RockSim Question- Short Fat Rockets

[vandy_club]

Hi all, first time poster...

We are using a modified 10'' thumper (https://www.polecataerospace.com/thumper_10.htm) for a university competition. We are trying to use RockSim to model our launches, but when we emailed Polecat to inquire whether they utilized RockSim in their design, they said that RockSim is very inaccurate when it comes to short, fat rockets. So, my question is twofold:

1) Why does RockSim not handle these type of rockets, and is there any way around it?

2) Given our 10'' diameter, and the ~3-4 ft. of height that we will be adding to the 7'6'' stock height, will this still be an issue for us?

Thanks to everyone for their help, cool forum!

 

[jj94]

Hey, welcome to the forum.

1. I don't know what the answer to this question is, but I'm sure that teflonrocketry (Bruce) will know that answer off the top of the head. He's extremely talented at Rocksim, an expert.

2. If your going to add 3-4 feet, I'm going to guess that Rocksim will be more accurate with the simulations, but still not extremely accurate.

 

[WillMarchant]

You should read issues 154, 158, and 162 at (https://www.apogeerockets.com/education/newsletter_archive.asp) on simulating short/fat rockets in RockSim.

 

[roadkill]

Use 'base drag correction', as described in those
apogee bulletins.

Its basically a transition with minimal front diameter like 0.0001",
rear diameter same as your max body tube diameter, and length
that is Pi x max diameter. Make it hollow with minimal thickness to make
it practically weightless (or use mass override).....

Simple as that and works pretty good...

 

[bobkrech]

Adding to the length won't be a problem. it will be more stable.

The normal issue with RockSim is in stability calculations for short stubby rockets where it tends to be conservative.

Bob

 

[jj94]

I tried out using the base correction drag method thing and I was wondering if I did it right on this Fat Boy. I have a question though, but it's not related to this - why is the altitude so high on the sim?

View attachment Fat Boy.rkt

 

[WillMarchant]

I tried out using the base correction drag method thing and I was wondering if I did it right on this Fat Boy. I have a question though, but it's not related to this - why is the altitude so high on the sim?

The Cd that got calculated is 0.3. I did a Cd override and set it at 0.75 and got 300 feet on the C6-3. Does that sound better?

 

[jj94]

Yes, it does. Now is there a trick in the book that can manipulate Rocksim for a more accurate Cd?

 

[WillMarchant]

Yes, it does. Now is there a trick in the book that can manipulate Rocksim for a more accurate Cd?

I don't know. :cry: I've always had to set the Cd to a "reasonable" number and then tune it based upon performance data. I've assumed it was nuances in selecting the "finish" options on the various components but I've never spent the time to try and figure it out.

I looked through the subject listing for the Apogee newsletter and didn't find an issue that appeared appropriate. I've sent Tim an email asking if there is one or if he could do one.
Best wishes,
Will

 

[WillMarchant]

Tim sent an email saying that improving the Cd estimation in RockSim is one their minds. He didn't mention any pointers to articles about improving our use of the current version of RockSim. So I think we just need to keep an eye on our simulations and use the manual Cd override when appropriate.
Best wishes,
Will

 

[jj94]

Tim sent an email saying that improving the Cd estimation in RockSim is one their minds. He didn't mention any pointers to articles about improving our use of the current version of RockSim. So I think we just need to keep an eye on our simulations and use the manual Cd override when appropriate.
Best wishes,
Will

Ok, that sounds fine. Thank you.