Static electricity rule threatens on-time liftoff
BY CRAIG COVAULT
Posted: October 25, 2009
A new constraint in the Launch Commit Criteria to prevent the buildup of static electricity on the Ares 1-X flight test vehicle as it accelerates away from Kennedy Space Center has reduced chances for favorable launch weather to 40 percent for the rocket's debut planned Oct. 27.
The conditions will improve to only a 60 percent chance of favorable weather for Oct. 28, according to NASA shuttle launch weather officer Kathy Winters.
Several factors are combined in that assessment, including the expectation that a weather system moving into the western Gulf of Mexico will pull a trough of active weather currently in the Caribbean across Florida and into the Northern Gulf.
That is expected by Winters to set up cloud conditions that could violate the new "tribo electrification rule" by creating conditions that would cause precipitation static. This would be the generation of static electricity on the vehicle that could disrupt the transmission of flight test data from the launcher and receipt of data by the Ares 1-X, such as a critical destruct by the 45th Space Wing if necessary.
Static electricity caused by tribo electric conditions are created by the rubbing together of two surfaces, in this case the Ares 1-X and the cloud vapor or precipitation in the cloud that it is flying through at increasing Mach numbers.
The static charge would occur if the vehicle flew through clouds with temperatures cooler than 14 deg. F. Winters says the Ares 1-X is likely to do that with the weather forecast for Tuesday and Wednesday. That forecast includes the likelihood of overlapping cloud decks all the more likely to violate the rule.
Winters said the design of the space shuttle made it immune to such static, but that other expendable vehicles flown from the Cape have always had to contend with the rule.
The rule is different than the triggered lightning phenomena experienced during the Apollo 12 launch to the Moon in November 1969. The AC-67 Atlas Centaur launch failure that occurred in 1987 also involved triggered lightning, but that vehicle also experienced the static electricity effects of flying through clouds.
The Atlas-Centaur mission was not as lucky. That vehicle exploded resulting in flaming debris falling near the pad which hardly anyone could see because it was raining so hard. The Atlas flight still generates raw feelings at the Cape to this day and is largely responsible for the exceptional weather capability and team now operated by the Air Force 45th Space Wing.
Rocket systems are not the only flight systems that can experience the effect. Aircraft can also, as can spacecraft diving through planetary atmospheres. The issue has been raised on Mars landings and the European Huygens probe that dove through the cloud tops of Saturn's moon Titan also underwent a major prelaunch design assessment for susceptibility to the tribo electrical phenomenon, according to the European Space Agency.
While the tribo electrical factor increases the likelihood of a countdown scrub, two other rules have been relaxed somewhat.
The rules that have been lessened involve the amount of time launch would have to be delayed if an electrical discharge occurs in a nearby anvil cloud or debris cloud near the Cape.
If an electrical charge is observed in the parent cloud to an anvil or associated cloud debris from the parent, then the vehicle can still launch as long it is within a safe distance from the debris or anvil.
Those rules were relaxed after a U. S. Air Force aircraft weather program assessed the relationship between Eastern Range electrical field mill readings with electricity observed in parent clouds to anvils extending to Florida's east coast from parent storm over central Florida.
The data found the ability of the parent clouds to hold a harmful charge extending to the anvil or debris -- and possibly risking a shuttle passing nearby -- is less that thought.