[AR] Re: Issues with operating at low chamber pressure
- From: David Weinshenker <daze39@xxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Mon, 5 Sep 2016 21:03:30 -0700
On 09/05/2016 08:49 PM, Troy Prideaux wrote:
Anyway, it turned out the combination producing the lowest Isp
actually produced the highest delta V return primarily due to the
respective densities on offer which directly affected the respective
mass ratios for the volume fixed analysis. Anyway, it raised a few
eyebrows if I vaguely recall.
I remember running some calcs for pure peroxide and different fuels -
it was interesting to see how much of an influence density did have
on overall stage delta-V when I assumed a given incremental tank mass
cost per unit volume (i.e., a given wall density and working strength).
Peroxide + DMAZ has noticeably better Isp than peroxide + a hydrocarbon
(such as RP-1, which being the liquid hydrocarbon fuel for which the
software had an entry in the data table; assuming commercial low-sulfur
#2 diesel might be fairly similar.) DMAZ (at about 0.92) has a slightly
better density than the kerosene. However, overall density, and hence
overall propellant quantity for a given total tank volume, is somewhat
worse, simply because the mixture ratio is less lopsided (about 3.5 vs.
7 for HP/DMAZ vs. HP/kero), and thus less of the propellant is the
denser peroxide. This tended to cancel out a significant fraction of
the Isp advantage. (I suspect, however, that DMAZ may be much easier
to ignite and burn smoothly in a peroxide rocket without the cat pack
commonly used with hydrocarbon fuels - this could have implications
for a lightweight low-pressure system.)
-dave w
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