Even more on Skylon

They stop airbreathing at 2 km/s. Let’s compare this to staging. Assuming an upper stage of the weight of Skylon at that point (230 t), what kind of a rocket would be required for that? With a hunch of 3000 m/s exhaust velocity and a tankage ratio of 8, the mass ratio is 2, first stage dry mass is 36 t and the stack’s total takeoff mass 520 t, about double of Skylon’s 280 t takeoff mass.

In reality of course, Skylon is not a good upper stage because it’s heavy. I also didn’t include gravity or drag losses (they’re not huge for a rocket though).

But anyway this points to how little propellant mass Skylon really needs to get up there.

The whole Skylon is actually the weight of an Atlas 5 and performance is about the same as well. (Though what if you want GTO?)

All this points to a Skylon type system being a good first stage for a two stage rocket. Launch a Centaur or something else from it that can then go directly to GEO.

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