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Japan's zero-gravity space drone sends first pictures from ISS

bbc.com

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Re: Japan's zero-gravity space drone sends first pictures from ISS

#11

Post by venning »

lordvon wrote: post #10
venning wrote: post #9

Earlier quoted context omitted.

From a related video: the reaction wheels spin up while keeping total angular momentum at zero, then apply a rapid (magnetic) brake to transfer the momentum to the body (from the wheels). I'm assuming if you do that precisely enough (and in three dimensions), you can generate linear movement. EDIT: I'm probably very wrong about the movement. EDIT: The video: https://youtu.be/n_6p-1J551Y

how does conservation of linear momentum apply? edit: this question was asked when the parent comment claimed that this machine could "generate linear momentum" from the wheels.

Imagine holding up a bicycle in the air from the midpoint. Now spin both wheels in opposite directions. No movement. Now apply one brake. Now you have movement. Now do that in three dimensions with precision and you have this thing.

Conservation of momentum applies when there are "no external forces", which wouldn't be the case here when you apply the brake.

EDIT: Yeah, my apologies. I didn't mean to write "momentum".

Re: Japan's zero-gravity space drone sends first pictures from ISS

#12

Post by [deleted] »

lordvon wrote: post #10
venning wrote: post #9

Earlier quoted context omitted.

From a related video: the reaction wheels spin up while keeping total angular momentum at zero, then apply a rapid (magnetic) brake to transfer the momentum to the body (from the wheels). I'm assuming if you do that precisely enough (and in three dimensions), you can generate linear movement. EDIT: I'm probably very wrong about the movement. EDIT: The video: https://youtu.be/n_6p-1J551Y

how does conservation of linear momentum apply? edit: this question was asked when the parent comment claimed that this machine could "generate linear momentum" from the wheels.

[deleted]

Re: Japan's zero-gravity space drone sends first pictures from ISS

#15

Post by averagewall »

venning wrote: post #11
lordvon wrote: post #10

Earlier quoted context omitted.

how does conservation of linear momentum apply? edit: this question was asked when the parent comment claimed that this machine could "generate linear momentum" from the wheels.

Imagine holding up a bicycle in the air from the midpoint. Now spin both wheels in opposite directions. No movement. Now apply one brake. Now you have movement. Now do that in three dimensions with precision and you have this thing. Conservation of momentum applies when there are "no external forces", which wouldn't be the case here when you apply the brake. EDIT: Yeah, my apologies. I didn't mean to write "momentum"…

The brake is an internal force though (the force and its reaction both act on the bicycle). The whole unit's only external forces are from the fans so that's all it can use to translate. That and bumping into things.

Re: Japan's zero-gravity space drone sends first pictures from ISS

#16

Post by TeMPOraL »

averagewall wrote: post #15
venning wrote: post #11

Earlier quoted context omitted.

Imagine holding up a bicycle in the air from the midpoint. Now spin both wheels in opposite directions. No movement. Now apply one brake. Now you have movement. Now do that in three dimensions with precision and you have this thing. Conservation of momentum applies when there are "no external forces", which wouldn't be the case here when you apply the brake. EDIT: Yeah, my apologies. I didn't mean to write "momentum"…

The brake is an internal force though (the force and its reaction both act on the bicycle). The whole unit's only external forces are from the fans so that's all it can use to translate. That and bumping into things.

> That and bumping into things.

Come to think of it - in air you're constantly bumping into things - air molecules. I think it could technically be possible to steer in zero-g within an atmosphere by pure rotation (e.g. consider that a fast-spinning object is kind of a (crappy) fan).

Re: Japan's zero-gravity space drone sends first pictures from ISS

#17

Post by lordvon »

venning wrote: post #11
lordvon wrote: post #10

Earlier quoted context omitted.

how does conservation of linear momentum apply? edit: this question was asked when the parent comment claimed that this machine could "generate linear momentum" from the wheels.

Imagine holding up a bicycle in the air from the midpoint. Now spin both wheels in opposite directions. No movement. Now apply one brake. Now you have movement. Now do that in three dimensions with precision and you have this thing. Conservation of momentum applies when there are "no external forces", which wouldn't be the case here when you apply the brake. EDIT: Yeah, my apologies. I didn't mean to write "momentum"…

though less technical, "movement" implies "momentum", no?

In the person + bike system, the brake action would result in internal forces. I don't see how this system could translate.

Re: Japan's zero-gravity space drone sends first pictures from ISS

#19

Post by ISL »

venning wrote: post #9
TeMPOraL wrote: post #4

Earlier quoted context omitted.

It seems to be rotating via reaction wheels (via this[0]). I haven't yet figured out how it translates, though. EDIT: if you have gravity available, the video on that site illustrates a clever way to use those reaction wheels to move around by tumbling. EDIT2: "for maneuvering around in space there are twelve electric micro-fans or μFans"[1]. The SE thread has a screenshot of a video which shows the placement of thos…

From a related video: the reaction wheels spin up while keeping total angular momentum at zero, then apply a rapid (magnetic) brake to transfer the momentum to the body (from the wheels). I'm assuming if you do that precisely enough (and in three dimensions), you can generate linear movement. EDIT: I'm probably very wrong about the movement. EDIT: The video: https://youtu.be/n_6p-1J551Y

The system in the (cool) video is in contact with a table.

The table exerts a "normal" force on the cube, allowing a change in vertical momentum. If the system in question is the cube alone, this is an external force.

This method could be used if the drone were cubic and near a wall of the space-station, by kicking off of the wall. That could get it moving, but until it hit the opposite wall, there's nothing it could do to stop.

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