STS-135 ascent Flight Dynamics Officer Mark McDonald from an earlier mission. (Photo: NASA)

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Shuttle trajectory design begins long before each mission’s scheduled launch date. As we covered previously, the flight dynamics team is responsible for the shuttle’s flight path from launch to landing. 

Mark McDonald is the ascent Flight Dynamics Officer (call sign FDO, or “fido”) for STS-135, the final shuttle mission. The FDO manages the flight dynamics team supporting the launch and during launch sits in the front row of the flight control room at the Mission Control Center in Houston.

Trajectory design involves a number of tradeoffs. One weather-related example has to do with the performance of the solid rocket boosters, or SRBs, that help lift the shuttle into orbit. 

The temperature of the SRB solid propellant affects their performance. SRB propellant is a mixture of mostly aluminum powder and ammonium perchlorate, along with a binder that gives it the consistency of a pencil eraser.

SRB temperature is estimated based on the average air temperature around the SRBs leading up to launch. McDonald says that in general, SRB performance is higher in the summer when the propellant burns hotter and lower in the winter when it burns cooler. 

During early planning, trajectory designers use generic atmosphere data for the time of year to simulate the launch to estimate performance and therefore how much payload the shuttle can carry to space.

There are many small uncertainties in both the shuttle’s performance and assumptions made in the computer models used to simulate the trajectory, so mission planners can’t fully pack payload to the brim. Some margin is built into the performance estimates to allow for these uncertainties. 

“Once you get closer to the launch and holding to specific launch date,” says McDonald, “you start talking about the real situation you’re going to be in and start applying deltas to your performance. You know what the PMBT [SRB propellant temperature] is, so you start saying, ‘Hey, our ascent performance margin is going to be higher or lower than we had predicted from the generic numbers for that time of year.’ Then you get close to launch and say, ‘Maybe we shouldn’t have loaded that extra payload because our PMBT is lower than we expected.’”

We previously discussed max Q, the period during the first two minutes of the flight when the dynamic pressure on the shuttle is largest. Part of the trajectory design involves deciding when to throttle down the shuttle’s three main engines during the first minute of flight to ensure that the maximum dynamic pressure does not exceed certified structural limits. 

Trajectory design also includes the pitch (think of the orbiter’s nose moving up or down) and yaw (when the nose pivots from side to side) steering commands flown by the shuttle’s guidance system. These commands, along with the shuttle’s “heads down” orientation, are designed to minimize structural loads on aerodynamically sensitive parts of the shuttle.

The throttling and steering commands are based on the wind between the ground and around 60,000 feet above the ground. A series of weather balloons are released at Cape Canaveral during the hours leading up to the launch. 

The wind profiles observed by these balloons are fed into a computer system that designs the commands. The full process takes around two hours after the release of the balloon, so to avoid last-minute changes the final updated steering commands are built on winds observed about four hours prior to launch.

Teams at Marshall Space Flight Center in Huntsville, Alabama use different computer systems to ensure that the wind profile has not significantly changed since the steering commands were built. A significant change in winds aloft could result in steering commands that put excessive loads on the shuttle. 

These teams also monitor wind shear, which can cause temporary excessive structural loads. Reports after the Challenger and Columbia accidents each mentioned wind shear as a possible contributing factor. The wind shear was not outside certified limits on either launch, but it may have played a role in each. 

If you have any questions about weather’s role in trajectory planning or any other part of the shuttle, send your questions to our Facebook page or post them to @twcspacewx or @weatherchannel on Twitter with #shuttlewx.

This process of building updated steering commands from balloon data on launch day is called Day-of-Launch Initialization-Load Update, or DOLILU, and has been used since 1991. DOLILU helps reduce the possibility of launch scrubs caused by unexpected changes in upper-level winds.

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