Canadarm Question

Comments and questions about the APOD on the main view screen.
ExtraTrstl
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Canadarm Question

Post by ExtraTrstl » Mon May 22, 2006 4:47 pm

If things are weightless in space, how can the Canadarm have a weight limit? Wouldn't moving a person be just as easy/difficult as moving an entire space shuttle?
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BMAONE23
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Post by BMAONE23 » Mon May 22, 2006 4:56 pm

It might not be a weight limit as in lift capacity but a possible structural limit for joint capacity or even the limits of motors within the arm to start and stop. A kinetic energy limit.

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Qev
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Post by Qev » Mon May 22, 2006 5:53 pm

Probably a mass or force limit would be a better term for it. Things in orbit have no weight, but their mass and thus, their inertia, remain unchanged. Trying to move too massive an object would apply forces to the arm that it's not designed to withstand.
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Pete
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Post by Pete » Mon May 22, 2006 8:34 pm

Endeavour, the heaviest of the three currently operational space shuttles, masses about 78 metric tons without fuel or payload. The highest gross launch weight I found was 145 tons, which is pretty close to the 116-ton mass limit mentioned in the APOD. Couldn't the Canadarm2 simply move the entire shuttle around any object, no matter how massive the object is? :)

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Post by l3p3r » Tue May 23, 2006 2:58 am

Couldn't the Canadarm2 simply move the entire shuttle around any object, no matter how massive the object is?
thats a good point! if they take it real slow and careful... I don't see why not

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BMAONE23
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Post by BMAONE23 » Tue May 23, 2006 2:01 pm

I would imagine that without the use of maneuvering or stationing thrusters during loaded operation of the arm, the mass of shuttle and sattelite would rotate around what might be considered to be their combined center of mass.

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Post by Rincewind » Wed May 24, 2006 4:04 pm

Even if using maneuvering thrusters to keep one object stationary while the other is moved, the net force on the arm is the same. So that is not the limmiting factor.

The limit is in how slow. They are using electric motors which work by lots of little magnetic shoves. Electric motors don't like running a varible speeds, there is only a narrow range of speed for the motor that will work. Below a certain momentum in the drive shaft of the motor, and there will not be enough power to kick round to the next shove point. Also if the motor is running slowly, then it is jerky. Not smooth. To move big things slowly you need smooth.

To get round the speed and jerkyness, the motors are run quite quickly, then geared down to the required speed. To change speed you have to change gear. These are mechanical gears. So they are heavy. So only only take a limited range of gears into space with you. Limited gears = limited speed range = limited momentum range.

They could probably change the load capacity by simply changing the gears without altering the structure of the arm. But since the Canadarm only has to deal with the shuttles own payload, why build it to handle larger.

The Canadarm2 on the space station has enough capacity to move the shuttle about. Indeed could move two shuttles about. This version 2 arm can have more motors and better gearing because it's in space for a long time and is not coming back and forth so it not so expensive to launch. But there is still going to be some limiting factor on how gently the arm can move things. So there will always be a mass limit.

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Post by l3p3r » Thu May 25, 2006 1:01 pm

Would a hydraulic arm have the same problem? for that matter, would a hydraulic arm work in space?

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Post by nomystics » Tue May 30, 2006 4:31 am

l3p3r wrote:Would a hydraulic arm have the same problem? for that matter, would a hydraulic arm work in space?
No, I don't think it would. First you need a massive amount of power to take advantage of Pascal's law, plus it would be prohibitively heavy. Secondly, at -250C or whatever, the hydraulic oil would probably be a solid unless heated throughout the system somehow. My car wouldn't even turn over at -45C. :D