What information do you use from OpenRocket simulations?

cecil · Nov 22, 2014

First try at a simulation. Given:

Looking at the projections for each engine, of those simulated the D 12 with 5 sec delay appears to put the least stress on the rocket and still allows the rocket to rise to the highest apogee of the engines tested. Right?

I keep getting the message "Discontinuity in the rocket body diameter." I don't see any "discontinuity" in the body diameter. What does OR mean?

Now, what are some other results that you experienced users find most helpful from OR?

Rex R · Nov 22, 2014

is this instance it means your nose cone size(shoulder iirc) is a bit off, and the flight sim might be a foot or two off. I wouldn't worry about it. the two important bits are speed off the rod and deployment speed. the rest is pretty much gravy, I always round the apogee to the nearest 100, I figure that is close enough for my purposes.
I have found that people have problems with the simple concepts, we all try to over complicate matters

.
Rex

rharshberger · Nov 22, 2014

I use the plotting features quit a bit, especially when 2 staging rockets since it gives a rough idea of when to ignite the second stage. It also allows me (once I figured out how to read them) to understand the flight characteristics a bit better. One of the neat features is the ability to adjust the simulations data for your area and launch equipment, Long/Lat, wind velocity (I set mine at 15 mph which is real common for Eastern Washington), launch rod length, lots of good stuff.

molten_dragon · Nov 22, 2014

I used it to figure out whether my first couple rockets would be stable without adding nosecone weight, and to figure out delay times for motor deployment. And to get a rough idea of altitude.

rocketgeek101 · Nov 22, 2014

molten_dragon said:
I used it to figure out whether my first couple rockets would be stable without adding nosecone weight, and to figure out delay times for motor deployment. And to get a rough idea of altitude.

+1 to that.

Sent from my iPad using Rocketry Forum

ThirstyBarbarian · Nov 22, 2014

I use the Optimim Delay column to help pick my delays.

Look at the Optimum Delay column for your sims for the D12-3, D12-5, and D12-7. They are all about the same within a fraction of a second, right around 4.2 seconds. That means that the perfect delay for a D12 motor in this rocket is 4.2 seconds. The motor is not sold in that particular delay, so you need to pick from what is available. You can see that 3 seconds is more than a second early, so the deployment happens when the rocket is still coasting upward, and that's why the apogee is a bit lower and deployment speed is high. The 7-second delay is long by almost 3 seconds, so the rocket is already falling down pretty fast at deployment. The 5-second delay is your best option. The deployment is less than one second after apogee, and the rocket is moving at a safe speed.

The first two sims you have are for motors with zero delay. Those are booster motors for 2-stage rockets and won't work in your single-stage rocket. As soon as the motor burns out, the chute will deploy when the rocket is moving upward at its fastest velocity. That's why the sims show such low apogees and fast deployment speeds. To see what a C11 motor will do in this rocket, change the C11-0 motor to a C11-3. You can just delete the sim for D12-0 --- you don't need that one.

It looks like your motor tube is long enough for E motors, assuming there is no motor hook limiting the length to D motors. You might want to do sims for an E9-6 and an E12-6. If you are feeling adventurous, try an E20-7!

gna · Nov 22, 2014

I play around with it to get an idea of stability but also performance. I use the Flight Configuration and Optimum delay column to help me pick the correct motor, or in most cases, check if a motor I have will work.

I have a Semroc Vega, and the kit instructions say to fly it on B6-4 or C6-5 motors. I didn't have either motor, and the family CFO wasn't about to allow me to buy any, but I had a pack of B4-4s. I found a Roc Sim for the Vega on Rocket Reviews, opened it in Open Rocket, and chose a simulation with a B4-4:

It looked good, so I went ahead and launched it:
[video=youtube_share;zCiis77SAJE]https://youtu.be/zCiis77SAJE[/video]

I also used it to check a modified build, and used the 3D finishing to help decide how to paint it:

It looks to be too stable, so it will probably windcock, and I'll keep that it in mind.

cecil · Nov 22, 2014

Rex R said:
is this instance it means your nose cone size(shoulder iirc) is a bit off, and the flight sim might be a foot or two off. I wouldn't worry about it. the two important bits are speed off the rod and deployment speed. the rest is pretty much gravy, I always round the apogee to the nearest 100, I figure that is close enough for my purposes.
I have found that people have problems with the simple concepts, we all try to over complicate matters .
Rex

It is a rather ugly nose cone. The dynamics of nose cone shape must be very interesting and no doubt complicated. What speed off the rod or rail do you consider ideal?

What is "shoulder iirc"?

Thanks again for coming to the rescue.

cecil · Nov 22, 2014

rharshberger said:
I use the plotting features quit a bit, especially when 2 staging rockets since it gives a rough idea of when to ignite the second stage. It also allows me (once I figured out how to read them) to understand the flight characteristics a bit better. One of the neat features is the ability to adjust the simulations data for your area and launch equipment, Long/Lat, wind velocity (I set mine at 15 mph which is real common for Eastern Washington), launch rod length, lots of good stuff.

All that makes sense and obviously you've found a pretty good match between OR and actual launches. Good to hear.

cecil · Nov 22, 2014

rocketgeek101 said:
+1 to that.

Sent from my iPad using Rocketry Forum

Thanks Molten_Dragon and rocketgeek for confirming the basic uses of OR. RG, you've got some great shots posted. I really envy those wide open, level spaces.

cecil · Nov 22, 2014

>>The first two sims you have are for motors with zero delay. Those are booster motors for 2-stage rockets and won't work in your single-stage rocket. As soon as the motor burns out, the chute will deploy when the rocket is moving upward at its fastest velocity. That's why the sims show such low apogees and fast deployment speeds. To see what a C11 motor will do in this rocket, change the C11-0 motor to a C11-3. You can just delete the sim for D12-0 --- you don't need that one.

It looks like your motor tube is long enough for E motors, assuming there is no motor hook limiting the length to D motors. You might want to do sims for an E9-6 and an E12-6. If you are feeling adventurous, try an E20-7!

I expect to be surprised with your suggestions. Thanks for the analysis, too.

cecil · Nov 22, 2014

Is the "too" stable a function of the fins? Too large, perhaps, putting a torque on the rocket from the wind?

Really enjoyed the video of you and your daughter. What a way to bond, learn, and have fun. I don't understand why more fathers don't rear their children this way. :clap:

gna · Nov 22, 2014

cecil said:
View attachment 247236

Is the "too" stable a function of the fins? Too large, perhaps, putting a torque on the rocket from the wind?

Really enjoyed the video of you and your daughter. What a way to bond, learn, and have fun. I don't understand why more fathers don't rear their children this way. :clap:

That rocket is what I dubbed "Murphy's Rocket" in a different thread. It's missing a few inches of length because of my incompetence. Yes, the fins are probably too big, so the wind will catch it and it will act like a weathervane. I used the simple rocket feature that came with OR, and modified it till it matched my rocket. I'll have to see how close the balance point is to reality when I get to launch again.

Yeah, my daughter and I have fun with rockets. Unlike many of the guys here, I wasn't really into rockets as a kid. I built two in summer camps. I was mostly interested in airplanes then. Anyway, my daughter had rocket camp for a week this past summer, and her Estes Wizard got damaged. I helped her rebuild it, and I built a launchpad out of an old camera tripod that I trashpicked and some odds and ends. We lost her Wizard to some trees, but we've built more rockets and had a good time. I hope I can keep her interested as she gets older.

cecil · Nov 22, 2014

gna said:
That rocket is what I dubbed "Murphy's Rocket" in a different thread. It's missing a few inches of length because of my incompetence. Yes, the fins are probably too big, so the wind will catch it and it will act like a weathervane. I used the simple rocket feature that came with OR, and modified it till it matched my rocket. I'll have to see how close the balance point is to reality when I get to launch again.

Yeah, my daughter and I have fun with rockets. Unlike many of the guys here, I wasn't really into rockets as a kid. I built two in summer camps. I was mostly interested in airplanes then. Anyway, my daughter had rocket camp for a week this past summer, and her Estes Wizard got damaged. I helped her rebuild it, and I built a launchpad out of an old camera tripod that I trashpicked and some odds and ends. We lost her Wizard to some trees, but we've built more rockets and had a good time. I hope I can keep her interested as she gets older.

You're definitely on the right track with your daughter. Curious, when I was in school no one but me was interested in rockets, everyone in planes. Creative use of a camera tripod; might be a bit top heavy. I wonder whether the legs could be anchored for safety?

I was thinking the other day how it isn't so strange to start learning by copying someone else's model, more or less. That's exactly what we did first after we got von Braun into the country, if my history is correct.

gna · Nov 23, 2014

The tripod has the legs out really far; it's stable. At least for these little rockets. It's funny, when I was building the Vega I thought it was really big, but one trip to a good hobby shop corrected that notion. I showed that video at work and one of my coworkers has a big launch pad his dad built and offered to give it to me.

I thought it was interesting that while we were in the alley painting the Vega the only neighbors who were interested in what we were doing were women! Several told me about building rockets in 4H, or in school, and quite a few wanted to come launch with us sometime.

What I'm curious about is whether the weights OR gives are accurate, and if the thicknesses for materials are correct. I'm not sure if the plastic nosecone on this rocket is really as heavy as it seems. I'll need to get a scale.

I experimented in clipping the fins on this rocket, and it seems to be stable and flies even better:

I may clip the fins a bit and touch up the paint.

Rex R · Nov 23, 2014

the given weights will get you in the ballpark, some times fairly close. you do have to pick the right materials though, body tubes are paper not cardboard. keep in min the variations humidity, manufacturing, growth etc. will cause differences in weight, don't forget that OR doesn't include glue or paint in its' estimate.
Rex

K'Tesh · Nov 23, 2014

cecil said:
It is a rather ugly nose cone. The dynamics of nose cone shape must be very interesting and no doubt complicated. What speed off the rod or rail do you consider ideal?

What is "shoulder iirc"?

Thanks again for coming to the rescue.

IIRC = "If I Remember Correctly"

As to the nose cone issue. I've created a library of common nosecones and a tutorial on how to create more accurate ones yourself. I'd suggest you download the correct nosecone for your build, then build the rocket from there.

ThirstyBarbarian · Nov 23, 2014

Rex R said:
the given weights will get you in the ballpark, some times fairly close. you do have to pick the right materials though, body tubes are paper not cardboard. keep in min the variations humidity, manufacturing, growth etc. will cause differences in weight, don't forget that OR doesn't include glue or paint in its' estimate.
Rex

Rex is right. If you are using OR to DESIGN a rocket, then the materials and densities in OR will give you an aproximate estimate of the final weight and center of gravity (CG). And OR can give a good estimate of the center of pressure (CP) based on the shape of the rocket, not the materials, and use the CP and CG to calculate the rockets's stability.

You can get even better estimates for the design if you can measure the components of the rocket and enter mass overrides for each component. For example, if you weigh the nosecone and enter a mass override for the nosecone component, then OR will use the override and make a more accurate estimate of the weight, CG and stability of the final rocket.

So, that is all related to the DESIGN. Once the rocket is built, glued together, and fully painted, the best way to get accurate FLIGHT SIMULATIONS is to weigh the entire rocket, minus the motor. And then find the CG of the entire rocket, minus motor, by finding the point where the rocket balances. And then do a mass and CG override of the entire rocket. OR will use those overrides, plus the known masses of commercial motors that you pick for your simulations, and the CP it calculates from the shape of the design to calculate the final stability of the rocket and give pretty good flight simulations.

So individual component weights and material densities are important mostly for design if you are making a scratch built rocket or modifying a kit. For flight sims, that doesn't matter much, and it's the final mass and CG that are important, along with accurate shapes and sizes of components.

cecil · Nov 23, 2014

K'Tesh said:
IIRC = "If I Remember Correctly"

As to the nose cone issue. I've created a library of common nosecones and a tutorial on how to create more accurate ones yourself. I'd suggest you download the correct nosecone for your build, then build the rocket from there.

Thanks for the translation, one of which I'm often in need. The nosecone suggestions should be helpful,too.

cecil · Nov 23, 2014

ThirstyBarbarian said:
Rex is right. If you are using OR to DESIGN a rocket, then the materials and densities in OR will give you an aproximate estimate of the final weight and center of gravity (CG). And OR can give a good estimate of the center of pressure (CP) based on the shape of the rocket, not the materials, and use the CP and CG to calculate the rockets's stability.

You can get even better estimates for the design if you can measure the components of the rocket and enter mass overrides for each component. For example, if you weigh the nosecone and enter a mass override for the nosecone component, then OR will use the override and make a more accurate estimate of the weight, CG and stability of the final rocket.

So, that is all related to the DESIGN. Once the rocket is built, glued together, and fully painted, the best way to get accurate FLIGHT SIMULATIONS is to weigh the entire rocket, minus the motor. And then find the CG of the entire rocket, minus motor, by finding the point where the rocket balances. And then do a mass and CG override of the entire rocket. OR will use those overrides, plus the known masses of commercial motors that you pick for your simulations, and the CP it calculates from the shape of the design to calculate the final stability of the rocket and give pretty good flight simulations.

So individual component weights and material densities are important mostly for design if you are making a scratch built rocket or modifying a kit. For flight sims, that doesn't matter much, and it's the final mass and CG that are important, along with accurate shapes and sizes of components.

Good suggestions. If it's going to be done, it should be done right.

What information do you use from OpenRocket simulations?

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