thin wall tubes

[MadMat]

I have acquired a LOT of 7/8 i.d. cardboard tubing. It is very straight, consistent in size and an accurate 7/8 I.D. the only problem is the walls are only 3/32" and a test rocket failed when the sides blew out. I've been thinking of trying to wrap it with filament strapping tape or possibly even "painting" it with polyester resin. Any suggestions? The accuracy of the I.D. fits my tooling great and I would like to salvage this tubing if at all possible.

[mikeee]

You could try rolling several layers of additional paper on the exterior of the tube and use a paste

to adhere it to the existing tube. You will need to let the tubes/glue dry completely before using. You

can use a 7/8" wood dowel and slide the tubes over the dowel and use this as a mandrel to maintain

the shape of the tube and make it easier to wrap the additional paper onto the tubes tightly.

[Col]

8 turns of 90gsm kraft will add another mm to the wall, the strip of paper will be about 686mm long. I`d rough up the surface of the tube to ensure a good bond.

Edited September 12, 2015 by Col

[schroedinger]

Also you could just buy propper rocket tubes and use these tubes for other things. E.g. inserts or use them for cake items.

[Col]

If they are convolute wound it`d be cheaper to increase the wall than buy new 7/8" tubes.

[MadMat]

They are convolute wound, another one of the reasons I'm trying to salvage them for rockets. Being wound that way is the only reason I attempted a test rocket with that thin of a wall in the first place. I think I am going to try "painting" one with polyester resin. It probably won't work, but I got a bunch of that resin, so its not really a waste. If it doesn't work I guess I'll be doing a bunch of paperwork.

[Mumbles]

What sort of rockets are you trying to make? I'm sure you can make them work, you might just need to back off the violence of the fuel a bit.

[Col]

Using a support will help alleviate potential damage to the tube during the ramming/pressing operation.

[MadMat]

I'm using Rcandy that is cast in the tube. I usually add a small amount of RIO. I cast the rockets in what I call a short core (only 15% of the length of the grain). This gives me a bigger boost at takeoff for stability and the rest of the flight it is an endburner. I actually tried 2 test rockets with these tubes. One was a standard core burner with no RIO added. The other was described above. Both rockets failed about 10 feet from launch when the sidewalls blew out. Inspecting the remains, I found the tubes broke almost exactly in the middle.

Edited September 13, 2015 by MadMat

[stix]

I'm using Rcandy that is cast in the tube. I usually add a small amount of RIO. I cast the rockets in what I call a short core (only 15% of the length of the grain). This gives me a bigger boost at takeoff for stability and the rest of the flight it is an endburner. I actually tried 2 test rockets with these tubes. One was a standard core burner with no RIO added. The other was described above. Both rockets failed about 10 feet from launch when the sidewalls blew out. Inspecting the remains, I found the tubes broke almost exactly in the middle.

 

Hey Mat, when you say "One was a standard core burner with no RIO added. The other was described above." you must have thought you posted it or deleted the reference. I can't see what you are referring to.

 

Are your motors nozzled, or nozzle-less?

 

Cheers.

[MadMat]

My 7/8 I.D. rockets have a nozzle, I have tried both 3/16 and 7/32 diameters on the nozzles. What I meant by a standard core is one which starts out with the same diameter as the nozzle with a 3 degree taper and runs into approx. 90 % of the length of the grain. The "short core" with RIO was the one I was referring to "above". Maybe this way of making rockets may seem unconventional, but through experimentation, I have gotten good results doing this.

 

Maybe upping the nozzle diameter a bit would help with these thin walled tubes; I guess its worth a try.

Edited September 14, 2015 by MadMat

[stix]

My 7/8 I.D. rockets have a nozzle, I have tried both 3/16 and 7/32 diameters on the nozzles. What I meant by a standard core is one which starts out with the same diameter as the nozzle with a 3 degree taper and runs into approx. 90 % of the length of the grain. The "short core" with RIO was the one I was referring to "above". Maybe this way of making rockets may seem unconventional, but through experimentation, I have gotten good results doing this.

 

Maybe upping the nozzle diameter a bit would help with these thin walled tubes; I guess its worth a try.

 

Yes, upping the nozzle diameter may do the trick. You could also drop the RIO to slow the burn rate.

[MadMat]

I fixed the problem! Upping the nozzle diameter worked. Still, I wrap the tube in two layers of filament tape. I do this especially to strengthen the tube around where I press my nozzle and bulkhead. I know the larger diameter nozzle reduces the exhaust velocity (lower specific impulse?) but since this tubing is approximately 1/3 the weight of the standard heavy walled tubing, my payload capability is probably the same.

Edited September 22, 2015 by MadMat

[stix]

Good to hear Mat. Yeah, you'll lose specific impulse but not necessarily total impulse (average thrust over time). Theoretically if you are using the same amount of the the fuel, then with a larger nozzle, the pressure drops, the thrust drops but the burn time is extended. Of course this all depends on the pressure the motor is under to begin with.

 

There's a certain pressure where various fuels perform at their best. Sugar motors can deliver a specific impulse of around 130 seconds. I keep my own motors at around 100 just for safety reasons.

 

I'd be interested to know what sort of performance you get out of these motors, do you test them?

 

Cheers.

[MadMat]

By testing, do you mean measuring thrust ect.? Sorry but no. I haven't gotten that involved in rocket making.... yet. For the time being, my rockets are just carriers for a header. I have a ways to go to find out things such as maximum payload, ect. All I can tell you for now is they fly like all hell and to a pretty dam good altitude.

[stix]

Yes, I did mean thrust. I guess it doesn't really matter though at the moment. "fly like all hell and to a pretty dam good altitude" seems like a reasonable and positive description.

[MadMat]

Thanks to all of you, in particular, stix.

I'm not a total noob to rockets, it's just been a long time, and for the most part I've been having to clear the cobwebs out of my memory. Back in my late teens and early twenties (a LONG time ago), I actually built a small scale rocket car. It was modelled after the "Blue Flame" (a long time land speed record holder). It was four feet long and constructed with an aluminum frame. The engine was a liquid fueled rocket, fueled by a combination of a solution of sodium borohydride and Albone peroxide (trade name for 35% hydrogen peroxide). I machined the engine out of 304 SS and even went so far as to design spring loaded ball check valves in the injectors. After a lot of testing, I got a measurement of 65-67 Lbs. thrust from the engine with a "burn time" of 4 sec. It's too bad I didn't put more time into a stabilization/guidance design for the car itself as it crashed and was destroyed on it's maiden run. After that, life, marriage and job requirements took up all my time and I stopped with my experiments.

[stix]

Not a problem Mat. The rocket car sounds like it was a lot of fun. Good to see you're back into it - I had a similar break until a few years ago I stumbled across sugar rockets on the net. And for the record, reading your posts, I didn't think you were a noob

 

I just realised that this statement I made in post #14 is incorrect.

 

...Yeah, you'll lose specific impulse but not necessarily total impulse...

 

If you lose on one, you'll also lose on the other.

 

The basic equations being:

Total Impulse = Average Thrust x Burn Time

Specific Impulse = Total Impulse / Fuel Weight

 

What I should have said was, "a reduction in pressure doesn't necessarily equate to a reduction in total impulse or specific impulse".

 

I did some tests a while back with 'scaling theory' ie. scaling up my standard (3/4" ID) motor. I had some tooling where I was able to make a larger motor (1.5" ID). The scale was almost perfect if you looked at it from the side. It ended up being almost exactly 7 times the amount of fuel than the original, although Kn ratios were slightly out.

 

Theoretically 7 times the amount of fuel, should give you 7 times the total impulse (specific impulse should be the same). I did the test on an accurate load cell and was very pleased with the results. The total impulse was almost exactly 7 times and the specific impulse almost exact. The only thing that was different than predicted, was the burn time - this makes sense because I would have had to 'scale up' the burn rate of the fuel as well - which I didn't.

 

The upshot was that due to the fuel rate not being scaled, and the slight reduction in Kn ratios, there was a drop in pressure and the motor burned a bit longer. Nevertheless this didn't affect the predicted total impulse. What this proved to me was that my fuel was working well within a range of pressures.

 

The tests were done using some basic equations, a software calculator, combined with practical and measurable results.

 

I've attached an image of the "prediction" calculations and results. The grey column is a base profile which was derived from an average of five tests. The yellow column is the prediction based on the base profile using the basic formula's above. The green column shows the actual measured results.

Cheers.

Edited September 26, 2015 by stix

[MadMat]

Nice! Yeah, I like when predictions come real close to actual results. I still have a lot of "what ifs" with my liquid fueled engine, such as; what if I could have acquired 50% hydrogen peroxide instead of 35%; or what if I would have had more room in the car's frame for larger fuel tanks; or the big one, what if I had designed some type of gyro stabilizer/guidance in the car. Oh well..... some day

Edited September 26, 2015 by MadMat