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.)

Comments

Marc13Bautista's picture
Marc13Bautista

That's a lot of science!

Free Elusive Skill = Athletic x4 in ATN Enclave encounter

matsy's picture
matsy

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
Rovlad

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
dcfedor

@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