Why are rocket launches so sensitive to weather?

Question

I've been following the upcoming launch of the Antares rocket to resupply the ISS (scheduled for today, July 13, 2014). This launch was originally scheduled for May, and has been rescheduled multiple times before this, due to poor weather.

However, airliners regularly take off and land even during snowstorms and thunderstorms. What is it about rockets that makes them so sensitive to bad weather? Is it simply a question of wanting to avoid any risk at all, with such an expensive piece of equipment?

Answer 1

Lightning and wind are the primary weather concerns that preclude rocket launch.

A rocket's control system needs to compensate for lots of un-modeled or difficult to model factors. For example, uncertainty in mass properties, engine performance, mechanical alignments, errors in inertial measurement units (IMU), gyroscopes, GPS, etc. If any of these errors become too large the system will not longer be able to compensate, the system will become uncontrollable, and the mission will be lost.

Wind, unsurprisingly, is one of these uncertain factors, and especially wind shears, which are rapid changes in wind direction as a function of location, are difficult for control systems to handle because they can put enormous aerodynamic loads on a vehicle very quickly.

Lightning is dangerous for entirely different reasons. Rockets are full of sensitive electronics, attract lightning nicely (even when not grounded), and are vulnerable to direct lightning strike. Even something as typically benign as a reset or temporarily glitch that could potentially be tolerated in a satellite would be disastrous to a rocket under powered flight. Airplanes avoid lightning storms for the same reason, although they have more margin available to incorporate lightning mitigation into their designs.

For these reasons (and to a lesser extent others, such as the extreme temperature example provided in another answer), rockets launches are frequently delayed for ideal conditions. There's no sense in risking a rocket and payload worth hundreds of millions of dollars for a week.

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In contrast, rockets that must be able to be launched in at a moments notice (think ICBMs and SLBMs) are designed with very different requirements. Here, the performance hit taken to design robustness to weather is accepted because a weapon isn't very useful if you have to wait for the storm to pass before it can be fired.

Answer 2

It's mainly just bad luck.

There is really only one relatively common weather condition that you don't want to launch a rocket into and that's a thunderstorm. Granted, high winds can also be problematic. And it should go without saying that you don't want to launch during a hurricane or tornado.

Rockets are perfectly capable of launching directly into a thunderstorm, just as a civil airliner might do, but there's a wee bit of a problem with doing so. Rockets are made of metal, and their exhaust is partially ionized, so during liftoff the entire rocket+exhaust trail is like a giant conductive wire tying the rocket to the ground. If the rocket flies into a thunderstorm then it will attract lightning strikes. Rockets can be made to withstand such strikes, and most of the current flows over the skin of the vehicle (just as with an airliner) but that adds a lot of difficulty. This actually happened during the Apollo 12 launch, and the lightning strike caused a huge number of problems with the onboard computers and with the telemetry link until folks were able to fix the telemetry and then slowly get everything else in order (fortunately the flight computers continued running normally).

As with all inherently dangerous activities the basic principle is generally to "stack the odds in your favor", so if you can avoid launching a hypersonic rocket packed with literally kilotons of explosive fuel into a scenario where it is almost certain to draw a lightning strike then you do so.

So why does weather cause so many launch delays? Well, that's mostly just an issue for the US. For orbital dynamics reasons it is most beneficial to launch in an Easterly direction from lower latitudes. In the continental US the lowest latitudes are in Florida, which are conveniently on the coast facing the entire Atlantic Ocean to the East, so that's where the bulk of America's space launches occur from.

Coincidentally, South Florida also has the highest level of thunderstorm activity in the entire US. The area around Cape Canaveral typically sees a thunderstorm on one out of every five days of the year (for a total of 70 or more annually).

So a high risk of thunderstorms right on top of the hub of US space launch activity translates to a high volume of weather related scrubs.

The spaceport up in Virginia has it slightly better than South Florida but the high incidence of inclement weather there still makes for an unfortunate combination.

Answer 3

Why are rocket launches so sensitive to weather?

It could make the difference between an O-ring failing, or not.

In the Challenger disaster..

Disintegration of the vehicle began after an O-ring seal in its right solid rocket booster (SRB) failed at liftoff. The O-ring failure caused a breach in the SRB joint it sealed, allowing pressurized hot gas from within the solid rocket motor to reach the outside and impinge upon the adjacent SRB attachment hardware and external fuel tank. This led to the separation of the right-hand SRB's aft attachment and the structural failure of the external tank. Aerodynamic forces broke up the orbiter.

This was partly attributed to the low temperature weather.

This was believed to be the result of supercooled air blowing on the joint from the liquid oxygen tank vent. It was much lower than the air temperature and far below the design specifications for the O-rings. ..

The temperature on the day of the launch was far lower than had been the case with previous launches: below freezing at 28 to 29 °F (−2.2 to −1.7 °C); previously, the coldest launch had been at 53 °F (12 °C). Although the Ice Team had worked through the night removing ice, engineers at Rockwell still expressed concern. Rockwell engineers watching the pad from their headquarters in Downey, California, were horrified when they saw the amount of ice. They feared that during launch, ice might be shaken loose and strike the shuttle's thermal protection tiles, possibly due to the aspiration induced by the jet of exhaust gas from the SRBs. Rocco Petrone, the head of Rockwell's space transportation division, and his colleagues viewed this situation as a launch constraint, and told Rockwell's managers at the Cape that Rockwell could not support a launch. However, Rockwell's managers at the Cape voiced their concerns in a manner that led Houston-based mission manager Arnold Aldrich to go ahead with the launch. Aldrich decided to postpone the shuttle launch by an hour to give the Ice Team time to perform another inspection. After that last inspection, during which the ice appeared to be melting, Challenger was finally cleared to launch at 11:38 am EST.[12]

Aircraft need to trim weight and get by with the minimum strength with which they can provide a high level of safety for machines that might travel thousands of kilometers in a journey at speeds of 100s of kilometers per hour.

Spacecraft operate faster & higher and must be closer still to the limits of the materials technology. As the high-tech increases, so do the risks and the fragility (to weather, among other things) of the vehicle. At the same time, the margin for error and safety decreases.