Steve Jackson Games - Site Navigation
Home General Info Follow Us Search Illuminator Store Forums What's New Other Games Ogre GURPS Munchkin Our Games: Home

Go Back   Steve Jackson Games Forums > Roleplaying > GURPS

Reply
 
Thread Tools Display Modes
Old 09-17-2020, 11:54 AM   #11
Fred Brackin
 
Join Date: Aug 2007
Default Re: [Spaceships] Martian “Gas” Station

Quote:
Originally Posted by TGLS View Post
Low efficiency rockets could plausibly "island hop" from space rock to space rock, as adding more propellant to a rocket has declining gains.
Generally not. Even if you can net gain a little more Delta-V from decelrating, refuelling and then accelerating again it won't be worth the time and/or the wear and tear.

Hard science ships can't just "stop". They have to spend Delta-V. So the efficient thing is usually to accelerate once and then decelerate once while you coast at maximum speed in between.
__________________
Fred Brackin
Fred Brackin is offline   Reply With Quote
Old 09-17-2020, 01:54 PM   #12
TGLS
 
Join Date: Jan 2014
Default Re: [Spaceships] Martian “Gas” Station

Quote:
Originally Posted by Fred Brackin View Post
Generally not. Even if you can net gain a little more Delta-V from decelrating, refuelling and then accelerating again it won't be worth the time and/or the wear and tear.
Well, it depends on how you set up the case. For example, say we have rocket with a dry mass of 1 ton and ISP of 1 km/s. It can travel from A to B for a ΔV of 4 km/s, or from A to M for 3 km/s then M to B for another 3 km/s. While the A to B via M route appears less efficient (taking 2 more km/s), if you can refill propellant tanks at M, it would save 15 tons of propellant compared to the A-B route.
TGLS is online now   Reply With Quote
Old 09-17-2020, 02:28 PM   #13
AlexanderHowl
 
Join Date: Feb 2016
Default Re: [Spaceships] Martian “Gas” Station

An exhausted velocity of 1 km/s is a weak drive. Assuming that the drive tosses out 3 kg/s/metric ton of mass, that would only allow an object with a dry mass of 1 metric ton and a wet mass of 2 metric tons an acceleration of 0.1 g for 1000 seconds (a delta-v of only 980 m/s).
AlexanderHowl is offline   Reply With Quote
Old 09-17-2020, 02:50 PM   #14
Fred Brackin
 
Join Date: Aug 2007
Default Re: [Spaceships] Martian “Gas” Station

Quote:
Originally Posted by TGLS View Post
Well, it depends on how you set up the case. For example, say we have rocket with a dry mass of 1 ton and ISP of 1 km/s. It can travel from A to B for a ΔV of 4 km/s, or from A to M for 3 km/s then M to B for another 3 km/s..
If A to B is 20 million kilometers and you can get there on 4 m/s of Delta-V then you are accelerating to 2 km/s then drifting for 5 million seconds and then decelerating for a total trip time of c.58 days.

If M is exactly between A and B you're doing 5 million kilometers on each leg at an average velocity of 1.5 million km/s and it will take you 3,333,333 seconds on each leg or about 77 days ignoring any time you spend finding and refining.

Note that these are the maximally favorable assuptions for the relative positions of A,B and M. Zig-zagging will take even longer.

Spending almost 3 weeks longer and putting 50% more wear on your engines will be less efficient by most standards.
__________________
Fred Brackin
Fred Brackin is offline   Reply With Quote
Old 09-17-2020, 03:16 PM   #15
TGLS
 
Join Date: Jan 2014
Default Re: [Spaceships] Martian “Gas” Station

Quote:
Originally Posted by Fred Brackin View Post
Spending almost 3 weeks longer and putting 50% more wear on your engines will be less efficient by most standards.
I'll admit I'm a bit biased towards "Time is Cheap, Mass is Expensive", though looking at near-future space travel that seems plausible enough (traveling outward would take months if not years). I'm also a little biased towards one-way flights.
TGLS is online now   Reply With Quote
Old 09-17-2020, 03:29 PM   #16
DaltonS
 
DaltonS's Avatar
 
Join Date: Aug 2004
Location: Hamilton, Ont. CANADA
Default Re: [Spaceships] Martian “Gas” Station

Okay, we are getting a little off focus here. If there was a reliable source of reaction mass, oxygen, food and water in Mars orbit incoming ships from Earth could re-provision themselves there for the return trip and reserve more room for cargo and passengers. Note that without a CO2 atmosphere there would be no methane production so that capacity on the moons would be dedicated to processing ice. A big import from Earth would probably be certain organic compounds to promote food production.
Quote:
Mars Needs . . .
Um . . .

The oxidant chemicals in the Martian soil aren’t very good for growing plants. To make the Martian surface material more fertile, humans have to combine it with water and carbon-rich compounds. The water will come from rain or irrigation, but the carbon is a bit more tricky. One excellent source of carbon is human waste. Mixing waste products with Martian soil solves two problems at once: the toxic soil kills bacteria and sterilizes the waste, while the carbon, water, and nitrogen compounds in the wastes make the barren red soil into something plants can use. Assuming a mix of three parts Martian soil to one part waste, a human can “terraform” about one square foot of Mars per day. Even with several million inhabitants, sewage disposal on Mars won’t be a problem for millennia!
GURPS Mars, page 75
Even hydroponics needs something to work with. So do humans, making food from Earth another big import item (at least initially). At least everyone (no matter how useless) would have something to contribute. ;D
Dalton “another political comment avoided” Spence

Last edited by DaltonS; 09-17-2020 at 03:33 PM.
DaltonS is offline   Reply With Quote
Old 09-17-2020, 06:24 PM   #17
Anthony
 
Join Date: Feb 2005
Location: Berkeley, CA
Default Re: [Spaceships] Martian “Gas” Station

I would note that you may want to rethink ratios of production to storage.

A basic refinery complex is 3 mining modules feeding one refinery, plus power plants capable of providing 4 energy points. At SM+10, that costs $40M for the mining modules, and $120M for fission reactors (solar power is vastly more expensive -- it starts at $200M, but because of martian sunlight and the fact that it only operates during the daytime, you actually need more than 5x as much. Note that both reactors and solar panels have options that are cheaper but heavier). It produces 150 tons per hour.

15,000 tons of fuel tanks costs $100M, so if your normal business cycle is 100 hours of business and then 10,000 hours of inactivity, you can drop down to SM+8 mining modules (total cost is $116M) and still refill your tanks in six weeks.
__________________
My GURPS site and Blog.
Anthony is online now   Reply With Quote
Old 09-17-2020, 09:20 PM   #18
DaltonS
 
DaltonS's Avatar
 
Join Date: Aug 2004
Location: Hamilton, Ont. CANADA
Default Re: [Spaceships] Martian “Gas” Station

Looks like I may have to redesign the whole thing. My problem is the first step: breaking down water into hydrogen and oxygen. The result is eight tons of oxygen for every ton of hydrogen. I see this as two standard and two small tanks for oxygen, one small tank for hydrogen, a standard mining system, two small refineries (LOH, CH4), a small fission reactor (powers one refinery at a time), a regular methane tank and a regular fission reactor (powers the mine). The H2 and O2 tanks take 30 hours to fill while the methane tank takes ten.
That takes care of the bottom section. I think I'll leave the top level much the same and work on the middle one tomorrow.
Dalton “Martian “Gas” Station 2.0?” Spence
DaltonS is offline   Reply With Quote
Old 09-17-2020, 10:03 PM   #19
Agemegos
 
Agemegos's Avatar
 
Join Date: May 2005
Location: Oz
Default Re: [Spaceships] Martian “Gas” Station

Quote:
Originally Posted by DaltonS View Post
Yes, by volume this may be true (see my quote from GURPS Mars above) but I expect hydrogen would be sold by mass, not volume. It's up to the user to have a gasbag big enough to provide the necessary lift.
Dalton “who decided not to make a political comment” Spence
The mass and cost of the gasbag envelope go with volume. Where the air is very thin you would need a vast, massive, costly envelope, perhaps impractical or uneconomic.
__________________

© copyright Brett Evill
Discussion of FLAT BLACK
Agemegos is offline   Reply With Quote
Old 09-18-2020, 07:33 AM   #20
DaltonS
 
DaltonS's Avatar
 
Join Date: Aug 2004
Location: Hamilton, Ont. CANADA
Default Re: [Spaceships] Martian “Gas” Station

Quote:
Originally Posted by Agemegos View Post
The mass and cost of the gasbag envelope go with volume. Where the air is very thin you would need a vast, massive, costly envelope, perhaps impractical or uneconomic.
True, but I have no idea how to model that. Sadly the "Gasbag" description in Spaceships 7 seems rather generic. I understand your concern about the expanded volume (I'd add 3 to the SM of the gasbag based on my first quote from GURPS Mars) but I think the increased area of the envelope would be amply countered by Mars lower gravity and Hydrogen's increased efficiency as a lifting gas there. The cost issue I'll grant (what sort of premium should I tack on?) but impracticality is a tech issue I'm not qualified to comment on. I stand by my preference for Hydrogen as a lifting gas on Mars for reasons I've previously stated and consider it an intrinsic part of my "Domed Mars" setting.
Dalton “So let's pop this balloon and get back to my station.” Spence
DaltonS is offline   Reply With Quote
Reply

Tags
mars, spaceships

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Fnords are Off
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -6. The time now is 05:51 PM.


Powered by vBulletin® Version 3.8.9
Copyright ©2000 - 2021, vBulletin Solutions, Inc.