From: Johnson Space Center
Posted: Friday, June 13, 2003
I thought I'd write next about what it is like to fly. That of course is how we get around up here on the space station. The main difference between life up here and life down there is that things don't drop to the floor here when you let go of them, and that includes yourself. Rather than walking around as we do on the ground, we fly around inside the station. It takes some practice getting used to it, but you get better rapidly. I thought I was pretty good at flying after 2 short shuttle flights, but after working with Ken, Nikolai, and Don (who had been up here for almost 6 months), I realized I have a lot to learn.
On about our second day onboard ISS, Sox and I had the task to pump some water into a container for use in the galley, so off we flew from the Service Module (the main Russian living compartment) to the node (where all our water is stored). We were each carrying some items in one hand (pumps, hoses, cables, etc.), so we only had one hand free. Off he flew, and I couldn't keep up with him over the 70 feet or so to the node. We had to fly through a module called the FGB, which is a narrow 35-foot long corridor with equipment strapped and "velcroed" to the walls, ceiling, and floor. The faster I tried to go, the more I bumped into stuff. Sox flew straight as an arrow down to the node, while I moved along the handrails down the corridor, leaving a cloud of debris I had knocked off the walls behind me. It turns out flying with one hand tied behind your back isn't so easy!
Our flying up here takes place under the jurisdiction of Newton's laws of motion. Over 300 years ago Isaac Newton wrote down his famous laws of mechanics - which was a great stroke of genius at the time, but you quickly realize up here how obvious they are if you are weightless and don't have pesky gravity utterly dominating the mechanics of moving around. Unfortunately Newton didn't have the advantage of living on a space station. His first law, which states that objects in motion will tend to remain in motion, and objects at rest will tend to remain at rest, is the very first thing you have to deal with when learning how to fly. When flying across the module, you will continue in a straight line until you grab onto something or you hit the far wall. Similarly, if you are floating in the middle of a module and not moving, you will stay there until you push off some other object (like a handrail or a wall). I am ignoring the effect of air resistance and air currents because it doesn't have too much effect on human flying (it does for much lighter objects).
Flying can be broken down into two tasks: getting from here to there, and keeping yourself facing the way you want. Engineers call this translation and orientation, and they are exactly the same tasks that a spacecraft like the ISS, space shuttle, or Soyuz needs to do when flying about space. In effect when flying around inside the ISS you are like a miniature spacecraft. When we talk about translation, we mean moving your center of mass (also called the center of gravity). For humans the center of mass is around your belt buckle. And as my old wrestling coach Mr. Yengo used to say - "Wherever your center of gravity is going is where you are going." As for orientation, when you spin an object here, it will rotate around its center of mass, so that means if you do somersaults here you will see that you will rotate around a point near your belt buckle. Controlling which way you are facing means controlling your rotation around your center of mass.
First, getting from place to place. If I want to get from one end of the laboratory module to the other, all I have to do is push off from the wall to get my center of mass moving, fly across the lab, and stop myself on the other side. Easy! But remember that since you fly in a straight line, you can't make midcourse corrections unless you grab onto something along the way, which is fine but you lose style points for that. The next thing to think about is how hard you push off. If you push too hard, you end up going really fast, and the next thing you know you are crashing into something on the other wall. Again, with nothing to slow you down in the middle of the module you are kind of helpless until you hit the far wall. It turns out that you don't need to push off from the wall as hard as you might think. On the ground, it takes a lot of work to move around because you are constantly fighting the force of gravity trying to make you fall to the floor. Up here, a push of maybe a few pounds is about right to fly across the module at a comfortable speed. It is easy to fall to the temptation of really flying fast, but you have to be careful to not knock your head on the many hatches and bulkheads here. Every bit of momentum you put into moving your center of mass (i.e., flying somewhere) has to be taken out at the other end when you stop, so flying slowly takes less energy.
In reality, there are lots of handrails and other objects to grab onto so you can actually just move your way slowly along them making continuous adjustments to your speed and direction. Most of the time your initial push is enough to give you enough speed to get where you are going. The handrails along the way end up being used to make those midcourse corrections to your trajectory and to control which way you are facing. If you get your initial push-off about right then you shouldn't need to push or pull very hard on any of the handrails from there to your destination (unless of course you have to turn a corner). I think the closest thing on the ground to this is swimming, but it is very different since in the water if you don't keep pushing yourself along you stop pretty quickly.
The next problem when flying is to keep yourself pointed the way you want and to control your rotation. If the force you impart from a push-off point is directed through your center of mass you will not spin yourself up. But if you don't push through that point, you end up making your body rotate around your center of mass which can mean doing flips or rolls across the room. So if it turns out that a straight line from your push-off point through your center of mass is exactly where you want to fly, then great - all is well and you will fly there without rotating. If not, think of rowing a boat with one paddle - it is hard to both go straight and keep the boat pointed where you want. In space what you have to do is apply a torque to any handrail you push off by twisting it as you push. This counteracts the twisting moment you get from simply pushing off the handrail. This is the equivalent, for you pilots, of flying a twin-engine airplane with one engine out and having to kick in a whole lot of rudder. Of course by using two hands on separate handrails you can make this much easier because you can apply a lot bigger torque than you can by twisting one handrail with a single hand.
Which gets me back to chasing Sox down to the node with all that equipment in the one hand. I had the equipment in my left hand and was holding it close to my chest, and my right hand was outstretched in front of me grabbing onto and pushing off of handrails. The problem with that is that with the big lever arm (distance between my right hand and my belt buckle), you have to torque the handrail really hard to control your motion. Which is why I had problems the harder I pulled to try to catch up to Sox. One way to minimize this problem I've found is to keep your pushing hand closer to your waist (this makes sense since it is then closer to your center of mass).
I've since been experimenting with different ways to fly around while carrying equipment. If the object is big and bulky, the easiest thing to do is to simply let it fly on its own. Push it in the direction you want to go, and follow along with it while not actually carrying it, giving it a nudge here and there to keep it on track. Another way to carry something like a bag is to simply hold it in your legs while you use both hands to fly around. Yuri and I have decided that a monkey with a prehensile tail would do very well up here since it can grasp handrails with not only his hands, but with his feet and tail!
Lately, I've been trying to perfect my technique flying across the node. When flying from the FGB there is a handrail right at the entrance to the node. If you grab that handrail for a split second as you pass and immediately tuck, then you will do somersaults across the node into the lab. If you time it just right you can grab a handrail right at the entrance to the lab to stop your rotation. If you time it wrong, you crash into the wall or go careening into the lab. Single flips are pretty easy now, but when doing double flips I still end up hitting something about half the time. I guess I need more practice.
In the end, you don't even think much about the mechanics of flying any more than you do about the mechanics of walking on the ground. It is pretty intuitive, so you don't need to be a physicist to figure it all out. But I am still having fun thinking about the physics of flying!
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