First Open Rocket Sim: Where Did I Go Right/Wrong, How Can I Do Better?

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Asked around on Reddit as a neophyte rocketeer what my first steps should be as someone who's read through "The Handbook of Model Rocketry" and picked up an Alpha III. They suggested I take each component, weigh it and put it together in OpenRocket. Did as suggested, here are the results:

antares first run.png

antares model.png


As I'm used to looking at audio spectrum analyzers instead of rocket results, I'm not entirely sure what went right/wrong (outside of that it took off). I ran the test using the default simulation settings. From the box it looks like it should be going 10 meters higher, but I'm not sure if I'm under that much because of something I can improve or if it's because what's written on the box is it's optimal performance with no wind, no turbulence, etc.

Can't wait for you feedback. Also would love to know if I'm missing anything component-wise: As this is my first time putting a rocket together in OpenRocket.

Thank you so much in advance :)

File attached to thread.
 

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TRFfan

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Asked around on Reddit as a neophyte rocketeer what my first steps should be as someone who's read through "The Handbook of Model Rocketry" and picked up an Antares III. They suggested I take each component, weigh it and put it together in OpenRocket. Did as suggested, here are the results:

View attachment 301021

View attachment 301022


As I'm used to looking at audio spectrum analyzers instead of rocket results, I'm not entirely sure what went right/wrong (outside of that it took off). I ran the test using the default simulation settings. From the box it looks like it should be going 10 meters higher, but I'm not sure if I'm under that much because of something I can improve or if it's because what's written on the box is it's optimal performance with no wind, no turbulence, etc.

Can't wait for you feedback. Also would love to know if I'm missing anything component-wise: As this is my first time putting a rocket together in OpenRocket.

Thank you so much in advance :)
Hi,

Welcome to the forum. In response to your question I do think that you did do everything right in openrocket, however most of the time the results on the box do differ with the results in sims, due to Estes using different methods to find the altitude.
 

K'Tesh

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First thing, you'll need to upload the file itself so we can look it over.

Next, default settings for nosecones are rarely good. If you look at a PNC-60AH nosecone in OR, it will look nothing like what the actual nosecone.

I have a nosecone library that may be able to help you.
 
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First thing, you'll need to upload the file itself so we can look it over.

Next, default settings for nosecones are rarely good. If you look at a PNC-60AH nosecone in OR, it will look nothing like what the actual nosecone.

I have a nosecone library that may be able to help you.
Updated the description with a Zippyshare link to the file. Will give the library a lookover, much appreciated!
 

neil_w

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I do not feel inclined to download that file, nor any other file for that matter, from Zippyshare. Can you just attach it to a posting here?

In general, there are a lot of variables in an ORK file that can cause variations in the sim, and since Estes doesn't use OR to produce its altitude estimates (as far as I know), you're bound to get some discrepancy. If you're within 15% you're doing OK.

Anyway, if you post the ORK file here I'll take a look.
 

Rex R

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have to agree with neil, post it to the forum after you run an av scan, zippy appears to be infested. (not to mention that it is not a family friendly site).
Rex
 

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A couple useful tips: measure the length of each component as close as you can, especially the exact shape & width of the fins. Start first with the component presets, then adjust length and position. If you're not going to weigh components, make sure you have the correct materials selected. Also use a reasonable surface finish. I use "regular paint 60µm". If the rocket isn't built yet, get real weights of the components and override, if it's already built then just override the total weight and CG.

After all this though, sims should be relatively close although real world results WILL differ. I've compared altimeter readings with sims and get +/- 10% difference.
 
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have to agree with neil, post it to the forum after you run an av scan, zippy appears to be infested. (not to mention that it is not a family friendly site).
Rex
Weird: Music production forum I'm on uses it all the time to send demos around, will pass the word on to them.

Any idea how I upload it directly to the thread? All I'm seeing is the link/embed photo/video options.
 

K'Tesh

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Weird: Music production forum I'm on uses it all the time to send demos around, will pass the word on to them.

Any idea how I upload it directly to the thread? All I'm seeing is the link/embed photo/video options.
click "Go Advanced"
scroll down till yyou see "Manage Attachments" click that
 
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Rex R

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my AV program was screaming 'unclean' (also unsafe download, takes some doing when the file size is less than 3kb). but yes use the manage attachments button to upload photos, design files etc.
Rex
 

Rex R

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well, in general you have the basics down...however I trust you noticed that getting the motor in and back out(well no back out is easy, just light the motor :)) is going to be tricky as your engine block is on the wrong end of the tube...and the mount is recessed by about 1/2" which puts the motor roughly 3/4" inside a 1"(roughly) tube. Estes tubes are closer to being 'office paper' for material density. I suspect that either your weights may be off or your measured cg is off(quite likely your nose cone cg is off, if the weight includes any clay). missing is a shock cord and launch lug.
open rocket does have a parts database for many common parts(fin units and likely not the plastic nose cone). for comparing your efforts to what others have done rocketreviews.com does have a library of open rocket/rocksim files. note open rocket can import rocksim files.
Rex
 

neil_w

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OK, you're #1 culprit there is the fins. You've got the default thickness in there (0.118", who knows why OR chooses that) which is almost guaranteed thicker than the actual fins. Also, you have the profile as "square". Both of those together mean the sim is calculating a lot more drag than on the actual rocket, more than enough to account for 10m of altitude.

General comments on the file:
It looks like you have some "approximate" measurements in there, which I especially noticed in the body tubes: you have 0.9" OD and 0.8" ID, which are fairly far off the actual values (.976" OD/ .95 ID). If you select your components from the database, you can get accurate dimensions without much effort. Nose cones may not always be perfectly accurate, but body tubes generally are. Or you can use this handy table that shows Estes body tube dimensions. More accurate dimensions = more accurate sim. Your materials are all over the place as well (the NC says "cardboard"); although you're overriding the weights it's still better IMHO to try to get the basics correct so it's as close as possible before you start overriding.

You're missing the shock cord. The stock Estes rubber bands are pretty light (and short), but as you start beefing them up their weight becomes enough to matter in the sim.

The motor mount and motor placement looks wrong, they should not be recessed like that. Generally the motor will stick out from the back a bit, although I don't know the exact positioning for that kit.

Hope this helps!

[ninja'ed by Rex!]
 

Rex R

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as I recall the alpha has 1/16" balsa, I suspect that the alpha 3's plastic fins are similarly thick.
Rex
 
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Made those tweaks, man Neil you were right about the Apogee jumping up MUCH higher when I adjusted the fins to rounded and turned down the thickness a bit. Good to know what tweaks I can make when I start making my own.

And Rex I read the description of the engine block: Don't know how I got the impression that it was supposed to go on the end xD

Mad props to you both. Gonna record a video of me building it and shooting it off today, fingers crossed!
 

Rex R

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well we all make mistakes :). anyway I put together a 'quicky' file showing roughly how one might rebuild an alpha3 after the fin(s) broke and if one wanted to reuse the airframe north of the 'fincan'. I did have to make a couple of assumptions 1) had to guess where the cg of the nose cone and 2) another guess on the fin mass since I routinely paper my fins and that adds a bit of weight(if you want the individual fin weight, divide the mass over ride by 3). hope this helps your understanding
Rex

View attachment alpha 3 real.ork
 
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