The 100/10/1 Rule.
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.
Jan
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