Open Rocket weight estimates

[Jacktango22]

I have been using open rocket for almost a year. Today, I added a scale to the desk where I keep my computer, and I compared the open rocket estimate of the rocket‘s weight to the actual measurement on the scale. There was an immense difference I can’t account for this. Anyone else see this before?

 

[Scott_650]

I have been using open rocket for almost a year. Today, I added a scale to the desk where I keep my computer, and I compared the open rocket estimate of the rocket‘s weight to the actual measurement on the scale. There was an immense difference I can’t account for this. Anyone else see this before?

OR uses a table of common materials to calculate weight that obviously didn’t match up with your real world weights. Doing an override to correct for weight is a recommended practice for any sim.

 

[neil_w]

Actual numbers and the ORK file would be helpful.

In general, there are a number of factors that contribute to errors in weight estimate:
1) Errors in creating the ORK file (e.g. using wrong materials, getting BT wall thickness wrong, etc.)
2) Variability of material density, particularly of wood parts
3) Additional materials in the final build that don't show up in the ORK model, including (but not necessarily limited to) paint and glue.

Usually, built models weight more than the estimates because of (3), but (1) and (2) can cause significant errors in either direction. But if (1) is taken care of, and all dimensions and materials are entered correctly, the weight should come out pretty close. Of course there is no substitute for overriding the total rocket mass and CG after it's built.

 

[bjphoenix]

In general, there are a number of factors that contribute to errors in weight estimate

And the center of gravity comes from the component weights, and stability analysis comes from center of gravity. So it is worthwhile to check final weight AND center of gravity, and enter overrides in the data to give the correct mass and center of gravity. For my most recent model I entered a small mass component to bring the total mass up to what I measured, and I move the mass component around until I get the correct CG location.

 

[bad_idea]

One huge contributor to phantom mass I've seen before has been hollow nose cones being assumed to be solid.

 

[aerostadt]

Depending on how much glue and epoxy is used can also make a difference in the predicted weight versus the measured weight . Over the years I have seen that my actual model weight is usually heavier than the predicted RocSim weight. As a result when I want an accurate predicted simulation of a model, I take the measured weight and the measured c.g. location and manipulate the RocSim model to mimic those actual measured conditions, then I use the RocSim predicted altitude, velocity, etc. In a pinch pay attention to the actual measured weight. If the measured weight is way higher than the predicted weight, this is a red flag not to fly until you have resolved the situation.

 

[SolarYellow]

I have put together a rocket where OR said the body tube and launch lug together should weigh 9 grams. Estes BT-60 and Estes standard launch lug. Painted, they weighed 19.4 grams. That's a lot of paint.

I've been listening to the Model Rocketry Show podcast and the n00b frequently mentions his studies where he found out he was building rockets that were more than half paint and glue by weight.

I do find that if I do a decent job measuring stuff and pick the right materials, the finished weights and c.g. without paint comes in really close.

 

[H. Craig Miller]

Ultimately, every build needs to have some adjustments made in OpenRocket to simulate more accurately. The OpenRocket Wiki may be helpful in doing so: Using Overrides and Surface Finish to Create More Accurate OpenRocket Simulations

 

[Jacktango22]

Thank you all for comments. Hoping for more of them. I will use inputs to refine my design and built process. I am recently retired and am getting back into this hobby (addiction) and plan to design and build a Level one rocket in 2023.

 

[lakeroadster]

Thank you all for comments. Hoping for more of them. I will use inputs to refine my design and built process. I am recently retired and am getting back into this hobby (addiction) and plan to design and build a Level one rocket in 2023.

Of course there is no substitute for overriding the total rocket mass and CG after it's built.

Well, actually there is a substitute. Track the variations and update the Open Rocket model while you build the rocket. Allow me to explain:

Before I build the rocket I create an initial model of the rocket in Open Rocket.

Then I create an "As Built" Open Rocket model. As I build the rocket, I weight each component, and then over-ride the weight of each component in Open Rocket. For anything that is glued, I'll weigh the sub-ass'y., after it is glued and the glue is dry, I then add a mass object, called "wood glue" to adjust the weight of the simulation to accurately reflect the as built component.

Since I mostly scratch building odd rocs, keeping an eye on stability as the build continues is a good way to make sure the build doesn't become unstable to the point that the rocket needs modified.

I personally don't like waiting until the rocket is built, and then just over-riding the cg, especially on a complex rocket. That can produce inaccurate pitch, roll and yaw rates. Most folks don't even look at that data in Open Rockets "Export" function... but I do.

 

[BigMacDaddy]

Is there a way to set default materials for certain parts so that it starts at the material you usually use? Maybe this is obvious but I have just not delved into the setup that much.

I noticed that many if not all parts default to cardboard but it would be great if nosecones and transitions defaulted to styrene (which is also comparable in weight to ABS 3D printed parts) and if fins defaulted to balsa (or better yet for me, birch plywood).

 

[lakeroadster]

Is there a way to set default materials for certain parts so that it starts at the material you usually use? Maybe this is obvious but I have just not delved into the setup that much.

I noticed that many if not all parts default to cardboard but it would be great if nosecones and transitions defaulted to styrene (which is also comparable in weight to ABS 3D printed parts) and if fins defaulted to balsa (or better yet for me, birch plywood).

Sure.

Create a Point of Departure, POD, file with a very basic rocket that utilizes components made of the materials you want. Save that file. Now when you want to create a new rocket, always start with that POD file.

Want to add a fin? Copy and paste the fin in the model, and its materials property will remain.

 

[neil_w]

Is there a way to set default materials for certain parts so that it starts at the material you usually use? Maybe this is obvious but I have just not delved into the setup that much.

I noticed that many if not all parts default to cardboard but it would be great if nosecones and transitions defaulted to styrene (which is also comparable in weight to ABS 3D printed parts) and if fins defaulted to balsa (or better yet for me, birch plywood).

The existing defaults annoy me constantly. Will look into doing something to improve it in the next release.

In the meantime, @lakeroadster 's suggestion above is one way to work around it.

 

[JoePfeiffer]

And the center of gravity comes from the component weights, and stability analysis comes from center of gravity. So it is worthwhile to check final weight AND center of gravity, and enter overrides in the data to give the correct mass and center of gravity. For my most recent model I entered a small mass component to bring the total mass up to what I measured, and I move the mass component around until I get the correct CG location.

Why not use mass/cg overrides on the components and component assemblies?

 

[Jacktango22]

This has all been great information. What I have learned is that a best practice would be for me to weigh each component part as I add it to the rocket, and then use the override to make sure the data input matches reality. I can then, when the project is finished and painted, take a reading and see how that compares to what open rocket projects. This seems an indirect way to get a gauge on the paint and glue. This is a great exercise for me and will only improve my building from this point forward.

 

[neil_w]

You can certainly do all that, and it would undoubtedly be educational to do on a couple of rockets. Usually, though, it's more than you need.

You still haven't provided your original ORK file that was "way off" on the mass estimate, so we don't know what specifically caused your problem with that file. But generally, if you specify the parts and materials correctly in OR, the mass estimate for the whole rocket will be pretty close. Generally it will be a little low, and the CG will be a little farther back than predicted, due to the preponderance of glue being applied in the back of the rocket.

Again, there's absolutely nothing wrong with weighing every part as you assemble and overriding... but I don't usually do that, unless it's looking like stability is going to be borderline, or if the mass is coming up on the high end of what my motor mount can support.

No matter what approach you take while building, when you're done you absolutely must weigh the rocket and measure CG, and override them in whatever fashion you prefer (use the global overrides, or add mass elements and position them appropriately). When making the final determination of rocket stability and getting flight sim info, the only thing that matters is the mass and CG as-constructed.

 

[bjphoenix]

Why not use mass/cg overrides on the components and component assemblies?

If I had a very accurate scale so I could weigh all of the parts before building...
But that doesn't account for additional materials such as glue and paint. And it doesn't account for my laziness- it is too easy for me to just find a sim file for my rocket and go from there.

 

[lakeroadster]

And remember @Jacktango22 .... simulations are great, but the old adage that became popular in the late 1950's is sage wisdom, GIGO: Garbage In, Garbage Out... the concept that flawed, or nonsense (garbage) input data produces nonsense output.

Since this post is in the "Scratch Build" sub-forum, I assume you can't follow member @bjphoenix 's advice and find a sim file for your rocket on the internet.

If at all possible, when you have finished building that scratch built bird, follow Vern Estes's advice and do a swing test. It's a great way to build some confidence that all your hard work and due diligence, design wise, has produced a rocket worthy of launching.

https://estesrockets.com/wp-content/uploads/Educator/Estes_Model_Rocketry_Technical_Manual.pdf

 

[Scott_650]

The method neil_w outlined is exactly how I simmed my L1 attempt rocket. The data my altimeter recorded from my practice flights and qualification flight matched my sim data almost exactly. So is weighing everything a best practice versus doing overrides based on physical measurements? More knowledgeable experts will have to answer that question, I do the overrides because it’s easier and it works for me, which is all part of the fun of hobby rocketry, finding methods and techniques that work consistently for you.

 

[lakeroadster]

The method neil_w outlined is exactly how I simmed my L1 attempt rocket. The data my altimeter recorded from my practice flights and qualification flight matched my sim data almost exactly. So is weighing everything a best practice versus doing overrides based on physical measurements? More knowledgeable experts will have to answer that question, I do the overrides because it’s easier and it works for me, which is all part of the fun of hobby rocketry, finding methods and techniques that work consistently for you.

Did you build a kit... or a scratch build?

 

[bjphoenix]

Since this post is in the "Scratch Build" sub-forum, I assume you can't follow member @bjphoenix 's advice and find a sim file for your rocket on the internet.

I do this because I'm pretty much a beginner at OpenRocket. I'm building fairly simple rockets cloned from kits so it is fairly easy for me to find a sim and modify it. I'm hoping that by the time I want to create sims from scratch I'll have enough experience and confidence with the program to take it to the next level.

 

[Scott_650]

Did you build a kit... or a scratch build?

I modded a slightly off-beat kit. Discount Rocketry used to sell a full kit using the 4” diameter Fantazia “Crayon Bank” - they still have a conversion kit available - I added nose weight, changed the recovery gear, added motor retention, nothing too radical. I know plenty of people have successfully built and flown 4” crayon bank based rockets but since it was my L1 certification rocket and I didn’t build it exactly per the instructions I wanted the experience of building the sim file along with building the rocket.

 

[lakeroadster]

I modded a slightly off-beat kit. Discount Rocketry used to sell a full kit using the 4” diameter Fantazia “Crayon Bank” - they still have a conversion kit available - I added nose weight, changed the recovery gear, added motor retention, nothing too radical. I know plenty of people have successfully built and flown 4” crayon bank based rockets but since it was my L1 certification rocket and I didn’t build it exactly per the instructions I wanted the experience of building the sim file along with building the rocket.

Sounds like a fun build.

The method neil_w outlined is exactly how I simmed my L1 attempt rocket. The data my altimeter recorded from my practice flights and qualification flight matched my sim data almost exactly. So is weighing everything a best practice versus doing overrides based on physical measurements? More knowledgeable experts will have to answer that question, I do the overrides because it’s easier and it works for me, which is all part of the fun of hobby rocketry, finding methods and techniques that work consistently for you.

For a modified kit, sure, that makes sense.

The difference, as I see it, is that a one off scratch design / scratch build needs a more thorough simulation, followed up with some actual real world testing, such as a swing test.

LPR.. not a big deal
MPR.. yes, could be a big deal.. heads up!
HPR.. Find a bunker.. incoming mortar round possible.

 

[tsmith1315]

This seems an indirect way to get a gauge on the paint and glue. This is a great exercise for me and will only improve my building from this point forward.

And use the Comments to your advantage, they can be useful build notes for reference:

View attachment 556715

 

[DerRedMax]

I'm also learning how to use OpenRocket and this discussion has been very helpful.

 

[4regt4]

I'm just finishing up a Leviathan build. I carefully monitored each piece in OpenRocket, in particular I measured tube thicknesses, which can be way off depending upon where you source the tubes. And at the very start, I added 2oz to the "stage" entry at the top for general paint and glue.

The build consisted of 3 parts: 1. The fin can, which was donated from a Big Daddy kit. It's quite short, and was nice to work with instead of having to manage a big tube. OR's estimate of the mass of the plywood fins was spot on. 2. The nose cone, which i modified to have a small payload bay for altimeter and tracker. 3. And the body tube. The only mass departure here was a baffle. Also, I lined the fin can and most of the body tube with couplers for added strength. Probably overkill.

Here's where I'm going with all this. Each of the 3 sub assemblies were weighed at their completion. I was pleasantly surprised that each was quite close to the OR estimates. I only had to make a small mass adjustment (less than 1oz) to the fin can.

As it's now complete, I just weighed the fully assembled rocket. Turns out my 2oz addition at the top "stage" level for paint was excessive. I was able to lower it to 1/2 oz.

So, at least in my case, the OR weight estimates were very close.

Hans.

 

[tsmith1315]

in particular I measured tube thicknesses, which can be way off depending upon where you source the tubes

Have you found density to be pretty close, so that correcting the dimensions gets a reasonably accurate mass?

Measuring tube dimensions, including thickness, can be touchy.

 

[4regt4]

Have you found density to be pretty close, so that correcting the dimensions gets a reasonably accurate mass?

Measuring tube dimensions, including thickness, can be touchy.

Yeah, for cardboard tubes I've found the density to be reasonably accurate.

Re: Tube measurements. I think it is difficult to measure diameter directly, because calipers or whatever can squish the tube slightly. Which really throws off the accuracy. It's probably more accurate to wrap a cloth tape measure around the tube and measure circumference. But what I simply do is take the nominal measurement - in the case of motor tubes, it would be, say, 24mm or 29mm I.D. - and use a dial caliper to measure thickness. That seems to nail the mass estimate every time.

Hans.

 

[neil_w]

Are you really getting significantly different measurements vs. the parts out of the preset library built into OR?

 

[4regt4]

Are you really getting significantly different measurements vs. the parts out of the preset library built into OR?

I wasn't sure if that was directed at my comments. The problem with the preset library is that I don't know the part number, nor often even the source of whatever tube I grab. Even if I know the source, it's often easier and faster to just use the dial caliper to measure thickness and apply that to what *should* be a known measurement. (i.e. 24mm I.D. for a motor tube, 3" O.D. for an Estes Pro Series tube, etc.) So for me, it's all measurement.

Hans,