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.