Travel Time and Propellant Usage

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Offline Kedamono

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Travel Time and Propellant Usage
« on: July 28, 2020, 03:20:34 PM »
So, I was playing around with a constant thrust calculator, making a spreadsheet and chart to make it easier to figure out how long it would take to travel a set distance. In this case, Astronomical Units, or 149,598,000 km. (Sorry, no Imperial Units here, you go into to space, you go metric.)

Travel Time

I used the Space Travel Calculator to get all my numbers. It uses the rocket equation to correctly calculate the time it takes to go a set distance, accelerating halfway there and then decelerating the other half. And it does one other thing: It determines propellant usage, though he uses the term "fuel mass". And guess what? When you go real fast, you use a metric butt tonne of propellant to do so.

The spreadsheet just goes out to about Saturn's orbit, roughly 10AU, and you can see that at 1G constant acceleration, you'd need 4 times your ship's mass in propellant. Let's say you have a ship that masses as much as the ISS, about 420 metric tonnes. That means you'd need about 1,680 tonnes of propellant, hydrogen, a volume about 3,360 cubic meters or a cube about 14 meters on each side. That's a five story building thereabouts.

To get out to Uranus, you'd need about 4.6 times your ship's mass in propellant at 1G. Fuel usage and travel times in FTL are not what you'd call realistic. Yes, revamping how space travel works in FTL will take some time, but I want to make it as easy and low math as possible.

The two charts can be used to figure out how long it takes to travel a specific distance. Just find the distance on the horizontal axis, find where it intersects the acceleration the ship is using, and that will show you how long it takes. For example:

The ship Example is traveling 2.25 AU at .1G. Looking at the chart, it's just under 13 days, about 12.8 days travel time.

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Offline ORtrail

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Re: Travel Time and Propellant Usage
« Reply #1 on: August 07, 2020, 01:27:36 AM »
I'm usually more about the speed of drama, but I'm also waiting on my Kickstarter copy of the hard sc-ifi RPG, High Colonies.  I might at least get an idea of what it would take in a more realistic setting. 

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Offline Kedamono

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Re: Travel Time and Propellant Usage
« Reply #2 on: August 07, 2020, 11:03:24 PM »
The speed of spaceships in FTL:2448 are hard to nail down, thanks to the Zone and Slot system in the game. Mercury, Venus, Earth, and Mars occupy the first 1.5 AUs of the system. The Sol System has 4 slots in the A Zone, 4 Slots in the B Zone. Venus is just outside the inner portion of the habitable zone, and Mars is just at the edge of the zone. So the B zone goes from .725 AU to 1.5 AU. Venus is in slot A4, Mercury is in slot A2. Earth is in slot B2 and Mars is in slot B4.

So it takes an hour to cross a slot, and it's three slots to go from Earth to Mars at closest approach, about 3 hours. (Yes, I'm counting the slot Earth is in, space is big.) That means to get there, the ship would have to do 250 Gs, and have 11 times its mass in propellant. And it would have the impact of 500MT per 25 tonnes of ship's mass. Yes, it's only a measly 0.045% the speed of light, but still... Ships are way too fast in FTL.