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PREDICTIVE THERMAL ANALYSIS OF THE COMBAT SENTINEL SATELLITE PREDICTIVE THERMAL ANALYSIS OF THE
COMBAT SENTINEL SATELLITE


Blake A. Moffitt
Dr. J. Clair Batty (Advisor)

Department of Mechanical and Aerospace Engineering
Utah State University, Logan, UT 84322-1600


ABSTRACT

One of the major technical challenges facing small satellites in low earth orbit is the design of an adequate thermal
control system. As the mass of a satellite decreases, the satellite becomes increasingly vulnerable to temperature
changes induced by varying orbital heat loads. In order to assure that on-orbit satellite component temperatures are
maintained within manufacturing limits, satellite heat transfer must be accurately modeled during the design and
analysis of any small satellite. This paper addresses thermal modeling used during the design and analysis of the
Combat Sentinel Satellite (CSSAT). An overview of the analysis used to make design decisions and create working
thermal models is discussed. A thermal model of the satellite developed using SDRC I-deas Thermal Model
Generator (TMG) is also explained. Results from thermal vacuum chamber (TVAC) tests are presented and
compared to the analysis. Finally, the correlated thermal model is used to predict temperatures during extreme
orbital heating environments.

1.0 INTRODUCTION

As technology advances, the capability of small
satellites is increasing. This increasing capability is
allowing more complex missions to be performed by
satellites that are decreasing in volume and mass.
Unfortunately, as satellites decrease in mass, the
thermal inertia of the satellite also decreases. This
decrease in thermal inertia causes small satellite to be
subjected to more severe on orbit temperature swings
than larger satellites of the past. These temperature
swings are further increased by the growing trend of
mounting solar cell arrays on the body of the
spacecraft as opposed to using extendable panels. As
a result, the thermal control system for small
spacecraft must play a primary role in determining
the final satellite design. In order for this thermal
control system to be successful, accurate thermal
modeling must be performed throughout the design
and testing of the satellite.

One of the recent satellite programs that has focused
on small satellite thermal analysis is the Combat
Sentinel program. Combat Sentinel is funded by the
Air Force Space Battle Lab (AFSBL) based in
Colorado Springs, Colorado. The Space Dynamics
Lab (SDL) in Logan, UT was contracted under the
Combat Sentinel program to provide a small satellite
for use as a thermal test article. SDL was also
contracted to build and calibrate a detailed thermal
model to be used during the TVAC testing of the
satellite.

This paper describes the thermal analysis that was
used to develop the final calibrated thermal model of
the Combat Sentinel satellite. Several steps of the
analysis are discussed that aided in development of
the thermal model as well as the satellite thermal
design. Specific thermal vacuum chamber testing
performed at SDL and Air Force facilities are also
discussed as pertaining to the thermal analysis of the
satellite. Finally, the calibrated thermal model is
used to extrapolate expected temperatures for on orbit
conditions.

2.0 SATELLITE OVERVIEW

The CSSAT is fundamentally based on the design of
the Ionospheric Observation Nanosatellite Formation
(ION-F) USUSAT. The external dimensions,
structural design, data processing unit, power control
system, and command and telemetry systems were all
taken from the USUSAT design. Some small
modifications of these systems were made during the
design. These modifications mainly consisted of
removing components that were not required for
basic operation of the satellite during testing. The
final CSSAT design resembles a simplified USUSAT
B. Moffitt


16
th
AIAA/USU Conference on Small Satellites

1 consisting of a data processing unit, a power system,
a telemetry transmitter, a camera, and a
magnetometer. General specifications of the satellite
are given in Table 1.

Table 1. Combat Sentinel Specifications
Maximum Diameter
50.165 cm (19.75 inches)
Thickness
15.24 cm (6 inches)
Total Mass
13.6 kg (30 lb)
Solar Cells
Techstar 3-junction
Gallium Arsenide
(GaAs)
Battery
4500 mAh NiMH

2.1 Thermal Control System

The most noticeable departure from the USUSAT
design is in the thermal control system. The AFSBL
requested that CSSAT contain external surface
coatings that are similar to other larger satellites used
by the Air Force Space Command. Specifically, the
inclusion of addition temperature sensors, the use of a
radiator surface, and the use of multi-layer-insulation
(MLI) external blanketing were requested. These
requests required changes to the mounting locations
of some internal components and also required the
addition of internal resistive heaters.

The final thermal design of the CSSAT contains both
passive and active thermal control systems (see Table
2). Heat is passively removed from the spacecraft via
a Z-93P painted radiator panel. Satellite panels that
directly face the earth are externally blanketed using
21 layer aluminized-mylar radiation shields.

Components that were attached to structural panels
containing solar cell arrays were moved to the bottom
radiator panel. This reduced component temperature
swings by eliminating a direct heat path to the highly
emissive and absorptive solar panel arrays. This also
located a majority of the satellite internal components
on the radiator panel. This configuration further
reduced temperature swings by locating a majority of
the satellite thermal inertia in a central location. This
configuration also facilitated the removal of excess
heat through direct contact to the radiator panel.

During seasonal orbit cycles where the satellite
spends a significant time in the earth eclipse, heaters
were needed to assure that the battery temperature
remained within its operational limits. During the
coldest scenario, the addition of 15 Watts of heater
power was needed to maintain the battery in its
operating regime. To control the addition of the
heating power, thermostats were installed.

In addition to the power needed during cold orbit
cycles, additional survival heaters were needed
during thermal vacuum chamber testing. A second
set of resistive heaters powered by an adjustable
external power source capable of producing 100
Watts of heating power was installed. This system
was used for periods when the satellite spent
extensive time in a cryogenically cooled vacuum
environment.

Table 2. Combat Sentinel Thermal Control
External Surface
Properties
Irridite Aluminum, Solar
Cell Arrays, Z-93P,
Aluminized Mylar MLI
Temperature Sensors
30 Sensors
Maxim-Dallas
Semiconductor DS1820
Thermostats
2 Klixon Thermostats
#3BTL6-3
On Orbit Heaters
Ohmite #TCH35P100RJ
Survival Heaters
Ohmite #TCH35P100RJ

2.2 External Structure

The external structure of the CSSAT consists of six
rectangular side panels and two hexagonal panels
(top and bottom panels). All of the panels are made
of irridited aluminum 6061-T6. From a vantage point
looking into the satellite with the top panel removed,
the sides are labeled clockwise from 1 to 6 with side
1 being the closest panel to the electronics enclosure.
The top panel contains a solar panel array of 40 solar
cells. The bottom panel is painted with Z-93P paint
and is connected to the majority of the internal
components. Sides 1, 2, and 6 are covered with MLI
using an external layer of either aluminized-mylar or
beta cloth. Sides 3, 4, and 5 each contain solar arrays
with eight solar cells. In addition to the solar cell
array, side 3 contains a camera and a G10 mounting
boom. An identical G10 mounting boom is also
located on side 6.

Figure 1 shows the Combat Sentinel numbering
scheme. The lower left picture shows the solar cell
arrays on the top, side 3, and side 4 panels. The
lower right picture shows the bottom Z-93P panel.




B. Moffitt


16
th
AIAA/USU Conference on Small Satellites

2


Figure 1. Combat Sentinel panel numbering.

2.3 Orbital Thermal Environment

As a baseline for the design of the Combat Sentinel
thermal control system, the orbital conditions defined
for the USUSAT were used.

The USUSAT is scheduled for delivery to orbit via
the Space Shuttle using the Air Force Multipl