"Paul F. Dietz" wrote in
:

Herb Schaltegger wrote:
On Sat, 10 Mar 2007 00:54:39 -0600, Henry Spencer wrote
(in article ):

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.

Um, what?

He explained that. What part of the explanation didn't you understand?

He kinda sorta explained that. The actual delta-V required to reach orbit
is purely a function of orbital mechanics and is independent of propellant
density. But the rocket equation does not account for things like gravity
and drag losses, and those things do depend on propellant density. Rather
than expand the rocket equation to explicitly include those terms, the
convention is to apply a fudge factor to the delta-V term. Dense-propellant
SSTOs have lower gravity losses so they need a smaller fudge factor. (They
also typically have lower drag losses since the dense propellants allow
smaller tanks, but that's not as significant as the lower gravity losses.)

So a dense-propellant SSTO doesn't really need less delta-V to reach orbit,
but you use a smaller delta-V term when modelling one using the rocket
equation.

--
JRF

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