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I-Roc stability, 8oz in the nose, really?

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jrkennedy2

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I'm running RocSim (Version: 9.6.0f202) on a new critter I'm building, an LOC I-Roc. I have the beastie all built there and with a 4gr 38mm CTI in her, I have to add 8oz of weight to the nose to get 1.9 calibers of stability. That's not even the >=2 I'd prefer. Whoa, almost 1/2 pound! I'm using the RocSim calc and sim methods. With Barrowman it's worse.


I had to add quite a bit of weight to my Minie Mag so is this just a challenge with short-wide rockets? The I-Roc is quite a bit longer but still only 55" OAL. I read that on the Minie Mag, the short / wide actually imparts greater stability with some suggesting that no weight was required from their experience. I thought, great and tried it even though I didn't very much like the sims I saw with that "phantom cone" on the tail. It was a sky-writer until it burned off enough fuel to become a TOW missile. :y: Fortunately, heading away from everyone. Not going there again as I must have misunderstood other folks' results, since mine were quite bad.


Anyone else have an I-Roc and how much weight did you add?
 

3stoogesrocketry

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I'm running RocSim (Version: 9.6.0f202) on a new critter I'm building, an LOC I-Roc. I have the beastie all built there and with a 4gr 38mm CTI in her, I have to add 8oz of weight to the nose to get 1.9 calibers of stability. That's not even the >=2 I'd prefer. Whoa, almost 1/2 pound! I'm using the RocSim calc and sim methods. With Barrowman it's worse.


I had to add quite a bit of weight to my Minie Mag so is this just a challenge with short-wide rockets? The I-Roc is quite a bit longer but still only 55" OAL. I read that on the Minie Mag, the short / wide actually imparts greater stability with some suggesting that no weight was required from their experience. I thought, great and tried it even though I didn't very much like the sims I saw with that "phantom cone" on the tail. It was a sky-writer until it burned off enough fuel to become a TOW missile. :y: Fortunately, heading away from everyone. Not going there again as I must have misunderstood other folks' results, since mine were quite bad.


Anyone else have an I-Roc and how much weight did you add?
I have flown mine up to a J570 with no nose weight and no problems . The Iroc is a sweet flying rocket built stock.
 

jd2cylman

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I've flown my IROC on small motors such as the AT I195J with NO nose weight at all. It's also flown on CTI 6xl J520SK with no nose weight. Remember, the further the motor goes up the tube the more stable it is. In fact, until the chute release came out, the AT 38/600 case were the only reloads I used in it (well except for one J350 for L2 :wink: ).
 

markkoelsch

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You will have no issues without the nose weight. Unless you have added a bunch of weight to the rear of the rocket.

Base drag in short and stocky rockets is an issue in Rocksim and OR. They tend to under calculate it, so really the Cp is likely a little farther back. There is an article by Bruce Levinson in Apogee Newsletter 154 that will be very worth your time to read.
 

jd2cylman

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Unless the new LOC nose cone is something super light compared to our old original kits from the late '90's... :confused2:

Weigh your cone and let us know the weight. I can weigh mine, although it's painted, it should be close.
 

MikeyDSlagle

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Why <=2? If it is 55" long and 5.5 diameter your looking at standard 10:1 length:diameter which would be classic 1 Cal stability.

I've had my eye on the IROC for over a year now. Just never bought one. So no experience with it myself.

And 8oz IS 1\2 lb. ;)
 
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qquake2k

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I'm curious as to why you prefer greater than 2 calibers of stability. I always shoot for >1.
 

UhClem

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I'm running RocSim (Version: 9.6.0f202) on a new critter I'm building, an LOC I-Roc. I have the beastie all built there and with a 4gr 38mm CTI in her, I have to add 8oz of weight to the nose to get 1.9 calibers of stability.
I suggest that you use Rocksim to plot the movement of the CP. Use high wind settings and the lowest thrust motor you are likely to use. Then see if the CP ever moves ahead of the CG.
 

georgegassaway

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2 calibers of stability is a lot to purposely add noseweight for. Also calibers of stability is a screwy thing for short large diameter rockets, i keep thinking a great R&D project would be to explore using percentage of length instead, or perhaps some formula that considers both length and diameter. As there are short large diameter rockets that are stable with half a "caliber" of stability. And long skinny super-roc type models that can go unstable even with three "calibers" of stability (and that is for stiff superrocs, not counting the ones that bend and cause their own aerodynamic pitch problems).

Anyway.....

Everyone who has mentioned that theirs have flown fine, I think what would be most useful is to measure the launch CG fully prepped with the heaviest motor used, and the recovery system as used before, prepped as usual, and post the CG location. I realize that unless you took notes before, that would be a hassle to do right now so that may not be practical for now (Maybe measure the next time you prep to fly on a heavy motor).

But that's always the best indicator, a proven-stable CG location. For example I have found thru the years on two scale vehicles, Saturn-IB and Little Joe-II, a proven CG location where at that point or forward it is always stable, and behind that point it may or may NOT be stable (finding out the hard way). I am pretty sure the Little Joe has less than one caliber of stability, although determining the CP is very complicated due to the aerodynamic flow past the LES system and tower, takes more like NASA than Rocksim to sort out an accurate CP for that (which again is why a known-to-be-stable CG location is most useful).

Really wish the kits listed a "here or forward" CG location.


FWIW - an example of the Saturn-IB, CG at middle of the interstage adapter is known to be reliably stable. Aft of that, sometimes not stable (the more aft, the more likely it'll flip). This is for a model with scale-sized fins, like the old Centuri 1/100 kit. The original Estes 1/70 IB and Semroc clone have somewhat larger fins so their CP is more aft and their CG can be aft of that point (But I have no known to be reliably stable CG for that kit)




Link to original drawing showing the CG:

https://georgesrockets.com/GRP/AOL/GCGassaway/SCALE/saturns/Saturn_IB_drwg.GIF
 
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jrkennedy2

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Thank you all for the guidance!

Don't know why I got 2 caliber as the goal. It is indeed 1 caliber. :blush: I guess after that sky-writer I did (and felt really bad about) I just got twitchy.

:duck:

Thanks again! Man I love this stuff!!!!! It just makes me smile.
 

jrkennedy2

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Why <=2? If it is 55" long and 5.5 diameter your looking at standard 10:1 length:diameter which would be classic 1 Cal stability.

I've had my eye on the IROC for over a year now. Just never bought one. So no experience with it myself.

And 8oz IS 1\2 lb. ;)
I had been looking at her for about a year too. I pulled the trigger as a last salute to Countyline Hobbies (online store departing). It was drop-shipped from LOC. I was hoping to get one of the newer styles with the LOC-N-fins but I got an older one with the simple thru-fin design. It's Ok cause I'm used to that style. But, the LOC-n-fin I had on the Minie Mag was very cool and tough as nails.

Regardless, it is going together well and looks really nice! Still pondering paint as it has plenty of real estate to work with. And yes, 8oz = 1/2lb. I was still stunned at my attempt to make 2 calibers stability (I know, I know) to make much sense. I seldom do, really...
 

LOC

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Unless the new LOC nose cone is something super light compared to our old original kits from the late '90's... :confused2:

Weigh your cone and let us know the weight. I can weigh mine, although it's painted, it should be close.
The batch of 5.38L cones we just received in January this year weigh in at 1lb 3.5oz...
 

MikeyDSlagle

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I would've got one for Christmas from County Line but when the near wife called to order, she was told...well something that upset/disappointed her. So she called Mike Fisher and I got number 2 on my list, a Tyrannosaur.

I'll get an Iroc eventually, but it has moved down my list. One from MAC will likely be next, something different. Just trying to decide which one, and hoping I can catch a sale when I actually have money to spend. Or quit spending on stupid stuff and just buy one already.
 
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