Rocket stick problem

[wncranger]

I've gotten to the point I can make motors by hand ramming (75:15:10 for the 2 oz). I'm using unwaxed tubes and haven't gotten any more CATO's since lightly moistening my bp with water. Now, I have a new problem, lol. My rockets don't fly straight.

 

I'm using 3mm, 12" dowels (I tried both square and round) for the 2 oz rockets and balancing just behind the motor using a thin blade as a pivot point. The motor is 3 inches long, nozzle and cavity centered. Flight conditions were zero wind, 75% humidity and 88° temperature. The launcher is a pvc tube, driven into the ground and angled approximately 40° NNE to open ground. The rockets leap out of the launch tube, fly straight up for about 50 feet then suddenly veer backwards, flying South!

 

Any clue what is going on and how I can correct it?

[SharkWhisperer]

Try moving your CG back a little bit by lengthening your dowels. I'd try 18" of your 3mm wood. Unclear if your square and round (less mass if same diameter & same wood as square) dowels are making rockets do the same thing. I generally balance a few inches behind the motor, perhaps a third the motor length in distance rearward, not right at the motor end. Else stick a second 12" dowel on the opposite side, though that'd add a little weight (total 24" dowel) and risk decreasing overall performance/range. I'd be aiming those suckers straight up until you get your stability sorted out....Reducing the clay at the front will also move your CG back a little bit, if feasible.

 

Curious what kind of tooling you're using. Is this a nozzled coreburner running 75% nitrate BP or an endburner??? Unusual to run 75% in a coreburner unless your BP's kinda slow. What are your core dimensions?

Edited August 18, 2020 by SharkWhisperer

[davidh]

Try a longer stick. 12" is a bit short. for very small rockets like 2oz rockets, I like to take a piece of bamboo (with the joints cut off) and crack off a piece to use as a stick. It's light and fairly stiff.

[wncranger]

SharkWhisperer, I really, really like your name, lol. The BP I'm using is a hot blend using willow charcoal. I'm using the 3/8" tooling from Caleb at WoodysRocks. I've tried both square dowels and round dowels and as soon as I can get some longer ones, I'll give it a whirl with them. I was thinking that might be the case, but I want to trim weight where I can. I'm rolling my own tubes from 90 lb virgin kraft paper with a wall thickness of 0.0485 inches. They are nozzled coreburners. I was told to use hot BP for them as well as my 4 oz rockets, which I've tested but not flown. I use ~60:30:10 with a charcoal blend for my 1 lb motors, again tested but not flown. My blackpowder is ball milled, willow charcoal which I also press into pucks on a hydraulic press and then corn them. It's much hotter than any commercial brand which isn't proving very useful except for some burst charges and, of course, my cannon, lol.

 

davidh, I like the way you think. I've got some bamboo growing next door. I'll ask my neighbor if he wouldn't mind me harvesting a stalk or two for testing. It's the kind that spreads like mad. My yard looked like spears were shooting out of the ground this spring, lol. If they prove useful, I might just let a stand grow, lol.

 

I suspected the moment arm was too short but was concerned about additional mass. I would love to characterize my motor and stick and eventually create a simulator. It would be awesome if we could model them and get them in the ballpark before construction. What would be really cool is to model a variety of fuels, such as BP, whistle and gilsonite (which I can't wait to experiment with).

 

Thank you both. I truly appreciate this community and can't wait until I can share something useful. I'm well into my journey into this art form and it has given me new life. I'm not so much into the booms as the amazing displays. Rockets offer me the chance to explore two subjects I love, rocketry and pyrotechnics.

[SharkWhisperer]

Cranger hey,

 

You'll get it dialed in. And if bamboo is free, all the better. Dry it out or cut the dead stalks so it's lighter. Glad you'd getting into the finer details--you'll learn a lot from experimenting. And great you're rolling your own tubes, too; that'll save ya some if you end up making many, especially your 1-pounders. And Caleb makes great tools--I've got a 1-lb set (core and end) that I love. I shoot some coreburners, but this year (apart from July) I'm mostly making ramped up end-burning model rocket motors with way more power and for pennies on the dollar compared to Estes prices. I use willow, too and love it. Never bothered pucking/corning--more of a wet granulating guy because it's faster for me to do quantity and makes some ass-whipping hot BP!

 

Keep it up & we'll all look forward to some video of your test launches!

 

p.s. "SW" is because I spend half the year as a scuba/spearfisherman in the FL Atlantic (and making bright lights in the remote Everglades), and tend to meet up with sharks that want me to share my catch from time-to-time. Like a testy woman, you just gotta use the right communication skills to get a happy ending! Bull sharks are dime-a-dozen but can get aggressive so they deserve your attention (reef sharks, lemons, blacktips don't really count--like big babies), especially if you don't see them coming right away, but if you whisper nicely then usually everybody ends the day on a happy note. In some areas they've learned the sound of the rubber bands on the gun firing or a spear hitting coral/rock (avoid) and that's like a dinner bell to them. But I've never shared and never will--that's just training them to expect a free lunch and increases their aggressiveness to the next guy...

[davidh]

BTW, center of pressure is very important. See https://www.grc.nasa.gov/www/k-12/rocket/rktstab.html. Just because it's fireworks doesn't mean it's not rocket science. A longer stick moves the CP aft. A lighter stick moves the CG forward.

Edited August 19, 2020 by davidh

[SharkWhisperer]

BTW, center of pressure is very important. See https://www.grc.nasa.gov/www/k-12/rocket/rktstab.html. Just because it's fireworks doesn't mean it's not rocket science. A longer stick moves the CP aft. A lighter stick moves the CG forward.

Good point. You want your CP behind your CG. This is absolutely essential in model rocketry--I make a lot of homemade finned model rockets with cheap and powerful homemade pyro motors. Don't forget that as your motor burn fuel and gets lighter, the CG moves backwards towards your CP, which can decrease stability if they overlap or reverse desired relative position along your rocket. Select your sticks so CG is mostly where ya want it and CP is plenty behind so they don't overlap as CG moves backwards during the burn. Most pyros I know that don't also blast finned rockets don't even know what CP means, and just use trial and error with different sticks to find one that works.

[Arthur]

One rule of thumb used is to ensure that the balance point of the rocket and stick is only just behind the motor nozzle.

[SharkWhisperer]

One rule of thumb used is to ensure that the balance point of the rocket and stick is only just behind the motor nozzle.

Arthur, with all respect, I contest that "rule of thumb". In dude's first post he clearly states that his pivot point is immediately at the nozzle end of his motor and he's getting unstable flight. I typically balance about 1/3 motor length behind the nozzle, moving that CG back just a little (which means a longer stick vs using nozzle region CG); this also moves your CP backwards, too. Some balance 1/2 motor length behind the nozzle, but then you need to add a lot of mass in longer stick. 1/3 back is the sweet spot, at least in my hands. I've had many CATOs over the years from grain defects or estimating the wrong nozzle diameter for my propellant and coring, but never have had a rocket change direction in flight.

[wncranger]

SharkWhisperer, I think scuba diving is pretty awesome. My wife and I took the PADI course but after an incident during her closed water test disclosed a heart arrhythmia, I never took the open water portion. Sharks remind me of dogs in a lot of ways and are really fascinating. I respect your ethics (and wisdom!) in regards to not giving them your catch. Have you seen or encountered any whites? I can tell you, from the deck of a fishing boat, a 14 to 15 footer looks much bigger than you think. I can only imagine one in the water, lol. I'd have a heart arrhythmia, myself!

 

davidh, yes, the CG/CP phenomenon is something I deal with in designing model rockets. I figured it was the problem but wanted to ask as I'm not familiar with designing skyrockets. I've read about balance points from 2/3rds the length of the motor to just behind the nozzle but I think with the mass of the stick being off center to the center of thrust, the problem is a bit more complicated. I'll try longer sticks and continue testing to dial it in. Then I'll try to work out the physics .

[SharkWhisperer]

SharkWhisperer, I think scuba diving is pretty awesome. My wife and I took the PADI course but after an incident during her closed water test disclosed a heart arrhythmia, I never took the open water portion. Sharks remind me of dogs in a lot of ways and are really fascinating. I respect your ethics (and wisdom!) in regards to not giving them your catch. Have you seen or encountered any whites? I can tell you, from the deck of a fishing boat, a 14 to 15 footer looks much bigger than you think. I can only imagine one in the water, lol. I'd have a heart arrhythmia, myself!

 

davidh, yes, the CG/CP phenomenon is something I deal with in designing model rockets. I figured it was the problem but wanted to ask as I'm not familiar with designing skyrockets. I've read about balance points from 2/3rds the length of the motor to just behind the nozzle but I think with the mass of the stick being off center to the center of thrust, the problem is a bit more complicated. I'll try longer sticks and continue testing to dial it in. Then I'll try to work out the physics .

Hey Cranger,

 

I wouldn't worry so much about the stick being off center to the center of thrust with smaller rockets, or there'd never be a commercial Chicom rocket flying in a straight path. It will probably become more of an issue when you step up your motor size to something seriously large, particularly if it's pushing a heavy header. With 1 lb (3/4" ID) motors I never seem to have any issues with stable flight, and I've used the round thin bamboo sticks for gardening and HD's poplar dowels most often. I think it was our colleague MinamotoKobayashi who shows some threads with dual sticks on his motors. Look 'em up--he's bright and good at a variety of pyro effects and rockets, and has posted useful information over the years. But again, on small motors, I wouldn't expect the center of thrust/stick non-centering makes a huge difference between straight up and 5 degrees from straight up, which you probably wouldn't notice. Think your stick's just too short and the CG might move dangerously close to your CP (or past it), which is about the only way to turn a rocket the opposite direction in flight. You are into model rocketry, too? Cool beans. I make/blast shells and whatnot, but my primary efforts go to making motors for various mid-power finned model rockets, whether from kits (plenty) or from scratch (much much cheaper and more reward because more investment). Some come home and some are not meant to, tsk tsk. There are not many who merge the hobbies. If interested in the hybrid hobby aspect, there are several threads by a fellow rocketeer/pyro "HCB", who hasn't been active for awhile, but who fastidiously tracked his efforts to run Estes Patriot Missiles with his own motors (and salvaging the fin ends of failures to make a two-stager, all with homemade motors). Dude's been quiet lately (personal stuff going on it seemed---enough about that), but a lot of back-n-forth about technical details of merging homemade motors with finned "model" rockets. I love that shit. You might enjoy it. You're in the desert, so a short drive outta Phoenix and you can blast away to your heart's content. Give the UFO club in Sedona something to talk about!!

 

Off topic and should be relegated to PMs, but yes, I've dived with whites. But in a cage. Off Gaansbai, South Africa, where I used to work and visit. Beautiful creatures, but glad I had a cage--a super aggressive 14-foot female white chased off 2 10-foot adolescent males, which was cool to watch. Prefer not to meet them underwater without protection, though many do (there's many YouTube videos of some hot girl diving alongside whites--love to meet the woman, but she can keep the sharks). Like pyro, if things head south, it's gonna be unexpected, fast, and possibly serious. Bull sharks are a real concern in FL, sharkbite capital of the world. They bite more people than any other species, often surfers and divers (stats are not hard to find online). They always get/keep my attention, especially if I'm a half mile swim out from Lauderdale by the Sea's beach on a shore dive and not boat diving. They can be persistent, too. I've jabbed (not stabbed) several bulls that dropped their pectoral fins (classic shark aggression sign) and came in close for a look-see/taste-see. The most important thing is to finish off any speared fish that wasn't immediately "brained" and motionless. Blood in the water? Meh. Don't care. Wriggling fish on a 5-foot spear...that's attractive to both sharks and barracuda. Speaking of barracuda, that, and invasive/venomous lionfish, are my two favorite eating fish. Barracuda are generally harmless and I don't shoot anything over 3 feet because of possible ciguatera toxin accumulation in their meat (same with any reef fish in FL), and because any bigger and you need to reel them in, straddle them like a bull rider, and stab their tiny brain with my kill knife to end the fight--that's a lot going on in a short time, and their teeth indeed are like razor blades (reason why you try to shoot through gills at a 45-degree angle from behind--so you can handle them on the spear underwater/brain them without them facing you). Sorry to hear of your wife's A-fib issues (doctorate in cardiovascular pathology), but it might be able to be controlled with typical A-fib drugs like amiodarone?

 

Anyways, good luck with your pyro adventures--I believe you'll really enjoy the challenges and the successes, and it's just a whole lot of fun!

[wncranger]

SharkWhisperer, awesome advice, thank you. I'll definitely check those threads!

[BetICouldMake1]

If using shorter sticks you can use two rather than one to help balance the rocket. I prefer doing this on all my rockets. For example, I use two 18-24" sticks on my 3/4ID motors, which is much shorter than would typically be used. The other thing to consider is how long of a delay you have. The bulk of your thrust is from the core burning, which is almost instant, the solid powder above the core mostly just creates a tail and/or serves as the delay for a header. If your powder is weak it could be your rockets have enough power to get up, but not enough for a decent coast and the weight of the delay/clay is just dragging your rocket down while the delay is still burning. Unbalanced rockets might have an erratic flight but if yours are literally going up and back down that sounds more like a thrust issue than a stick issue. There are lots of pyro rocket guys that don't bother balancing or worrying about CG and CP and make great rockets. I check the balance on mine, but unless it's way off I don't worry too much. There's a fairly wide window that will work.

[dagabu]

Some 10 years back I showed 1/2" rockets launched on plastic drinking straws, 3 fin rockets with no sticks, 2 stick rockets like the above post etc. I pay absolutely no attention to the CG or thrust center, blah, blah. blah....

 

In the end, sticks are a crutch, they are attached to MAKE the rocket go straight but often they don't. Take a stick and bend it along it's length, see how it bends MUCH easier one way that the opposite? What effect does that create when the thrust hits? 12 inch, 36 inch, 48 inch, a stick that will yield to make a corkscrew will still not fly straight.

 

Problem solved! Strong sticks, OK, heavy... Less flight height. Sigh.

 

Forget about all the "Rocket Science" and go back to what works for you, extend your stick some and see if it helps, if not, try 2 sticks 180º apart but shorter.

 

Also, keep in mind that there are layers to the air above us, most places it's about 100 feet above ground level and the wind can change drastically, you need to consider that wind when you launch, a longer stick will 'buck' the wind layer a dna shorter stick will 'yaw' allowing it to turn into the wind. Both have their uses.

 

Good luck

[BetICouldMake1]

Dag makes good points. Another reason I like the double stick method is because it's far more rigid than a single stick. I tie or tape the bottoms of the stick together. Makes it look like a crutch 😁

[davidh]

The bulls in Tultepec don't need no stinkin' sticks!

Otherwise, I'll argue that sticks are useful.