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The ASTP deserves special attention and is discussed at length here. The triggering of lightning by Apollo 12 in 1969 alerted the community to the threat of launch vehicles (and their ionized exhaust plumes) triggering lightning when launched into clouds containing strong electric fields. This in turn, raised questions about which clouds produce these fields and where these fields are located inside those clouds, particularly at the Kennedy Space Center where thunderstorms and lightning are frequent. Plans had been made for an earth- orbiting rendezvous of an Apollo vehicle with a Soyuz vehicle to take place in July 1975. Both launches were planned to take place on the same day. The Soyuz vehicle would launch first from the Soviet Union, and if that was successful, the Apollo would launch 7 ½ hours later from KSC. Because of the low Earth orbit, the time- window for a successful Apollo launch was only 5 - 8 min (Arabian, 1976), and it needed to take place at 3:50 PM local time on July 15, the diurnal maximum for lightning at KSC in a month of peak lightning activity. Given the international politics of that era, NASA felt intense pressure that this Apollo launch not be delayed unless it was absolutely necessary (Kanter, 1975). At that time, a large network of surface electric field sensors was in place at KSC to identify impending lightning hazards to space vehicles, but “very little data were

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available to correlate the field intensities aloft to those measured at ground level”. Consequently a research program was conducted jointly by KSC and NOAA using four aircraft to make airborne electric field

measurements at different altitudes in clouds over the surface network (Arabian, 1976). The four aircraft were the New Mexico Institute of Mining and Technology (NMIMT) Schweizer powered glider instrumented with rotating-vane field mills, a NOAA T-29 aircraft instrumented with 2 cylindrical field mills, one on the nose and one atop the fuselage (Kasemir, 1972); a NRL/NOAA S-2D instrumented with 2 cylindrical field mills, and a NASA C-45 (NASA 6) instrumented with 2 cylindrical mills. The C-45 was also equipped to dispense chaff if it was necessary to reduce the electric fields over the launch path just before the Apollo launch. The results from the aircraft study “indicated that, below an altitude of 3050 meters, the airborne data compared favorably with the ground instrumentation. Above this altitude, however, the ground-based sensors could not be relied upon to provide an accurate measurement of the electric field intensities aloft” (Arabian, 1976). Therefore airborne measurements of electric field were deemed necessary to support the Apollo launch. In order to be able to measure the electric field along the entire portion of the flight path of the Apollo vehicle where it might be subject to lightning strikes, four aircraft in addition to those used in the KSC/NOAA study were used: A NASA/Ames Learjet and a NASA/JSC T-38, both instrumented by the Stanford Research Institute with rotating-vane field mills, an Air Force C-130 instrumented with rotating-vane mills by Air Force Cambridge Research Laboratory (AFCRL) and a RF-4C from Kirtland Air Force Base instrumented by the Air Force Weapons Laboratory. A detailed flight pattern with exit and entry points for all aircraft was worked out for the hour immediately preceding the launch. The flight pattern and altitude assignments of the eight aircraft are shown in figure 3.3.3-1.

Figure 3.3.3-1 Flight plan to determine electric fields prior to the Apollo-Soyuz Test Project Note: [Reproduced from Arabian, 1976]

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For the 10 days prior to July 15 there had been thunderstorms every afternoon with much rain and lightning, some hail and even one tornado, but the evening before the scheduled ASTP launch the prevailing winds changed from southwest to southeast. This greatly reduced the potential for thunderstorms at the time of the launch. Although there were showers in the early afternoon on July 15 (Kanter, 1975), the launch proceeded on schedule at 3:50 PM local time (Arabian, 1976).

The conclusions drawn from these studies and from the multiple aircraft instrumented for the ASTP launch were that launch delays due to the presence of clouds could be avoided by measuring the electric field aloft along the flight path just prior to launch using multiple instrumented aircraft and that further research was required to characterize the electrical properties of various cloud types (Arabian, 1976).

The ASTP launch rules were as follows (Heritage, 1988, page 4-4): Apollo-Soyuz Lightning LLCC 1975 Space vehicle will not be launched if the nominal flight path will carry vehicle:

A. Through a cumulonimbus (thunderstorm) cloud;

B. Within 5 sm of a cumulonimbus (thunderstorm) cloud or within 3 sm of an associated anvil. This rule may be relaxed at the discretion of the Launch Director if the electric field at the launch pad is less than 1 kV//m with a very narrow launch window

C. Through cold front or squall line clouds which extend above 10K feet;

D. Through middle cloud layers 6000 feet or greater in depth where the freeze level is in the clouds; E. Through cumulus clouds with tops at 10,000 feet or higher;

F. Rules C, D, and E above may be relaxed at the discretion of the Launch Director when electric field measurements in the launch pad area are stable and measure less than 1 kV/m. [Editor’s note: It seems likely that the 1 kV/m threshold was only for a foul-weather polarity field. Explicit use of the absolute value of the electric field was not made in the operational rules until 1991.]

G. Rules C, D, and E above may be further relaxed provided that airborne and ground electric field measurements are less than or equal to 3 kV/m at the surface and less than or equal to a vertical profile of electric field, EC(H), where EC(H) varies linearly from 3 kV/m at the surface to 15 kV/m at 25K feet and remains constant above that altitude. This rule may be applied only if vertical field measurements along the flight path in a 3-mile area are within the described envelope and there are no rapid fluctuations of about 3 kV/m at about 1 minute intervals within the 3 mile area measured by the ground mills.