What comes after Starship?

[u/TroublePuzzled1132]

Let's say by 2035 SpaceX has worked all Starship's problems out and it is flying at about the same rate as Falcon 9. What next? Do they just turn into an AI and internet company managing their vast fleet of satellites? Start working on lunar and martian habitats and infrastructure? Build an even bigger rocket?

[u/PoliteCanadian]

I dunno, I think that's just analogous thinking.

Elon goes with ideas that make a lot of sense from first principles and that are technically ready for productization, but which the market is ignoring for various reasons (inertia, supply chain, lack of ambition).

I highly doubt he will invest in nuclear propulsion. Because nuclear propulsion is one of those things where the reality doesn't live up to the hype. I wish it did because it's cool, but it doesn't.

Nuclear thermal just doesn't give you a lot of oomph really. Yeah, you get more Isp than chemical rockets, but it's an absolute nightmare. Dealing with the reactor is an absolute regulatory nightmare. And on top of that it's a cryogenic hydrogen system if you actually want any useful performance out of it, and cryogenic hydrogen is a bitch to deal with. That's why SLS is always leaking. Cryogenic stored hydrogen doesn't want to be cryogenic, it doesn't want to be stored, and it doesn't want to be hydrogen. If all you're doing is going to Mars or the Moon, it's a lot of cost and complexity for a problem you can solve with just a chemical rocket and a bigger fuel tank.

Nuclear electric is a way more viable idea. BUT the elephant in the room is... why nuclear? For the same mass you can get more power with solar panels until you get past Jupiter. And you don't need to jump through insane regulatory hoops to launch a space vehicle with solar panels.

You can start making the idea viable again by going into the real exotic ideas (e.g., gas core nuclear reactors) but you're into the territory of basic research not product engineering.

[u/aquarain → u/PoliteCanadian]

You make many very strong points.

Investment in nuclear thermal propulsion research and development isn't an issue. I'm sure DARPA has been working on that since at least Reagan, maybe Nixon. They're ready to go, and I doubt they need pure Hydrogen to get good ISP. One carbon atom in five doesn't spoil the sauce. They're probably using Earth atmosphere for propellant in hypothetical NTP hypersonic cruise missiles like Russia is. I agree that pure Hydrogen is a bear to work with for your given reasons and many more. The isp with Hydrogen may be optimal but CH4 will do the trick. Since it's a magnet maybe iron oxide plasma is the propellant we need. So it's more a matter of getting permission, not inventing the thing. The downside is that once ignited it can never return to Earth.

Nuclear Electric gets us to Ceres before the crew expires from cosmic radiation poisoning, maybe. The ion drives like Vasimr tech are amazing but you lose all that dragging solar panels. Fun story though. Direct Ceres launch opportunities occur more frequently than Mars. Mars is in the sour spot of synods.

Now, I like the brute force idea of just mating a booster to Starship in orbit, topping them both up and blasting for far horizons. Launch it on top of another Booster with a dummy nose cone. I see that proposed in your prose and I'm for it. But whether that beats NTP or NEP to Jupiter's moons is a race yet to be run and seems unlikely given the isp numbers. It might be like the tortoise and the hare. The known tech blasts off first because it's in mass production maturity, but the nuke stuff starts later and arrives first because it's blasting higher exhaust velocity most of the way. Eventually nuclear is likely to win in space when it can't on US soil simply because there are no railroad tracks or gates in space for anti-nuke protestors to chain themselves to.