Rear-eject pyramid simulation

[JoePfeiffer]

THE BASE DRAG HACK DOESN'T WORK IN THIS CASE, JUST LIKE THE BASE DRAG HACK DOESN'T WORK IN A LOT OF CASES. We have no real reason to believe the base drag hack ever works right. As far as I can tell no one has ever done a careful comparison of a measured CP vs a CP calculated with the base drag hack. Ever. For any design at all.

In fairness, I've seen precious little actual experimental evaluation of just about any of our CP calculations, not just the base drag hack. We really need an army of high school students doing science fair projects with wind tunnels.

I am curious why you are asserting the hack doesn't work in this case, however. As I understand the aerodynamics of the pyramid, and the base drag hack, it seems like it ought to work fine.

 

[Buckeye]

T We have no real reason to believe the base drag hack ever works right. As far as I can tell no one has ever done a careful comparison of a measured CP vs a CP calculated with the base drag hack. Ever. For any design at all.

Yes, correct. As discussed starting here, the initial goal of Base Drag Hack was to artificially create a static margin of 1.0 caliber in the simulations of known stable rockets, so the data matched the rule of thumb.

 

[Buckeye]

We really need an army of high school students doing science fair projects with wind tunnels.

Never going to happen. An army of college students, preferably grad students, running parametric models in CFD would have a far better chance of success.

There are a few threads in this forum of people trying to compute model rocket CP with a CFD model. Nobody I know of is trying to build or use an existing wind tunnel. It just isn't practical.

 

[jqavins]

That sounds like a challenge! (Sadly, I'm neither a high school teacher no someone with access to a wind tunnel, so I'm in no position to take that challenge up.)

 

[Buckeye]

That sounds like a challenge! (Sadly, I'm neither a high school teacher no someone with access to a wind tunnel, so I'm in no position to take that challenge up.)

Exactly. You would be more likely to undertake CFD yourself or find someone who already does than trying to conduct reliable wind tunnel tests. CFD and digital models are better anyway - less costly, more throughput, and more data to analyze. And accurate.

 

[jqavins]

I made an offer to my local high school to mentor a rocket club if they'd like to start one up. They never got back to me. If this were happening, I'd be happy to contact someone at RIT to ask if they've got a tunnel that some high schoolers could get some time on, and I would expect a positive response if we're able to work around their schedule.

 

[Jeff Lassahn]

In fairness, I've seen precious little actual experimental evaluation of just about any of our CP calculations, not just the base drag hack. We really need an army of high school students doing science fair projects with wind tunnels.

I am curious why you are asserting the hack doesn't work in this case, however. As I understand the aerodynamics of the pyramid, and the base drag hack, it seems like it ought to work fine.

The original Barrowman papers have a section on comparison of the calculation with wind tunnel data from real sounding rockets. It's only about three rockets so it's not nearly enough coverage to be ideal, but it's something anyway.

I don't think the base drag hack works in this case, because I don't think the CP of that pyramid from the first post is almost a full body length behind the tail of the rocket. The version without the hack that says CP is about 2/3 of the body length back is much more believable and seems to agree with some other people's practical experience.

 

[Daddyisabar]

Dear Mr. Owl, how do Pyramids and Spools fly?

Mr. Owl says: Sheer dark arts and oddroc necromancy!

No mindsiming. If you can't computer simulate it, don't fly it! Be safe and sound.

 

[thzero]

https://openfoam.org/
There you go for your cfd needs.

 

[Buckeye]

https://openfoam.org/
There you go for your cfd needs.

Of course. OpenFOAM is well-known as the "free" CFD code. However, getting free CFD is like getting a free puppy. Your work is only starting.

I used OF extensively at the workplace. However, it was a commercialized version, with a custom front-end GUI and meshing schemes specific to our business application. Something similar will have to happen for model rocketry. OpenFoam can be at the core, but some enterprising fellow needs to build out a preprocessor to set up the correct/efficient settings for model rocket simulations by hobbyists.

AeroCFD used to be a thing, with reduced-order models utilizing 2D, symmetry, Euler methods, etc. I hope something similar can be created but with fewer limitations and still run on a "home" computer. Software as a Service and cloud computing are options if the computations remain too intensive.

 

[JoePfeiffer]

Of course. OpenFOAM is well-known as the "free" CFD code. However, getting free CFD is like getting a free puppy. Your work is only starting.

Speaking as someone whose wife is on the road to pick up an Australian Shepherd puppy in Oklahoma even as I type, let me assure you your work is also just beginning when the puppy isn't free.

 

[dpower]

I still fly a Sunward Khufu pyramid, with a thin-mil 18" nylon chute. Only real mod is to tie the motor mount with a thin kevlar line, so it falls together as one unit. To aid clean separation, I use a couple partial pieces of BT-80 around the chute. Always fly on a D12-3. It's mostly balsa, with a plywood rear plate. There is a fair amount of clay nose weight, but no idea how much.