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Jacktango22

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Retired last year and having a blast. I have been designing and flying mid and low power with success, and this winter I decided to design and build a rocket to use for a Cert 1 attempt.

I have not flown it yet and was thinking about a first flight with a low G or maybe a high F to see how it goes. I swear that as I was building it all seemed to work out .... but as I started to think about doing this at our next club launch I pulled it out, updated and checked sim data on Open Rocket and the simulations are all a disaster.

At the end of the day, I'm ok having this as a nonflying display rocket (I love to build) but hope someone can help with this.

I attached a photo and the Open Rocket file.

Appreciate any thoughts and suggestions.

Thanks!
 

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Should be ok, or not far off.

Fin span (height of fin to top from body tube) looks a little short. Usually want at least 1 caliber. (Fin height at least body tube diameter) But it looks close. This keeps the fins out in "clean air".

Did you weight it on a scale? What is ready to fly weight, with no motor? Where is measured center of gravity from tip of nose? (Again ready to fly except no motor.)
 
That rocket looks fine to me. If it fails the swing test, add some nose weight. I've never used a sim so far and I scratch build quite a few rockets.
 
Looks fine-ish.
But....... Are your chute and lines that far back? You could probably easily move your Cg further forward just by putting a bulkhead in that while still allowing the chute and lines, to keep them further forward. The weights of some of the components seem unusual. 2Kg of tether??? Are you sure you need 5/8 diameter solid Kevlar rope? 2.4 or 3.2 mm will be fine.( changed) You've overridden that weight, but wouldn't have helped with the pre-build sim.
on the custom parachute, you had 12mm diameter solid Kevlar shroud lines.
You chose filled polystyrene for the nosecone. I've changed that to a 3"blown pvc like a purchased one would be or a printed one would be close enough to. Polystyrene is solid plastic and not as you may have thought, polystyrene foam.

Does this eject from the nosecone or rear coupler? The rear coupler was done as a solid log. Not a cardboard tube. Fixed.
I've moved the mass component into the nosecone, where you would add any weight you needed. You don't need to override the weight of a mass component, it has a line where you can specify it in the general tab.
The body tube needs to be in 2 pieces if you plan to separate it at the rear coupler.(not fixed.)
I've added an additional bulkhead to hold your parachute and lines forward. Depends if you can or not. Don't forget to drill some holes to let the ejection gas pass.

I've removed all the mass overrides which now work as the component masses are close to what they will be.
Once you have your final weight, and know where your Cg is. Override the mass of the Nosecone and tick the override all subcomponents, with the correct weight and Cg location.

Hope that helps.
Good luck with the launch. Your rocket weel fly. (forged in fire)

I've attached the updated file.

Norm
 

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Norm,

This is all helpful. I may not fully understand the way the override function works. I have a scale and the all-in weight w/o motors (I corrected the Kevlar cord size which is actually 1/8 inch) is 936 grams. So, I used a "body tube" override (all components) to adjust for that -- so that 936 is there when launch is computed. . I built it "heavy" as is not uncommon.

I uploaded what I just did and OR shows it flying well with (as examples) a G74, G 78, and G 76.

However, I don't like the stability on this version (1.04) without motor) and will add some nosecone weight (as you suggested). E.g., with a G 76 the stability at launch is only .428 ---- and though I realize that the CG will move forward as it burns, that still seems too close.

I'll do this (add weight to Nose Cone) to get no motor stability to about 1.8 to 2.2 w/o a motor. I don't care about altitude anyway -- just want it to fly well and to get it back!

Sound OK? I'd buy you a beer for your time if you're nearby (Connecticut). :)

Jack
 

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However, I don't like the stability on this version (1.04) without motor) and will add some nosecone weight

Does the CG position in OR agree with the actual CG for the complete rocket, chute, etc? I load it all up and just balance it by hand for the real CG.

If not, you can specify that point with Norm’s method.
 
Norm,

This is all helpful. I may not fully understand the way the override function works. I have a scale and the all-in weight w/o motors (I corrected the Kevlar cord size which is actually 1/8 inch) is 936 grams. So, I used a "body tube" override (all components) to adjust for that -- so that 936 is there when launch is computed. . I built it "heavy" as is not uncommon.

I uploaded what I just did and OR shows it flying well with (as examples) a G74, G 78, and G 76.

However, I don't like the stability on this version (1.04) without motor) and will add some nosecone weight (as you suggested). E.g., with a G 76 the stability at launch is only .428 ---- and though I realize that the CG will move forward as it burns, that still seems too close.

I'll do this (add weight to Nose Cone) to get no motor stability to about 1.8 to 2.2 w/o a motor. I don't care about altitude anyway -- just want it to fly well and to get it back!

Sound OK? I'd buy you a beer for your time if you're nearby (Connecticut). :)

Jack

Take the ORK file I've fixed(ver2) up for you and work with that. There are so many fundamental flaws with your version, it would be difficult for you to fix.

The rear centring rings are set at 1/4" ply They'd be perfectly ok at 1/16" ply. CP is the same as the design but will be slightly further back(more stable) due to base drag. Just build it and see where the CG ends up. Then add weight to the nose if required. Moving atll your parachute and lines as far forward as they will go with a bulkhead to prevent them from moving back under thrust usually gets you the same or better stability with a lower weight penalty than adding weight. As long as you can keep the final weight under 1.2Kg you'll be ok and get to about 1000ft-ish on a G64 or 78 at 1.5Kg you'll be down to about 700ft.
Good luck with the launch
Don't forget if you don't reply to this post, the poster(me) doesn't get notified. So you need to click onthe "reply" on the RHS on this post rather than the Post Reply at the bottom of this series of posts.
Norm
 
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Here it is with base drag accounted for and mass in the nosecone added to take it to the maximum permissible weight for LPR. The Cp is behind the centring ring. (It moves about 50mm or 2 bananas in US measurement. Probably too stable as shown, which could cause windcocking if the launch speed at the end of the launch rod was too slow and it was windy. So short version, build it and balance it out with weight in the nose to make it stable by at least 1 calibre. Make sure the chutes and lines don't get packed backwards due to acceleration on take-off. That could move the Cg into a bad place. Might have to rename it Signwriter, skywriter or something similar if that happens.
1682198410556.png
 
Why did you override the mass of the nose cone and the rest of the rocket separately without overriding the CG? I assume the mass was what was measured, but what about CG. Without the CG overridden accurately also, your stability is going to be off.

BTW, I sim +1600 ft. on a baby H, H128W. This is really a MPR, not a L1 HPR rocket. If you really want a L1 HPR, you need to account for the full range of L1 motors, not just a baby H for a cert flight. A 38mm MMT and 4 - 6 lbs total weight would be a much better flier on H & I motors.

Remember, the full range of E through G motors is less Ns than the range of H motors alone. When you add I motors, it's more than 3 times as much.
 
Personally, I'd just stick a minimum H motor in it for certification. But you can certainly test it on a G if you need to. Few people would take a Ferarri to do their driving test. So as much as I like high thrust or sparkies or others, I just wouldn't get the Ferarri keys out for that driving test.
Once the rocket is built, you weigh it without motor or casing and use that built weight to override the weight of the stage( right click the stage at the top of the component tree and select "Edit" this is above the nosecone!!!!). Measure the Cg without the motor fitted and override that in the stage.(And all subcomponents)
Then sim the motors you want to use. The mass of the motors will be outside of the overridden mass and Cg should be correctly calculated.
Norm
 
Thanks guys. I think I do not understand the CG and CP overrides functions. Will they not adjust automatically after I override mass of either the individual components or the entire rocket? Or is the task to do an independent physical test (find the balance point on desktop) and change CG to that?

_
 
Thanks guys. I think I do not understand the CG and CP overrides functions. Will they not adjust automatically after I override mass of either the individual components or the entire rocket? Or is the task to do an independent physical test (find the balance point on desktop) and change CG to that?

_
When you have completed the rocket, without the motor in it, update the mass and measure the balance point(Cg) measured from the nose tip, and override them in the stage which is at the very top of the component tree.... Glue has weight. Don't underestimate its weight. The actual weight of the finished item, is no longer an estimate. I wouldn't adjust the Cp. Let OR calculate it.
 
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Is this rocket already built? If so, then I’m going to echo what Norm said.

I would not worry much about getting all the component weights and Cgs right. Those are primarily for the pre-build process, not for a rocket that has been built. Now that it is built, all you really need to do is to physically load the rocket up the way you intend to fly it. Put EVERYTHING inside the rocket, except the motor. Put the shock cord and chute in, put in the blanket if using, or dog barf. Connect the quick links and swivels, etc. Screw on the motor retainer, even though there is no motor. Get it ready like you are going to fly it, and then weigh the whole rocket — that’s your mass. And then balance the rocket and measure the distance from the nose to the balance point — that’s your Cg. Next, do as Norm said and enter the weight and Cg as your mass and Cg overrides for the whole rocket “stage” in the component tree.

You don’t really need to worry about Cp. As long as all the external components that touch the air are the right size and shape, the program will calculate the Cp for you. Is the nosecone the right length diameter and shape? Are the body tubes the right length and diameter? Are the fins the right shape, size, position and number? If that’s all correct, the program will calculate the Cp for you.

The program will use the Cp it calculates and the Cg you entered for your override to calculate the stability. And as you add different motors to your flight configurations, the program will automatically account for the mass and Cg of the motor to calculate a new stability for the rocket on each motor.

Just looking at the rocket and doing a “mind sim”, I’d say it looks like it should be stable, unless there’s some unusual build techniques or materials.

As long as the combined mass of the rocket and motor are less than 1,500 grams (53 oz), you can fly it at an unwaivered (non-HPR) launch without your certification. So you could try a G motor — pick a fast one, like a G77 or G80. But if it’s over the weight limit, you will need your certification, and you should make your first flight your cert flight. Try something like a H115 or H135.

Good luck!
 
I don’t have OR available at the moment, so I can’t see the file. But looking at screenshots, it looks like you’ve named the top- level component “BT70 S”. That’s actually the “stage“ we are talking about. It represents the whole rocket. You can name it whatever you like, but you have a component in the tree for your Body Tube, so I’d suggest you rename “BT70 S” to “stage” or something that represents the whole rocket to you, or leave the name as is if it makes sense to you. And when you have weighed the rocket and measured the balance point, right click the “BT70 S” component and enter the mass and Cg overrides.
 
Thanks guys. I think I do not understand the CG and CP overrides functions. Will they not adjust automatically after I override mass of either the individual components or the entire rocket? Or is the task to do an independent physical test (find the balance point on desktop) and change CG to that?

_

To answer these questions specifically.

Yes, to your first question. The program calculates individual component weights and Cgs based on materials and dimensions. And it calculates the overall weight and Cg of the rocket based on those individual components. If you enter mass and/or Cg overrides for components to override those calculated values, then the program will recalculate the overall mass and Cg of the entire rocket based on your overrides. But it’s almost impossible to accurately weigh each bit, account for glue, primer, filler, paint, clear coat, etc. and have the calculated mass and Cg come out exactly right and match the actual built rocket. Something is always a little bit off.

That’s why you should always weigh and balance the completed rocket and do a global override for mass and Cg for the whole rocket. When you do that, the program ignores all the calculated masses for components and the overrides for components that are nested inside the “stage”. So the answer to your second question is Yes too. Do a physical test and enter those values.
 
OR is a great program. But it will let you overdesign a rocket. By that, I mean that it is possible to account for every nut, bolt and even paint weight by adding in parts to account for these. You really only need to account for the major components initially to see if your design is in the right ballpark. And I've built many without going any further than that. If you want to get the maximum from your design you can go further, but if it's just an "is it safe" calculation, that's usually enough. Just make sure you have all your major bits accounted for. Don't forget about the hampster squid that will be at the top.
When you start with a new design you have your known knowns, your known unknowns and your unknown unknowns. As you design and start building, things move more toward the known knowns section.

The objective is to get all those into a flyable single known known. A rocket as we know it. :)

Good luck in your journey to L1 and let us know how it goes.
Norm
 
We have users who use it to maintain their BOM for them.
At that stage though we're past the initial design phase. Which was the discussion. This is a fairly generic rocket.

OR has capabilities way beyond the majority of daily usage of it. The team has done a great job on the update. :)
 
Looks like a nice clean design, if the structure is put together as well as it looks in the picture, I think you are in for many good flights!

may want to have a launch checklist, hate to see something happen on first or any flight that is one of those, :facepalm: moments.


straight trails!
 
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