Author Topic: ITS: Modeling passengers vs. cargo  (Read 808 times)

Offline Rei

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ITS: Modeling passengers vs. cargo
« on: 01/17/2017 03:45 pm »
Hi all. I was trying to set up a model the other evening which involved some way to determine how many passengers you could carry in a capsule of a given mass going on a voyage of a given length.  So I turned to ITS.  They cite "100+" passengers for ~80-140 day trips using 6 m/s delta-V, which corresponds on their chart (and to my calculations given their ISP, dry mass and propellant specs) to a payload of 300 tonnes.

I also looked other, lower-end things, like the proposed passenger version of the shuttle, to try to get some "short term flight" crew figures.  So that was something like 30-70 passengers, if I recall correctly, and it was to replace the payload bay, so the shuttle's cargo plus some extra for replacing the bay and that's maybe 30k tonnes.  But they don't go anywhere, just to LEO, with no more than a few days onboard.

Food, carbs are 4kcal/g, protein 4kcal/g, fat 9kcal/g.  But water and fibre are none.  I'm assuming an average of 2kcal/g, and 3000kcal/day.  100 people on a ~120 day trip need .  Call it 240 for round trip.  That's 36 tonnes.  Bringing the available payload on ITS down to 264 tonnes. 

We could assume sent oxygen and disposable CO2 scrubbing and no water recycling rather than reusable, but that would be silly for a craft in nearly constant usage in space, given that all of that is high TRL. 

We could (and should) inflate the mass of the craft on a per-person basis to account for more space to move around, recreation, etc because the voyage is so long.  Countering this is that the bigger you make your craft, the less mass per unit volume takes - and also they're planning to use predominantly composites in construction, which are lighter than the aluminum that's used on most proposals.

So how much mass exactly are they assuming on a per-passenger basis?  They're clearly also sending cargo.  How much?  Is most of their shipped mass cargo - say, a lifetime's worth of food?  Or is it mostly "inflate the mass of the capsule to account for people having to spend long periods of time in it", at a rate greater than the mass savings of going big and using composites?  Or is the extra mass all for the plus sign in the term "100+"? 

In short, what would you say is a reasonable formula for a function "estimate_passenger_capacity(capsule_mass, days)"?  I made a formula on my own but it only comes up with about 70 tonnes for 100 people for ~120 days
« Last Edit: 01/17/2017 04:11 pm by Rei »