View Full Version : Rocksim vs. Barrowman
12th June 2009, 01:20 AM
When using Rocksim I noticed that there is a substantial difference in the location of the CP when switching between "use Rocksim stability equations" and "use Barrowman stability equations" Which one do most people use? I would like to build a scratch HPR rocket soon and I want to be sure I have my numbers right.
12th June 2009, 02:04 AM
I'm pretty sure Rocksim calculates using extended Barrowman equations for dynamic stability. Barrowman's original equations only apply to zero angle of attack. I'd go with Rocksim over standard Barrowman.
Note--there will still be some blind spots if using really high or really low aspect ratio designs under either approach, but "normal" rockets would be fine.
12th June 2009, 05:17 PM
I know this is amateur, not model, rocketry, but a friend of mine is wondering why a home-made hybrid that he launched tumbled and went horizontal - does anyone think that this might be due to change of moment of inertia due to propellant depletion?
12th June 2009, 05:52 PM
For hybrid rockets, as the N2O is depleted, the CG actually moves backwards. So if the rocket was maginally stable at the start of the flight, it might have been unstable at the end. I have seen this happen a couple of times with hybrids.
12th June 2009, 06:14 PM
There's a lot of information on this topic on this thread:
13th June 2009, 04:22 AM
For hybrid rockets, as the N2O is depleted, the CG actually moves backwards. So if the rocket was maginally stable at the start of the flight, it might have been unstable at the end.
I think Crazyrocket is onto something here, and that this is very likely the cause of the problem.
Reduced pitch/yaw inertia (as propellant burns off) would make a rocket more responsive to external pitch/yaw forces (it would rotate more quickly to new flight headings when external airloads hit it from the sides). This is a bit different than the effects that a shifting c.g. would have. You could have reduced inertia while the c.g. remains in the same location, or while the c.g. shifts forward, or while it shifts aft. A change in inertia will generally not make a rocket stable or unstable, while a change in c.g. will greatly affect stability.
Did any of that make sense?
19th June 2009, 03:04 AM
A while back I email Mr. Van Milligan about it, and this is what he told me:
"The RockSim method is the Barrowman method without all the assumptions
that are traditionally used.
I use the RockSim method for everything."
I take that to mean the RockSim method allows for some more real-life versitality than the rigid requirements that the Barrowman method has in it.
I have a feeling that the RockSim method uses calculus to figure out its CP. Barrowman just noticed trends in basic rocket shapes and came up with mathematical expressions to total up the aerodynamic force exerted on its basic parts.
20th June 2009, 01:57 AM
Thanks for the replies. There seems to be some debate on the other forum. For now I think I'll make sure the CG is ahead of both calculated CP points.
20th June 2009, 03:09 AM
That's usually what I do - I simply take the worst case scenario between the two values, and assume it is correct.
1st July 2009, 02:47 PM
If you're curious, Mr. Van Milligan of Apogee Rockets just posted Newsletter 238, and he gives a detailed explanation of the difference in the two methods:
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