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Old 07-18-2019, 02:06 PM   #1
cvannrederode
 
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Join Date: Apr 2015
Location: Pennsylvania
Default [Spaceships] Regenerative Fuel Cells

Reading through the rules in Spaceships, I can infer the following points regarding fuel cells (and by extension, MHD turbines).

1. Fuel cells vent their "exhaust" off the ship (presumably in a non-propulsive manner).
2. The "fuel" tanks for a fuel cell are dedicated 100% to holding reactants for the cell (i.e. hydrox).
3. Refueling involves purchasing fresh reactants in the proper proportions.

Now, I was designing an minimalist, SM +4 vehicle for bounding around the surface of a world like Ceres: rotary reactionless drive and fuel-cell power, open frame armor and no life support, yada yada yada. At TL9, the fuel cell and one fuel tank gives 60 hours (2.5 days) of endurance. I also thought it seems wasteful for the little craft to vent 1,250 lbs of water during that time, and then have to buy that much hyrdox back at base. So I did a little looking, and regenerative fuel cells are a thing...meaning a fuel cell can be run in reverse to get the reactants back (although probably not in the same cryogenic form they were stored in). Even without that, real spacecraft tend to tank the water, at least to use it as fresh water on board (Apollo, Space Shuttle).

So here's my little house rules...
1. Normally, fuel cells operate as written (see above).
2. Optionally, the system can be designed to save and tank the cell by-products (water for a H2/O2 cell). This reduces endurance by half, since half the tankage (by weight) is for water. Water tanks and hydrox tanks can mix and match.
3. Without any other modifications, the operator of the fuel cell can buy reactants at a discount by providing the byproducts.

I was originally going to work out rules for having the power cell working in reverse, but I then I had an epiphany: have it be a reconfigurable system that can work as either a fuel cell, or chemical refinery. This makes the final system 6x cost, and allows it to process the reactants at the rate of one fuel tank every 90 minutes (22.5 minutes for the fuel tank built into the fuel cell), as long as it can be provided with 1 power point (at the proper SM).

So our $55K space truck would have its endurance halved to 30 hours, but then when it comes time to refuel, you can sell back 625lbs of water as you're buying 625lbs of hydrox. *Or* you can spend an extra $75K on the truck, and have it refuel itself in less than 2 hours with suitable external power.

My only sticky point is that 1/4 the mass of the fuel cell is the integral tankage, and how that will reconfigure in the refinery mode. I'm thinking of just saying the regenerative fuel cell has no integral tank, and drop the cost to x5 of a normal fuel cell.

Anybody have thoughts on this?

(Had to edit, because I fudged the time for the refinery to process, by a lot).

Last edited by cvannrederode; 11-11-2019 at 04:04 PM. Reason: Messed up my math on one part.
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