Sim'ing fin set masses in RockSim [#$@!]

Sim'ing fin set masses in RockSim [#$@!]

Post by Chuck Pierc » Sat, 27 Oct 2001 12:01:11



I don't know how I manage to***up entering the masses for fin sets
every time I sim a new rocket in RockSim, but I somehow manage to do
it.  I know that on the component/construction screen, it says "fin
set", which certainly implies that the mass of the entire fin set
should be entered, but doesn't it seem more logical to enter the mass
of only one fin and let RS do the math for the whole fin set?  It
would be nice if Apogee would at least put a parenthetical note next
to the fin set saying that the fin-set mass is entered for the entire
fin set, even though the fin dimensions are entered for only one fin.

Well, anyway, I was entering my final, actual component masses into RS
for my new two new builds for this weekend, when I caught the error.
Man, was it ever a big error!  It gave me more static margin on my big
canard'd Sempre Fi, but it now requires a motor with an average thrust
greater than ~150 N to get it off the 8-foot rail with a pretty good
velocity (~30mph / 44 fps).

Does anyone else have trouble remembering how to enter fin masses in
RockSim, or am I the only dork who does?

Chuck
--
Chuck Pierce
NAR 78629, Level 2
cpierce_AT_knology.net

 
 
 

Sim'ing fin set masses in RockSim [#$@!]

Post by Jon Ros » Sat, 27 Oct 2001 12:55:22


Chuck,
Maybe you started off with a bad assumption. Even though you are only
entering the fin dimensions for one fin (Rocksim assumes they are all the
same) you also enter the number of fins. If you change the number of fins
you can see the fin mass increase or decrease proportionally. Once you've
cut fins and weighed them, you can adjust the mass accordingly using mass
override to get it exact.

I've always felt that its what Rocksim is best at. It does the dirty work
(read: math) for you and lets you handle the creative end of things.

Jon


Quote:
> I don't know how I manage to***up entering the masses for fin sets
> every time I sim a new rocket in RockSim, but I somehow manage to do
> it.  I know that on the component/construction screen, it says "fin
> set", which certainly implies that the mass of the entire fin set
> should be entered, but doesn't it seem more logical to enter the mass
> of only one fin and let RS do the math for the whole fin set?  It
> would be nice if Apogee would at least put a parenthetical note next
> to the fin set saying that the fin-set mass is entered for the entire
> fin set, even though the fin dimensions are entered for only one fin.

> Well, anyway, I was entering my final, actual component masses into RS
> for my new two new builds for this weekend, when I caught the error.
> Man, was it ever a big error!  It gave me more static margin on my big
> canard'd Sempre Fi, but it now requires a motor with an average thrust
> greater than ~150 N to get it off the 8-foot rail with a pretty good
> velocity (~30mph / 44 fps).

> Does anyone else have trouble remembering how to enter fin masses in
> RockSim, or am I the only dork who does?

> Chuck
> --
> Chuck Pierce
> NAR 78629, Level 2
> cpierce_AT_knology.net


 
 
 

Sim'ing fin set masses in RockSim [#$@!]

Post by Chuck Pierc » Sun, 28 Oct 2001 05:21:14




Quote:
>Chuck,
>Maybe you started off with a bad assumption. Even though you are only
>entering the fin dimensions for one fin (Rocksim assumes they are all the
>same) you also enter the number of fins. If you change the number of fins
>you can see the fin mass increase or decrease proportionally. Once you've
>cut fins and weighed them, you can adjust the mass accordingly using mass
>override to get it exact.

>I've always felt that its what Rocksim is best at. It does the dirty work
>(read: math) for you and lets you handle the creative end of things.

>Jon

Thanks for step response, Jon, but now, I'm really confused!  <g>  I
had entered the mass for one fin for each fin set (I have two fin sets
on the rocket in question) and real weights for each of the components
(having to make some minor adjustments for my liberal use of epoxy)
into RockSim.  However, the RS-calculated mass for the whole rocket
was WAY low.  I eventually disassembled the rocket into sections
(forward airframe, avionics bay, and fin can) and for each section
added up the component weights in RS and compared that weight to the
actual weight of the section.  Only when I tripled (or quadrupled,
depending on step fin set in question) the per-fin mass in RS, could I
get the total RockSim-caluclated weight to get close to the actual
weight.  

Tonight I'll go back and re-evaluate my component masses to make sure
that I haven't made a critical somewhere along the way.

Chuck
--
Chuck Pierce
NAR 78629, Level 2
cpierce_AT_knology.net

 
 
 

Sim'ing fin set masses in RockSim [#$@!]

Post by Chuck Pierc » Sun, 04 Nov 2001 01:03:21




Quote:
>Chuck,
>Maybe you started off with a bad assumption. Even though you are only
>entering the fin dimensions for one fin (Rocksim assumes they are all the
>same) you also enter the number of fins. If you change the number of fins
>you can see the fin mass increase or decrease proportionally. Once you've
>cut fins and weighed them, you can adjust the mass accordingly using mass
>override to get it exact.

>I've always felt that its what Rocksim is best at. It does the dirty work
>(read: math) for you and lets you handle the creative end of things.

>Jon

Jon,
I finally got around to playing with RockSim to try to figure out how
it calculates fin masses.  What I found is that if you override the
fin mass in RockSim (which I almost always have to do), the mass you
enter must be for the entire fin set, not just for a single fin.  I
also think that when RS calculates it's own mass for the fin set, the
mass displayed on the fin screen is for the entire fin set.  I don't
know how everyone else feels about this, but in my book it's a short
coming of RS.  Since the mass is determined by weighing one fin, RS
should display the mass for one fin, then maybe have a second window
that displays the mass for entire fin set, taking into account the
number of fins.  It seems like this would be an easy thing to do and
would avoid a lot of confusion.

Chuck
--
Chuck Pierce
NAR 78629, Level 2
cpierce_AT_knology.net