Hello fellow ROCstars,
I have a question about BP charge calculations i.e. how much to use
based on tube size. There are plenty of calculators out there where you
put in the tube diameter, length, PSI, number of shear pins etc. and
then it tells you how much BP to use.
My question is, how do you account for the volume taken up by the
parachute and shock cord that's all folded up inside that same tube? Is
there a rule of thumb here? I haven't seen this mentioned in any
calculators or online articles etc. They all seems to base their
equations on an empty tube i.e. using the entire volume when computing
the pressure from the expanding gas due to the burning BP.
The parachute and cord obviously take up some space, which reduces the
volume that needs to be pressurized, and thus less BP is needed than
with an ideal / empty tube. In most of my rockets the cord, parachute,
deployment bag, Nomex etc. pretty much fill up the tube section, albeit
folded loosely. My guess is their volume is around 1/3 - 1/2 of the
available tube volume, and thus to correctly compute the amount of BP
needed I should model the tube as if it were 1/3 - 1/2 shorter than it
really is.
Any thoughts? Of course I don't want to cut it too close and not have
enough BP, but I also don't want to over-pressurize the system and shoot
my nose cone over the mountains north of the lake :).
A second question: How much loss of BP do you assume? My guess is some
amount gets ejected by the expanding gas as the first grains ignite, and
some just won't burn for whatever reason -- a few grains at least. Is
this significant? Do you add a safety margin on your BP calculations,
maybe 10% or so?
Maybe the two terms just cancel each other out: the reduced volume do to
the presence of the parachute and cord vs. the unburned portion of the BP.
Any thoughts?
Thanks
Terry
--
ROC-Chat mailing list
roc-chat@xxxxxxxxxxxxx
//www.freelists.org/list/roc-chat
