OpenRocket software questions

[JohnCoker]

Looks like you are up and running. I put together documentation on the RASP format in case you want to know the details:
thrustcurve.org/raspformat.shtml

TcTracer just saves some arithmetic if all you have is an image of the thrust curve. (In the past, we used to have only printed manufacturer's thrust curves, but lately we tend to get data directly from the certification organizations so it's less often necessary to work from an image.)

Welcome to the hobby!

 

[John Ross]

I LOVE this program!

I especially like how you can watch onscreen as the CG and CP locations shift as you're altering fin dimensions.

When I think I have it right, I can select a commercially available motor, find one that gives an appropriate apogee, run a simulation, and see what delay increment I need. Then I can build a motor that matches those specs. This is going to take so much trial and error out of development I can't believe it!

Now I have another question, but first a little background on my launch site and testing procedures:

Other than the PGI Convention, I launch all my rockets on my own property on the bank of the Mississippi River. It is over 1/2 mile wide at that point.

Second, if I've built a motor to lift a size/weight of shell I've not done before, I make up a dummy to the same dimensions, filled with the appropriate amount of bentonite clay cat litter, and observe the results.

Third, I launch at an angle that insures that the rocket will always be over the water. Displays are at dusk or later, when there is never more than a light breeze. Testing is on days where the forecast predicts winds of less than 5mph.

For safety, I launch out of HDPE pipe that is larger than the outer fin diameter of the rocket. If a motor CATOs, the HDPE pipe contains it without damage.

The HDPE pipe I use for my largest rockets is 6" ID and 7 feet long. Here's my question: What should I shoot for as a stability factor when designing a rocket, or minimum velocity when the back of the fins exit the launch pipe (or what have you) in a simulation, to ensure a good flight?

Attached is an OR file for a rocket carrying a payload of 3 pounds of cat litter. I'd like an apogee of 350 to 450 feet. Simulation (using the default settings) with an Aerotech G55-5 motor shows stability 1.64 cal., velocity off rod 24.9 FPS, altitude 433 feet, optimum delay 3.7 seconds.

Is stability of 1.64 cal too low for safety, much higher than needed, or what?

Comments?

JR

 

[neil_w]

Stability is probably fine, although again with the weirdness of that rocket it's possible that the OR CP calculations are not totally accurate.

We would not normally fly with 25 fps off the rod; 50 fps is a common target, but a bit lower can be OK in calm conditions.

 

[Kelly]

A couple things on thrust curves:
1) I usually load the data from my test stand into a spreadsheet, then it's easy to do some simple manipulation and dump it out as a thrust curve that OpenRocket can use.
2) I've noticed that, once I've put a thrust curve where OpenRocket can see it, I need to restart OR (if it's already open). It seems as though OR only looks for thrust curves when you start it up.

Welcome to the forum!

 

[John Ross]

Stability is probably fine, although again with the weirdness of that rocket it's possible that the OR CP calculations are not totally accurate.

We would not normally fly with 25 fps off the rod; 50 fps is a common target, but a bit lower can be OK in calm conditions.

Changing rod length to 84" gets 35 fps off the rod with the G55, and changing to the slightly lower total impulse G80 motor with higher peak thrust gives 44 fps, and virtually the same apogee.

 

[John Ross]

I usually load the data from my test stand into a spreadsheet, then it's easy to do some simple manipulation and dump it out as a thrust curve that OpenRocket can use.

My test stand was built by rocket aficionado Pete Hand and outputs the data in .csv (comma separated values) file format. The software that comes with the test stand will display not only thrust curves but estimated flight curves for that motor based on rocket weight at launch and heading size and type, ball or cylinder.

Here's the manual, if you are interested.

JR

 

[Kelly]

My test stand was built by rocket aficionado Pete Hand and outputs the data in .csv

Well, a thrust curve for OR is basically a space-separated-values file of time and thrust, with a header, so if you've got CSV, you're most of the way there. Here is the thrust curve (.ENG) file format:

https://www.thrustcurve.org/raspformat.shtml

 

[John Ross]

NEW PROBLEM with OR:

Today, when I open my design files and choose "Motors & Configuration," then select a motor, it shows "N/A" for apogee, whereas before it always showed an altitude number. If I run a simulation with that motor (ANY motor), I get error messages of "Motor data out of date, run simulation." I run the simulation again, and the message says "no motors ignited."

Joking and sarcasm aren't always appreciated in printed form online, but I'll try anyway: "Boy, OpenRocket really simulates real life! Today it simulated that I went to my launch site alone, not realizing the batteries in my launch controller were dead."

What am I doing wrong today compared to yesterday?

JR

 

[neil_w]

Please post the latest version of the file.

 

[John Ross]

I downloaded and used the "packaged installer" from here after I discovered these problems, and it made no difference. Here's a screenshot of the contents of the "Downloads" folder on my computer:

 

[neil_w]

Please post the latest version of your ORK file.

 

[John Ross]

Just before you posted that, I went to Control Panel, uninstalled my Java stuff and OR, then downloaded the installer package and reran it. Works right again.

JR