Fuel and Propellant Storage

Hey Folks! Hope everyone had a good weekend. Pretty uneventful here, though I did fire up a bit of both Alien Isolation and Warlock for some fun. Forgot how much I enjoy those!

Back at the office, I resumed work on end-to-end flight systems. Last week, I managed to get everything on the ship assigned a mass, as well as an estimated thrust for the typical reactor, and that's a major step towards getting the flight computer to plot an actual course based on ship parameters.

Fuel and propellant are still a question, though. And how much we carry directly controls how long we can burn. Given the flight characteristics we want (~1g or more acceleration constantly over ~28 days on a ship with a mass ~300-500 tonnes), we can do some estimation about the amount of mass we need to throw out the back. And the current estimate is about 0.05kg/s over 2419200s (28 days in s), for a grand total of 117765 tonnes.

A lot of this morning was spent rounding up data and research to get a sense for how fuel/propellant storage would look in our scenario. I'm assuming we're running an He-3 (helium missing one neutron) + Deuterium (hydrogen with an extra neutron) fusion reaction, so our inputs are He-3 and D. And as far as I can tell, the most density we can get from either is to liquify them.

And curiously, they aren't very dense as liquids. D weighs-in at about 162kg per cubic meter, while He-3 is only 75! For comparison, liquid water is 1000! Imagine you've got a cube 1 meter (about 1 yard) on a side, filled with liquid He-3. It only weighs about as much as an average human!

Why is this significant? Well, take a look at that image above. The right side is the overall ship so far, the left is a zoom-in of the tanks. Each of those tanks is about 40m3 (m3 = cubic meters). And depending on whether you fill them with liquid D or He-3, they hold ballpark 2800kg and 3000kg, respectively. Now go back up to our earlier fuel calculations. That 28-day trip at 1g for a ship weighing ~400 tonnes (about the weight of the ship in the picture as-is)? That needs 117765 tonnes of propellant, or 117,765,000kg! For reference, using these style of tanks, we'd need 20 of each fuel type, for a grand total of 40 tanks.

See anywhere left on the ship to place 'em? :)

Granted, this is a single-deck ship, with about 3m height, not counting whatever nozzle is under that reactor. (Note: this ship is a "belly lander," meaning the thruster is on the underside. It "flies" with the roof pointed toward/away from the destination to simulate gravity during the trip's constant acceleration.) So we could have more decks. But still, this ship is all fuel (by volume, anyway). Not exactly a Millennium Falcon, Firefly, or even Rocinante at this rate.

And speaking of the Expanse, it sounds like even with this ultra-high-tech inertial confinement D-He3 fusion reaction/thruster, they're cheating with propellant somewhere. Because the Roci does not have room for this kind of reaction mass. Unless maybe they have a more compact storage solution than liquid tanks. Metallic hydrogen? Water electrolysis? Bag of holding? :)

I don't necessarily have to solve this problem. I can just hand-wave it away like The Expanse does. Heck, the NEO Scavenger mythology certainly supports it! But it should at least seem plausible, whatever excuse I make up. (Or vague enough to dance around without details.)


Marc13Bautista's picture

That's a lot of science!

Free Elusive Skill = Athletic x4 in ATN Enclave encounter

matsy's picture

Could you not fudge individual types being more dense, so then have a fuel processing step that turns the base resources into H-3 and D before it goes into the reactor?

Or alternatively have a need to make more stops at the gas station.

Rovlad's picture

I'd say just name it "fuel" without going into any details, then it could have any properties you want.
I mean, every space game in existence does exactly that.

dcfedor's picture

@Marc, indeed! That's important to me in games like this. I like to know that a lot of thought went into the things I play, and that helps me suspend disbelief and really get into them.

Plus, I'd be lying if I didn't admit this research is one of the parts of gamedev I like best :)

@matsy, yeah, I was thinking along those lines last night after work. I haven't run the numbers yet, but I could theoretically store the D as (D2O) heavy water if that's a more dense configuration. The power from the reactor could be used to separate the deuterium from oxygen much like a reverse fuel cell. Plus, the O2 byproduct could be used in air processing on the ship, and leftover D2O could be used for life support in a pinch. (Though, it presents the risk of increased radiation absorption if ingested.)

I'd still need some pure D and He-3 to kickstart the reactor, so to speak, but once it's running, it's generating more power than the ship (or even an entire city) could use, so a paltry electrochemical process isn't going to affect much.

Doesn't solve the He-3 issue, but I might find a similar solution there.

@Rovlad, I could, but part of the allure of a game like this (or NEO Scavenger) is the grounding in reality. Even though some weird-ass shit happens in the game world, there's a supporting framework based on real-world stuff :)

That said, I might be able to compromise a bit. Like, maybe the fuel is an ultra-dense compound that is rich in D and He-3, solving the storage problem. And like in my comment to matsy above, maybe it can open up some interesting tricks like the O2 byproduct and heavy water.

Dan Fedor - Founder, Blue Bottle Games