Questions about motor selection for Level 2 attempt.

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Astrofox

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I have a Loc-IV rocket that I used to obtain a level 1 certification. I was thinking about using a CTI J425 Blue Streak motor for my level 2 certification. However, I recently have been thinking about the concerns associated with using such a high thrust motor. I already purchased the motor in the spring but haven't been able to fly due to being busy with university work and the pandemic. At the time I didn't consider aeroelastic flutter and only now have been thinking about how thin the fins are and whether or not it would withstand such a flight. If it wouldn't survive a flight without much modification from the level 1 flight, then is there a way for me to avoid this issue? Is it possible for me to get another motor that's more suitable for the rocket as it is now and just set the J425 aside for a different rocket?

The construction of my Loc-IV was followed from John Coker's video on the rocket, and so I've epoxied the fin tabs to the motor mount tube and airframe well, but I don't believe that would ultimately have much of an effect on avoiding issues with aeroelastic fluttering. With the current simulations I've run in OpenRocket, if I were to fly it without much modification other than some additional work to make the nosecone more suitable for handling the shock chord and using a 42" chute instead of a 36", I would be hitting around a Mach number of 0.91, and an acceleration of 326 m/s^2, which in truth also concerns me looking in retrospect.

Thanks for your time on this. I feel like I've made some rookie mistake and just realized it before throwing myself out there and having to spend (much) more time and money rebuilding.
 

heada

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Tip to tip the fins with fiberglass. It'll stiffen them up and keep them attached even if they do flutter some.

Check the CP to CG relationship after adding mass to the aft end. You'll probably need to add mass to the nose to offset it.
 

Astrofox

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My Stability Caliber currently is at 1.87 without the motor installed, however, if I am able to do fiberglassing then I'd definitely check and recheck my CG. My concern is that I've never fiberglassed anything, and I'm uncertain where to get the correct fiberglass for the job or how I would cut them into the appropriate shapes for a tip to tip glassing. There's already thick epoxy fillets on the fins but I'm uncertain if that changes anything related to the process of fiberglassing. Further, I'd assume I would need to do a lot of sanding to remove the current coat of paint (at least, in the region I'll be fiberglassing) before doing anything substantial. Not too much of an issue but annoying nonetheless, but hey, what can you do?
 

heada

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There are several threads here that go over the process to do tip to tip fiberglassing. Your filets should be fine but you'll have to remove the paint. Depending on your location, fiberglass can be gotten from many different places. Online will probably be cheapest.
 

Tim51

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I agree with Heada's points. FWIW my 4" Loc L2 bird was unglassed, and featured 3mm ply fins. The flight was on a 6 grain 38mm motor with not quite as much kick as the J425 but a similar burn time. However, one thing you don't mention is recovery. I'm deducing from the simmed velocity you mention that you're intending to fly this rocket either with motor eject or a JL chute release, rather than full dual deploy (...?) An unglassed 4" Loc rocket with the extra weight of an Av bay will max out around Mach 0.60 / 4200' on a 38mm 6G; a fully glassed one around 3800' - I've flown both those combinations). So my point is that you could build a Loc Av Bay and payload extension kit for the rocket, get an altimeter, do some ground testing and practice dual deploy flights on some high thrusting Level 1 motors, then do the L2 cert with the J425 when you're ready. To get the most out of L2 you'll need to understand dual deploy, Av bay construction and be comfortable using electronic deployment anyway, so now is a good time.
 

Astrofox

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I have considered building a payload/avionics bay and doing dual deploy. I do have (some of) the materials because I initially intended to go down that route (primarily because level 3 requires experience for multiple dual deploy flights), however, recently I've been on the fence about it since I don't currently have access to the necessary tools for me to complete the bay and other components, and I wouldn't be able to start work on it for a month - primarily assuming that my university isn't going to close again out of concerns over covid and that I would have enough free time available to complete and test the systems appropriately. I can probably design the sled, refine the payload bay's design, and do a few other things in the meantime but that's as much as I can really do at the moment.

Regarding recovery, I was just thinking about using a 42" chute that would be ejected by the motor's charge. It'd still be the same old three fins and a nosecone design with some minor modifications to the nose for better shock chord attachment and potentially the aforementioned fiberglassing.
My only question is, what motors would you consider to be high thrusting if you could quantify that? My level 1 was done with a CTI I285 classic but I don't believe that could constitute as "high thrust" compared to say, a 350N thrust motor or so. Predicted Mach with the I285 might be around 0.7 on the rocket.
 

Tim51

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By 'high thrusting' I was thinking of propellants like blue streak or white thunder. (Eg I303, I470 or I540), motors that would easily push a fully laden 4" dual deploy rocket into the air but not so high that you couldn't see the apogee event. But if you're trying to do your L2 cert without either DD or chute release, simply relying on motor eject at apogee I'm afraid I can't advise you further - it's something you should discuss with your club officials who will be signing off your L2 certification.
 

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I have considered building a payload/avionics bay and doing dual deploy. I do have (some of) the materials because I initially intended to go down that route (primarily because level 3 requires experience for multiple dual deploy flights), however, recently I've been on the fence about it since I don't currently have access to the necessary tools for me to complete the bay and other components, and I wouldn't be able to start work on it for a month - primarily assuming that my university isn't going to close again out of concerns over covid and that I would have enough free time available to complete and test the systems appropriately. I can probably design the sled, refine the payload bay's design, and do a few other things in the meantime but that's as much as I can really do at the moment.

Regarding recovery, I was just thinking about using a 42" chute that would be ejected by the motor's charge. It'd still be the same old three fins and a nosecone design with some minor modifications to the nose for better shock chord attachment and potentially the aforementioned fiberglassing.
My only question is, what motors would you consider to be high thrusting if you could quantify that? My level 1 was done with a CTI I285 classic but I don't believe that could constitute as "high thrust" compared to say, a 350N thrust motor or so. Predicted Mach with the I285 might be around 0.7 on the rocket.
I thought L2 required electronic ejection? If I were you I would be building the electronics anyway as once you have the reliability of altimeter ejection you will never use motor eject again :). You could use the motor ejection as a backup.

Your speed with an I looks to be relatively low, so that should be no problem. M0.91 for the L2 cert is likely not too bad either, and around 10G or so is not very stressful. Relatively tame actually. I don't have any experience with the LOC IV, but I suspect you will be ok if you have decent-size fillets. They seem to be through-the-wall fins so strength should not be a problem I would think.

Going to the larger 42" chute is a good idea since the fins project past the rear end of the rocket. Slower descent speed protects the fins more on landing.

I would probably go for it myself, but the flight is your's, not mine. As rocketeers this is the sort of things we get to choose all the time. Good luck with the selection and certification flight :).
 
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Tim51

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Some time ago we a guy at our
No such requirement till L3.
This is true, but 54mm L and 75mm K or L motors tend to be plugged of course, so in terms of the OP's question, if say a flyer wishes to only use single deploy motor eject at apogee, then they will face limitations (eg J-K impulse motors in 54mm, certain diameters of mmt / airframe and possibly flying field etc). Hence my advice.
 

timbucktoo

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This is true, but 54mm L and 75mm K or L motors tend to be plugged of course, so in terms of the OP's question, if say a flyer wishes to only use single deploy motor eject at apogee, then they will face limitations (eg J-K impulse motors in 54mm, certain diameters of mmt / airframe and possibly flying field etc). Hence my advice.
He's not going to be flying the LOC IV on anything bigger than 38mm motors.
 
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boatgeek

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Me, I wouldn’t worry about reasonably sized 1/8” plywood fins until you crossed Mach. You might check with LOC and see how fast they have experience driving this kit. Speed is probably more of an issue than acceleration for reasonable motors (no VMax!).

If you have a spacer, you might also choose a 5-grain J. There are several around 300N average thrust. Save the big one for another flight.
 

Astrofox

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I do have (two) spacers available to me, however I've already purchased the J425 and don't think I could get my hands on a lower thrust motor because of that. However, I obtained it way back around the beginning of March. I don't know if that has any impact on whether or not I could purchase a different motor.

If I can do so through Apogee, I would definitely consider that route and have already done some preliminary looking for more appropriate motors.

In either case, I don't think it would be difficult to do a tip-to-tip fin fiberglassing or to work on an e-bay for the rocket. It just takes more time that I might not have, primarily based on my university's stance on dealing with the COVID pandemic in the fall.
 

Glasspack

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I have a Loc-IV rocket that I used to obtain a level 1 certification. I was thinking about using a CTI J425 Blue Streak motor for my level 2 certification. However, I recently have been thinking about the concerns associated with using such a high thrust motor. I already purchased the motor in the spring but haven't been able to fly due to being busy with university work and the pandemic. At the time I didn't consider aeroelastic flutter and only now have been thinking about how thin the fins are and whether or not it would withstand such a flight. If it wouldn't survive a flight without much modification from the level 1 flight, then is there a way for me to avoid this issue? Is it possible for me to get another motor that's more suitable for the rocket as it is now and just set the J425 aside for a different rocket?

The construction of my Loc-IV was followed from John Coker's video on the rocket, and so I've epoxied the fin tabs to the motor mount tube and airframe well, but I don't believe that would ultimately have much of an effect on avoiding issues with aeroelastic fluttering. With the current simulations I've run in OpenRocket, if I were to fly it without much modification other than some additional work to make the nosecone more suitable for handling the shock chord and using a 42" chute instead of a 36", I would be hitting around a Mach number of 0.91, and an acceleration of 326 m/s^2, which in truth also concerns me looking in retrospect.

Thanks for your time on this. I feel like I've made some rookie mistake and just realized it before throwing myself out there and having to spend (much) more time and money rebuilding.

Astrofox,

I wish you all the best on your LVL2 I am also worrying about my LVL2 flight for all the same reasons. I have scratch built a 4inch diameter heavy wall, paper wound airframe model that stands 90 inches. I am constantly playing with Rocksim and Open Rocket to verify flight sims. It is not very easy to find a motor that pushes this off the rail at a speed greater than 35 miles and hour, yet keeps the altitude below 4000 feet like I would like. I am very confident that for my LVL2 Cert Flight it will be fine, and plenty strong enough. I used Canvas Phenolic for the fin sets instead of Birch Ply. I am curious if you have already weighed your model fully assembled ? I am finding out through some test weighing, that Rock Sim and Open Rocket have my model on the heavier side of the scale, than I am actually seeing when I weigh the pieces.....

One of the things I found to be interesting, but not so much help, was the package for a kit will say "flies on J&K motors"..... (I.E. Binder Design Raptor/Cobra Kit) … But does not give out impulse range or number. My sim with w K1440 says it will do Mach 1.1 but; ….. I've read a K1100 was cardboard killer !! I have read many articles and got feed back from people that have flown cardboard models at Mach 1. I am NOT interested in that for a cert flight ….BUT, I am very interested in trying it later on with the Antares Model I am building. I may just be to scared to attempt it and save that flight for one of my Fiberglass kits. I am enjoying learning along with you, as more experienced modelers offer advice and comments.
 
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Steve Shannon

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You’re not required to admit you already have one J motor on hand when ordering from a vendor and nobody is going to check. Do your research, look at manufacturer recommendations, and get acquainted with Finsim.
Or, and in my opinion better yet because you’ll get more experience, build another rocket that’s sized right for the motor you have. Good luck with your certification!
 

Glasspack

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You’re not required to admit you already have one J motor on hand when ordering from a vendor and nobody is going to check. Do your research, look at manufacturer recommendations, and get acquainted with Finsim.
Or, and in my opinion better yet because you’ll get more experience, build another rocket that’s sized right for the motor you have. Good luck with your certification!

Steve,
I have been meaning to ask you about the Big Sky Rocketry Club. Are any of your Certifying members part of NAR or are you all members of Tripoli only.
I only have a NAR membership, but I would love to fly possibly Cert... with your club in Montana

Thank You
Paul

Oh **** ….. there it is …. right on the website Sorry
 

Steve Shannon

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Steve,
I have been meaning to ask you about the Big Sky Rocketry Club. Are any of your Certifying members part of NAR or are you all members of Tripoli only.
I only have a NAR membership, but I would love to fly possibly Cert... with your club in Montana

Thank You
Paul
Hi Paul,
We’re both a NAR section and Tripoli Prefecture. You’re welcome to come fly with us. I’m a NAR L3CC and we can help you certify at any level. Our launch yesterday was possibly the clearest, calmest skies I’ve seen. Smoke trails just hung in the air and didn’t waft away. Not one cloud all day.
 

Glasspack

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Sounds great...…….. Your site at Twin Bridges is even 4 hours closer to me than going back to my moms house in Wisconsin.
I am getting ready to start sanding the crap out of my model and begin the chore of trying to get a nice finish.
Have you seen the thread ? Any comments / Advice?
 

Astrofox

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Astrofox,

I wish you all the best on your LVL2 I am also worrying about my LVL2 flight for all the same reasons. I have scratch built a 4inch diameter heavy wall, paper wound airframe model that stands 90 inches. I am constantly playing with Rocksim and Open Rocket to verify flight sims. It is not very easy to find a motor that pushes this off the rail at a speed greater than 35 miles and hour, yet keeps the altitude below 4000 feet like I would like. I am very confident that for my LVL2 Cert Flight it will be fine, and plenty strong enough. I used Canvas Phenolic for the fin sets instead of Birch Ply. I am curious if you have already weighed your model fully assembled ? I am finding out through some test weighing, that Rock Sim and Open Rocket have my model on the heavier side of the scale, than I am actually seeing when I weigh the pieces.....

One of the things I found to be interesting, but not so much help, was the package for a kit will say "flies on J&K motors"..... (I.E. Binder Design Raptor/Cobra Kit) … But does not give out impulse range or number. My sim with w K1440 says it will do Mach 1.1 but; ….. I've read a K1100 was cardboard killer !! I have read many articles and got feed back from people that have flown cardboard models at Mach 1. I am NOT interested in that for a cert flight ….BUT, I am very interested in trying it later on with the Antares Model I am building. I may just be to scared to attempt it and save that flight for one of my Fiberglass kits. I am enjoying learning along with you, as more experienced modelers offer advice and comments.
OpenRocket does give you the option of manually changing the entire "dry" mass of the rocket and distance the CG is from the nose. You just need to double-click the sustainer option and you should be able to put in more appropriate parameters. As for my rocket, it's fully assembled and unless I need to make modifications, it's also weighed out.

You’re not required to admit you already have one J motor on hand when ordering from a vendor and nobody is going to check. Do your research, look at manufacturer recommendations, and get acquainted with Finsim.
Or, and in my opinion better yet because you’ll get more experience, build another rocket that’s sized right for the motor you have. Good luck with your certification!
Ah, okay. I was worried that they would ask about that sorta stuff. I know they require a level 1 certification card in order for you to purchase a motor but didn't know if they'd be adamant about whether or not I could get another motor if I found that the previous one purchased just wasn't appropriate. I'll look into finsim to see what's up with that. I had thought about purchasing the LOC 163 min diameter as I would expect it to be a decent intro into fiberglassing fins to the body tube and an introduction to minimum diameter rockets. It's also fairly cheap too (~$55, not including s&h), but I don't know what to expect with using the J425 on that at all, or if I could. Or maybe I could go up to the 54mm version, but of course, that's twice the price and I feel oddly "unfinished" with the current rocket I own somehow. Might just be my own bias for wanting to fly it again for level 2 lol, but I'll look into any solutions that I feel confident about.
 

Steve Shannon

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OpenRocket does give you the option of manually changing the entire "dry" mass of the rocket and distance the CG is from the nose. You just need to double-click the sustainer option and you should be able to put in more appropriate parameters. As for my rocket, it's fully assembled and unless I need to make modifications, it's also weighed out.



Ah, okay. I was worried that they would ask about that sorta stuff. I know they require a level 1 certification card in order for you to purchase a motor but didn't know if they'd be adamant about whether or not I could get another motor if I found that the previous one purchased just wasn't appropriate. I'll look into finsim to see what's up with that. I had thought about purchasing the LOC 163 min diameter as I would expect it to be a decent intro into fiberglassing fins to the body tube and an introduction to minimum diameter rockets. It's also fairly cheap too (~$55, not including s&h), but I don't know what to expect with using the J425 on that at all, or if I could. Or maybe I could go up to the 54mm version, but of course, that's twice the price and I feel oddly "unfinished" with the current rocket I own somehow. Might just be my own bias for wanting to fly it again for level 2 lol, but I'll look into any solutions that I feel confident about.
I’ve seen a lot of people build a second rocket to get more experience. Once they fly a few different L2 motors then they have a better idea of whether their L1 rocket is sufficient, plus they can fly more often on a wider variety of motors, which is really what it’s all about. My L1 rocket eventually flew just fine on a J350 which is a great L2 gateway motor.
 

Off Grid Gecko

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Have you run a fin flutter equation on your rocket at all?
My spreadsheet is for trapezoidal, but it's showing pretty low Mach numbers if I split the difference for a root chord. The fins are really tall and thin, not a good combo for high speed flight. It's also really difficult to consult a sheer modulus for plywood, but using 200,000 I get around Mach 0.6 (probably way too low). Douglas fir (solid) has a mod of 1.9 million psi and gives Mach 2 as the flutter point. Lots of variation in wood, so maybe a good time to invest in the fin software already mentioned.

Also I would look around and maybe ask people with the kit what kind of motors they have flown it on, estimated speed, and the results. I think experience would be the best thing here. Or perhaps if anyone can suggest a sheer modulus for 1/8" birch ply I can run the numbers again. Should be fairly close, just not super precise because I have to adjust the fin shape a bit to fit my formula.
 

kbRocket

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Ah, okay. I was worried that they would ask about that sorta stuff. I know they require a level 1 certification card in order for you to purchase a motor but didn't know if they'd be adamant about whether or not I could get another motor if I found that the previous one purchased just wasn't appropriate.
I recommend against breaking the rules. Most in rocketry are for a good reason. If you choose to purchase a second J+ impulse motor prior to your L2 cert you could hand off your first motor to someone else for safe keeping. Any L2 or even an L1 not in possession of a L2 motor.
 

blackjack2564

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I recommend against breaking the rules. Most in rocketry are for a good reason. If you choose to purchase a second J+ impulse motor prior to your L2 cert you could hand off your first motor to someone else for safe keeping. Any L2 or even an L1 not in possession of a L2 motor.
There are times when leniency for a rule like this are in order. Over the coarse of many years certifying fliers, things like this have been encountered:

Cert flight planned with motor X which under normal conditions would be fine, but day arrives for flight is windy and I recommend motor Y with more thrust for conditions at hand & safe stable flight. I want 20 to 1 with wind over 10mph.

Conversely times when motor chosen turns out to have too much thrust for project like above.

Motor does not fit when time to fly.
Flier does not have case to fly reload [for whatever reason ,person who promised loaner doesn't show up etc.etc.]

Ubfortunatwely L-1 and L-2 fliers are not required to consult or have project approved before building. So issues like these will ARISE FROM TIME TO TIME.

Just a few issues I have encountered, there are many more with lengthly explanations. Suffice to say there are enough legitimate reasons, I'm for exceptions occasionally.
It not like violating "recovery chute mandatory" rule...no exceptions!

By the time U get to L-3, better have learned all your lessons and get it right.
 
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