Modular Rocket Design

[ollie366]

I've been making 3/4" ID paper tube rockets for a few years, using R Candy or pressed BP. They work well but building them takes quite a bit of time and I sometimes got inconsistent results. With a long tube, I find it difficult to make a decent nozzle with convergent and divergent sections before or after the fuel is in place. With R Candy, pouring in the melted fuel can result in small voids with inconsistent burning due to the long tube. Then there's the issue of getting in a good sealed delay charge and ejection charge.

My approach was to make 3 separate sections- nozzle, fuel and recovery. Since each section is separate, I can make consistent pieces. The key is to tightly connect each section so the assembly stays together through the flight and landing. To take care of this, I use 2" wide maximum thickness aluminum tape with 5 wraps around the tube at the section joints. The added weight is not significant. Each tube section end is carefully cut to 90 degrees so the section ends align. At the nozzle-fuel joint, I apply a small bead of high temperature RTV. It works!

WRT to nozzle construction, the tube is 1-1/2" long and is made with Rocktite with a steel washer imbedded in the throat (1/4 to 3/8" hole). I use a #3 cake frosting tip with a light coating of silicone grease to form the divergent end. The convergent inlet is made using a 1/2" twist drill moved around in the Rocktite as it is hardening. I make 4 at a time.

WRT to the delay/ejection charges, the delay charge is just there to slowly burn for maybe 6- 8 secs and then ignite the ejection charge. To get a good seal around delay charge so the hot gas doesn't prematurely ignite the ejection charge I use a 1-1/4" long 3/4" dowel with a 3/8 hole that is epoxied into the recovery section tube. Before setting it in place, I ram in the delay charge powder so it's solid. The ejection charge can be added at the other end of the tube and then pushing in paper wadding to prevent the charge from coming out.

The fuel section can use whatever fuel mix you prefer. Whether it's R-Candy or BP (maybe something else), cut the length of tube you need just for the fuel, add the fuel and integrate the finished section into the assembly. I find it's easier to make just a fuel section without trying to work in the nozzle and/or the delay part.

I'll be very interested in comments you might have,

[cmjlab]

I had two thoughts.

 

1. Your method seems to be similar to model rocketry, but I'd love to see a video of your rocket flying. I'm surprised that your nozzle doesn't fall off, unless the whole thing goes inside a rocket body afterwards (in which case disregard everything after this).

 

In model rocketry it's a similar concept up to that all components are in fact built separately, just like you suggest. However, the rocket body acts a holder and case for the R-Candy motor, and what is usually a graphite or Al machined reusable nozzle. The finished R-Candy motor is cast and cored separately, then placed inside the rocket body, and held in by retainer clips built into the rocket body. If a nozzle is used, then it is typically a screw on nozzle to the motor before the motor is cast, but sometimes after (like pressed end-burner motors). The rocket body also has motor mounting rings, and a smaller retainer ring which holds the top of the motor in place and transfers thrust to the whole rocket. The ejection charge is built similar to how you describe yours.

 

I am not doubting you, I have no reason to, but I am surprised that Al tape and some RTV silicone adhesive holds your components together. Pretty cool though!

 

2. If you are making firework style rockets (i.e. 1lb tube, with a stick or two), why not use a single tube, press your clay nozzle (divergent nozzles for this style don't typically need a true 30/60 taper, as long as there is a bit of taper inside and if possible outside, then the gasses will be directed to the vent and prevent erosion for the short burn - at least for B.P.), then you can plug the nozzle with a dowel and proceed to cast R-Candy, and create the core like normal; with B.P. you can just form the clay nozzle like normal on a spindle. Then finish with ejection charge.

[cmjlab]

I'm my hurry to write down my thoughts before I forgot them :-), I forgot to say that I do like that you came up with a unique outside the box approach and figured out how to make it work. I certainly respect that!

[ollie366]

Thanks for the great reply and thoughts.

I wasn't clear that at this stage, I'm just making and testing the motors using the modular approach to make sure they are viable. Initially tested them upside down just to see if they would hold together (they all did) and then launched the assembled motors - not in a rocket body- with just a long stick or fins attached directly to the paper tube. No RUDs yet! After a few more successful bare modular motor launches I'll use them inside a proper rocket body. The nozzles have remained intact and I've seen very little erosion of the throat.

I have tried making R Candy motors as you described- press a clay/grog nozzle and then added the fuel with the nozzle throat plugged. A few issues- the nozzle eroded quite a bit and while I tried to get some convergence/divergence to the nozzle, it wasn't very good, so it results in some performance loss. Also the pouring the fuel into the top of the tube resulted in a mess on the side of the tube at the top- where the delay was located. I covered the inside of the top of the tube with tape or paper but it was still messy and sometimes resulted in the residual fuel igniting the ejection charge.

With my BP motors, I pressed the BP and put an 3/4" diameter aluminum blank in the bottom so when the fuel pressing was finished, the bottom of the tube had an empty section where I would make a nozzle using Rocktite or other material around a form to get some shape to the outlet. Still not ideal and some nozzles blew out. The BP residue at the top of the tube is also a potential short circuit pathway to prematurely ignite the ejection charge.

The modular approach seems to mitigate these issues.

Making segments of R Candy ID tube fuel for a 3/4" hasn't worked out for me- perhaps it's my technique- but getting them to seal at the walls is difficult, so I just pour it into the bare tube.

WRT to the concern of nozzles falling out, the wood ring at the nozzle outlet plus the Rocktite expanding cement seems to hold. The RTV is used mainly as a seal to block hot gas from leaking through any gaps in the tube-to-tube joint. I don't think there's enough burn time for a gas jet to burn through the aluminum tape so this is the "belt & suspenders" approach.

I hope that provides the info I overlooked in my 1st post. I look forward to additional comments/suggestions.

[cmjlab]

Interesting concept. Are you melting and casting R-Candy Motors or pounding them as a sieved combination? I never had luck ramming them, it always had to be melted, which of course comes with its own issues (like cooking off in the pan). I look forward to seeing a finished product!

[ollie366]

I melt it and pour it in, rapping the sides of the tube. I'm using KNPSB which has reasonably low viscosity at temp. I use a 1/4" core about 1/2 way up the completed fuel length. Hope to launch this version tomorrow.

[ollie366]

A quick update. I launched a modular 3/4" ID KNPSB rocket the other day. 75 g of fuel, 5" long, 1/4" core 3" long. 9/32 diameter nozzle throat made as earlier described. 

Excellent take-off and then the 5 sec delay worked. Lost sight of it and could not find it.  However I'm convinced the modular approach works reliably and makes it much easier to put these together and to optimize each section.