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02-27-2020, 03:02 AM | #1 |
Join Date: Nov 2018
Location: Live in Seoul, Korea and I have never been abroad.
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[Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.
When I played Kerbal Space Program, only 1.2G-1.6G was sufficient to escape Kerbin. And I know that TWR of real-world Saturn V is 1.15G.
But at GURPS Spaceships p21, it says that TL7 Chemical Rocket provides 3G accelerations. It seems that it is an astounding overspec. It is greater than real-world TL7 Chemical Rockets, and it is greater than even TL9 HDEM Chemical Rocket (it provides 2G). Last edited by Pectus Solentis; 02-27-2020 at 03:06 AM. |
02-27-2020, 03:56 AM | #2 | |
Join Date: Aug 2004
Location: Wellington, NZ
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Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.
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Also, the F-1 rocket used in the first stage has a thrust/weight ratio of 94-to-1 on its own, so Spaceships is actually being conservative - an F-1 weighing 8.4 tons in a ship weighing 168 tons (twenty times as much) would have an initial acceleration of 4.7G. Likewise, the 2nd stage J-2 had a 73-to-1 thrust/weight ratio. Russian launch rockets have thrust/weight ratio of anywhere from 75:1 up to at least 137:1. I'm not sure how generous (or not) Spaceships is with delta-vee, but it's conservative with rocket engine accelerations. As for the TL9 HDEM rocket - it trades raw thrust for efficiency. I doubt anyone would think it very unreasonable for you to decide that chemical and HDEM rockets can use the High Thrust option available to most other reaction engines (x2 acceleration, x1/2 delta-vee per tank). Now, if you want numbers that don't make much sense, consider jet engines. Being generous a modern jet manages a thrust/weight ratio of 5:1, or 8:1 with afterburning, which should give 0.25G per system (0.4G per system with afterburner). Also, they should burn an entire system of fuel per quarter hour of normal thrust and every 7.5 minutes on afterburner.
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02-27-2020, 04:03 AM | #3 |
Join Date: Jul 2008
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Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.
Just to make it explicit, that means that if you want a chemical rocket with less thrust, the natural approach is simply to use a smaller system as described in Spaceships 7.
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I don't know any 3e, so there is no chance that I am talking about 3e rules by accident. |
02-27-2020, 04:13 AM | #4 | |
Join Date: Nov 2018
Location: Live in Seoul, Korea and I have never been abroad.
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Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.
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02-27-2020, 04:18 AM | #5 |
Join Date: Jul 2008
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Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.
It is an initial acceleration, and also a final acceleration, because while Spaceships doesn't entirely ignore the way decreasing rocket mass in flight affects delta-V, it does entirely ignore the way it affects acceleration.
__________________
I don't know any 3e, so there is no chance that I am talking about 3e rules by accident. |
02-27-2020, 04:21 AM | #6 |
Join Date: Nov 2018
Location: Live in Seoul, Korea and I have never been abroad.
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Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.
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02-27-2020, 04:32 AM | #7 |
Join Date: Nov 2015
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Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.
Could you calculate the acceleration every time you empty a fuel tank for example?
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02-27-2020, 04:40 AM | #8 | |
Join Date: Aug 2004
Location: Wellington, NZ
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Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.
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At some point you may as well just do all the calculations from scratch, and use real rocket science. (But not me - that's too much effort given the minimal return I'd get.)
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Rupert Boleyn "A pessimist is an optimist with a sense of history." |
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02-27-2020, 04:41 AM | #9 |
Join Date: Nov 2018
Location: Live in Seoul, Korea and I have never been abroad.
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Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.
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02-27-2020, 05:10 AM | #10 | |
Join Date: May 2005
Location: Oz
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Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.
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A Saturn V all-up launch weight was 3,270 US tons. Its first-stage engines, five Rocketdyne F-1, totalled 46.25 US tons mass. That's one seventieth of the vehicle mass (1.4%), not one-twentieth of vehicle mass (5%). In Spaceships' terms a Saturn V had less than one third of a system of reaction engine, chemical rocket (TL7). Build it as a smaller system using the rules in Spaceships 7 on page 4 — then you will be complaining that the TL7 chemical rocket is actually slightly under-spec¹ and that a Saturn V cannot take off until it has burned part of its fuel and if you write house rules for the changing acceleration as ship mass diminishes. ___________ ¹ TL7 chemical rockets in Spaceships have a thrust-to-weight ratio of 60 g₀, Rocketdyne F-1 engines had a thrust-to-weight ratio of 84 g₀.
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