How Does the International Space Station Maintain Its Orientation?


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.

What Does 'State of Emergency' Really Mean?

Firefighters battle a state of emergency.
Firefighters battle a state of emergency.
Phonix_a/iStock via Getty Images

Local and state officials across the U.S. are declaring states of emergency in their efforts to manage the coronavirus pandemic. Some entire countries, including Italy and Japan, have also declared a state of emergency. But what does this phrase really entail?

Local and State Response

The answer varies a bit from state to state. Essentially, declaring a state of emergency gives the governor and his or her emergency management team a bit of extra latitude to deal with a situation quickly and with maximum coordination. Most of these powers are straightforward: The governor can close state offices, deploy the National Guard and other emergency responders, and make evacuation recommendations.

Other powers are specific to a certain situation. For example, in a blizzard, a governor can impose travel restrictions to clear roads for snowplows and other emergency vehicles.

Calling in the Feds

If a disaster is so severe that state and local governments don’t have the cash or the logistical ability to adequately respond, the governor can ask for a declaration of a federal emergency. In this case, the Federal Emergency Management Agency (FEMA) does a preliminary damage assessment to help determine whether the governor should petition the president for a federal emergency declaration.

When the declaration from the president comes through, state and local governments can get funding and logistical help from the feds. What makes a crisis a federal emergency? The list is pretty broad, but FEMA shares some criteria here.

Why Does Hand Sanitizer Have an Expiration Date?

Hand sanitizer does expire. Here's why.
Hand sanitizer does expire. Here's why.
galitskaya/iStock via Getty Images

The coronavirus pandemic has turned hand sanitizer from something that was once idly tossed into cars and drawers into a bit of a national obsession. Shortages persist, and people are trying to make their own, often to little avail. (DIY sanitizer may not be sterile or contain the proper concentration of ingredients.)

If you do manage to get your hands on a bottle of Purell or other name-brand sanitizer, you may notice it typically has an expiration date. Can it really go “bad” and be rendered less effective?

The short answer: yes. Hand sanitizer is typically made up of at least 60 percent alcohol, which is enough to provide germicidal benefit when applied to your hands. According to Insider, that crucial percentage of alcohol can be affected over time once it begins to evaporate after the bottle has been opened. As the volume is reduced, so is the effectiveness of the solution.

Though there’s no hard rule on how long it takes a bottle of sanitizer to lose alcohol content, manufacturers usually set the expiration date three years from the time of production. (Because the product is regulated by the Food and Drug Administration, it has to have an expiration date.)

Let's assume you’ve found a bottle of old and forgotten sanitizer in your house somewhere. It expired in 2018. Should you still use it? It’s not ideal, but if you have no other options, even a reduced amount of alcohol will still have some germ-fighting effectiveness. If it’s never been opened, you’re in better shape, as more of the alcohol will have remained.

Remember that sanitizer of any potency is best left to times when soap and water isn’t available. Consider it a bridge until you’re able to get your hands under a faucet. There’s no substitution for a good scrub.

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