L1 Cert choices

Slightly irrelevant, but just weighed my 3" DS booster and it tips in at 1950g or so without a nose. This is the old, thick-wall carbonate/profusion model. The newer thin wall FG from Tim's new supplier will likely weigh less.

Andrew - thank you. All is good. I understand perfectionism.

David - thanks for looking despite the change in plans. This confirms my OR file and helps me going forward. DS 2.6 could be interesting also.

grouch - yet another kudo for JLCR. I'm very impressed - and equally impressive was the owner's support for a failure that was clearly due to misuse. I feel good about it, and good about landing nearby. lol, something in this build needs to be high tech.

Eric - you're building one too? That's cool, will you maiden it next Saturday? I have to order a lot of parts today, Priority Mail delivery. I have a question... you picked two DMS motors. Do you prefer DMS over RMS, or is this an availability decision? The H115DM has a nice thrust curve, only 10.6G's for 1840'. The H113SK hits harder: 11.8G's for 1800',

So all this got me thinking about an eyebolt in the NC. That's very HPR but then all that mass will stress the BT even more. Now I'm thinking of bracing it with horizontal CF rod and epoxy. Probably no Quick-link; just wrap the shock cord twice and epoxy/heatShrink the tag end, so the stress isn't concentrated on essentially a single point.

Eric, is that nylon or Kevlar? I worry about an MT-mounted lanyard zippering the BT, so I was thinking of mounting it on a coupler higher in the BT (a la Crazy Jim in one of his builds, maybe BH38). Then again, I don't want to add nose weight haphazardly. Is there any validity to my question of an MT-mounted lanyard zippering the cardboard?

If I fly it next Saturday, it will just be white. Without the fins painted, decal or clear coat.

The H115DM is an amazingly beautiful sparky motor for $30. And it does have a great thrust curve. I mention DMS motors for simplicity. No upfront hardware needed. No motor assembly issues to worry about. Just stick the motor in and go (do adjust the delay and add deployment charge).

I use both RMS and DMS motors equally, and AT or CTI. Mike always has a good selection with no HAZMAT.

I added a 1/8" kevlar Y harness to mine. Having a Y harness with some side wraps of masking tape will stop a zipper. Never been worried about zippers. Never had a zipper. I love long shock cords. The stock elastic will perform fine. Or double it and make it longer. Or replace it with 25' of 1/8" kevlar. I still might use the elastic in mine.

A longer shock cord should also stop any nose cone mount tearing off. But if you are still worried about it. Install a safety strap as shown in the picture. Connect to harness to both in case the plastic fail. (Only seen them fail when people use short 8' kevlar with no stretch). Go super long.

The end goal is to get you L1 on the first try. Build it nice and strong like you are doing. Keep the motor choice simple. Add a chute release if you want to walk less(come find me if you want to barrow one). Fill out your paperwork ahead of time. Get an easy L1 then play around with more complex.

Come find me with your rocket and your paperwork and I'll do your cert.

Looking for the thin wall FG will help keep weight down as well. The 2.6" FG Arcas from Madcow uses the thin wall FG and flies beautifully. Mine weighs in at 2.2 lbs fully loaded, but I bought mine before Madcow made the "payload bay/dual-deploy version." Also means mine is the older 29mm MMT version. I've flown it on an F52 all the way up to I224. F52 was a bit of a nail biter. Tad slow looking off the rod, and ~550' altitude.

I'm guessing with a little caution in construction, it's possible to get it into DD territory and still be under 1500g for class 1.

If using a Jolly Logic Chute Release instead of full DD setup, it's easily done.

The H115DM is an amazingly beautiful sparky motor for $30.

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Since it fits the rocket so well, that's a great choice for the cert flight. I'll bring my Nikon, maybe we can talk a friend into taking a photo so I can watch it. Did anyone ever mention how beautiful Snow Ranch is?

Which makes me think that I should get a Madcow rocket soon. Cows are to LUNAR as Wolverines are to U of M.

Thanks for all the answers WRT recovery systems. I'll order the must-haves tomorrow - do you know of somewhere in Sac that sells Kevlar cord? R/C Country didn't. Although I know where to get it online, it's in the critical path and I'd like to get it soon.

All 4 fins are finished. Weights came out about the same, given that I did one at a time and didn't weigh the microbubbles. All are strong, rigid, and safe-feeling. But no more building today. Find parts, place orders. I don't care if it's an AWR, I haven't flown a rocket in years and want some things "just so". Friends coming for dinner tonight, too. 2/3 of Midtown is closed though, post-St-Patty's-Day-traumatic-shutdown.

Come find me with your rocket and your paperwork and I'll do your cert.

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That would be grand - thank you!

My wife doesn't see the style interplay in our Victorian that I do. I think it makes the table and painting look nice.


(*AWR = Average White Rocket)

How many feet of kevlar do you want? I can bring some out to Mather tomorrow if you want it tomorrow.

Wow. Thank you! Maybe 5 feet for the harness. I'll order 25' online - I can hook that up a few days from now no problem. Let's do email to coordinate. (Hmmm... 5' for the looped part, so there is ~2.5' from MM to the point I'd attach the 25' section of Kevlar to the chute and NC. I wasn't very clear here.)

75 fps off rail is decent on skid mark. In general anything excess of 45fps off launch rail at 1.5-2.0 Stability cal is great. Then it's up to matching delays. If you're a second off don't fret.

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Andrew, with all the time you've put into stability, please let me know your opinion. With the heavier fins and selection of H motors that make sense, the stability is 1.79-1.84 cals. If there was 15g of epoxy at the tip of the NC, the stabilities are 1.98-2.03.

Is it worth adding the nose weight?

Rockethound said:

Andrew, with all the time you've put into stability, please let me know your opinion. With the heavier fins and selection of H motors that make sense, the stability is 1.79-1.84 cals. If there was 15g of epoxy at the tip of the NC, the stabilities are 1.98-2.03.

Is it worth adding the nose weight?

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If the stability is already at 1.98 or more don't add any additional weight. Honestly we see to much nose weight added on too many rockets when its not needed simply because people over do it. Most of my rockets unless they are going to be going through mach transition only receive nose weight if they are .5 or less, if the rocket will be going through mach transition then I worry about it a bit more. The Nike Smoke is a very stable rocket as built (Estes version,and the original too since it was designed as a free flight sounding rocket). Aim for a stability of 1, any more than that and its overstable (not necessarily a bad thing, but making it overstable intentionally without good reason is pointless). If going through mach transition then use additional sim programs like RAS Aero II and plot stability vs. time/velocity for the calibres of stability the rocket is built for to back stop OR or RocSim, it may be necessary to add nose weight beyond 1 calibre to maintain stability through the mach transition, however a stock built PSII Nike Smoke may not survive a mach velocity flight once or more than once.

Nose weight serves three good purposes and several bad ones:

Good ones first: First improves stability, Two brings a rocket to optimal mass for maximum altitude (generally an issue in non-fiberglass minimum diameter rockets), Three adds mass to limit altitude if needed for a given motor on a given field (better option is to fly a better choice of motor and forgo the extra mass).

Bad ones: Increases weight of rocket and by extension requires larger chutes for safe recovery, requires heavier construction to deal with heavier nosecone (not in all cases), requires bigger motors for similar altitudes as a lighter built rocket, and limits lower powered motor choices, a stock built PSII Smoke should be flyable on the recommended F motors up to moderate thrusting I motors, if the rockets weight is increased motor choices are limited more and more.

Just noticed that Tough Mudder is taking up all of Gibson Ranch on the 24th. Sent a note to the SARG FB page to alert them. We may not be launching this weekend...

Rich - clear and thorough analysis of my situation. For some reason I thought 2x was a good target, but now I recall 1x was stable unless >mach when CP moves forward. You mentioned optimum mass - I got a feel for that with the Smoke, how more weight can increase the coast phase after the boost phase. All goes well, but for...

Swatkat, say it ain't so! The Gods Must Be Crazy!

Rockethound said:

Rich - clear and thorough analysis of my situation. For some reason I thought 2x was a good target, but now I recall 1x was stable unless >mach when CP moves forward. You mentioned optimum mass - I got a feel for that with the Smoke, how more weight can increase the coast phase after the boost phase. All goes well, but for...

Swatkat, say it ain't so! The Gods Must Be Crazy!

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I *do* have a coke bottle rocket.

Rockethound said:

Andrew, with all the time you've put into stability, please let me know your opinion. With the heavier fins and selection of H motors that make sense, the stability is 1.79-1.84 cals. If there was 15g of epoxy at the tip of the NC, the stabilities are 1.98-2.03.

Is it worth adding the nose weight?

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I personally prefer a range of 1.7-2.0 on supersonic L-1 multistages predicting under M2.4. I had the booster more stable than sustainer for supersonic flight. I have limited experience to three HPR flights. 1 of those flights being single stage subsonic missile kit that could do M=0.8 on an I motor at 1.73 stability with 488 grams noseweight, which tracked completely straight (vertical) and true in a light breeze off rail. The RB05 had negative stability numbers when constructed no nose weights which is why I added that much. Granted it's a tad Overstable at 1.77 by rich's over 1 definition on an H around M=0.43 but it as a kit wasn't as stable as a Nike for example without adding counterweights to it. The I300 now sims 1.55 with the heavier fillets since I rebuilt it. I noticed on multistage projects when booster stability was slightly above 2.1 stability as over stable in a cross wind up to 15mph the entire rocket tilted about 6 degrees into wind when launched vertically when using H motors and lower end of 50-60 ft per second off rail. On other multistages that had a much higher exit velocity into two hundreds, there was no weather cocking despite booster stability was 2.2. I always kept the sustainers around 1.7 being conservative. I read IREC reports of student teams hitting trees by over stability way over 2 and destroying fins under 1.6 on larger team rockets. The Blackhawk MD kit our prof certified on had a stability of 2.4 with a higher launch rail exit velocity and was flown in no winds. Being new we were all HENK it's Overstable. But given enough launch rail exit velocity and very light winds it won't matter as much. Needless to say I prefer 1.7-2.0. On the scratch rockets you can design the rocket so you don't need nose weights but by final assembly you may be off by .1-.2 over or under a "target" goal by final construction. When you buy a kit you just try to stay acceptable with a range of motors or add mass or adjust motor overhang as needed. On a design you can tweak fins and fin location to change cp.


My opinion is your 1.79-1.84 is completely acceptable and will fly completely stable when with a light crosswind. You are not very overstable nor very under stable. The minimum stability on TRA l-2 exam was 1.0 or greater. A bunch of guys complain you can never be Overstable too much but I disagree with these lower L-1 models they get picky when you exceed 2.0 at certain winds and launch rail velocities by a CP aero force acting on airframe from fins compared to the CG position weight force creating a torque by the distances of the CG to cp affecting the self centering ability of the rocket. And you do not want it to self center into the wind for example and fly horizontally or wobble a bunch and destroy fins. So if I were you I would not add any nose weight until you decide to change motors.

One thing to keep in mind as motor burns the CG shifts forward. This is not supersonic but on supersonic models they have a CP shift at certain Mach ranges noticeably. I am personally certifying again at 1.77 Stability cal if that makes you feel better. I would not fly if winds are over 7-8mph at surface.

The L-3 dudes were saying you want a lot more stability once models exceed M2.5. I noticed a lot of their stabilities on large fast rockets were over three to account for the CP shifting.

Thanks, Andrew. In reality, after adding the Y harness the stability may drop to 1.75-1.80. There's no need to oversim the model; it will be built soon, I'll weigh it and find the CG, override everything and get on with life.

Oh, hmmm, it won't work very well to put pencils under the wings of a rocket to find the CG. I'll have to poke around TRF and see how CG is determined without wings to balance on.


Eric - thanks for the Kevlar!

Rockethound said:

Thanks, Andrew. In reality, after adding the Y harness the stability may drop to 1.75-1.80. There's no need to oversim the model; it will be built soon, I'll weigh it and find the CG, override everything and get on with life.

Oh, hmmm, it won't work very well to put pencils under the wings of a rocket to find the CG. I'll have to poke around TRF and see how CG is determined without wings to balance on.


Eric - thanks for the Kevlar!

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You really shouldn't need more than 1 cal stability, and there isn't much of an upper limit. Andrew is talking about his "supersonic L-1 multistages", which have to have a lot more margin for error due to CP shift through Mach.

Most Estes stuff is stable on any motor you put in it out of the box. I wouldn't worry about it too much- check OpenRocket, but don't worry about extra (>1) margin or anything.

For CG, I usually just balance the model on my finger to get a rough idea... It's not as crucial as in the airplane world in terms of exact numbers, you just need to check that it's forward of where it has to be.

... accidental resubmit ...

For CG, I usually just balance the model on my finger to get a rough idea... It's not as crucial as in the airplane world in terms of exact numbers, you just need to check that it's forward of where it has to be.

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Thanks, LR. Yes, for the big or fast gliders, 1mm change in CG changes flight characteristics dramatically. 5mm too far rearward and you need to wash your underwear. 5mm forward and you're flying a porpoise.

Rockethound said:

Oh, hmmm, it won't work very well to put pencils under the wings of a rocket to find the CG. I'll have to poke around TRF and see how CG is determined without wings to balance on.

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I usually do two steps. First one is with the rocket all up but no motor. Weight and measure CG location by balancing on your finger. Put that into your OR file as a mass override on the top level of sustainer, overriding all subcomponents. That lets you do all of the sims to make sure you're happy with the speed off the pad and the delays. Second step is to mark the CP as calculated by OR on the rocket. When you are ready to take the rocket to the RSO table, balance it again and make sure you have a caliber plus of stability.

I've flown rockets with ~6 calibers of stability successfully. You just have to get them moving before they leave the pad.

Yeah. What hurts people on Overstable stability is extreme crosswind velocity or a gust of wind and not leaving pad faster than RSO min off rail velocity spec. It has to be right at leaving pad that's when it's likely to tilt from crosswinds or gust. For subsonic boosters going slow off pad slower than your Nike with 15mph wind, I observed at 6 degree from predicted path,84 degrees from ground tilt at 2.1 stability margin. It flew at that six degree angle and no further tilt. The real screw ups are when these things don't exceed or meet min rail velocity and a gust smacks it hard. Rockets like CG infront of CP. Else crash. And supersonic just moves cp which complicates crap but you do not worry about it. Most kits are Overstable out of box and fly fine when sense is used and not flown in high winds. They are Overstable to adapt to a vast range of motor classes which have different CG configurations. I would fly your rocket at 1.75-1.8 and give the 1.0 Stability snobs the middle finger proudly for winds under 5-8mph.

I routinely fly my mini magg at .5 cal

It's motor burns so fast and violently. If the motor burns out by pad rail height then it's stability is more than 0.5, David. Sim it with a empty motor slot config and tell me what stability cAl is? I guess it leaves pad greater than 1.0 stability cal at high velocity or you have found an exception.

Andrew_ASC said:

It's motor burns so fast and violently. If the motor burns out by pad rail height then it's stability is more than 0.5, David. Sim it with a empty motor slot config and tell me what stability cAl is? I guess it leaves pad greater than 1.0 stability cal at high velocity or you have found an exception.

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nope, G76G burns well past the rail.

It's motor burns so fast and violently.

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"It is motor burns so fast and violently."??? Did you mean "Its motor..."? Why are you doing all this? Please don't take over the thread with OT nonsense. None of these gentlemen you refer to deserves to be flipped off. Your comment was unwelcome.

David, I agree with you. I routinely fire a .44 cal mag(num) and it's very stable.

Boatgeek, you outlined the steps I was considering. Thanks for letting me know the ropes. Re 6 cals, maybe that's the answer to David's "I don't want to dissuade you about the BH, but..." comment. Small fins, 5 cal overstable, could that maybe work out?

I know about angle of attack from gliders and GA aircraft. Here's it's like the vector dot product of the rocket axis and the direction of travel. More fun rocket science to learn!

Sorry about the unwelcome comment. About the 0.5 cal stability I accept it is an exception to a rule of thumb. The "workings" of why it works are beyond my knowledge. CG ahead of CP and as motor burns CG would only move more than the static location although below 1.0 and eventually I assume increase over 1.0 by burn out hopefully.

I assume the motor burns and CG changes but since it isn't burnt by pad height, who knows what its dynamic not static stability is. An equivalent was seeing a transistor in a lab and being told it was pixy dust by a prof when we asked how it worked defying rules we were taught. These were two diodes. Connected the current flowing opposite directions. The rules explained to us as mechanical engr students in an electrical lab that current flows one way in a diode. This device made no sense. It broke the rules and it increases current out. Three years later at another uni they had material science with electron hole mobility equations finally explaining the exception with math and material options that the guy called pixy dust. It explained how current in semiconductors could flow both ways at slower rates permissibly through materials. The don't understand thing and getting upset about not understanding was a natural feeling.

Base drag. Not many people account for it. You think a spool, pyramid or saucer has a stable CG/CP relationship. I think my crayon and "running with scissors" are in the .4 cal range. But simming it with base drag cone adds almost one full cal. And they fly great.

Its kinda funny how a simple "what should I use for L1" draws comments that are obviously misplaced and well about what the OP was asking. And I have read the same comments about a failed rocket launch in 3 different posts...

Sent from my VS988 using Tapatalk

About the 0.5 cal stability I accept it is an exception to a rule of thumb.

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Andrew, I'm sorry, but you don't have enough Years On Earth to know the envelope of anything intimately. Look up Qix in wikipedia; I did the HW and SW. You've learned a little about semiconductors, OK, but they don't break rules. They work according to quantum mechanics and you don't get QM yet. Who defines an envelope, anyhow? This is the ASDEX-400:


Just one of the hundreds of individually designed and fabricated cores for the superconducting containment electromagnet:


A picture of a picture of the internals during its construction:


The ASDEX was the prototype for ITER, designed by a dear friend of mine as MD of Max Planck Institute for Plasma Physics. Is fusion energy from a Tokamak "outside the envelope"? Whose envelope? What technology and specifics? Some people carefully push the envelope, then the next generation learns a new envelope. If I want to know something about math or physics, I trust this man. His results speak for themselves.

You are beginning the exciting journey of life. But you are at the beginning, when you should be gathering, processing, refining and personalizing that which you can learn. You are not yet in a position to be guiding people. There are people here who have both the knowledge and the experience to provide meaningful guidance, while keeping the recipient's best interests in mind. They have nothing to prove, only something to share.

Give it time. Then give back.

Maybe watch "Little Miss Sunshine" then "find" all the girls you can. That would be good, too.

Two things going on. First is that technically, anything at 0 or above is capable of being stable. So it’s not like, “oh no under 1, gonna crash”.

Second, with that particular rocket is it’s a short, fat rocket. 5.5” wide. The answer in this case, is that base drag creates stability, and in short fatties, it’s a significant amount.

Short version- stability margins are a guide meant to give room to absorb effects like wind , etc. in a single stage sub sonic, it’s a pretty forgiving thing.

Overstability has come up a few times. It’s common for long skinny rockets to fly well overstable, with relatively small fins. The only real downside is that is coning. Which, I probably only have a tiny grasp on myself. Short version is the long distance between the CG and CP acts a a longer lever, and in some cases can cause a slight spinning/gyrating

Never learned base drag as mechanical engineering student in fluid mech its mainly pipe flows and stuff plate drag, similitude, and some wind tunnel and hydrostatic labs. Learn about bernoullis, pitot tube design, manometers, etc. I took compressible gas dynamics for nozzle design and to learn about oblique shocks and other aspects. That had supersonic nozzle design and diffuser design then variable throat supersonic wind tunnels those sucked. Found some rocketry airfoils were symmetrical for supersonic flow to reduce oblique shocks you could size those as sharp wedges. Had to do a bunch of self reading on wing design books when I made those rocket fins. I could compute the drag given CFD info manually with a Cd in a force equation. I've flown Cessna 152 and done spin recovery. They never once mentioned base drag. Damn. It's not even like plate drag cross sectional area and velocity and density, Cd related as much. It's just new to me. Guess it's an Aero Dork thing. Heard a Von Karman has minimum pressure wave drag. I understand fin geometry affects cp by wing design books and playing with OR and equations exist to get sweep angles for Mach. Done some Cd vs Cl charts in fluids for airfoils NACA but nothing professional worthy. Anyways thanks guys. Guess it's gonna be like the P-51 faster with the oil cooler deployed and drag related as to why. Plate area says more drag then you guys got no there's more drag concepts, well sh*t. Man I really want to take some Aero courses someday. LOL. They never offered it here. Base drag... Yeah IDK. More aero flavored pixy dust.

What color is the sky in your world?

Just never heard of base drag.