Careful.
While it is certainly a general rule that an _amateur_ rocket's cost will
tend to go down if one pulls dry mass out that is only accurate up to a
point.
The whole theory behind big dumb boosters is that cost goes down as dry
mass increases when one starts with a very high propellant fraction.
However, even that relationship is subject to other factors, notably
business model optimization: Falcon 9FT is both low cost and offers a high
structural fraction....
Bill
On Tuesday, September 6, 2016, ignacio belieres <
ignacio_belieres@xxxxxxxxxxx> wrote:
Henry,
Well if you accept the correlation that cost tends to go up
with dry mass, and that volume is in a direct relationship to dry mass,
then using a smaller vehicle for a same payload will reduce cost. The
difference is even more pronounced than that because with a room
temperature liquid propellant you dont have to deal with cryofluids,
prechills, cryovalves, insulation, reaally good thermal control, lox vents,
and extra annoying stuff. A peroxide compatible 1 inch ball valve will
probably be 20 times less expensive than an equivalent Lox valve.
IB
Date: Tue, 6 Sep 2016 14:31:25 -0400<javascript:_e(%7B%7D,'cvml','hspencer@xxxxxxxxxxxxx');>
From: hspencer@xxxxxxxxxxxxx
To: arocket@xxxxxxxxxxxxx<javascript:_e(%7B%7D,'cvml','arocket@xxxxxxxxxxxxx');>
Subject: [AR] Re: Issues with operating at low chamber pressureactually
On Tue, 6 Sep 2016, Troy Prideaux wrote:
Anyway, it turned out the combination producing the lowest Isp
forproduced the highest delta V return primarily due to the respective
densities on offer which directly affected the respective mass ratios
denserthe volume fixed analysis...
There are occasional observations going back to the 1950s about how
fuels often seemed to produce higher performance in real rockets, evenif
they incurred an Isp penalty.in
That said, any constant-volume analysis is going to be somewhat biased
favor of denser fuels, in the same way that any constant-gross-massbut
analysis is going to favor the highest possible Isp: the conditions of
the analysis have stacked the deck somewhat. The yardstick that's really
of the most interest is not volume or mass, but cost -- defined broadly,
to include issues like operations difficulty as well as cash outlay --
that's harder to estimate well for a paper design.
Henry