| Ed
Lu | 
NASA
ISS Science Officer Ed Lu floats in the International
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Expedition
7
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Flying
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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