Rocksim 10 and short fat rockets like Big Daddy

[Bill S]

I'm building an Big Daddy with my wife, and as usual, I enter everything into Rocksim to ensure things go well. I am seeing stability numbers that seem awfully low. Doing some research, I found that Rocksim seems to have issues with short and fat rockets. I saw discussions of using a base drag hack in Open Rocket to trick the program into providing more reliable numbers.

I have found some videos that talk about using them in Rocksim 8, but I'm not sure if those hacks are usable in Rocksim 10?

The only changes I have made from stock kit are to cut down the nosecone shoulder to 1.125, put an 1/8" plywood bulkhead and bolted in eyelet inside the nosecone, an 21" nylon parachute and 1/4" cotton elastic shockcord (with a short piece of kevlar inside the body tube).

 

[DigBaddy]

What is "awfully low?"

With the mods you're making, there won't be any concerns about stability with any 24mm motor. Send it.

 

[mhwillis]

Full disclosure-I think this is my first post, and I know less than a lot of people, so I'm foolish for taking on such a contentious subject!

First, if you are using a kit, and are using engines that the kit recommends, you should be fine. Especially if you are adding weight to the nose cone, which I think you are doing with the bulk head and eye.

I believe that the rocksim equations essentially use the "base drag hack", since I can get OR and RS to match very closely for my simple rockets when I add the base drag hack in OR. I personally trust the Rocksim Cp calculation. It will show more stable than the Barrowman equations. Also, while I like to have 1.5 calipers of stability, for a relatively fat and short rocket, it just isn't going to happen. It is not really needed anyway. For example, right now I'm doing a LOC Mega Magg. The stability margin with the a K1275 motor is like .85. That is with 2 pounds of weight in the nose cone. That is going to have to be good enough. The key will be to use a high thrust motor so that, as the rocket leaves the rod, it is going much faster than any wind coming from the side. That way the angle of attack stays small, and will keep going staight. Good luck!

 

[DaHabes]

The "hack" is usable in Rocksim, just as in OR, if you are talking about adding a transition at the ass-end of the rocket design with a front diameter of zero (OK, .0001 of whatever ruler you are using), a rear diameter = to the rocket diameter and a length of pi*d. I use it all the time.

 

[Gus]

The base drag hack is absolutely accurate in Rocksim 10.

My daughter and I just won the team World Championship in FAI S1B altitude using Rocksim 10 and the Base Drag hack extensively. Our altitude sutainers pushed stability to truly ridiculous margins and, if anything, the Base Drag Hack still slightly underestimated stability for our 4:1 models.

The articles and technique described on Apogee's website were written by the brilliant, and very much missed, Bruce Levinson, Phd. Bruce was the first true uber-expert on Rocksim and in the early days of the program came up with many of the techniques that were later incorporated into the program itself. Bruce was one of the very first of our fellow rocketeers we lost to Covid. Bruce made a huge contribution to so much of what we now take for granted in model and high power rocketry simulation.

As I said, you can absolutely believe what you find in the Base Drag Hack technique. The nice thing is that with small short stubby rockets, like the Big Daddy, you can verify the data with a swing test. Emma and i also used that technique extensively in creating out S1B models.

Steve

 

[Bill S]

Dig Baddy, I'm seeing stability numbers of .99 with an D12-5, and .91 with an E12-6. I'm using estimated weights for glue and paint, since I haven't built the kit yet.

I haven't yet glued in the nosecone bulkhead just in case some weight were to be needed, but we'll see.

 

[DigBaddy]

Dig Baddy, I'm seeing stability numbers of .99 with an D12-5, and .91 with an E12-6. I'm using estimated weights for glue and paint, since I haven't built the kit yet.

I haven't yet glued in the nosecone bulkhead just in case some weight were to be needed, but we'll see.

oh yeah, you're fine!

 

[Buckeye]

Start at this post for some discussions on the accuracy/need for the "hack." It does not necessarily make CP more accurate, but rather artificially moves it into the 1 caliber comfort zone on stable designs. In my view, it is unnecessary, especially if you measure stability margin in terms of % length rather than calibers.

https://www.rocketryforum.com/threads/zephyr-build-open-rocket-concerns.181659/post-2475946

 

[JoePfeiffer]

My understanding (I don't have a copy) is that RockSim 10 can use either the Barrowman equations or what it calls "RockSim improved Barrowman" in its stability calculations. My impression is that "improved" incorporates base drag, though I don't have confirmation of that. So I'd be leery of using the hack unless I was using straight Barrowman.

@Buckeye 's reading of the base drag hack article doesn't correspond to mine -- to me it's pretty clear that accounting for an effect Barrowman doesn't is making the results more accurate. If you're curious I'd suggest reading it; I think the thread he referenced has a link to it.

 

[Bill S]

I always use the Rocksim stability calculations, not Barrowman.

 

[kalsow]

My understanding (I don't have a copy) is that RockSim 10 can use either the Barrowman equations or what it calls "RockSim improved Barrowman" in its stability calculations. My impression is that "improved" incorporates base drag, though I don't have confirmation of that. So I'd be leery of using the hack unless I was using straight Barrowman.

The RockSim enhancements to standard Barrowman equations deal primarily with the fins: shape, placement on boattails, interference, etc. The "base drag hack" is something else. Lots of this stuff is documented on Apogee's web site.


"RockSim versus the Barrowman Equations":
https://www.apogeerockets.com/education/downloads/Newsletter238.pdf

"Determining Rocket Base Drag"
https://www.apogeerockets.com/education/downloads/Newsletter103_sm.pdf
"Implementing Base Drag CP Correction"
https://openrocket.info/tutorials/base-drag.html

 

[Capt. Eric]

Don't know about the sims, but I've built 2 Big Daddy's. The first according to the directions. It flew ok usually, but was always tough to get the nylon chute I used into it, especially with a JLCR. It finally took a little turn at liftoff into the wind and staying too low, crashed before ejection. So for it's rebuild, I shortened the shoulder an inch or so, and added a little nose weight. Flies much better than the original. So I think you're on the right track doing what you've said.

 

[Alan15578]

By many reports the hack is useful. My objection is the name "base drag". I'd feel better if it was called Dr. Levinson's hack.

 

[neil_w]

By many reports the hack is useful. My objection is the name "base drag". I'd feel better if it was called Dr. Levinson's hack.

I advocate calling it "Base Drag CP Correction", because that's exactly what it is.
https://openrocket.info/tutorials/base-drag.html

 

[JoePfeiffer]

The RockSim enhancements to standard Barrowman equations deal primarily with the fins: shape, placement on boattails, interference, etc. The "base drag hack" is something else. Lots of this stuff is documented on Apogee's web site.

"RockSim versus the Barrowman Equations":
https://www.apogeerockets.com/education/downloads/Newsletter238.pdf

That was written in 2009. I hope a lot has happened since then.

"Determining Rocket Base Drag"
https://www.apogeerockets.com/education/downloads/Newsletter103_sm.pdf
"Implementing Base Drag CP Correction"
https://openrocket.info/tutorials/base-drag.html

That last one isn't on Apogee's site, it's ours!

 

[JoePfeiffer]

I advocate calling it "Base Drag CP Correction", because that's exactly what it is.
https://openrocket.info/tutorials/base-drag.html

When we get it into OR directly calculating the normal forces from base drag I'll agree with you. As long as we're having the user add a cone after the body tube to get those forces it's a hack.

 

[kalsow]

That was written in 2009. I hope a lot has happened since then.

That last one isn't on Apogee's site, it's ours!

Indeed!

 

[neil_w]

When we get it into OR directly calculating the normal forces from base drag I'll agree with you. As long as we're having the user add a cone after the body tube to get those forces it's a hack.

Well, you are free to add the word “hack” to the end of it if you like. I prefer to save the syllable.

The reason I advocate that wording is because “base drag hack” has in the past left some folks confused about what it was accomplishing. Specifically, it led some to think that the hack was somehow adding missing base drag, not realizing that OR already calculates base drag. The purpose of the hack is CP correction, and nothing more.

 

[Bill S]

Guys, I am asking about Rocksim, not Open Rocket.

 

[jmasterj]

What's your reason for paying for Rocksim over using Open rocket?

 

[JoePfeiffer]

Guys, I am asking about Rocksim, not Open Rocket.

The hack is applied the same way in both (using Barrowman equations in RS, at least).

 

[Buckeye]

The RockSim enhancements to standard Barrowman equations deal primarily with the fins: shape, placement on boattails, interference, etc. The "base drag hack" is something else. Lots of this stuff is documented on Apogee's web site.


"RockSim versus the Barrowman Equations":
https://www.apogeerockets.com/education/downloads/Newsletter238.pdf

Correct.

That was written in 2009. I hope a lot has happened since then.

I found no mention of a base drag correction for CP in the Rocksim version history.

https://www.apogeerockets.com/RockSim/Version_History
BTW, here is the original R&D paper on the "RockSim Method." It's all about free-form fins.

https://www.apogeerockets.com/downloads/Technical_Publications/Tech_Pub_17.pdf

 

[Buckeye]

Dig Baddy, I'm seeing stability numbers of .99 with an D12-5, and .91 with an E12-6. I'm using estimated weights for glue and paint, since I haven't built the kit yet.

I haven't yet glued in the nosecone bulkhead just in case some weight were to be needed, but we'll see.

Bear in mind, the 1.0 caliber stability margin is merely a rule of thumb. Some rockets need more, some (stubby rockets) can do with less. The original goal of the Levison method was to make the calculations for short stubbies agree with the rule of thumb. It was a clever mathematical implementation, but the rationale was a bit odd, imo.

https://www.apogeerockets.com/education/downloads/Newsletter154.pdf
https://www.apogeerockets.com/education/downloads/Newsletter158.pdf

If you use % length (8% to 15% is deemed a good range for stability) instead of calibers, you won't get freaked out by a 0.99 margin.

https://www.rocketryforum.com/threads/question-about-or-23-09.183715/

 

[Alan15578]

Well, you are free to add the word “hack” to the end of it if you like. I prefer to save the syllable.

The reason I advocate that wording is because “base drag hack” has in the past left some folks confused about what it was accomplishing. Specifically, it led some to think that the hack was somehow adding missing base drag, not realizing that OR already calculates base drag. The purpose of the hack is CP correction, and nothing more.

Then I would prefer "base cone hack", or "base cone CP hack".

 

[Bill S]

What's your reason for paying for Rocksim over using Open rocket?

At the time, I needed pod support for some rockets I was designing, and OR didn't have that. RS also seemed like a more refined program. Little did I know that it would be "updated" into a bug-fest which turned into a hassle (v10 of the program).

 

[jmasterj]

Sounds like it's time to switch sim software!