# Rolling a spaceship along its horizontal axis

by dbaezner
Tags: axis, horizontal, rolling, spaceship
 P: 3 This is probably a silly question, but I want to make sure I get it right for a story I'm writing. I want to roll an elongated spaceship "around" it's horizontal axis (as opposed to flipping it end over end). Is it equivalent to say "along" the horizontal axis? In this case, I'm assuming that the horizontal axis is the one that is parallel to the length of the ship. I hope I got at least that part right. Thanks. Dirk
 Emeritus Sci Advisor PF Gold P: 4,500 Let's use this picture as an example http://www.google.com/imgres?imgurl=...QEwAw&dur=2398 If you said you were rolling that car along the 'a' axis, then I would think you are pushing it forward. If you said you are rolling it around the a-axis, I would say you are rolling the car onto its roof, then back onto its wheels, then onto its roof again etc. the shorter way (such that the doors are occasionally touching the ground). I don't think the word "horizontal axis" has a technical definition to the point where you have to be worried. In the sense that horizontal is perpendicular to vertical, both the 'a' and 'b' axes would be acceptable as "horizontal axis" though I think most people would think of the 'a' axis before the 'b' one
 P: 3 Got it. Thanks.
HW Helper
P: 2,280
Rolling a spaceship along its horizontal axis

Roll, Pitch, and Yaw are the terms usually used for both aircraft and spacecraft.

The roll axis would be parallel to the length of the ship as most commonly depicted in movies. (This is a little anachronistic in an environment with little to no atmospheric drag, but it makes it easier for the viewers to figure out what they're looking at.)

Unless your story is a little more on the technical side and you're talking about some difficulty stemming from spinning about your minor axis instead of the major axis.

 To the surprise of mission experts, Explorer 1 changed rotation axis after launch. The elongated body of the spacecraft had been designed to spin about its long (least-inertia) axis but refused to do so, and instead started precessing due to energy dissipation from flexible structural elements. Later it was understood that on general grounds, the body ends up in the spin state that minimizes the kinetic rotational energy for a fixed angular momentum (this being the maximal-inertia axis). This motivated the first further development of the Eulerian theory of rigid body dynamics after nearly 200 years—to address this kind of momentum-preserving energy dissipation.
This is why college graduates should design spacecraft. They tend to spin their class rings a lot and would surely wonder why the jeweled part almost always winds up pointing up as the ring spins.

Or why little kids should design spacecraft. Surely they've spun at least a few of those plastic Easter eggs.

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