12-05-2017, 12:55 PM | #21 | |
Join Date: Aug 2008
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Re: [Blog] n-Body Politics
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1) Eric and I are addressing cost efficiency, not dv efficiency - which itself doesn't necessarily translate into mass efficiency or cost efficiency. And what will ultimately determine which engines get used in some hypothetical futurist setting is the combination of cost and demand. 2) Hohhman, bi-elliptic (that ration is 11:1, btw), and other high-thrust transfers are the least time-efficient transfers available. This makes them highly undesirable for most applications of space flight, both because no one wants to sit on a spaceship for a decade while they drift toward Saturn and because no company wants to pay a crew a decade of wages to drift toward Saturn once. 3) The fastest transfers available use constant thrust to attain high velocities over the course of the transfer. This necessarily increases dv, which in turn, increases the fuel costs. It, however, saves time, and thus allows more trips (meaning more revenue) and fewer paid crew hours (meaning lower expenses). This is desirable when maximizing profit, and thus, monetary efficiency. 4) When dealing with brachistochrone transfers, the higher the craft's acceleration, the higher the maximum velocity attained, and thus the more dv required to execute it. Thus, a fusion torch can pull 0.5G, it would need approximately 800 mps - meaning 13 fuel tank systems - to transfer from Earth to Mars, and you would have to refuel there. But, even a regular fusion rocket could pull 0.005G, need only 100 mps and make the trip there and back again on less than 2 fuel tank systems. They use the same type of fuel, so it's pretty plain to see that a fusion rocket is more cost efficient than a fusion torch. Or in other words, the lower-thrust drive is more cost efficient than the higher-thrust drive. 5) High-thrust rocket engines have existed since 1926. They had a high-thrust system available for the SMART-1 mission and chose to use a high-efficiency, low-thrust ion engine instead. It had a higher dv because it went faster, but it had a far lower fuel-mass percentage (3.33%) specifically because the ion engine was more efficient.
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12-05-2017, 01:29 PM | #22 | ||
Join Date: Jul 2013
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Re: [Blog] n-Body Politics
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Specifically, ignoring orbits, your goal is to move to a target at 1 AU. Your available engines are a fusion rocket (0.005G, v_exh 10290km/s or 350mps/525km/s dV per tank) or a hydrogen-fuelled fusion torch (0.5G, v_exh 1323km/s or 45mps/67.5km/s dV per tank). I'm also ignoring acceleration changes due to fuel expenditure. A brachistochrone transfer with the fusion rocket will take you just over 40 days (20 each spent accelerating and decelerating). In total, you spend about 175km/s dV for this (1/3rd of a tank). A brachistochrone transfer with the fusion torch will take 4 days, spending an astronomical ~1750km/s of dV (surprisingly, that's just ~15 tanks). I believe that's where your numbers came from. However, you can also use the fusion torch to accelerate to 87.5km/s, coast for some time, then decelerate. Total dV spent is 175km/s too (~2.5 tanks), but you spend slightly less than five hours each to accelerate/decelerate, and ten days coasting. In total, you spent the same dV, but cut your transfer time by 75%. In fact, you can achieve the same transit time as with the fusion rocket's constant acceleration by spending just 45km/s dV (2/3rds of a tank)! Quote:
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12-11-2017, 10:06 AM | #23 | |
Join Date: Jul 2013
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Re: [Blog] n-Body Politics
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12-21-2017, 07:32 AM | #24 | |
Join Date: Jul 2013
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Re: [Blog] n-Body Politics
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12-21-2017, 07:41 AM | #25 | |
Night Watchman
Join Date: Oct 2010
Location: Cambridge, UK
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Re: [Blog] n-Body Politics
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12-21-2017, 09:17 AM | #26 | |
GURPS FAQ Keeper
Join Date: Mar 2006
Location: Kyïv, Ukraine
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Re: [Blog] n-Body Politics
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12-21-2017, 01:53 PM | #27 | |||
Join Date: Jul 2013
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Re: [Blog] n-Body Politics
I'll answer to the relevant points here.
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12-21-2017, 03:08 PM | #28 | ||
Hero of Democracy
Join Date: Mar 2012
Location: far from the ocean
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Re: [Blog] n-Body Politics
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Of course now when I look at costs I realize that engines are among the most expensive things in the book, and two systems, the engine and the stardrive, cost around 70% of the ship already. Quote:
The equations got really complex and than all cancelled out. It was stunning. dv where a planet's influence is g and the mass is M is dv = (GM)^.25 * g^.5 * .001 Where G is the gravitational constant.
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12-21-2017, 04:09 PM | #29 |
Join Date: May 2009
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Re: [Blog] n-Body Politics
When I clicked on the continue reading link, it said "significant costs of surging". What's surging?
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12-22-2017, 06:30 AM | #30 | |||
Join Date: Jul 2013
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Re: [Blog] n-Body Politics
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This hopefully makes it clearer. |
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