Jan Vorbrüggen wrote:

You would be hard pressed to get an RS-68 to go single stage.
The T/W ratio is twice the SSME, and the Isp is lower.

Isn't the engine T/W somewhat irrelevant? You need a suitable mass
fraction of the whole stage to get to orbit, and the engine mass surely
is only a small fraction of the total dry weight.

You can easily simulate this in orbiter to first order approximation. I
just haven't bothered to do it yet, because the way it stands right now,
if I want to fly either the RS-68 or the RL-10, I can fly the Delta IV.

In my SSME based test vehicle, engine weight is 20%.

Everything that has mass is relevant to SSTO.

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