Linear Aerospike SR-71 Experiment (LASRE): Aerospace ... · NASA/TM-1998-206561 Linear Aerospike SR-71 Experiment (LASRE): Aerospace Propulsion Hazard Mitigation Systems Masashi Mizukami,

NASA/TM-1998-206561

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

Masashi Mizukami, Griffin P. Corpening,Ronald J. Ray, Neal Hass, and Kimberly A. EnnixDryden Flight Research CenterEdwards, California

Scott M. LazaroffNASA Johnson Space CenterHouston, Texas

July 1998

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NASA/TM-1998-206561

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

Masashi Mizukami, Griffin P. Corpening,Ronald J. Ray, Neal Hass, and Kimberly A. EnnixDryden Flight Research CenterEdwards, California

Scott M. LazaroffNASA Johnson Space CenterHouston, Texas

July 1998

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

LINEAR AEROSPIKE SR-71 EXPERIMENT (LASRE): AEROSPACE PROPULSION HAZARD MITIGATION SYSTEMS

Masashi Mizukami,* Griffin P. Corpening,† Ronald J. Ray,† Neal Hass,† Kimberly A. Ennix†

NASA Dryden Flight Research CenterEdwards, California

Scott M. Lazaroff†

NASA Johnson Space CenterHouston, Texas

Abstract

A major hazard posed by the propulsion system ofhypersonic and space vehicles is the possibility of fire orexplosion in the vehicle environment. The hazard ismitigated by minimizing or detecting, in the vehicleenvironment, the three ingredients essential toproducing fire: fuel, oxidizer, and an ignition source.The Linear Aerospike SR-71 Experiment (LASRE)consisted of a linear aerospike rocket engine integratedinto one-half of an X-33-like lifting body shape, carriedon top of an SR-71 aircraft. Gaseous hydrogen andliquid oxygen were used as propellants. AlthoughLASRE is a one-of-a-kind experimental system, it mustbe rated for piloted flight, so this test presented a uniquechallenge. To help meet safety requirements, thefollowing propulsion hazard mitigation systems wereincorporated into the experiment: pod inert purge,oxygen sensors, a hydrogen leak detection algorithm,hydrogen sensors, fire detection and pod temperaturethermocouples, water misting, and control roomdisplays. These systems are described, and theirdevelopment discussed. Analyses, ground test, and flighttest results are presented, as are findings and lessonslearned.

Nomenclature

canoe aerodynamically-shaped pod, housing fluid and other systems in LASRE

GH2 gaseous hydrogen

hydrogenH2

1American Institute of Aero

*Aerospace Engineer, AIAA Member.†Aerospace Engineer.Copyright 1998 by the American Institute of Aeronautics and

Astronautics, Inc. No copyright is asserted in the United States underTitle 17, U.S. Code. The U.S. Government has a royalty-free licenseto exercise all rights under the copyright claimed herein for Govern-mental purposes. All other rights are reserved by the copyright owner.

water

He helium

LASRE Linear Aerospike SR-71 Experiment

liquid nitrogen

liquid oxygen

model top portion of LASRE, shaped like one-half of a lifting body

nitrogen

oxygen

P hydrogen tank pressure, psi

ambient pressure, psia

pressure during calibration, psia

psi pounds per square inch

psia pounds per square inch, absolute

psig pounds per square inch, gage

sea level pressure, psia

R gas constant

T hydrogen gas temperature, R

TEA-TEB triethylaluminum-triethylborane

V hydrogen tank volume, in3

Z compressibility

change in mass of hydrogen in tanks, lbm

volume fraction of hydrogen

volume fraction of hydrogen, measured

volume fraction of oxygen

volume fraction of oxygen, measured

H2O

LN2

LO2

N2

O2

Pamb

Pcalib

PSL

∆mH2

%H2

%H2meas

%O2

%O2meas

nautics and Astronautics

Introduction

For hypersonic and space vehicles that are reusableor piloted, safety is an exceptionally importantconsideration. For piloted vehicles, this consideration isa result of the possible tragic consequences of a mishap.For reusable launch vehicles, cost is the major driver;the total operational costs must be minimized, includingthe high cost of losing an expensive vehicle and itspayload as a result of a mishap. For a flight researchproject, safe flight can be considered as important as thetechnology being investigated.

A major hazard posed by the propulsion system ofhypersonic and space vehicles is the possibility of a fireor explosion in the vehicle environment. These systemsstore a large amount of potential energy in the form offuel, oxidizer, high pressures or a combination ofthese.1,2 The three ingredients essential to theproduction of fire are fuel, oxidizer, and an ignitionsource (except in the case of hypergolics andpyrophorics, which can combust without an ignitionsource). Minimizing or detecting those three ingredientsin the vehicle environment mitigates the hazard. Inaddition to sound design and operational practices, it isoften necessary to provide dedicated hazard mitigationsystems.

The Linear Aerospike SR-71 Experiment (LASRE) isa flight experiment of a reusable launch vehicle

propulsion concept done in cooperation with theindustry. NASA Dryden Flight Research Center,Edwards, California was responsible for the flight test.3

The experiment consisted of a linear aerospike rocketengine integrated into one-half of an X-33-like liftingbody shape, carried on top of an SR-71 aircraft (figs. 1,2). Unlike conventional bell nozzle rockets, the linearaerospike engine can compensate for ambient back-pressure effects to provide higher performance over awide range of altitudes. However, external flowslipstream effects on the nozzle flow may reduceperformance. Therefore, the primary objective ofLASRE was to evaluate flight effects on aerospikeengine performance at several altitudes and Machnumbers. Although LASRE is a one-of-a-kindexperimental system, it had to be rated for piloted flight,which demands a high degree of safety. A rapidprototyping approach was taken to hardwaredevelopment. This approach resulted in an extendedseries of tests to troubleshoot and validate the integratedsystem. These tests included various functional testssuch as ‘cold flows’ of inert fluids, ‘ignition tests’ using

and triethylaluminum-triethylborane (TEA-TEB),and actual ‘hot fires’ of the engine.

A simplified schematic of fluid systems in the LASREpod is shown in figure 3. Essentially, this was a flyingrocket engine test facility. The model is the top portionshaped like half of a lifting body. The canoe is an

LO2

2American Institute of Aeronautics and Astronautics

Figure 1. LASRE configuration mounted on SR-71 aircraft.

980345

EC 97-44295-108

Figure 2. SR-71 and LASRE in flight.

Figure 3. Simplified schematic of LASRE fluid systems.

He

SR-71

Engine cooling H2O

Purge LN2

GH2

TEATEB

HeHe

He HeHe GH2 GH2

Pod cooling H2O

980346

Canoe

ModelEngine

LO2

aerodynamically-shaped pod, housing several fluid (andother) systems. A 4000 psi gaseous hydrogen ( )blowdown system, containing about 20 lb of ,provided fuel to the engine. A liquid oxygen ( )system, containing about 330 lb of , andpressurized to about 365 psi, provided the oxidizer. Both

and could be dumped overboard through theengine. could also be expelled overboard throughdump lines, and pressure could be releasedthrough a vent line. Triethylaluminum-triethylborane(TEA-TEB), a pyrophoric, was used to initiatecombustion in the engine. Engine coolant water,pressurized to 800 psi, flowed through channels drilled

in the engine block and out the water exit line. Two 9000psi gaseous helium (He) systems provided pressurant, water pressurant, line purges, TEA-TEBcartridge expulsion and pneumatic valve actuation. TheSR-71 contained water for pod misting, and liquidnitrogen ( ) for pod inert purge.

Even before hazard mitigation systems wereincorporated, the soundness of the underlyingpropulsion system itself had to be assured. The systemwas intended to be fail-safe for single-point failures.Component burst and proof factors were equal to orgreater than those used in typical missile and space

H2H2

LO2LO2

H2 LO2H2

LO2

LO2

LN2


systems.4 The and systems were leak checkedas much as was feasible on the integrated pod, usinglong-term pressure decay, bubble fluid, an ultrasonicleak detector, a thermal conductivity leak detector, anelectrochemical hydrogen sensor, and blowdown testing.

Flammability limits for - - mixtures at sealevel are about 4 percent and 4 percent , and arefairly independent of each other (fig. 4). The limitsremained about the same throughout the LASRE flightenvelope, extending up to an altitude of 50,000 ft, andwere not significantly narrowed by reduced pressures atthose altitudes.5 Calculation of combustion conditionsat equilibrium or for detonation showed that even forlow flammable concentrations of and ,temperatures and pressures high enough for catastrophicconsequences could develop. Therefore, a ground rulewas established that and concentrations in thepod should be maintained below 4 percent during theentire mission, because any combustion in the pod wasconsidered unacceptable.

To help meet safety requirements, propulsion hazardmitigation systems were incorporated in LASRE. Thesesystems were: a pod inert purge, sensors, a leakdetection algorithm, sensors, fire detection and podtemperature thermocouples, water misting, and controlroom displays. These systems are described, and theirdevelopment discussed. Analyses, ground test, and flight

test results are presented, as are findings and lessonslearned. Use of trade names or names of manufacturersin this document does not constitute an officialendorsement of such products or manufacturers, eitherexpressed or implied, by the National Aeronautics andSpace Administration.

Propulsion Hazard Mitigation Systems

The following sections discuss the systems andmethods used to mitigate propulsion hazards. Findingsand lessons learned as a result of tests performed arealso presented.

Pod Inert Purge

The pod cavity was purged with inert gas, forseveral reasons. The primary purpose was to minimizethe presence of oxidizer in the pod environment tobelow 4 percent by displacing air out of the pod.The purge may also reduce the presence of fuel bydisplacing any leaked . The and valves andsome of the electronics boxes were purged with , tominimize ignition sources. This purge surrounded theelectronics with inert gas, and prevented moisturecaused by water misting from coming into contact withthe electronics.

Figure 5 shows a schematic of the purge system. was supplied from two Dewar flasks onboard

H2 LO2

H2 O2 N2H2 O2

H2 O2

O2 H2

O2 H2H2

N2

O2

H2 H2 LO2N2

N2N2 LN2


Figure 4. Flammability limits of – – mixtures at sea level.

H2 concentration, percent volume

N2 concentration,percent volume

O2 concentration,percent volume

403020100 50 60 70 80 90 100100

90

80

70

60

50

40

30

20

10

0 100

90

80

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60

50

40

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0

980347

Air

Nonflammable region

Flammable region

H2 O2 N2

5

American Institute of Aeronautics and Astronautics

Figure 5. Nitrogen purge system schematic.

the SR-71. was vaporized by electric heaters, itwas dispersed into the pod through perforated piccolotubes near the front of the pod, and also into theelectronics boxes and valves. The two Dewar flasks helda total of 100 liters (180 lb) of . The flow rate wasabout 2.5 lb/min, or 34 standard cubic feet per minute,for an operating duration of 72 minutes. These flowrates gave about 0.3 volume change-outs per minute onthe ground, and 2.4 change-outs per minute at analtitude of 50,000 ft. For ground operations, wasdirectly provided from a ground cart to conserve theonboard supply. During flight, flow could beturned on or off from the SR-71 rear cockpit, and theoperating pressure was monitored in the control room toverify flow.

Purge effectiveness was evaluated primarily by using sensors distributed in the pod. Preliminary ground

testing revealed that it took a long time to establish apurged environment, and the pod panels had to be well-sealed to maintain it. Therefore, a separate monsterpurge was implemented to establish an initially purgedenvironment by using ground service equipment that fedlarge quantities of directly into the pod. Duringsubsequent ground tests an adequate purge wasconfirmed. In flight however, substantial infiltration ofexternal air occurred shortly after takeoff and purgeeffectiveness was quickly lost, with levels reachingnearly 21 percent. Therefore, attempts were made toseal gaps in the pod by using elastic materials such asfoam rubber and caulking. Also, the second Dewarflask was added, and purge flow rate was doubled tocurrent levels (2.5 lb/min). In addition, a valve wasplaced in the purge vent line, which could be remotelyactuated from the cockpit. With the valve open, thepurge gas flowed through the pod and out the vent asoriginally designed, which also allowed any major leaksto be vented out of the pod. With the valve closed, thepurge gas maintained a positive pressure in the pod, thusreducing air infiltration. The purge gas flowed out fromany remaining leak paths. A subsequent flight testdemonstrated adequate purge performance for flight, upto about Mach 0.9 and an altitude of 26,000 ft, with levels below 4 percent. Figure 6 shows Mach number,altitude, and the measured levels during a portion ofthe flight. The canoe sensor that reads consistentlylow levels is located close to a purge piccolo tubenear the front of the canoe. The spike above 4 percent isbelieved to be telemetry noise. A preliminaryassessment of additional data suggested that purgeintegrity may be maintained up to Mach 1.6 and analtitude of 50,000 ft. It was observed that with the ventvalve closed, pressure was maintained in the pod duringclimb and descent. During climb, ambient pressure

decreased, and pod pressure was maintained aboveambient pressure, which aided purge effectiveness.However, during descent the opposite effect occurred,which made it difficult to maintain an adequate purge.These tests demonstrated that it is important to providean adequate purge flow rate, and to seal the vehicle wellagainst outside air infiltration.

Oxygen Sensors

Twelve oxygen sensors were located in the pod todetect the presence of oxidizer, eight in the canoe andfour in the model. The purpose of these sensors was todetermine inert purge effectiveness, infiltration ofoutside air, and the presence of any leaked .Commercially available electrolytic sensors were used,originally intended for automotive applications. Eachsensor was installed in a temperature-controlled canisterwhich was heated to about 115 °F, to reduce effects ofambient temperature changes on the readings. Thesesensors were point measurements that would not detectair or oxygen which could have been trapped in otherareas, but the number and distribution of the sensorswere judged sufficient to characterize overall levelsin the pod.

It was evident from data at altitude that these sensorsactually measured the partial pressure of , butvolume fraction is of greater interest than partialpressure for determining the flammability of mixtures.Therefore, volume fraction of ( ) wascalculated by correcting the measured partial pressureof ( ) to sea level pressure ( ), usingambient pressure ( ), as follows:

(1)

Accuracy of the sensor at sea level, according to thespecification sheet, was 1 percent. But for a givenvolume fraction of , as altitude increased, ambientpressure decreased, and the partial pressure of decreased, approaching sensor accuracy limits. As aresult, at high altitudes volume fraction determinedfrom sensor readings appeared erratic.

A test program was undertaken to better characterizesensor accuracy at altitude. In a small test chamber,sensors were exposed to a range of concentrationsvarying from 0 percent to 21 percent, at a range ofpressure altitudes from sea level to 80,000 ft. Becausesensors have a life of about 2 years, new sensors as wellas older ones were tested to help determine aging

LN2

LN2

N2

LN2

O2

N2

O2

LN2

O2

O2O2

O2

LO2

O2

O2

O2 %O2

O2 %O2measPSL

Pamb

%O2 %= O2meas

P

SL

P amb ------------

O2O2

O2

O2


7


Figure 6. Nitrogen purge effectiveness in flight, from takeoff up to Mach 0.9 and an altitude of 26,000 ft.

CanoeO2 sensors

ModelO2 sensors

30 x 103

10.5 11.0 x 10310.0

Altitude,ft

Mach

% O2

% O2

Time, sec

0

2

4

6

8

10

0

2

4

6

8

10

0

0

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.4

.6

.8

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5

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25

980349

effects. Representative results for 0 percent, 1 percent,2 percent and 5 percent concentrations are shown infigure 7(a). Accuracy at sea level appears much betterthan the specified 1 percent accuracy, but a distinctdrop-off is seen above an altitude of 30,000 ft. Tocompensate for the drop-off, a fifth-order polynomialcorrection was applied to the average of the sensorreadings (fig. 7(b)). Ninety-nine percent confidenceintervals for the corrected readings, based on thet-distribution, were less than 1.3 percent up to analtitude of 50,000 ft (fig. 7(c)).

Rules were established for interpreting and acting on sensor data during flight. If any sensor, plus its

uncertainty, exceeded a specified threshold level(nominally 4 percent), steps would be taken to securethe system and mitigate the hazard, typically bydumping the overboard.

Clearly, it was important to validate sensoraccuracy before use, especially because these sensorswere being used outside their originally intendedenvironment. Characteristics of other types of sensors,or sensors from other manufacturers, might be entirelydifferent.

Hydrogen Pressure Decay Leak Detection

In order to infer the presence of fuel in the pod, areal-time pressure-decay method was used to detectleaking hydrogen. Conventionally, pressure-decay leakdetection takes a long time to perform, and is doneduring system checkout. A noticeable pressure drop inthe gaseous hydrogen tanks during flight operationwould indicate a large leak, or perhaps a thermaltransient. However, as a result of the unavailability ofhydrogen sensors, a pressure-decay method wasimplemented for real time in-flight use. It was fordetection of leaks from the tanks in a static modeonly, and not for detection of leaks in the lines orduring flow.

In theory, the hydrogen tank leak rate can bedetermined from the change in mass of hydrogen in thetanks ( ) as follows:

(2)

Tank pressure (P) was measured by a pressuretransducer. Hydrogen gas temperature (T) was measuredby two redundant thermocouples mounted on probes

O2

O2 O2

H2

O2

H2H2

∆mH2

∆mH2

VR---- P

ZT-------

P0

Z0T 0-------------–

=

8


(a) Pressure altitude compared with percent , for 5 percent, 2 percent, 1 percent and 0 percent mixtures.

Figure 7. Oxygen sensor altitude chamber test data.

% O2

Pressure altitude, ft

Sensor123456

10 20 30 40 50 60 x 1030

1

2

3

4

5

6

7

8

980350

O2 O2


(b) Pressure altitude compared with percent with fifth order correction, for 5 percent, 2 percent, 1 percent and0 percent mixtures.

(c) Pressure altitude compared with 99 percent confidence intervals for 5 percent, 2 percent and 1 percent mixtures.

Figure 7. Concluded.

60 x 103

% O2fifth ordercorrection


Sensor123456

10 20 30 40 500

1

2

3

4

5

6

7

8

980351

O2O2

60 x 103

% O2uncertainty


Sensor125

10 20 30 40 500

.5

1.0

1.5

2.0

980352

O2

inside the tanks, which were averaged together. Note,that to obtain an accurate measurement of gastemperature under changing conditions, a tank surfacemeasurement would have been inadequate. Tank volume(V) was assumed to be constant. Compressibility (Z)was a function of P and T, and the subscript 0 denotesinitial conditions. R is the gas constant for .

In software, digital low-pass filtering was applied tothe signal, to remove high frequency randomnoise, 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 could beconfigured for either hydrogen or inert helium as theworking gas. The resulting signal was displayedon a scrolling time history display (fig. 8).

The ability to detect leaks depended on discerningsmall changes in pressure and temperature, which was afunction of instrumentation accuracy. The trace showedgood stability under the varying ambient conditions offlight. Uncertainty analysis indicated that a mass loss of0.15 lb or more could be detected. Ground testing, doneby releasing controlled small amounts of gas from thetanks and observing the trace, showed that mass loss aslow as 0.03 lb could be detected.

In the control room, if a mass loss rate of greater than0.03 lb was seen in 8 minutes or less, it would beconsidered a positive determination of leakage, andsteps would be initiated to secure the system bydumping propellants overboard. This leak ratecorresponds to about 4 percent of the purge flow ratein the vicinity of the tanks, or about 1460 standardcubic inches per minute. This amount was judged to be

the minimum leak rate reliably detectable in areasonable timeframe. However, this is still a substantialand potentially hazardous quantity of hydrogen that iscapable of locally forming combustible mixtures in thepod. Lower leak rates could possibly be detected over alonger period of time.

The leak detection algorithm was a useful tooland was the only available means of leak detectionon LASRE, but could not be relied upon to detect allhazardous leaks. This algorithm could detect moderateto large tank leaks, or smaller leaks over a long period oftime. However, it could not detect small but stillhazardous leaks in a timely manner, nor did it detectleaks in the lines.

Hydrogen Sensors

Although sensors were not used, they would haveconstituted a more direct way to detect the presence offuel. Sensors were planned to be strategically positionedwithin the canoe and model. The sensor proposed foruse was based upon a design developed by SandiaNational Laboratories, Albuquerque, New Mexico.6 Thesensor element was a Palladium-Nickel patch that has acurrent passing through it. When present, hydrogenfreely infiltrates the lattice network of the crystal,causing a change to the electrical resistance of theelement proportional to the local concentration. Theelement was temperature regulated by a controller,which aided in the response time of the sensor andremoved one of the operational variables. Theadvertised range of detection was 1 percent to100 percent volume fraction, with 0.5 percent accuracy.

The suitability and accuracy of the sensor in a flightenvironment needed to be ascertained. A test chamberwas used to simulate pressure altitudes. Testing wasconducted with pure and with calibration gases ofspecific concentrations of in , at discrete ambient

H2

∆mH2

∆mH2

N2H2

H2H2

H2

H2

N2H2 N2


Figure 8. Hydrogen tank leak detection algorithm scrolling time history plot display.

∆mH2,

lb

Time, min0 5 10 15 20 25 30

.05

0

– .05

– .10

980353

Current time

pressure points ranging from 2 psia to 13.5 psia. Thesensor heating element mitigated temperature effects.

Preliminary data for seven sensors with a 3 percent concentration by volume are shown in figure 9(a). It

is clear from the data that when the ambient pressurewas reduced the sensor measured concentration wasalso lowered and a linear relationship existed betweenambient pressures and the measured concentration.Therefore, the sensors appear to measure partialpressure of the gas to be detected, much like the sensors. If this phenomenon was not accounted for, andthe sensor readings ( ) were used withoutcorrection during flight, the readings could potentiallylead to the false conclusion that the flammability limitshad not yet been reached. Like the sensors, thefollowing correction algorithm could be applied real-time during flight using ambient pressure measurements( ) in the pod and the pressure at which the sensorswere calibrated ( ), to obtain percent volume ofhydrogen ( ) present.

(3)

Results of applying this simple correction to the rawdata are shown in figure 9(b). Stable readings were seenup to nearly 27,000 ft pressure altitude. Deviation from3 percent actual concentration indicates a bias error, butit was believed this could be corrected with a morerigorous calibration program. Above an altitude of27,000 ft, the accuracy limitation of the sensor isreflected in the nonlinear deviation and data scatter. Verysimilar results where achieved using a 10 percent concentration.

Unfortunately, once installed in the pod, the systemcould not be made to function correctly, apparentlybecause of design flaws in the sensor controller.Programmatic and schedule constraints ruled out thepossibility of troubleshooting to correct the system, sothe sensors were not installed for flight.

Certainly, a detection system would be highlydesirable for hazard mitigation. Bench test results werepromising for future flight applications. However, sensors and systems that are qualified for use in inertbackground, with varying pressures and temperaturesand that are robust and compact enough for flightapplication are difficult to find.7,8 detectiontechnologies can be pursued for future projects, as thistype of fuel becomes more prevalent.

H2

H2O2

%H2meas

O2

PambPcalib

%H2

%H2 %H2meas=

P

calib P

amb --------------

H2

H2

H2

H2

H2

H2


(a) Pressure altitude compared with percent measured, for 3 percent mixture.

Figure 9. Hydrogen sensor altitude chamber test data.

60 x 103

% H2meas


Sensor1234567

10 20 30 40 500

1

2

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980354

H2 H2

(b) Pressure altitude compared with percent , for 3 percent mixture.

Figure 9. concluded.

60 x 103

% H2

Pressure altitude, ft10 20 30 40 500

1

2

3

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6

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8

980355

Sensor1234567

H2 H2

Fire Detection and Pod Temperature Thermocouples

Fifteen thermocouples were distributed throughoutthe pod for sensing heat from nearby combustion.Thermocouples are commonly used for local firedetection. Procedurally, if high temperatures that werefar above expected ambient total temperatures wereseen (especially locally), fire would be indicated andsteps would be taken to dump the propellants and securethe aircraft.

The thermocouples were also used to monitor podenvironment temperatures, because certain componentswere temperature limited, such as the composite tanks and electronics. Flight data (fig. 10) showed that,except for some areas near cryogenic components, thepod thermal environment was quite benign throughoutthe mission profile. During ground operations and inflight up to Mach 1.6 and an altitude of 50,000 ft,temperatures typically ranged from 45 °F to 85 °F. Thisbenign environment was unexpected, given the widerange of external static and total temperaturesthroughout the mission. If pod temperatures hadapproached the tank temperature limit of 130 °F,steps would have been taken to cool the pod bydecelerating the aircraft or turning on the water mist. Ifthese measures had not been successful, the tanks

would have been depressurized by dumping the contentsoverboard.

Water Misting and Coolant System

The coolant water system was used to provide acooling water mist for the pod, and to recirculatecoolant for the electronics. The recirculating coolantwas required anytime the pod was powered up. The mistwas to be turned on only when necessary andappropriate for pod cooling, such as during highstagnation temperature flight conditions

Figure 11 shows a schematic of the system. About20 gallons of water was carried aboard the SR-71. Therecirculating coolant flow rate was 3 gallons per minute(gpm), and the mist flow rate was 0.18 gpm. The waterwas chilled by cold air from the SR-71 environmentalcontrol system. It was important to verify that the purge was on before turning on the water mist, toprevent moisture from entering the electronics boxes.Figure 12 shows the altitude and temperature regimeswhere the water mist boils into vapor; condensing waterwas undesirable, because it could cause hardwareproblems. The upper temperature limit for thecomposite high-pressure tanks is also indicated. Clearly,the usable range for water mist is limited if

H2

H2

H2

N2


13


Figure 10. Pod in-flight thermal environment, up to Mach 1.6 and an altitude 50,000 ft.

Canoe

thermocouples

Modelthermocouples

60 x103

12 14 x 10310

Altitude,ft

Mach

°F

Time, sec

– 20

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Figure 11. Pod water-misting and coolant system schematic.

Figure 12. Pod water-misting envelope, pod temperature compared with pressure altitude.

Pressurealtitude,

ft

Temperature, °F

Vapor

Tank temperature limit

80 100 120 140 160

60 x 103

50

40

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980358

Liquid

condensation is to be avoided. Also, water mist was notfor use as a fire suppressant, and could aggravateelectrical fires.

The original dual coolant pumps experienced repeatedfailures, because they were not designed for continuousservice. These pumps were replaced with a more robustsingle pump. In practice, the water mist system wasnever used in flight because additional cooling was notnecessary and because there were concerns abouthardware damage from water condensation.

Control Room Displays

Propulsion systems and fire hazard mitigation systemparameters were monitored in real time using controlroom displays. Two displays (not shown) werededicated to monitoring propulsion system parametersand general system health. These displays schematically

represented the propellant feed systems, and were usedfor decision making during tests and missions.Additional displays and strip charts showed moredetailed and time history data of specific parts of thepropulsion and fire hazard mitigation systems.

One display was primarily used for monitoring of thefire hazard mitigation systems and parameters (fig. 13).This display included: a time history plot of the leakdetection algorithm; bar charts and digital values of thefire detection thermocouples and sensors; and digitalindicators of purge and water misting and coolantsystems. The operator of this display could easily switchto time histories of the fire detection, thermocouples,

sensors, or sensors by selecting the appropriateon-screen button. A keyboard interface allowedreal-time setup and resets of the leak detectionalgorithm. All critical parameters changed from green,

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Figure 13. Primary control room display for fire hazard mitigation systems.

980359

O2 SENSORS FIRE TCs ENG_60SEC SYS_60SEC ???

11:09:12 + 5

Fire TC's 0– 50 °F 50 100 150 200 250

0– 50

≤ 32°F 32°> < 120° 120°≥ < 140° < 2.0% 2.0-4.0%≥140°F ≥4.0%

°F 50 100 150 200 250

GH2 TANK LEAKAGE (lb)

+ 10 TIME (MIN) + 20

MAXDMGH2ALLOWED

+ 25 + 30

MACHALTKEASPAMBTAMBTTO

MODELLOCATION

GH2TKPGH2TKT1GH2TKT2

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H2O_MST_PH2O_OUT_PH2O_OUT_TH2O_RTN_TH2O_RMN

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PINFMODEL_PPOD ∆PCANOE_PVENT_PVENT ∆P

XX.XXX.XX.XXXX.XXX.XX.XX

0.05

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– 0.05

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O2 SENSORS 0 1 2 3 4 5%

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BAL.FLXGHeMOD

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XX.X O2S02

XX.X O2S03XX.X O2S04

XX.X O2S05XX.X O2S06XX.X O2S07

XX.X O2S08XX.X O2S09XX.X O2S10XX.X O2S11XX.X O2S12

to yellow, then red, as they approached and thenexceeded predetermined limits.

Ground tests and inert flight tests helped validate andmature the control room displays and operatingprocedures. Control room displays, although they mayappear overly detailed, proved indispensable during theextended development and troubleshooting program,and for determining system status and performance.

Conclusions

The LASRE experiment incorporated a number ofpropulsion hazard mitigation systems. Hazards weremitigated by minimizing or detecting the threeingredients essential to the production of fire: fuel,oxidizer, and ignition source. The major findings wereas follows.

For inert purge of the vehicle cavity, it was importantto provide an adequate purge flow rate and to seal thevehicle well against outside air infiltration. Those effortsreduced the in-flight cavity levels from nearly21 percent to below 4 percent.

Oxygen and hydrogen sensor accuracies in flightneeded to be considered. Accuracy in terms of volumefraction-of-gas degraded at high altitudes. This wasbecause the sensors actually measured partial pressureof the gas. For a given volume fraction-of-gas, asaltitude increased, ambient pressure decreased, and thepartial pressure of the gas also decreased, approachingsensor accuracy limits. Sensor accuracy could be bettercharacterized by calibration at reduced pressures.Oxygen sensors were a mature technology and werevalue-added for safety monitoring. Accurate and robusthydrogen sensors able to operate in an inert flightenvironment were identified as an important safetytechnology, requiring further development.

A pressure- and temperature-based gaseous propellanttank leak-detection algorithm was found to be a usefultool, but could not be relied upon to detect all hazardousleaks. This algorithm could detect moderate to largetank leaks, or smaller leaks over a long period of time.However it could not detect small, but still hazardous,leaks in a timely manner, nor did it detect leaks in the

lines or during flow.

The pod thermal environment was found to besurprisingly benign throughout the mission, up to

Mach 1.6 and an altitude of 50,000 ft, given the widerange of static and total temperatures. The water mistsystem was never used in flight, because additionalcooling was not necessary, and because of concernsabout hardware damage from water condensation.

Detailed control room displays proved indispensableduring the extended development and troubleshootingprogram, and for determining system status andperformance.

References

1

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

2

“Safety Standard for Oxygen and Oxygen Systems,”NASA NSS-1740.15, Office of Safety and MissionAssurance, Washington DC, 1996.

3

Corda, Stephen, David P. Lux, Edward T. Schneider,and Robert R. Meyer Jr., “Blackbird Puts LASRE to theTest,”

Aerospace America

, vol. 36, no. 2, pp. 24–29,Feb. 1998.

4

“Standard General Requirements for Safe Design

and Operation of Pressurized Missile and SpaceSystems,” MIL-STD-1522A (USAF), May 1984.

5

Benz, F. J. and P. L. Boucher, “FlammabilityCharacteristics of Hydrogen/Oxygen/Nitrogen Mixturesat Reduced Pressures,” in:

Proceedings of the Workshopon the Impact of Hydrogen on Water Reactor Safety

,vol. 3, M. Berman, ed., SAND-81-0661, vol. 3, 1981.

6

Miller, S. L., K. L. Hughes, J. L. Rodriguez, and P. J.McWhorter, “Calibration and Characterization of WideRange Hydrogen Sensors,” in:

Advanced Earth-to-OrbitPropulsion Technology Conference

, CP-3282, vol. 1,Huntsville, AL, 1994.

7

Hunter, Gary W.,

A Survey and Analysis ofCommercially Available Hydrogen Sensors

, NASATM-105878, 1992.

8

Hunter, Gary W.,

A Survey and Analysis ofExperimental Hydrogen Sensors

, NASA TM-106300,1992.

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NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18298-102

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

WU 242-33-02-00-33-00-T15

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

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

H-2268

National Aeronautics and Space AdministrationWashington, DC 20546-0001 NASA/TM-1998-206561

A major hazard posed by the propulsion system of hypersonic and space vehicles is the possibility of fire orexplosion in the vehicle environment. The hazard is mitigated by minimizing or detecting, in the vehicleenvironment, the three ingredients essential to producing fire: fuel, oxidizer, and an ignition source. The LinearAerospike SR-71 Experiment (LASRE) consisted of a linear aerospike rocket engine integrated into one-halfof an X-33-like lifting body shape, carried on top of an SR-71 aircraft. Gaseous hydrogen and liquid oxygenwere used as propellants. Although LASRE is a one-of-a-kind experimental system, it must be rated for pilotedflight, so this test presented a unique challenge. To help meet safety requirements, the following propulsionhazard mitigation systems were incorporated into the experiment: pod inert purge, oxygen sensors, a hydrogenleak detection algorithm, hydrogen sensors, fire detection and pod temperature thermocouples, water misting,and control room displays. These systems are described, and their development discussed. Analyses, groundtest, and flight test results are presented, as are findings and lessons learned.

Fire prevention, Flight safety, Hypersonic vehicles, Reusable launch vehicles,Rocket engines A03

22

Unclassified Unclassified Unclassified Unlimited

July 1998 Technical Memorandum

Presented at the 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Cleveland, Ohio, July 13–15, 1998.Masashi Mizukami, Griffin P. Corpening, Ronald J. Ray, Neal Hass and Kimberly A. Ennix, NASA Dryden FlightResearch Center, Edwards, California; Scott M. Lazaroff, NASA Johnson Space Center, Houston, Texas.

Unclassified—UnlimitedSubject Category 20

WU 242-33-02-00-33-00-T15

Documents

Linear Aerospike SR-71 Experiment (LASRE): Aerospace ... · NASA/TM-1998-206561 Linear Aerospike SR-71 Experiment (LASRE): Aerospace Propulsion Hazard Mitigation Systems Masashi Mizukami,