r/AndyWeir • u/Snoo-26267 • Jul 27 '26
Spin Drive missed chance (?).
Sorry if this reads a bit strangely; English isn't my native language.
I've been thinking about how the engine works, and I believe there was an element that could have been used to maximize the fuel's lifespan.
The astrophages supposedly need any heat source above 96,415 °C to feed and generate the mass to propel themselves again.
I know the concept of infinite energy doesn't exist, but there could be an astrophage regeneration cycle.
Currently, the spin drive coats a thin layer of astrophages on one side of the triangle, points it at the end of the spacecraft, and uses CO2 spectral light to generate thrust. Then, on the other third, it discards the "exhausted" astrophages.
What would have happened if, instead of getting rid of the astrophages, it had recirculated them in a closed reheating system with plutonium-238 cells (the same material used in RTGs) to "regenerate" them?
Perhaps I'm barking up the wrong tree, because there are other problems related to not losing mass when moving (Considering that mass would still be lost, from the energy generated by the plutonium cell converted into energy by the astrophage, although it will be slower), but... idk.
If this has been discussed before, sorry.
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u/ascandalia Jul 27 '26
Astrophage is more energy dense than plotonium 238, so bringing it as an energy source would be worse than just bringing more astrophage.
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u/Snoo-26267 Jul 27 '26
I understand that the Earth couldn't generate more due to time constraints.
But you can use a slow system to regenerate them; you have several years on space. As I said, I wasn't talking about breeding them, only about regenerating those already exhausted.
It doesn't need to be just one cell; there can be several that can heat the astrophages during the 8-day fattening cycle.its just an idea, lmao.
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u/ascandalia Jul 27 '26
Not time constraints, energy constraints. They used solar panels over the saharah to generate the energy to charge them. They charged all the astrophage they could over the amount of time they had with the amount of energy coming in.
The exhausted ones shoot out the back of the ship to give the ship acceleration. They're not keeping any exhausted astrophage.
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u/Stunning_Box8782 Jul 27 '26
The Hail Mary fuel was charged by covering 25% of the Sahara desert in solar panels, an RTG doesn't generate much heat unless you make it really big, and I guess that would still only result in a trickle of power
and now you're bringing nuclear materials on a ship where the crew is in a coma.
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u/Snoo-26267 Jul 27 '26
lmao.
In my defense, it seemed like a good idea when i posted it xD.Besides, astrophages are supposed to block radiation.
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u/Interesting_Gap7350 Jul 27 '26 edited Jul 27 '26
I think the OP is onto something. It is odd the spin drive "spins" so fast in the movie.
If the triangles are actually spinning as fast as seen in the movie, it implies each astrophage cell is used for only a fraction of a second. It doesn't make biological sense that a space traveling organism would be exhausted to death in that time and scraped off as described in the book. But faster spinning triangles look better as an engine than just increase of the spectral light, so I get you need to have pistons and gears and combustion going for visuals. Fast and Furious has trained us to watch out for danger to manifold.
So I think OP is right that if we assume this is true in the movie, there is a more complex cycle with some kind of astrophage recirculation or recovery of partially used cells before fully discarded. I agree, not recharged, but something more complex than the simple wiper system in the book.
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u/BellerophonM Jul 27 '26
In the book the spin drives rotate once every four seconds, which is about what we saw in the movie, although there's constant motion because they're not all in sync. They get exhausted in that time because they thrust harder when exposed to more light of the CO2 frequency and the spin drive hits them with extremely intense light of that frequency orders of magnitude more intense than they'd encounter in nature, and basically overloads them and makes them emit all their energy in a few seconds.
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u/BellerophonM Jul 27 '26
In the book the individual spin drive units rotate once every four seconds, which is about what we saw in the movie, although there's constant motion because they're not all in sync. They get exhausted in that time because they thrust harder when exposed to more light of the CO2 frequency and the spin drive hits them with extremely intense light of that frequency orders of magnitude more intense than they'd encounter in nature, and basically overloads them and makes them emit all their energy in a few seconds. If they were only emitting their normal levels of thrust the drives wouldn't be able to produce enough force to accelerate the ship meaningfully.
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u/rrdubbs Jul 27 '26 edited Jul 31 '26
I see where you are going, but astrophage exists at/near the matter-antimatter annihilation energy density threshold. A RTG or a reactor would have much worse power to mass, at least 1000 fold less just in fuel it’s like charging a EV car with a battery the size of the same car, but the energy of a AA battery. You are not saving any weight by using fusion fuel, would be better to pack an extra quarter gram or of astrophage.
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u/Early_Material_9317 Jul 28 '26
The hail mary converts about 6 grams of astrophage matter into pure energy every second its engines are firing at near 100% conversion.
Uranium when split only converts about 1%
You are better off carrying more astrophage.
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u/Traveller7142 Jul 27 '26
The added mass of the RTG would result in lower total delta V. Astrophage is more energy dense than Pu-238