Observational Analysis E1A
Official RCC Panel Debris Impact Test
a number of months the public waited for the C.A.I.B. to provide an
answer to one of the most burning questions which was, where did
the foam strike the left wing of the Columbia? There was a rather
complicated shell game of where the foam may have impacted the
orbiter. Possibly near one of the
RCC panels or maybe it plowed into the main landing gear door.
There was also the possibility that a magic piece of foam similar to
that magic bullet that entertained us so well in the Warren Commission
Report ricocheted off different surfaces producing several different
holes. Of course the most likely candidate for a breach location needed
to be near temperature sensors that trended upward rather than those
that went low, stayed neutral or fell off scale during reentry. The final
location for the hole placed it somewhere on RCC panel 7, 8 or 9.
Conveniently there were several sensors near the area that collected
data confirming the existence of a breach in that area. There
was a delay in getting that information because the sensors sent their
data only to the OEX data recorder which was found later.
Miraculously once again the OEX recorder saves the day.
News story with comments from Admiral Gehman on impact testing
few observations made by the C.A.I.B.:
STS-107 Accident Investigation Final Report
Vol. I, Pg. 60, Fig. 3.4-4
image to the left,
Fig. OA-E1A-1, is directly from the
STS-107 Accident Investigation Final Report
Vol. I. The graphic simply shows a couple of
different scenarios of how foam
traveling parallel to the orbiters body might impact the leading edge of a shuttle wing.
An impact near the apex of the RCC panel would result in a large
angle of incidence, nearly 90°, and will impart a much greater
percentage of its kinetic energy to the panel than a foam strike
that occurs further back on the panel.
also explains that the strongest part of the panel tends to be
at the apex near the tip where the dome like structure is the
most pronounced. The panel then becomes weaker as you move
back from the apex towards the midpoint of the surface where the
greatest damage is likely to occur.
STS-107 Accident Investigation Final Report
Vol. I, Pg. 55
Investigation Final Report Vol. I, Pg. 55
This is just a brief
description of how the RCC panels are fabricated. It is
only posted here for the interest of the reader.
OA-E1A-2 is a brief summary of design requirements for the RCC
panels probably written near the beginning of the shuttle
program. Per the design requirements the RCC panels are to
have the same resistance to kinetic energy impacts as do the TPS
ceramic tiles. The kinetic energy impact they must be able to survive
has a value of 0.006
Ft.-Lbs. One engineer studying this specification during
the investigation discovered that the kinetic energy released by
a regular wood No. 2 pencil dropped from a height of 6 in. would
be equal to 0.006
Ft.-Lbs. Because this value for the maximum allowable
energy for impact is almost jokingly low, it might be deduced
that the engineers responsible for it did not expect that an
object of any significance would ever come near the leading edge
of the shuttles wings. The following statement was also
part of the design requirements.
wing leading edge would not need to withstand impact from debris
or ice since these objects would not pose a threat during the
the Boeing engineers performing the 1ascent analysis on STS-107
needed a maximum allowable value for debris impact. they referred
to the statement that required the RCC panels to have the same
value as the TPS ceramic tiles and the 2test
report done on the ceramic tiles. whether or not
the new information on the RCC panels would have led to a
different conclusion for those reports will require another
Orbiter Impact Tile Testing,
SwRI Project No. 18-7503-005, March 5, 1999
absolutely nothing else at all was accomplished in the creation of the
STS-107 Accident Investigation Final Report Volumes I Through VI, the
team testing the RCC panels established some significant guidelines that
may provide a starting point for developing a true analytical method to
predict impact damage to the panels. This was quite an
accomplishment given that they started from scratch with absolutely no
material strength data available for the Reinforced Carbon Carbon and no
loading data of any kind available for the panels themselves unless you
count the specification for surviving a Kinetic Energy Impact of 0.006
Ft.-Lbs. presented above in Fig.
problems found in the impact testing that was done are detailed in the
following sections, the areas of concern are the test methods used and
the material properties. Although the test methods as described in
the STS-107 Accident Investigation Final Report Vol. II, D.12 are
somewhat flawed, they provide a template that any follow up work may
build on. The allowable values for the material need to be independently
verified and most likely determined using standard empirical methods for
finding materials properties.
E1A-4 through E1A-6 are from STS-107 Accident Investigation Final Report Vol.
II, Appendix D.12, Pgs. 375 to 390
The strength values tabulated in Fig.
were created by the RCC panel testing team based on the strength properties of the raw materials used
to makeup the RCC composite.
The next step
was attempting to develop an analytical
solution that would predict the extent of damage to an RCC panel
resulting from a debris impact. The results of the analytical
solution are represented in
OA-E1A-5 by equation 12
below. Equation 12 calculates the maximum boundary stress seen by
the RCC panel during impact.
OA-E1A-6 is a graph of pressure Vs.
Velocity for equation 12. The
units on either side are kPa on the left and psi on the right.
OA-E1A-7A shows the most likely scenario for a debris impact on
RCC panel 8 during launch and ascent. The final report states that the debris was
traveling parallel to the orbiters body which makes the
trajectory angle 0° as represented by the graphic. The
final report also indicates that the geometry or shape of the
underside of an RCC panel would result in angles of incidence to
the panel's surface that are at the most 20° and usually much
OA-E1A-7B shows the angles and impact locations that were used in the
test that caused failure to a #8 RCC panel. This is an
extreme worst case scenario that probably would not and possibly could
not ever happen during an actual launch ascent.
Structural Strength of the RCC Panel
As shown in
Fig. OA-E1A-1 above the strength of an RCC panel changes as
to move across it. The strongest point is at the apex or
the very front of the panel. The weakest point occurs
midway along the surface of the panel. Both Fig.
OA-E1A-7A and Fig.
OA-E1A-7B upper and lower range of panel impact locations
along with the point of greatest damage.
the angle of the foams trajectory decreases its original kinetic
energy. If a 0° course change means the foam retains all
of its original energy then a 90° course change would mean that
all of the original kinetic energy has been lost. Any
values in between can be calculated using simple trigonometry.
STS-107 using new data:
can be reanalyzed using the analytical method,
12 and graph
developed in the final report with the added advantage of knowing that
the official damage assessment placed the breach in RCC panel 8 of the
left wing. The final report uses the total velocity multiplied
by the Sin of the angle of incidence to get the velocity component
normal to the surface. This analysis based on
stated to be more conservative than a computer modeled solution but
might be used as a preliminary determination of whether more extensive
analysis is required.
working stresses for the RCC panels are given in the final report as
29 ksi (200 MPa) for the panel rib and 27 ksi (185 MPa) for the panel
face. These values seem significantly lower than what would be
expected but no other literature is currently available.
values are computed for impact incidence angles ranging from 5
to 20° and a velocity of 774 Ft./ Sec. The principle
stresses found for angles of incidence 5, 10, and 20° are 34,
42 and 56 ksi respectively. These values are conservative
but fall short of the allowable working stress of 27 to 29 ksi.
Damage to the panel would have resulted in each case.
this calculation were done for the Columbia on STS-107 it would
have shown that a more complete analysis should be done
including computer modeling and visual inspection.
Velocity Vs. Pressure graph along with
12 may provide a quick
and reliable method for determining the potential damage done to RCC
panels after a debris strike. It might be used to get a
preliminary result before more time consuming and expensive methods
are used. As the equation and graph appear to work now with the
allowable working stresses found in the final report, Almost every
possible debris strike would be shown as significantly damaging a
panel. Even though the preliminary method needs to be, and has
been shown to be, conservative, the current values will show damage
for every event of a debris impact. After 100+ shuttle missions
this is known not to be the case. Therefore the allowable values
and or the results of the calculation need to be compared more closely
to actual impact damage and modified to increase accuracy.
nature of this test itself may be misleading. What the
test shows is the extent of damage done to an RCC panel when
struck with foam debris at a given velocity and angle of
incidence. What the test does not show is the maximum
possible impact velocity and angle of incidence of foam
debris with some given initial aerodynamic properties on the
leading edge of a wing. The actual impact velocity on
the RCC panel in question is then debatable based on how the
aerodynamic properties of the debris change when it
encounters the boundary layer of the wing shockwave.
Since there is no information on how the velocity and
trajectory of foam debris may change when coming into
contact with such a boundary layer this possible difference
can only be considered for discussion purposes.
was stated in the description of the test procedures that
both fiberglass WLE panels from Enterprise were used along
with RCC panels from other shuttles that had similar flight
histories to Columbia. The report describes damage to
both the fiberglass and RCC panels and It is often unclear
which type of panel is being tested. The fiberglass
panels were designed and fabricated to look like the RCC
panels for display purposes but were supposed to act only as
a template for the actual test. Anytime you have such
an item in a test environment that does not provide any
usable data for future design purposes it should be clearly
marked as something other than a test specimen. It
does not appear anywhere in the test description that the
fiberglass panels were marked in such a way as to separate
them from the RCC panels and no different markings can be
observed in any of the associated images.
"smoking gun" that was found during the impact tests was
the large hole, 16" x 17", created on the lower
side of an RCC panel 8, Fig.
OA-E1A-10. The test procedures stated that this
test was performed with an angle of incidence of 25.1° to
the lower surface of the panel. One of the
conclusions in the test report was that increasing the angle of incidence was one of the single
greatest factors in increasing the amount of damage done to
an RCC panel during an impact test. Based
on information within the official final report the
trajectory angle of the foam during the actual event was
most likely 0° and not
more than 5° with the
resulting angle of incidence being between 5°
and 20°. To achieve
an angle of incidence of 25°
requires that the trajectory angle of the foam debris to be
between 15° and 20°.
The actual angle of incidence is dependent on exactly where
along the lower surface of the RCC panel the impact occurs.
The impact tests may be conducted using any velocity and
angle of incidence. In a flight environment that
represents the actual debris impact any change in the
trajectory angle of the debris will result in a reduction of
its kinetic energy and velocity.
given parameters and test conditions for the result known
as, "The Smoking Gun", were far too conservative to
represent an event that occured durring the launch and
ascent of Columbia on January 16, 2003.
The debris strike video
never clearly shows the piece of foam hitting the wing of
the shuttle. The piece of debris appears to disappear
below the shuttle wing followed by the breakup and
scattering of the foam debris. Therefore the video
evidence is at best inconclusive.
STS-107 Accident Investigation Final Report Vol. I,
Chap. 3, Pg. 60
[ Image analysis determined
that the foam was moving almost parallel to the Orbiter's
fuselage at impact, with about a five-degree angle upward
toward the bottom of the wing and slight motion in the
outboard direction. ]
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references for RCC panel testing