Joshua Finkbeiner, Pat Dunlap, and Bruce Steinetz

Glenn Research Center, Cleveland, Ohio

Jeff DeMange

University of Toledo, Toledo, Ohio

Daniel Newswander

Johnson Space Center, Houston, Texas

Investigations of Shuttle Main Landing Gear

Door Environmental Seals

NASA/TM—2005-213860

September 2005

AIAA–2005–4155
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Joshua Finkbeiner, Pat Dunlap, and Bruce Steinetz

Glenn Research Center, Cleveland, Ohio

Jeff DeMange

University of Toledo, Toledo, Ohio

Daniel Newswander

Johnson Space Center, Houston, Texas

Investigations of Shuttle Main Landing Gear

Door Environmental Seals

NASA/TM—2005-213860

September 2005

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

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AIAA–2005–4155
Acknowledgments

The authors would like to acknowledge Richard Tashjian for his significant contributions involving

seal installation and test setup.

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Available electronically at http://gltrs.grc.nasa.gov
NASA/TM—2005-213860 1

Investigations of Shuttle Main Landing Gear Door 

Environmental Seals 

Joshua Finkbeiner, Pat Dunlap, and Bruce Steinetz

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Jeff DeMange 

University of Toledo 

Toledo, Ohio 43606 

Daniel Newswander 

National Aeronautics and Space Administration 

Johnson Space Center 

Houston, Texas 77058 

The environmental seals for the main landing gear doors of the Shuttle Orbiters were raised by the Columbia 

Accident Investigation Board as a potential safety concern. Inspections of seals installed on the Shuttle Discovery 

revealed that they were permanently deformed and no longer met certified seal compression requirements. 

Replacement of the seals led to the inability to fully close the main landing gear doors. Johnson Space Center 

requested that Glenn Research Center conduct tests on the main landing gear door environmental seals to assist in 

installing the seals in a manner to allow the main landing gear doors to fully close. Further testing was conducted to 

fill out the seal performance database. Results from the testing indicated that the method of bonding the seals was 

important in reducing seal loads on the main landing gear doors. Also, the replacement seals installed in Shuttle 

Discovery were found to have leakage performance sufficient to meet the certification requirements. 

I. Introduction and Background

During the investigation into the loss of the Shuttle Columbia (STS–107), the Columbia Accident Investigation 

Board requested an investigation into the Main Landing Gear (MLG) door environmental seals to evaluate their 

potential contribution to the loss of the Shuttle. The resulting investigation determined that while the seal 

certification documents specified a minimum compression level to be applied to the seals (after maximum flight 

deflections had been taken into account), neither the seal installation drawings nor the landing gear rigging drawings 

specified how to properly rig the MLG doors to achieve the certified seal compression. Furthermore, there was no 

documentation specifying procedures between flights to ensure continued compliance with the seal certification 

documentation. 

Visual inspections of the MLG door environmental seals on OV–103 (Shuttle Discovery) revealed permanent 

deformation of varying degree along the length of the seal such that the seal no longer met the certification drawing 

requirements for minimum height. A clay compression test was performed on the environmental seals and the results 

indicated gaps larger than the required limits (i.e., too little compression on the seals). These two findings led to a 

concern that flow could penetrate the MLG bays, resulting in thermal damage to the underlying structure. To 

alleviate this concern, the flown OV–103 environmental seals (Rev. M) were replaced with new seals (Rev. P) and 

constant-thickness shims were added to the sealing surface of the MLG bays to satisfy the certified seal compression 

requirements around the door perimeter. However, the new configuration resulted in higher loads sufficient to 

prevent the full closure of the MLG doors. Past experience (ca. 1991) with the MLG door rigging mechanism 
NASA/TM—2005-213860 2

Figure 1.—Photograph of Space Shuttle Orbiter with Main Landing Gear Doors highlighted. 

demonstrated that similar loads caused structural damage to the mechanism. Therefore, a solution was required on 

how to install the seals to achieve seal compression requirements without generating excessive loads that could 

damage the rigging mechanism. 

NASA Johnson Space Center (JSC) requested that the NASA Glenn Research Center (GRC) perform a number 

of engineering evaluations of the MLG environmental seals (both the new design and the old seals removed from 

OV–103) to obtain data to guide seal installation and aid in closing the main landing gear doors. In addition, more 

systematic testing was requested to fill out the seal performance database. A series of exploratory tests were 

performed on as-received revision P seal specimens to assist in installing the seals so that the MLG doors could be 

fully closed. These tests evaluated seal performance under various configurations, and the findings from these tests 

provided NASA Kennedy Space Center (KSC) engineers with data which was valuable in ultimately closing the 

MLG doors. After the exploratory tests, a systematic series of compression and flow tests were performed on the 

seals to assist in filling out the seal performance database and evaluating the new revision P seals as replacements 

for the revision M seals. 

II. Description of Seal Specimens 

The Thermal Protection System (TPS) of the Space Shuttle Orbiter protects the vehicle and crew from the 

extreme temperatures of reentry. The TPS contains breaks to allow for opening the landing gear doors and extending 

the landing gear as shown in figure 1. The MLG doors are highlighted in the photograph. The MLG doors include 

both a thermal barrier and an environmental pressure seal to prevent hot reentry gases from entering the MLG bay 

and damaging the interior of the vehicle. Figure 2 depicts a close-up view of one of the MLG doors with its installed 

environmental seal. 

The environmental seal design consists of a bulb seal with an attached tail, both constructed of a silicone rubber 

(ZZ-R-765, Class IIIa, Grade 50) core overwrapped with Nomex fabric. Small vent holes are punched into the seal 

material in the front of the seal bulb and are spaced roughly 6 in. apart. These holes allow pressure in the seal bulb to 

vent to vacuum while the Shuttle is in orbit. The seal was produced by Kirkhill Rubber (Brea, CA) under Northrop 

Grumman specification number GS179M10. A schematic of the seal design is shown in figure 3 with specific 

dimensions presented in table 1. The seal certification documents (Rockwell International 05-GS17M10C) assert 

that the seal leakage may not exceed 3 SCFM/ft for a differential pressure of 1 psid. 
NASA/TM—2005-213860 3

Figure 2.—Photograph of environmental seal installed on Main Landing Gear Door. 

The tape behind the seal is used for masking and is removed before the door is 

closed. 

Tape removed 

before door closure 

Figure 3.—Schematic of Shuttle MLG door environmental seal (taken from Northrop Grumman 

Standard GS179M). 

TABLE 1.—DIMENSIONS OF SHUTTLE MLG DOOR ENVIRONMENTAL SEAL 

[All dimensions are in inches, tolerances are specified as ±0.020 in.] 

A 1.55 

B 0.44 

C ----- 

R 0.12 

T1 0.08 

T2 0.06 

T3 0.010 
NASA/TM—2005-213860 4

Forward 

Aft 

Outboard 

(Hinge side) 

Inboard 

22” 8” 

38” 33” 

0” 

0”

7”

12”

25”

“Good” 

“Bad” 

Specimens: 

Compression 

Flow 

Figure 6.—Schematic of starboard MLG 

environmental seal removed from OV–103 

along with locations of the flow and 

compression specimens removed from the 

seal. Locations of the flow specimens are 

shown in red, and locations of the 

compression specimens are shown in green. 

Two seal revisions were evaluated in this study and are 

shown in figure 4, both in their as-received conditions. The first 

seal, revision M (Rev. M), featured a Nomex fabric mesh 

impregnated in the silicone rubber core material. This contrasted 

with the new revision P (Rev. P) seal, which did not contain the 

fabric impregnation. Furthermore, the coloration of the rubber 

material in the Rev. P seal was slightly different than that of the 

Rev. M seal, indicating a possible difference in the composition 

of the rubber. The Rev. P seal was noticeably less stiff to the 

touch than the Rev. M seal. 

Several samples of both Rev. M and Rev P. seals were tested 

in this investigation. Two manufacturing lots of as-received 

Rev. P seal were evaluated, having lot numbers 

024025753450019 (lot -0019) and 024025753450021 (lot 

-0021). In addition, several Rev. M seals were investigated, 

including both as-received and flown specimens. The flown seal 

was removed from the starboard MLG door of OV–103. The 

bulb of the flown seal had sustained “permanent” deflection 

which varied in severity along its length. The two extremes of 

the permanent deformation are shown in figure 5. The “good” 

seal specimen exhibited approximately 0.035 in. of permanent 

deformation in the bulb compared to the as-received state, while 

the “bad” specimen had sustained heavy permanent deformation 

varying between 0.110 and 0.160 in. with a near-fully collapsed 

bulb. 

A schematic of the flown seal layout is shown in figure 6. 

Note that the outboard direction refers to the direction pointing 

toward the wing tips of the vehicle relative to the door and is 

synonymous with the hinge line of the door. As shown in the 

figure, the seal was received in six sections. The forward and aft 

Figure 4.—Photograph comparing as-received Rev. P seal to asreceived Rev. M seal. Note the presence of Nomex fabric impregnation 

of Rev. M seal bulb. 

Rev. M Rev. P

“Bad” “Good” 

Figure 5.—Photograph of flown Rev. M seals removed from OV–103. 

Both seals show signs of permanent deformation, with the “bad” 

specimen showing heavy deformation and the “good” specimen 

showing light deformation. 
NASA/TM—2005-213860 5

Stationary lower 

loading platen

Movable upper 

loading platen

Seal specimen

Figure 7.—Photograph of compression specimen in 

contact with loading platens. 

sections were each a single length of seal while the inboard and outboard sections were each comprised of two 

lengths of seal. Also noted in figure 6 are the locations where the flow and compression specimens were removed 

from the seal. Flow specimens are denoted in red and compression specimens are denoted in green. Where 

necessary, the seal samples were trimmed to 5-in. lengths for compression tests and 12-in. lengths for flow tests. 

III. Experimental Setup 

A. Compression Tests 

1. Test Setup 

Room temperature compression tests were performed on samples of the MLG environmental seal to assess load 

versus compression behavior. The compression rig consists of a servohydraulic load frame, a high-temperature 

furnace (which was not used), and a laser extensometer (accuracy of ±0.00025 in.) and is described in-depth by 

Dunlap, et al. (ref. 1). Due to the relatively high loading and unloading rates, the laser extensometer could not be 

used and was monitored only during dwell segments. During the loading and unloading segments, a linear variable 

displacement transducer (LVDT) (accuracy of ±0.0022 in.) was used in place of the laser extensometer. A 500-lb 

load cell (accuracy of ±0.03 percent FS) was used to monitor the load applied to the seal. Samples of the seals were 

cut to 5-in. lengths and bonded to aluminum compression test plates according to a JSC-provided procedure which is 

also used to bond the environmental seal to the MLG door on Shuttle. 

Figure 7 depicts a seal compressed between the two loading platens. Prior to insertion of the seal specimens, 

parallelism of the upper and lower compression platens was adjusted to within 0.001 in. After the bonded seal 

specimens were placed on the stationary lower platen, the movable upper platen was brought into contact with the 

seal. For these tests, contact was defined using a 0.2 lbf 

preload applied to the seal. A set of gage pins with 

±0.0002-in. accuracy was then used to check the gap size 

(distance between surface of aluminum mounting plate 

and contact surface on upper platen), which was 

nominally measured to be 0.455±0.006 in. This 

corresponded to the bulb diameter of approximately 

0.430 in. plus 0.02 in. of room temperature vulcanized 

silicone rubber (RTV). 

2. Test Procedure 

As shown in table 2, initial exploratory tests were 

performed on the as-received Rev. P seals to assess the 

effects of loading rate, mounting plate configuration 

(groove or no groove), and amount of RTV behind the 

seal bulb. Figures 8 and 9 depict the different mounting 

plate configurations and amounts of RTV, respectively. 

These tests were conducted to better simulate the actual 

installation conditions of the MLG environmental seals 

and to provide a roadmap for the systematic tests 

performed later. 

Three types of systematic tests were conducted in 

the compression rig. Based on the results from the 

exploratory tests, the first systematic test compressed 

the seal to 63 percent compression at a loading rate of

TABLE 2.—EXPLORATORY TEST PARAMETERS 

Test 

name 

Plate 

configuration 

RTV 

amount 

Loading rate, 

in./sec 

MLG-RTC-P0.1a No Groove No fillet 0.002 

MLG-RTC-P0.2a 0.165-in. Groove 0.002 

MLG-RTC-P0.3a 0.002 

MLG-RTC-P0.4a 0.200 

MLG-RTC-P0.5a 0.200 

MLG-RTC-P0.6a Fillet 0.200 
NASA/TM—2005-213860 6

0.2 in./sec, held the compression for 1 min, and unloaded the seal. The cycle was then repeated once. The platen 

displacement and load were recorded during the test. 

The second type of test conducted in the compression rig was a 24-hr load decay test. In this test, the seal was 

compressed to 63 percent of the original bulb diameter. The compression was then held for 24 hr while the 

displacement and load were recorded. 

Finally, a 31-day uninstrumented compression test was conducted by first using a separate fixture. In this test, a 

5 in. length of Rev. P seal was compressed to 63 percent compression between two aluminum plates for 31 days. 

After 31 days, the seal was removed from the fixture and placed in the compression rig, where the load on the seal 

was measured at several discrete compression levels. The maximum load values at each compression level were then 

compared to the loads generated in an as-received seal under the same loading conditions. 

B. Flow Tests 

1. Test Setup 

Flow tests were conducted on the MLG environmental seals in order to understand the seal leakage rate as a 

function of seal construction and permanent deformation, pressure drop across the seal, and compression applied to 

the seal. Figure 10 shows the assembly of the flow test. The seal specimen was first bonded to a modular test 

cartridge which included a 0.250-in.-wide by 0.165-in.-deep groove adjacent to the seal bulb location to better 

simulate actual flight setup. The seals were cut and the ends were lightly sanded on a belt sander to achieve a final 

length of slightly over 12.000 (±0.030) in. This enhanced sealing of the ends against the aluminum spacer blocks, 

described below. The seal was bonded to the cartridge using the same procedure as for the assembly of the 

compression tests. The seal compression level was set by installing appropriately-sized spacer blocks onto the ends 

of the seal cartridge. RTV (RTV-577 with 3 percent DBTT catalyst) was dabbed onto the cut ends of the seals to 

minimize end leaks between the seal and the spacer blocks. The spacer blocks were then attached using a thin coat 

of the same RTV and were bolted down to the cartridge. To permit bonding of the seal ends to the spacer blocks, the 

seals were compressed using a pair of Plexiglas beams temporarily bolted to the end blocks. The RTV on the spacer 

blocks was then allowed to cure for ~3 to 5 hr. Faster cure times (e.g., 1.5 hr) could be achieved by placing the 

 (a) (b) 

Figure 8.—Photographs of mounted MLG environmental seal (a) without a groove and (b) with 

a groove in the aluminum mounting plate.

 (a) (b) 

Figure 9.—Photographs of MLG environmental seal (a) with a clean bond line (i.e., no RTV fillet) (b) 

and with an RTV fillet behind the seal bulb.
NASA/TM—2005-213860 7

Figure 10.—Assembly of Linear Flow Fixture. 

assembly on a hot plate (~130 °F). Post-installation analysis showed that the seal was well-bonded to the cartridge 

with an RTV thickness of ~0.020 in. 

Once the spacer blocks had cured, the compression level of the seal was determined. The height of the spacer 

blocks above the seal mounting surface was measured with a depth gage. From this measurement, the percent 

compression of the seal could be calculated by 

100 G g %Comp percent D

− = ×

where G is the gap size at 0 percent compression, g is the measured gap size of a given test setup, and D is the nominal 

bulb diameter (0.430 in.). The gap for zero percent compression was estimated by observing when the amount of light 

passing between the seal bulb and a long bar (supported by appropriately sized gage pins) was minimal. 

The cartridge assembly was then installed in the linear flow test fixture as shown in figure 10. An elastomeric 

o-ring sealed the perimeter of the cartridge (fig. 10 (a)), preventing leakage from passing around and behind the 

cartridge. A second o-ring sealed below the cover plate and partially through the spacer blocks (fig. 10(b)), ensuring 

that the primary leakage path was across the test specimen. Small metal-to-metal contacts existed between spacer 

blocks and the cover plate where the o-ring terminated. This junction was sealed with a small amount of RTV which 

was allowed to cure before testing commenced. The cover plate was then bolted onto the fixture. 

2. Test Procedure 

Figure 11 shows a cross-section schematic of the test setup. As observed in this figure, the gap setting 

established the amount of compression. The seals were installed on a flat plate which simulated the actual 

configuration of the MLG door seal. At each compression level, the seal was tested in two orientations. In the first 

Flow

Cartridge

Cover plate

Gap

 

Flow

Cartridge

Cover plate

(a) (b) 

Figure 11.—Schematic of flow test setup for MLG door seal simulating (a) the bulb-first descent case 

and (b) the tail-first ascent case for the Shuttle Orbiter.
NASA/TM—2005-213860 8

orientation, the seal was pressurized from the bulb side (fig. 11(a)) which simulated the pressurization imposed on 

the seal during Orbiter descent (i.e., high pressure outside of vehicle). The entire cartridge was then rotated 180° and 

pressurized from the tail side (fig. 11(b)) to simulate the Orbiter ascent case (i.e., high pressure in MLG bay). 

The data gathered during the tests included the differential pressure across the seal, the flow rate across the seal, 

and the inlet air temperature to the plenum. The differential pressure across the seal was measured by a Setra model 

239 5-psig transducer (accuracy of ±0.14 percent FS). Since the tests were performed in ambient conditions, the 

differential pressure across the seal was measured in psig. A bank of Hastings Model HFM-200 (±1 percent fullscale) series flow meters were used to measure the flow through the fixture with ranges of 10 SLPM (0.35 SCFM), 

100 SLPM (3.5 SCFM), 750 SLPM (26.5 SCFM), and 3000 SLPM (106 SCFM). Only one flow meter was used at 

any given time. A data point was taken before pressure was applied to the seal in order to determine the zero-offsets 

of the differential pressure transducer and the flow meter. Then, the pressure was increased in steps of 0.1 psig up to 

a maximum pressure of 2.0 psig. At each pressure step, the pressure and flow rate were allowed to stabilize, after 

which a data point was recorded. Leakage rates were calculated using a 12 in. seal length. The downstream pressure 

was ambient room pressure for all tests. 

Many tests had sufficiently high flow rates to require switching to different flow meters while the tests were in 

progress. When a switch to a different flow meter was required, the supply pressure to the fixture was shut off and 

the fixture was allowed to depressurize. Then, the active flow meter was changed by opening and closing relevant 

valves. A zero-offset of the new flow meter was recorded, and the test was then allowed to proceed from the last 

recorded pressure step. 

A total of six seal specimens were tested in the flow rig as part of this study and are described in table 3. Rev. P 

seals MLG-RTF-P3 and MLG-RTF-P4 were from the same manufacturing lot (lot -0019) while Rev. P seal MLGRTF-P5 was from a separate lot (lot -0021). Table 4 presents the test matrix for the as-received Rev. P seals. Note 

that the compressed height also includes the 0.02-in. thickness of the RTV. 

TABLE 3.—FLOW TEST IDENTIFICATION AND INFORMATION 

Seal 

identification 

Seal 

revision Description 

MLG-RTF-P3 Rev. P Manufacturing lot #024025753450019 

MLG-RTF-P4 Rev. P Manufacturing lot #024025753450019 

MLG-RTF-P5 Rev. P Manufacturing lot #024025753450021 

MLG-RTF-M1 Rev. M “Good” flown seal removed from OV-103 

MLG-RTF-M2 Rev. M “Bad” flown seal removed from OV-103 

MLG-RTF-F1 Rev. M As-received seal tested earlier and included for comparison purposes 

TABLE 4.—GAP SIZE AND ASSOCIATED SEAL COMPRESSION FOR 

REV. P. THE COMPRESSED HEIGHT INCLUDES THE THICKNESS 

OF THE RTV BONDING LAYER (0.020 IN.). 

Compressed height, 

in. 

Nominal compression,

 percent 

0.452 0.0 

0.441 2.5 

0.430 5.0 

0.410 10 

0.387 15 

0.365 20 

0.345 25 

0.270 42 

0.195 60 
NASA/TM—2005-213860 9

IV. Results 

A. Compression Results 

1. Exploratory Tests 

Figure 12 presents the first and second loading cycles of the exploratory test on the Rev. P seal test MLG-RTCP0.3a, the case with an included groove, no RTV fillet, and a loading speed of 0.002 in./sec. Note that the arrows in 

the figure indicate the loading and unloading portions of the curve. Five photographs of the seal under compression 

are displayed in the graph with lines pointing to the reference compression level for each photograph. Of note in the 

photographs is the buckling-like behavior along the top of the seal bulb. The center of the upper portion of the bulb 

moved away from the platen at high compression levels, forming a “kidney-like” shape. Figure 12 is representative 

of the first five exploratory tests listed in table 2, with these five tests demonstrating nearly identical behavior. The 

increased loading rate and the inclusion of the groove in the aluminum did not affect the load versus compression 

behavior of the seal. 

The sixth test listed in table 2 demonstrated divergence from the other tests and is shown in figure 13 along with 

photographs of the seal under various levels of compression. The RTV fillet restricted movement of the front wall of 

the seal, which maintained a more vertical position and did not roll over until the seal had reached a higher 

compression level than was seen for the other exploratory tests. When the bulb approached a fully collapsed state 

(e.g.. roughly 55 percent compression), the RTV fillet appears to have restricted the buckling behavior of the top 

surface of the bulb, and the top of the bulb maintained better contact with the platen than for cases without the RTV 

fillet. However, the presence of the RTV fillet increased the loads generated by the seal at all compression levels. 

The largest difference occurred at a compression of 40 percent where the seal with the RTV fillet generated a load 

three times higher than a seal without a fillet. 

Figures 12 and 13 both demonstrate a drop-off in load in the loading portion of the second cycle when compared 

to the first cycle and show signs of permanent deformation in the seal bulbs. The second loading cycle of figure 12 

shows no applied load until roughly 2.5 percent of the original seal bulb diameter. The second loading cycle of 

figure 13 demonstrates similar behavior until a compression of 5 percent is attained, followed by a larger reduction 

in second cycle load throughout the remainder of the loading portion of the cycle. Even though the overall loading 

for the second cycle of the seal with the RTV fillet is lower than the first cycle, the general shape of the loading 

curve is maintained. This is demonstrated by a slight increase in the slope of the curve at approximately 30 percent 

compression. 

2. Single Compression Tests 

Figure 14 presents the loading portion of the first compression cycle for various seals, including an as-received 

Rev. M seal, “good” and “bad” flown Rev. M specimens, and a single Rev. P seal. The as-received Rev. M data is 

plotted twice: first with as-measured data, and second with the curve shifted 0.035 in. to the right (i.e., higher 

compression), which is equal to the amount of permanent deformation measured in the “good” flown specimen. 

During compression testing of the “good” flown specimen, the loading platen was not in contact with the seal during 

the first 0.035 in. of compression because of the permanent deformation of the seal. The shift in the as-received data 

simulates the effect of an out-of-contact condition for the seal. 

Several relationships between the as-received and flown Rev. M seals can be determined from figure 14. First, 

both flown specimens generated lower loads than the as-received Rev. M specimen, demonstrating a decreasing load 

with increasing permanent deformation. The “bad” flown seal generated lower loads than the “good” seal, although 

the “bad” seal had a higher rate of load increase at high compression levels (>0.265 in. compression) than the 

“good” seal. This sudden increase in load generation from the “bad” seal illustrates the second point from figure 14, 

which is that the “bad” specimen exhibited a dissimilar loading curve when compared to the other seal specimens. 

This behavior may have been due to the near-fully collapsed bulb of the “bad” flown specimen. If the “bad” seal 

bulb closed almost immediately after contact with the test platen, compression of the silicone rubber material would 

have been the primary loading mechanism instead of compression of the bulb structure, ultimately leading to higher 

loading rates. Finally, figure 14 illustrates similarity between the as-received data shifted by 0.035 in. and the 

“good” flown seal data. The lightly damaged seal generated loads similar to the as-received seal after the damaged seal 

was placed under compression. The primary difference between the as-received seal and a lightly damaged seal was the 

lightly damaged seal’s initial lack of contact between the loading platen and the seal bulb due to permanent set. 

In addition to the Rev. M seals described above, figure 14 also includes data from an as-received lot -0019 Rev. P 

specimen. The as-received Rev. P seal had a similar loading curve as the “good” flown Rev. M seal specimen, both 

of which generated lower loads than the as-received Rev. M specimen. The as-received Rev. M seal generated five 

times more load than the as-received Rev. P seals at 63 percent compression. The presence of a Nomex fabric
NASA/TM—2005-213860 10

0

2

4

6

8

10

12

14

16

18

20

0% 10% 20% 30% 40% 50% 60% 70%

Compression

Load per inch of seal [lbf/in]

Cycle 1

Cycle 2

Figure 12.—First two cycles of exploratory load versus compression results of Rev. P seal (Test 

MLG-RTC-P0.1a): groove in mounting plate, 0.002 in./sec loading speed, no RTV fillet. 

0

5

10

15

20

25

30

35

40

45

50

0% 10% 20% 30% 40% 50% 60% 70%

Compression

Load per inch of seal [lbf/in]

Cycle 1

Cycle 2

Figure 13.—First two cycles of exploratory load versus compression results of Rev. P seal (Test 

MLG-RTC-P0.1f): groove in mounting plate, 0.200 in./sec loading speed, included RTV fillet.
NASA/TM—2005-213860 11

impregnation of the silicone rubber material in the Rev. M seals is the most likely cause for the higher load 

generation of the Rev. M seal. Based on this data, as-received Rev. P seals can be expected to generate lower loads 

than as-received Rev. M seals previously used on the Orbiter. 

3. 24-hour Test 

Figure 15 shows the load versus time plot for 24-hr compression tests of the “good” and “bad” flown Rev. M 

seals, an average curve of three as-received Rev. P lot -0019 seals, and a curve from a single as-received Rev. P lot 

-0021 seal. The seals were compressed to a level of 63 percent of the as-received diameter during the duration of the 

test. 

The “good” flown Rev. M seal exhibited the highest loads for the duration of the 24-hr test, followed by the 

as-received Rev. P seals, and finally by the “bad” flown Rev. M seal. The “bad” flown Rev. M seal exhibited an 

initial increase in load followed by a drop-off shaped differently than the other seals, which may have been due to 

drift in the actuator amounting to approximately 0.001 in. For the other three seals, the load decreased by ~20 to 

25 percent during the duration of the test. Most of the drop in the load occurred during the first hour, during which 

the load dropped by roughly 15 to 20 percent. 

Of note is the small drop-off in load in the Rev. P lot -0021 seal between 4 and 5 hr into the test. During this 

time, the temperature in the laboratory dropped slightly, resulting in a thermal contraction of the load train and a 

subsequent drop in load. The temperature in the lab was brought back to nominal conditions when this drop was 

recognized and the test proceeded similarly to the other tests (except for the “bad” Rev. M seal). 

4. 31-day test 

For the 31-day compression test, an as-received Rev. P lot -0019 seal was held at 63 percent compression for 

31 days in a static, uninstrumented test fixture. It was then removed from the test fixture and the bulb was measured 

to be permanently deformed by ~0.050 in., which was equivalent to a compression level of 12 percent. 

Figure 16 presents a bar chart comparing the peak loads of the first two cycles of an as-received Rev. P seal to 

the first two loading cycles of the post-31-day test seal for various compression levels. The 31-day test seal 

generated no load for compressions of 10 percent and lower; this is an expected result because the seal took on 

approximately 12 percent permanent deformation while it was compressed for 31 days. Upon contact with the platen 

0

5

10

15

20

25

30

0.00 0.05 0.10 0.15 0.20 0.25 0.30

Compression [in]

Load [lbf/in]

Rev. M (as-received)

Rev. M (as-received, shifted 0.035 in.)

Rev. M (flown, good)

Rev. P lot -0019 (as-received)

Rev. M (flown, bad)

Rev. M (as-received)

Rev. M (as-received, shifted 0.035 in.)

Rev. M (flown, good)

Rev. P lot -0019 (as-received)

Rev. M (flown, bad)

Figure 14.—Load versus compression data for as-received Rev. M and Rev. P seals as well as flown Rev. 

M seals. The as-received Rev. M data is plotted twice: once in an as-measured state, and once with its 

curve shifted 0.035 in. to the right (i.e., higher compression).
NASA/TM—2005-213860 12

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25

Time [hrs]

Load per inch of seal [lbf/in]

Rev P (lot 0019 avg)

Rev P (lot 0021)

Rev M (flown, good)

Rev M (flown, bad)

Temporary temp. drop in lab resulted in load train

shrinkage and subsequent drop in load

Rev. P lot -0019

Rev. M (flown, good)

Rev. P lot -0021

Rev. M (flown, bad)

Figure 15.—Load versus time plot of 24 hr compression tests on as-received Rev. P and flown Rev. M 

seals. The seals were held at 63 percent compression for the duration of the test.

0

2

4

6

8

10

12

14

16

18

20

5% 10% 15% 20% 37% 55% 63%

Compression

Peak load per inch of seal [lbf/in]

As-received Cycle 1

As-received Cycle 2

31-day Cycle 1

31-day Cycle 2

Figure 16.—Peak loads versus compression of first and second incremental loading cycles of 

as-received and 31-day Rev. P seals. 
NASA/TM—2005-213860 13

and throughout the remainder of the test, the 31-day test seal generated loads lower than those generated by the 

as-received seal. At the highest compression level of 63 percent, the 31-day test seal generated loads amounting to 

~80 percent of those generated by the as-received specimen. 

For all compression levels, the change in load between the two loading cycles was less for the 31-day test seal 

than for the as-received specimen. This may be due to the permanent set already present in the 31-day specimen. 

From figure 13, it is known that much of the permanent deformation of a seal occurs during the first hour of 

compression. Figure 16 appears to indicate that a seal with some permanent deformation takes on additional 

permanent deformation more slowly than a seal in the as-received state. 

B. Flow Results 

Figure 17 presents a sample data set of flow versus pressure differential across the seal from Rev. P seal MLGRTF-P3 in both the bulb-first (i.e., pressurized from the bulb side) and tail-first (i.e., pressurized from the tail side) 

orientations with a nominal gap size of 0.387 in. and a measured compression of 15.8 percent (nominal compression 

was 15 percent). The seal in the bulb-first orientation demonstrated a concave-down shape in its flow versus 

pressure curve, indicating an improvement in seal performance with increasing differential pressure. Conversely, the 

leakage across the seal in the tail-first orientation was measured to have a concave-up shape with increasing 

pressure, indicating a reduction in seal performance with increasing differential pressure. 

Figure 18 presents flow versus compression data for the three Rev. P seals and an as-received Rev. M seal in the 

bulb-first orientation at a differential pressure of 1 psig. This pressure was selected because it matches the 

differential pressure used in the certification documents to specify the maximum seal leakage rate (e.g., 3 SCFM/ft 

at 1 psid). Note that the as-received Rev. M seal has fewer data points than the other seals because it was tested 

before all of the desired compression levels had been defined. A common feature among the Rev. P seals was the 

plateau or increase in flow rate between 10 and 25 percent compression. The Rev. M curve did not exhibit this 

plateau or increase in the flow rate and maintained a lower flow rate in this compression region than two of the three 

Rev. P seals. However, the four seals demonstrated similar leakage responses at high compressions (>40 percent). 

Regardless of the shapes of the flow curves, the flow across all of the seals was universally below the certified 

allowable flow rate of 3 SCFM/ft for a differential pressure of 1 psid. 

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

∆P [psig]

Flow per foot of seal [SCFM/ft]

Bulb-first

Tail-first

Figure 17.—Flow versus ∆P data for Rev. P seal MLG-RTF-P3 at 15.8 percent compression, pressurized 

from both the bulb side and the tail side. 
NASA/TM—2005-213860 14

Tail-first flow data from the Rev. P seals at a differential pressure of 1 psig is plotted in figure 19 on a 

logarithmic scale. Note that the as-received Rev. M seal was tested before tail-first tests were considered, and thus 

no data is available for this seal. The tail-first flow rates were higher than the bulb-first flow rates (fig. 18) for equal 

compression levels except for a compression of 60 percent, where seals in the two orientations were measured to 

have approximately the same flow rate. Seals in the tail-first orientation demonstrated a plateau or small increase in 

the flow rate between 10 and 25 percent compression, although to a lesser magnitude than the bulb-first cases in 

figure 18. The Rev. P seals in the tail-first orientation met the certified allowable flow rate (3 SCFM/ft at 1 psid) for 

compressions greater than 2.5 percent. 

Figure 20 presents flow results for the “good” flown Rev. M seal as well as an as-received Rev. M seal. The 

“good” flown seal was tested with a pressure drop of 1 psig in both the bulb-first and tail-first orientations, and the 

as-received Rev. M flow data is identical to that shown in figure 18. Note that the results are presented in terms of 

compressed height (i.e., final compressed height of the seal bulb) instead of percent compression. This is because 

percentage compression is defined in terms of an as-received seal bulb diameter, whereas the flown seals exhibited 

varying degrees of deformation. The compressed height axis is reversed from the percent compression axis in that a 

low percent compression equates to a high compressed height. The “good” flown seal in the bulb-first and tail-first 

orientations produced comparable flow rates except for a compressed height of 0.430 in., where the flow rate across 

the seal in the tail-first orientation was higher than that for a seal in the bulb-first orientation. In addition, the “good” 

flown seal had comparable flow performance to the as-received specimen. 

Figure 21 presents flow results for the “bad” flown Rev. M seal. The “bad” specimen demonstrated flow rates 

much higher than the either of the “good” flown seal or the as-received Rev. M seal. The flow rates across the “bad” 

flown seal were comparable for the two orientations at a compressed height of 0.195 in., while the tail-first 

orientation produced a higher flow rate than the bulb-first orientation at a compressed height of 0.270 in. 

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0% 10% 20% 30% 40% 50% 60% 70%

Compression

Flow per foot of seal [SCFM/ft]

Rev. P MLG-RTF-P3 (lot -0019)

Rev. P MLG-RTF-P4 (lot -0019)

Rev. P MLG-RTF-P5 (lot -0021)

Rev. M MLG-RTF-F1 (as-received)

∆P = 1 psig

Figure 18.—Flow versus percent compression for the as-received Rev. P seals as well as as-received 

Rev. M seal at ∆P of 1 psig in the bulb-first orientation. 
NASA/TM—2005-213860 15

0.001

0.010

0.100

1.000

10.000

100.000

0% 10% 20% 30% 40% 50% 60% 70%

Compression

Flow per foot of seal [SCFM/ft]

Rev. P MLG-RTF-P3 (lot -0019)

Rev. P MLG-RTF-P4 (lot -0019)

Rev. P MLG-RTF-P5 (lot -0021)

∆P = 1 psig

Figure 19.—Flow versus percent compression for as-received Rev. P seals at ∆P of 1 psig in the 

tail-first orientation. Flow values are plotted on a logarithmic scale. 

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.15 0.20 0.25 0.30 0.35 0.40 0.45

Compressed Height [in]

Flow per foot of seal [SCFM/ft]

"Good" seal, bulb-first

"Good" seal, tail-first

As-received Rev. M, bulb-first

∆P = 1 psig

Figure 20.—Flow versus compressed height for “good” flown Rev. M seal (bulb-first and tail-first) as 

well as as-received Rev. M seal for 1-psig pressure drop. 
NASA/TM—2005-213860 16

Flow data could not be measured for the “bad” flown seal at compressed heights greater than 0.270 in. due to an 

open gap between the seal and the cover plate. For a compressed height of 0.343 in., supply pressures below 0.5 psig 

generated flows consistent with a small open gap. When the supply pressure was increased above 0.5 psig, the seal 

bulb suddenly inflated and closed the gap. The supply pressure immediately climbed to over 2 psig and was 

accompanied by a reduction in the flow rate. When the supply pressure was subsequently reduced below 2 psig, the 

seal bulb suddenly deflated and was accompanied by an immediate drop in supply pressure to below 0.5 psig. The 

pressure drop was followed by an increase in flow rate consistent with a small open gap. Thus, a pressure of 1 psig 

could not be applied across the seal for a compressed height of 0.343 in. At higher compressed heights, an open gap 

existed between the seal and the cover plate for all differential pressures with resulting flow rates greater than the 

capabilities of the flow meters (e.g., >106 SCFM). 

V. Discussion of Seal Bulb Inflation 

As mentioned above, the “bad” flown seal in the bulb-first orientation was observed to exhibit bulb inflation, 

where the seal bulb expanded to fill an open gap for particular compressed height and pressure combinations. 

Similarly, other seals in the bulb-first orientation appear to have an increasing sealing effectiveness with increasing 

pressure, while seals in the tail-first orientation appear to have a decreasing sealing effectiveness with increasing 

pressure. These effects appear to be related to the same mechanism. 

The increase in effectiveness of all Rev. P seals in the bulb-first orientation and the sudden inflation of the “bad” 

Rev. M seal is hypothesized to be due to the presence of vent holes in the forward side of the seal bulbs. When a seal 

is oriented bulb-first, the vent holes are located near the stagnation point of the flow. The stagnation pressure inflates 

the seal because the pressure between the seal and its working surface is less than the internal bulb pressure. This 

increases the seal footprint against its working surface and enhances the seal’s flow blocking capability. Conversely, 

a tail-first orientation aligns the vent holes away from the high pressure, keeping the internal bulb pressure low 

(perhaps slightly below ambient) and preventing bulb inflation. If the seal is not sufficiently compressed, the bulb 

may deflate (and possibly “bend over”) and open a gap between the seal and its working surface. This effect is seen 

in figure 21 for the 1-psig flow result from the “bad” seal specimen at a compressed height of 0.270 in. The increase 

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.18 0.19 0.20 0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28

Compressed Height [in]

Flow per foot of seal [SCFM/ft]

"Bad" seal, bulb-first

"Bad" seal, tail-first

∆P = 1 psig

Figure 21.—Flow versus compressed height for “bad” flown Rev. M seal (bulb-first and tail-first) 

for 1-psig pressure drop. 
NASA/TM—2005-213860 17

in flow rate compared to the other results is likely due to the gap opening up under the loads imposed by the high 

differential pressure. 

Seal bulb pressurization explains several observed patterns in the flow data. The bulb inflation hypothesis 

predicts that the seal should become more effective at high pressures in a bulb-first orientation and less effective at 

high pressures in a tail-first orientation. Figure 17 demonstrates concave-down behavior for the bulb-first orientation 

indicating an improvement in seal performance with increasing pressure, and it demonstrates concave-up behavior 

for the tail-first orientation which is indicative of decreasing seal performance with increasing pressure. 

Furthermore, the tail-first flow data in figure 19 at low (<10 percent) compressions demonstrates flow rates orders of 

magnitude higher than the corresponding bulb-first cases in figure 18, providing experimental verification that the 

presence of downstream (i.e., on the low pressure side) vent holes play a role in opening the gap between the seal 

and its working surface. The sudden expansion of the bulb-first “bad” flown Rev. M seal bulb provides a directlyobserved instance of bulb inflation. 

VI. Implications for Return to Flight Effort 

As mentioned previously, the investigations of the MLG seals were initiated to assist JSC and KSC engineers in 

closing the OV–103 MLG doors. After careful inspection of the seals on OV–103 revealed that they had become 

permanently deformed and no longer met requirements for minimum seal height, the old seals (Rev. M) were 

removed and new (Rev. P) seals were installed. In an effort to achieve seal compression requirements, constantthickness shims were added to the MLG door sealing surface. However, these changes to the seals generated loads 

in the MLG door rigging hardware which approached maximum loading limits. 

Results from the exploratory compression tests performed at GRC suggested steps to be taken to facilitate the 

closure of the MLG doors. Seals with an extra fillet of RTV along the bond line below the bulb demonstrated loads 

up to three times greater than those for seals with a clean bond line (i.e., no RTV fillet), as shown in figures 12 and 

13. The presence of the RTV fillet prevented the “rolling over” of the front of the seal bulb material and reduced the 

magnitude of the buckling behavior observed at the top of the seal bulb. 

In response to the GRC exploratory test results, engineers at KSC removed the excess RTV fillet from the bulbs 

of the newly-installed Rev. P MLG door environmental seals on OV–103. This allowed the closure of the starboardside MLG door, although the port-side door was still unable to fully close. KSC engineers then shortened the door 

retract link on the door rigging to increase the preload on the retraction mechanism without overloading it. 

Additionally, custom-height shims were added to seal locations where seal compression levels were measured to be 

below certified limits. The combination of the removal of RTV, increased preload in the door retract link, and the 

addition of shims only where the seal compression was below certified limits allowed the closure of the port-side 

MLG door. 

For future flights, seal compression will be monitored between flights to ensure that the seals have not taken on 

an excessive permanent deformation. Installation procedures for future missions were also amended to include the 

removal of excess RTV from under the seal bulb. 

VII. Conclusions 

1. The presence of an RTV fillet under the seal bulb resulted in increased seal loads compared to a seal 

without a fillet at the same compression level. Based on this information, KSC engineers removed excess 

RTV from under the newly installed seal bulbs, which immediately allowed one of the two MLG doors to 

fully close. Closure of the port-side door was achieved after preload adjustment of the door retract link 

mechanism. 

2. As-received Rev. M seals were up to five times stiffer than the Rev. P seals. This was most likely due to the 

presence of an impregnated Nomex mesh in the Rev. M seals that was not present in the Rev. P seals. 

3. Rev. P seals loaded for 24 hr at 63 percent compression demonstrated a 20 to 25 percent decrease in load 

compared to the initial peak load. During the first hour of compression, 15 to 20 percent of the initial peak 

load was lost. A Rev. P seal held at 63 percent compression for 31 days took on 0.050 in. (12 percent) of 

permanent bulb deformation. Incremental loading tests on this seal revealed that it generated loads 

80 percent of those of an undamaged specimen at equal compressions. 

4. The new Rev. P seals met the certified seal flow requirements of 3 SCFM/ft at a differential pressure of 

1 psid. In the bulb-first orientation (i.e., simulating reentry pressurization), the seal leakage was less than 

the limiting flow rate for all compressions including cases with 0 percent compression (i.e., very light 

contact). The tail-first orientation (i.e., simulating Orbiter ascent pressurization) produced leakages below 

the certified limits for compressions greater than 2.5 percent. 
NASA/TM—2005-213860 18

5. As-received Rev. P seals demonstrated flow blocking characteristics comparable to those measured from 

tests on as-received Rev. M seals. The primary difference between the two seals was that the Rev. P seals 

exhibited either a plateau or an increase in the flow rate between 10 and 25 percent compression, while a 

Rev. M seal did not exhibit this behavior. 

6. Seal bulb inflation occurred when the seal was oriented in the bulb-first orientation (i.e., pressurized from 

the bulb side and simulating reentry pressurization), which improved seal leakage performance. Inflation 

and subsequent reduction in leakage rate was not observed for seals in the tail-first orientation (i.e., 

pressurized from the tail side and simulating orbiter ascent pressurization). 

Reference

1. Dunlap, Patrick H. Jr., Steinetz, Bruce M., DeMange, Jeffrey J., and Taylor, Shawn C., “Toward an Improved 

Hypersonic Engine Seal,” NASA TM–212531, July, 2003. 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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AIAA–2005–4155

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WBS–22R–617–44–0376–01

24

Investigations of Shuttle Main Landing Gear Door Environmental Seals

Joshua Finkbeiner, Pat Dunlap, Bruce Steinetz, Jeff DeMange, and

Daniel Newswander

Sealing; Seals (stoppers); Spacecraft structures; Leakage reduction; Space shuttle orbiters;

Landing gear; Doors; Reentry shielding

Unclassified -Unlimited

Subject Category: 37

Prepared for the 41st Joint Propulsion Conference and Exhibit cosponsored by the AIAA, ASME, SAE, and ASEE,

Tucson, Arizona, July 10–13, 2005. Joshua Finkbeiner, Pat Dunlap, and Bruce Steinetz, NASA Glenn Research Center;

Jeff DeMange, University of Toledo, 2801 W. Bancroft Street, Toledo, Ohio 43606; and Daniel Newswander, NASA

Johnson Space Center. Responsible person, Joshua Finkbeiner, organization code RSM, 216–433–6080.

The environmental seals for the main landing gear doors of the shuttle orbiters were raised by the Columbia Accident

Investigation Board as a potential safety concern. Inspections of seals installed on the Shuttle Discovery revealed that

they were permanently deformed and no longer met certified seal compression requirements. Replacement of the seals

led to the inability to fully close the main landing gear doors. Johnson Space Center requested that Glenn Research

Center conduct tests on the main landing gear door environmental seals to assist in installing the seals in a manner to

allow the main landing gear doors to fully close. Further testing was conducted to fill out the seal performance database.

Results from the testing indicated that the method of bonding the seals was important in reducing seal loads on the main

landing gear doors. Also, the replacement seals installed in Shuttle Discovery were found to have leakage performance

sufficient to meet the certification requirements.