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Sensitivity of the Space Program to Weather Elements On November 14, 1969, the Apollo 12 space vehicle was launched from complex 39A at the NASA Kennedy Space Center (KSC), Florida. At 36.5 seconds into the flight, and again at 52 seconds, major atmospheric electrical disturbances occurred that were subse- quently attributed to vehicle-triggered lightning. Temporary Thrum tions of normal operations included the loss of attitude reference by the inertial platform in the spacecraft, illumination of many warning lights and alarms in the crew compartment, disconnection of the electronic circuitry to three fuel cells, loss of communication, and disturbances to the timing system, clocks, and other instrumenta- tion. Nine nonessential instrument sensors with solid-state circuits were permanently damaged. It was most fortunate that the triggered lightning damage did not have disastrous consequences. On March 26, 1987, an Atia~Centaur unmanned vehicle was launched from pad 36B at Cape Canaveral Air Force Station. The weather conditions were similar to those present at the time of Apollo 12, and this time the outcome was calamitous. At 16:22:49 EST, about 48 seconds after liftoff, the vehicle initiated a four-stroke light- ning flash to ground. This discharge caused a memory disruption in the vehicle guidance system that, in turn, initiated an unplanned yaw maneuver. The resulting exaggerated angle of attack produced stresses that caused the vehicle to break apart. About 70 seconds 9
10 after liftoff, the range safety officer ordered that the AtIa - Centaur be destroyed, in order to protect those below from large faring debris. Both of these events illustrate that triggered lightning is cur- rently one of the major forecasting problems at KSC. This threat may have already caused NASA managers to adopt an attitude of overconservatism to the extent that almost any cloud overhead may now merit the delay of a launch. Thus it is also important to know when clouds are benign and safe to fly through. There are also other weather phenomena (such as wind, wind shear, and precipitation) that may be hazards and that at present are not being observed or forecast adequately. Space vehicle encounters with adverse weather conditions have been quite limited over the Midyear history of the space program, owing to a judicious selection of launch days, landing sites that usually favor benign weather environments, and the relatively short periods of tone when the flight is in the weather-bearing layers of the atmosphere. The accumulated "exposure" time, amounting to a few minutes during each launch and up to an hour on manned reentry and landing, makes the total base of weather experience a few days at most. Until recent years, this limited weather experience had led to a belief that weather was of secondary importance in space operations. The pane} hopes this perception no longer prevail. Meteorologists realize that the space program has been relatively lucky with respect to weather hazards. Research in the last decade has revealed the occasional existence of various small-scale weather phenomena that could be dangerous to space flight, but often cannot be observed or forecast with existing operational instrumentation and techniques. The previous absence of encounters with these features over KSC has been partly a matter of chance. In view of recent temperature effects on O-rings and triggered lightning strikes, our run of good luck may have ended. Good luck need not be a requisite for acceptable space flight weather. It is the opinion of the pane! that, with the introduction of new and upgraded observing, analysis, and forecasting tools, critical weather variables can be observed and launch conditions successfully predicted. HISTORICAL PERSPECTIVE If we reflect on the magnitude of the problem faced by the pim neers of space exploration and the history of the space program, it is
11 understandable that, when faced with the need to develop unprece- dented mechanical, control, and communications systems, weather was not consid~erecl a high-priority problem. Prior to late 1987, no of- fice was designated to coordinate weather-related operational needs, research, and related issues. As entry into space has become more common, the character of the space program has changed in that the emphasis is turning to frequent launches, economical operations, reusable vehicles, and manned missions. These trends have increased the sensitivity of the space program to weather. If the space program progresses into the 19908 as planned, two points are certain: (1) space flight wait be more frequent, with delays and cancellations more intolerable and costly, and, as a result (2) encounters with potentially hazardous weather environments wiD be more Sequent. With more frequent launches and an expected decrease In the weather safety margins it is imperative that NASA (1) more rigorously define the effects of weather on the space program and (2) take steps to upgrade its weather observing and forecasting program into a state-of-the-ecience system tuned to serve In this new era In space flight a system that can confidently and reliably identify hazards as weB as define launch windows with a him Tepee of weaker skew. Historically, NASA has dealt with weather-related problems (1) by avoiding recognizable hazardous weather situations, (2) by re- ducing the sensitivity of the space vehicle systems to the weather ("system hardening"), and (3) by examining ways to change the weather. The pane! certainly endorses further hardening of space- craft systems. The Apollo 12 and Atias-Centaur accidents have clearly demonstrated the vulnerability of spacecraft electronics to triggered lightning. A similar experience by NASA astronauts fly- ing a NASA T-38A on February 24, 1987, in wintertime stratiform clouds near Los Alam~tos Army Aviation Facility, California, shows that triggered lightning is pervasive. Since the avoidance and hardening options have practical limits that fan short of ensuring total weather "nnmunity" and since mod- ification of the weather does not appear to be practical at this time, the pane! advocates improving weather observing and forecasting capabilities. Fortunately, bold initiatives are nothing extraordinary for the space program, and there is already evidence that NASA and
12 the cooperating agencies are taking steps to improve meteorological support. In the remainder of this chapter the pane] will lay the foundation for the future weather system by assessing the unpact of numerous weather elements on various aspects of space operations. This will also provide the background for the subsequent chapters, which will map a strategy for implementing an effective, state-of-the-science weather observing and forecasting system. WEATHER FACTORS IMPORTANT FOR SPACE OPERATIONS Weather elements influence all phases of space operations, from mission planning through actual launch, booster rocket recovery (in the case of the Space ShuttIe), and landing. Weather information is needed on tune scales ranging from seasonal averages to seconds and spatial scales ranging from global size to meters. Each phase of the space program has weather sensitivities, some of which are described below. Mission pl~nn~g Years in advance of launch, space vehicles are designed and configured based upon climatological factors such as wind and tem- perature. Climatological wind profile statistics, which indicate the range of stresses that the vehicle is likely to encounter, are used in determining payload limits, flight trajectories, fuel requirements, and crew configuration. Other factors can influence the season or even the time of day scheduled for launch. Ground Operations ~ . . . Trounce activities are sensitive to a number of weather phe- nomena. The temperature and wind profiles are critical factors in determining the hazards from fueling accidents because they deter- mine the concentrations and trajectories of released gases. Activities involving toxic substances are curtailed when the resultant plume would threaten workers or a nearby population. Activities are also curtailed during the presence of nearby lightning or strong inversions (layers of air In which temperature increases with height) that could focus sound energy from explosions and cause window breakage.
13 Transport of equipment to and from the launch pads is curtailed during precipitation, lightning, strong winds, and blowing sand or dust. Fueling or detanking, as well as work on scaffolds, is halted by nearby lightning or winds exceeding 35 knots. Precautions must be taken for static electricity discharges during periods of low humidity. Launch Weather hazards encountered during launch can jeopardize the safety of the entire mission: launch pad, spacecraft, payload, and crew. Extended periods of low temperatures can inhibit the oper- ation of some essential components. For example, temperatures on January 28, 1986, which were far colder than during any previous shuttle launch, have been deterrn~ned to have contributed to the failure of the O-rings that led to the Challenger accident.* Stresses (wind loads) on structural members of the spacecraft that deviate significantly from those anticipated during planning stages (from cli- matological data) could cause the vehicle to deviate from course or break apart. Aerodynamic loads, from wind shears comparable to the largest previously encountered during launch and from vehicle response maneuvers, may have contributed to the final failure of the O-ring seals.~* Precipitation drop impact during flight can darnage heat-insulating tiles on the exterior of the Space Shuttle vehicle. A direct lightning strike can damage the exterior of the space vehicle or the external tank of the shuttle. A nearby or direct strike can cause damage to the digitally controlled flight systems and other instrumentation, and even cause uncontrolled ignition of fuel. Both natural and triggered lightning are safety threats. Common cumu- lon~mbus clouds and their anvils and deep nonconnective clouds can pose a threat of triggered lightning. In order to avoid hazardous situations, weather ~ periodically reviewed during the countdown prior to launch. Weather conditions must meet stated criteria in order for the launch to proceed. If necessary, a launch can be delayed or postponed at any time until seconds before liftoff. The specific lists of weather launch criteria and flight rules have been under revision during the past several years, *Report of the Presidential Commission on the Space Shamir Challenger Accident, June 6, 1986, pp. 70-72. **Ibid.
14 with extra safety margins added to address the lightning threat and other hazards. The proposed launch commit criteria and flight rules are included as Appendix D. Reentry and [andmg Landing operations include "normals landings of the Space Shut- tIe involving reentry and ens] of mission (EOM), and Abnormal landings, including missions aborted during ascent (return to launch site (RTES)), trans-AtIantic landings (TAL), and abort once around (AOA) maneuvers. Unlike the ground and launch procedures, which can be delayed and resumed when conditions unprove, the landing procedure, once begun, is irreversible. Thus the final weather deci- sion and site selection must be made at least 90 rn~nutes before the vehicle is due to land. Complicating the situation Is that landing is the most sensitive phase of the space flight mission. ~ the landing phase, all of the weather factors discussed with respect to launches are again important. In addition, many previ- ously unimportant weather conditions become critical because the spacecraft may be piloted visually below 8000 feet. Low clouds and fog, haze or other sources of low visibility directly affect the suit- ability of sites for landing. These constraints present a significant susceptibility to even weak weather systems. Because the spacecraft has limited control capability during this stage, clear-air turbulence, or strong headwinds or crosswinds, can present difficulties. More obvious weather threats such as thunderstorm-related wind shears and lightning present an even greater risk to the spacecraft during the landing phase. :Rescue and Recovery at Sea Booster rockets from the Space Shuttle normally fall into the sea and are recovered by ship. Observations or forecasts of adverse weather in the recovery region, such as high winds, low visibility, thunderstorms, or high sea conditions, would affect the launch deci- sions. Postlandmg Procedures The landing does not end the weather threat to the spacecraft or space program personnel. In loading the orbiter onto the Shuttle Carrier Aircraft (SCA) and readying the SCA for transport, the
15 orbiter may be exposed to weather elements for a number of days. High winds, sandstorms, lightning, and precipitation can produce damage. Wind sensitivity is maximized while the orbiter is installed piggyback onto the SCA. The SCA flight itself can be dangerous. Flights are limited to daylight hours at low altitudes, maximizing potential interaction with thunderstorms arid turbulence. Quantified Hazards from Weather Elements As discussed previously, there are launch criteria and flight rules for a number of weather variables. It was very difficult, and perhaps beyond the scope of this panel's task, to determine how much data had been collected on the response of the shuttle (or other) vehi- cle in a range of possible values of some of these parameters (e.g., precipitation types and sizes). Many of the weather elements are potentially disastrous to space flight, and the extent of the danger should be quantified as exactly as possible. Unfortunately, it appeared] to the pane} that there is only crude quantitative data regarding the risk posed by some weather hazards, such as the values of cloud electric fields that are capable of pros ducing spacecraft-triggered lightning. One dangerous byproduct of inadequate information on weather element-risk relationships may be a tendency for the launch director to issue waivers of launch criteria when conditions seem to be marginal; on average, two waivers have been issued for each shuttle launch to date. It would be far better to base Sections on the analysis of a complete data base. New launch and landing weather flight rules have been de- veloped that effectively prevent launch or landing if there is any thunderstorm-produced cloud nearby. The pane} is concerned that the implementation of overcautious flight rules will so constrain the opportunities for launch that the launch director will ultimately have no choice except to issue waivers. The development of quantified weather element-risk relationships advocated above would provide the best basis from which to define launch criteria and flight rules. Only through use of these relationships can optimum flight rules be attained, balancing the need to launch (i.e., the acceptable risk), the need for safety, and the extent of risk posed by a given weather situation. A review Should be concocted to determine whether or not the detailed responses of the Space Shuttle and other space
16 vehicles to expected ranges of meteorological parameters are known and are accurate. The rents of weB-posed studies should be quantified and published and Fed as the basis for launch commit criteria. If the review show e that previous studies of weather hazard have been inadequate, then new data shown be collected to quantify the chances of vehicle damage and/or a catastrophe as a function of the observed values of various meteorological parameters and their tm~e-space distributions. In some cases existing data bases are not adequate to estate fish appropriate flight rules. An example of a phenomenon where additional data are needed is lightning triggered within and near the clouds produced by distant thunderstorms. In order to make real-time launch decisions, data are needed to show the probability of triggered lightning as a function of distance to the parent thun- derstorm, in combination with surface and airborne electric field measurements and other parameters that can be observed. CLIMATOLOGY OF CRITICAL WEATHER ELEMENTS Climatological data can provide important guidance in schedul- ing activities to minimize weather hazards. For example, Figure 1, a graph of the number of lightning strokes as a function of time of day, reveals that threats from natural lightning could be minimized by scheduling launches only between 0300 and 1500 UT (10:00 p.m. and 10:00 a.m. EST). New climatological data bases are needed for weather elements used in the newly revised flight rules and launch criteria. Most of the data needed for an effective climatology of this type do not yet exist and require obtaining data sets from new sensors. Among the data needed are the types and sizes of precipitation elements in various kinds of clouds, the electrical fields within and near detached anvils and a variety of other cloud types, the electric fields that are required to produce triggered lightning, and the magnitude of wind variations on a variety of time scales. The sensors that may be used to collect these data bases are discussed in Chapter 3. As the extent of new weather hazards is quantified, and as new launch criteria and flight runes are established, climato- logical data bases should be generated that show their sea- sonal ant diurnal frequencies. It is clear that data bases
2800 2400 2000 m 1 600 it 1 200 800 400 o 0000 0600 FLORIDA 1 i.. I ,,L,r an ~ or .~n i 17 ETR , .,- l i\ 1 ~1 2000 0200 1200 1800 2400 UNIVERSAL TIME 0800 EASTERN DAYLIGHT TIME 1 400 2000 FIGURE 1 Variations in the number of summer lightning discharges as a function of time of day. The dashed plot represents the cumulative number of lightning flashes detected over the Eastern Test Range (ETR) using the field mill network (Launch Pad Lightning Warning System (LPLWS)) during the summers of 1976 to 1980. The solid plot represents the cumulative number of lightning dashes detected over southern Florida using the Lightning Location and Protection System (LLP) from June 15 to August 31, 1978. Data were tabulated in 10-minute intervals. (prom Maier, L.M., E.P. Krider, and M.W. Maier. 1984. Aver age diurnal variation of summer lightning over the Florida Peninsula. Mon. Breather Rev. ~ tt! :1134-1140.) are needed that characterize the triggered lightning hazard, electric fields within and near a variety of cloud types, pre- cipitation types and sizes, and ~ort-term wind variability. One existing climatological data base may need expansion. The present use of winds in the loads program has some inherent limita- tions. Wind variability statistics invoked in determining the likeli- hood of hazardous loads on the spacecraft ("knockdown loads") are based upon pairs of jimsphere wind profiles obtained about 3.5 hours and 1.7 hours apart. (The jimsphere is a roughened balloon designed
18 KM 9 - 8 7- i100 120 130 ~ \\ 1/ ,0 \!140j 1 ~ 1 · R AWI NSONDE 130 120 KTS I ; 120 /i \ l ~ :120\ 120 5- 4- 3- 2- O FEET '3O,OOO -25,000 80. 51'\\\411 10\ ~ 40 4o 1 ~ \ - \:J/~10-; ~ t1 / /<10 J / ~ 1'___,' ~-60 20 KTS \J -20,000 ~ HEIGHT -15,000 -10,000 1 1 0000 2100 1800 1500 20 JAN 1987 1 200 0900 0600 0300 0000 -TIME (UTC) 19 JAN 1987 FIGURE 2 Time-height section of wind speeds (knots) obtained from hourly wind profiler data at Pennsylvania State University on January 19-20, 1987. The stippling indicates a 100-knot change of wind speed in 2 hours. For comparison, also indicated are the times of twice-daily National Weather Service rawinsonde launches at sites across the United Staten. The latter observations, taken about 400 km and 12 hours apart, can easily miss significant atmospheric features. (Courtesy of G. Forbes, Pennsylvania State University.) to respond rapidly to wind changes.) Pairs are pooled] by season, and sample sizes range from 37 to 65. The pane! is concerned that the jimsphere-pair sample is biased toward fair weather days in general, and to warm days during the winter season, the types of days most typically used for launches in the past. Sharp, dangerous jet streaks, relatively small (500 to 1000 km) wedges of high wind speeds with strong vertical shears, such as illustrated in Figure 2, are typically associated with disturbed weather and may not have been adequately represented in a sample biased toward warm, fair weather occasions. An accelerated launch schedule will tend to require launches on some les~than-perfect occasions, and the present jimsphere pairs underestimate the wind shear hazard on those types of days. The
19 winter season is likely to bear the brunt of the heightened schedule, as thunderstorms, their rapidly changing weather, and the associated forecasting difficulties make it difficult to increase the pace during the warm season. The jimsphere pair data base should be expanded during the winter season to include all types of days that meet the other weather criteria for launch. It ~ especially critical that the data base include cases of clear skies immediately following cold front passage, where strong turbulent jet streaks axe often found. The jm~phere-pan data base should be expanded, e~eciaDy ding the winter season, and shown be supplemented by wind profiler data.