What did you do rocket wise today?

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Joharvey1

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attempted to build an OR file from scratch for my Prometheus 5. Trying to get the calibers value below 2... let the trial and error begin. Stock steel tipped nosecone making things complicated. I wasn't intending to fly this rocket on an M1350...lol. Even then, the stability is still not below 2 calibers...


prometheus2.jpg
 

Sooner Boomer

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Sunday was a beautiful day to launch in Western Oklahoma. A bunch of folks showed up, including a few students from a nearby university's physics club. (competition better watch out in the next Argonia Cup Challange!).

My two best launches were my Raspberry Ice (Fatboy rebuild) which went up on an E28 reload, and a Baby Bertha on a 3-B4-4 cluster. The Raspberry is a great flier. I'm glad I added baffles in the last rebuild! I just stuff an engine in one end and jam the chute in the other (it's hard to get the two mixed up). The BB flew straight and somewhat slowly. I could track it by eye until it was almost out of sight, then it layed over and ejected the streamer. Unfortunately, it landed on the runway. The BB fins are long, and the impact caused the balsa to split down the grain in several places. I'm not sure how I'll fix it: glue the splits with thin CA or paper over them. I think this is a perfect engine combo for this rocket! If someone wants to get into clusters, this would be hard to beat! OR says it went to a tad over 650 feet. This would make a great park flier with the streamer (about 12 feet of 2" crepe paper), but if you recover on a hard surface, a small chute (or larger chute with spill-hole) would be better (expect drift with chutes).
 

jqavins

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Stock steel tipped nosecone making things complicated.
Is it possible to change out the steel tip for alumineinium? That'd certainly help, though it's probably not enough. You could always pack the space between centering rings with something.

I'm not sure how I'll fix it: glue the splits with thin CA or paper over them.
Are the splits completely separated, or hanging on. If the latter, then here are my two cents: Get some wood glue, not CA, into the splits and make sure everything is aligned. Place good flat pieces on both sides of the fin with wax paper and clamp them.

If they're fully separated, it's harder, because I'd be afraid the pieces might slide out of alignment while the clamp is being tightened and you can't see it. Perhaps you can make flats that are a little smaller than the fin so you can see movement while tightening, and make adjustments when the clamp is snug and not tight. But I don't know how well that would work.

Another possibility - and I'm not too sure about this and really just thinking out loud, is a hybrid of wood glue and how I like to use CA for fin mounting when I can't use my jig. what I do then is use two drops of thick CA, one near but not at each end. I place one corner of the fin against the body tube, and I can keep it straight and aligned as I rotate it down and the drops hit. Hold it a few seconds while the CA kicks (thick is slow) and that has it tacked in place until I can attach it permanently. That's done by applying the wood glue fillets, which is where the strength always really lies anyway. sometimes I run some thick CA along the joint first, but I really don't think that's necessary.
1634650770800.png

So, in this case, since there aren't going to be fillets, Maybe use the same method to tack the pieces together and the, with a fresh (or freshly sharpened) blade, make a tiny V groove over the joint and fill that with the wood glue. Let it dry fully then repeat on the other side.

For either fix, papering afterword can't hurt.

Can you tell I really don't trust CA?
 

Joharvey1

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Is it possible to change out the steel tip for alumineinium? That'd certainly help, though it's probably not enough. You could always pack the space between centering rings with something.
I actually am now not sure if the tip is steel or aluminum. I said that offhand because it felt heavy without context to support my claim. The whole nose cone actually weighs 1519 grams stock. However, I noticed it has a screw in the bottom of the point, so its possible it could be removed for a different tip type. I also noticed today that Composite Warehouse sells replacement nosecones that contain a composite tip as opposed to metal. I wonder if this tip would be any lighter?

Anyone have experience with metal vs composite nose cone tips?

I"ll try to experiment with adding mass objects in open rocket to see if I can produce a difference in the stability.
 

jqavins

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1) Weigh the tip alone. It should be easy to tell steel from aluminum.
2) A composite tip would be hollow, so most certainly lighter than a solid one of either metal.
 

Doug Holverson

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I'm rebuilding my Arcie II pop-pod for rear ejection. I'm also replacing the shock cord in my rear ejection Condor pop pod. I'm wondering if elastic cord just ain't what it useta be: he new cords are weak and frail and the old cords are brittle and a bit dry rotted? Or maybe it's just that 1/16"and under cord is just wimpy Chinesium these days? Anybody else have issues with shock cords just snapping off after a couple of uses in the past two or three years?
 

Lee

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I actually am now not sure if the tip is steel or aluminum. I said that offhand because it felt heavy without context to support my claim. The whole nose cone actually weighs 1519 grams stock. However, I noticed it has a screw in the bottom of the point, so its possible it could be removed for a different tip type. I also noticed today that Composite Warehouse sells replacement nosecones that contain a composite tip as opposed to metal. I wonder if this tip would be any lighter?

Anyone have experience with metal vs composite nose cone tips?

I"ll try to experiment with adding mass objects in open rocket to see if I can produce a difference in the stability.
 

tsmith1315

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I actually am now not sure if the tip is steel or aluminum. I said that offhand because it felt heavy without context to support my claim. The whole nose cone actually weighs 1519 grams stock. However, I noticed it has a screw in the bottom of the point, so its possible it could be removed for a different tip type. I also noticed today that Composite Warehouse sells replacement nosecones that contain a composite tip as opposed to metal. I wonder if this tip would be any lighter?
For comparison, here's how my 3" metal tipped fiberglass nosecone shakes out:

Nose cone shell with coupler368.5 g12.99 oz60.9% of nosecone mass
Aft-mounted bulkplate37.7 g1.33 oz6.2%
Eye nut with washer and jam nut64.5 g2.27 oz10.7%
Solid metal tip35.3 g1.24 oz5.8%
All thread connecting tip to bulkplate98.5 g3.47 oz16.3%

Yes, that's only 99.9%, close enough for a comparison. I think the composite tips may be for launch sites that forbid metal-tipped nosecones.
 

Joharvey1

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For comparison, here's how my 3" metal tipped fiberglass nosecone shakes out:

Nose cone shell with coupler368.5 g12.99 oz60.9% of nosecone mass
Aft-mounted bulkplate37.7 g1.33 oz6.2%
Eye nut with washer and jam nut64.5 g2.27 oz10.7%
Solid metal tip35.3 g1.24 oz5.8%
All thread connecting tip to bulkplate98.5 g3.47 oz16.3%

Yes, that's only 99.9%, close enough for a comparison. I think the composite tips may be for launch sites that forbid metal-tipped nosecones.
That's good to know. So essentially by swapping nose cone tips I am not really gaining much in terms of weight reduction. Might save me a hundred dollars buying a new nose cone.
 

jqavins

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For comparison, here's how my 3" metal tipped fiberglass nosecone shakes out:

Nose cone shell with coupler368.5 g12.99 oz60.9% of nosecone mass
Aft-mounted bulkplate37.7 g1.33 oz6.2%
Eye nut with washer and jam nut64.5 g2.27 oz10.7%
Solid metal tip35.3 g1.24 oz5.8%
All thread connecting tip to bulkplate98.5 g3.47 oz16.3%

Yes, that's only 99.9%, close enough for a comparison. I think the composite tips may be for launch sites that forbid metal-tipped nosecones.
But remember, it's not about weight reduction, it's about moving the CG, in this case trying to move it aft. If you could add a negative eighty gram mass object right at the tip that might very well be significant. Might not be worth another $100. Surely not if other measures are required anyway.
 

Joharvey1

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But remember, it's not about weight reduction, it's about moving the CG, in this case trying to move it aft. If you could add a negative eighty gram mass object right at the tip that might very well be significant. Might not be worth another $100. Surely not if other measures are required anyway.
Sorry I should have said: when I reduce the weight of my 1519gram nose cone to 0 grams, it drops my CG under 2 to around 1.9 calibers. So anything that reduces my nose cone weight significantly would improve my static stability. Can you describe a negative gram mass object? Not familiar with the term.
 

Banzai88

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But remember, it's not about weight reduction, it's about moving the CG, in this case trying to move it aft.
Why not both? Weight costs newtons, newtons cost dollars. Weight reduction makes for higher flights and more flexibility in motor selection and thrust:weight safety margins.

Due the the magic of Open Rocket, one can play with both weight and location. Usually, the solution isn't to ADD weight, but to REDUCE the length of the rocket pieces to effectively move CG into the range that one is looking for.

Case in point, when I designed my L2 rocket, I built it in OR, added all the J and K motors currently available.....and adjusted the length of the body tubes to ensure a minimum of 1 caliber stability with the largest K available. Magically, with the available J motors, still only just slightly above 1.5! The only thing one really needs to be cognizant of is the minimum space necessary in the payload bay for the main parachute and recovery harness, and the same caveat for the drogue and harness.

If you design a rocket with a certain length, and are committed to it no matter what, you necessarily must accept that your stability margin might well ALWAYS be higher than 1 caliber unless you add a tremendous amount of weight, perhaps a dangerous amount, in all the wrong ways or places.

Bottom line for the OP, since it's currently a planning exercise, to leverage the weight of the current component selection, you necessarily must reduce body tube length or add weight to shift the CG rearward. Adding weight, you have to accept the penalty that must be offset with larger motors. Or the shape and size of the fins can be changed to advance the CP location forward with regard to the other components selected. It's all about trade offs.
 
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Joharvey1

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....Usually, the solution isn't to ADD weight, but to REDUCE the length of the rocket pieces to effectively move CG into the range that one is looking for.

If you design a rocket with a certain length, and are committed to it no matter what, you necessarily must accept that your stability margin might well ALWAYS be higher than 1 caliber unless you add a tremendous amount of weight, perhaps a dangerous amount, in all the wrong ways or places.
This was actually my first thought. The rocket was about 10ft long stock as you can see in the below OR screenshot. But if I were to eliminate the payload bay and just stick the nose cone on the end of the booster, I am thinking it will fix the stability issue. Or at least make it where I am adding nosecone weight instead of trying to remove it unsuccessfully.

Can you also describe a situation where you would "add a tremendous amount of weight, perhaps a dangerous amount, in all the wrong ways or places?" I think you meant a weathercock, unless I'm mistaken. What I typically do is always try to always keep my upper section weight below my booster weight so this does not occur by default. Basically just add enough weight to my rocket to fix any stability issue to above 1 caliber, but not so much it over balances the rocket.

If you had an alternate example in mind, I'd be interested to hear it, so I don't inadvertently make a mistake I didn't realize I was making.
 

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Banzai88

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This was actually my first thought. The rocket was about 10ft long stock as you can see in the below OR screenshot. But if I were to eliminate the payload bay and just stick the nose cone on the end of the booster, I am thinking it will fix the stability issue. Or at least make it where I am adding nosecone weight instead of trying to remove it unsuccessfully.

Can you also describe a situation where you would "add a tremendous amount of weight, perhaps a dangerous amount, in all the wrong ways or places?" I think you meant a weathercock, unless I'm mistaken. What I typically do is always try to always keep my upper section weight below my booster weight so this does not occur by default. Basically just add enough weight to my rocket to fix any stability issue to above 1 caliber, but not so much it over balances the rocket.

If you had an alternate example in mind, I'd be interested to hear it, so I don't inadvertently make a mistake I didn't realize I was making.
Weathercocking is one of the more manageable effects of 'overstability' and perhaps the least offensive.

Anywhere you add weight, it takes more energy to move that part. If you add weight to a nosecone, it takes more ejection force to get it moving. Subsequently, there is more energy to dissipate, which leads to a longer recovery harness, which leads to more weight and volume used......and you can see how it cycles. Further, adding a significant amount of weight in a nose cone leads to weight forward recovery issues....where the super heavy nose cone (relative to the rest of the rocket components) can effectively drag the entire rocket down in a linear manner. Mitigating that leads to an excessively large drogue (again, more weight, volume, and expense AND wind drift). Greater weight also adds to the necessary recovery upsizing, with the added bulk and expense that comes with it.

Also, adding weight means that you necessarily have to spend at least half a though on material strength of everything else due to planning for a worst case scenario deployments and recovery scenarios. You DO plan for off optimal deployments, right? High mass falling objects are more dangerous than low mass ones.

Most folks that add weight don't add structural reinforcement when, in many cases, you could do both. For example, instead of adding weight forward as a mass object in the nose, why not fiberglass wrap the tube? Need weight aft, why not tip-to-tip fiberglassing instead? The higher performance the rocket is, or is intended to be, the more important that the weight be put into structural integrity and not useless cargo.

Or if you simply MUST add weight, make it something useful than just useless cargo. How about a universal nose cone bay and use it for a GPS tracker?
 

Joharvey1

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Weathercocking is one of the more manageable effects of 'overstability' and perhaps the least offensive.

Anywhere you add weight, it takes more energy to move that part. If you add weight to a nosecone, it takes more ejection force to get it moving. Subsequently, there is more energy to dissipate, which leads to a longer recovery harness, which leads to more weight and volume used......and you can see how it cycles. Further, adding a significant amount of weight in a nose cone leads to weight forward recovery issues....where the super heavy nose cone (relative to the rest of the rocket components) can effectively drag the entire rocket down in a linear manner. Mitigating that leads to an excessively large drogue (again, more weight, volume, and expense AND wind drift). Greater weight also adds to the necessary recovery upsizing, with the added bulk and expense that comes with it.

Also, adding weight means that you necessarily have to spend at least half a though on material strength of everything else due to planning for a worst case scenario deployments and recovery scenarios. You DO plan for off optimal deployments, right? High mass falling objects are more dangerous than low mass ones.

Most folks that add weight don't add structural reinforcement when, in many cases, you could do both. For example, instead of adding weight forward as a mass object in the nose, why not fiberglass wrap the tube? Need weight aft, why not tip-to-tip fiberglassing instead? The higher performance the rocket is, or is intended to be, the more important that the weight be put into structural integrity and not useless cargo.

Or if you simply MUST add weight, make it something useful than just useless cargo. How about a universal nose cone bay and use it for a GPS tracker?
Thanks! This is helpful.

It had occurred to me to fiberglass some of my paper and plywood rockets in this way for this reason. In this case however, the rocket is already fully fiberglass, so I would just be adding more fiberglass or some other material.

Nose cone dragging drogue down in a linear manner is a concept I was just beginning to explore at the request of my recovery supplier. He was describing how the booster and upper section should come down at a certain angle relative to the drogue, and to not overdue the drogue due to putting undue stress on the recovery gear, if I said that correctly. So there has to be an upper limit to the size of the drogue, and relatedly, the weight of the two components...I think... relative to each other, if I said that correctly.
 

jqavins

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Can you describe a negative gram mass object? Not familiar with the term.
They are hypothetical objects that can only possibly exist in silico. In other words, it's when you add a mass object to an RS or OR model and set its mass to a negative number. Except the software won't let you do that (I think) so it has to be done as a thought experiment, or by using mass and GC overrides for the nose cone, which requires estimating the modified nose cone CG.

Why not both?
No reason. The subject that got the nose cone tip mass discussion started was the desire to reduce the static margin. That doesn't mean that overall mass reduction isn't a good thing too. It does mean that something that might be insignificant in one regard may still be significant in the other.
 

T-Rex

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@Joharvey1 You have me curious, and maybe I missed it somewhere, why are you concerned about a stability of 2? I fly a BSD THOR with a stability of 4.5 and it's just fine. 2 is nothing to get excited about.
 

tsmith1315

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But remember, it's not about weight reduction, it's about moving the CG.
Yeah, I'm working that subject on my L2 project now.
It's a simple design that I built previously and flew many times on some of the motors I'm simulating. But it looks a little overstable in OR compared to what I remember. The original was built a bit heavy in the can, so I'm thinking that may smooth things out.
 

GrouchoDuke

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@Joharvey1 You have me curious, and maybe I missed it somewhere, why are you concerned about a stability of 2?
I’m curious too. 2 is a nice happy place for many rockets. For some longer rockets or faster rockets a 2.0 at OpenRocket’s default of Mach 0.3, isn’t enough. Unless I’m missing something, I wouldn’t worry about that as a problem.
 

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@Joharvey1 You have me curious, and maybe I missed it somewhere, why are you concerned about a stability of 2? I fly a BSD THOR with a stability of 4.5 and it's just fine. 2 is nothing to get excited about.
This is pretty much what I was thinking. As long as you have good acceleration off the pad, you can accept a higher stability margin. I have a rocket with 5 calibers that flies straight as an arrow.
 

Joharvey1

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@Joharvey1 You have me curious, and maybe I missed it somewhere, why are you concerned about a stability of 2? I fly a BSD THOR with a stability of 4.5 and it's just fine. 2 is nothing to get excited about.
just me being over cautious. I currently have two rocket projects on the table in the 90-120 inch range or thereabouts. These are the first of my large rockets, so I wanted to be sure I was correct when the values of stability were a bit higher than I wanted them to be. I was previously advised that some rockets as high as 5 calibers can be flown. I didn't know if I should push my stability above a certain margin for this reason just to be on the safe side.
 

heada

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Yesterday I glued a fin onto a rocket. Yup, 1 fin. Was a forward canard on a 4" BBX. Wasn't a difficult glue up or a complex fin, just was busy and only glued on 1 fin. At least this time I didn't CA my finger to the fin first.

Some days, all you can do is glue on 1 fin.
 

jqavins

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just me being over cautious. I currently have two rocket projects on the table in the 90-120 inch range or thereabouts. These are the first of my large rockets, so I wanted to be sure I was correct when the values of stability were a bit higher than I wanted them to be. I was previously advised that some rockets as high as 5 calibers can be flown. I didn't know if I should push my stability above a certain margin for this reason just to be on the safe side.
Y'know, I just jumped in with ideas on achieving what you said you want to do, but I should have asked the same question. Your rocket's length to diameter ratio (L/D) is pretty large, so a high static margin may not be merely OK, but actually required. A rocket's CP migrates up as its angle of attack increases, and that effect grows greater as the length increases. It can be a fair fraction of the length, and when L/D is very large that can become several diameters, i.e. a large static margin figure. A stock Mean Machine has a really big static margin because it needs to, or it becomes unstable with only a modest angle of attack.

See Peak of Flight #470.

Some say that referring the CG-CP distance to the model's length rather than diameter is a better indication of a happy situation, and I think this is the reason. For that measure, 10 to 15 percent is what people recommend, as I recall. Which makes sense to me. A "normal looking" rocket often has L/D of about 10, so 10% to 15% of the length is 1 to 1.5 diameters, which we all know is a good place to be. Since your L/D is about 25, it could be that 2 calibers is a little low, as it's only 9.4%. Probably OK, I think, but maybe 2.5 to 3 calibers would be better.
 

DigBaddy

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Have been mulling some winter 29mm builds to give us more to fly on club days where we have the room and noticed Estes got some PSII parts back on their site that were gone a few days ago. So, I ordered some 2.5" tube, couplers and a nosecone to build an upscaled Nike-X clone and grabbed the MDRM body tube and nose cone. Had been thinking BT80 for the Nike-X, but the 2.5" PSII tube should be a bit stronger. Got plywood here for rings and fins, so not a whole lot else needed to get these going.
 

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I glued the sub-spars on a Stratoblaster nearly three decades later. I'm waiting for them to set up. I made scans of the die-crunched parts and messaged them to Jim Z.
 

Joharvey1

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Y'know, I just jumped in with ideas on achieving what you said you want to do, but I should have asked the same question. Your rocket's length to diameter ratio (L/D) is pretty large, so a high static margin may not be merely OK, but actually required. A rocket's CP migrates up as its angle of attack increases, and that effect grows greater as the length increases. It can be a fair fraction of the length, and when L/D is very large that can become several diameters, i.e. a large static margin figure. A stock Mean Machine has a really big static margin because it needs to, or it becomes unstable with only a modest angle of attack.

See Peak of Flight #470.

Some say that referring the CG-CP distance to the model's length rather than diameter is a better indication of a happy situation, and I think this is the reason. For that measure, 10 to 15 percent is what people recommend, as I recall. Which makes sense to me. A "normal looking" rocket often has L/D of about 10, so 10% to 15% of the length is 1 to 1.5 diameters, which we all know is a good place to be. Since your L/D is about 25, it could be that 2 calibers is a little low, as it's only 9.4%. Probably OK, I think, but maybe 2.5 to 3 calibers would be better.
Thats interesting food for thought. Thanks for pointing that out. I can now feel a bit more comfortable with the stability the way it is. Or, I can try to make it more in the range of what you suggest if that is avisable.
 
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