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sgirard

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Hi Everyone

This is my first post on this forum so I will introduce myself in a few words as I can. I flew my first Estes model rocket back in 1964. I have been flying them off and on ever since. I got into R/C airplanes and helicopters for many years. I have reached a point that my physical limitations are making me cut back and so I decided to go with model rockets. I don’t have any interest in kit building anymore and don’t care about high speed or high-altitude flights. So I guess that leaves low and slow and that suits me fine. My plan is to see how big I can safety fly in Class D, Class E, and maybe Class F. Getting them off the launch rod at a safe speed but keeping the altitude 500 feet or less (I like to see the whole flight). From what I have seen looking around on the forum I suspect my type of project is of little interest to most but I want to try it anyway.

The first thing I would like to know in how accurate is the data generated by OpenRocket . My first design I weighted each and every component and entered to correct weights in the program. My overall weight is a match and the CG is the same on both the rocket and the plan. So can I trust the data to be reasonably correct?

Next, I can only assume that the ejection charge in these motors are only good to have reliable ejection on a given volume. I have calculated the volume in my 1/100 scale Saturn V knowing it works with a D12-3 and have use that as a guide but know there has to be a better way. I know the answer is not simple so if I could be directed to some sites that will explain ways to figure out if I have enough ejection charge for recovery it would be a start.

I don’t consider myself a beginner in model rocketry itself but will admit to this is my first attempt at design and scratch building.

Thanks for any help you can provide.

Steve
 

Cape Byron

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The first thing I would like to know in how accurate is the data generated by OpenRocket . My first design I weighted each and every component and entered to correct weights in the program. My overall weight is a match and the CG is the same on both the rocket and the plan. So can I trust the data to be reasonably correct?
Yep. OR is as accurate as the data you enter. Weighing each component and overriding the mass with a real world figure will give you very accurate results.
 
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RocketTree

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Hi Everyone

This is my first post on this forum so I will introduce myself in a few words as I can. I flew my first Estes model rocket back in 1964. I have been flying them off and on ever since. I got into R/C airplanes and helicopters for many years. I have reached a point that my physical limitations are making me cut back and so I decided to go with model rockets. I don’t have any interest in kit building anymore and don’t care about high speed or high-altitude flights. So I guess that leaves low and slow and that suits me fine. My plan is to see how big I can safety fly in Class D, Class E, and maybe Class F. Getting them off the launch rod at a safe speed but keeping the altitude 500 feet or less (I like to see the whole flight). From what I have seen looking around on the forum I suspect my type of project is of little interest to most but I want to try it anyway.

The first thing I would like to know in how accurate is the data generated by OpenRocket . My first design I weighted each and every component and entered to correct weights in the program. My overall weight is a match and the CG is the same on both the rocket and the plan. So can I trust the data to be reasonably correct?

Next, I can only assume that the ejection charge in these motors are only good to have reliable ejection on a given volume. I have calculated the volume in my 1/100 scale Saturn V knowing it works with a D12-3 and have use that as a guide but know there has to be a better way. I know the answer is not simple so if I could be directed to some sites that will explain ways to figure out if I have enough ejection charge for recovery it would be a start.

I don’t consider myself a beginner in model rocketry itself but will admit to this is my first attempt at design and scratch building.

Thanks for any help you can provide.

Steve
If the model is simulated correctly in OpenRocket, the data is usually accurate. This is easily verified by comparing onboard altimeter flight data to the OR simulation. In my experiences, it is very close. Any major deviation is likely due to other outside factors.

You don't really have to weigh each piece as you build the rocket (plus, paint and glue will add additional weight). Instead, you can override the MASS and CG when the rocket is completed. This would be more accurate, in my opinion.

I have not been able to locate the ejection charge equivalency of Estes motors to FFFFg black powder, but I do know that Aerotech disposable motors in the F-G range, have 0.65-0.75g black powder. From there you can use BP calculator to determine the pressure.


Best of luck.
 

sgirard

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ok thanks. I figured if I weighted each part and it was placed correctly on the plan my CG would be closer. but your comment about glue and paint explains the small difference I ;had. I made small adjustments the the mass override to get the total weight and the CG correct and I do plan on flying with an altimeter to check things. thanks for the information.
 

dhbarr

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ok thanks. I figured if I weighted each part and it was placed correctly on the plan my CG would be closer. but your comment about glue and paint explains the small difference I ;had. I made small adjustments the the mass override to get the total weight and the CG correct and I do plan on flying with an altimeter to check things. thanks for the information.
Weighing each piece and then adding a paint/glue object which you shift to tweak the loaded CG is supposed to be the most accurate. But I'm super lazy.
 

KenECoyote

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...
So I guess that leaves low and slow and that suits me fine. My plan is to see how big I can safety fly in Class D, Class E, and maybe Class F. Getting them off the launch rod at a safe speed but keeping the altitude 500 feet or less (I like to see the whole flight). From what I have seen looking around on the forum I suspect my type of project is of little interest to most but I want to try it anyway.
...
Welcome Steve! :welcome:

Regarding the "low and slow" as well as the D-F motors, I love them and most of us here love and appreciate rockets of all types. I'm Level 2 and considering L3, but I still mostly fly A-F because I love designing my own rockets, more easily seeing them launch (rather than disappearing into the clouds or sun for 5+ minutes), low prep time (allows a lot more flights), as well it being much cheaper!

Sorry, I haven't used OR so I can't provide me an opinion on that, but I'd definitely recommend you get a Flightsketch altimeter (tiny and should work perfectly for D-F) or similar so you can compare vs. your sim and then that should allow you to quickly adjust your predictions.

Best luck to you and please share your flights and results! :)
 

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Welcome to the forums...

If you have any questions, just post your questions, a lot of folks here can try to help.

Pointy Side Up!
 

neil_w

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If the model is simulated correctly in OpenRocket, the data is usually accurate. This is easily verified by comparing onboard altimeter flight data to the OR simulation. In my experiences, it is very close. Any major deviation is likely due to other outside factors.
The simpler the rocket, the more accurate it will be. As the rocket becomes more complex, the assumptions built into OR get less accurate.

For complex rockets, it is good to assume a greater degree of uncertainty on first flights, and then adjust accordingly.

You don't really have to weigh each piece as you build the rocket (plus, paint and glue will add additional weight). Instead, you can override the MASS and CG when the rocket is completed. This would be more accurate, in my opinion.
Overriding mass and CG of the completed rocket is *always* most accurate, because it is... well, 100% accurate, taking into account absolutely everything.

Careful measuring of pieces in advance can be very useful for planning purposes, but you should *always* measure mass and CG of the finished rocket.

This is my first post on this forum so I will introduce myself in a few words as I can. I flew my first Estes model rocket back in 1964. I have been flying them off and on ever since. I got into R/C airplanes and helicopters for many years. I have reached a point that my physical limitations are making me cut back and so I decided to go with model rockets. I don’t have any interest in kit building anymore and don’t care about high speed or high-altitude flights. So I guess that leaves low and slow and that suits me fine. My plan is to see how big I can safety fly in Class D, Class E, and maybe Class F. Getting them off the launch rod at a safe speed but keeping the altitude 500 feet or less (I like to see the whole flight). From what I have seen looking around on the forum I suspect my type of project is of little interest to most but I want to try it anyway.
I would suggest that 500', while doable, is a bit low for E and F motors. You might consider upping your target to about 800', which is is still very easily viewable for a larger LPR or smaller MPR rocket, and will give you a lot more design flexibility.

High-drag rockets are good for limiting altitude. I have one that'll go about about 500' on an E20.
 

sgirard

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Thanks everyone for the input. Been out of pocket for a few days so I did not know I had additional post. my 500 foot target is a pure guess. I have never flown with an altimeter before so I really don't know how high 500' is, so build it big and get it off the rod safely and I will take whats left (a long as I can get a safe recovery). If I were to be honest I am thinking the larger the rocket the lower if will fly (I didn't want to open a can of worms but you know what I mean). I suspect my biggest hurtle will be getting it off the rod at 44fps to be safe (on calm days).
Again thanks for the welcome.
 

Funkworks

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My plan is to see how big I can safety fly in Class D, Class E, and maybe Class F. Getting them off the launch rod at a safe speed but keeping the altitude 500 feet or less (I like to see the whole flight). From what I have seen looking around on the forum I suspect my type of project is of little interest to most but I want to try it anyway.
I'm interested. In fact, I spent the better part of last year exploring all the nooks and crannies of low and slow, and pushing on every limit. But I'm still not sure my project can fly so don't take it for granted (look up my scratch-built "rocket for a smurf" for more details than you could possibly want). Good luck!
 

sgirard

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OK I will. glad to hear someone else is thinking like me. I don't care some much about the altitude I just like BIG but can afford to do it in high power. I will say i haven't tried to launch yet but can'nt believe how much I have learned. Having fun and for me that's what its all about.
 

jqavins

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From what I have seen looking around on the forum I suspect my type of project is of little interest to most but I want to try it anyway.
Quite a few of us like low-and-slow, and nearly all of us like the challenge of uncommon design goals. You won't have any trouble getting interested parties on board.

The first thing I would like to know in how accurate is the data generated by OpenRocket. My first design I weighted each and every component and entered to correct weights in the program. My overall weight is a match and the CG is the same on both the rocket and the plan. So can I trust the data to be reasonably correct?[
That depends on what you mean by accurate and reasonably correct. Factors like wind and variation in motor performance will have a greater effect than the deficiencies in the software, so there's only so good any simulation can possibly be. (I'm not knocking OR; every program has its deficiencies.) Yet there are things you can do to get the best results.

One of the biggest is one that I'm really surprised no one has mentioned yet: the coefficient of drag, CD. OR will have a reasonable guess, but for the best simulations you should do some test launches, all with the same engine type, compute the average altitude reached, and then override the model's CD to make the results match. Then when you use different engines the results will be well dialed in.

Regarding the weighing of each component vs. the weighing of the whole built rocket, Neil is absolutely right (he usually is) about the greater mass and CG accuracy from weighing the whole. However, there's benefit to doing both, because that gives you the most accurate distribution of the mass throughout the rocket, and that affects the rocket's performance to some degree. So again, how much accuracy are you looking for in your simulations?

Next, I can only assume that the ejection charge in these motors are only good to have reliable ejection on a given volume.
You can assume that if the rocket's size and engine chosen are a good match for performance then the ejection charge is probably just fine. If the rocket is oddly shaped in some way or has abnormal weight for its size then you might want to take a closer look, but in general you really don't need to sweat it.

One good way to achieve safe rod exit speed and low altitude is to pick the best thrust vs. time profile for the purpose. An engine with a high average thrust for its impulse letter will generally get you off the rod fast and then peter out soon. But then, with composite propellant motors it can be more complicated and you want to look for either a high average thrust with a flat thrust profile or a profile that starts high then drops off as it goes (call a regressive burn). There are a ton of engine options available. I recommend a web site called ThrustCurve.org.

By way of example, here are two E motors with nearly the same total impulse:
1619802100381.png
All else being equal, the higher thrust motor, the E30, will get you off the rod at a whole lot higher speed (don't blink) and will probably get you a lower apogee. If you're building draggy as previously suggested, it will all but certainly give you a lower apogee. Low and slow? Well, low, but not so slow.
 
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Ez2cDave

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The first thing I would like to know in how accurate is the data generated by OpenRocket . My first design I weighted each and every component and entered to correct weights in the program. My overall weight is a match and the CG is the same on both the rocket and the plan. So can I trust the data to be reasonably correct?

I don’t consider myself a beginner in model rocketry itself but will admit to this is my first attempt at design and scratch building.

Thanks for any help you can provide.

Steve
Steve,

The most important piece of information to have is an accurate location for the "CP" ( Center of Pressure ) . . .

The "CG" ( Center of Gravity ) is merely the Balance Point of the rocket.

For stable flight, the Center of Gravity MUST be IN FRONT of the Center of Pressure a MINIMUM of 1 Body Diameter ( 1 "Caliber" ) . . . If it is not, the result is an unstable rocket, which can be very dangerous.

Back in the Day, CP calculations were done by hand, using the Barrowman Equations, which was great, provided that one was proficient in Calculus.

Fortunately, in 1995, Bruce Lee, a Tripoli HPR Rocketeer devised a system to greatly simplify the daunting math . . . His 10-year old son could easily do the math.

I have attached a PDF of his system, to calculate the Center of Pressure, below . . . I hope it helps !

Dave F.
 

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sgirard

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Thanks Joe & Dave for your comments!

I have to admit that I have flown Estes kits since the 60’s (off and on) and until I started down the road on this project I never knew anything about CG and CP, I’m having a ball. When I ask about accuracy in OR I was mainly concerned about speed off the rod and altitude. My current design (and my first) shows speed off the rod at 41 fps (60” rod) with a D12-3. I know that’s below the 44 fps that is said should be the minimum and I don’t want to have to build a bunker and I’m to old to run. The total weight with motor is 14.7 oz and that’s .7 oz over what Estes said is the maximum lift off weight for a D12-3. OpenRocket said apogee will be 157 ft. All these numbers give me pause, so I built another one same size but lighter (11.2 oz) and the numbers look good. That’s not to say a won’t launch the first one as I fly alone in the country so feel I can hide behind the pickup and be safe. My stability is at 1.96 on the heaver one and 2.31 on the lighter version. I have an Estes altimeter currently because I want to do exactly what you described in running test flights. I would like to have the Jolly logic altimeter two so I can have more data but figure I should see if I can get one off the ground first.

I have been looking and trying to understand all the thrust curves, total impulse, average thrust, and so on but haven’t put it all together yet but I will. I have also been looking at the reloads but figure I will stick with the single use motors first before making that kind of investment.

I’m waiting for the best launch day (as calm as I can get) before giving them a test flight and at least that should give me a point to move forward.

Thanks again for your input

Steve
 

Ez2cDave

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I have been looking and trying to understand all the thrust curves, total impulse, average thrust, and so on but haven’t put it all together yet but I will. I have also been looking at the reloads but figure I will stick with the single use motors first before making that kind of investment.

Steve
Steve,

Staying with the Estes BP motors is a good solid plan . . . You can always move into Composite motors ( single-use & reloads ) later.

I will be happy to help you . . . So, ask away !

Dave F.
 

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Back in the Day, CP calculations were done by hand, using the Barrowman Equations, which was great, provided that one was proficient in Calculus.

Fortunately, in 1995, Bruce Lee, a Tripoli HPR Rocketeer devised a system to greatly simplify the daunting math . . . His 10-year old son could easily do the math.
While Barrowman used calculus to derive his equations, the equations themselves only require basic algebra at most. Admittedly giving Lee's article only a cursory look, he appears to have translated the equations as derived by Barrowman into a series of step by step instructions. While it will make it easier to calculate them, it isn't really a simplification in the sense of being new, easier calculations that would give close results.

Quite possibly drawing a distinction without a difference, but one I felt should be made.
 

jqavins

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My current design (and my first) shows speed off the rod at 41 fps (60” rod) with a D12-3. I know that’s below the 44 fps that is said should be the minimum
While I agree with Dave that sticking with BP motors is a good plan, I do have to say that single use composites are only a little different. And the Aerotech D22 single use 24 mm engine will solve your rod exit speed speed and then some. Chek it out on ThrustCurve and try it in OpenRocket.
 

sgirard

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If your talking about the D22W from AreoTech I have been trying to find out if it had been released yet. I first saw it in the NARCON 2021 Manufactures forum of January 29, 2021. It shows up in “thrustcurve” but I can not find anyone that knows anything about it. The Manufactures forum release I saw said spring of 2021. Yes your right, I think it would be ideal for me and don’t mind waiting if I could find out some idea of when they will be available.
 

jqavins

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Oh, shoot, that's exactly the one I meant. I picked it from TheustCurve, which shows its status as "Available", and I didn't think to check that with vendors.
 

neil_w

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If your talking about the D22W from AreoTech I have been trying to find out if it had been released yet. I first saw it in the NARCON 2021 Manufactures forum of January 29, 2021. It shows up in “thrustcurve” but I can not find anyone that knows anything about it. The Manufactures forum release I saw said spring of 2021. Yes your right, I think it would be ideal for me and don’t mind waiting if I could find out some idea of when they will be available.
When I asked recently on the Aerotech Open Thread they said late April-Mid May estimate... that would suggest "soon" at the very least, barring unforeseen delays.
 

Ez2cDave

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That D22W looks like an interesting motor . . . Haven't seen them in stock anywhere, yet, though.

Dave F.
 

Ez2cDave

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That D22W sounds like an interesting motor . . . Has anyone seen them available, anywhere, yet ?

Dave F.
 
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Ez2cDave

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I, mainly, suggested the use of Estes BP motors to keep operating costs down.

More flights = More practice = Experience & proficiency !

Dave F.
 

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Nor is any "basic Algebra" in sight, either.

View attachment 462633
Yes, you're right. There isn't even as much algebra as in this worksheet. You just plug the numbers in the equations, turn the crank, and get the answer. Nothing but arithmetic. I stand corrected.

I'm not quite sure what the rest of your examples are for, unless you're trying to exhaustively catalog all the math you don't need in order to apply the equations.
 

Ez2cDave

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I'm not quite sure what the rest of your examples are for, unless you're trying to exhaustively catalog all the math you don't need in order to apply the equations.
I was just showing some of the "levels" above X + 2 = 3 "basic algebra".

The beauty of Bruce Lee's work is, as you said, the elimination of the need for anything above simple arithmetic . . . A great contribution by him !

Dave F.
 
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