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Post by Sojourner on Jul 14, 2004 9:41:17 GMT -5
On the other hand, you have a force-multiplier effect when the thruster is further from the centre of rotation...
I don't know. We'd have to test it.
Given the rarity of Kroot Warspheres on 21st-century Earth, this may be problematic.
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Post by Kage2020 on Jul 14, 2004 11:38:44 GMT -5
Reguarding volumes... you have been reading my design thread posts with the stupidly long calculations, havn't you? You're talking about the Retribution? I'm amused that CELS came up with a totally different length to you... somewhere around the 2.5km mark rather than the 4.6km one that you advocated (which incidentally comes out to 510M v-displacement tons, which is ridiculously large! )...
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Post by CELS on Jul 14, 2004 12:10:14 GMT -5
Right.. there seems to have been a misunderstanding here. When working up size estimates of various ship types, based on BFG models, I worked with the assumption that Cruisers were 1,4 km long, which I believed Minister had told me.
Apparently, his original post was this; We do have some factoids for the Lunar and it's fellow cruisers (these are the ones about 3km). The battleships are about 5km.
For a cruiser (without prow ram): Length: 3,214m Beam: 1,402m Height (excluding comms array): 1,383 these are in rough proportion, by the way. For this I took a shot-in-the-dark guesstimate of Mass (unladen): 474 million tons
So, working from a roughly 3km length for the cruiser and assuming the picture posted is to scale, we have a length of about 4.6km without the ram for the Retribution.
Now, before I start reconsidering my estimates... where do these "factoids" come from? Are they from Black Library fiction, or from a Fanatic article by one of the big guys themselves (Andy Chambers, Gav Thorpe, Jervis Johnson)
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Post by ErnestBorgnine on Jul 14, 2004 12:12:30 GMT -5
For what its worth, taking a 2E6 m3 volume cylindrical ship that is 5 times as long as it is high, we get a width of 50m and a length of 250 meters.
For the same volume, a spherical ship would have a radius of 78m, or a total diameter of 156m.
For equal masses, the torque from the thrusters on the cylindrical ship is greater for equal force applied, as you noted. However, this relationship is linear - the torque of the cylinder is 125/78 greater for equal forces due to its distance from the center of mass. However, the moment of inertia grows as the square of the radius (sphere) or length (cylinder), so the increase in length more than offsets this. The sphere's moment of inertia is 1/2.14 that of the cylinder. Given that these numbers are in an inverse relationship to angular acceleration, the torque advantage doesn't outweigh the momentum penalty.
I believe this will hold up for arbitrary spheres and cylinders of equal volumes, but physics 1055 was so long ago, I could be wrong . Anyhow, I believe based on the above (and common sense real world experience) that a spherical ship should always be quicker and more responsive on the helm than a cylindrical ship unless you put in a much more powerful maneuvering system.
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Post by CELS on Jul 14, 2004 12:16:37 GMT -5
Alright, I only really understood the last sentence of that post, but... are you suggesting that the ideal shape of a space ship is that of a ball?
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Post by ErnestBorgnine on Jul 14, 2004 12:26:40 GMT -5
In terms of turning speed (and hence maneuverability), yes. A sphere has the smallest moment of inertia of any shape. In terms of other factors not necessarily. In a cargo ship in particular, if you're going to be placing square/rectangular containers in it, the sphere really cuts into available volume more than a boxier shape. If you haven't already seen it, www.projectrho.com/rocket/rocket3f.html and the other pages of that site.
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Post by Minister on Dec 2, 2004 18:42:48 GMT -5
The length, IIRC, was from Execution Hour, which is one of the best 40K novels as far as fitting in with things goes.
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Post by Kage2020 on Dec 2, 2004 22:32:00 GMT -5
The shape of ships is one of those things that, I'm afraid, must be accepted. Same with the size...
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Post by Minister on Dec 3, 2004 7:30:17 GMT -5
I maintain that, for a science-fantasy setting, the size is realistic. We're not talking Death Stars here, people...
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