CLOSE
iStock
iStock

How Does the International Space Station Maintain Its Orientation?

iStock
iStock

How does the ISS keep its orientation?

Robert Frost:

Nominally, attitude control is provided by four control moment gyroscopes (CMGs). Each CMG contains a wheel that is 220 pounds (100 kg). That wheel spins at 6600 rpm, resulting in an angular momentum of 3500 ft-lb-s (4742.5 N-m-s). The basic idea is that if a torque induces a rotation on the ISS, those wheels can rotate about their gimbals to change the angular momentum of the ISS, creating a counter torque. Using CMGs is much more subtle than using thrusters, so microgravity experiments are not impacted. CMGs do have limits, though, so thrusters can assist, if needed. That assistance is needed whenever the torques are large.

To minimize thruster assists, during quiescent operations, we do a type of attitude control called momentum management (MM). This is done by maneuvering the ISS to a torque equilibrium attitude (TEA) that was analyzed by the ground a year or more in advance. This TEA is an attitude that, with meanderings of up to 15 degrees, will result in the gravity torques and atmospheric torques adding up, over an orbit, to close to zero. The CMGs then take up the slack to make that zero.

We often can't be in a TEA during critical operations. For those we need to be in an attitude hold (AH). An example of this is a docking or berthing. Attitude holds are challenging because they require a lot more work, often too much for the CMGs to handle alone, and yet firing thrusters during critical operations can be problematic.

For these operations we design a matrix for the flight rules to ensure safety. For example, we do not allow thrusters to fire whenever the end of the robotic arm is within 2 feet (0.6 m) of the vehicle. The last thing we need is for a thruster firing to shake the arm and cause it to hit the side of a module, puncturing the module. If the timeline indicates the arm will be that close, ADCO (the attitude control flight controller) will inhibit thruster assist.

Dockings and berthings can produce sudden changes in momentum. During these activities we inhibit the entire attitude control system to ensure we do not introduce forces that could damage a docking or berthing mechanism. You might notice, on NASA TV, that the vehicle can get considerably out of attitude at these times.

The attitude control computer (GNC MDM) contains the software that does all of the necessary calculations for attitude control. It takes in the actual attitude and subtracts the commanded attitude to determine the error it needs to correct. It knows the rates of the ISS. That is very sensitive, so sensitive that we can tell when the crew wake up by watching the behavior of the CMGs as the crew start to move around the vehicle. The software also needs a set of user provided parameters such as the vehicle mass properties and inertia tensors. These are located in data slots called CCDBs (controller configuration databases). We have a stockpile of these CCDBs for different vehicle configurations. For example, if a Progress cargo vehicle arrives and docks to the Russian Segment, we will have a CCDB slot designed for that configuration. When it leaves, we will swap to another one.

This post originally appeared on Quora. Click here to view.

nextArticle.image_alt|e
Getty Images
arrow
Big Questions
How Are Balloons Chosen for the Macy's Thanksgiving Day Parade?
Getty Images
Getty Images

The balloons for this year's Macy’s Thanksgiving Day Parade range from the classics like Charlie Brown to more modern characters who have debuted in the past few years, including The Elf On The Shelf. New to the parade this year are Olaf from Disney's Frozen and Chase from Paw Patrol. does the retail giant choose which characters will appear in the lineup?

Balloon characters are chosen in different ways. For example, in 2011, Macy’s requested B. Boy after parade organizers saw the Tim Burton retrospective at the Museum of Modern Art. (The company had been adding a series of art balloons to the parade lineup since 2005, which it called the Blue Sky Gallery.) When it comes to commercial balloons, though, it appears to be all about the Benjamins.

First-time balloons cost at least $190,000—this covers admission into the parade and the cost of balloon construction. After the initial year, companies can expect to pay Macy’s about $90,000 to get a character into the parade lineup. If you consider that the balloons are out for only an hour or so, that’s about $1500 a minute.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

nextArticle.image_alt|e
Rey Del Rio/Getty Images
arrow
Big Questions
Why Do the Lions and Cowboys Always Play on Thanksgiving?
Rey Del Rio/Getty Images
Rey Del Rio/Getty Images

Because it's tradition! But how did this tradition begin?

Every year since 1934, the Detroit Lions have taken the field for a Thanksgiving game, no matter how bad their record has been. It all goes back to when the Lions were still a fairly young franchise. The team started in 1929 in Portsmouth, Ohio, as the Spartans. Portsmouth, while surely a lovely town, wasn't quite big enough to support a pro team in the young NFL. Detroit radio station owner George A. Richards bought the Spartans and moved the team to Detroit in 1934.

Although Richards's new squad was a solid team, they were playing second fiddle in Detroit to the Hank Greenberg-led Tigers, who had gone 101-53 to win the 1934 American League Pennant. In the early weeks of the 1934 season, the biggest crowd the Lions could draw for a game was a relatively paltry 15,000. Desperate for a marketing trick to get Detroit excited about its fledgling football franchise, Richards hit on the idea of playing a game on Thanksgiving. Since Richards's WJR was one of the bigger radio stations in the country, he had considerable clout with his network and convinced NBC to broadcast a Thanksgiving game on 94 stations nationwide.

The move worked brilliantly. The undefeated Chicago Bears rolled into town as defending NFL champions, and since the Lions had only one loss, the winner of the first Thanksgiving game would take the NFL's Western Division. The Lions not only sold out their 26,000-seat stadium, they also had to turn fans away at the gate. Even though the juggernaut Bears won that game, the tradition took hold, and the Lions have been playing on Thanksgiving ever since.

This year, the Lions host the Minnesota Vikings.

HOW 'BOUT THEM COWBOYS?


Kevin C. Cox/Getty Images

The Cowboys, too, jumped on the opportunity to play on Thanksgiving as an extra little bump for their popularity. When the chance to take the field on Thanksgiving arose in 1966, it might not have been a huge benefit for the Cowboys. Sure, the Lions had filled their stadium for their Thanksgiving games, but that was no assurance that Texans would warm to holiday football so quickly.

Cowboys general manager Tex Schramm, though, was something of a marketing genius; among his other achievements was the creation of the Dallas Cowboys Cheerleaders.

Schramm saw the Thanksgiving Day game as a great way to get the team some national publicity even as it struggled under young head coach Tom Landry. Schramm signed the Cowboys up for the game even though the NFL was worried that the fans might just not show up—the league guaranteed the team a certain gate revenue in case nobody bought tickets. But the fans showed up in droves, and the team broke its attendance record as 80,259 crammed into the Cotton Bowl. The Cowboys beat the Cleveland Browns 26-14 that day, and a second Thanksgiving pigskin tradition caught hold. Since 1966, the Cowboys have missed having Thanksgiving games only twice.

Dallas will take on the Los Angeles Chargers on Thursday.

WHAT'S WITH THE NIGHT GAME?


Patrick Smith/Getty Images

In 2006, because 6-plus hours of holiday football was not sufficient, the NFL added a third game to the Thanksgiving lineup. This game is not assigned to a specific franchise—this year, the Washington Redskins will welcome the New York Giants.

Re-running this 2008 article a few days before the games is our Thanksgiving tradition.

SECTIONS

arrow
LIVE SMARTER
More from mental floss studios