A large structural model of a reentry vehicle has been built incorporating design concepts applicable to a radiation-cooled vehicle. Thermal-stress alleviating features of the model are discussed. Environmental tests on the model include approximately 100 cycles of loading at room temperature and 33 cycles of combined loading and-heating up to temperatures of 1,6000 F. Measured temperatures are shown for typical parts of the model. Comparisons are made between experimental and calculated deflections and strains. The structure successfully survived the heating and loading environments

Helms, Bobbie F.

Pride, Richard A.

Royster, Dick M.

English

NASA Technical Reports Server

.............................. NASA TM X-314
m
H
:l
<
Z
TECHNICAL MEMORANDUM
X,314
EXPEIM ENTA L-STU_A_O T STI_UC_E
FOR A RE:E:NTI_Y::VEHICLE i
- 1 7 77 _Z TZ U _ ZZII_7_ ZIZLL L
................. By Richard A. Pride, Dick M. Royster;
and Bobbie F, Helms
Langley Research Center
Langley Field, Va.
NATIONAL AERONAUTICS AND_SPACE ADMINISTRATION
WASHINGTON September 1960
Declassified September 28, 1961
https://ntrs.nasa.gov/search.jsp?R=19980223616 2020-06-15T22:59:30+00:00Z
- --
..... k
"z
!
i
J
!
i
J
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
TECHNICAL MEMORANDUM X-314
EXPERIMENTAL STUDY OF A HOT STRUCTURE
FOR A REENTRY VEHICLE
By Richard A. Pride, Dick M. Royster,
and Bobbie F. Helms
SUMMARY
A large structural model of a reentry vehicle has been built incor-
porating design concepts applicable to a radiation-cooled vehicle.
Thermal-stress alleviating features of the model are discussed. Environ-
mental tests on the model include approximately lO0 cycles of loading
at room temperature and 33 cycles of combined loading and heating up to
temperatures of 1,600 ° F. Measured temperatures are shown for typical
parts of the model. Comparisons are made between experimental and cal-
culated deflections and strains. The structure successfully survived
the heating and loading environments.
INTRODUCTION
Reference 1 indicates that the construction of reentry vehicles
of low wing loading could be based on radiative-type structure. The
load-carryingmaterial of such vehicles would be subjected to high
temperatures with large variations in temperature throughout the struc-
ture. These conditions present difficult thermal-stress problems in
conventional stressed skin design (ref. 2) and have prompted investiga-
tion of structural concepts which are more suited to the thermal
environment.
This paper describes the available results of a test program on
a large sheet metal structure designed to cope with the heating and
air loads encountered by a reentry vehicle.
2STRUCTURALDESIGN
The sheet metal structure used in the present investigation is
shownin figures I and 2. It is triangular in planform and cross sec-
tion with a length of 12 feet_ a width at the back of 7 feet, and a
height at the back of 2_ feet. These dimensions provide a planform
I
area of 47 square feet with a sweep angle of 75 °. In order to provide
the greatest amount of test information 3 this structural model was
designed with one side flat and the other V-shaped. Skin panels and
internal structure were symmetrically designed so that the model could
be heated and loaded from either side. All parts of the structural
model were fabricated from the superalloy, Inconel X.
The exterior of the model is covered with corrugated skin panels
with the axis of the corrugations parallel to the flight direction.
These skin panels serve a dual purpose; they carry the air loads fore
and aft to the supporting internal frames, and they provide torsional
stiffness for the model. Expansion Joints which extend around the
model cross section at 2-foot intervals help to alleviate thermal
stresses but prevent the skins from contributing any bending stiffness
to the model.
The internal structure is shown in figure 3. It consists of an
approximately orthogonal arrangement of transverse frames and longitudi-
nal beams. The skin-panel loads are transmitted to the transverse
frames. These frames in turn transmit the loads to the two longitudi-
nal beams. The load-carrying members operate at high temperatures
because the heated skin panels radiate heat throughout the internal
structure.
Corrugated shear webs are used in both transverse frames and
longitudinal beams to carry the shear loads and at the same time to
permit differential thermal expansion between top and bottom spar caps
without a large buildup of thermal stress. Well-designed corrugated
shear webs have efficient load-carrying strength and stiffness values
(ref. 3). The average weight of the internal structure in this model
is 1.5 ib/sq ft of wetted area, and the average weight of the skin
panels is 1.0 ib/sq ft.
A closeup view of the circled region in figure 3 is shown in fig-
ure 4. This figure gives a view of an intersection of a transverse
frame, longitudinal beam, and skin panels. The longitudinal shear web
is a standard 60 ° corrugation. The spar cap consists of two channels
with a cover plate and is spotwelded to the corrugated web on both
sides.
3The transverse shear web is a specially designed corrugation to
provide extreme flexibility normal to the web between the two channels
which make up the transverse cap. This flexibility is needed to allow
the skin panels to exp_d or contract freely without large buildup of
thermal stress. The transverse shear webs are spliced to the longitudi-
nal webs at their intersection.
The skin panels are attached to the outside flanges of the trans-
verse caps. Skin panels are fabricated by seam welding together two
pieces of 0.010-inch-thick sheet. The outer sheet is beaded lightly to
stiffen it against local buckling and to preset a pattern which deforms
uniformly when thermal expansion across the corrugations is restrained
by the underlying transverse channels. The inner sheet is formed to a
60 ° , i/2-inch flat corrugation and stops short of the edge of the outer
sheet. A Z-stiffener provides the transition from the inner sheet to
the outer sheet along the attached edges as shown in figure 5. The
skin panels are attached only to the transverse frames and do not come
in contact with the longitudinal spar cap. The expanslon-Joint tie
allows the transverse frame channels to move relative to each other as
the skin panels expand or contract. The tie also provides continuity
between skin panels so that their shear stiffness can be utilized for
torsional stiffness of the model. For aerodynamic smoothness_ the
expansion Joint is covered with a strip which is fastened on the upstream
side.
In order to help isolate the longitudinal spar caps from the skin
panels, the transverse frame channels cross the longitudinal spar cap
on the outside. One channel in each frame is rigidly attached to the
longitudinal cap but the other channel is freely floating so that the
expansion-Joint action is not restricted.
A skin sheet thickness of 0.010 inch was selected as minimum gage
based on general concepts of noise failures and panel flutter. In order
to better define design criteria for these phenomena, noise and flutter
tests of this skin panel have been undertaken. Results to date of the
noise tests indicate that cracking starts near the rivet line after
20 minutes exposure to 157 decibels of random frequency noise from an
air jet. After 2 hours the cracks had lengthened but the panel had not
broken up disastrously.
Flutter tests were run in the Langley Unitary Plan wind tunnel on
2-foot-square skin panels. Although the panel with the corrugations
parallel to the air flow was more flutter resistant than one with the
corrugations perpendicular to the air flow, both orientations withstood
a dynamic pressure greater than 2,000 ib/sq ft.
TESTSANDDISCUSSION
In order to study the behavior of the structure when subjected to
a simulated reentry environment, the laboratory test setup shownin
figure 6 was constructed. The 12-foot-long model is cantilevered from
a support at the left in figure 6. The model is mountedwith the flat
side up for convenience in testing. The testing simulates reentry con-
ditions for a flat-bottom configuration. It should be pointed out that
in the figures which follow, whenever top or bottom is mentioned, it
refers specifically to the testing orientation of the model; thus the
top skin refers to the flat side.
Raised above the model is a large radiator which heats the flat
planform area. Planform temperature variations of 250° F are programed
into seven control channels, and this variation of heat intensity can
be seen in the radiator (fig. 6). Before a test, the radiator is low-
ered so that it is parallel to and about 4 inches above the flat sur-
face of the model. Programed loading is applied hydraulically by Jacks
through the whipple-tree system into the underside of the model.
Figure 7 shows a typical programed test environment to which the
structural model was exposed. Temperature and load are shown as a
function of time up to 20 minutes. Since the model was a research
specimen, no attempt was made to duplicate the effects of any specific
reentry trajectory. Ramp function inputs were used to simplify the
analysis of results. The temperature along the structural leading edge
was increased at a rate of i0 ° F per second up to a maximum for the
test series of 1,600 ° F, then held constant for about 15 minutes, and
finally decreased at a maximum programed rate of i0 ° F per second.
These temperatures are comparable to structural temperatures which would
be achieved in a lightly loaded unshielded vehicle or in a more heavily
loaded vehicle behind a metallic heat shield. About midway in the test,
a load pulse of 150 ib/sq ft was applied which produced maximum stresses
of about 30 ksi in the structure.
Prior to the maximum cycle of heat and load at the level shown in
figure 7, the model was subjected to about lO0 cycles of loading up to
212 lb/sq ft at room temperature and 32 cycles of combined heating and
loading at various levels of peak temperature between 400 ° F and
1,600 ° F.
The measured distribution of temperature around the model cross
section at a location 9 feet from the nose is given in figure 8. The
general trend of the programed temperature variation of 250 ° F in the
flat planform is evident in the top of the figure with the leading
edges at about 1,600 ° F and the center at 1,350 ° F. The presence of
the two main spar caps lying beneath the heated skin showsup in the
two intermediate peaks in the temperature curve. These spar caps are
not conductive heat sinks because they are not in contact with the skin;
but, since their proximity to the skin blocks the radiation, higher
skin temperatures occur above the spars than on either side of the
spars. A similar effect is not evident in the bottom skin inasmuch as
the temperature levels are lower and radiation heat exchanges are much
less. The temperatures shownin figure 8 are for 7 minutes of test
time and correspond to approximately steady-state equilibrium for the
skin panels.
Variation of temperature with time is shownin figure 9 for selected
points on the top and bottom skins and top and bottom spar caps of the
longitudinal beams. Temperature is plotted against time in minutes.
The top skin shows a response very similar to the programed input(fig. 7) with the exception of the region near the end of the test when
natural cooling took place at a slower rate than the maximumprogramed
cooling rate. Heating of the spar caps and bottom skin occurred primarily
by radiation from the top skin through the interior of the model. From
5 to i0 minutes elapsed after the top skin reached equilibrium before
the other elements essentially reached equilibrium. The maximumtem-
perature difference between the top and bottom spar caps occurs after
approximately 7 minutes of test time.
Since the spar caps provide the longitudinal bending stiffness
for the model, their temperature differences are reflected in the model
deflection as shownin figure i0. Deflection of the model nose
is plotted against time for the simulated reentry test. The total
measuredexperimental deflection is given by the solid curve. It reaches
a peak value of nearly 3 inches due to spar-cap temperature difference
at about 7 minutes of test time corresponding to the maximumspar-cap
temperature difference shownon the previous figure. Beyond this point,
the deflection due to temperature difference decreases even though the
absolute spar temperatures are still increasing.
The humpin the deflection curve is produced by the load pulse
applied during the heating cycle. Note that deflection due to heating
is about three times the deflection due to load. The rapid dropoff in
deflection at 17_ minute[: corresponds to the time whenpeak heat input2
ceased and the entire model began to cool rapidly.
Computeddeflections based on beamtheory and using the measured
temperatures inside the model are shownby the dashed curves in fig-
ure i0 for the conditions of heat and load which were superimposed
during the test. The agreementbetween experimental and computeddeflec-
tions is satisfactory considering the timewise and spacewise temperature
variations.
A comparison of experimental and computed strains in the longitudi-
nal compression spar cap is shownin figure ll for a loading of
212 lb/sq ft on the planform area. Average load strain in the spar-
cap cross section is shownas a function of distance from the rear.
The Solid curve is the computedvalue of average strain. The circles
are average values of room-temperature strain measuredwith resistance
strain gages located in several places on the cross section of the spar
cap. The square is a measured load strain for the sameapplied load
during a 1,000° F test. Agreement is shownbetween theory and experiment.
Figure 12 showsthe condition of the structural model at the con-
clusion of the heating cycles. The only damageobserved was a few local
buckles in the hottest portions of the back two panels, that is,
those with the largest planform temperature differences. Most of the
buckles occurred in the two outlined regions and are small and confined
to the beaded portion of the outer skin.
CONCLUDINGREMARKS
A large structural model has been built incorporating design con-
cepts applicable to a radiation-cooled vehicle. Someof the results of
the test experience have been presented. The agreement shownbetween
theory and experiment for deflections and strains indicates that beam
bending theory is adequate to describe the average response of the model
to applied loads and temperatures. On the basis of these results com_
bined with visual inspection, it is concluded that these design concepts
enabled the structure to survlvm successfully the simulated reentry
heating and loading environments.
Langley Research Center,
National Aeronautics and SpaceAdministration,
Langley Field, Va., April 12, 1960.
REFERENCES
i. Anderson, Roger A., and Swarm,Robert T.: Structures for Reentry
Heating. NASA_MX-313, 1960.
2. Pride, Richard A._ Hall, John B., Jr., and Anderson, Melvin S.:
Effects of Rapid Heating on Strength of Airframe Components.
NACA TN 4051 , 1957.
3. Peterson, James P., and Card, Michael F.: Investigation of the
Buckling Strength of Corrugated Webs in Shear. NASA TN D-424,
196o.
8STRUCTURAL - MODEL DIMENSIONS
_, 12 FT
Figure 1
STRUCTURAL- MODEL EXTERIOR
Figure 2 L-59-4915
9STRUCTURA_MODELINTERIOR
Figure 3 L-59-3377
INTERSECTION OF MODEL COMPONENTS
Figure 4
lO
EXPANSION- JOINT DETAIL
I
0
i I
I 2
INCHES
I
3
Figure 5
TEST SETUP
Figure 6 I,-59-6745
ll
TEMPERATURE,
oF 80O
PROGRAMED TEST ENVIRONMENT
HEAT
L LLL.
LB/SQ FT
0 I0 20
TIME, MIN
Figure 7
TEMPERATURE VARIATION IN MODEL CROSS SECTION
STATION, 9 FEET FROM NOSE; TEST TIME, 7 MINUTES
1,550 ° F 1,620 = F
8 0 ° F
5
Figure 8
12
TIME-TEMPERATURE VARIATION IN MODEL CROSS SECTION
STATION, 9 FEET FROM NOSE
TEMP., °F
2'000I TOP SKIN
,,.oop; \
i I TOPSPAR __
,.oot/ _I--- -t\
,ooi-I _=,,, \_\\.._
o ,_o 2_o _o
TIME, MIN
Figure 9
MODEL DEFLECTION DURING SIMULATED REENTRY
i _, "
8, IN. 2
0
4
HEAT
I0 20 30
TIME, MIN
Figure lO
13
LOAD
STRAI N
LOAD STRAIN IN COMPRESSION SPAR CAP
LOAD, 212 LB/SQ FT
600
0
COMPUTED
TEST
o 80 ° F HEAT
[] ,,ooooF _:_:t_':t
,,2oo[_,o-6 _TT_o_ D
'm----" X, IN.
I I
48 96 144
x, IN.
Figure ii
CONDITION AFTER SIMULATED REENTRY
Figure 12 L-99-8649
NASA- Langley Field, "Ca. L-lOS1



Experimental Study of a Hot Structure for a Reentry Vehicle

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server