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congratulations

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i got a question here from brian watt

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from phoenix arizona says

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greg why does the space station have to

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be in a specific orientation in order to

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dock the shuttle or resupply spacecrafts

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this is a really good question and not

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easy to explain uh quickly but i'll

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generally do my best

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basically what's happening is you know

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the orbital mechanics that's going on

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when you fire thrusters with a

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spacecraft uh have kind of a strange

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effect when you look at what the what

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the motion is between two spacecraft

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that are near each other so for example

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if if two spacecraft are right behind

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each other in in the same orbit

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and the one behind fires thrusters to

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move forward

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what's going to happen really is it's

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that spacecraft's changing its orbit and

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the new orbit

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is going to be an

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ellipse with a higher apogee the

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furthest point away halfway around the

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orbit uh from the time of the the

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thruster firing the orbit is going to be

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a little bit higher

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and

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so what's going to happen is

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45 minutes later basically at this

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altitude

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what's going to happen is that

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spacecraft would move up rather than

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forward towards the spacecraft it's

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trying to catch up with

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and also since that orbit now is a

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little bit bigger

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it takes longer to go around and

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actually it'll fall behind so thrusting

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forward results in

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floating up and falling behind which is

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very counter-intuitive and a lot of the

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orbital mechanic

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effects are kind of like that now those

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are things that happen in the plane of

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the orbit but out of the plane it's

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something completely different if you if

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you fire a thruster to move the

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spacecraft out of the plane of the orbit

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what happens is it kind of creates an

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angle between

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the two orbits one of the first

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spacecraft one of the other and you know

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two orbits that are at an angle with

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each other they cross each other twice

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uh one you know per orbit

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so what actually would happen is the

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spacecraft would move out of the orbit

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to the side and then half an orbit later

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come back and cross over again go to the

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other side and then half an hour but

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later come back and across again so

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these are kind of weird things to take

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into account when you're trying to

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manually fly a rendezvous or you can

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automatically fly around

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the in-plane motions in the auto plane

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are are different from you different

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effects from each other

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but there's a couple of places in

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that you can be relative to other

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spacecraft where you can stay for a

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while and have it not cost you fuel for

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example if you if you're below another

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spacecraft and you're trying to

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rendezvous with it but you want to stay

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there for a while if you're below in

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other words closer to the earth

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you're in a different orbit you're at a

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different altitude you're in a different

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orbit and that orbit you know takes

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different amount of time to go around

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and and um and you have to apply

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constant uh thruster firings not

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constant but you have to keep firing

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thrusters and keep yourself there so

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that costs you energy

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one place for sure you can stay relative

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to another spacecraft and not have it

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cost you any energy to stay there is

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right in front or right behind in

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exactly the same orbit

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and so these are really good places to

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approach from

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if something goes wrong when the shuttle

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approaches if something goes wrong or

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you want to check that everything is

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right right before you you know finally

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do your final maneuver to come in you

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can have time to stay in that place for

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a while and not have it cost you a lot

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of fuel to stay there so that's one of

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the main reasons the shuttle approaches

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from that side theoretically you could

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approach from any angle

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and for other reasons it's helpful you

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know if you can stay in the plane in the

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orbit plane

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you minimize you know coupling between

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axes other factors have to do with

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firing thrusters that might impact

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parts of the spacecraft you know for the

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space station with solar rays we don't

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want to impact the arrays with um with

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uh the thrust exhaust from the plumes

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from the thrusters so a lot of things

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combined but

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essentially it helps to keep things in