Cryogenic Moisture Uptake in Foam Insulation for Space ... - AIAA ARC

JOURNAL OF SPACECRAFT AND ROCKETS Vol. 49, No. 2, March–April 2012

Cryogenic Moisture Uptake in Foam Insulation for Space Launch Vehicles James E. Fesmire,∗ Brekke E. Coffman,† Jared P. Sass,‡ Martha K. Williams,§ and Trent M. Smith¶ NASA Kennedy Space Center, Florida 32899 and Barry J. Meneghelli∗∗ QinetiQ North America, Kennedy Space Center, Florida 32899 DOI: 10.2514/1.43776 Rigid polyurethane or polyisocyanurate foams used for existing space launch vehicles such as the space shuttle and Delta 4, and planned for use on future space vehicles, were tested under cryogenic conditions and found to gain an extraordinary amount of water. A cryogenic moisture uptake apparatus was developed to determine the amount of water taken into the specimen under actual use propellant loading conditions. After 8 h of test duration with a simulated launch pad environment on one side and liquid nitrogen temperature on the other, these foams gained at least 30% (new condition) to 75% (weathered condition) by mass. This effect can translate into an additional mass of over 1000 kg for space vehicles at liftoff. To determine the thermal performance and moisture uptake of foam insulation systems, three different materials were tested, including NCFI 24-124, NCFI 27-68, and BX-265. Results are presented for testing of both aged specimens and weathered specimens. The trends of increasing mass gain are clear for both aging exposure and weathering exposure durations up to 24 months. The water accumulation in these flight quality, closed cell polyurethane foams is shown to be water vapor driving into the subsurface due to the extreme thermal gradient imposed by the cryogen.

Conditions,” was conducted to investigate the thermal performance and fire performance of SOFI under simulated actual use conditions [4]. The six main elements of the study were 1) environmental exposure testing (aging and weathering), 2) cryogenic moisture uptake (CMU) testing under actual use conditions, 3) thermal conductivity testing under cryogenic vacuum conditions, 4) physical characterization of materials, 5) thermal conductivity testing under ambient conditions, and 6) fire chemistry testing. This study was part of a larger project entitled Technologies to Increase Reliability of Thermal Insulation Systems for space launch and exploration applications [5]. Although this paper focuses on exposure tests and CMU test results, the total scope is important for understanding the design implications and flight performance effects on space launch vehicles. Previously, little information was available on the intrusion of moisture into SOFI under large temperature gradients [6]. New cryogenic and fire chemistry test capabilities, established by the research testing laboratories of NASA Kennedy Space Center (KSC) in recent years, were the basis for the actual use performance characterization of the materials. The study focused on SOFI materials currently used on a majority of the surface area of the space shuttle external tank (ET). This approach was taken to answer some long-time questions from the space shuttle’s flight history and provide baseline information for the new Constellation program designs. The following SOFI materials were tested: NCFI 24-124 (acreage foam), BX-265 (closeout foam, including intertank flange and bipod areas), and NCFI 27-68 (alternative acreage foam) [7]. The NCFI 27-68 was a formerly proposed alternative acreage foam material with a slightly altered formulation. Weathering and aging durations ranged from one week to two years. The total test plan calls for durations extending up to six years.

I. Introduction

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OAM insulation on cryogenic tanks for space vehicles is now in standard use because of its light weight, mechanical strength, and thermal insulating performance. It is necessary to provide thermal shielding between either the liquid hydrogen at 20 K (423 F) or liquid oxygen at 90 K (297 F) and the ambient temperature at 300 K (80 F). These two extremes provide a temperature difference of 280 K (503 F), which makes thermal insulation necessary for two main reasons: 1) to make it possible to control the propellant loading systems and 2) to preserve the mass and density of the propellant at the levels needed for flight propulsion. In the late 1960s, a major breakthrough in technology developed new foam materials such as rigid polyurethane foams and polyisocyanurate foams and new application techniques, such as spray-on foam insulation (SOFI) [1,2]. Although updated for compliance with environmental regulations, this technology remains the standard thermal insulation system today for the space shuttle, Delta 4, and other space launch vehicles. Similar SOFI technology is planned for use on the cryogenic stages of the new Constellation program vehicles, Ares 1 and Ares 5 [3]. A comprehensive experimental study, “Long-Term Moisture/ Aging Study of SOFI Under Actual Use Cryogenic Vacuum Presented as Paper 2008-7729 at the AIAA Space 2008 Conference and Exposition, San Diego, CA, 9–11 September 2008; received 12 February 2009; revision received 30 November 2011; accepted for publication 2 December 2011. This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0022-4650/12 and $10.00 in correspondence with the CCC. ∗ Senior Principal Investigator, Cryogenics Test Laboratory, Mail Code NE-F6. † Senior Research Engineer, Cryogenics Test Laboratory, Mail Code NEF6. ‡ Lead Engineer, Cryogenics Test Laboratory, Mail Code NE-F6. § Lead Scientist, Polymer Science and Technology Laboratory, Mail Code NE-L6. ¶ Senior Research Scientist, Polymer Science and Technology Laboratory, Mail Code PH-I2. ∗∗ Principal Investigator, Cryogenics Test Laboratory, Mail Code ESC-830.

II.

Experimental Test Apparatus and Method

The CMU apparatus is designed to determine the amount of water/ ice taken into the specimen under actual use cryogenic conditions. Actual use in this sense means that one side of the specimen is fixed at the temperature of liquid nitrogen, whereas the other side is exposed to moist air that is temperature and humidity controlled using an environmental chamber as shown in Fig. 1 [8]. The backface (or cold side) represents the interface with the tank wall while the front face (or warm side) is exposed to the ambient environment. A standard 220

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Fig. 1 CMU apparatus: schematic view (left) and overall view of test apparatus including the environmental chamber (right).

in-house laboratory test method has been established for CMU testing [9]. Relative humidity level is kept at 90% and a heater control system maintains an air temperature of 295 K. The surface temperatures of the specimen and apparatus are measured using thermocouples. A National Instruments data acquisition system with LabVIEW is used for data monitoring and recording. The edges are guarded from moisture intrusion and from substantial heat leakage. Moisture uptake occurs when water or ice is taken into the specimen in the vertical direction through the thickness of the insulation material. The mass of the test specimen is monitored with respect to time. Each hour, the specimen is briefly removed from the test apparatus, weighed on a precision scale, and replaced. Figure 2 shows the installation of a SOFI test specimen onto the CMU apparatus. The weathered side (dark orange) is placed downward into the environmental chamber whereas the nonweathered side (light yellow) is facing the preceding cold mass assembly. Although the exposed surface is placed downward, a small amount of water (condensate) will sometimes collect on the surface. This surface water is shaken from the test specimen before it is weighed. The specimen is placed upright inside a plastic sealable bag at the end of the test to melt any surface ice or frost so that it can be weighed. This end-of-test correction is typically a small amount of water weighing from 1 to 2 g, at most. The cold mass of the CMU apparatus provides a cold contact surface diameter of 152 mm. The SOFI test specimens have a diameter of 203 mm and a nominal thickness of 25.4 mm for the shaved materials (BX-265) or 31.8 mm for the net spray materials (NCFI 24-124 or NCFI 27-68). Based on a mean diameter of 178 mm, the effective heat transfer area is therefore 249 cm2 . All tests were conducted at ambient pressure (760 torr). The typical run time, which simulates a space shuttle launch loading timeline, is at least 8 h from start of cooldown. Multiple runs are performed for each test series. The warm boundary temperature (WBT) is approximately

295 K and the cold boundary temperature (CBT) is approximately 78 K. The temperature difference for the cryogenic testing is therefore approximately 215 K.

III. A.

Preparation of Foam Test Specimens

Materials

Foam materials were sprayed at NASA’s Marshall Space Flight Center in accordance with nominal flight specifications [10]. Baseline (new condition) specimens were allowed to cure for approximately one month. Several 610 610 mm samples of the three types of foam, NCFI 24-124, NCFI 27-68 (formerly proposed new acreage foam without flame retardant), and BX-265 (closeout foam, used in the critical bipod area of the ET), were packaged and shipped to KSC. The 610 610 mm samples of foam were then machined into round test specimens of specific thicknesses. The acreage foams, NCFI 24-124 and NCFI 27-68, were machined on the backface to obtain a nominal 31.8 mm thickness. The backface represents the cold boundary surface of the tank wall and is oriented upward in the CMU apparatus. Specimens of 203 mm diameter were then cut. The front faces of these specimens were further machined to have a 25.4 mm thickness on a 12.7-mm-wide periphery, leaving a 178-mmdiam central area with the nominal 31.8 mm thickness. The front face represents the warm boundary surface exposed to the ambient environment and is oriented downward in the CMU apparatus. The closeout foam, BX-265, was machined on all surfaces to obtain a thickness of 25.4 mm over the entire 203-mm-diam specimen. All specimens were prepared in this way so that testing would be performed under simulated actual use conditions in representative thicknesses, that is, with rind (net spray) for acreage foams and without rind (machined) for closeout foam. The measured densities of all specimens were comparable, ranging from 37 to 40 kg=m3 before testing. Baseline specimen masses were approximately 40 g for NCFI 24-124, 36 g for NCFI 27-68, and 30 g for BX-265.

Fig. 2 Installation of a SOFI test specimen shown in the photographic sequence from left to right.

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Environmental Exposure (Aging and Weathering)

Environmental exposure of the SOFI materials was an integral part of the performance testing. The approach was aided by prior work on the extreme weathering of polyimide foams [11,12]. The aging and weathering simulations were performed at two exposure sites. The platform level “A” within the KSC Vehicle Assembly Building was used for the aging test. The aging site simulates the area where the ET is stored before the vehicle stacking and launch preparations begin. The conditions in this area are mild with ambient humidity levels and no direct sunlight. The Corrosion Beach site at KSC was used for the weathering test. The weathering site simulates the conditions to which a mated ET is exposed while it is on the pad awaiting launch. These conditions are harsh. The aging time for an ET can be several years whereas the typical weathering exposure is about 35 days. The maximum weathering exposure, as recorded in the space shuttle flight history, is about six months [13]. Specimens were mounted on custom-designed aluminum stands with a protective enclosure of the backface, edge, and periphery as shown in Figs. 3 and 4. The test specimen mounting enclosure, shown in Fig. 3 (left), was designed to expose only the center portion of the top face of the material. Figure 3 (right) shows a SOFI test specimen type NCFI 24-124 after only one month of weathering. Views of both the aging exposure test stand and the weathering exposure test stand are shown in Fig. 4. Exposure time durations are as follows: baseline; two weeks; 1, 3, 6, 12, and 18 months; and two years. Previous studies have typically been limited to aging and only for relatively short periods as necessary to determine the effect of diffusion of the spray-foam blowing agent [14]. Current plans for this study are to test a limited number of specimens yearly for up to six years.

IV. Testing of Spray-On Foam Insulation Materials This experimental study of SOFI materials included physical characterization, cryogenic thermal performance, and CMU. Fire chemistry testing was also performed on a separate set of similarly

aged and weathered SOFI specimens. This comprehensive study emphasized testing methods that accurately represent the actual use conditions to the extent possible and provide experimental repeatability. The testing and testing sequences were designed to yield as much thermophysical information as possible with the main focus placed on the CMU. Brief summaries of the physical characterization tests, thermal performance testing, and fire chemistry testing are given as follows. A.

Physical Characterization Tests

Physical characterization tests included percent open cell content and surface area (and pore size). The cell content testing (43 specimens and a total of 237 tests) was performed in accordance with ASTM D-6226 using a Quantachrome Instruments UltraFoam pycnometer [15]. These tests were performed at ambient temperature. The open cell content was found to be approximately 6% for the baseline foam specimens. The average measured open cell contents for the aged and weathered specimens were 10 and 12%, respectively. The surface area testing (13 specimens and a total of 112 tests) was performed using a Quantachrome Instruments Nova surface area analyzer. These tests were performed under vacuum in sequence with LN2 bath immersion in accordance with standard laboratory methods. The average surface areas for the baseline specimens were measured to be approximately 130 m2 =g for the BX-265, 170 m2 =g for the NCFI 24-124, and 180 m2 =g for the NCFI 27-68. The surface area was found to increase by approximately 35% after three months of aging or weathering. The surface area then diminished somewhat with further aging but continued to increase with further weathering exposure [16]. B.

Thermal Performance Testing

Cryogenic thermal performance testing included a total of over 100 tests of 4 baseline (new condition) test specimens using Cryostat-100 (absolute apparent thermal conductivity, k-value) and a

Fig. 3 Test specimen mounting enclosure (left) and one month weathered specimen (right).

Fig. 4 Environmental exposure testing at the KSC: aging inside the Vehicle Assembly Building (left) and weathering at the Corrosion Beach site (right).

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() ranged from 19 mW=m-K (baseline) to 32 mW=m-K (aged). Results show that the actual thermal performance is significantly degraded from aging durations of up to 24 months with the sharpest change occurring within the first six months of aging. The thermal conductivity is widely variable based on many factors such as test method, test conditions, aging, weathering, density, closed cell content, surface area, pore size, foam chemistry, and spraying conditions. The baseline results are therefore an important and necessary starting point for thermal performance comparison among foam materials.

total of 147 tests of 19 aged or weathered specimens using Cryostat-4 (comparative k-value) [8,17]. The actual use, cryogenic vacuum test conditions are listed as follows: CBT of 77 K, WBT of 293 K, and pressure range from ambient pressure to high vacuum with nitrogen as the residual gas. The Cryostat-100 test apparatus uses 25-mmthick cylindrical clamshell test specimens about 1 m long [18]. The Cryostat-4 test apparatus uses flat disk specimens 203 mm in diameter and 25 mm thick [19]. The k-values for the baseline case ranged from 21:1 mW=m-K at ambient pressure down to approximately 7:5 mW=m-K at high vacuum [20]. Prior work on a similar SOFI specimen of BX-250 material (51 mm thickness and a density of 38 kg=m3 ) shows a generally similar result [21]. Further testing and analysis for materials with and without rind, different aging/ weathering periods, different thicknesses, and various thermal cycling effects is an ongoing effort. Thermal conductivity () testing under ambient conditions of both baseline and aged specimens (26 specimens and a total of 30 tests) was performed using a heat flux meter in accordance with standard test method ASTM C-518 [22]. The ambient thermal conductivities

C.

Fire Chemistry Testing

Fire chemistry testing included a complete study of flammability with respect to weathering and aging [23]. All foams after a 12 month weathering period showed little or no change in ease of flame extinction as measured by oxygen index (OI). The OI test indicates that NCFI 24-124 is an inherently flame retardant material, whereas the other two materials are not. These materials were also tested by

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A. Cryogenic Moisture Uptake Test Results for Spray-On Foam Insulation BX-265

radiant panel and cone calorimeter methods. Flame spread was quite high for NCFI 27-68 and BX-265. In addition, the flame spread rate was found to increase in aged specimens. The heat release rates of BX-265 and NCFI 27-68 materials were not affected by aging or weathering as measured by cone calorimetry. However, weathering did reduce the heat release rate of NCFI 24-124. Specimens with a rind did have an increased flame spread index, heat evolution factor, and a decreased time to peak heat release rate. Simply put, specimens with a rind burned faster and hotter.

V.

Summaries of the CMU results for the BX-265 specimens are given in Figs. 5 and 6 for the aged and weathered conditions, respectively. Aging times include 0, 0.75, 3, 6, 12, 18, and 24 months. Weathering times include 0, 0.25, 0.5, 3, 6, 12, 18, and 24 months. Nine different aged specimens were tested in a total of 18 runs with cold soak times ranging from 8 to 10 h. Seven different weathered specimens were tested in a total of 19 runs with cold soak times ranging from 8 to 14 h. Moisture uptake ranged from approximately 35% for the baseline (new) specimens up to approximately 83% for the three month weathered specimens.

Cryogenic Moisture Uptake Test Results

CMU testing included a total of 91 tests of 43 specimens. The test conditions for CMU are listed as follows: CBT of 77 K (top side), WBT of 295 K (bottom side), and relative humidity of 90% air exposure to bottom face. The CMU results are summarized in the charts of Figs. 5–10. Each chart shows the CMU results by foam type and either aging or weathering for a range of exposure durations.

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Summaries of the CMU results for the NCFI 24-124 specimens are given in Figs. 7 and 8 for the aged and weathered conditions, respectively. Aging times include 0, 0.6, 3, 6, 12, 18, and 24 months.

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CMU results for the NCFI 24-124 specimens in the weathered condition.

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CMU results for the NCFI 27-68 specimens in the aged condition.

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Fig. 10 CMU results for the NCFI 27-68 specimens in the weathered condition.

Weathering times include 0, 0.25, 0.5, 3, 6, 12, 18, and 24 months. Ten different aged specimens were tested in a total of 16 runs with cold soak times ranging from 8 to 11 h. Eight different weathered specimens were tested in a total of 21 runs with cold soak times ranging from 8 to 15 h. Moisture uptake ranged from approximately 34% for the baseline (new) specimens up to approximately 88% for the three month weathered specimens.

Table 1 Material

C. Cryogenic Moisture Uptake Test Results for Spray-On Foam Insulation NCFI 27-68

Summaries of the CMU results for the NCFI 27-68 specimens are given in Figs. 9 and 10 for the aged and weathered conditions, respectively. Aging times include 0, 3, 6, 12, 18, and 24 months. Weathering times include 0, 3, 6, 12, 18, and 24 months. Nine different aged specimens were tested in a total of 16 runs with cold

Summary of test results for SOFI materials given in percentage mass gain (8 h cold soak) Baseline

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soak times ranging from 8 to 10 h. Six different weathered specimens were tested in a total of 17 runs with cold soak times ranging from 8 to 14 h. Moisture uptake ranged from approximately 36% for the baseline (new) specimens up to approximately 83% for the three month weathered specimens. D.

Summary of Test Results

The multiple numbers of specimens, data points, and test runs give an indication of the repeatability of the method and build confidence in the overall trends. Table 1 presents a general summary of averaged values of the CMU test results for both the aged and weathered specimens for various exposure durations. The baseline designation indicates a nonexposed specimen. The values are averaged for test runs with an 8 h cold soak and are given in percentage mass gain based on a 165-mm-diam effective heat transfer area (mean of the 152-mm-diam cold mass surface and 178-mm-diam exposed face). As previously discussed, the specimens of the two NCFI materials have a nominal thickness of 32 mm with a net spray front face while the BX-265 material specimens have a nominal thickness of 25 mm with a machined front face. Moisture uptake increased significantly even for weathering durations as short as one week. Weathered specimens of six months or more showed that the moisture uptake continued unabated for the entire duration of each test. Thicknesses of the specimens were gradually being reduced by the corrosive effects of the environment in the longer durations from 12 to 24 months; the moisture uptake results for the weathered specimens reflect this change. E.

Additional Test Results


The mass gain test results for the aged specimens (see Figs. 5, 7, and 9) have all been plotted from 0 to 100% for ease of comparison among the three foam material types. In all cases, the moisture uptake after eight hours was more than one third by mass for baseline foam with no aging or weathering. The two NCFI foams (with rind) show increased mass gain with aging and give similar results. Compared to the NCFI foams, the BX foam (without rind) specimens show an even more pronounced increase in mass gain with increased aging durations. Extended test runs for up to 15 h were performed for select test specimens in the weathered condition. In general, the rate of moisture uptake increased with weathering duration. The results were consistent in showing initial increases in mass with soak time as shown in Figs. 6, 8, and 10. Most specimens, excluding the baseline and short duration weathered specimens, showed fairly linear increases in mass gain for the duration of the test. That is, the moisture uptake

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continued unabated for the entire test up 14 h total duration. The mass increases are presented in varying scales from 190 to 280% to provide better clarity among the different weathered specimens. Further details on the location and the morphology of the water/ice inside the foam are needed in order to fully understand this effect and when stabilization is likely to occur. Repeat runs (up to four runs total) were performed on all test specimens. Between runs, the specimens were reconditioned in a dry nitrogen environment overnight. Some data curves are not shown in the graphs for the sake of clarity. Although the measurement variation is quite large for a few of the individual test data points, the repeatability of the test runs was excellent. The large number of specimens and multiple runs of each specimen establish confidence in the method used for the CMU test. Figure 8 also shows one curve for a three month weathered specimen with the outer surface (6.4 mm) machined off. The moisture uptake is less than that of the normal three month weathered specimen, but is still more than the nonexposed baseline specimen. This result gives further evidence that while the moisture is penetrating into the thickness of the specimen over the duration of the cold soak, weathering is still largely a surface/subsurface effect.

F.

Cumulative Effect of Successive Launch Attempts

A long-duration test was performed to determine the additive effect of the moisture mass gain. This experiment was a three-day test simulating two back-to-back scrubs and a launch on the third day. The environmental conditions were the same as for previous CMU tests. The liquid nitrogen tank of the CMU apparatus was maintained nearly full for the first ten hours and was depleted at approximately the 17 h mark each day. Figure 11 summarizes the results of this longduration CMU test series. The special NCFI 24-124 test specimen had been aged for approximately two years and then weathered for one month, a combination that is closely representative of a typical ET case. The slope of the mass change data on the first day of this long-duration test closely agrees with the previous NCFI 24-124 data as shown by the curves for the bounding weathering durations shown in Fig. 11. The cumulative specimen mass change from the original starting mass to the 10 h mark of each day was 75% on day one, 131% on day two, and 167% on day three. The additive effect of CMU is explained by the fact that, upon cryogenic tanking, the moisture rapidly penetrates the foam due to the strong cryopumping effect that occurs during cooldown. The vast majority of the accumulated moisture, which takes much longer to come out as a result of natural heating from the surrounding environment, remains in the foam on subsequent days. After the

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long-duration CMU test was completed, the test specimen was kept inside the apparatus under the same environmental conditions and weighed once per day to monitor the natural desorption rate of the moisture. The specimen did not return to its original mass for approximately 30 days. Therefore, for consecutive launch attempts, or scrub turnarounds, moisture in the SOFI from prior cryogenic propellant tanking operations is expected to remain in the foam with additional moisture gained during subsequent loadings that occur within a few days.

VI. Analysis and Discussion

Whereas surface water mass remains a significant factor, the amount of water inside the foam is found to be much more substantial. The finding that SOFI can nearly double its mass during a single launch operation of loading the cryogenic propellants has two main implications. The first is in regard to performance of the flight propulsion system. The second is in regard to performance of the thermal protection system. Also, determining the location of moisture inside the foam is important to understanding the morphology of the foam moisture system and the physics of vapor drive in the CMU process. A.

Mass gain in SOFI due to moisture uptake under subambient temperature conditions is often referred to as vapor drive or vapor permeance [24]. Even good quality, closed cell polyurethane foams with a vapor retarder can accumulate water under moderately cool (compared to cryogenic propellants) conditions of industrial refrigeration applications. Because all foam insulation materials are permeable to some degree and vapor retarders are less than perfect, the industry standard practice states that “the eventual result is that a significant percentage of the void space within the insulation will contain water, both water and ice, or ice alone, depending on whether the operating temperature is above 273 K (32 F) or how far it may be below 273 K [25].” Although the thermophysical transport mechanisms of vapor drive, condensation, and ice formation within insulation subjected to cold boundary conditions are still to be determined, the accumulation is clearly due to water vapor driving into the foam in the presence of a thermal gradient [26]. When the cold side is extended to the extreme low temperatures of liquid oxygen or liquid hydrogen, this effect will produce an even greater driving force (sometimes referred to as cryopumping), especially during the cooldown process. Surface water has also been addressed in prior work. While no mass allowance is given for acreage ice formation on the ET, water in the foam is to be considered [27]. The space shuttle flight performance prediction adds approximately 181 kg (400 lb m), or approximately 8% of the total foam mass of 2188 kg (4823 lb m), to account for the additional mass of water that has condensed on the exposed surface of the ET at liftoff [28,29]. Simple experimentation with running water over the face of a test specimen confirms this result, with a mass increase of only about 8% relative to the dry mass. The end-of-test correction for the CMU test method also confirms this result, indicating a typical increase of 5 to 10% in surface water for a test specimen in the thickness range from 25 to 32 mm. The mechanism of vapor transport, not liquid water, has been shown by others to be the dominant mechanism of water absorption by permeation from a warmer, humid environment to a colder, dryer environment [30]. Although previous studies of cryogenic foam insulation systems have cautioned against mass increases by the absorption of atmospheric constituents, none have focused on the permeation of water vapor under large temperature gradients that are representative of the actual use condition [31].

Relevance to Flight Propulsion System Performance

Models of the main propulsion system performance indicate that the space shuttle has been carrying a significant amount of additional mass since its first flight [32]. Flight performance data reconstructions also show that the additional mass is reduced during the ascent to orbit. The missing mass is that amount carried into orbit and is termed the “performance collector” by propulsion system analysts. The performance collector was widely variable, but in later years approached a more stabilized value of around 544 kg (1200 lb m) [33]. Determining a connection between CMU in SOFI and the additional mass is a follow-on subject of investigation. A rudimentary calculation of water mass carried in the foam based on the CMU test data and the total SOFI mass of the ET thermal protection system results in at least 635 kg (1400 lb m), or 30%, of additional mass for new unweathered foam and at least 1633 kg (3600 lb m), or 75%, of additional mass for SOFI that has been weathered for three months. A preliminary estimate, based on the results of this study, is illustrated by Fig. 12. The SOFI material NCFI 24-124 is the acreage foam used for thermal protection on the ET and covers the majority of its surface. Because the weathering exposure for an ET is typically around one month, the potential mass increase at liftoff is conservatively estimated to be an average of these two amounts, about 1134 kg (2500 lb m), or 52% [13]. The results given in Fig. 9 indicate that the CMU for one month weathered foam is only slightly lower than for three month weathered foam. The additional mass at liftoff could therefore be closer to the 1633 kg (3600 lb m), or 75%, figure depending on actual weathering exposure, weather conditions during launch operations, duration of cold soak, and a scrub turnaround event. The estimate is also conservative in that the CBT for the test data is at 77 K (liquid nitrogen temperature) although the majority of the ET is at 20 K (liquid hydrogen temperature) and the remainder at 90 K (liquid oxygen temperature). With an estimate of 1134 kg (2500 lb m) of moisture in the foam at liftoff, the question remains as to how much of this additional water mass is carried to orbit. The on-orbit amount is related to the performance collector and the overall flight performance reserve. Preliminary testing of representative, CMU-conditioned foam specimens for launch ascent profile simulation was performed using the Cryostat-100 vacuum chamber at the Cryogenics Test Laboratory. The results show that roughly half of the water mass is driven out by the evacuation and heating process of 8.5 min duration. Although further testing and analysis is needed, these findings provide a starting point for explaining the space shuttle’s missing mass.

4000 3 months weathered

Total ET Foam Mass (kg)

3500 3000

1641 New

2500

2000

656 Ideal case no water

1500 1000

2188

2188

2188

0%

30%

75%

500 0

Moisture Mass Gained During 8-hours Cryogenic Propellant Loading

Fig. 12 Estimated effect of CMU on the space shuttle’s launch mass.

B.

Relevance to Thermal Protection System Performance

The additional moisture inside the foam due to CMU should naturally provide some degree of enthalpy margin for any external heating effects applied to the thermal protection system. The space shuttle’s flight history also shows a consistent but unusual charring behavior on the aft dome area of the ET. According to foam recession test data from the Improved Hot Gas Facility at Marshall Space Flight Center, this aft dome area, insulated with NCFI 24-57, a modified formulation from NCFI 24-124, chars much later in flight than expected in comparison to ground testing [34]. Also, in-flight video footages show large amounts of vapor coming off the aft dome in the early minutes of flight. Water present within the foam could explain the gradual receding of the “steam ring” and the suppressed charring behavior observed on the aft dome of the ET during flight. Figure 13 presents photos of the aft dome before and after flight. The new CMU

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Fig. 13 Views of the ET aft dome before and after flight. A gradual receding of a steam ring on the aft dome can be observed during the first few minutes of flight.

results can be examined for other thermal protection system performance considerations such as the allowable debris masses used in calculating potential impact damages during flight. C.

further degrades the cellular structure of the material. The longduration CMU test shows the additive effect of moisture penetration. Moisture enters the foam and causes a progressive change in the overall thermal conductivity. The total moisture content can be composed of various combinations of liquid water, water vapor, frost, and ice. The k-value for SOFI is nominally 21 mW=m-K, whereas the thermal conductivity for frost ranges from 80 to 700 mW=m-K, and for ice is approximately 2210 mW=m-K [37]. Therefore, the overall thermal conductivity of the new foam moisture system is increasing slightly and tending to shift the 273 K isotherm, the freezing point of water, toward the outer surface. Thermal equilibrium with the environment would be reached only when there is no net change in makeup of the insulation system, which is ultimately composed of some combination of foam and moisture components.

Location of Moisture Within Spray-On Foam Insulation

Specimens were dissected while in the fully cold condition at the end of an 8 h cold soak cycle to determine the location of the additional mass within the thickness of the specimen. Figure 14 depicts the coring process. The moisture was found to exist mainly in the outer third (warm side) with progressively smaller amounts in the middle third and inner third (cold side). This result confirms that the test apparatus and method does not allow water intrusion from the backface and that the moisture is in fact migrating into the material from the warm side to the cold side. Researchers at the National High Magnetic Field Laboratory used a 900 MHz ultrawide bore nuclear magnetic resonance (NMR) spectrometer to perform an evaluation of the moisture accumulation within SOFI specimens. Samples of NCFI 24-124 were conditioned to match launch pad conditions following methods similar to those described in this report [35]. The NMR imaging results clearly show that the moisture is migrating toward the cold side and accumulating between the knit lines within the foam [36]. SOFI materials, although a closed cell type, have some open cell content and are not impermeable to water vapor. Aging or weathering

D.

Thermal Performance Considerations

Thermal performance is governed by the combined effects of solid conduction, cellular convection, residual gas conduction, and radiation heat transfer. The true thermal performance of the total foam insulation system under actual use cryogenic conditions is further variable with physical characteristics such as aging, weathering, cellular structure, internal gas composition, and presence of voids/ cracks within the foam. Vapor drive effects due to the cryogenic cold 2”

203-mm

203-mm

Thermocouple region 50-mm

WBT = 293K (+68°F)

32-mm

CBT = 77K (-321°F)

Fig. 14 Coring process for SOFI specimen used to determine the location of moisture through the thickness of the material after a CMU test.

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boundary condition provide the mechanism for continual addition of moisture in vapor, liquid, or frozen forms. This CMU, with its highly variable amount, distribution, and morphologies within the foam, must necessarily affect the thermal performance of the foam insulation system.

VII.

Conclusions

A study of SOFI under actual use cryogenic conditions, with a focus on the CMU phenomenon, was completed at the Cryogenics Test Laboratory. New information on the intrusion of moisture into SOFI under large temperature gradients was produced. Experimental data for a significant number of test specimens were produced and the overall trends are clear. The SOFI materials were found to gain an extraordinary amount of water mass during the cryogenic propellant loading (cold soak) phase leading up to launch. However, more intensive investigation and analysis is warranted to understand CMU and its connection to flight performance. Furthermore, these findings provide a plausible explanation for the space shuttle’s missing mass over the life of the program. Determining the amount of water, or “moisture,” uptake within three SOFI materials was the main focus of the study. The study included exposure tests with aging and weathering, cryostat thermal performance tests, physical characterization of the materials by surface area and open cell content, and fire chemistry properties. Both aged specimens and weathered specimens were used to determine the amount of moisture added to a given launch vehicle during a simulated cryogenic propellant loading operation. The NCFI 24-124 material (with rind) and the BX-265 material (machined) both take up a substantial amount of water inside the material. The alternate acreage foam material, NCFI 27-68, was tested for comparison and gave similar CMU results. Moisture uptake results have been expressed in terms of percentage mass gain because all three foams tested have a similar density. Moisture uptake for the NCFI 24-124 specimens (acreage foam) averaged 34% for the baseline condition and 88% after three months of weathering. CMU continued to increase for both aging and weathering. With a mass of SOFI on the space shuttle ET of 2188 kg (4823 lb m), the potential added liftoff mass corresponds to 1134 kg (2500 lb m), by the most conservative estimate, for an on-time launch with no scrubs. Space shuttle flight performance history, including both main propulsion system models and thermal protection system observations of the aft dome area, also affirm the reasonability of the presence of a large amount of moisture within the insulation. Combining aspects of materials science and cryogenic engineering, the experimental data sets include the complex interactions of the material microstructure, the polymer chemistry, and the launch environment. This information includes cryogenic thermal conductivity under the full range of vacuum pressure conditions from ambient pressure to high vacuum. The test methods used in this study give results for the total system performance of foam insulation materials as tested under simulated actual use cryogenic conditions and following the appropriate duration of aging and weathering exposures. Through multiple CMU tests of an extensive array of specimens, the water accumulation in these flight quality, closed cell polyurethane foams is shown to be water vapor driving into the subsurface due to the extreme thermal gradient imposed by the cryogen. The basic data are now presented, but the combined effects of thickness, surface finish, environmental conditions, liquid hydrogen CBT, and multiple thermal cycles may also have significant impacts on the system performance. A three-day simulation test with thermal cycling due to two consecutive scrubs and a launch demonstrates that the moisture uptake is fully additive in the time frame of a few days. The results of this experimental study provide additional understanding of the thermal performance of existing cryogenic foam insulation materials in the real-world space launch environment and an improved basis for the research and development of new materials. The CMU phenomenon has both design and operational

implications for future space launch vehicles that use thermal insulation systems exposed to the ambient environment.

Acknowledgments This work was part of the NASA Internal Research and Development project, Technologies to Increase Reliability of Thermal Insulation Systems, funded by the Space Operations Mission Directorate. The CMU foam coring study was funded by the NASA Engineering and Safety Center with support from Leslie Curtis. The authors wish to acknowledge Gweneth Smithers of NASA Marshall Space Flight Center and Nancy Zeitlin of NASA Kennedy Space Center for their guidance of this project. The authors especially recognize Wayne Heckle of Sierra Lobo, Inc., at NASA Kennedy Space Center for his work in performing the majority of the tests.

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K. Wurster Associate Editor