- Jul 14, 2015
- Reaction score
- Northern NJ
Fully loaded is with motors.
Ok, so my CG fully loaded is still pretty low down. ThanksFully loaded is with motors.
No to both unfortunately on the instruction manual.Do the instructions talk about balancing and adding weight before flight? Do the instructions give you a CG to shoot for? And I'll ask because you put CG and swing test in same sentence, you measured your CG by just locating the point where the rocket balances, right?
I believe that the source info from Apogee said something like anything over the 10:1 there were insignificant returns, or something of that nature.Indeed.
The same base drag should be present regardless of the length of the rocket, and so the hack should apply no matter what.
As rockets get longer, the effect of the hack on CP diminishes, but I don't see why it should strictly only apply below a certain aspect ratio. If someone could explain why it would, I'd appreciate it. We are investigating building some sort of native support for this into a future version or OR.
But the important thing implied here is that it doesn't *hurt* to do it on longer rockets. If, say, the software automatically applied it to the blunt ends of all rockets then it would OK. Does this sound correct?In that, applying it to anything over the 10:1 ratio is a waste of time since the equations apparently have much better resolution and reflection of reality beyond that. To know for sure you'd have to run a few comparative sims and maybe pull back the curtain behind the original RS/OR code.
No idea, I've never personally used it on anything over 10 : 1, only on things less than that. You'd have to run a series of sims on a less than 10 : 1, a 10 : 1, and a more than 10 : 1 rocket model to see how WITH and WITHOUT the hack adjusted the CP to know for sure if coding in the 'hack' on 100% of sims is detrimental, or if it's even necessary. (I would run a few and start a new thread on it doing the comparison, but I won't have much free time until next week).But the important thing implied here is that it doesn't *hurt* to do it on longer rockets. If, say, the software automatically applied it to the blunt ends of all rockets then it would OK. Does this sound correct?
With the exception of hybrids and some minimum diameter rockets that are mostly motor, adding a motor will almost always drive the CG lower.Ok, so my CG fully loaded is still pretty low down. Thanks
With the exception of hybrids and some minimum diameter rockets that are mostly motor, adding a motor will almost always drive the CG lower.
Technically speaking the only way adding a motor will shift CG forward is if the CG of the motor falls forward of the CG of the rocket.
On simple 3FNC or 4 FNC rockets the CP falls just ahead of the forward end of the fin root. A large blunt aft end will shift it slightly rearward but not until the rocket is flying.Of course, just making the point that my CG fully loaded is 40.7" from nosecone, which is still substantially lower than the 38.5 that the other individual was worried about.
Apogee lists the CP as 46" from nosecone which just seems wholly inaccurate, even according to their own software.
On simple 3FNC or 4 FNC rockets the CP falls just ahead of the forward end of the fin root. A large blunt aft end will shift it slightly rearward but not until the rocket is flying.
I would probably trust the CP that Apogee published. They have a lot of experience.
The Fosse modification to the Barrowman equations has been pretty accurate in my experience. If you’re still nervous hang your rocket on a string and see whether it points into the wind. If it does, just fly the damned thing.
3FNC = 3 fin and nosecone, the simplest design.
4FNC = 4 fin + nosecone. Second simplest design and one which shifts the CP rearward slightly compared to the 3FNC.
Your model still shows the CP just ahead of the fins 44.8 inches, just over one inch forward from the Apogee model, and the CG nearly 6.5 inches ahead of that at 38.5. Too bad you didn’t insert the picture that has the motor loaded, but it probably doesn’t shift the CG rearward more than 2 or three inches. That’s easy enough to test once you’re loaded.
Mark the CP and test the CG. If it needs nose weight improvise. I had a candidate cut a hole in the base of a nosecone and put in a few rocks, followed by packing it with a sweat sock he had in his car. He flew his cert flight in memory of his father who had died of a brain tumor. He and his mother cried when he succeeded. I think I just had some dust in my eye.
You’ll do just fine.
FNC = Fin and Nose Cone. As in 3 or 4 Fin and nosecone rocket describing a very simple rocket.
Your question and concern is understandable, but I would listen experienced advice you have been given and trust the flight. You've built an established kit, I'd trust it to fly. If you want to further ease your mind, do a swing test and then fly it on a G before your cert attempt. If it were my rocket, I'd just relax and fly it.
I built a Darkstar Jr for my L1 amd flew it without the payload and AV bay. Come launch day, I noticed it was windy and other rockets were flying top far away. I took that into consideration and bought a motor from one of the vendors from several I simmmed ahead of time. I showed the rocket to the LCO, answered a few standard questions along with why I had 2 CP points marked (I used decals for CP with and without the payload bay.) It was approved and the rocket flew and was recovered down range by the away pads.
It was an uneventful flight that was almost a letdown from some other people's experience with L1. Whoosh, pop, recover - just like every other nominal low and mid powered flight I have had. It was really nothing to stress or worry about.
Based on this I would add a little nose weight.Here's the same model loaded (my model).
Rocksim's method for calculating CP
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Based on this I would add a little nose weight.
Ground test. There’s no reason to assume that a little nose weight will interfere with ejection of the nosecone.I have a second G-force here literally ready to build (yes I am that paranoid) in case something went wrong with my initial L1 flight.
I think I'm just going to rebuild the damn thing, weigh all the parts first, add some nose weight and HOPE that the additional nose weight does not prevent proper ejection.
Ground test. There’s no reason to assume that a little nose weight will interfere with ejection of the nosecone.
That’s the plan. Dry fit everything together as well and test the CG prior to gluing anything. Should tell me a lotIf you have a second kit, weigh and measure all of the parts and make your own sim file. You'll now have the dry weights which should be comparable to the one you built already. You'll still have to override for your glue, but it will be close. See how far apart your sim file is from the one provided and go from there.
Why not just cut it s bit lower and then reattach the pieces with a coupler afterwards?If you really feel that you need to add nose weight, your sim shows the nosecone shoulder as 2". Cut the forward tube at the bottom of the shoulder. Losing 2" from the end of the body tube is not all that significant. Most likely you would be the only one to notice. Don't re-glue the nose until you are sure......
Because I'm not smart enough to think of that... I like your way better.Why not just cut it s bit lower and then reattach the pieces with a coupler afterwards?
(Iff it is necessary to add nose weight)
If you really feel that you need to add nose weight, your sim shows the nosecone shoulder as 2". Cut the forward tube at the bottom of the shoulder. Losing 2" from the end of the body tube is not all that significant. Most likely you would be the only one to notice. Don't re-glue the nose until you are sure......
This is definitely an option I will look into.Why not just cut it s bit lower and then reattach the pieces with a coupler afterwards?
(Iff it is necessary to add nose weight)
Heyyy, now that’s a great idea. A small 3mm hole and a syringe should be more than enough and a lot less work. Thanks for the idea!You could drill a small hole into the nosecone a bit from the tip add some lead shot and squeeze in some polyurethane glue.
Do you still have it by any chance? Could you do me the favor of checking your CG by manually balancing it? (No motor loaded) nice paint job btwI got my L1 on the G Force. H128W motor. No added weight at all.
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