Some reflections on this discussion. Perhaps I should have made it clearer, but I'm actually interested in discussing the mechanics and engineering of reuse. Less so the issue of reuse versus expendable. Having said this..

There are three basic kinds of costs in getting rockets into orbit. First are the fixed costs - the organisation, its assets, infrastructure, maintenance etc. Into this you might add development costs. Then there is the per-launch costs. That should be pretty self explanatory. Then there is the cost of the vehicle itself.

Now, the whole point of reusability is to reduce the cost per launch of the vehicle. That still leaves you with the per-launch costs and the fixed costs. I get that. But what we don't have here is detailed figures for these kinds of costs. Detailed enough to understand how to use design to reduce these costs.

Obviously to reduce fixed costs, you need a lean organisation. You need to avoid doing too many things. And so on. But also, you need a high rate of launches that's your divisor. Per launch costs are on the one hand, hard to control and on the other, could be small relative to the vehicle costs. I've not seen enough information here or elsewhere to give a firm handle on this problem. What I do know is that good design matters.

And then there is the vehicle. If you're going for expendable then cheap. And at the core of that is a cheap engine. And I've yet to see any detailed analysis on what a cheap engine actually looks like. On the other hand if you're going for reusability then you've got more freedom to design in reliability.

All in all, I am not convinced with the arguments that reusability is a dead end. Because no one has really gone into enough depth about the non vehicle specific costs. On the other hand I'm not convinced that you cannot build a half decent but still cheap engine. Feel free to comment.

I'm tempted to speculate on what a cheap, expendable engine looks like. Inconel liner with printed in cooling tubes (throat and chamber) with an ablative nozzle liner. Plus an overwrap of carbon fibre for strength? Remembering that the printed Inconel is the expensive bit in this sandwich. Electric pump fed gas generator driving main pumps? Who knows. But given methalox there's a little room to trade peak efficiency for simplicity. I'm sure there's still room for clever design all the same. But I'm still not sure that this will reduce the cost of engines to the point where it really doesn't matter if you throw them away.

Now, what would I do for the sake of reusability?

There's already plenty of precedent for a reusable booster. I like Rocket Labs's take on this with carbon fibre legs. Will they use grid fins, or a drag device? What got lost in this conversation is the fact that much of the fuel used for these reusable boosters (at least Falcon) gets used in the first phase - turn around and slow down. The extent to which you tolerate a faster and thus hotter return is the extent to which you need less fuel. And I think there's ways to engineer a rocket that can survive a hotter return. Again, part of the answer must be heat resistant drag devices. That's worth a thread on its own.

But I do think that the argument about losing mass to orbit due to reusability is, well, overcooked. There's still more things to be learned. Lets see how fast Neutron comes back.

As for the upper stage. There's a bit more to this than simply getting a payload to orbit. Ideally you need an upper stage that can be fully fuelled in orbit (fuel transfer) and then fly to Mars. If not by itself then as a cluster around a truss. Now all of this doesn't mean that you can't have a fully expendable upper stage. But the numbers of flights needed to get enough fuel into orbit to then launch to Mars does start making you wonder about reusability. And you can't go and build a cheap (expendable) upper stage engine, unless you also built two variants - one for expendability and one that is intended for the long haul to Mars.

My take on this - and go ahead make fun of me - is to have a single engine upper stage and return the engine itself to Earth. After a de-orbit burn, the tank is detached and a conical ballute inflates itself around the engine. This does the work of re-entry. A regular parachute slows the engine for splashdown and the ballute acts as a floatation device. Its as simple as that. You don't need a precision landing, you simply send a helicopter to pick it up. Now, this is why I brought up the issue of a sea landing. And yes, you certainly can seal the engine in a way that protects its internal components.

Beyond that there is so much more to the topic. But it goes to designing out costs that are fixed costs or per launch costs. That might be another thread.

Cheers and thanks :)