Help with OR Warlock File Mystery

I am having a problem with my Warlock file in OpenRocket, and I can't figure it out. It seems like the CP is being calculated incorrectly, and I do not know why. The program is showing the CP at 35.9", but I've checked with others, and it is should be closer to 39". On such a short stubby rocket that already has narrow stability margins, this makes a big difference, and for some motors, the rocket looks dangerously close to unstable in OR. The rocket is definitely a proven design and is not unstable on the motors selected, so something is amiss.

What is going on?

I thought maybe there was possibly some kind of override that could be set for CP, and maybe that was throwing it off, or maybe there were options for the method of calculating CP, and some odd option had been selcted. But I do not see anything like that in the program.

I'm attaching the file, and if anyone can help me out with it, I'd really appreciate it. This is a file I originally downloaded from somewhere several months ago, and I do not remember where it came from. I deleted all the original component mass and CG overrides and entered my own mass and CG override for the whole rocket based on actual weight and balance test of the finished rocket. My CG is very close to what other people measure for their Warlocks, and my rocket is built very close to stock. I've also double-checked the dimensions of NC, BT, and fins, so the shape of the rocket should be accurate for purposes of calculating CP. (There had been a small error in the fin dimensions in the orignal file, but it did not make much difference, and I think they are correct in the version I am posting.) This version of the file has been saved with only the "primary figures" to make the file size acceptable to upload as an attachment.

Thanks for taking a look!

View attachment warlock-small.ork

Thirsty, there were some Apogee newsletter articles about simming short rockets, recommending adding a fake cone to the rear to simulate the base drag...I just did that to your model, and get CP= 42.8"

here is what I found on Apogee
https://www.apogeerockets.com/education/downloads/Newsletter154.pdf
https://www.apogeerockets.com/education/downloads/Newsletter158.pdf
https://www.apogeerockets.com/education/downloads/Newsletter162.pdf

some folks don't agree with this method, but for the short fat rockets that appear to be unstable in the sims but fly well, it seems to work.

rocketguy101 said:

Thirsty, there were some Apogee newsletter articles about simming short rockets, recommending adding a fake cone to the rear to simulate the base drag...I just did that to your model, and get CP= 42.8"

here is what I found on Apogee
https://www.apogeerockets.com/education/downloads/Newsletter154.pdf
https://www.apogeerockets.com/education/downloads/Newsletter158.pdf
https://www.apogeerockets.com/education/downloads/Newsletter162.pdf

some folks don't agree with this method, but for the short fat rockets that appear to be unstable in the sims but fly well, it seems to work.

Click to expand...

Thanks for the links. I was aware of the cone method, but had lost the details. You don't think there is anything off with the file I posted, correct? The issue is just the short stubby shape, and all I need to do is add the cone for a more accurate sim? I had spoken to another member, and his sims for his Warlock had calculated a CP of closer to 39", and I thought that was without a cone, but I may have misunderstood. Now I've looked at another file from Rocket Reviews, and it is showing a CP similar to what I have in my file --- around 36" without a cone.

Just in case, for the first launch, I think I will wear a cone.

Two points: short stubby rockets typically need less than 1 caliber of margin to be stable, and Rocksim uses a different, less-conservative method to compute CP than Openrocket, and many CPs were computed with Rocksim.

I know that short stubby rockets need less than 1 cal. I'm not a firm believer in cal anyway, and tend to think a ratio of the length of the rocket makes more sense that a ratio of the diameter. But I was just struck by the sim results:

Stability = .528 cal without motors. But when I add a motor, the CG shifts quite a bit, and stability drops significantly.


I345 --- Stability = .321
I540 --- Stability = .279
J290 --- Stability = .257

For some of the other lower thrust motors I simmed (under 200 N average thrust I motors), OR puts up a warning and says "Rocket Tumbles under Thrust" Yikes!!!!

(Those numbers were all before I made a slight correction to the fin shape that actually made the stability move in the wrong direction by a very tiny amount. More accurate numbers are in the sims in that attached ork file.)

I am sure the Warlock is a proven design, and I have nothing to worry about. And I accept that this kind of rocket needs less than 1 cal. I just wasn't expecting it to me THIS low. I guess I'll trust in the cone method and the proven design for the initial test flights.

But one of the reasons I want to be sure I know exactly what is going on with this rocket is that I had intended to use it for an L2 attempt. I'm probably not going to do that this weekend as planned, so it's not as urgent. But if asked, I want to be able to to explain why I think the rocket will be stable. If OR shows a stability of .257, and the CG and calculated CP are only separated by 2 inches, I want to have an explanation that is better than, "It's a proven design that flies OK for other people." I want to demonstrate understanding.

Thanks, everyone, for helping me out on this.

Here's some new info.

I received a Rocsim file from another member. I opened it in OR, and the CP is very close to what OR calculated for my file: 36.284". According to the member who sent me the file, Rocsim calculated it as 39.3". So I guess OR and Rocsim are using a different method for calculating the CP, and these methods yield very different results. Maybe Rocsim recognizes a short stubby rocket and does something to account for base drag.

Next I added a base drag cone to my OR file and got another very different result. Here's how it breaks down:

• OR without cone: 35.8"
• Rocsim without cone: 39.3"
• OR with cone: 42.6"

That is quite a spread of values!

And with the various methods for calculating CP, plus all the various motor sizes affecting CG, the stability of this rocket could be calculated as dangerously marginal to well over 1 cal, depending on the methods used and motors picked.

Everything I've heard about this rocket would suggest that it is stable as I built it, and I have nothing to worry about with regards to flying it. But I did have a concern about being able to explain the stability of the rocket if asked during the L2 cert process. I think I can just explain it is a short stubby rocket, and different methods yield different values for CP. Solid flyer, but hard to sim.

I think that perhaps RS is correcting for stability as per https://argoshpr.ch/joomla1/articles/pdf/sentinel39-galejs.pdf where OR is not. Would be nice if OR had an option to do this.

I had lots of trouble getting my TARDIS to fly straight due to it both being very short and square. Also ran into issues with how RS calculates body thickness.

I just simmed it in VCP and came up with CP at 35.793" from nose cone. Adding a cone, it gives a CP at 42.814

That matches the OR with and without a cone pretty close. I would use the OR with a cone.