.10 m (8 ft x 20 ft) X-band phased array
membrane, populated with approximately 5000 thermal representations of transceiver
modules, that is automatically deployed utilizing a lightweight frame. This configuration is
similar to that envisioned for large space-fed phased array SBR concepts which employ the
single-axis roll-out aperture deployment technique. The frame is comprised of an end beam,
two continuous-longeron canister-deployed side masts, and a storage drum which is supported
on trunnions extending from the drum support beam that would also serve as the main
spacecraft of a free-flying satellite, in this particular X-b_d antenna, the transceiver module
is a four element subarrayed design: thus an active radar of this size would have about
20,000 radiating patch elements.
156
The membrane is fabricated as four .61 m (2 ft) wide panels that extend from the
storage drum to the end beam. The two central panels are connected during deployment by
an automatic seaming device that would be required in a large satellite which is folded to fit
within the shroud of a launch vehicle. The outer strips are connected to the adjacent
membrane through permanent, but very compliant interpanel splices. These connections
provide RF isolation from one side of the membrane to the other.
It should be noted that as a shuttle-attached payload, the LADD test article was
designed with considerably high margins of safety than would be required in a free-flying
satellite. Therefore, as a structural frame the LADD test article is somewhat stiffer than a
typical satellite.
I"RUNN[ ON
SUPPORT
CANI STI[N
Figure 3. Stowed LADD test article Figure 4. Deployed LADD test article
TEST PROGRAM
The LADD test article belongs to a class commonly referred to as large precision
space structures. These structures typically have low spatial density, modes of vibration well
under 1 Hz, many closely spaced modes, and interaction between the structure's dynamic
behavior and the performance of the operational system. Since the LADD test article was
originally designed as a shuttle-attached space experiment and is representative of operational
designs for SBR, it provides a unique test opportunity to address issues of concern relating to
157
this class of structure. Specific areas of interest include testing of large flexible space
structures in a gravity environment, modeling and analysis of low-mass pretensioned
membranes, and system identification. To address these issues, a series of ground tests are
proposed which includes static, dynamic, and thermal measurements in a simulated space
environment.
Static tests will include multiple deployments of the LADD test article with
corresponding membrane flatness and tension measurements taken after each deployment.
These tests will establish the repeatability of the membrane static shape. To date, the LADD
test article has been cycled through more than 100 deployments and retractions.
Thermal tests will include differential heating and deployment/retraction at
temperature extremes. Membrane flatness, membrane tension, and membrane structure
temperature measurements will be taken in each test case. The differential heating tests will
establish the ability of the LADD test article to maintain required membrane flatness in and
out of periods of solar illumination and eclipse while on-orbit. The thermal tests at the
expected temperature extremes will establish the performance of the deployment mechanisms
as a whole at those operating l:_ints.
Dynamic testing will be performed to determine the structural modal properties. Test
results will be used to validate the analytical model, which will in turn be used to perform
transient response analyses to predict the behavior of the structure in the presence of on-orbit
disturbances.
All tests will be performed in a vacuum chamber with radiative heating and cooling
capability. A non-contact displacement measurement system will be used to quantify the
flatness of the LADD test _icle membrane during static tests. Photogrammetry techniques
are being considered for this purpose. Both photogrammetry and multiplexed laser
vibrometer system techn!ques are being considered for measuring vibration at numerous
points on the membrane during modal tests. Conventional accelerometers will be used to
measure vibration at points on the structure other than the membrane. The tension in the
membrane will be measured using load cells. Thermocouples and calorimeters will be used
to measure the temperature and heat flux during thermal tests.
An equally important objective of this test program is the verification of the analytical
tools used to design and develop large precision space structures, such as the LADD test
article. Testing of the LADD test article provides an opportunity to compare predicted
behavior with actual test data to verify those analytical tools. A successful test program
coupled with a detailed, comprehensive modeling effort will evaluate the ability of existing
analytical methods to predict behavior of new designs.
A modeling effort was carried out by Grumman Space Systems Division under terms
of the original LADD contract. This analysis considered the LADD test article attached to
the shuttle in a microgravity environment. Although details of that modeling effort are
considered proprietary, it can be said that the procedures and tools used are considered
standard by the industry. Good correlation was obtained between the finite element
158
formulation and numerical solution of the characteristic equation derived from thin plate
vibration theory for the membrane. The first five natural frequencies predicted by the finite
element model are listed in Table 1. Figure 5a illustrates the finite element model and the
corresponding mode shapes for the first five natural frequencies are illustrated in Figures
5b-f.
Table 1. Predicted natural frequencies of shuttle-attached LADD test article
Mode Frequency (Hz)
1 0.205
2 0.270
3 0.307
4 0.312
5 0.325
Figure 5a. LADD finite dement model
(772 elements)
Figure 5b. Mode 1
159
Figure 5e. Mode 2 Figure 5d. Mode 3
Figure 5e. Mode 4 Figure 5f. Mode 5
REFERENCES
1. J. Ruze, "Pattern Degradation of Space Fed Phased Arrays", Lincoln Laboratory,
ESD-TR-79-278, Dec 79, DTIC ADA-083264.
2. Grumman Space Systems Division, "Membrane Deployment/Retraction Test Article
Design Study", Final Report, Jun 1986, Contract No. NAS 8-36113.
3. Grumman Space Systems Division, "Lens Antenna Deployment Demonstration Critical
Design Review", Report No. 31.18, Nov 1987, Contract No. NAS 8-36621.
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The development and testing of the Lens Antenna Deployment Demonstration (LADD) test article

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