Leak checking and evaluation of pressure vessels by observing the slight temperature changes resulting from structural anomalies has been made possible through developments in high resolution infrared cameras and advanced image processing. These developments have made thermal nondestructive analysis a very practical and efficient method to determine material consistency and structural quality as well as monitor processes. The Space Shuttle Main Engine Nozzle has regions which can not be inspected with standard leak check methods. The Thermographic methods being developed to nondestructively test the Nozzle for leaks in inaccessible regions are reported. Also, a flash heating Thermographic investigation of the braze line bonding the cooling tubes to the outer structural jacket of the nozzle is reported

Russell, Samuel S.

Walker, James L.

English

NASA Technical Reports Server

DETECTION, LOCATION, AND CLASSIFICATION OF
SPACE SHUTTLE MAIN ENGINE NOZZLE LEAKS BY
TRANSIENT THERMOGRAPHIC INSPECTION
Samuel S. Russell
Nondestructive Evaluation Team
NASA Marshall Space Flight Center
256-544-4411
sam.russell@msfc.nasa.gov
James L. Walker
Center for Automation and Robotics
University of Alabama in Huntsville
256-890-6578 ext. 207
walkerj @email.uah.edu
ABSTRACT
Leak checking and evaluation of pressure vessels by observing
the slight temperature changes resulting from structural anomalies
has been made possible through developments in high resolution
infrared cameras and advanced image processing. These
developments have made thermal nondestructive analysis a very
practical and efficient method to determine material consistency
and structural quality as well as monitor processes.
The Space Shuttle Main Engine Nozzle has regions which can
not be inspected with standard leak check methods. The
Thermographic methods being developed to nondestructively test
the Nozzle for leaks in inaccessible regions are reported. Also, a
flash heating Thermographic investigation of the braze line
bonding the cooling tubes to the outer structural jacket of the
nozzle is reported.
INTRODUCTION
Modem infrared video cameras are capable of measuring temperature differences
of less than 0.025°C. Using this extreme detection capability and creative digital image
processing methods to control noise, Thermography can solve many nondestructive
evaluation problems. Inspection methods based upon resolving r.apid transient thermal
processes in particular are made possible. This has allowed inspection of material by
flash heating with photographic flash lamps and observing the thermal response images at
every 0.017 seconds per frame. Also, this instrumentation allows the visualization of
minute surface temperature changes due to some internal process mapped upon a
geometric image of a part. Two examples of using Thermographic methods to solve
inspection problems with the Space Shuttle Main Engine Nozzle are investigated and
reported. One of these problems, interstitial leaking, has no other nondestructive
solution.
The nozzles of the Space Shuttle's Main Engines (SSME) consist of 1080 tapered
Inconel coolant tubes brazed to an A-286 structural jacket (Figure 1). Three potential
https://ntrs.nasa.gov/search.jsp?R=19990008765 2020-06-18T00:40:37+00:00Z
problemsoccurwithin theSSMENozzlecoolanttubesasaresultof manufacturing
anomaliesandthehighly volatile serviceenvironment.Theproblemsincludepoor or
incompletebrazingof thetubesto thestructuraljacket, tubesthat leakinto the interstitial
zone,andtubesthat leakinto theflamesurface(hotwall). Thehotwall leaksare
accessible,andareeasilyidentified,located,andclassifiedby theapplicationof a liquid
leakchecksolutionandvisual inspection.
Inspectionsfor determiningbrazeline integrity canbeperformedby standardx-
raypractices.Theproblemthoughis that theprocessof performinganx-ray inspection
on theentirenozzleis very labor intensiveandexpensive.A proposedalternativeto
locatetheunbondis flashthermographyfrom theouter(cold wall) sideof the nozzle. In
regions were the jacket is exposed, so that it can be flash heated, the potential to map the
braze integrity exists. The regions obscured by hat bands and external plumbing will still
have to be inspected using x-ray techniques.
Inconel 718 jacket
}_!_i Brazed
. .,_ A-286 coolant
Figure I. SSME Nozzle with cutaway showing cooling tubes and outer structural jacket.
DISBOND BETWEEN TUBES AND JACKET
Initial experiments were performed to determine if it was possible to identify
disbonds between the coolant tubes and structural jacket of the SSME nozzle. The
disbond was simulated by milling out a portion of the tubing with a moto-tool from the
hot wall side of the nozzle. The defect tapered from a 1 inch opening at the inner surface
of the nozzle to the brazed edge of the skin. Although this did not produce a true
disbond, the artificial defect did demonstrate the same thermal behavior that a disbond,
i.e. both have the effect of reducing the effective flow path of heat. The artificial defect
was easily detected.
An SSME nozzle segment was then inspected by flash heating its cold wall side
using an Amber Radiance It camera (25 mm lens) running under Thermal Wave Inc.
software. Figure 2 shows the apparatus in place during the inspection of the nozzle. The
flash unit and infrared camera were mounted on a tripod with the flash hood against the
part and the camera imaging the part through the hood. Flash Thermography images
were acquired by viewing the jacket side of the nozzle after flash heating the same side.
Areas of greatest structural integrity conducted the heat away from the jacket quite well,
andappeardarkin theseimages.Areascontainingdiscontinuities(suchasthe
programmeddefector the interstitial gapsbetweenthecoolingtubesandjacket)did not
conducttheheatawayfrom thejacket aswell, andappearbright in theseimages.Figure
3 showsaregionof largeunbrazebetweenthetubesandthestructuraljacketdueto a
manufacturingdefectin anozzle. However,largeregionsof thenozzlecannotbe
inspectedusingFlashThermographydueto theinterferenceof structuralstiffeners,
insulation,or piping. But,mappingof brazeintegrity in theopenbaysis possible.
Figure2. FlashThermographyEquipmentInterrogatinganSSMEnozzle.
LEAK DETECTION PROBLEM
The origin of leaks into the interstitial zone can not be determined with leak check
solution or other traditional nondestructive methods. Due to the geometry and location of
the defects, eddy current and visual inspections are not possible and the crack or pin hole
responsible for the leak is normally too small for radiographic, ultrasonic, or computed
tomographic detection. The current inspection technique for locating interstitial leaking
is the application of leak check solution in the openings where the interstitials vent below
the bottom of the nozzle, while the tubes are pressurized to 25 psig. When a leak is
found, it is classified, and if the leak is severe enough the suspect tube is cut open so that
a boroscope can be inserted to find the leak point. Since the boroscope can only cover a
finite tube length and since it is impossible to identify which tube (to the right or left of
the identified interstitial) is leaking, many extra and undesired repairs must be made to fix
Unbrazedregion
Hat Band9
Figure3. FlashThermographyofunbrazedarea.
just oneleak. In certaininstanceswhentheinterstitialsareinterlinkedby poor braze
bonding,manyinterstitialswill showindicationsof leakingfrom asinglesource.Whatis
desiredis a techniquethat canidentify theleaksourcesothat asinglerepaircanbe
performed. Theleaksof interestareclassifiedin four categoriesasdescribedin the
following Table. It wasproposedto try thermographicmethodsto locatethe interstitial
leak,from thehot wall sideof thenozzle,by detectingthelocalcoolingcreatedby
escapinggasat the locationof adefect.
ClassI
ClassII
ClassIII
Blower
Volume ratein scim
basedupon25psig
Helium
0.001-1.3
1.2- 4.0
4.0 -50
Greaterthan50
Descriptionusingacertainleakchecksolution
Foaming
Randomsizebubblesof moderatepersistence
Largefast-formingbubblesof shortpersistancemost
of whichbreakasthenext forms.
Blowing (Bubblesdo not formbecauseof largegas
flow)
THERMOGRAPHY FOR LEAK DETECTION
Artificial defects were fabricated into a nozzle segment cut from the lower region
of a scrap SSME nozzle including the eighth and ninth hat bands. By cutting open the
tubing on the hot wall side of the nozzle, then puncturing the tube into the interstitial
region and finally resealing the tube with a cover patch through a welding operation,
phantom defects were fabricated, see Figure 4. The leak type and interstitial location
were determined by individually pressurizing each tube and checking with leak solution.
4
In all casesnosurfaceleaksweredetectedwith leakchecksolution. For brevity only
resultsfor Region1 arepresented.
During the reweld operation the hot wall surface of the tubes had to be cleaned
with a wire brush. This left the surface shiny, which greatly limited the thermographic
inspection capability. To dull the surface it was painted with three coats of a water
washable fiat black paint. The paint was applied by first heating the metal surface with a
hot air gun, applying a light coat of paint, and then reheating the surface to speed
evaporation of the water cartier. This process provided a consistent finish and permits a
thinner coat of paint be applied and still have adequate dulling of the surface. The
normal, as fired, surface of the nozzle has an emissivity high enough so that no surface
treatment is required.
Figures 5 shows the resulting Thermograms over 4 time intervals obtained by
rapidly pressurizing the nozzle segment to 40psig (less than 2 seconds to reach maximum
pressure) with Nitrogen gas. These Thermograms were made by subtracting an image
prior to pressurization from the later images. Hence, the temperature change is accented.
Also included (Figure 6) is the Time-Temperature plot for four points on the image which
provide an indication of the degree to which the defects can be detected. By re-running
the test at several pressure levels, it was determined that the temperature change observed
at a leak source increased approximately linearly with increasing pressure. Also, it was
determined experimentally that Nitrogen and Argon gas produced larger thermal
indications as compared to the Helium gas used for the liquid leak check investigations.
A second panel cut from the upper end of a nozzle was prepared with artificial
weld defects that were classified as Class II and Class III as shown in Figure 7. The
cooling tubes grow continuously from the top to the bottom of the nozzle creating a some
what different problem at the top because of the much smaller tubes and interstitials. The
Thermograms generated by rapidly (< 2 seconds) pressurizing the segment with 40 psig
Nitrogen are shown in Figure 8. The difference between a Class II leak on the right and a
Class III on the left is very apparent.
To Nitrogen tank
SSME Nozzle tubes
Figure 4.
Manifold
Flexible air lines
Configuration for panel from lower nozzle region with manufactured leaks.
5
2.369 seconds 7.574 seconds
16.583 seconds 25.092 seconds
Figure 5. Thermograms of Region 1 of panel shown in Fig. 4.
Figure 6.
Figure 7.
Locations and colors
correspond to left chart.
Time vs. Temperature for Class II and Class III leaks in test panel.
Upper manifold
Location of defects in panel cut from the upper portion of a nozzle.
6
Original image Image(AT) difference
Figure8. Thermogramof leaksin panelcut from upperportion of nozzle.
FULL SCALE SSME NOZZLE INSPECTION
An SSME Nozzle which contained a Class II interstitial leak was inspected at the
manufacturing facility. Weld repairs were painted with three coats of water washable fiat
black paint as previously described for the nozzle segments. The nozzle was pressurized
with Argon gas from a standard welding "K" bottle. The general radial location had been
determined with a leak check solution and was marked with brass foil tape, placed on 6
inch centers from the forward manifold, to track image location. The pressurization with
Argon took an average of 16 to 20 seconds to reach the desired 40 psig level. This is a
much slower pressurization rate than that utilized for the experiments run on the panel
segments cut from a scrap nozzle. Faster pressurization was possible in the proof
pressurization test cell, but it was not available for these tests.
As shown in the Figures 9, a leak indication was found under the cover patch of a
previously repaired location. It was not possible with the slower pressurization rate to
define which tube under the cover patch was leaking. Due to the slow thermal transient,
the effects of the leak dissipated into the heavy cover patch before its source could be
pinpointed. There was though, a thermal indication of the effects of the leak at the cover
patch seen in the tubes above the suspect area. Here, due to the increased flow rate, the
tube with the leak appears to be colder than its neighbors. By zooming in on the region
of the tubes where they enter the upper manifold it was possible to identify two tubes
(labeled A and B) that appear to be cooler "leaking". These tubes are shown in Figure 10,
and have been marked on the nozzle with foil tape. The nozzle manufacturer now knows
the location of the leak and can open those tubes and repair them.
7
Figure9. Thermogramof SSMENozzlewith Argon pressurization,_ 18seetime to 40psig).
Thermalindication(0.084°AC)
Figure 10. Thermogramof upperedgeof nozzleshowingleakingtubes.
CONCLUSION
Thermography can locate and classify leaks that occur in the interstitial locations of
the Space Shuttle Main Engine Nozzle. Prior to this method no means of
nondestructively inspecting interstitial leaks existed.
• Braze integrity between the hat stiffener and structural jacket can be determined on
the Space Shuttle Main Engine Nozzle with Flash Thermography.
Dueto increasedthermalsensitivityandadvancesin imageprocessingtechniques
manyproblemsin materialtestingor processverificationmaybesolvedwith modem
videoThermographicmethods.
ACKNOWLEDGMENTS
Messieurs Albert Avila, A1 Bemat, Caleb Branscomb, Gary Schock Ron Daniel
and Ben Coby of Boeing for the excellent support at Canoga Park, CA and very
especially Messieurs Paul Caraccioli and Melvin Bryant of NASA/MSFC for their
patience in explaining the problem and for help coordinating activities with Boeing.
9


Detection, Location, and Classification of Space Shuttle Main Engine Nozzle Leaks by Transient Thermographic Inspection

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