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Submitted by Arthur - N1ORC

Expedition Seven Letter #2


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

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

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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