Florida Airborne Field Mill Research May Improve Launch Weather Criteria


KSC Release No: 56 - 00

A weather study conducted at Kennedy Space Center last month could lead to improved lightning avoidance rules and fewer launch scrubs for the Space Shuttle and other launch vehicles on the Eastern and Western Ranges.

A team of NASA and university scientists is gathering data both from the air, using a specially equipped Cessna Citation jet aircraft, and from the ground with the Cape Canaveral Spaceport's extensive weather monitoring system.

"We believe this new study will help us get the evidence we need to demonstrate that weather criteria for launch can be made more flexible and at the same time be as safe or safer than now," said Dr. Hugh Christian, a senior Marshall Space Flight Center scientist who is principal investigator in the study at KSC. "When we better understand the physics behind lightning generation, we can better predict what weather conditions preclude launching."

Weather in general is the single greatest cause for launch delays and scrubs. About 30 percent of weather delays and scrubs are related to natural and triggered lightning avoidance rules, called lightning launch commit criteria (LCC), said Dr. Frank Merceret, KSC's Applied Meteorology Unit chief and program manager for the research project.

"Those national criteria prohibit launching any space vehicle under certain lightning danger conditions," Dr. Merceret said. "Because many factors related to the genesis of lightning are incompletely understood, the criteria have been set conservatively."

A launch vehicle and its plume ascending through an anvil cloud can trigger lightning at lower electric field levels than required for natural lightning. That's because the vehicle and the plume act as a conductor and thus decreases the electric field strength necessary to initiate a lightning flash. Such triggered lightning can disrupt or damage vehicles and their electronics. An Atlas-Centaur rocket and its payload, for example, were destroyed in 1987 when the launch of the vehicle triggered lightning.

To prevent such accidents the lightning LCC (a strict set of lightning avoidance rules) were modified by the national Lightning Advisory Panel. The panel, which is made up of representatives from various government agencies and academia, continues to review and modify those lightning launch commit criteria. These rules apply to all launches from both the Eastern and Western ranges.

The current study, which might lead to significant changes in the criteria, will use airborne devices that measure electric fields, called field mills. Six of the field mills, attached to a Cessna Citation aircraft owned and operated by the University of North Dakota, are being flown into anvil clouds in the KSC area. The aircraft is also equipped with cloud physics probes that measure the size, shape and number of ice and water particles in the clouds. Electric fields within anvil clouds are a major focus of the new study because the LCC relating to these anvil-shaped storm clouds show significant potential for improvement as soon as the behavior of these fields is better understood.

The electric field data generated from the airborne field mills will be correlated with the cloud physics data and data generated from ground field mill stations at KSC as well as a mobile field mill unit being driven by graduate student researchers from the University of Arizona. The field mill data will also be compared to data generated by the rest of the Eastern Range's weather monitoring system, including radar, wind profilers and weather towers.

"We are hoping to see clear patterns in the data from our ground-based monitoring system that correlate with data generated by the airborne field mills so that during a launch we can more accurately predict what the actual conditions in the cloud are," said Dr. Merceret. "If we can do that, then the criteria could become more flexible."

The payoff of the study could be significant. It costs about $300,000 extra for mission costs, for example, when a Shuttle launch is scrubbed. In addition, weather delays for one launch vehicle on a range can cause launch delays for other vehicles.

The current airborne field mill research project is being funded through savings created from KSC and the 45th Space Wing's Joint Base Operations Contract.

NASA plans to conduct additional related airborne electric field studies at KSC during 2001 if funding is available. Range weather monitoring is one of KSC's strategic areas for research and development in its growing role as a Spaceport Technology Center.

"KSC is an incredible location for studying lightning," said Dr. Phil Krider, a KSC visiting professor from the University of Arizona, who is the head of the Lightning Advisory Panel. "You have a combination of the world's best instruments and one of the most active lightning areas."

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