posted September 10, 2013
When it comes to lightning, it doesn’t take three strikes for you to be out.
Lightning poses a risk to spacecraft, and Aerospace has developed new guidelines for the Federal Aviation Administration regarding the best way to avoid a particular type of lightning during launches. Now, the Department of Defense and NASA are interested in using those same guidelines.
Lightning striking a launch vehicle can have serious consequences. In 1987, Atlas-Centaur 67 was struck, and the vehicle guidance system malfunctioned. The rocket started breaking apart and had to be destroyed due to safety concerns.
No one wants this to happen to their rocket. Therefore, precautions are taken to avoid launching into a lightning bolt.
The Office of Commercial Space Transportation, which is part of the FAA, asked Aerospace to assess the situation for commercial launch vehicles at four different locations: Spaceport America in New Mexico, the Oklahoma Spaceport, the Mojave Air and Space Port in California, and the West Texas Spaceport.
After completing two studies in 2006 and 2010 for the FAA, Aerospace developed a set of lightning flight commit criteria — basically a list of rules for when to postpone a launch to avoid lightning. Although the DOD and NASA already have their own criteria that are very similar in content, they are thinking of changing them to match the FAA criteria.
“What’s happened recently is that the DOD has reviewed the updated rules and likes their clarity and improved precision,” said Bob Seibold, the Aerospace program manager for this project.
Lightning avoidance is an ongoing area of research, and Aerospace is not new to the field. In fact, Aerospace scientists Dr. Richard Walterscheid and Dr. Paul O’Brien are part of the Lightning Advisory Panel for the Air Force and NASA.
Prior to the FAA request, much study had already been done on how to keep spacecraft safe from lightning. It might seem like the easy solution is to not launch during a thunderstorm, but it turns out the problem is a little more complicated than that.
There are two types of lightning that present a threat to launches — natural lightning and triggered lightning. Natural lightning is pretty much what it sounds like — lightning that occurs naturally during a storm.
However, it turns out that a launch can actually cause lightning, in situations when it otherwise would not have happened.
“The primary lightning threat to launch vehicles as they ascend are lightning discharges induced by the vehicle,” Walterscheid said. “Launch vehicles can intensify the ambient electric field.”
This so-called triggered lightning presents an additional problem for launches to overcome.
“Triggered lightning is a greater threat than natural lighting because it can occur in conditions that are electrically more benign,” said Walterscheid. “In addition, situations where natural lightning is occurring are obvious. Triggered lightning on the other hand must be inferred from meteorological conditions that are conducive to charging.”
So over the years Walterscheid and other experts from Aerospace and elsewhere have studied the topic and come up with guidelines for when to postpone a launch due to the risk of triggered lightning.
“The criteria involve avoiding flight through (or standing off) from clouds that are known to be associated with enhanced electric fields. These include cumulus clouds of various types, thick layered clouds, and raining clouds associated with weather systems,” Walterscheid said. “The rules require standing off from natural lightning (not necessarily because of the threat of natural lighting, but because it is a very good indicator of large ambient fields).”
Following these and other rules provides protection from lightning. However, in the effort to elude lightning, sometimes launches are postponed unnecessarily, which costs money and delays schedules.
So the issue is how to avoid lightning when there is really a risk, and not postpone a launch when there isn’t. The effort to have the best guidelines is ongoing.
Aerospace’s work for the FAA is one more step forward in the effort to understand lightning and the best ways to avoid it.
That work was carried out by Walterscheid and the team of Dr. Lynette Gelinas, Dr. Frederick Simmons, Dr. Paul Zittel, Dr. Grace Peng, and Glenn Law, as well as University of Arizona professor Philip Krider, and consultant Dr. John Willett. FAA senior meteorologist Karen Shelton-Mur was the government technical monitor.
Now, Walterscheid and O’Brien are pressing forward with more research in an attempt to find an alternative to a part of the lightning flight commit criteria called the Volume Averaged, Height Integrated Radar Reflectivity.
“Larger values of VAHIRR are correlated with a higher risk of triggered lightning as an ascending rocket passes through clouds,” Walterscheid explained. “The problem is that VAHIRR has proven difficult to calculate in some situations, making it unavailable during some launch conditions.”
If VAHIRR cannot be calculated for a given launch, then the launch team must rely on other criteria that are more restrictive, possibly delaying a launch unnecessarily.
“We are working with the lightning and radar experts on the Lightning Advisory Panel to develop quantities that are easier to calculate,” Walterscheid said. “The goal is to replace VAHIRR and increase launch availability.”
Thus, the research keeps going. Walterscheid and other Aerospace experts will continue their study of the fascinating field of lightning as they strive to minimize its effects on launches.