Moonport: A History of Apollo Launch Facilities and Operations|
Lightning Protection for Apollo Launch Operations
KSC officials had been concerned about lightning strikes since the start of the Apollo program. The Cape Canaveral area averaged more than 70 thunderstorms per year, twice the national average. Although there had been little lightning damage to missiles during the 1950s, the height of the Saturn vehicle greatly increased the chances of a strike. Studies made in 1962 pointed up the hazard to LC-39. General Electric engineers predicted that the VAB would receive five lightning strikes per year, the mobile service structure and mobile launcher four strikes per year. The potential for lightning damage had prompted a Marshall-LOC meeting in August 1962. The group recommended contracting with General Electric's High Voltage Laboratory for a lightning protection study and appointed LOC the technical supervisor. In February 1963, Petrone set up a committee on lightning protection, under Hayward D. Brewster, to review the GE proposals.49
The GE study served as the basis of the committee report submitted to Debus in July 1963, which concentrated on four problems, all during prelaunch operations: protection of the vehicle against a direct strike, the induced effects of a strike, mobile launcher grounding in transit, and corona. The placement of a lightning mast atop the mobile launcher was the straightforward solution to the first problem. Some authorities, however, did not think the traditional "cone of protection" applied to a structure as tall as the mobile launcher.* Electrical engineers also differed as to whether bonding and shielding on the mobile launcher would lower induced voltages to an acceptable level. Unless KSC could protect the launcher from both a direct strike and the secondary induced voltage, some other lightning diverter, such as balloons, would have to be used. GE module tests demonstrated that the cone-of-protection theory did apply to the mobile launcher and that practical measures would protect the vehicle and support equipment circuits from induced voltages. Besides a retractable mast for the launcher,** the committee recommended "general grounding, shielding, and bonding techniques . . . throughout the LC-39 area in order to keep the high voltage imposed by lightning strokes anywhere on the complex to a safe level."50 An extensive underground counterpoise of rods and interconnecting conductors was eventually built into LC-39. Thousands of ground rods, driven deep enough to achieve a one-ohm resistance, tied together the crawlerway, service structure parking area, perimeter fence, and pad. Similar counterpoises protected cross-country cabling.51
GE engineers recommended certain precautionary measures when the mobile launcher was in transit. If a threat of lightning existed, personnel would stay inside the mobile launcher, or at least six meters from the crawler. Insulated ladders would be used for movement on or off the crawler. The committee proposed a backup warning system to alert personnel of approaching storms. The actual grounding of the crawler was simple - it would drag a chain along a conductor buried in the crawlerway.
A bluish electrical discharge, sometimes called St. Elmo's fire, occurs frequently when storm clouds pass over tall structures. GE investigated the possibility of this phenomenon igniting a hydrogen explosion, but found that the corona would likely appear on the top outer edges of the mobile launcher and mobile service structure. This posed no threat, since the S-IVB lines ran 30 meters below the top of the launcher. The hydrogen lines to the Apollo service structure would shield the spacecraft connections during loading. The GE team rated the corona hazard a "negligible risk."52
During the next three years, Brewster's committee implemented the safety features on LC-39 while KSC's Instrumentation Division set up a system to collect more data. A GE study of LC-34's and LC-37's needs led to a second set of committee proposals approved by Debus in November 1964. At a September 1965 meeting of the Lightning Protection Committee, R. H. Jones, an Instrumentation Division engineer, reported thirteen measured strikes during the previous year. One bolt had killed a construction worker on LC-39, pad B. Another strike on the Cape side had delayed Gemini II operations at LC-19 by several weeks (the lightning had damaged a number of electrical components in the spacecraft and supporting equipment). E. R. Uhlig of GE's High Voltage Laboratory pointed out the correspondence between the measured incidents and GE's earlier predictions.53
When Apollo launch operations began in 1966, KSC applied strict safety rules for lightning protection. All launch personnel evacuated the mobile launcher, mobile service structure, and space vehicle when lightning was detected within five miles of the pad. A half dozen lightning storms delayed operations but never for more than a few hours. KSC relaxed its provisions somewhat in 1970 as experience demonstrated the safety of the mobile launcher and service structure. Thereafter operations on the tall structures, excepting electrical work, continued in the face of an approaching storm.54
* In the early 1960s experts disagreed about the generation and incidence of lightning and about its behavior and effects. The cone-of-protection theory held that all strokes would terminate on a tall structure in preference to a shorter structure located within the conical volume whose apex was the height of the tallest structure and whose base radius was equal to the apex height. Evidence from lightning strikes on skyscrapers and church steeples indicated the theory applied to the top half of the cone; the disagreement concerned the protection provided to the lower half.
** The mast retracted so that the mobile launcher could get into the VAB.