NASA Technical Reports Server (NTRS) 20070003680: Orbiter Gap Filler Bending Model for Re-entry

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by  NASA Technical Reports Server (NTRS)

AIAA-2007-???
Orbiter Gap Filler Bending Model for Re-entry
Charles H. Campbell*,
NASA Johnson Space Center, Houston, TX 77058
Abstract Pressure loads on a protruding gap filler during an Orbiter reentry are investigated
to evaluate the likelihood of extraction due to pressure loads, and to ascertain how much bending
will be induced by re-entry pressure loads. Oblique shock wave theory is utilized to develop a
representation of the pressure loads induced on a gap filler for the ISSHVFW trajectory,
representative of a heavy weight ISS return. A free body diagram is utilized to react the forces
induced by the pressure forces. Preliminary results developed using these methods demonstrate
that pressure loads, alone, are not likely causes of gap filler extraction during reentry. Assessment
of the amount a gap filler will bend over is presented. Implications of gap filler bending during re-
entry include possible mitigation of early boundary layer transition concerns, uncertainty in
ground based measurement of protruding gap fillers from historical Orbiter flight history, and
uncertainty in the use of Orbiter gap fillers for boundary layer prediction calibration. Authors
will be added to the author list as appropriate.
I. Introduction
F
light history on the Orbiter vehicle illustrates that gap fillers can and do come out during the ascent/reentry
cycle. Two occurrences of protruding gap fillers, on STS-28 and STS-73, during Orbiter reentry are believed to
have caused very early boundary layer transition (BLT), near a mach number of 18. During STS-114, two gap
fillers were removed by an Extra Vehicular Activity (EVA) crewmember because of the uncertainty involved in
determining if those protuberances at the locations identified on Discovery would lead to early BLT and potential
catastrophic consequences during reentry. After Discovery returned from the STS-114 mission, the Orbiter Thermal
Protection System (TPS) community engaged in a concerted activity to evaluate the installation processes and
design involved with the TPS gap fillers. As a direct result of that STS-114 post-flight activity, significant changes
have been made in the gap filler installation process. In addition, due to previous occurrences of protruding gap
fillers, and the concern with potential consequences of early BLT as well as the mission impacts to perform
additional EVA activities to remove gap fillers, a long term activity to remove and replace gap fillers on the Orbiter
fleet has begun. However, due to the large number of gap fillers on the fleet and the time involved with re-
installation of the gap fillers, the TPS vehicle processing is a significant schedule concern. Prioritization of the
Orbiter regions to have the gap fillers reinstalled has occurred with significant inputs from the Orbiter reentry
technical community. Recent activity by the Space Shuttle Program (SSP) Systems Engineering and Integration
(SE&I) community investigating the risk involved with debris transport of gap fillers has elevated the SSP concern
in regard to extraction of gap fillers during ascent. Because of lessons learned during the Return To Flight process
in regard to foam and ice loss on ascent, the SSP Shuttle Engineering Review Board (SERB) has requested that
engineering analyses be performed on the extraction processes involved with gap filler removal during both Orbiter
ascent and reentry. In order to support a simple engineering analyses framework, analytical flow field methods are
utilized to develop a representative reentry pressure loading history on a protruding gap filler. A heavy weight
trajectory representative of an Internation Space Station (ISS) return mission is utilized to evaluate pressure loads on
a protruding gap filler. Loads on the gap filler are assessed in order to evaluate the likelihood that the gap filler can
be extracted due to pressure, and the amount that a gap filler may bend due to pressure.

* Aerospace Engineer, Aeroscience and Applied CFD Branch, M/C EG3, Senior Member.
1
American Institute of Aeronautics and Astronautics
AIAA-2007-???
II. Analyses
An analytical framework for evaluating pressure loads has been assembled in Excel. To the authors knowledge, this
model represents the first attempt to model the effects of re-entry pressure loads on an Orbiter protruding gap filler
during re-entry. Representative deflection angle of a protruding gap filler is provided to demonstrate that the
pressure load model has been implemented. Representative geometry of a gap filler bending due to reentry pressure
loads are also included to demonstrate that a simple structural analyses has been implemented to evaluate the
geometry of a deflected, protruding gap filler.

The intent of this paper will be to provide a summary of the methodology utilized for determination of the pressure
load distribution, assessment of the force balance induced on the gap filler, and an assessment of the geometry of the
gap filler during re-entry conditions. This information will be used to understand the possible uncertainty in the
height of gap fillers that were documented on the ground for the STS-28 and STS-73 Shuttle missions, which have
also been attributed with early boundary layer transition.

Gap Filler Deflection
Flow Direction Triple Ply Gap Filler Geometry E = 40000 [psi]
Poisson Ratio = 0.3
1" Gap Filler Protruding
Full Cp
Mach 15
45˚Gap Filler Sweep
E=400000 [psi] Tip
1 Angle
~ 10˚ Mach 21 Mach 18 Mach 12
0.9
~ 45˚
0.8 ~ 60˚ ~ 66˚
] 0.7 Cp ~ 71˚
s Mach 15
e 0.6 front
h
c 0.5
n
i 0.4
[ Cp wake = 0.0
Y 0.3
0.2
0.1 Recessed Gap Filler OML
0
-0.05 0.05 0.15 0.25 0.35 0.45 0.55 0.65
Note: Results have not been validated by detailed ISSHVFW Trajectory
structural analyses X [inches]
Note: X-scale is magnified ~250%
Cp front is determined with a Newtonian methodology

2
American Institute of Aeronautics and Astronautics
AIAA-2007-???
Results
E = 400000 [psi]
GapG aFpil lFeirl lDere Tfliepc Ationng lAengle
Poisson Ratio = 0.3
Full Cp
50 8
45 Cp 7
40
s] 6
e 35
e Single-Ply
r 5
g 30
e
d
[ 25 4
e
gl 20 Triple-Ply
n 6-ply 3
A
p 15 2
Ti 10
1
5
0 0
5.0 10.0 15.0 20.0 25.0 30.0
Mach Number
Richardson Angle [degrees] Richardson Angle [degrees] RicharRdsiconh aExrdtraspoonla tAionn gwlaes u[dseedg troe deeste]rmine theCsep v alues

3
American Institute of Aeronautics and Astronautics