In article ,
Pat Flannery wrote:
Using any other propellant combination than LOX/LH2 (barring exotics
like fluorine or ozone) will give you an inferior Isp, and make the
whole SSTO concept probably unworkable with any useful payload at all.

Not necessarily. The lower Isp is more than made up for by the higher
propellant density and the better engine T/W ratio. Sure, you need a
higher mass ratio, but it's actually easier to do. Dense-propellant
stages with SSTO-class performance were built years before LOX/LH2 ones
started to show up, and with considerably less difficulty.

Also note that the mass-ratio disparity is not as large as you'd think,
because a dense-propellant SSTO needs less delta-V to reach orbit. Lower
Isp, and thus higher propellant mass-flow rate, means it loses mass
faster, thus reaches higher accelerations sooner, thus suffers smaller
gravity losses. It's not a big advantage -- circa 300m/s -- but it's on
the steep part of the curve, so it drops the required mass ratio quite
substantially. (This effect was known in the early 60s, but got forgotten
in the rush of enthusiasm for hydrogen the wonder fuel.)
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