Fun with Solar Arrays

Another week on console, another couple of solar array feathering events.

Monday we have a Prop Purge from the Progress. This Progress has been with us since January, and is leaving on Thursday morning.

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A Progress is basically a big empty (unmanned) Soyuz. They launch from Kazakhstan full of fuel, food, water, air and other consumables. This one docked to the docking compartment (PIRS) which is on the earth facing (we call that nadir) side of the ISS. We could just say bottom, but really there's no up or down in space so bottom, top, left, and right are irrelevant.

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After the Progress docks, astronauts spend some time moving cargo out of the Progress; then before it's time to leave, they spend a bunch more time moving in trash. Before the Progress leaves, whatever extra propellant it has left is transferred to the station tanks. After that, we must vent out the fluid lines safely, to avoid it spraying any which way when the Progress undocks... that's the Prop Purge that we did this morning. Since this involves both propulsive attitude control and contaminants being vented to space, we move the US solar arrays to safe positions where they won't be damaged by the propellant.

What does propulsive attitude control mean? Well, ordinarily we use non-propulsive attitude control- large spinning gyroscopes rotated about an axis. Remember that experiment in high school physics where you spin the bike wheel and try to turn it sideways and it resists? Or when you do that on a chair and the chair spins in the opposite direction.  Or watch the video below, where a spinning wheel resists gravity to stand upright even though its only anchored by the rope to the axle.

These gyroscopes work like that bike wheel to impart forces on the ISS and keep it, essentially, straight. But they are only so powerful. When you vent something, you are subject to Newton's Third Law- every action has an equal and opposite reaction. For example, spraying propellant to the left would cause the ISS to move right, the opposite reaction. These same laws are acting on you when you hold a running garden hose, but other forces (like your weight and the friction between your feet and the floor) keep it from pushing you back. The ISS is just floating there in space, so no friction and no weight are going to help it out. Instead the station's thrusters must act to keep the ISS from moving right. Those thrusters can damage the solar arrays, so we must move the arrays to a safe position.

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Thursday morning we will undock this Progress (number 46). It will float away and burn up in the atmosphere, taking our trash with it. Then, a few days later, a new Progress (number 47) will come take it's place. The circle of life.