Something strange with my Open Rocket

Lt72884

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Since i am still new at OR, i have built a basic rocket, and for some odd reason, it only hits 65m in height with an L sized motor

wtf.JPG

I noticed the CP is in front of my Cg, which may be the case. i have tried to change a few things and my flight sims are pretty bad. the body is made of fiber glass. I am still really new to this software so it may be just that haha

thanks
 

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rharshberger

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Its unstable for some reason the CG (blue target looking dot) should ALWAYS be ahead/forward of the CP (red dot), the 65m altitude is optomistic...that rocket will skywrite...

Add noseweight to correct, or length, or both.
 

Lt72884

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Its unstable for some reason the CG (blue target looking dot) should ALWAYS be ahead/forward of the CP (red dot), the 65m altitude is optomistic...that rocket will skywrite...

Add noseweight to correct, or length, or both.
ok, ill lengthen the rocket a bit, and make it a little skinnier. I know CP is a function of surface area so ill try and lengthen that BT a little bit and see.
 

Lt72884

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Added 9.9 lbs to the nose, made the body tube fiberglass, you have 9,300 feet. You're welcome.
im not sure how adding weight to the nose effect the CP? CP is all based on surface area. Or did adding weight to the nose change the CG and therfor stabilized it?
 

PayLoad

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Rule number one - in my book and I would dare say everyone else's - Center of gravity must be 1 body diameter aft of the center of pressure. So I changed your CG by adding your payload. (I better pass that damn Level 2 cert exam)
 

Lt72884

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Rule number one - in my book and I would dare say everyone else's - Center of gravity must be 1 body diameter aft of the center of pressure. So I changed your CG by adding your payload.
thanks for that rule of thumb. I have taken that note. I have also made the changes like you have and now its 3032m.

So, now how do i add 9.9lbs to a real plastic nosecone? Drill a hole and add buckshot? or sand? or resin?
 

rharshberger

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thanks for that rule of thumb. I have taken that note. I have also made the changes like you have and now its 3032m.

So, now how do i add 9.9lbs to a real plastic nosecone? Drill a hole and add buckshot? or sand? or resin?
Either method will work however given you relative inexperience (or at least thats how I perceive it) I highly recommend watching the following video then checking out all of John Cokers videos on High Power Rocket construction. Lead shot, tungsten powder, sand, all mixed with epoxy will work some require more volume that others.

Also if you add 9lbs of noseweight to a plastic nosecone you need to be thinking about shock cord attachment as the normal plastic loops wont be a good idea (they aren't usually a good idea anyways) and John has methods for that too.

 

Lt72884

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Either method will work however given you relative inexperience (or at least thats how I perceive it) I highly recommend watching the following video then checking out all of John Cokers videos on High Power Rocket construction. Lead shot, tungsten powder, sand, all mixed with epoxy will work some require more volume that others.

Also if you add 9lbs of noseweight to a plastic nosecone you need to be thinking about shock cord attachment as the normal plastic loops wont be a good idea (they aren't usually a good idea anyways) and John has methods for that too.

ill watch the video and post it for my team. But i figure its a lot like some of the other projects i have done. Use buckshot mixed with epoxy, rough up the inside of the cone or install some sort of matrix to help hold the epoxy mix to.
10lbs of buckshot seems like it would fill up a very large portion of the nose cone? its been a while since i have used this method, but 10lbs of lead buckshot is pretty large?
 

Lt72884

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ok, i got the rocket to 13,000 feet by dropping the weight in the nose to 6 lbs. this will make it easier to build.
 

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Lt72884

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thanks for all the support and help these last few days.I know it has been alot of questions haha
 

dhbarr

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ok, i got the rocket to 13,000 feet by dropping the weight in the nose to 6 lbs. this will make it easier to build.
You're still dramatically understable at 0.2 calibers, especially if you're going to get that close to transonic. Likely you can get a lot of airmiles out of just making your fins as tall as your bodytube diameter ( looks like about 10-12cm ) -- I think you'll find once you do that you can keep the total noseweight down around 7-8lbs which in the end makes recovery easier.

EDIT: Also, why the nonstandard bodytube size with custom printed cone? Is there a certain size payload you need to keep interior?
 
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Lt72884

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You're still dramatically understable at 0.2 calibers, especially if you're going to get that close to transonic. Likely you can get a lot of airmiles out of just making your fins as tall as your bodytube diameter ( looks like about 10-12cm ) -- I think you'll find once you do that you can keep the total noseweight down around 7-8lbs which in the end makes recovery easier.

EDIT: Also, why the nonstandard bodytube size with custom printed cone? Is there a certain size payload you need to keep interior?
i had to add a cutom density to the list because my version of open rocket doesnt have plastic listed as an option for material, so i just picked PETG as the materials density as a rough number

Ill research what the stability number is and then redesign the fins to match that. Im guessing it has to do with the CP and CG differences. I know the rule of thumb was stated above so seeing a design not follow that has helped me understand more.
 

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One thing that I think was mentioned in one of your previous threads, I would have to go back through and check, is get a copy of Mark Canepa's book Modern High Power Rocketry 2. I think Apogee might still sell it. The book is a little dated, but it gives some great design and build tips. I would also get a copy of Make: High Power Rockets. It is available on Amazon for around $25. While I have not seen this book in person yet, I have heard it is a good source of information for people getting into tbe hobby.
 

QFactor

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Lt72884's rocket is missing all the standard components for a proper design and simulation.

It's just a tube with fins, a nose cone, and a motor - and 9 lbs. of mass. There is no avionics
bay, there are no harnesses, there are no chutes, etc.

Attached is an OpenRocket file for introducing students to a basic rocket design that could
possibly be entered in a competition. I share this with HPR teams when they decide to run
before they can walk in HPR design.

The design is not perfect. It's not meant to be perfect. It's meant to show one of several
common layouts, how to organize the sim file, use proper component names, etc.

The "overstable" condition reduces significantly once actual weights & locations are finalized.
So don't focus on the overstable. An experienced HPR designer can overcome this.

This current file has mixed materials because it was used to highlight design conditions
for the last HPR team I worked with.

Lt72884 - please go and get a mentor. And go read Mark Canepa's book on High Power
Rocketry as others have suggested. It's an easy read, and better use of your time versus
messing around with software.

Note: The payload location, in the OR file, was selected by the students. This was based on their payload
experiment, and the competition's requirement of payload access for inspection. This is what created
a high overstable condition in the simulation. Just one of many things to address in a design.
 

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neil_w

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Lt72884's rocket is missing all the standard components for a proper design and simulation.

It's just a tube with fins, a nose cone, and a motor - and 9 lbs. of mass. There is no avionics
bay, there are no harnesses, there are no chutes, etc.

Attached is an OpenRocket file for introducing students to a basic rocket design that could
possibly be entered in a competition. I share this with HPR teams when they decide to run
before they can walk in HPR design.
Ooh, this is fantastic. Any chance you'll allow us to include this as an example design inside OpenRocket?
 

QFactor

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Ooh, this is fantastic. Any chance you'll allow us to include this as an example design inside OpenRocket?

I won't say Yes or No. I could think of better sim files as an example. This has not been updated and cleaned up since working
with the last team. And your new Beta version would allow for some component improvements (rail buttons, etc.).

My hesitation is that teams will look at is as a template and not a teaching example. I don't normally post a file like this
to such an open forum. I try to target it to a team I'm working with, their particular design issues, and their issues with
the software (OR & RockSim).

I posted the file because I am seeing some of the impatience that inexperienced students display when they
are presented these types of HPR challenges along with some new software. Like I said in another post, they
like to run before they walk. (It would be nice to hear from a regular Utah club member that a university
group has recently reached out to them.)

I know you have some very capable HPR people in your OR group, so you may want to get their input on using this sim.

I greatly appreciate you asking first.
 

Lt72884

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Lt72884's rocket is missing all the standard components for a proper design and simulation.

It's just a tube with fins, a nose cone, and a motor - and 9 lbs. of mass. There is no avionics
bay, there are no harnesses, there are no chutes, etc.

Attached is an OpenRocket file for introducing students to a basic rocket design that could
possibly be entered in a competition. I share this with HPR teams when they decide to run
before they can walk in HPR design.

The design is not perfect. It's not meant to be perfect. It's meant to show one of several
common layouts, how to organize the sim file, use proper component names, etc.

The "overstable" condition reduces significantly once actual weights & locations are finalized.
So don't focus on the overstable. An experienced HPR designer can overcome this.

This current file has mixed materials because it was used to highlight design conditions
for the last HPR team I worked with.

Lt72884 - please go and get a mentor. And go read Mark Canepa's book on High Power
Rocketry as others have suggested. It's an easy read, and better use of your time versus
messing around with software.

Note: The payload location, in the OR file, was selected by the students. This was based on their payload
experiment, and the competition's requirement of payload access for inspection. This is what created
a high overstable condition in the simulation. Just one of many things to address in a design.
yeah i know its missing all that stuff:) i did that on purpose to learn the software and to see what effects each item has on the rocket. If i can make a tube with fins reach 13,000 feet with a stability of 2.35, then i am in a good place to start adding the other items and see how each thing effects the GC,CP, altitude, etc etc. One of the openrocket tutorial videos says to aim for a stability between 2 and 3.

As for the payload section on our rocket, We will figure that out soon. I relate it to trying to balance a stick on your finger, if you add mass to close to the bottom, the mass moment has dramatic effects on balance, but if the mass is closer to the top of the stick, it finds balance easier.
Its like taping a rocket motor to a stick and launching it, which i have done many times, just to see how it flies. Then i add other things to it. Line upon line my friend. One step at a time. I cant get to the top of the empire state building by skipping steps, you get to the top by taking it a step at a time, and this is my process for doing so. Seeing how each thing effects the other.
I have had lots of information from this forum in the last 3 days, that i need to sift it and organize it. I know that avionics, electronics, and of course a chute or a recovery system for is needed for the rocket. Seeing a example like you provided is great, and i need to disect it because an example like that feels like being rushed to the finish line without understanding the in-betweens. So i will use your example like you say, to see how it all fits:)

EDIT:: Im not upset you gave me an awesome example, im not upset at all. The example in OR is great because it shows a final product. I am still currently in my expiermental stage where i see what things do what. For example, the first post in here said i was correct with my CP in the wrong place, so i moved it, and then it flew to 847m, then someone said add mass, so i did to the nosecone and it hit 3000m, so i kept designing and seeing, thats my process of how i work with building something from the ground up:)
 
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JoePfeiffer

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You didn't learn the material in your classes just to pass the final exam, you were taught it because that's how things work. If your sim says your CG is behind your CP, you don't need to ask us why your rocket didn't go very high.
 

SolarYellow

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Newbs in another forum I participate in are often counseled to read more and post less in the beginning, because you don't know what you don't know.

Another example from my student days:
At one of my summer jobs, another engineering student intern was hired to work on a project to build an R/C aircraft that would perform to a specific, unconventional flight envelope as a proof of concept. Dude worked all summer: custom designed and fabricated airframe with unusual powerplant that was a learning process all on its own to get to run right (with probably the most knowledgeable bunch of people who ran more normal powerplants all the time that you could ask for around to help). Got it running, got it flying, and the thing failed to meet the performance targets. Not long after, a more experienced tech (not an engineer) who was very familiar with the technology space went and bought an almost-ready-to-fly model and off-the-shelf, ordinary powerplant. About as cheap and simple as you could possibly get. Threw it together in a few hours one evening and went out and nailed the performance targets the next day.

Another engineer in the same shop who built really cool stuff on his own time said, "I'd never build anything if I can just go buy it." The point being, a factory that's set up to churn stuff out in volume will be many times more efficient and can reasonably be expected to produce higher quality at lower cost. Consider the classic essay, "I, Pencil." https://fee.org/media/14940/read-i-pencil.pdf My point is not to indoctrinate you in free market libertarianism, but to point out that a vast swath of engineering and manufacturing turns out to be just figuring out what to buy, and where and how to buy it.

Another example: At a previous job, we wanted a piece of hardware that was similar to, but not quite the same, as one we already had in inventory. Engineering created a print and sent it over to production. It turned out that an $1800 lathe attachment with no other use in our operations was required to convert the existing parts into the new parts. And the labor costs per part were high. Total cost ended up being several dollars each, after weeks of delay getting it all up and running. When I found out about that whole situation, I took some photos and made some notes on the desired part. Had the purchasing manager reach out to her suppliers. One of them came back and said, "I have 100 here. They're 35 cents each. How many do you want now, and what do you forecast for the future?"

If you want to do stuff that hasn't been done before, make sure you have a good idea what's already been done, so you don't waste your time doing it over again, and poorly.

My advice is to spend a bunch of time reading. Learn all you can about what has already been done. Come up to speed on the technology space. Figure out who the vendors are, what products are available, what technology solutions are out there as plug-and-play and as people DIYing. There are thousands of people doing things like you're trying to do, and thousands of people together are probably smarter than the few people on your team. Unquestionably, they must have more experience, and you can profit by that. It's likely that there's an off-the-shelf kit that comes close to being able to do what you need, and is designed by someone with many years of experience. It may be that enough modifications would be required that you'd be better off buying materials and components and building from scratch, but getting really familiar with what is already proven is a good place to start. You'll save more time and money than you can imagine.

My start with OR was buying an Estes Hi-Flier for $6.99 from Hobby Lobby and building it in OR. Got out simple measuring tools and a milligram scale and looked up a lot of specs for the standard components like tubing. I just wanted to understand the model and why it was what it was. Do the work to build the whole thing with every component included. You'll have to figure out all the basics, and then you'll be able to make changes and see the results in your sim. Play with all the parameters. Develop sweep matrices for sims, where as you sweep one parameter from high to low, you simultaneously sweep another that trades off against it, and see where the optimum overall solution lies. You probably won't need to fill the entire x*y grid, but you'll see how the optimal region is shaped and hopefully find the global maximum. An example is main body tube length versus weight required to achieve a stability target.

The classic manuals on rocketry that have been recommended are worthwhile. Much of them may not seem relevant, but they will bring you wisdom and perspective that will serve you well. After three decades in my field, I still learn new things that have me wondering why something so basic wasn't in the first book I read.

What is the plan for every person on the team to become L2 certified? My understanding is every person being certified has to build their own rocket. How many rockets is that? Do they come out of the $3500 budget? Is everyone paying for their own? It would be good experience for everyone. On the other hand, most of the Formula SAE teams I know about have division of labor. There are people doing software and electronics, people doing powertrain, people doing suspension, someone driving, etc.

In the automotive world, being able to go fast is a relatively small part of knowing how to race.

The biggest weakness I see on most FSAE teams is program management: Someone herding all the cats to ensure that all aspects of the project are done well enough and on time. Someone needs to develop an overall plan, working backward from launch, including testing so that everyone has a high confidence level based on prior performance when you get to the big show that the bird will fly as expected. Figure out who's doing what, when, and then have your smaller do-or-die thrashes along the way to stay on schedule, rather than letting it all build up until the end and crash and burn with a rocket that's not ready and proven. Include test motors and travel in your budget.

The bottom line is, you probably aren't going to have to invent anything. This is established territory. You just have to integrate a bunch of stuff that's already out there into a system that works efficiently and reliably as a whole.

Apply functional requirements trees. You have an overall goal. Break it down into functions and subfunctions that the rocket must perform to achieve that goal. Then explore all the established solutions you can find for each of those functions and subfunctions. Look at weight, cost, packaging. This automatically provides a structure for division of labor: you can have an airframe team, a fin can team, a motors team, a nose cone team, a recovery team, an electronics team. Each team does a deep dive in its area, starting off just being an information sponge.

You could work up presentations of your findings for each other, so the whole group can be up to speed and understand how each area fits in with the whole, and have a chance to propose possibly helpful ideas and ask helpful questions due to a different perspective. You will probably be expected to produce a final presentation, so these small internal presentations will be good practice, as well as generating and tuning up most of the needed material. The not-so-public speaking is also awesome experience for when you start real work.

Some functional areas will be obviously related, such as recovery and electronics, and recovery and airframe. Some functional areas will compete with other functional areas, and you may have to balance performance between them rather than it being possible to optimize everything. You should end up with a vast amount of information about things you didn't do. Come together as a group often. Many of the best ideas seem to happen when you have a bunch of competent people just sitting around bouncing ideas off each other.

Good luck!
 
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Lt72884

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You didn't learn the material in your classes just to pass the final exam, you were taught it because that's how things work. If your sim says your CG is behind your CP, you don't need to ask us why your rocket didn't go very high.
Sure i do, thats why i asked:) Knowing theory is one thing but seeing it in action is another. Its validates my math. Take for example, a 3rd order diff eq, putting it into laplace domain is easy, then solving for the natural frequency etc etc is easy, but until you actually see a physical system imitate the math, it then makes sense. So when i saw that the CP was behind my CG i knew that could be part of the issue, and when i did move it back behind, i saw the rocket flew higher, but it was only a portion of the problem.
 
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