Modeling an egg-shape in OpenRocket...or anything

JordanT · Nov 29, 2012

I'm considering building the rocket modeled after the moon rocket from Wallace and Gromit's "A Grand Day Out." The shape is essentially an egg (0.8:1 ogive front and rear) with much-too-small fins at the aft end. I don't mind building some small models to test, but I'd like to see if I can get a ballpark before I start.

In a way, it's a little like the Fat Man (

Can Open Rocket model an egg-shaped body like this accurately? My first attempt with a nosecone and tailcone, plus simple oversized fins, resulted in a CP which was about 15% of the rocket length in front of the nosecone (a negative number). I would have expected it closer to the centroid (like a sphere).

mwtoelle · Nov 30, 2012

It would probably have to be built and swing tested. OpenRocket uses the Barrowman equations for determining CP. You are going to get weird results because Barrowman's equations do not like fins on a transition or tail cone.

Pem Tech · Nov 30, 2012

You can make a Fore and Aft transition and using shape control get the parts pretty close. If I remember correctly you will need to slip in a phantom tube between the transitions to be able to simulate chutes and stuff.

Pem Tech · Nov 30, 2012

Like this...

View attachment Eggo.rkt

Oppsss...
Just noticed it was for OR not RS....

But here is an updated flying version if someone can translate it into OR.

View attachment Eggo Leggo.rkt.

JordanT · Nov 30, 2012

Thanks, Layne! I'll give it a shot. I have a demo of rocksim around here somewhere, but I've taken to trying out OR because I've never quite reconciled the cost/benefit of RS to my endeavors. Maybe this is the tipping point.

duckhead · Nov 30, 2012

Open Rocket can read and write Rocksim files directly. I opened Layne's design just fine.

Cheers,
Doug

Sampo · Nov 30, 2012

Hi,

Short answer: OR doesn't simulate such rockets well.

mwtoelle said:
It would probably have to be built and swing tested. OpenRocket uses the Barrowman equations for determining CP. You are going to get weird results because Barrowman's equations do not like fins on a transition or tail cone.

It's not that Barrowman equations don't like fins on transitions, it's that the Barrowman equations screw things up overall with boat tails with large contraction ratios.

The Barrowman equations predict a negative CNa for boat tails, meaning that they destabilize a rocket. (CNa is the value that weights the individual CP positions on a rocket when averaging the CP position, similar to how mass is used when computing the total CG from the CG of its parts.) If the boat tail reduces most of the diameters, the CNa becomes so great (while negative) that it pushes the weighted-average CP in front of the rocket, which is clearly unphysical. (A boat tail that reduces the diameter to zero would produce a CP at minus infinity.)

I haven't found any references on how to solve this issue. I'm not sure how RockSim handles it (or whether it does). If someone has RockSim at their hands, I'd be interested in hearing it's CP prediction when using the Barrowman method and when using the RockSim method.

Cheers,
Sampo N.

Pem Tech · Nov 30, 2012

Sampo said:
Hi,

Short answer: OR doesn't simulate such rockets well.

It's not that Barrowman equations don't like fins on transitions, it's that the Barrowman equations screw things up overall with boat tails with large contraction ratios.

The Barrowman equations predict a negative CNa for boat tails, meaning that they destabilize a rocket. (CNa is the value that weights the individual CP positions on a rocket when averaging the CP position, similar to how mass is used when computing the total CG from the CG of its parts.) If the boat tail reduces most of the diameters, the CNa becomes so great (while negative) that it pushes the weighted-average CP in front of the rocket, which is clearly unphysical. (A boat tail that reduces the diameter to zero would produce a CP at minus infinity.)

I haven't found any references on how to solve this issue. I'm not sure how RockSim handles it (or whether it does). If someone has RockSim at their hands, I'd be interested in hearing it's CP prediction when using the Barrowman method and when using the RockSim method.

Cheers,
Sampo N.

OK, here is one I whipped up while on painmeds this morning...
Based on a plastic egg from HL.
Had to cheat and create a pahtom nosecone to make it fly.

RS CP= 11.5076
Barrowman CP= 7.9754

View attachment Fushia HenFruit of Questionable Lineage.rkt

markkoelsch · Dec 1, 2012

I think the Rocksim method works better for odd shapes. I designed a Fat Man for a buddy of mine, and it seemed to work rather well.

View attachment Fat Man 2.rkt

Modeling an egg-shape in OpenRocket...or anything

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