Propellant Feed System Leak Detection Lessons Learned · PDF fileNASA Dryden Flight Research...

NASA/TM-1999-206590

Propellant Feed System Leak Detection—Lessons Learned From the Linear Aerospike SR-71 Experiment (LASRE)

Neal Hass, Masashi Mizukami, Bradford A. Neal, Clinton St. John NASA Dryden Flight Research CenterEdwards, California

Robert J. Beil

NASA Kennedy Space CenterKennedy Space Center, Florida

Timothy P. Griffin

Dynacs Engineering Co., Inc.Kennedy Space Center, Florida

November 1999

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NASA/TM-1999-206590

Propellant Feed System Leak Detection—Lessons Learned From the Linear Aerospike SR-71 Experiment (LASRE)

Neal Hass, Masashi Mizukami, Bradford A. Neal, Clinton St. John NASA Dryden Flight Research CenterEdwards, California

Robert J. Beil


Timothy P. Griffin


November 1999

National Aeronautics andSpace Administration

Dryden Flight Research CenterEdwards, California 93523-0273

NOTICE

Use of trade names or names of manufacturers in this document does not constitute an official endorsementof such products or manufacturers, either expressed or implied, by the National Aeronautics andSpace Administration.

Available from the following:

NASA Center for AeroSpace Information (CASI) National Technical Information Service (NTIS)7121 Standard Drive 5285 Port Royal RoadHanover, MD 21076-1320 Springfield, VA 22161-2171(301) 621-0390 (703) 487-4650

1

American Institute of Aeronautics and Astronautics

*Aerospace Propulsion Engineer, Member AIAA†Aerospace Propulsion Engineer, Member AIAA‡Operations Engineer§NASA Co-op Student, Purdue University, Student member AIAA

¶Senior Engineer#Space Shuttle Main Propulsion System Lead Engineer

Copyright

1999 by the American Institute of Aeronautics andAstronautics, Inc. No copyright is asserted in the United States underTitle 17, U.S. Code. The U.S. Government has a royalty-free license toexercise all rights under the copyright claimed herein for Governmen-tal purposes. All other rights are reserved by the copyright owner.

PROPELLANT FEED SYSTEM LEAK DETECTION—LESSONS LEARNED FROM THE LINEAR AEROSPIKE SR-71 EXPERIMENT

(LASRE)

Neal Hass,

*

Masashi Mizukami,

†

Bradford A. Neal,

‡

Clinton St. John

§

NASA Dryden Flight Research CenterEdwards, California

Robert J. Beil

¶


Timothy P. Griffin

#


Abstract

This paper presents pertinent results and assessment ofpropellant feed system leak detection as applied to theLinear Aerospike SR-71 Experiment (LASRE) programflown at the NASA Dryden Flight Research Center,Edwards, California. The LASRE was a flight test of anaerospike rocket engine using liquid oxygen and high-pressure gaseous hydrogen as propellants. The flightsafety of the crew and the experiment demanded proventechnologies and techniques that could detect leaks andassess the integrity of hazardous propellant feedsystems. Point source detection and systematic detectionwere used. Point source detection was adequate forcatching gross leakage from components of thepropellant feed systems, but insufficient for clearingLASRE to levels of acceptability. Systematic detection,which used high-resolution instrumentation to evaluatethe health of the system within a closed volume,provided a better means for assessing leak hazards.Oxygen sensors detected a leak rate of approximately0.04 cubic inches per second of liquid oxygen. Pressuresensor data revealed speculated cryogenic boiloff

through the fittings of the oxygen system, but location ofthe source(s) was indeterminable. Ultimately, LASREwas canceled because leak detection techniques wereunable to verify that oxygen levels could be maintainedbelow flammability limits.

Nomenclature

Cda theoretical orifice size

GH

2

gaseous hydrogen

GHe gaseous helium

GN

2

gaseous nitrogen

GO

2

gaseous oxygen

HST Helium Signature Test

kPa kiloPascals

KSC NASA Kennedy Space Center, Florida

LASRE Linear Aerospike SR-71 Experiment

lbm pounds mass

LN

2

liquid nitrogen

LO

2

liquid oxygen

LOXMAIN electrical signal name for valve command

MPS Space Shuttle main propulsion system

mdot mass flow

N

2

nitrogen

O

2

oxygen

P pressure

2


ambient pressure

PODPRES electrical signal name for pod pressure

ppm parts per million

PRECHILL electrical signal name for valve command

psi pounds per square inch

psia pounds per square inch, absolute

Q volumetric flow rate

R specific gas constant

°R degrees Rankine

Re Reynolds number

scim standard cubic inches per minute

slpm standard liters per minute

T temperature, °R

WSTF NASA White Sands Test Facility, New Mexico

0 (zero) as a subscript denotes initial conditions

change in mass of hydrogen

(rho) density

Introduction

The Linear Aerospike SR-71 Experiment (LASRE) wasa semi-span 20-percent scale model of the LockheedMartin X-33 vehicle with an integrated linear aerospikerocket engine fed by liquid oxygen and gaseoushydrogen. The model was mounted to the upperfuselage of the SR-71 aircraft and flight tested with theintention of obtaining data on the pressurecompensation performance of the aerospike engine. Theprogram evolved as a partner flight test experiment thatdirectly supported the Lockheed Martin X-33technology demonstrator, a NASA “Access to Space”program. It was the task of NASA Dryden FlightResearch Center to perform the flight tests of theLASRE, and do so in the safest manner possible.Coupling the hazards of this propellant feed system witha piloted flight test vehicle posed a challenging andinteresting safety problem. Important issues includedearly detection of and quantification of the leak ratefrom high-pressure gaseous hydrogen and liquid oxygenfeed lines in a closed, nitrogen purged environment.This hazardous combination required significantvalidation for confidence in the integrity of the designand operation of the systems. It was not without somerisks, and those risks had to be clearly defined byapplying practical techniques and tools that would

gather data and assess the leak rate of each system.Unfortunately for LASRE, the systematic leak detectionsystems indicated an unacceptable leak rate of liquidoxygen into the pod environment. The leak source(s)could not be found using what little instrumentation wasavailable and techniques available for detection. Theseresults had a major influence on the decision todiscontinue the project. A discussion follows of thoseleak detection techniques and tools, some of the data,and an analysis of results.

A great deal of experimental research has already beenconducted upon the flammability of hydrogen andoxygen mixture ratios. This research demonstrated thatthe limits for non-flammability of an oxygen andhydrogen mixture is an oxygen volume fraction

≤

4 percent and a hydrogen volume fraction

≤

4 percent

.

1, 2

These limits are suggested as the rule ofthumb for mission safety for any operations using acombination of these gas components.

Investigations into reduced-pressures flammability-limit testing with oxygen and hydrogen mixtures havebeen conducted as well. Figure 1

3

shows resultscompiled by NASA White Sands Test Facility (WSTF)on the flammability of reduced-pressure hydrogen-oxygen-nitrogen mixtures. Regions to the left and belowthe isobars are non-flammable at that pressure.Figure 2

4

shows similar results of Benz’s work onvolume sensitivity and reduced pressures toflammability limits. The results from both of theseresearch efforts indicate that increases in altitude causelittle, if any, favorable relief in the flammability limits ofhydrogen and oxygen mixtures until altitudes in excessof 80,000 feet (0.4 psia/2.8 kPa pressure altitude) areachieved. Even then, low leak rates from the propellantfeed systems might provide enough volume fraction andpressure, if contained, to create a hazardous mixture.Mixed concentrations above these flammability limitsare clearly defined as hazardous and require extremelylow ignition energy to initiate combustion. In someinstances these conditions have led to detonation. TheLASRE operational limits of oxygen and fuelconcentrations follow the purge discussion in theExperiment System Description section.

Ballistic profiles flown by most launch vehicles shortenthe exposure to hazards of flammability with hydrogenand oxygen concentrations by getting very quicklyabove the pressure altitude where flammability issuesexist. Aerospace vehicles designed to operate over flightprofile requirements similar to those of conventionalcommercial aircraft and using hydrogen and oxygenmixtures for fuel, must address flammability as a safety-

Pamb

∆mH2

ρ

3


of-flight issue any time they are within the earth’satmosphere. To meet the safety requirements ofprograms like LASRE, a higher priority is set on leakdetection and health assessment capabilities forpropellant feed systems than what has been acceptablein the past.

It is the purpose of this paper to provide an assessmentof particular leak detection methods and techniques forhigh-pressure gaseous hydrogen and liquid oxygenpropellant feed systems based upon lessons learnedfrom LASRE. In addition, the paper providessuggestions for needed future research and new insightinto leak detection strategies, methods, andtechnologies. One goal of this work is to provideinformation that will be useful for improving thesuccess of leak detection and assessment for current andfuture research programs employing hazardouspropellant feed systems for aerospace vehicles.

A special thanks is extended to the personnel atKennedy Space Center and White Sands Missile Rangethat supported the LASRE program. The insight,experience, and wisdom from these personnel was awelcome addition to the team and they provided asignificant contribution to the effort.

Use of trade names or names of manufacturers in thisdocument does not constitute an official endorsement ofsuch products or manufacturers, either expressed orimplied, by the National Aeronautics and SpaceAdministration.

Experiment System Description

The LASRE was a propulsion flight researchexperiment that intended to obtain hot plumeperformance data at reduced pressures and multipleMach numbers for validation and calibration of designpredictions.

5

The linear aerospike rocket engine issignificantly different than the conventional bell nozzledesign, in the respect that the aerodynamic flow over thevehicle alters with altitude changes resulting in a nozzlethat compensates with altitude. Theory and groundtesting suggests that the aerospike engine operates withgreater efficiency over operating altitudes whencompared to the conventional single design point bellnozzle. Data collected by the LASRE flight testing wasto be used to support computational fluid dynamicsmodels used for development of the engine and itsintegration with the X-33 lifting body design. The feedsystems designed for LASRE do not reflect those to beemployed by the X-33 program.

The LASRE was, in essence, a unique, flightworthy,self-contained, rocket engine test stand. The system wascomposed of a 20-percent scale, semi-span lifting bodymodel of the Lockheed Martin X-33 attached to theupper fuselage of the SR-71 aircraft. A photograph ofthe LASRE is shown in figure 3. A simplified schematicof the propellant feed systems internal to the LASRE isshown in figure 4. As can be seen from the schematic,the flight test hardware is comprised of several partsidentified as the “canoe,” the “kayak,” the “reflectionplane,” and the “model.” The system, as a whole, wasreferred to as the “pod.”

The canoe is a long and sleek fairing design that directlymounts to the SR-71 upper fuselage and is the base ofthe flight test structure. Contained within this canoe arefive gaseous hydrogen (GH

2

) tanks capable of storing upto 27 lbm at 6000 psi, two cooling water tanks, and three10,000 psi helium (He) pressurization tanks.

The kayak is a structure above the canoe that sets theangle of incidence for the model. Atop the kayak is areflection plane that acts to isolate the flow-field effectsbetween the model and the SR-71 aircraft and pose asthe plane of symmetry.

The model is mounted to the top of the reflection plane.Contained within the model are the liquid oxygen (LO

2

)tanks capable of storing 335 lbm and two additional10,000 psi helium storage tanks used for pressurization.Integrated vertically across the base of the model areeight linear, aerospike thruster segments, arranged fourby two running spanwise.

A hollow structure known as the “sewer pipe” extendsbetween the canoe and model (fig. 5). This structure isopen-ended and facilitates the instrumentation leads,hydrogen feed lines, cooling water lines to the engine,and allows for purge to flow freely between the modeland the canoe. The sewer pipe also carries the loads tobe measured by the force balance.

The fuel used by LASRE was gaseous hydrogen (GH

2

).The gaseous hydrogen propellant system operated as ablowdown type of feed system employing a regulatorand flow control valve. Once testing was completed,helium purged the feed system and pressurized the tanksto an inert state.

The oxidizer used was liquid oxygen (LO

2

). The liquidoxygen propellant was a pressure-fed system using theonboard helium to provide positive pressure in theoxygen tanks. Upon completion of a test, the remaining

4


liquid oxygen was forced overboard and the linespurged by the onboard helium supply.

To mitigate some of the hazards associated with thesetypes of propellant feed systems, the volumes within thecanoe and model were purged with nitrogen. Thenitrogen was supplied from two liquid nitrogen Dewarflasks that were stored within the SR-71 aircraft. EachDewar flask has a capacity of 1.76 ft

3

.

All of the systems employed within the pod wereheavily instrumented to obtain status information. Alltanks and significant stages in the feed lines wereinstrumented with pressure and temperatureinstruments. Flight safety instrumentation included fire-detection thermocouples and oxygen-detection sensors.

A simplified schematic showing the approximatelocation of each of the twelve oxygen sensors is shownin figure 5. The oxygen instruments were electrolytic-type sensors originally intended for automotiveapplications. The sensors were temperature controlledto 115 °F with a heater wrap to reduce any drifting thatwould have resulted from the extreme temperaturechanges over the flight profile. The instruments had anaccuracy specification of ±1 percent volume fraction ofoxygen measured. After thorough calibration theoxygen sensors demonstrated an uncertainty that wasmuch better than the advertised accuracy, even over analtitude range from sea level up to 60,000 ft (seefigure 6).

6

There were pressure transducers at two locations withinthe pod. These instruments monitored the inside-to-outside pressure differential. This monitoring was toensure that the surface panel stress limits were notexceeded as a result of the pressure differential createdby the purge. These ambient pressure transducers werealso used as a monitor for over-pressurization resultingfrom plumbing bursts or leaks, and deflagration ordetonation of combustibles. For the purposes of thisdiscussion, the focus is on the unit monitoring the modelvolume.

Leak Detection Techniques and Results

A considerable amount of effort has been put forth todevelop techniques, tools, and strategies for identifyingleaks in propellant feed systems. The fundamentalprinciples and techniques behind leak detection ofpropellant feed systems that were developed in the dawnof the space age are still in use today. Improvements ininstrumentation resolution provide better accuracy in

assessing hazards and increased bandwidth capabilitiesallow more information to be attained nearlyinstantaneously for clarification of the hazard.

The methods and techniques for detection were brokendown into two classifications for this paper. The firstclassification is the point source leak detection whichencompasses detection and evaluation of leakage atindividual sources. The second classification, systematicleak detection, detects and evaluates the overall leakagefrom the system.

Point Source Detection Tools and Techniques

Point source methods for leak detection of gaseous andcryogenic systems include: visual inspection, bubblesolution, ultrasonic leak detectors, thermal conductivitysensor, joint bagging, and mass spectrometers. Inaddition to these tools, inspection of cryogenic lines andfittings with the naked eye is always a useful tool. Whencryogenics are flowing drips and vapor trails can oftenbe seen. All of these techniques and tools require directaccess to each of the fittings and components of thepropellant feed system in order to evaluate the integrityat each junction.

For LASRE, the leak-rate specification for each fittingwas to be no greater than the equivalent of bubble tight(a bubble formation within one minute of time). As aresult of the way the LASRE system was designed, alarge portion of the fluid system fittings and tubing wereunable to be leak-checked under static conditions. Inaddition, the length of time for which the propellantfeed systems operated (3 to 5 sec) made it difficult toleak check the total system while flowing. A flowingleak test, where cryogenic fluids were used in the liquidsystems and gases were used in the gaseous systems,was performed. A flow test using the actual propellantswhen checking for leak detection is a dangerousexercise and is not recommended. The safety of thepersonnel performing the inspection is at risk. Asubstitution of inert cryogenics and gases (liquidnitrogen and helium) were used to check the system inorder to remove the hazards of flammability. Thisallowed all of the systems to be inspected in conditionsas near as possible to the actual operating conditions ofpressure and temperature. To yield a more stringent testcondition, these tests also took into account thetransients associated with the operation of the system.During these tests personnel were located strategicallyaround the system to perform visual and point sourceinspections.

5


Bubble Check

The minimal detectable leak rate of bubble checking istypically on the order of 10

–3

standard cubic inches perminute (scim).

7

For LASRE, only a modest number ofthe fittings could be reached to apply this technique.Even though a fitting could be reached, there was noassurance that a full inspection could be performed.Using high-pressure helium gas as a substitute, thistechnique was applied during a pressure decay test onthe tank and line leading to the main shutoff valve tocheck for leakage. Several leaks were detected on thehydrogen main feed line and downstream of the split toeach side of the engine during transient testing.Application of this technique to the cryogenic flow testsis not practical because the bubble solution freezes onthe joint under inspection.

Ultrasonic Detector

An ultrasonic leak detector was employed on LASREearly on in the program to evaluate leakage from fittingsduring both static and transient testing of the gaseoushydrogen system. This instrument measures the high-frequency pressure waves that emit from a small sonicgaseous jet. Its minimal detectable leak rate is on theorder of 10

–3

scim. This leak detector could only reach alimited number of fittings because of accessibility. It isnot known if any significant leakage was detected usingthis instrument during static pressure tests. Backgroundnoise during the transient testing cycles rendered theultrasonic leak detector ineffectual.

Thermal Conductivity Detector

A thermal-conductivity-based leak detector was alsoused for leak detection of LASRE. This is a small,inexpensive handheld device. It can detect lowconcentrations of leaking gas by measuring smallchanges in the air thermal conductivity. However, it isnot gas-specific, and must be set for the specific gasbeing used for leak testing, such as helium. For thisdevice to read correctly, the background environmentmust be steady (such as air) and there should beno contaminant gases in the region. Minimumdetectable leak rate is comparable to bubble checking,about 10

–3

scim. Accessibility to hidden components isbetter than with bubble checking and it does not get thesystem wet. This device was useful for locating a fewleaking fittings, and for determining leaks through thevent valves and out the vent lines.

Joint Bagging

The technique of joint bagging was attempted byLASRE. It was unsuccessful in obtaining any resultsbecause of the lack of means to adequately measure thebag volume and sample the volume captured. Lack ofaccessibility to much of the systems made this techniquevery impractical.

Mass Spectrometer

A mass spectrometer that was on loan from KSC wasused by LASRE, but never as a point source leakdetection instrument. The minimum detectable leak rateof the instrument is several orders of magnitude betterthan these other point source detectors. It has beensuccessful in detecting leaks on the order of 10

–7

scim,and is a little better in gaining accessibility. By attachinga wand to the sniffer line it can be used to probe into thevolume up to the fittings in question.

Systematic Detection Tools and Techniques

The specialized instrumentation used for LASRE assystematic leak detectors were: pressure transducers,gas species detectors, and a mass spectrometer. Wellcalibrated and strategically placed, these highlyspecialized sensors can provide a wealth of informationfor assessing the environment in which the feed systemsoperate. A drawback of these techniques is that thecoverage must be dense enough to be able to makeconservative estimations about the overall leak rate ofthe system. With testing and familiarization a highdegree of confidence can be achieved with thesetechniques for manned and autonomous programs thatanticipate multiple flight tests.

Hydrogen Tank Pressure Decay Leak Detection

8

In order to infer the presence of fuel in the pod, a real-time pressure decay method was used to detect theoverall hydrogen leakage from the wetted portion of thesystem while the operations were static. Conventionally,pressure decay leak detection takes a long time toperform, and is usually done during system checkout. Anoticeable pressure drop in the gaseous hydrogen tanksduring flight operation would indicate a large leak, orperhaps a thermal transient. However, due to the lack offlight-qualified hydrogen sensors, a pressure decaymethod was implemented for real time in-flight use.This method was for leaks from the hydrogen tanks andlines up to the main shutoff valve in a static mode only,and not for leaks in the hydrogen lines or during flow.

6


In theory, hydrogen leak rate can be determined fromthe change in mass of hydrogen in the tanks asfollows:

Tank pressure (P) was measured by a pressuretransducer. Hydrogen gas temperature (T) wasmeasured by two redundant thermocouples mounted onprobes inside the tanks and the data was averagedtogether for one reading. To obtain an accuratemeasurement of gas temperature under changingconditions, a tank surface measurement would havebeen inadequate. Tank volume (V) was assumed to beconstant. Compressibility (Z) was a function of P and T,and the subscript 0 denotes initial conditions. R is thegas constant for hydrogen.

In software, digital low-pass filtering was applied to the signal, to remove high-frequency random noise

and to facilitate data interpretation. The filter timeconstant was adjustable and was set by the user to obtaina readable signal, while preserving reasonable responsetime and quick recovery from data spikes. Magnitudelimits were imposed to prevent telemetry data spikesfrom corrupting the calculation. The calculation couldbe configured for either hydrogen or inert helium as theworking gas.

The ability to detect leaks depended on discerning smallchanges in pressure and temperature, which was afunction of instrumentation resolution. The traceshowed good stability under the varying ambientconditions of flight. Uncertainty analysis indicated thata mass loss of 0.15 lbm or more could be detected.Ground testing, done by releasing controlled smallamounts of gas from the tanks and observing the trace,showed that a mass loss as low as 0.03 lbm could bedetected.

In the control room, if a mass loss rate of greater than0.03 lbm was seen in 8 minutes or less, it would beconsidered a positive determination of leakage, andsteps would be initiated to safe the system by dumpingpropellants overboard. This leak rate corresponds toabout 4 percent of the nitrogen ground operations purgeflow rate in the vicinity of the hydrogen tanks, or about1460 scim. This was judged to be the minimum leak ratereliably detectable in a reasonable timeframe. However,it is still a substantial and potentially hazardous quantityof hydrogen and a quantity that is capable of locally

forming combustible mixtures in the pod. Lower leakrates could possibly be detected over a longer period oftime.

The hydrogen leak detection algorithm was a useful tooland the only available means of real-time hydrogen leakdetection on LASRE, but could not be relied upon todetect all hazardous leaks. This algorithm could detectmoderate to large tank leaks, or smaller leaks over along period of time. It could not detect small but stillhazardous leaks in a timely manner.

Concentration Assessment Techniques

It was determined through the interpretation of theindustry and NASA safety practices that the acceptablelimits of oxygen and hydrogen concentration for theflight safety of this particular flight test was to be lessthan or equal to 1 percent at any time the system wasstatic. This represented one-quarter of the lowerflammability limit providing a safety factor of four tothe test. Post engine testing, the oxygen levels wereallowed to reach 4 percent, but hydrogen acceptabilitylimits were to remain unchanged. Exceeding theselimits meant a violation of the safety of flight rules, andthe program was not allowed to proceed to actual hotfire testing unless the violations were corrected.

To clearly interpret the concentrations measured so thatproper assessment of the hazard can be accomplished,the purge flow rates must be known and flow pathscharacterized. The design of the nitrogen purge massflow rate for the system was set to approximately38,000 scim while conducting test operations. Thepurge rate by volume breakdown was approximately24,000 scim for the canoe and 14,000 scim for themodel. Based upon these purge rates, the maximumconcentration allowable, and the assumption ofachieving a homogeneous mixture, the maximum-allowable leak rates could be established. Themaximum-allowable leak rates, in order to maintain the1-percent volume fraction for the hydrogen system, is240 scim into the canoe. During the transients, thehydrogen system was not to leak more than 140 sciminto the model. For the oxygen system, the maximum-allowable leak rate was determined to be 140 scim forthe static period. The transient period maximum-allowable leak rate for oxygen was 560 scim.

The concentrations of escaped fuel and oxidizer withinthe LASRE were assessed by field sampling the volumewith gas species sensors. Commercial oxygen andhydrogen detectors are available for this purpose.Some detectors output partial pressures over the rangeof 0–100 percent. Other types of detectors sense very

∆mH2

∆mH2

VR---- P

ZT-------

P0

Z0T0-------------–

=

∆mH2

7


low levels, typically 10–2000 parts per million (ppm),and that may or may not be within the range ofsensitivity for adequately assessing leak rateacceptability. These detectors may also require aminimum oxygen concentration in the background forproper operation of the sensor element. The oxygenatedbackground requirement may conflict with the use offlammable propellants for testing the system integrity.

Most detectors available are designed with the intentionof being used in industrial applications operating atstandard atmospheric conditions. Employment in aflight environment requires a significant qualificationeffort and calibration at reduced pressures andconcentrations to determine range of detection andaccuracy. This information is useful in understandingand determining mission flight safety rules. Tocompensate for altitude effects, some of these sensorpackages may require an ambient pressure measurementunless a pressure compensation has already beenintegrated into the detector. Upon satisfactorycompletion of a thorough qualification test, sensors maybe placed strategically within a vehicle in the hopes ofproviding adequate coverage to confidently assess thevehicle environment and overall leakage from thesystems.

For the LASRE program, six hand-held industrialhydrogen detectors were used to help evaluate the healthof the propellant feed system. Typical characteristics are0–1000 or 0–2000 ppm hydrogen range, with anaccuracy of 1 percent of the full-scale reading. Theseunits were used in conjunction with a mass spectrometeron loan from KSC for a 3-percent gaseous hydrogenblowdown test to assess the gaseous hydrogen feedlines. All of the units used aspirators to draw samplesfrom surface ports. The sample lines were teed, so thatboth the handheld units and the mass spectrometer couldsample simultaneously. These locations are shown infigure 7. The positions were chosen to represent thoselocations where leaking gaseous hydrogen were mostlikely to occur within the volume post-testing. All of themain feed lines and flow control valves existed belowsample ports 4, 5, and 6. Sample ports 1 and 2 wereintended to catch any gaseous hydrogen coming up thesewer pipe and leaking from the main feed lines. Sampleport 3 was intended to catch any leakage from theengine manifolds. The gaseous hydrogen tank wasserviced to 4000 psi with certified 2.4-percent gaseoushydrogen in a helium balance. The failure criteria wasobservation of levels that exceeded 240 ppm, whichrepresents 1 percent of the 2.4-percent gaseoushydrogen within the canoe volume. The nitrogen purgewas not activated for the test because the handheld units

required an oxygenated background. Background andnoise levels measured from zero to ten ppm.

Figure 8(a) shows the transient data from the main flowtest. The maximum concentration detected wasapproximately 67 ppm with handheld unit no. 6.Figure 8(b) shows the transient data of the autosafefunction, which empties the contents of each systemindividually through the engine. The maximum valueattained for this test was approximately 170 ppm.Neither of these tests breached the maximum operatingconcentration of 240 ppm, which related to a localconcentration in excess of 1-percent gaseous hydrogen.For both tests, lack of concurrence with the massspectrometer values could not be rationalized, but it isspeculated that there might have been time lag andsample line switching issues. Regardless, even theaddition of the handheld units levels to massspectrometer levels measured did not breach the240 ppm acceptability limit set for the test. There is nodirect way to relate these measurements to aconservative estimate of leak rate given the smallnumber of sample points. If the system were allowed toreach equilibrium, and more sample locations wereavailable, an estimate of the mean concentration couldbe surmised. Using this surmised value and assumingthat this is a homogeneous mixture at standardconditions within the canoe volume (72 ft

3

), thenmultiplying by the percentage ratio of pure hydrogengas to the test gas, dividing these results by the timeperiod (~3 sec) of the main flow, and doing thenecessary unit conversions; will result in anapproximate leak rate from the system. Based upon the1-percent accuracy of the handheld units used for thistest, this is equivalent to a 20-ppm measurementcapability within the volume. If the system wereallowed to stabilize after the main flow, the leak rateminimally detectable by this method would beapproximately 50 scim. This leak rate was well withinthe limits of acceptability.

Onboard hydrogen detectors for in-flight assessmentpurposes were researcher requested, but not flight safetycritical to the program. Commercial systems areavailable, but were not proven to be robust equipmentwith a history for this type of application. One suchsystem was provided for the project that used apalladium-nickel sensor design developed by SandiaNational Laboratories of Albuquerque, New Mexico,and had a temperature compensation control package.This system was taken to bench tests for calibrationwhich would check out the useful pressure altituderange of the system and sensitivity of the sensors.Calibrations were performed with uncertified

8


customized nitrogen and hydrogen mixtures. Thepreliminary results of these tests were encouraging, butas a result of schedule issues, it only was accepted to usethese instruments as a discrete indicator of a hydrogenleak. Once the system was integrated into the pod, thedifference in instrumentation setup from what was usedon the calibration bench in some way corrupted thesignals. The problems were elusive and wentunresolved. The system was disregarded as anindication of in-flight hydrogen detection, which addeda higher level of risk to the program.

As stated previously, the oxygen sensors chosen forimplementation were put through rigorous qualificationtesting before they could be employed. The strategybehind their emplacement (see figure 5) was as follows.Sensors 1 and 2, being forward of the gaseous hydrogentanks, were intended to verify purge and canoe hullintegrity. Sensors 3 through 8 monitored oxygenmigrating with the purge down the length of the canoeand flowing over the gaseous hydrogen tanks and mainfeed lines. Sensor 9 was to verify that the model purgewas active and that there was no oxygen intrusionthrough the model hull. Sensor 10 was the liquid oxygenspill monitor. Sensors 11 and 12 monitored theatmosphere around the manifolds where both thegaseous hydrogen and liquid oxygen main feed linessplit into the engine block.

The detectors performance was surprising and provedbetter than expected. As a confidence check of theinstruments before activation of the nitrogen purge, theoxygen sensors were compared to standard atmosphericconditions for nominal operation checks. Afteracceptance checks, an inert environment wasestablished. Figure 9 shows a time history response ofthe oxygen sensors (with the canoe and model volumesinert) to the flight dynamics of takeoff, throughestablishment of an altitude at 31,000 ft.

Figures 10(a)–(d) show the correlation of liquid oxygenprechill and main valve flow startup with oxygendetection sensor data. Parameter PRECHILL representsthe prechill valve command and parameter LOXMAINis the main valve command. Figure 10(a) is of oxygenleakage that was detected during a first main flowthrough the system from a ground test. Figure 10(b)shows oxygen leakage detected during the second mainflow through the system from the same ground test.After the first, and sometimes second, main flow wasexecuted, the autosafing function was executed to emptythe tanks of their contents. Figure 10(c) demonstratesthe detection of oxygen leakage that occurred during theoxygen autosafing process from a ground run. Based

upon these results, the purge rate was increased. Thisincrease was expected to improve the purgeeffectiveness and flight testing resumed. Figures 10(d)and 10(e) are the oxygen leakage detection data fromthat flight, in which a single main flow and then theautosafing function were executed, respectively.

As previously stated, the oxygen sensors had anuncertainty of approximately 0.1 percent at mean sealevel (see figure 6). Using this uncertainty as theminimum detectable oxygen concentration by thesensor, knowing the volume of the model (51 ft

3

), andassuming that a 0.1-percent concentration that wasmeasured represented a homogeneous mixturethroughout the model volume; an estimate of theminimal detectable leak rate from the system could bemade. The minimum detectable leak rate of liquidoxygen is estimated to be approximately0.04 in.

3

per sec. From the data in figure 10(a), aconservative approximation of the homogeneousconcentration peak is 1.6 percent within the modelvolume after a first main flow of liquid oxygen lasting3 seconds. The leak rate of liquid oxygen from thesystem for this test is estimated to be 0.63 in.

3

per sec ofliquid oxygen. From the data in figure 10(e), aconservative approximation of the homogeneousconcentration peak is 3.5 percent within the modelvolume after a liquid oxygen autosafe blowdown testlasting approximately 30 seconds. The leak rate fromthis test is approximately 0.14 in.

3

per second of liquidoxygen. Each of these test runs show leak rate datarepresenting a composite leak rate from a transientsystem. This is what made the identification of the leaksource such a difficult task. It is believed that thequantity of liquid leakage that has been estimated, whensplit up by the number of possible leak paths from thesystem, was not detectable by visual inspectiontechniques. This data reinforces the position thatsensitive instrumentation, when strategically positioned,can provide a wealth of information on the health of thesystems.

Helium Signature Test

NASA KSC has performed a significant amount ofresearch on the use of a mass spectrometer for leakdetection of the Orbiter main engines and propellantfeed lines. Of notable interest is the technique that wasdeveloped for leak detection of the main engines andfeed lines, known as the “Helium Signature Test(HST).” The minimal leak detection capability achievedwith this technique, as applied to the Orbiter propellantfeed systems and the Space Shuttle main engines purgerate, has been measured to be on the order of 6.0 scim.

7

9


HST is a two-step process that characterizes leakagefrom the system, and then using actual leakage data,allows one to extrapolate what the operationalequivalent leakage may be. Applying the purge rate ofthe model to the results of the signature test, one canascertain the flammability hazard.

The first step of characterizing leakage is to insertgaseous helium at known flow rates into a purgedcontrol volume, using a wand at predeterminedlocations where leakage is most likely to occur,i.e. joints, instrumentation, section fittings, and valves(see figure 11(a)). The mass spectrometer then pullssamples of the purged environment from a fixed effluentlocation for each insertion point, and characterizes themass spectrometer response for a known leak rate at thatpoint. This data is used to develop calibration curves forthe known injected leak rate.

The second step involves applying back-pressure withgaseous helium to the feed system and sampling thepurge effluent from the same fixed effluent location thatwas used during the characterization test points (seefigure 11(b)). Once the readings have stabilized and arerecorded, several more back-pressure settings are usedto add more spread to the data points. The back-pressuredata is then used with the characterization data togenerate a response curve of leakage for givenoperational pressures. In theory, it is then possible toextrapolate what the overall leakage will be at theoperating pressures of the system (see Appendix).

The mass spectrometer instrument was used only for the3-percent gaseous hydrogen blowdown test discussedpreviously and the HST. Figure 12 shows theapproximate locations within the pod volume wheregaseous helium was inserted to simulate leakage for theHST. A panel near the aft left-hand side was crackedopen (~6 in. vertical slit) to act as a sink for the purge.The probe for the mass spectrometer was inserted intothis slit.

The data from the HST was separated into the canoe andthe model. Only the model data is fully analyzed in thispaper because the model volume is where the mostsignificant potential for a hazard exists by having theliquid oxygen and gaseous hydrogen systemscoexisting. The model gaseous helium insertion data forthe characterization portion of the test is plotted infigure 13(a). The average of the data was computed andplotted as well. A linear curve fit of the average data wasgenerated and overlays the real data. The worst-caselinear curve fit was generated from determining theequation of the line through the y-intercept (the

baseline) and the worst-case data point. The differencebetween the average and the worst case is used as theuncertainty of the data set. The data plotted infigure 13(a) shows a tight grouping that lends credibilityto the statement that a leak at any location along thesystem within the model volume will be readilydetectable by the mass spectrometer during the back-pressure portion of the test.

Gaseous helium was then applied through the engine,using special throat plug fittings. Samples were taken atseveral back-pressure settings in an attempt to providesome data spread. Using the concentrations measuredduring the gaseous helium back-pressure test, the leakrate at each back-pressure point can be correlated to thegaseous helium insertion rates average leak rateequation to estimate the whole system leak rate from theback-pressure setting. This data is shown in figure 13(b)with the uncertainty shown by the vertical bars. It isimportant to remember for the LASRE test case that theleakage measured during the gaseous helium back-pressure test includes any possible leakage from theliquid oxygen system. This is a result of the inability toisolate the liquid oxygen system from the gaseoushydrogen system for the gaseous helium back-pressuretest. It means that the results from the back-pressuretest, as related to the resulting gaseous hydrogen leakrate computations, are a conservative estimation,because they include any leakage from the liquidoxygen system, but the extent of the liquid oxygencontribution was not quantifiable.

Once the gaseous helium leak rates have beencalculated, it is possible to relate the gaseous heliumleak rates to gaseous hydrogen leak rates. This can onlybe achieved if a fixed-geometry choked orifice flow isassumed. This method is detailed with application of theactual data in the appendix, given the known mass flowrate of gaseous helium and its source pressure andproperties, it is possible to calculate the approximatesize of the leak orifice based upon these assumptions.Then, knowing the leak orifice size, and the gaseoushydrogen system operating pressure and properties, onecan calculate what the equivalent gaseous hydrogenmass flow rate is for this same gaseous helium leak rate.The results of this conversion are shown in figure 13(c).

Now having an estimation of the leak rates from thegaseous hydrogen system and knowing the purge flowrate, the hazard can be scaled. This is accomplished bytaking the gaseous hydrogen leak rate and dividing it bythe purge mass flow through the model. This, of course,assumes a homogeneous mixture throughout the modelvolume.

10


For the LASRE experiment, the main flow cycle wasscheduled to run for only three to five seconds. Teamconsensus was that the volume would have remainedinert and begun to nearly completely purge the leakagewhich occurred during the test within a short amount oftime thereafter. So, a better estimation of the true hazardis to take the leak rate estimated and integrate it over thethree seconds of main flow. This quantity, in proportionto the model volume, is a better assessment of thegaseous hydrogen concentration hazard. The results ofthis calculation are shown in figure 13(d).

Pressure Transducers

It is speculated that the ambient pressure sensor thatmonitored the pressure of the volume inside the modeldetected cryogenics leaking during the prechill andmain flow. Figure 14(a) shows ground run data,beginning with the command of the prechill valvePRECHILL, that reveals a very low-amplitude, high-frequency noise signal present on the pressuretransducer signal that samples the pod internal pressure.Once the main flow valve (LOXMAIN) wascommanded closed, the noise component on the signaltrace (PODPRES) disappears. This event was observedroutinely when cryogenics flowed through the oxygensystem and only when the volume was closed up.Figure 14(b) shows the in-flight data of the samephenomenon with the amplitudes a bit less than theground data. The low-frequency high-amplitude signal

that this noise is superimposed upon, is a result of thepurge performance (i.e. – liquid nitrogen flowingthrough a heater exchanger to vaporize it beforeinjection into the volumes). No other correlation to anyother system events could be made with this data.

In summary, table 1 displays leak detection techniques,brief descriptions, notes on limitations andconsiderations, and some leak rate detectionquantification broken into the two classificationsdiscussed previously. It also shows conservativelycalculated quantification of the leak rates detectable bythe techniques employed on LASRE as compared topast experiences and test results. Quantification of theleak rates is very difficult to achieve with the pointsource measurement techniques and there was very littlehistorical precedence available on the leak ratequantification of the systematic techniques. Note thatthe HST LASRE results are not indicative of theminimal detectable leak rates for the system andinstrumentation configuration because there was nocharacterization done for this technique. Only thePressure Decay Monitor Algorithm had characterizationtesting done in order to determine its minimal detectableleak rate based on the instrumentation resolution anduncertainty. The minimum detectable leak rates fromthe oxygen system is based on oxygen sensor resolution,uncertainty, and the assumption of a homogenousmixture that is based upon concentration fieldmeasurements within the model volume.

11


Tabl

e 1.

Lea

k de

tect

ion

tech

niqu

es.

Tech

niqu

eB

rief d

escr

iptio

nLi

mita

tions

and

cons

ider

atio

nsH

isto

rical

prec

eden

ceLA

SR

Ere

sults

a

a. B

ased

upo

n pu

rge

rate

, vol

ume

of L

AS

RE

com

part

men

ts, a

nd h

omog

eneo

us m

ix a

ssum

ptio

ns.

Poi

nt s

ourc

e de

tect

ion

Bub

ble

chec

kLe

akag

e in

dica

tion

by b

ubbl

es

afte

r ap

plic

atio

n of

hig

h su

rfac

e te

nsio

n flu

id to

join

ts, fi

tting

s,

etc.

Req

uire

s di

rect

acc

ess

and

full

insp

ectio

n, m

ay h

ave

slow

tim

e re

spon

se.

Sug

gest

ed u

se a

s a

disc

rete

indi

cato

r.

10.0

E-3

sci

mU

sed

succ

essf

ully

for

disc

rete

indi

catio

n.

Ultr

ason

ic le

ak d

etec

tor

Leak

age

indi

catio

n by

m

easu

rem

ent o

f ultr

ason

ic

freq

uenc

ies

gene

rate

d by

son

ic

gase

ous

jet.

Req

uire

s di

rect

acc

ess

to a

ll co

nnec

tions

, no

t rel

iabl

e fo

r tr

ansi

ent s

yste

ms

due

to

back

grou

nd n

oise

leve

ls.

10.0

E-3

sci

mIn

effe

ctua

l for

tran

sien

t te

stin

g du

e to

ba

ckgr

ound

noi

se, b

ut

usef

ul fo

r st

atic

che

cks.

The

rmal

con

duct

or g

asde

tect

orLe

akag

e in

dica

tion

by m

easu

ring

smal

l cha

nges

in a

ir th

erm

al

cond

uctiv

ity.

Req

uire

s di

rect

acc

ess

and

non-

cont

amin

ated

bac

kgro

und

gase

s.10

.0E

-3 s

cim

Use

ful f

or s

tatic

ch

ecks

.

Join

t bag

ging

Leak

age

indi

catio

n by

infla

tion

of b

agge

d jo

int,

fittin

g,

com

pone

nt.

Req

uire

s di

rect

acc

ess,

mea

ns to

cap

ture

vo

lum

e on

ce in

flate

d, ti

me

cons

umin

g.N

o da

ta a

vaila

ble

Atte

mpt

ed, b

ut

unsu

cces

sful

ly

impl

emen

ted.

Mas

s sp

ectr

omet

erS

niff

join

ts w

ith m

ass

spec

trom

eter

pro

be.

Req

uire

s di

rect

acc

ess,

tim

e co

nsum

ing.

10.0

E-7

sci

mN

ever

per

form

ed.

Sys

tem

atic

det

ectio

n

Pre

ssur

e de

cay

test

Mon

itors

mas

s lo

ss o

f sta

tic

syst

em.

Req

uire

s ch

arac

teriz

atio

n, n

ot a

pplic

able

fo

r tr

ansi

ent e

vent

s.N

o kn

own

data

1240

sci

m

Oxy

gen

gas

sens

orIn

dica

tion

by c

alib

rate

d se

nsor

(s).

Req

uire

s a

sign

ifica

nt s

enso

r ca

libra

tion

effo

rt a

nd a

fine

est

imat

ion

of v

olum

e,

poin

t sou

rce

mea

sure

men

t.

No

know

n da

ta17

60 s

cim

GO

2

2.37

cu.

in./m

inLO

2

Indu

stria

l hyd

roge

n ga

sse

nsor

Indi

catio

n by

cal

ibra

ted

sens

or(s

).M

easu

rem

ent l

evel

s lim

ited

to 2

000

ppm

, po

int s

ourc

e m

easu

rem

ent.

No

data

ava

ilabl

eN

ot e

noug

h da

ta.

Hel

ium

Sig

natu

re T

est

Cha

ract

eriz

e w

ith fa

lse

leak

age

from

sys

tem

, the

n in

stig

ate

leak

age,

follo

wed

by

scal

ing

for

fuel

.

Mea

sure

s on

ly to

tal s

yste

m le

akag

e, m

ay

not b

e ap

plic

able

for

cryo

geni

c sy

stem

s.6.

00 s

cim

b

b. B

ased

upo

n pu

rge

rate

and

vol

ume

of O

rbite

r S

pace

Shu

ttle

mai

n en

gine

s ba

y.

168

scim

12


Concluding Remarks

This document describes the results of propellant feedsystems leak detection tools and techniques applied aspart of the validation testing of the Linear AerospikeSR-71 Experiment (LASRE) program flown at NASADryden Flight Research Center, Edwards, California.This testing was conducted in order to meet the safetyrequirements associated with the use of combustible anddetonable gases aboard a piloted aircraft. The goal ofthis paper is to provide an assessment of leak detectiontechniques for hazard mitigation of hydrogen-oxygenpropellant feed systems, based upon the experiences ofthe LASRE program team. This document also providessome insight into strategies that may improve leakdetection for the success and increased safety of otherprograms, and provides suggestions for future research.

The conclusions and suggestions based upon theLASRE experience are as follows:

Propellant Feed Line Design Considerations

•

Build in provisions to leak check the majority of afluid system lines and fittings under staticconditions. There was no provision built into theLASRE design for blocking valves close to the endof the feed lines, leaving the system open toambient pressure from the source once the mainblocking valve and flow-control valves wereopened. This feature would have made it possible todo systematic static leak tests on the system. Inparticular, the design lacked the means todetermine whether the liquid oxygen leakage wascoming from the main feed lines upstream of themain and prechill junction, the prechill line, ordownstream of the main and prechill junction. Italso left no means to isolate the liquid oxygensystem from the gaseous hydrogen system whenthe Helium Signature Test was being conducted.

•

Successfully isolating the feed systems wouldreduce the instrumentation requirements necessaryto assess the health of the systems and furtherreduce the hazards. Design the individual systemssuch that the oxidizer, fuel sources, and feed linesare separated by as much purged space as isreasonably allowable. If possible, isolate thesesystems with sealed boundaries and create separatepurge paths. LASRE did take advantage of thisisolation in the sense of the gaseous hydrogenbeing stored in a compartment separate from theliquid hydrogen and the intention of the design wasto have the purge operate as two separate flowpaths.A physical boundary existed between the canoe and

model, with the exception of the sewer pipe, whichdid not preclude liquid oxygen from dripping fromits feed lines down into the compartment where thegaseous hydrogen was housed. A plate over the endof the sewer pipe with feed-throughs would havebeen an easy isolation solution early on in thebuildup phase that would have added confidence tothe system.

Purge

•

An acceptable leak rate must be clearly defined andis based predominantly upon the mass flow rate ofthe purge system. As a rule of thumb, size the purgefor the volume accordingly, so that a high-enoughReynolds number is generated to ensure turbulentflow for all phases of operation. This procedure (orhigh Reynolds number) enhances the rate ofdilution to bring the mixture to an inert state andtransports it from the volume quickly. This wouldalso maximize the acceptable leak rate, whichwould make the validation of the systems that mucheasier to achieve. The negative impact is that itmight require more stores to be carried thaninitially planned. For LASRE, the volume change-out rate was 1.5 volumes per minute for groundoperations. At test altitude (~50,000 ft), thischanged favorably to nearly 10 volumes of change-out per minute which was encouraging.

•

It is desirable to characterize the purge and validateleak detection techniques by introducing quantifiedleaks. This is accomplished by activating the purgewith the system in the configuration intended fornominal operation and inserting known leak ratesinto the volume, one at a time until stable data isgenerated. Enough source locations should beattempted to convince one that any leakage fromthe propellant systems will be indicated by thedetection system and identify flowpath patterns andcharacteristics. None of this was conducted byLASRE until the Helium Signature Test wasconducted, and as a result it indicated a purgeflowpath from the model into the canoe when thecanoe’s manual vent was opened. This was not theintention of the original design and increased thedanger of the system operation by bringing togetherpurge laced with oxygen and hydrogen. It was thenthe intention not to operate the vent unless anoverpressure was eminent to the pod.

•

Be sure to re-characterize the limits of the systemwhen changing the purge settings because the newsettings may not have achieved the desiredresponse. In the case of LASRE, it was assumed

13


that increasing the purge mass flow rate betweenthe last successful ground test and the followingflight test would improve the performance andsafety of the system operation. Unfortunately, thesegood intentions might have contributed tounfavorable results. It is speculated that the LO

2

leak indication from the flight data was causedeither by the resulting change in the purge flowpath, where indications from oxygen sensors ofprevious ground run data were not seeing theoxygen leak at its full potential, or that the liquidoxygen system had in some way degraded from thelast ground test. The purge mass flow rate change,without characterizing the results with a groundtest, added a variable that could not be accountedfor in the flight test results.

Techniques and Tools

•

Visual inspections methods are valid for gross leakchecks of cryogenic systems only. Extremely lowflow rate leakage from multiple cryogenic fittings,which vaporize nearly instantaneously, cancontribute significantly to an overall violation. Thiswas where LASRE repeatedly made its mistakes.The inspection team was looking for some kind ofvisual indication during flow tests. Most likely, theycould not detect these with the naked eye.

•

Bubble checking, joint bagging, and ultrasonicdetectors were not sufficient leak detectiontechniques to meet acceptability requirements forLASRE.

•

The pressure decay algorithm must be applied withcaution. The resolution of the instrumentationlimits this technique to detecting only grossleakage. The greater the tank volume in question,the greater the leakage before detection by thealgorithm.

•

Commercially available hydrogen detector systemsshow promise, but in this application they remain tobe proven. Considerable research in qualificationand validation testing still needs to be done toimprove this technology before achievingintegration. The full range of altitude as arequirement needs to be met as well.

•

Commercially available oxygen sensorsdemonstrated good leak detection performance, buta thorough calibration and qualification of thesesensors are a prerequisite.

•

Using high-precision pressure transducers thatmeasure ambient environmental conditions, it maybe possible to detect what might be cryogenics

leaking through fittings and boiling off. The use ofmultiple sensors might allow approximation of thelocation of the leak source. More research is neededto validate this technique. This method may belimited to single-source leak detection. To improveaccuracy in determining locations, the purge mightneed to be non-operational once the environment isinert.

•

The location and the quantity of sensors is criticalfor gathering the optimum amount of informationand assessing the health of the system. Thephilosophy generated from the LASRE experiencewas to: (1) place a higher number of oxygensensors along the fuel lines and fewer in thesurrounding regions of the oxidizer lines and(2) place a higher number of hydrogen detectorsalong the oxygen lines and fewer in the proximityof the hydrogen systems. The danger to eithersystem exists when one component is above theflammability limits and in close proximity to itscomplement system. It is unlikely that a highconcentration of fuel or oxidizer will persist at anylocation remote from the leak source as long as avalidated purge is doing its job. Therefore, anyleakage from one system may never reach the otheror do so at levels below flammability. This does notpreclude the need to validate leak detectioncapability if you have a leak of oxygen, hydrogen,or both. This may or may not be acceptabledepending upon the philosophy of the design andtest team and the risk levels associated with theprogram.

•

Because transient cryogenic systems are so difficultto leak check, a total mass capture technique mightbe employed to quantify the leak rate of the system.This could be achieved by capturing purge gasvented from the volume, beginning from the eventstart until a portion of time after the event. Onesuggestion would be to perform this capture for thetime necessary to complete at least one volumeexchange. Then knowing the timespan of the event,it would be possible to estimate the total mass lossduring the event from a mass spectrometer sampleof the mass captured.

•

Although the fundamental theory behind the HSTtechnique makes sense, further research is neededto validate the assumptions made and quantify theaccuracy. It was noted that during the LASRE HST,the effluent from the cracked panel was stratified.For improved accuracy on the effluentmeasurements, a total mass capture of steady stateconditioned effluent might be done and then a mass

14


spectrometer reading performed on this totalsample. A suggestion for mass capture would be alarge vacated bottle or simply a bag to be filled andthen samples withdrawn.

•

The HST data, supported by 3-percent blowdowndata, lent confidence that the LASRE gaseoushydrogen feed lines met acceptability criteria. Thedesign of the feed systems do not allow forseparation of the gaseous hydrogen system fromthe liquid oxygen systems during the back-pressureportion of the Helium Signature Testing. As a resultof this design shortcoming, it is speculated that theresults are composed of leakage from both feedsystems. The analysis of the data is presented asleakage from the gaseous hydrogen system onlyand believed to be a conservative estimation basedupon this information.

•

The HST was effective for characterizing leakagefrom the liquid oxygen system because it does nothave the ability to replicate the transient effects ofoperating the liquid oxygen system. It is believedthat thermal shock and loading issues inherent tothe use of cryogenics cannot be adequatelyaddressed by the results of the back-pressureportion of the test.

•

The detected leak rates by the HST at lower sourcepressures may not actually linearly extrapolate to ahigher source pressure. The extrapolation may ormay not be an appropriate assumption, dependingon the flow regime, dominant physics of the leak,and whether the leakage is linear as a result ofloading.

References

1

“Safety Standard for Hydrogen and HydrogenSystems,” NASA NSS 1740.16, Office of Safety andMission Assurance, Washington, D.C., 1995.

2

“Safety Standard for Oxygen and Oxygen Systems,”NASA NSS 1740.16, Office of Safety and MissionAssurance, Washington, D.C., 1995.

3

“Ignition and Thermal Hazards of SelectedAerospace Fluids,” NASA JSC White Sands TestFacility, RD-WSTF-0001, 1988.

4

“Ignitability of Hydrogen/Oxygen/NitrogenMixtures at Reduced Pressures,” NASA JSC WhiteSands Test Facility, TR-243-001, 1980.

5

Corda, Stephen, Richard C. Monaghan, Leonard S.Voelker, Griffin P. Corpening, Richard R. Larson, andBruce G. Powers,

Flight Testing the Linear AerospikeSR-71 Experiment (LASRE)

, NASA/TM-1998-206567,September 1998.

6

Ennix, Kimberly A., Griffin Corpening, MicheleJarvis, Harry Chiles, “Evaluation of the LinearAerospike SR-71 Experiment (LASRE) OxygenSensor,” AIAA-99-4815, 9th International Space Planesand Hypersonics Systems and TechnologiesConference, Norfolk, Virginia, November 1999.

7

Bilardo, Vincent J., Jr., and Francisco Izquierdo,“Development of the Helium Signature Test for OrbiterMain Propulsion System Revalidation BetweenFlights,” AIAA-87-0293, AIAA 25th AerospaceSciences Meeting, Reno, Nevada, January 12–15, 1987.

8

Mizukami, Masashi, Griffen P. Corpening, Ronald J.Ray, Neal Hass, Kimberly Ennix, and Scott M.Lazaroff,

Linear Aerospike SR-71 Experiment (LASRE):Aerospace Propulsion Hazard Mitigation Systems

,NASA TM-1998-206561, July 1998.

15


Appendix – Helium Signature Test Leak Rate Computation

The following calculates the worst-case leak rate fromthe gaseous hydrogen system at operating pressuresbased upon the leakage measured from the gaseoushelium back-pressure test and the calibration curvesgenerated by the characterization test. The zero point isforced through zero, again providing a worst-casecondition (background noise is not subtracted).

Using the line equation, and the 0 slpmand 3.635 slpm points (worst-case slope fromfigure 13(a)) for the 60 psia gaseous helium back-pressure results:

(maximum leakage seen during GHe back-pressure test @ 60 psia)

slpm or27.923 scim.

This leak rate is then correlated to a gaseous hydrogenleak at operating pressure using the informationprovided by Tibor Lak of Boeing, Downey, California.This equation is an approximation that assumes the leakpath(s) to be small and choked flow occurs at the exitplane(s). The simplified coefficient for helium is 0.21and for hydrogen is 0.14.

G

He

@ 70 °F + 14.7 psia,

ρ

GHe

= P/RT

= (14.7 psia

×

144 in

2

/ft

2

) / (386.18 lbf

×

ft/lbm

×

°R

×

530 °R) = 0.01034 lbm/ft

3

GH

2

@ 70 °F + 14.7 psia,

Using the standard mass flow rate obtained above,

mdot

GHe

=

ρ

GHe

×

mdot

GHe@std

= 0.01034 lbm/ft

3

×

(27.923 scim)/

(1728 in.

3

/ft

3

×

60 sec/min)

= 2.785

×

10

–6

lbm/sec

The theoretical orifice size is calculated next, basedupon the mass flow rate.

For the gaseous helium,

Cda = (mdot

×

T

1/2

) / (0.21

×

P)

= (2.785

×

10

–6

lbm/sec)

×

(530 °R

1/2

)/((0.21)

×

(60 lbf/in.

2

))

= 5.089

×

10

–6

in.

2

This is then correlated to a gaseous hydrogen leak atoperating conditions (600 psia),

= (5.089

×

10

–6

in

×

0.14

×

600 psia) /

(530 °R–)

= 1.857

×

10–5 lbm/sec

the equivalent flow rate at standard conditions is,

@std = (1.857 × 10–5 lbm/sec × 1728 in3/ft3

× 60 sec/min) / (0.00515 lbm/ft3)

= 373.83 scim

This can then be correlated with the purge rate toestimate the hydrogen volume fraction of,

percent = /Qpurge

=374/14,000= 2.67 percent (assuming homogenousmixture)

y mx b+=

m y2 y1–( ) x2 x1–( )⁄=

6530 335–( ) 3.635 0–( )⁄=

1704.26=

b 0=

y 780=

x y b–( ) m⁄ 780 1704.26⁄( ) 0.4577= = =

ρGH20.00515 lbm/ft

3=

mdotGH2

mdotGH2

H2QGH2

16American Institute of Aeronautics and Astronautics

Figure 1. Flammability limits of hydrogen-oxygen-nitrogen mixtures at reduced pressures.

Figure 2. Ignition pressure limit with the standard excess oxygen and hydrogen mixtures.

N2 concentration,percent volume

O2 concentration,percent volume

100

100

90

90

80

80

70

70

60

60

50 50

40

40

30

30

20

20

10

10

0

0

0 10 20 30 40 50H2 concentration, percent volume

990205

Air

60 70 80 90 100

2.7 kPa

3.0 kPa

4.0 kPa

8.0 kPa12.0 kPa

80.0 kPa

10.0

5.0

1.0

.50

Pressure,psia

Nonflammableregion

Nonflammableregion

Flammableregion

Flammableregion

UntestedUntested

IgnitionNon-ignition

.10

.050 5 10 15

Percent hydrogen20 25 30

990206

0 5 10 15Percent hydrogen

20 25 30

(a) 240 ft3 chamber. (b) 10 ft3 chamber.


Figure 3. LASRE flight experiment.

Figure 4. LASRE systems general layout.

He

SR-71

Engine cooling H2O GH2

TEA–TEB

He

He HeHe

Pod cooling H2O

Purge LN2

GH2 GH2 Controller

990207

Canoe

ModelSewer pipe

Reflective plane

Engine

Kayak

LO2

EC98-44440-13


Figure 5. Oxygen sensor location.

Figure 6. Oxygen sensor uncertainty at altitude.

Canoe, top view

Pod, side view

H2O tanks GH2

GHeGHe

GHe

GH2

GHe

GH2

LO2

3 4 5 61

Sewer pipe

Model, top view

Oxygen sensorPurge source

990208

GHe

7 8

9 10 11 12

2

#

✺✺

✺

✺

✺

✺

✺

✺

✺

60 x 103

PercentO2

uncertainty

Pressure altitude, ft

Sensor125

10 20 30 40 500

.5

1.0

1.5

2.0

980352


Figure 7. Three-percent gaseous hydrogen sample port locations.

Canoe, top view

Pod, side view

H2O tanks GH2 GH2 GH2

LO2

4

Model, top view

990209

5 6

1 2 3


(a) After main flow.

(b) After hydrogen autosafe.

Figure 8. Three-percent gaseous hydrogen blowdown data

250Transient data after main flow

240

200

150

H2,ppm

100

50

0 1 2 3 4 5 6 7 8 9 10Time, min

11 12 13 14 15 16 17 18 19 20

990210

Handheld 1Handheld 2Handheld 3Handheld 5Handheld 6Mass. spec. 9

Maximum allowed H2 level,1 percent equivalent

10 2 3 4 5 6 7Time, min

8 9 10 11 12 13 14 15

990211

Handheld 1Handheld 2Handheld 3Handheld 5Handheld 6Mass. spec. 4Mass. spec. 5

250240

200

150

H2,ppm

100

50

Maximum allowed H2 level, 1 percent equivalent

Transient data after H2 autosafe


Figure 9. Purge effectiveness with altitude change.

12345678

Oxygensensor

9101112

Altitude

1.0

.9

.8

.7

.6

.5

.4

.3

.2

.1

0

1.0

.9

.8

.7

.6

.5

.4

.3

.2

.2

0

35 x 103

30

25

20Altitude,

ft

Percent O2,

model

15

10

5

0 100 200 300Time, sec

400

990212

500 600

Oxygensensor

Percent O2,

canoe


(a) Ground run 63 first main flow.

Figure 10. Oxygen sensor response.

9

10

1112

Model,percent O2,

sensors9-12

LO2valve

commands

LOXMAIN

0 20

1.2

40 60Time, sec

80 100 120

990213

1.1

1.0

.9

.8

.7

.6

.5

.4

.3

.2

.1

0

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

.8

.6

.4

.2

PRECHILL


(b) Ground run 63 second main flow.

Figure 10. Continued.

9

10

11

12

PRECHILL

LOXMAIN

0 20

1.2

40 60Time, sec

80 100 120

990214

1.1

1.0

.9

.8

.7

.6

.5

.4

.3

.2

.1

0

1.0

.9

.8

.7

.6

.5

.4

.3

.2

.1

Model,percent O2,

sensors9-12

LO2valve

commands


(c) Ground run 63 autosafe blowdown.


9

10

11

12

PRECHILL LOXMAIN

0

1.2

4020 60Time, sec

80 100 120

990215

1.1

1.0

.9

.8

.7

.6

.5

.4

.3

.2

.1

0

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

.5

Model,percent O2,

sensors9-12

LO2valve

commands


(d) Flight 51 main flow.


9

10

11

12

LOXMAIN

0 20

1.2

40 60Time, sec

80 100 120

990216

1.1

1.0

.9

.8

.7

.6

.5

.4

.3

.2

.1

0

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

.5

Model,percent O2,

sensors9-12

LO2valve

commands

PRECHILL


(e) Flight 51 autosafe blowdown.

Figure 10. Concluded.

9

10

11

12

0 20

1.2

40 60Time, sec

80 100 120

990217

1.1

1.0

.9

.8

.7

.6

.5

.4

.3

.2

.1

0

10

9

8

7

6

5

4

3

2

1

Model,percent O2,

sensors9-12

LO2valve

commands

PRECHILL LOXMAIN


Figure 11(a). Gaseous helium characterization insertion point examples.

Figure 11(b). Gaseous helium back-pressure testing.

GN2

GN2 and GHe

990218

GHe Mass spectrometer

Propellant feed tankP = Pamb

Transducer

Valve

Venturi

Regulator

GN2

GN2 and GHe

990219Mass spectrometer

GHeP >> Pamb

Transducer

Valve

Venturi

Regulator


Figure 12. Gaseous helium insertion and mass spectrometer sample locations.

Canoe, top view

Pod, side view

H2O tanks GH2 GH2 GH2

LO2

GHe

Model, top view

990220

GHe

GHe

GHeSample

port


Figure 13(a). Gaseous helium insertion response.

Figure 13(b). Gaseous helium leak rate due to back-pressure.

18,000

16,000

14,000

12,000

10,000

8,000

6,000

4,000

2,000

0 1 2

Concentration,ppm

3 4GHe flow rate, slpm

6

990221

5 7 8

Average trendy = 2170x +50

Worst casey = 1637x +50

Insert point 10Insert point 11Insert point 13AverageLinear (average)

Model HST characterization

.500

.450

.400

.350

.300

.250GHe

leak rate,slpm

.200

.150

.100

.050

040 45 50

GHe back-pressure, psia55 60 65

990222

Calculated GHe leak rate based on back-pressure data

Calculated GHe leak rate


Figure 13(c). Estimated 600 psi gaseous hydrogen leak rate.

Figure 13(d). Estimated gaseous hydrogen concentration post-test.

.450

.400

.350

.300

.250GH2leak rate,

slpm

Estimated 600 psi H2 leak rate, slpm

.200

.150

.100

.050

040 45 50

GHe back-pressure, psia55 60 65

990223

Estimation of 600 psi H2 leak ratebased upon back-pressure

1 percent acceptability limit

.016

40

GH2 in model volume,

percent

GHe back-pressure, psia990224

.014

.012

.010

.008

.006

.004

.002

045 50 55 60 65

Estimated model volume fraction of GH2 in 3-second main flow test

Estimated percent H2 of model volume in 3 sec


Figure 14(a). Model pressure sensor excitation during prechill and main flow.

Modelvolume

pressure,psia

LO2valve

commands

PODPRES

LOXMAINPRECHILL

0 5

1.2

990225

1.1

1.0

.9

.8

.7

.6

.5

.4

.3

.2

.1

14.220

14.215

14.210

14.205

14.200

14.195

14.190

14.185

14.180

14.175

14.170

14.165

14.160

10Time, sec

15 20


Figure 14(b). Model pressure sensor excitation during prechill and main flow in flight.

LO2valve

commands

PRECHILL LOXMAIN

0 5

1.2

990226

1.1

1.0

.9

.8

.7

.6

.5

.4

.3

.2

.1

Modelvolume

pressure,psia

PODPRES

5.150

5.145

5.140

5.135

5.130

5.125

5.120

5.115

5.110

5.105

5.100

5.095

5.090

10 15Time, sec

20 25

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NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)Prescribed by ANSI Std. Z39-18298-102

Propellant Feed System Leak Detection—Lessons Learned From theLinear Aerospike SR-71 Experiment (LASRE)

WU 242 33 02 00 23 00 T15

Neal Hass, Masashi Mizukami, Bradford A. Neal, Clinton St. John,Robert J. Beil, and Timothy P. Griffin

NASA Dryden Flight Research CenterP.O. Box 273Edwards, California 93523-0273

H-2378

National Aeronautics and Space AdministrationWashington, DC 20546-0001 NASA/TM-1999-206590

This paper presents pertinent results and assessment of propellant feed system leak detection as applied to the LinearAerospike SR-71 Experiment (LASRE) program flown at the NASA Dryden Flight Research Center, Edwards, California.The LASRE was a flight test of an aerospike rocket engine using liquid oxygen and high-pressure gaseous hydrogen aspropellants. The flight safety of the crew and the experiment demanded proven technologies and techniques that coulddetect leaks and assess the integrity of hazardous propellant feed systems. Point source detection and systematic detectionwere used. Point source detection was adequate for catching gross leakage from components of the propellant feedsystems, but insufficient for clearing LASRE to levels of acceptability. Systematic detection, which used high-resolutioninstrumentation to evaluate the health of the system within a closed volume, provided a better means for assessing leakhazards. Oxygen sensors detected a leak rate of approximately 0.04 cubic inches per second of liquid oxygen. Pressuresensor data revealed speculated cryogenic boiloff through the fittings of the oxygen system, but location of the source(s)was indeterminable. Ultimately, LASRE was canceled because leak detection techniques were unable to verify thatoxygen levels could be maintained below flammability limits.

Fire safety, Hypersonic vehicles, Leak assessment, Leak detection, Rocketengines

A03

38

Unclassified Unclassified Unclassified Unlimited

November 1999 Technical Memorandum

Presented at the AIAA 9th International Space Planes and Hypersonic Systems Conference, Norfolk, Virginia,November 1–5, 1999.

Unclassified—UnlimitedSubject Category 20

Propellant Feed System Leak Detection Lessons Learned · PDF fileNASA Dryden Flight Research...

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