AIAA 99-2763

Orbital Maneuvering Subsystem Engine Propellant Leakage Ball-Valve Shaft N. Buntain AlliedSignal Technical Services Carp. Team Johnson Space Center White-Sands Test Facility Las Cruces,-NM

K. Lueders YASA Johnson Space Center White Sands Test Facility Las Cruces, NM

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June 20-23, II999 / Los Angeles,

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AIAA 99-2763 ::

ORBITAL MANEUVERING Sl$SYSTEib@NGINE PROPELLANT LEAKAGE BALL-VALVE SHAFT SEALS

Nick Buntain* Kathryn Lueders+ AlliedSignal Technical Services Coy. Team

Johnson Space Center White Sands Test Facility Las Cruces, New Mexico

ABSbACT

Evidence of propellant leakage across ball- valve shaft seals has been noted during the repair of five flight engines and one test engine at the NASA Lyndon B. Johnson Space Center White Sands Test Facility. Based on data collected during the disassembly of these engines, the consequences of propellant leakage across the ball-valve shaft seals can be divided into four primary areas of concern: Damage to the ball-valve pinion shafts, damage to sleeved bearings inside the ball-valve and actuator assemblies, degradation of the synthetic rubber o-rings used in the actuator assemblies, and corrosion and degradation to the interior of the actuator assemblies.

The exact time at which leakage across the ball- valve shaft seals occurs has not been determined, however, the leakage most likely occurs during engine firings when ball-valve cavity pressures range as high as 453 to 550 psia. This potential pressure range for the ball-valve cavities greatly exceeds the 332-psia pressure used during acceptance testing of the series. valve.

Since redesign and replacement of the ball- valve shaft seals is unlikely, the near-term solution to prevent damage that occurs from shaft-seal leakage is to implement a routine overhaul and maintenance program for engines in the fleet.

BACKGROUND

The orbital maneuvering subsystem engine (OMS-E) Depot Team at the White Sands Test Facility (WSTF) has performed disassembly, inspection, overhaul, and acceptance testing of five

l Depot Project Leader, Propulsion Department + Depot Project Manager, Propulsion Test Offke.

Copyright*’ 1999 by AlliedSignal Technical Services Corp. Team. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission.

NASA Johnson Space center White Sands Test Facility Las Cruces, New Mexico

OMS engines (Serial Numb&s 111,2B, 106, 105, and lOS), and performed in-place repair on one engine (Serial Number 107) since 1995. Of the many anomalous conditions noted during these repairs, the majority are related to or have resulted from propellant or propellant vapor leakage that has occurred across the redundant shaft seals located on each of the ball-valve pinion shafts in the series valve assembly. Since the areas of the series valve downstream of the ball-valve shaft seals were, by design, not intended to be exposed to propellant residue or vapors, leakage of this type results in numerous problems. Because of the shaft seal leakage issue, NASA decided in 1996 to implement a process for overhauling each of the engines in the shuttle fleet over approximately a 6- year period.

OBJECTIVES

The intent of this paper is, to document the consequences that result when leakage of propellant or propellant vapors occurs across the ball-valve shaft seals on the OMS series valve. In addition, recommended repair, verification, and possible preventative maintenance measures are discussed. Figure 1 may be used as an aid in tracing the areas discussed in this report.

OMS-E Series Valve Svstem Description

The series valve assembly controls fuel and oxidizer flow to the OMS-E injector (Figure 2). The series valve consists of two bipropellant valve assemblies that are connected in parallel providing series redundant ball seals to prevent propellant leakage when the engine is in the OFF mode. Each bipropellant valve assembly has an actuator assembly,

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one upstream ball valve, and one downstream ball valve.

Each actuator assembly consists of an actuator, a linear variable differential transducer (LVDT) for indicating valve position, and a three-way control valve (CV). The actuator is spring-closed and pneumatically opened. It contains a piston rack that is pneumatically positioned by the regulated gaseous nitrogen (GN,) supply from the pneumatic pack assembly. The rack translation resulting from piston movement is converted to rotary motion by the pinion gear attached to each ball-valve shaft, causing one fuel and one oxidizer ball valve to rotate open or closed. The position of the rack is monitored by an LVDT. The LVDT is calibrated to show the percentage of ball-valve rotation as a function of the rack’s linear motion. The CV is a three-way, two- position, dual-solenoid operated valve. The valve is spring normally closed. When energized, redundant solenoids open in tandem allowing the flow of pressure regulated GN, to the actuator.

Two ball-valve assemblies, one upstream and one downstream, are mounted to each actuator housing with each ball-valve shaft pinion gear meshed with the actuator rack.

APPROACH

Inspection and characterization of the condition of the OMS-E are performed throughout the decontamination and disassembly process for an engine. During decontamination of the engine, propellant vapor measurements are performed to determine if leakage has occurred across the ball- valve primary shaft seals. During series valve removal and disassembly, optional steps are included in the procedures to allow for disassembly steps to be performed under the protection of a fume hood. If physical evidence of propellant leakage, or damage resulting from this leakage, has occurred, samples are taken for chemical analysis and/or photo documentation is performed.

SHAFT SEAL LEAKAGE

Each ball-valve pinion shaft has two seals (primary/inboard and secondary/outboard) that are intended to prevent the migration of propellant from the ball-valve into the bipropellant valve actuator. During the performance of acceptance test procedures (ATP) at the series valve and engine levels, leakage tests are conducted on the primary and secondary shaft seals. The tests are performed using gaseous helium at low (30 psig) and high pressures (320 psig). Allowable leakage varies depending on the ATP and

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Figure 2. OMS Engine Steady-State Operation

the firing status of the engine. For example, at the series valve level, the allowable leakage rate across any individual seal is 40 standard cubic centimeters per hour (scch). Leakage tests are performed with the ball valves in both the open and closed positions. At the engine level, allowable leakage across any individual shaft seal is 45 scch for an engine with new seals, and 50 scch for an engine that has been hot fired. At the engine level, however, leakage tests are performed only with the ball valves in the closed position.

apparent than oxidizer leakage due to the differences in vapor pressure. As a result, evidence of oxidizer leakage is much more likely to be discovered during disassembly and inspection than by checking for evidence at the OP-2A/2B-seal vent port.

During decontamination and cleaning of OMS- E S/N 111 by Aerojet in June of 1994, evidence of fuel leakage was noted at the FP-2A/2B port. After S/N 111 was completely rebuilt at the WSTF and successfully passed ATP and hot-fire tests, propehant vapors were registered both at the fuel seal port (FP- 2A/2B) and oxidizer seal port (OP-2A/2B).*ss During Evidence of shaft seal Ieakage has been

documented on all engines repaired at WSTF to d&e. A standard step in the engine decontamination procedure calls for the removal of caps from’the FP and OP-2A/2B ports (Figure 2) to check for evidence of propellant leakage (i.e., propellant vapor). Evidence of fuel ‘leakage will tend to be more

’ NASA Johnson Space Center Discrepancy Record NI-IB-OME- 970028.

’ NASA Johnson Space Center Discrepancy Record NHB-OME- 970029.

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ATP leakage tests performed prior to hot firing this engine, no detectable leakage was measured on the oxidizer ball-valve shaft seals and three of the four fuel shaft seals. Leakage within allowable specifications was measured at the upstream fuel ball- valve inboard shaft seal. When post hot-fire ATP helium leakage tests were performed on the primary and secondary shaft seals, no detectable leakage was noted at any of the shaft seal locations.n This indicates that the m igration of propellant vapors across the shaft seals can occur during engine hot-firing, yet not register during later leakage tests using helium as a test media.

There is a possibility that seal leakage is occurring during firing operations when pressure in the ball-valve cavities exceeds the pressure at which the shaft seal leakage tests are performed.’ ATP requirements for the ball-valve cavity relief valves (annotated as CKV- 1 and CKV-2 in Figure 2) call for a cracking pressure of between 100-200 psig. ** The Orbiter Maintenance Requirements and Specifications Document (OMRSD) allows for a cracking pressure of not greater than 300 psid.tt During firing operations, inlet pressure to the series valve is approximately 203 psia on the oxidizer side and 230 psia on the fuel side.*l During static conditions, line pressure at the inlet, or upstream ball valves is approximately 250 psia (nominal tank supply pressure). Since the cavity relief valves vent back into the propellant inlet lines, pressure inside the liquid locked ball-valve cavities could conceivably reach as high as 453 to 480 psia (using ATP allowable cracking pressures), or even 550 psia (based on OMRSD allowable cracking pressure) before the cavity relief valves open to relieve the pressure. Both of these possibilities greatly exceed the ATP shaft seal leak test pressure of 320 psig.

’ WSTF Job Instruction PROP-CTF-0111 .A. 0Ms-E A TP- Propellant System and Series Valve Internal Leak Tests. NASA Johnson Space Center Tests performed on March 24 and March 25, 1997.

’ Conversation with R. Burton, WSTF Fleet Leader Program Lead Engineer, April 24, 1997.

‘* ATC-47077AN. Acceptance Test Procedure for OMS Rocket Engine. Aerojet TechSystems, February 12, 1990.

” Space Shuttle Operations and Maintenance Regulation and Specifications Document (OMSRSD) Orbital Maneuvering Q,s/ettz, V43 FILE III, July 12, 1996.

zt Acrojet TechSystems. Propellant and Pneumatic System Design Pressures. Customer Engineering Memo (CEM) No. 3 13, November 1, 1976.

Another possibility is that seal leakage occurs across the shaft seals during the transition time when the ball valves are opening or closing. This possibility is one that is currently being investigated by the Fleet Leader Program at WSTF using low-pressure, high- accuracy transducers installed at the FP-2A/2B and OP-2A/2B ports on OMS Engine S/N 2B. Data gathered from future tests at these points will be evaluated to study if this phenomena is occurring.

Based on the possibilities discussed above, a review of the pressures when shaft seal leakage tests are performed would be prudent. To replicate actual possible operational pressure, leakage tests could be performed at pressures in the 450 to 550 psia range. Prior to implementing this, however, a review would need to be conducted of series valve maximum operating and working pressures.

Pinion Shaft Damage

Leakage of propellant and/or propellant vapors across the pinion shaft seals can ultimately result in damage to the ball-valve pinion shaft surface. This is particularly likely on the oxidizer pinion shafts where long-term exposure can result in the build-up of byproduct particles or tarnishing of the shaft. Oxidizer ball-valve pinion shafts on most engines repaired at WSTF have been severely tarnished (presumably due to long-term oxidizer exposure) and required polishing. Once contamination builds up between the shaft and the seals, the likelihood of scratching both the seal and the shaft itself increases dramatically. Scratching of these surfaces can, in turn, contribute to additional leakage across the seals. Leakage across both shaft seals on each of the four ball valves has been documented on most engines repaired at WSTF. Fuel and oxidizer vapors have been noted on a regular basis when the ball valves are removed from the actuator assemblies. At the fuel ball-valve interface joints with the actuators, tieI byproduct has been documented several times as literally standing inside the actuator housing (Figures 3 and 4). Laboratory data on this residue typically indicate that fuel leakage has occurred.

The accumulation of contamination and/or particles on the seals and seal&g surfaces of the pinion shaft can also result in damage during the disassembly process. Pinion bores on the ball valve housings have been scratched on numerous occasion during removal of and seals, as well as during removal of the seals from the shafts themselves. Since a repair

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. _. / .-_ . . . . Figure 3. Fuel Byproduct

Figure 4. Fuel Byproduct in Actuator Housing s *

method exists.for polishing of the pinion shafts and the ball-valve housing bores, the scratching that occurs during disassembly can be mitigated. Scratching, however, that occurs due to cycling of the valve during hot-fire operations can result in greater leakage as described above. .’

Sleeve Bearings

Each OMS engine series valve requires a total of 16 sleeved bearings that serve to prevent metal-to- metal contact between moving parts. These bearings are located at the ball-valve pinion shafts (12 bearings total) and the actuator rack assemblies (4 bearings

total). All of the bearings are titanium with a material sleeve lining. The sleeving is made up of two layers of a plastic film, one layer of fluoroglas fabric, and a final surface layer of PTFE. The PTFE layer is the actual contact surface with the ball-valve pinions and rack assemblies. The layers’of material are bonded by the original equipment manufacturer (OEM) process.

On several of the engines repaired. at WSTF the sleeved bearings were found to be deteriorated to the point that the sleeve material had actually separated from the metal bearing. As a result, consideration is being given to making these type of bearings mandatory replacement items any time an engine is sent for repair or overhaul.

A potential consequence of this anomaly is that metal-to-metal contact could be occurring each time a series valve is cycled OPEN/CLOSED. The area of greatest concern are the 1186734-29- and 1186735- 29-rack bearings located inside the series valve actuator assemblies. In some cases, the sleeving on these bearings have become completely separated from the titanium portion of the bearing and are traveling with the rack (Figure 5). As a result, large amounts of debris from the sleeves and bearings is generated inside of the actuator housing (Figure 6).

Figure 5. Oxidiier Bore Sleeve Separated from TitanicBearing

Loose sleeving,. particulate generation, and metal-to-metal contact can result in numerous performance-hindering scenarios within the actuators.

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Figure 6. Oxidizer Bore Particles Generated

Sticking or jamming of the rack could conceivably occur due to the additional travel friction associated with the sleeve separation problems. Evidence of this rnight be seen in reduced series valve performance; i.e., valve opening and closing times. Depending on the severity of the problem, it is conceivable that actual jamming could occur between the bearing and rack, preventing the valves from fully cycling. It is very likely that this is what was occurring on OMS engine serial number 2B when slower response times and out-of-specification opening positions were noted during Fleet Lead test firings. In addition, particulate generated from this anomaly could migrate either toward the actuator mid-body seals or, in the case of the oxidizer ball valve and actuator connections, toward the actuator LVDT’s. Damage or scratching that might occur at the mid-body actuator seals because of debris could ultimately result in cross leakage of fuel and oxidizer vapors. Particulate migrating into the LVDT areas could settle into the base of the spring guide and possibly affect LVDT reliability.

Synthetic Rubber O-Rings

O-rings used in the series valve actuators are made of a chlorobutyl compound manufactured by Parco, Inc. and are identified as type AS8040H3 (with a size suffix) per Aerojet TechSystems Standard AS8040. The actuator rack assembly has two o-rings. The primary seal on the rack assembly is in continuous contact with the walls of the actuators, while the secondary seal makes contact with a

shoulder in the actuator housing only when the control valves are energized and the ball valves are open. The secondary seal is also an integral part of the rack assembly in that it is permanently affixed to the underside of the piston. Two additional o-rings provide sealing capability for the actuator cap, while two more o-rings provide the seal between each control valve and the actuator caps. Finally, a single o-ring is installed at each vent check valve where it connects to a control valve.

Based on inspection of these various seals during disassembly of OMS engines serviced at WSTF; however, long-term exposure would appear to have some detrimental effects on the resistance of these items to propellant residue and vapors. Many of the o-rings that are removed from actuator assemblies during repair are sticky, as if deterioration occurred. In some cases, when these seals are removed, numerous small strings of sticky residue occurred as the o-ring was pulled from its sealing surface. On several rack assemblies, the permanently affixed secondary seal separated from the underside of the piston. All of these chlorobutyl o-rings are located on the fuel ball-valve ends of the actuator assemblies and are compatible or resistant to monomethyl- hydrazine (MMH). According to Aerojet documentation, these seals are “intended for use as an elastomeric part where long term contact with propellant. . . is necessary.“‘iB

Evidence of propellant residue and vapors migrating beyond the primary rack seal and into the pneumatic cavity above the piston (and therefore below the control valves) has also been noted on several engines. O-rings that are removed from the actuator caps, control valves, and CV vent check valves will typically register fuel vapors when measured with an Interscan unit. Since o-rings on the actuator cap, control valve, and vent check valves are of the same material as the rack o-rings, it is conceivable that degradation of these items could occur over time as well.

Shelf-life of the synthetic elastomer o-rings is another issue. Aerojet Standard AS8040 notes that these o-rings are to be age-controlled per a specific standard, Age Control of Age-Sensitive Elastomeric Material. This standard defines age control as “ . . the designation of a specific maximum period of age after

Aerojet TechSystems. Elastomers, Monomelhylhydrazine, and Hydrazine-Uns-Dimethylhydrazine (50% N2Hq - 50% UDMH) Resistant. Material Specification ATC-44329F. April 11, 1988.

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cure date or assembly date that will assure desired conformance characteristics of an elastomer. Age control is based on the premise that elastomers deteriorate upon exposure to ozone, oxygen, heat, sunlight, rain, and other similar environmental factors.” The standard indicates that o-rings greater than 8 quarters in age (based on receipt from the OEM) should not be installed in an assembly. The shelf life of the chlorobutyl o-rings was extended from 8 quarters (i.e., 2 years) to 15 years in October of 1996, based on testing of several 19-year old o-rings that had been sent to WSTF for installation onto OMS engine S/N 111 .nn The testing of the 19-year old 0:

’ rings was performed by Parco, Inc., and indicated that the samples passed all original hardness, tensile, elongation, and modulus requirements for the AS8040H3 type o-ring. Two of the three samples tested, however, had specific gravities slightly above the requirements of the AS8040H3 requirements.” A point to note is that the testing that was performed at. Parco, Inc., was not performed per the original Aerojet Material Specification (ATC-44329) which requires that some samples be subjected to propellant exposure prior to being tested.

phenomena has occurred on nearly every engine repaired at WSTF to date. The LVDT core is installed in a ball bearing which allows the core free movement (in rotation and translation). The bearing/LVDT core is, in turn, installed into the spring guide using a threaded ring. The installation procedure is dimension -critical in that shimming is performed both at the core to bearing connection and bearing to spring guide connection to ensure dimensional tolerances. When debris accumulates at the base of the core and spring guide connection, seizing of the LVDT core can result. Nine of the eleven LVDT cores removed from engine serviced at WSTF have been frozen in-place due to debris accumulated at the base of the spring guide (Figures 7, 8, and 9). Analysis of the debris taken from several LVDTs and spring guides indicates the presence of metal nitrate material corrosion byproducts that are due to oxidizer.leakage.

It has been recommended to the program that one or all of the following test scenarios be considered: 1) Testing of older o-ring specimens be repeated in accordance with ATC-44329; i.e., exposed to MMH, then tested for volume, change, weight change, hardness, tensile change, elongation change, and modulus change. 2) Testing, without additional propellant exposure, be performed on o-rings removed from OMS engine sent’to WSTF for overhaul. Performing this additional testing would provide valuable data relating to age/performance issues versus long term exposure to MMH:

Actuator/LVDT Bore Issues

Since the LVDT core is supposed to be able to rotate and move freely at the base, it is possible that being frozen into place could result in damage to the core and/or the transducer portion of the LVDT. In addition, build-up of metal nitrates on the core itself and the bore of the transducer could result in damage. Depending on the position and rigidness of the core

to base connection, the core could be frozen into a position such that it is scratching the interior of the transducer when the valves are cycled. Further, the friction created could actually result in galling of the core and transducer. In a worst case, the LVDT core itself could become bent and damaged beyond use, or even pulled out of the bearing, making the LVDT signal give an erroneous output. This was the case with OMS engine serial number 107. During mission STS-91, the core assembly was pulled completely through the ball bearing and rendered non-functional. An in-place removal and replacement of this LVDT was performed by WSTF personnel at KSC in July of 1998.

Because the spring guide in each actuator assembly does not seal against the actuator housing, debris that may be generated inside the assembly can easily migrate past the spring guide-and accumulate at the base of the spring guide and LVDT core. This

In Rockwell International. IDMRD Revision Request - ShelfLife of O-Rings. Internal Letter 284-200-96-349, October 17, 1996.

ifi! Letter from Ronald R. Campbell, Vice President of Research and Development. 70 Dwometer Chlorobuyl Compound 0823- 70 Samples Refztrnedfor Testing. Parco, Inc., September 20, 1996.

Migration of propellant vapors and residue into the actuators has also resulted in corrosive action on the springs that are used to close the ball valves. The springs are manufactured from 17-7 PH CRES, a material that is susceptible to corrosion when exposed to oxidizer propellant for long periods of time. Most springs removed from OMS engines sent for overhaul at WSTF exhibit signs of a rust colored build-up (Figures 10 and 11). Analysis of particle samples removed from the.springs typically indicates metal nitrate type material corrosion byproducts resulting from oxidizer leakage and ambient air exposure. The

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Figure 7. Actuator 1 Spring Guide

standard presence of iron, nickel, chromium, and aluminum almost completely match the material composition of the actuator springs (17-7 PH CRES). During disassembly of engine S/N 111 at the WSTF, standing liquid was noted on one of the actuator springs. Analysis of a sample from this spring indicated that the liquid was an aqueous and nitric acid solution, again the result of oxidizer leakage.

CONCLUSIONS

The discussion and evidence presented in this paper represent a simple cause and effect issue. The root cause of all of the problems discussed is the leakage of propellant and/or propellant vapors across the ball-valve shaft seals on the OMS-E series valve. Evidence of both fuel and oxidizer leakage into the series valve actuators is abundant. It is highly unlikely that an evaluation and re-design of the current shaft seal configuration will occur in the future, therefore, leakage of propellant and/or propellant vapors across the ball-valve shaft seals will continue to occur. The nearest term preventative solution to the issues discussed in this report is to reduce the damage that can occur over time by implementing a routine overhaul and maintenance program for the engine. This would allow for the replacement of all seals, polishing, and cleaning of all areas where propellant exposure has occurred, and in turn increase the productive life of the engine.

Figure 8. LVDT-1 Core and Bearing Assembly

Figure 9. LVDT- 1

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Figure 10. Rust on Springs

Figure 11. Actuator 2 Inner Spring

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