Aerospace Nuclear Safety at APL - The Johns Hopkins University ...

Y. CHANGstarting above 1920 m (6300 ft). The probabilities ofnear-pad ground impact conditions (orientation andimpact velocity) of the spacecraft and third stage as afunction of time of accident were reported in the SARdatabook. The FSAR used these impact conditions indetermining the responses of the RTG upon hitting theground.If the spacecraft attains park orbit, but an accidentprevents it from escaping orbit, then the spacecraft’sorbit will decay because of atmospheric drag and it willreenter within days. For reentry breakup from orbitaldecay, APL calculations showed that the RTG andspacecraft would break up and separate from the CSCsbefore the CSCs’ thermally induced auto-ignition.Reentry breakup analyses of the spacecraft and RTGwere also conducted for the launch contingency effort 5and are described later in this article.If the spacecraft is injected into a heliocentric orbitfollowing an accident, it could potentially impact Earthin the future. APL conducted a study on long-term Earthreentry probabilities, where “long term” is defined as 1000years. Considering that the half-life of Pu-238 is 87.7years, 1000 years translates into 11.4 half-lives, and theactivity of the Pu-238 at the end of 1000 years would beabout 0.04% of its activity at the beginning. The resultsof the study showed that the probability of a long-termEarth reentry, once the spacecraft was in heliocentricorbit, was on the order of 10 –4 ; this can be reduced by 3orders of magnitude if a trajectory correction maneuverwith a 4V of 100 m/s is applied. 6 These probabilities oflong-term reentry were used in the accident probabilitiessection of the New Horizons SAR databook.CSC TestingTwo CSCs, each with 500 g of plastic explosive, arepart of the New Horizons STAR 48B BUS, which in turnis part of the launch vehicle’s FTS. In the event of anFTS activation, including a BUS activation as describedin the last section, the CSCs could eject backside metalcasing fragments toward the RTG at estimated velocitiesof around 3.0 km/s (1.9 mi/s). Thus CSC testing was initiatedby NASA to determine if these fragments wouldhave any harmful effects. APL participated in the CSCtesting, which was conducted at China Lake, California.The CSC test setup (Fig. 2a) followed an April 2004APL-written test proposal, which also predicted thatthe CSC fragments would not be harmful to the RTGbecause of the oblique angles at which the fragmentswould impact the RTG housing. The test setup replicatedthe flight configuration, specifically the material,structural, and geometric arrangements of the two CSCsinside the bottom of the lower payload attach fitting(PAF) and the simulant packs for the RTG. Because ofcyclical symmetry, four simulant packs can each representthe RTG location for each test firing. APL designedand fabricated the simulators for the PAFs, as well asthe internal brackets for the CSCs and shields. SouthwestResearch Institute (SwRI) conducted independentCSC tests at their facility in San Antonio, Texas, whichprovided valuable insight in designing the China Laketests. For the China Lake tests, SwRI also providedthe design and fabrication of the boron carbide shields(Fig. 2b), which would be incorporated as third-stageflight hardware if the unmitigated CSC fragmentsturned out to be harmful.There were three identical test firings without theshields at China Lake in November 2004, for a total of12 RTG simulant test articles. These test results showedthat the fragments generated by the detonation of theCSCs were unlikely to damage the graphitic materialof the GPHS modules encasing the nuclear fuel of theRTG. Thus the proposed shields were not implementedon the flight hardware. These conclusions were reportedin the New Horizons SAR databook and cited by theINSRP in the New Horizons SER.(a)(b)Figure 2. (a) CSC test setup at China Lake. The RTG simulant pack is constructed with a 2219 aluminum face sheet and 1.27-cm-thick (0.5-in.-thick) Celotex sheets. (Any Celotex damage was a criterion for adding shielding to the next test.) (Photo courtesy of NASA/JPL.) (b) CSCboron carbide shield configuration in the third-stage PAF.256Johns Hopkins APL Technical Digest, Volume 27, Number 3 (2007)

SOLID PROPELLANT FIREThe 17 January 1997 near-pad in-air explosion ofthe Delta II 241 rocket over Cape Canaveral reinforcedthe concept that a near-pad launch accidentcan result in solid rocket propellant burning in ambientatmospheric conditions on the ground, possiblynear the RTG or RTG components. Characteristicsof burning solid propellant are high temperature, longduration (up to several minutes), and persistence (doesnot dissipate). Earlier characterizations of solid propellantfires were of less energetic propellants (about7000 kJ/kg) compared to the more energetic propellants(about 9300 kJ/kg) used in modern solid rocketpropellant formulations. 7 Earlier tests characterizedthe environment above a propellant fragment burningonly on its top surface. However, the at-risk hardwareof the RTG most likely would be on the ground, underneathor adjacent to a propellant fragment burning on itstop, bottom, and side surfaces. Thus there was a need tobetter characterize the nature of solid propellant fires.To address this need, APL conducted a solid propellantfire-testing program in 1999–2001 using more than25 individual burns with various geometries, burn configurations,and instrumentation. The test setup of a91-kg (200-lb) cylinder of propellant is shown in Fig. 3a.The vertical rod attached to the top of the propellanttest article allowed vertical movement but preventedlateral movement if the hot gasses emanating from thepropellant underside were forceful enough for the blockto self-levitate. The in situ instrumentation installed inthe ground under and surrounding the propellant massconsisted of rod calorimeters, slug calorimeters, witnessmaterials, bare thermocouples, and a two-color pyrometeroperating in the near-IR. The remote instrumentationconsisted of UV/visible and mid-wave IR (MWIR)spectroradiometer measurement systems and visible,MWIR, and longwave IR (LWIR) cameras. The MWIRwavelength range was 3–5 mm, and the LWIR rangewas 8–12 mm. A LWIR test image is shown in Fig. 3b.AEROSPACE NUCLEAR SAFETY AT APL: 1997 –2006Post-test analysis of the LWIR data and the calorimeterand thermocouple data indicated that plume temperaturesreached 3000 ± 100 K and heat fluxes reached 200± 80 W/cm 2 .Test results, which were reported in the databooksand in Ref. 7, provided the peak temperatures (up to3100 K) for the solid propellant fire specification inthe Mars Exploration Rovers (MER) SAR databookin 2002. APL’s analysis of the test results continuedinto 2004. The Laboratory developed a fire model foraluminized propellant fires based on the physics of thephenomena in an aluminized propellant fire. 8 Modelpredictions were consistent with test results. The APLfire model was then used to produce a solid propellantfire specification for the New Horizons SAR databooksin 2004 and 2005; this specification was carriedover into the Mars Science Laboratory (MSL) SARdatabook in 2006. The specification provided a rangeof propellant fragment sizes and masses, specified thetemperatures as functions of time and location for eachpropellant size, and was used to develop the New HorizonsFSAR. In addition, the test results were deemed tobe accurate by INSRP in the New Horizons SER andwere a sufficient basis to develop the specification.APL also provided test observations, based on posttestmaterials analysis, that were included in the SARdatabooks’ solid propellant fire specification. A calorimeterof solid molybdenum metal melted at 2896 K servedas a temperature marker. Aluminized solid propellantfires produce aluminum oxide (Al 2O 3, or alumina). Theresponses of various witness materials near the propellantshowed that aluminum oxide tended to dissolve andmerge with other oxides, such as silicon dioxide (silica)or sand, yttrium oxide (yttria), and cerium oxide (ceria).The merged liquid alumina and silica would be muchmore viscous than liquid alumina alone. An endothermicreaction of alumina with carbon was identified thatcould lower the temperatures of materials enclosed incarbon (such as a GPHS) by about 150 K.Figure 3. (a) Test setup with propellant block and in situ instrumentation. (b) LWIR image of a solid propellant fire test.Johns Hopkins APL Technical Digest, Volume 27, Number 3 (2007)257

Y. CHANGThe following test observations were also reportedin Ref. 7. Measurements of the deposits downstreamand on a vertical rectangular witness board (shown tothe right of the propellant in Fig. 3) suggested that fora ceria rod originally at a 28-cm (11-in.) radius, only34% of the mass merged with the local ground deposits,about 22% sprayed downstream onto the ground andthe board, and the remaining 44% was not recovered.At least part of the unrecovered ceria may have becomeairborne indefinitely. The post-test materials analysisprovided important insight into the behavior of RTGmaterials subjected to a solid propellant fire and theirthermal, chemical, and physical interactions with thecombustion products of the fire and the sand and concreteground materials.In March 2006, APL participated in a Joint Army/Navy/NASA/Air Force workshop on Ambient AtmosphereSolid Propellant Combustion. Participantsincluded national experts from government, universities,and industry. APL gave several invited keynote presentationson the solid propellant fire modeling, solidpropellant fire tests, optical diagnostics overview andimplementation, and calorimetrics. These were wellreceived by the workshop participants, who noted in theworkshop report that the Laboratory’s work provided avaluable contribution in that the bottom-burning environment,which is the most severe environment for anobject, had not been considered before.NUCLEAR RISK ASSESSMENTSAs discussed in the boxed insert, DoE provides theNRAs as input to a mission’s EIS. Tetra Tech NUS, Inc.,under subcontract to APL, provided the NRAs in 2002for the NASA MER missions, in 2005 for the New Horizonsmission, and in 2006 for the MSL mission. In 2005,ASCA, Inc., also under subcontract to APL, providedthe EIS databook for the New Horizons mission, whichconsisted mainly of accident conditions and probabilities.In 2002, Tetra Tech NUS also provided the FSARfor the MER missions. 9 (Each MER rover carried eightradioisotope heater units [RHUs] and no RTGs. TheRHUs provide heat to the rovers’ electronics to allowtheir survival during the cold Martian nights. Each RHUcontains approximately 2.7 g [about 0.1 oz] of PuO 2fuel,with an activity of 32.4 Ci, enclosed in a protective cladand graphitics. The cylindrically shaped RHU is 2.6 cm[1.0 in.] in diameter and 3.2 cm [1.3 in.] long, with a totalunit weight of about 40 g [1.4 oz]. 9 )LAUNCH CONTINGENCY EFFORTSThe failure of the fourth stage carrying the RussianMars 96 spacecraft with four Russian RTGs, each containing200 g (7 oz) of Pu-238, resulted in Earth reentryin November 1996. According to the WashingtonPost, 10 the U.S. Space Command informed PresidentBill Clinton, who then warned the Australian PrimeMinister that the spacecraft would impact Australia nearCanberra. But local Australian officials had alreadyalerted emergency teams 2 h earlier. Furthermore, thespacecraft debris had already landed a day earlier, not inAustralia but near the coast of Chile.This series of events demonstrated the need for realtimeproactive monitoring of a nuclear space launch andon-orbit trajectory until Earth escape. Specifically, thisexperience illustrated the need, after a reentry accident,for accurate timing (when to expect Earth impact), timeliness(warning before, not after), and accurate locationprediction (where Earth impact would occur). Thesecapabilities were provided by APL’s Cassini launchcontingency effort 11 for the October 1997 launch. TheCassini spacecraft carried three GPHS-RTGs and 117RHUs. Before launch day, APL distributed to the communitya set of ballistic coefficients of the GPHS andRHU modules so that the same information was used byall parties. (The ballistic coefficient of a body is a measureof its ability to overcome air resistance in flight.)APL provided the capability to predict the time ofspacecraft reentry from an orbital decay based on orbitalparameters from U.S. space tracking assets. In the eventof a suborbital or out-of-orbit reentry, U.S. space trackingassets would also provide the reentry conditions ofthe spacecraft. The breakup conditions of the spacecraftand RTG would be provided by JPL, the spacecraftmanager. APL would then predict the trajectory of theGPHS modules and RHUs released from the spacecraftas well as their Earth impact footprints via a threedegree-of-freedomtrajectory propagation code that usedthe ballistic coefficients of the GPHS modules and theRHUs, all within an hour of reentry. These predictionswould be used for notification and recovery purposes.The contingency function was stood up at APL, and thecapabilities for predicting the reentry time and impactfootprints were brought online and staffed for launch,through ascension to park orbit, to final Earth escape.The launch and interplanetary injection were successful,and no contingency activities were needed.The same launch contingency efforts were plannedfor the two MER launches in June and July 2003. TheMER rovers, Spirit and Opportunity, each carried eightRHUs, as noted earlier. In preparation, APL improvedthe prediction capabilities by using a six-degree-of-freedomtrajectory propagation code, incorporating winds,and validating the code to predict the time of spacecraftreentry from an orbital decay. 12 Again, two successfullaunches precluded the need for intervention by theAPL contingency team.To prepare for the New Horizons launch in January2006, APL’s launch contingency roles were expanded 13as part of NASA/DoE Radiological Contingency Planning.In addition to the tasks of the Cassini and MERlaunch contingency efforts, APL also wrote the contingencyplans for the debris impact footprint definition258Johns Hopkins APL Technical Digest, Volume 27, Number 3 (2007)

AEROSPACE NUCLEAR SAFETY AT APL: 1997 –2006and heliocentric orbit, participated in the RadiologicalControl Center (RADCC) at the Kennedy SpaceCenter as the APL Payload Representative, provided ahazardous elements picture book for first responders,developed capabilities for spacecraft and RTG breakuppredictions, 5 coordinated with U.S. space tracking assets,and reviewed pre-positioned cables to U.S. embassies.The Laboratory distributed to the community a set ofballistic coefficients of third-stage and spacecraft individualcomponents that could reenter so that the sameinformation was used by all parties. APL also completedthe database of full rotational aerodynamic coefficientsof the GPHS module to cover the supersonic and transonicflight regimes by computational fluid dynamicsanalysis. These coefficients supplemented existing subsonicand hypersonic wind tunnel data.The probability of a launch accident leading to asuborbital or orbital reentry was estimated at about 3%in the New Horizons FEIS, 3 with Earth impact possibleanywhere worldwide between 28.8°N and 28.8°S. Theconcept of operations for the debris impact footprintprediction following a reentry is shown in Fig. 4, with anominal launch shown by the solid red line. In case of areentry accident, APL was to provide an Earth impactfootprint prediction in less than an hour. The footprintpredictions would be made by personnel at the EmergencyOperations Center on the APL campus and forwarded tothe APL Payload Representative in the RADCC at theAPL-FTP = APL Footprint Prediction RepresentativeAPL-NAV = APL Navigation and Tracking RepresentativeAPL-RADCC = APL Payload Representative in the RADCCAPL-SCR = APL Spacecraft Reentry Breakup RepresentativeCAR = Coordinating Agency RepresentativeCCAFS = Cape Canaveral Air Force StationC.MECOx = Centaur main engine cutoff 1 or 2C.MESx = Centaur main engine start 1 or 2CNN = Cable News Network (proxy for all news media)DHS = U.S. Department of Homeland SecurityDoE = U.S. Department of EnergyDoS = U.S. Department of StateDSN = Deep Space NetworkEIF = Earth impact footprintFL = Florida state and local governmentofficialsGSFC = Goddard Space Flight CenterJPL = Jet Propulsion LaboratoryKSC = Kennedy Space CenterLV = Launch vehicleRADCC = Radiological Control Center at KSCSTA = U.S. space tracking assetsFigure 4. Debris impact footprint prediction concept of operations. 13Johns Hopkins APL Technical Digest, Volume 27, Number 3 (2007)259

Y. CHANGlaunch site. APL’s footprint predictions would then bepassed on to NASA and DoE officials for notification andrecovery purposes.The actual launch went off flawlessly. Thirteenmonths later, in February 2007, the New Horizonsspacecraft flew by Jupiter and gathered scientific data onthat planet, its moons, rings, magnetosphere, and magnetotail.New Horizons is expected to fly by Pluto andCharon in July 2015, then past additional Kuiper Beltobjects a few years after that, for our first close encounterswith these bodies.CONCLUSIONIt has been said that more than 99% of our solarsystem is unexplorable without nuclear power. Past missionsenabled by space nuclear power have revealed someof the secrets held by the planets, their moons, and theedge of the solar system. Future missions to bodies in thesolar system and beyond to interstellar space could alsouse space nuclear power. APL has contributed to the aerospacenuclear safety of past missions and maintains thetechnical expertise to continue to do so in the future.ACKNOWLEDGMENTS. The Aerospace Nuclear Safety Program(ANSP) at APL is sponsored by Lyle Rutger ofDoE. The NASA NEPA compliance, launch approval,and launch contingency activities for the New Horizonsmission were sponsored by Kurt Lindstrom ofNASA Headquarters. The New Horizons Concept StudyReport activities were supported by APL internal funds.I would also like to thank Donald Owings, retired fromDoE, Glen Fountain of APL, and Dr. Alan Stern, formerlyof the Southwest Research Institute, for their support.The accomplishments described in this article built onthose of past ANSP members, who worked on severalmissions and contributed to the development of DoE’sGPHS-RTGs. The Galileo GPHS-RTGs were recognized asmission-enabling power system technologies in a 2000commendation letter from the NASA Administrator tothe Secretary of Energy.REFERENCES1 Spacecraft Power for New Horizons, NASA Fact Sheet (Feb 2005);http://pluto.jhuapl.edu/common/content/pdfs/NHRTG_FS_100804.pdf.2 Bennett, G. L., “Mission of Daring: The General Purpose Heat SourceRadioisotope Thermoelectric Generator,” AIAA-2006-4096, 4th Int.Energy Conversion Eng. Conf., San Diego, CA (Jun 2006).3 Final Environmental Impact Statement and Record of Decision forthe New Horizons Mission, NASA (Jul 2005); http://spacescience.nasa.gov/admin/pubs/plutoeis/index.htm.4 McGrath, B. E., Frostbutter, D. A., and Chang, Y., “Probabilities ofGround Impact Conditions of the New Horizons Spacecraft and RTGfor Near Launch Pad Accidents,” in Space Technology and ApplicationsInt. Forum (STAIF-2007), AIP Conf. Proc. 880, M. S. El-Genk (ed.),Melville, NY, pp. 579–589 (2007).5 Lear, M. H., McGrath, B. E., Takashima, N., and Heyler, G. A., “JHU/APL Breakup Analysis Tool (APLbat) for the New Horizons RadiologicalContingency,” in Space Technology and Applications Int. Forum(STAIF-2007), AIP Conf. Proc. 880, M. S. El-Genk (ed.), Melville,NY, pp. 571–578 (2007).6 Guo, Y., New Horizons Earth Impact Study, SRM-TR-03-008, JHU/APL, Laurel, MD (May 2003).7 Hunter, L. W., Chang, Y., Oguz, H. N., Wilkerson, J. T., Lennon,A. M., et al., “The Environment Created by an Open-Air Solid RocketPropellant Fire,” Combustion Sci. Technol. 179(5), 1003–1027 (2007).8 Oguz, H. N., and Hunter, L. W., Fire Model for Propellants with a SizeDistribution of Aluminum Particles, RAS-04-071, JHU/APL, Laurel,MD (Aug 2004).9 Firstenberg, H., Rutger, L. L., Mukunda, M., and Bartram, B. W.,“Nuclear Safety Analysis for the Mars Exploration Rover 2003 Project,”in Space Technology and Applications Int. Forum, AIP Conf. Proc.699, pp. 285–299 (2004).10 Sawyer, K., “Craft? What Craft? Russian Mars Probe Already HadFallen Before Dire Warnings,” Washington Post (19 Nov 1996).11 Chang, Y., O’Neil, J. M., McGrath, B. E., Heyler, G. A., andBrenza, P. T., “Cassini Launch Contingency Effort,” in Space Technologyand Applications Int. Forum, AIP Conf. Proc. 608, pp. 732–739 (2002).12 McGrath, B. E., Frostbutter, D. A., Parthasarathy, K. N., Heyler,G. A., and Chang, Y., “Mars Exploration Rovers Launch ContingencyEfforts,” in Space Technology and Applications Int. Forum, AIP Conf.Proc. 699, pp. 300–307 (2004).13 Chang, Y., Lear, M. H., McGrath, B. E., Heyler, G. A., Takashima,N., and Owings, W. D., “New Horizons Launch Contingency Effort,”in Space Technology and Applications Int. Forum, AIP Conf. Proc. 880,pp. 590–596 (2007).The AuthorYale Chang is a member of the APL Principal Professional Staff. He has been the Program Manager of the Aerospace NuclearSafety Program since 1997, the Deputy Manager for NEPA/launch approval of the New Horizons mission at APL, and the APLPayload Representative in the Radiological Control Center at the Kennedy Space Center. He hasexperience on civilian and military space missions as well as DoD weapons systems. Mr. Chang receiveda master’s degree in mechanical engineering from the Southern Methodist University in Dallas, Texas,in 1984, and a bachelor’s degree in mechanical engineering from the University of Illinois at Urbana–Champaign in 1980. His e-mail address is yale.chang@jhuapl.edu.Yale Chang260Johns Hopkins APL Technical Digest, Volume 27, Number 3 (2007)