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Old 06-12-2019, 02:19 PM   #21
Ulzgoroth
 
Join Date: Jul 2008
Default Re: Approaching TL9?

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Originally Posted by TGLS View Post
I hate to break this to you, but the Chemical Rockets presented in GURPS Spaceships are almost implausibly efficient compared to chemical rockets we have today. Even assuming that Fuel Tanks have a negligible dry mass, the specific impulse ends up around 468.38 seconds, slightly more efficient than modern upper stages. Adding in an adjustment for fuel tank structure makes them even more efficient. On top of this, rocket engines designed to operate from launch aren't even close to this level of efficiency because of higher thrust demands and pressure differences.
Fuel tanks are clearly written to have zero dry mass in the rules, and I'm getting a value within easy rounding of 480 seconds (based on the one-tank performance).
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Old 06-12-2019, 04:44 PM   #22
AlexanderHowl
 
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I can guarantee you that the each stage is not getting an ISP of 480. A delta-v increase of 2.52 mps (~4.04 km/s) is an ISP of 412, which is well within LOX/LH2 design parameters. The first stage of the Saturn V used RP-1/LOX, so it had only an ISP of 263, but the upper stages used LOX/LH2, so they had an ISP of 421.

Last edited by AlexanderHowl; 06-12-2019 at 04:49 PM.
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Old 06-12-2019, 05:12 PM   #23
Ulzgoroth
 
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I can guarantee you that the each stage is not getting an ISP of 480. A delta-v increase of 2.52 mps (~4.04 km/s) is an ISP of 412, which is well within LOX/LH2 design parameters. The first stage of the Saturn V used RP-1/LOX, so it had only an ISP of 263, but the upper stages used LOX/LH2, so they had an ISP of 421.
You can guarantee anything you want to, but the stats you gave with stages getting 3.12 mps off of a 65% fuel mass imply an Isp of 488 seconds.
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Old 06-12-2019, 05:47 PM   #24
Anthony
 
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Default Re: Approaching TL9?

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I can guarantee you that the each stage is not getting an ISP of 480.
You have a figure of 13 fuel tanks giving 3.12 mps or 5.02 km/s. Delta-V is equal to ln( full/empty ) * ISp, ln( 1/0.35 ) = 1.05, so ISp = 4780m/s or 487s.

The RS-25 (SSME), which is still a quite respectable engine operating near theoretical limits for its fuel, achieves 452s vacuum, 366s at sea level. That corresponds to 0.138 mps per tank, and you should probably set aside one module per stage (call it armor) to account for structure, tank mass, and incidental weight associated with staging, which will wind up reducing your delta-V from 3.12 mps to 2.53mps.
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Last edited by Anthony; 06-12-2019 at 05:57 PM.
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Old 06-12-2019, 05:47 PM   #25
AlexanderHowl
 
Join Date: Feb 2016
Default Re: Approaching TL9?

If you notice, I mention that 2.52 mps were realistic for LOX/LH2 rockets. The 3.12 mps design would require a LOX/LF/LH rockets or something similar, which is still realistic.
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Old 06-12-2019, 05:58 PM   #26
Anthony
 
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If you notice, I mention that 2.52 mps were realistic for LOX/LH2 rockets. The 3.12 mps design would require a LOX/LF/LH rockets or something similar, which is still realistic.
No, it really isn't. There's a reason no-one actually uses those fuels.
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Old 06-12-2019, 06:30 PM   #27
AlexanderHowl
 
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They do not use it right now because we do not have the technology to support it because we are not that great when it comes to space, not because it is impossible. Anyway, there have been experimental LOX/LH2 designs (the old RL10B-X) that have gotten an ISP of 470 (back in the 1960s), so I am not sure that an ISP of 486 is impossible if the technology was improved upon (the maximum possible for LOX/LH2 is 528 at 100% efficiency).
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Old 06-12-2019, 06:34 PM   #28
Anthony
 
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They do not use it right now because we do not have the technology to support it because we are not that great when it comes to space, not because it is impossible.
They don't use it because it's crazy toxic and corrosive.
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Old 06-12-2019, 06:45 PM   #29
maximara
 
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Default Re: Approaching TL9?

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First of all, not everything appears at day 1 of a new TL. Second, well, no-one gets the future right. Tech has certainly changed substantially since 2006 and will change more in the next few years, but it's never in quite the ways people thought it would change.
Regarding the first point, he Basic set says as much: "The equipment listed for a TL does not appear all at once as soon as a society enters that TL. Instead, items appear gradually after that date."

I should mention that even within a TL there is a lot of change. For example, the Wright Flyer, Fokker Dr.I, and a B29 are all TL6 aircraft but the type of TL6 that went into them is vastly different.

As for the second one only has to look at the TL7+ future envisioned during TL5-6. For the most part as Futurerama of the Worlds Fair shows the level of progress tends to be envisioned to far higher then it turns off or the optimism causes thing that logically don't make sense to be proposed.

Also there can be a lot of TLx-2 stuff around. For example much of the sewer water systems in TL8 cities is from the 19th century (TL6). The powergrid has TL8 computers controlling TL7 nuclear/solar and TL6 conventional power plants delivering power over a largely TL6 power transfer system.

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Old 06-12-2019, 07:28 PM   #30
Fred Brackin
 
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For example, the Wright Flyer, Fokker Dr.I, and a B29 are all TL6 aircraft .
In 4e, the B-17 would be late TL6 but the B-29 is early TL7. I do not necessarily think 1940 is a great place to draw the line but that's where 4e puts it.
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