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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