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.

Take Advantage of Amazon's Early Black Friday Deals on Tech, Kitchen Appliances, and More

Amazon
Amazon

This article contains affiliate links to products selected by our editors. Mental Floss may receive a commission for purchases made through these links.

Even though Black Friday is still a few days away, Amazon is offering early deals on kitchen appliances, tech, video games, and plenty more. We will keep updating this page as sales come in, but for now, here are the best Amazon Black Friday sales to check out.

Kitchen

Instant Pot/Amazon

- Instant Pot Duo Plus 9-in-115 Quart Electric Pressure Cooker; $90 (save $40) 

- Le Creuset Enameled Cast Iron Signature Sauteuse 3.5 Quarts; $180 (save $120)

- KitchenAid KSMSFTA Sifter with Scale Attachment; $95 (save $75) 

- Keurig K-Mini Coffee Maker; $60 (save $20)

- Cuisinart Bread Maker; $88 (save $97)

- Anova Culinary Sous Vide Precision Cooker; $139 (save $60)

- Aicook Juicer Machine; $35 (save $15)

- JoyJolt Double Wall Insulated Espresso Mugs - Set of Two; $14 (save $10) 

- Longzon Silicone Stretch Lids - Set of 14; $13 (save $14)

HadinEEon Milk Frother; $37 (save $33)

Home Appliances

Roomba/Amazon

- iRobot Roomba 675 Robot Vacuum with Wi-Fi Connectivity; $179 (save $101)

- Fairywill Electric Toothbrush with Four Brush Heads; $19 (save $9)

- ASAKUKI 500ml Premium Essential Oil Diffuser; $22 (save $4)

- Facebook Portal Smart Video Calling 10 inch Touch Screen Display with Alexa; $129 (save $50)

- Bissell air320 Smart Air Purifier with HEPA and Carbon Filters; $280 (save $50)

Oscillating Quiet Cooling Fan Tower; $59 (save $31) 

TaoTronics PTC 1500W Fast Quiet Heating Ceramic Tower; $55 (save $10)

Vitamix 068051 FoodCycler 2 Liter Capacity; $300 (save $100)

AmazonBasics 8-Sheet Home Office Shredder; $33 (save $7)

Ring Video Doorbell; $70 (save $30) 

Video games

Nintendo

- Legend of Zelda Link's Awakening for Nintendo Switch; $40 (save $20)

- Marvel's Spider-Man: Game of The Year Edition for PlayStation 4; $20 (save $20)

- Marvel's Avengers; $27 (save $33)

- Minecraft Dungeons Hero Edition for Nintendo Switch; $20 (save $10)

- The Last of Us Part II for PlayStation 4; $30 (save $30)

- LEGO Harry Potter: Collection; $15 (save $15)

- Ghost of Tsushima; $40 (save $20)

BioShock: The Collection; $20 (save $30)

The Sims 4; $20 (save $20)

God of War for PlayStation 4; $10 (save $10)

Days Gone for PlayStation 4; $20 (save $6)

Luigi's Mansion 3 for Nintendo Switch; $40 (save $20)

Computers and tablets

Microsoft/Amazon

- Apple MacBook Air 13 inches with 256 GB; $899 (save $100)

- New Apple MacBook Pro 16 inches with 512 GB; $2149 (save $250) 

- Samsung Chromebook 4 Chrome OS 11.6 inches with 32 GB; $210 (save $20) 

- Microsoft Surface Laptop 3 with 13.5 inch Touch-Screen; $1200 (save $400)

- Lenovo ThinkPad T490 Laptop; $889 (save $111)

- Amazon Fire HD 10 Tablet (64GB); $120 (save $70)

- Amazon Fire HD 10 Kids Edition Tablet (32 GB); $130 (save $70)

- Samsung Galaxy Tab A 8 inches with 32 GB; $100 (save $50)

Apple iPad Mini (64 GB); $379 (save $20)

- Apple iMac 27 inches with 256 GB; $1649 (save $150)

- Vankyo MatrixPad S2 Tablet; $120 (save $10)

Tech, gadgets, and TVs

Apple/Amazon

- Apple Watch Series 3 with GPS; $179 (save $20) 

- SAMSUNG 75-inch Class Crystal 4K Smart TV; $998 (save $200)

- Apple AirPods Pro; $199 (save $50)

- Nixplay 2K Smart Digital Picture Frame 9.7 Inch Silver; $238 (save $92)

- All-New Amazon Echo Dot with Clock and Alexa (4th Gen); $39 (save $21)

- MACTREM LED Ring Light 6" with Tripod Stand; $16 (save $3)

- Anker Soundcore Upgraded Bluetooth Speaker; $22 (save $8)

- Amazon Fire TV Stick with Alexa Voice Remote; $28 (save $12)

Canon EOS M50 Mirrorless Camera with EF-M 15-45mm Lens; $549 (save $100)

DR. J Professional HI-04 Mini Projector; $93 (save $37)

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Why Do the Lions and Cowboys Always Play on Thanksgiving?

Elsa, Getty Images
Elsa, Getty Images

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 was founded 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 will host the Houston Texans.

How 'bout them Cowboys?

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 Washington Football Team on Thursday.

WHat's with the night game?

In 2006, because six-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 Pittsburgh Steelers will welcome the Baltimore Ravens.

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