Nicholas G. Garafolo and Christopher C. Daniels

The University of Akron, Akron, Ohio

An Experimental Investigation of Leak Rate 

Performance of a Subscale Candidate 

Elastomer Docking Space Seal

NASA/CR—2011-216829

November 2011

AIAA –2010–6907
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Nicholas G. Garafolo and Christopher C. Daniels

The University of Akron, Akron, Ohio

An Experimental Investigation of Leak Rate 

Performance of a Subscale Candidate 

Elastomer Docking Space Seal

NASA/CR—2011-216829

November 2011

AIAA –2010–6907

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared under Contract NNC08CA35C

Prepared for the

46th Joint Propulsion Conference and Exhibit

cosponsored by AIAA, ASME, SAE, and ASEE

Nashville, Tennessee, July 25–28, 2010
Acknowledgments

The authors greatly appreciate the contributions of Richard Tashjian. The material is based upon work supported by the 

National Aeronautics and Space Administration under Contract NNC08CA35C.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

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Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by NASA technical management.
An Experimental Investigation of Leak Rate

Performance of a Subscale Candidate Elastomer

Docking Space Seal

A novel docking seal was developed for the main interface seal of NASA’s Low Impact

Docking System (LIDS). This interface seal was designed to maintain acceptable leak rates

while being exposed to the harsh environmental conditions of outer space. In this experimental evaluation, a candidate docking seal assembly called Engineering Development

Unit (EDU58) was characterized and evaluated against the Constellation Project leak rate

requirement. The EDU58 candidate seal assembly was manufactured from silicone elastomer S0383-70 vacuum molded in a metal retainer ring. Four seal designs were considered

with unique characteristic heights. The leak rate performance was characterized through

a mass point leak rate method by monitoring gas properties within an internal control

volume. The leakage performance of the seals were described herein at representative

docking temperatures of -50, +23, and +50◦C for all four seal designs. Leak performance

was also characterized at 100, 74, and 48% of full closure. For all conditions considered,

the candidate seal assemblies met the Constellation Project leak rate requirement.

Nomenclature

a Regression coefficient

AO Atomic oxygen

B Bias error

CBM Common Berthing Mechanism

CV CM Collected volatile condensable materials

EDU58 Engineering Development Unit

h height

LIDS Low Impact Docking System

NASA National Aeronautics and Space Adminstration

m Mass

p Pressure

P Precision error

R Specific gas constant

RT D Resistance temperature device

t Time

TML Total mass loss

T Temperature

U Uncertainty

UV Ultraviolet

V Volume

x bulb location

Subscript

0 zeroth order

1 first order

bulb bulb

m ̇ Mass Leak Rate

I. Introduction

T

he National Aeronautics and Space Adminstration (NASA) developed a new docking system for spacecraft. This system, called the Low Impact Docking System (LIDS), was expected to provide the interface

between pressurized manned and autonomous vehicles.1 Novel elastomer seals were developed for the main

interface seal of LIDS. The characterization of developmental seals was necessary to ensure the safety of both

astronauts and spacecraft equipment.

Nicholas G. Garafolo and Christopher C. Daniels 

The University of Akron 

Akron, Ohio 44325 

NASA/CR—2011-216829 1
LIDS has three large seal interfaces: the bottom-tunnel seal, the mid-tunnel seal, and the main interface

docking seal. Illustrations of the seal locations are shown in Figure 1. One seal was located the bottom

of the docking system and provides the seal for joining the LIDS to the vehicle. The mid-tunnel seal was

integral to the LIDS and joins two components of the LIDS. The seal at the top of the LIDS was the main

interface seal and provides the interface for attaching one LIDS equipped vehicle to another LIDS equipped

vehicle.

Figure 1. Illustration of the Low Impact Docking System

The main interface seal assembly, located at the top of the approximate 58 in. diameter docking system, was a quasi-static counter-face seal. This interface is designed to be joined and separated when the

spacecraft docks and undocks during its mission. The seal must remain operational through repeated docking/undocking operations while also being capable of withstanding the operating load, pressure, and temperature environments.

Each of the three seals was a vacuum seal and must inhibit transport of atmospheric pressure air from

within the spacecraft to its exterior (i.e., the seals must prevent the loss of air from the inside diameter of the

seal while the exterior of the seal was exposed to the vacuum pressure of space). The maximum allowable

leak rate of the LIDS main interface seal was 0.0025 lbm,air per day. This volume of air (0.0025 lbm,air)

was equivalent to approximately 80 in.3 at standard temperature and pressure. For a 58 in. diameter seal,

the maximum allowable leak rate scales to 1.4E-5 lbm,air

day−in. . With the very low value of allowable leak rate,

permeation of gas through the seal material was of concern and was minimized. As the LIDS and its main

interface seal operate in a vacuum pressure environment, all materials must conform to NASA-STD-(I)-

6016.2 This standard mandates that outgas byproducts be limited to less than 1.0% total mass loss (TML)

and less than 0.1% collected volatile condensable materials (CVCM) when exposed to heat and vacuum

pressures, as tested following the ASTM E595-076.3

The amount of force required to fully compress the LIDS main interface seal must be less than 140 lbf

per linear inch of seal. This requirement was dictated by the latching capacity of the docking system. The

mid-tunnel and bottom-tunnel seals are not held to these requirements. The LIDS main interface seal must

be able to accommodated 80 docking and undocking cycles during its lifetime.

Elastomer seals provide a means to create a joint that has very low leak rates across the seal while

maintaining the ability to be used repeatedly. Additionally, elastomer seals can be relatively low cost

compared with other types of seals (e.g., metal seals). As the LIDS was developed to be used while in lowEarth orbit, during Trans Lunar travel, and while orbiting the Moon, the seal assemblies must be compatible

with a wide range of environments. Atomic oxygen (AO) is highly reactive species and is present in low-Earth

orbit. AO reacts with the surface of an elastomer seal and can embrittle the surface.

Ultraviolet (UV) radiation emitted directly from the Sun, combined with the albedo from the surface of

the Earth and Moon, penetrates the bulk solid of an elastomer seal and breaks the long flexible molecular

chains. The resultant short inflexible molecules tend to be brittle and can lead to cracking. The vacuum

pressure can evolve the elastomer compound and continually change its properties with exposure. The

NASA/CR—2011-216829 2
environmental temperature of the LIDS was expected to be -100 to +125◦C. The seal would not be required

to seal across the entire exposure temperature, but it would be expected to operate across a -50 to +50◦C

temperature range. These requirements and exposure environments severely limit the choices of compounds

suitable for docking seal applications. Therefore, the decision was made to pursue the development of

silicone elastomer compound seals. These types of seals are capable of achieving very low leak rates when

used as counter-face seal. They are compatible with the space environment for their expected lifetime. Select

compounds are able to meet the low outgassing specifications, while most silicones can withstand the thermal

environment.

The choice to use silicone, however, has the negative aspect of increased permeation through the material.

Silicone is known to have a high permeation rate with respect to other typical seal material compounds.

This increase in the permeation is mitigated through design.

One candidate seal is the Gask-O-Seal
R , manufactured by Parker Hannifin Corporation. Gask-O-Seals
R

are a composite design; the elastomer compound was vacuum molded into an aluminum retainer. The seal

was held into position using fasteners mounted through the metal member of the composite seal, see Figure

2.

Figure 2. Illustration of EDU58 series cross section.

Experimental studies have been conducted on Gask-o-Seals
R with several differing geometries and elastomer compounds, aimed at understanding the behavior of the elastomer and the leak rates corresponding to

the different operating conditions that may occur during the seals lifetime. Subscale versions of the Common

Berthing Mechanism (CBM) and initial Engineering Development Unit (EDU54) have been experimentally

evaluated by Smith.4It has been shown that the leak rate of elastomer seals depends greatly on temperature.

An increase increase in temperature results in an increase in leak rate. Previous docking seals, such as those

used on the CBM, have met the maximum leak rate requirement. Examinations have also been conducted

to characterize the leak rate and adhesion of elastomer after exposure to AO and UV radiation.5 Leak rate

dependencies on closure level have also been studied by Smith.4 An increase in the level of closure (i.e.,

incomplete compression) increases the sealing performance of the candidate seal. In addition to the subscale

studies noted above, near full-scale seals have been characterized and behaved similarly.6–8

The primary mechanism of air leakage from the main interface docking seal was permeation, where the

largest contributing leakage was found through the seal rather than flow through the seal/counter-face mating

interface.4 Permeability is the measure of momentum transport in porous media.9 Here, air travels through

elastomer bulb. Permeation consists of two components, diffusion and advection. Diffusive permeation is

transport driven by concentration gradients, whereas advective permeation is driven by pressure gradients.

A compressible permeation approach to space seal research that encompassed both diffusion and advection

was recently developed.10–12

The objective of the research presented herein was to characterize the sealing performance of the Engineering Development Unit (EDU58) series of docking seals under simulated operating conditions in temperature

and level of closure.

II. Experimental Setup

A series of experiments were conducted to characterize the leak rates of subscale candidate docking seals

at specific temperatures and differing levels of compression in an elastomer on metal plate configuration.

The experimental setup and procedure are detailed herein.

NASA/CR—2011-216829 3
A. Test Specimens

The subscale candidate docking seals were Gask-O-Seals
R manufactured by the Composite Sealing Systems

Division of the Parker Hannifin Corporation. The seals were composite seals comprised of four elastomer

bulbs vacuum molded into a metal retainer ring. An illustration of the seal showing a cross section of the

molded elastomer and retainer ring is shown in Figure 2. The elastomer compound was S0383-70, manufactured by the Parker Hannifin Corporation. The metal retainer ring was manufactured from aluminium

6061-T651, which was anodized for improved corrosion resistance. The subscale test specimens were approximately 12 in. on the outside diameter. Photographs of the front side and back side of a sample test

specimen are shown in Figure 3(a) and 3(b), respectively.

(a) Front side, dynamic seal (b) Back side, static seal

Figure 3. Photograph of a subscale EDU58 test specimen.

Four bulb designs were tested. Each design had a unique and proprietary bulb height. Furthermore, the

seal designs have increasing bulb heights and are noted as EDU58-1, EDU58-2, EDU58-3, and EDU58-4,

respectively. The elastomer compound used for the seal bulb was verified to meet the low outgas requirements

with total mass loss (TML) and collected volatile condensable materials (CVCM) values below the limits of

1.0% and 0.1%., respectively,13 as dictated by NASA-STD-I-60162 and tested per ASTM E595.3

B. Test Apparatus

The test apparatus consisted of two plates, hermetic plumbing, and the required sensors. The test specimen

was clamped between two stainless steel plates with a surface finish of 16 μin. or better, creating an internal

control volume. The seal was attached to the bottom plate with twelve 8-32 screws, compressing the back

side, static seals to full closure. An illustration of the two plates clamped onto the seal is shown in Figure

4. The top plate had a vent port located between the inner and outer bulbs. This allowed for isolation and

characterization of the inner seal. Dry air was supplied to the inner control volume. The pressure of this

control volume was monitored by two pressure transducers with a full scale accuracy of 0.75% at a range

of 0 - 35 psia. Temperature was monitored with an resistance temperature device (RTD) with an accuracy

of ±0.2◦C. The RTD was inserted into a small aluminum block, mounted to the stainless steel plate, and

insulated from the environment in a foam block. The apparatus was enclosed in a Tenney BenchmasterBTRC environmental chamber. The environmental chamber had a temperature range of -73 to +200◦C with

a control accuracy of ±0.3◦C. The installation of the test specimen to the bottom plate of the test apparatus

is shown in Figure 5.

In order to simulate different levels of closure of the front side, dynamic seal, shims of a known value were

placed between the two fixture plates. The shims were located around the circumference of the fixture and

equidistant from each other. The leak rates were characterized at 100, 74, and 48% closure, where closure

is defined in Equation 1.

closure =

hbulb − x

hbulb

× 100% (1)

NASA/CR—2011-216829 4
Figure 4. Illustration of two plates clamped onto a seal assembly.

Figure 5. Illustration of test apparatus with creation of inner control volume.

where x is the distance between the metal retainer and the mating counter-face at the desired closure level

and hbulb is the uncompressed bulb height above the retainer. Note, full closure was metal to metal contact

between the top plate and the metal retainer ring.

C. Leak Rate Calculation

The mass point leak rate method was implemented to quantify the leak rate of the subscale candidate space

seals. Pressure and temperature measurements of known inner control volume of gas were monitored over

time. Utilizing the ideal gas law and the definition of specific volume, the mass (m) was calculated through

measurements of pressure (p), temperature (T ), volume (V ), compressibility factor (Z), and the specific gas

constant (R).

m =

pabsoluteV

(ZRT)

(2)

The compressibility factor, Z, equal to unity was appropriate for this application, as the temperature was

below 2 × 103K.

14

Leak rate analyses require a population of mass sample points with corresponding time stamp. An

appropriate regression analysis on the population directly yields the leak rate in units of mass over time

(e.g., lbm,air

day ). As calculated in Equation 2, the mass at each time step was determined, (ti

, mi). A regression

analysis of the mass-time population yielded the coefficients of the linear decay model:15

m (t) = a1t + a0 (3)

where m was the mass of the control volume as a function of time (t), a1 was the leak rate, and a0 was the

initial mass of the control volume and was of no consequence. The coefficient a1 was determined through a

least-squares regression shown in Equation 4.

16

a1 =

n

Pn

i=1 (timi) −

Pn

i=1 (ti)

Pn

i=1 (mi)

n

Pn

i=1 (t

2

i

) − (

Pn

i=1 (ti))2

(4)

NASA/CR—2011-216829 5
The corresponding leak rate per linear inch of seal was determined by dividing the total leak rate by

the mean circumference of the inner bulb. All leaks were attributed to permeation though the seal, rather

than test apparatus leaks or flow across the seal interface. The complete methodology, including uncertainty

analysis was detailed by Garafolo17 and Daniels.18

D. Uncertainty Analysis

An uncertainty analysis is imperative for a quantifiable level of confidence in the leak rate measurement. In

addition to the value of the space seal leak rate, the system uncertainty (bias error), the random uncertainty

(precision error), and the confidence level must be reported.15 Each measurement has it own bias and

precision error. In this analysis, all bias and precision errors are assumed independent from each other.

A comprehensive explanation of detailed uncertainty analysis can be found in Coleman15 and with direct

application to leak rate measurements in Comprehensive Mass Point Leak Rate Technique.

17, 18

The uncertainty analysis of the leak rate calculation included the variability in the measurement devices

on the least-squares regression15, 19, 20 and is shown in Equation 5. The uncertainties for each elemental

measurement (i.e., pressure, temperature, time) were calculated from calibration records or manufacturer

stated accuracies and propagated through to the calculation of mass, Equation 2. The comprehensive uncertainty analysis contained measures of covariance, Bmimk

, Btitk, and Btimk. In this study, the covariances

were assumed neglectible, as each transducer was assumed completely independent of from another and that

there was no covariance with the data acquisition’s time control. Further discussions of these covariances

are found in Coleman15 and Brown.21 This uncertainty, Um ̇, is the uncertainty of the leak rate and is often

given as a percentage, um ̇.

22

U

2

m ̇ =

Xn

i=1



∂a1

∂mi

2

P

2

mi +

Xn

i=1



∂a1

∂ti

2

P

2

ti +

Xn

i=1



∂a1

∂mi

2

B

2

mi + 2

nX−1

i=1

Xn

k=i+1



∂a1

∂mi

  ∂a1

∂mk



Bmimk

+

Xn

i=1



∂a1

∂ti

2

B

2

ti + 2

nX−1

i=1

Xn

k=i+1



∂a1

∂ti

 ∂a1

∂tk



Btitk + 2

nX−1

i=1

Xn

k=i+1



∂a1

∂ti

  ∂a1

∂mk



Btimk(5)

III. Experimental Results and Discussion

The leak rate per linear inch of bulb length was determined for the 12 in. EDU58 -1,-2,-3, and -4 docking

seal assemblies at select temperatures and closure levels representative of operating environment. For all

conditions investigated, the leak rate performance for each seal assembly met all design requirements as

outline above. The maximum observed leak rate of 2.30E-6 lbm,air

day−in. , found at +50◦C and 48% closure, was

well below the requirement of 1.4E-5 lbm,air

day−in. . Results and discussion on the effects of temperature, design,

closure, and seal design repeatability are detailed.

A. Effects of temperature on leak rate

Temperature had as significant effect on the leak rate performance of each seal; an increase in leak rate

correlated to an increase in temperature. The average leak rate values per linear inch of bulb for candidate

seal designs at three test temperatures are shown in Figure 6. For a given design, the 95% confidence interval

for a given design’s leak rate did not overlap with the 95% confidence interval of the same design at a different

temperature. This suggested that there was a statistical difference between the leak rate values at the select

temperatures.

The increase in leak rate was attributed to the an increase in the permeability of the elastomer and was

supported by permeation modeling of elastomeric space seals.11 The elastomer undergoes thermal expansion

with an increase in test temperature, thus increasing the effective bulb width. However, the temperature

effects on permeability overcome any sealing benefits in geometric changes due to thermal expansion.

B. Effects of Design

Experimental testing suggested a dependency of the leak rate on the characteristic bulb height between

the EDU58 series seals. An increase in bulb height correlated with a decrease in the observe leak rate

NASA/CR—2011-216829 6
Figure 6. Comparison of average leak rates for four EDU58 seal designs at temperatures.

value, Figure 6. This was expected, as the effective seal width was wider for taller bulbs under the same

displacement.

Save for the EDU58-4 seal at +50◦C, there was no significant statistical difference at each temperature

for the four seal assembly designs. Although it was possible to differentiate the observed leak rate values

between each design, there was commonality in 95% confidence interval. It is suggested that if the fidelity

of the test apparatus and data acquisition system were increased, the differences in leak rate values of the

three designs would be statically significant.

It is important to note, however, that the leak rate performance is not the only metric used in the the

selection of a main interface seal. The force required for full closure also needs consideration.23

C. Effects of closure levels on leak rate

The effects of closure level on the leak rate were determined at 100, 74, and 48% of full compression, as

defined in Equation 1, for an EDU58-3 seal assembly design at each temperature. Result showed a strong

dependency of compression level on leak rate, Figure 7. A decrease in closure increased the leak rate; the

decrease in percent closure shortens the permeation pathway for the gas to travel and increases with wetted

area available. A strong correlation between leak rate and temperature, regardless of compression level, was

developed. The difference in the leak rate performance between the selected closure levels were statistically

significant for +23◦C and +50◦C operating temperatures; the confidence intervals did not overlap. For the

-50◦C, differences in the leak rate were observed, however, they were not statistically significant.

D. Design Repeatability

Design manufacturing repeatability was investigated using three different seal assemblies of the same design,

namely EDU58-3. The leak rates were characterized at three test temperatures (-50, +23, and +50◦C) and

in duplicate, for a total of six test points per temperature. Results are shown graphically in Figure 8. Given

the 95% confidence interval, there was commonality in each observed leak rate of the three different EDU58-3

seals for a given temperature. This measure suggests that leak rate values of multiple seals within a specific

design were not statistically different.

NASA/CR—2011-216829 7
Figure 7. Average leak rates for EDU58-3 seal assembly at temperatures and various compression

levels.

Figure 8. Leak rates of three EDU58-3 seal assemblies at temperatures.

IV. Conclusion

The performance characterization of pressure seals in spacecraft is imperative for manned spaceflight.

Seals must meet or exceed acceptable air losses in a harsh vacuum environment over extreme operating

conditions. A performance characterization of a candidate seal assembly for the main interface seal of the

Low Impact Docking System was performed with four seals of the EDU58 design series in various compression

and temperature conditions.

The following conclusions were supported with this experimental investigation:

• There exists a dependence of leak rate on temperature. An increase in test temperature was shown

to increase the leak rate. The temperature effects on permeability overcome any sealing benefits in

geometric changes due to thermal expansion.

• There exists a dependance of leak rate on compression. Results showed that a decrease in the level of

compression increases the leak rate.

• A dependency of the leak rate on seal design was suggested. However, the leak rates of the designs

were not statically different at a given temperature. It was suggested that an increase in the fidelity

of the test apparatus and data acquisition system may differentiate the seal designs.

• The EDU58 series seal design met program performance requirements for all seal designs and temperatures investigated.

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NASA/CR—2011-216829 8
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21Brown, K. K., Coleman, H. W., Steele, W. G., and Taylor, R. P., “Evaluation of Correlated Bias Approximations in

Experimental Uncertainty Analysis,” AIAA Journal, Vol. 34, No. 5, May 1996, pp. 1013–1018.

22Figliola, R. S. and Beasley, D. E., Theory and Design for Mechanical Measurments, John Wiley & Sons, Inc., 3rd ed.,

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23Bastrzyk, M. B. and Daniels, C. C., “The Mechanical Performance of Subscale Candidate Elastomer Docking Seal,”

Proceedings of the 51st AIAA Materials Conference, No. AIAA 2010-3129, AIAA, Orlando, FL, 12-15 April 2010.

NASA/CR—2011-216829 9
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1. REPORT DATE (DD-MM-YYYY)

01-11-2011 

2. REPORT TYPE

Final Contractor Report 

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

An Experimental Investigation of Leak Rate Performance of a Subscale Candidate Elastomer 

Docking Space Seal 

5a. CONTRACT NUMBER

NNC08CA35C 

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Garafolo, Nicholas, G.; Daniels, Christopher, C. 

5d. PROJECT NUMBER

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5f. WORK UNIT NUMBER

WBS 644423.06.31.04.01.03.22 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

The University of Akron 

302 Buchtel Mall 

Akron, Ohio 44325 

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 REPORT NUMBER

E-17457 

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National Aeronautics and Space Administration 

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NASA 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

A novel docking seal was developed for the main interface seal of NASA’s Low Impact Docking System (LIDS). This interface seal was 

designed to maintain acceptable leak rates while being exposed to the harsh environmental conditions of outer space. In this experimental 

evaluation, a candidate docking seal assembly called Engineering Development Unit (EDU58) was characterized and evaluated against the 

Constellation Project leak rate requirement. The EDU58 candidate seal assembly was manufactured from silicone elastomer S0383-70 

vacuum molded in a metal retainer ring. Four seal designs were considered with unique characteristic heights. The leak rate performance 

was characterized through a mass point leak rate method by monitoring gas properties within an internal control volume. The leakage 

performance of the seals were described herein at representative docking temperatures of -50, +23, and +50 °C for all four seal designs. 

Leak performance was also characterized at 100, 74, and 48 percent of full closure. For all conditions considered, the candidate seal 

assemblies met the Constellation Project leak rate requirement.

15. SUBJECT TERMS

Docking seal; Leakage; Elastomers; Permeation; Space seal; Permeability; Silicones 

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