Frequent viewers of live rocket launches will know the crucial role weather plays in the success, delay, or even cancellation/scrub of any orbital launch very well. But how exactly does the weather affect a rocket launch?
Weather is a crucial factor during a rocket launch, and adverse conditions with high winds or lightning pose a significant threat to a rocket, crew, and payload. On the launchpad, it can damage the vehicle and injure groundcrew. After launch, it can damage the rocket or impact its flight trajectory.
Anyone regularly flying on one of the world’s major airlines will know that the weather is not such a crucial consideration, and a flight seldom gets delayed or canceled, except in the most extreme weather conditions.
Modern aircraft are designed to fly in rain, snow, ice, thunderstorms, and even high winds to some extent. Their infrastructure & electronics are strengthened with several redundant systems in place to safeguard the plane and passengers even in the worst-case scenario.
With rockets, unfortunately, it’s not that simple. They are mainly designed to spend a very brief period of time in the atmosphere, just enough to withstand its dynamic forces (weight, thrust, lift, and drag) before entering orbit or traveling deeper into space.
Its structure can withstand immense pressures in a specific direction, the one it is traveling in. As a result, it is very vulnerable to forces applied from other directions, like powerful crosswinds. Strengthening a rocket to handle these pressures will simply make it too heavy.
Its shape also makes it more prone to reacting with the atmosphere (as explained later when discussing lightning), and they have no winglike structures to provide lift, which enables a conventional aircraft to glide back to the surface and make an emergency landing.
Rockets also have more sensitive navigational systems for complex maneuvering in the atmosphere and space. Redundant systems are in place, but their sensitivity, combined with a rocket’s structure, makes them vulnerable to extreme weather conditions like lightning.
As a result, much more stringent weather criteria need to be in place during a rocket launch to safeguard sensitive electronic equipment and a vehicle structure designed to withstand only a certain amount of force from a specific direction.
A number of different weather conditions can impact a rocket during a launch, including freezing temperatures, wind, lightning, and even forms of precipitation.
In the upcoming sections, we will take a look at the specific weather conditions that are especially dangerous for a rocket before and after launch and the measures in place to safeguard a rocket, launch site, crew, and payload against these forces.
(Learn more about precisely what wind is, how it forms, and its characteristics in this article.)
How Wind Affects A Rocket Launch
The saying “The Tallest Trees Catch The Most Wind” has a few different meanings but can literally be applied to rockets. Modern rockets are at least as tall as large trees (the Saturn Rocket was 363 feet tall) with a big enough surface area to be affected by air movement.
Even if little wind is present at ground level, it usually gains speed as altitude increases, meaning that a much stronger wind may be blowing at the top of a rocket on the launchpad. An even bigger wind hazard may be lurking higher up in the atmosphere, though.
There are mainly two types of wind conditions that can impact a rocket that needs to be monitored before a launch can take place:
- Wind Conditions On The Launchpad
- Upper-Atmospheric Winds
1) Wind Conditions On The Launchpad
The biggest danger of strong winds near or at the launchpad is not necessarily the danger to the launch vehicle or tower’s structural stability but its impact on the launch itself. A strong enough gust of wind can literally blow a rocket off course.
Modern rockets have “steering” ability through gimbaled thrust, which allows them to make course directions. If winds are severe & push the vehicle too far off-course, though, it may be unrecoverable, leading to an abort and the destruction of the rocket and payload.
(Learn more about how rockets are able to change direction and maneuver through gimbaled thrust in this article.)
To illustrate just how serious NASA is taking the presence of wind at the launchpad, it puts a clear limitation on the maximum wind speed allowed at a certain height for each launch as part of its Launch Weather Criteria.
During the Space Shuttle Program, the maximum wind speed allowed at the 60-level (18.2 meters) of the pad varied between 19 to 34 knots (35 – 63 km/h or 22 – 39 mph), depending on the wind direction.
Similarly, sustained winds must not exceed 33 knots (61 km/h or 38 mph) for the launch of an Atlas V rocket, while the SpaceX Falcon 9 Crew Dragon is not allowed to launch if winds exceed 30 mph (48 km/h) at the 162-foot level of the launchpad.
2) Upper-Atmospheric Winds
Higher up in the atmosphere, winds usually blow at much greater velocities. The biggest danger of strong upper-atmospheric winds, however, is not their speed but the vertical wind shear that is caused as a result.
Vertical wind shear can be defined as the sudden change in horizontal wind speeds experienced with a change in altitude. If a rocket traveling at high speeds suddenly flies into a layer of fast-moving air, it puts a sudden and tremendous strain on the vehicle.
This sudden impact can severely impact a rocket’s structure and even destroy it. A great analogy to illustrate the effect of such an impact is to imagine an automobile crossing an intersection only to be hit from the side by a truck that failed to adhere to a stop sign.
After a SpaceX Falcon 9 launch had to be aborted due to high wind shear in March 2016, CEO Elon Musk rightly stated that wind shear “hits like a sledgehammer when [the rocket is] going up supersonic.”
For companies like SpaceX, strong upper-atmospheric winds are not only a problem during launch. They can also be problematic for first-stage rocket boosters needing to navigate back to Earth to make a safe landing, sometimes on a small drone ship in the ocean.
Like all weather data/predictions provided to NASA before launching, space agencies in the United States rely on the United States Space Force’s 45th Space Wing (formerly Weather Squadron) to provide them with accurate data about upper-atmospheric wind conditions.
How Lightning Affects A Rocket Launch
Of all weather conditions, thunderstorms, specifically lightning, are probably of the greatest concern to anyone involved with a rocket launch. This is not just because of the damage it can cause to the craft and crew but also due to its unpredictability.
The possibility of a lightning strike near the launchpad can be determined with a high degree of accuracy by measuring lightning activity in clouds at and around the launch site and measuring the amount of electricity present in the air (even in the absence of any clouds).
Even with the most accurate sensors and advanced forecasting models available, it is still impossible to determine precisely when and where lightning will strike. As a result, NASA has dedicated a huge part of its Launch Weather Criteria to lightning activity alone.
(The complete list of NASA’s Launch Weather Criteria will be covered later in this article.)
Essentially, there are three types of lightning events that can pose a danger to a rocket:
- Cloud-To-Ground Lightning
- In-Cloud Lightning
- Electrically Charged Atmosphere (Rocket itself can trigger a lightning event)
1) Cloud-To-Ground Lightning
Cloud-to-ground lightning is the type of lightning most observers will be familiar with. It occurs when an electric discharge occurs between the base of a cloud and an object on the ground, visible as a bright lightning bolt.
On the launchpad, the rocket itself is relatively safe from a lightning strike due to the four lightning towers that surround most modern launch facilities. However, it still poses a danger to groundcrew and equipment in the vicinity and should always be monitored as a result.
2) In-Cloud Lightning
In-cloud lighting is the most common and frequently occurring type of lighting. Because it occurs within a thundercloud formation, it is not always visible from the ground, although it can sometimes be observed when one sees a cloud “light up” during such an event.
If a rocket flies into a thundercloud and gets hit by in-cloud lighting, it can severely damage the guidance system and other sensitive equipment. In a worst-case scenario, this can lead to the complete destruction of the vehicle and payload.
Fortunately, this form of lightning can also be detected and measured by modern equipment. Any cloud formation with this type of lighting present in the flight path of any rocket will immediately result in a violation of launch weather criteria.
3) Electrically Charged Atmosphere
Probably the most dangerous and unpredictable type of lightning occurs when there is no visible evidence of a potential lightning strike. Even on a clear day without any clouds present, a lightning strike can be triggered under the right conditions.
If the air at the launchpad or on the rocket’s flight path contains enough electrically charged particles, it can trigger a strike if the vehicle passes through it. The conductive surface of a rocket and the ionized exhaust plumes can act as the perfect impetus for a lightning strike.
NASA found this out the hard way during the Apollo missions of the 1960s and 70s. During the launch of the Apollo 12 spacecraft, the vehicle passed through a highly charged volume of air, which triggered a lightning strike.
The strike severely disrupted telemetry data and communication between the Saturn 5 rocket and Mission Control. Fortunately, the crew was able to rectify the problem, and the rest of the mission was completed successfully but could have ended in disaster.
In 1987, an Atlas-Centaur rocket was not that fortunate and had to be destroyed shortly after launch after it triggered a lightning strike.
As a result, the amount of electricity present in the air is measured before every launch, and if the measurements indicate levels high enough to pose a possible risk, a launch may be delayed or scrubbed.
(To learn more about what precisely a lightning strike is, what causes it, and its impact, read the full article here.)
Although wind and lightning may be the two biggest dangers for a rocket before and during a launch, they are not the only weather hazards. Consequently, space agencies monitor a broad range of weather conditions.
Before a launch, meteorologists monitor different aspects of cloud development around the launch facility, with specific attention given to cloud thickness and height. Cumulus and cumulonimbus cloud formations are of high importance for this very reason.
They have the potential to produce high wind and lighting activity, but if their height extends into freezing temperatures, the possibility of hail and other forms of ice forming is also likely. If a rocket hits pieces of ice at supersonic speeds, the results can be catastrophic.
It is, therefore, quite understandable why the bulk of NASA’s Launch Weather Criteria for any launch vehicle focus on cloud development in and around a rocket launch site, as will be illustrated in the next section.
Measures For Protecting A Rocket Against Adverse Weather Conditions
As a result of the hazardous weather conditions discussed in the previous sections, NASA and other space agencies have set out a comprehensive list of requirements, called the Launch Weather Criteria, that has to be met before any rocket launch can take place.
These requirements do not only apply to conditions around the launch facility but also to the weather behavior downrange, the area surrounding a rocket’s flight path. This is to protect both the launch vehicle and the population/objects on the ground.
During a crewed flight, the weather at designated abort/emergency landing sites must also be monitored in case they are needed. During the Space Shuttle Program, there were three abort locations where the weather conditions had to meet certain requirements:
- Return To Launch Site (RTLS): If a failure occurs shortly after launch and a vehicle is unable to make it to the next abort site, it must try to return to the launch/landing site.
- Transoceanic Abort Landing: If the vehicle cannot return to the launch/landing site, a location across the Atlantic Ocean was chosen as an emergency landing location.
- Abort Once Around: In some instances, it would make more sense to allow the vehicle to orbit the planet once and land at the launch/landing site or a suitable location nearby.
All these abort sites also had to adhere to a strict set of weather criteria. Modern-day crewed flights also have abort locations with a similar set of weather requirements.
To illustrate how seriously NASA is taking the weather conditions during a launch, the following is a word-for-word copy of its Launch Weather Criteria for SpaceX’s Crew Dragon that has to be met before they can commit to a launch:
Do not launch if the sustained wind at the 162-foot level of the launch pad exceeds 30 mph.
Do not launch through upper-level conditions containing wind shear that could lead to control problems for the launch vehicle.
Do not launch for 30 minutes after lightning is observed within 10 nautical miles of the launch pad or the flight path, unless specified conditions can be met.
Do not launch within 10 nautical miles of an attached thunderstorm anvil cloud, unless temperature and time-associated distance criteria can be met.
Do not launch within 10 nautical miles of a detached thunderstorm anvil cloud.
Do not launch within 3 nautical miles of a thunderstorm debris cloud, unless specific time-associated distance criteria can be met.
Do not launch within 5 nautical miles of disturbed weather clouds that extend into freezing temperatures and contain moderate or greater precipitation, unless specific time-associated distance criteria can be met.
Do not launch for 15 minutes if field mill instrument readings within five nautical miles of the launch pad exceed +/- 1,500 volts per meter, or +/- 1,000 volts per meter if specified criteria can be met.
Do not launch through a cloud layer greater than 4,500 feet thick that extends into freezing temperatures, unless other specific criteria can be met.
Do not launch within 10 nautical miles of cumulus clouds with tops that extend into freezing temperatures, unless specific height-associated distance criteria can be met.
Do not launch within 10 nautical miles of the edge of a thunderstorm that is producing lightning within 30 minutes after the last lightning is observed.
Do not launch through cumulus clouds formed as the result of or directly attached to a smoke plume, unless time-associated criteria can be met.
Do not launch if downrange weather indicates violation of limits at splashdown in case of Dragon launch escape.
Do not launch if downrange weather shows high probability of violating limits at splashdown in case of Dragon launch escape.
Downrange weather is monitored at more than 50 locations along the ascent track along the North American eastern seaboard and across the North Atlantic.
Probability of violation is calculated for each location including limit conditions for wind, waves, lightning, and precipitation.
The original NASA document can be found here.
All weather data/predictions are provided to NASA before a launch by the United States Space Force’s 45th Space Wing (formerly Weather Squadron) located at Patrick Air Force Base close to Cape Canaveral and Kennedy Space Center.
They work with partners across the world to provide NASA, the Department Of Defense, and other space agencies with crucial, accurate, and up-to-date weather data needed for safe rocket launches and all related activities.
Conclusion
Thousands of enthusiasts flock to Florida each year to observe a rocket launch, and since the advent of the Internet and the growth of Social Media sites like YouTube, millions more are watching live rocket launches online.
It is not uncommon to hear complaints from observers whenever a much-anticipated launch is delayed/scrubbed (sometimes for the 2nd or 3rd time). The situation is made even worse when there is no sight of any adverse weather conditions near the launch facility.
However, with so many (but necessary) weather restrictions in place both near a rocket launch site and various related locations, it is surprising that any launches take place at all.
(Which is made even worse by the fact that Florida experiences more thunderstorms per year than any other place in the United States.)
This article detailed the various weather restrictions in place that can prohibit a rocket launch from taking place but also highlighted the various reasons these restrictions are in place.
This article was originally published on headedforspace.com. If it is now published on any other site, it was done without permission from the copyright owner.
