I think we're mostly in agreement now, just we disagree on our guesses of benefits vs risks.
The key point seems to be the complexity penalty of adding airbreathing to the engine vs the weight savings of less reaction mass. If we're comparing reusable apples to apples, this is really the value proposition. I'm clearly more optimistic on this point.
Also any engine that uses ram effect becomes more efficient at higher speeds. The SR71 uses less fuel per unit of distance the faster it goes, which is a bit counterintuitive. How big a benefit this is for space launch I can't really guesstimate but it's probably minor.
Reaction mass savings means more than just oxidizer material cost though. It ripples through the whole design. There aren't many times when the mass fraction of a rocket is working for you instead of against you.
I think we skipped over that a horizontal takeoff requires a lot less launch infrastructure. But being smart with rockets and launching from a barge in the ocean can equalize things.
As a summary, I think you and Elon may be right about Skylon for space launch. Mass produced rockets can get pretty cheap, and SpaceX does aspire to full reusability.
But space launch is only one of the two applications of a design like Skylon. Nothing SpaceX develops will be used for terrestrial transport. You aren't going to take a rocket to visit your family for the holidays, so Skylon may find a market there.
Skylon also could be used as a WhiteKnight style carrier for a more traditional second stage, which might still be interesting for space launch, but I'm pessimistic on this point because I think if the numbers worked the air force would already be using such systems instead of Deltas.
Skylon can also hedge that their high flow flash chiller is useful in other applications, and apparently they've developed an interesting high temperature composite material.
So on the whole I think it's interesting to watch what happens to them, even if it's not a sure bet.
The key point seems to be the complexity penalty of adding airbreathing to the engine vs the weight savings of less reaction mass. If we're comparing reusable apples to apples, this is really the value proposition. I'm clearly more optimistic on this point.
Also any engine that uses ram effect becomes more efficient at higher speeds. The SR71 uses less fuel per unit of distance the faster it goes, which is a bit counterintuitive. How big a benefit this is for space launch I can't really guesstimate but it's probably minor.
Reaction mass savings means more than just oxidizer material cost though. It ripples through the whole design. There aren't many times when the mass fraction of a rocket is working for you instead of against you.
I think we skipped over that a horizontal takeoff requires a lot less launch infrastructure. But being smart with rockets and launching from a barge in the ocean can equalize things.
As a summary, I think you and Elon may be right about Skylon for space launch. Mass produced rockets can get pretty cheap, and SpaceX does aspire to full reusability.
But space launch is only one of the two applications of a design like Skylon. Nothing SpaceX develops will be used for terrestrial transport. You aren't going to take a rocket to visit your family for the holidays, so Skylon may find a market there.
Skylon also could be used as a WhiteKnight style carrier for a more traditional second stage, which might still be interesting for space launch, but I'm pessimistic on this point because I think if the numbers worked the air force would already be using such systems instead of Deltas.
Skylon can also hedge that their high flow flash chiller is useful in other applications, and apparently they've developed an interesting high temperature composite material.
So on the whole I think it's interesting to watch what happens to them, even if it's not a sure bet.