35 replies to this topic

[Kembang Api]

I am making a rocket that carry a round shell. For my safety concern, my initial goal is to fly the rocket without the shell.

I make a rocket from a paper tube made by rolling a straw paper of 115 grams in 7 turns. with the length 100 mm, a nozzel 7mm ( 10 mm clay) ID 20 mm and a bore of 70 mm above the clay. Filled with meal BP (59% Pot. 32% of pine charcoal and 10% sulfur). There is no end plug, cause intended to attach the shell with a delay fuse.

Sometime the rocket took off very well to about 200 meter high, but some other rocket just bust off from the other end.
From the nozzel onward I fill it with the above BP (70 mm), and the rest of it I used different composition ( slower BP). I had it dry over night.

Could it be that the slower BP should add dextrin? Cause BP with dextrin would be very hard, when dry. It will hold the pressure and slow down the burning of the shell's fuse. I had try to ram this area very hard, but still the rocket burn through before the rocket reach to the height.

Thank you very much for any suggestion.

[Phoenix]

It sounds like a clay plug at the top would help. Ram about 10-15mm of clay on top of the last increment of BP, and then carefully drill a small hole (about 2.5mm) through the clay at one side. Try to just touch the BP with the drill bit, rather than drill into it. Before you attach the shell, put a little meal or fine grain BP in the section of empty tube above the clay (normal 75:15:10) and then attach the shell with the fuse sticking in the BP. The clay plug will be more effective than rocket propellant at creating a strong seal. Once the propellant has burned to the clay plug, the pressure in the rocket motor will send a jet of flame and sparks through the hole, igniting the loose BP, which ignites your shell fuse. Note that the fuse must be quite short, or the shell will fall a long way to the ground before bursting. You can just use an empty spolette with a bit of black matchg through it to ignite the shell as soon as the rocket finishes.

Another possibility may be that the core is too long, so you could try reducing it, although 70mm sounds OK.

You may not need the slower BP on top of the core. What may happen with the extra weight of the shell is that the rocket will tip over once the slow BP starts burning, and head back down to the ground for several seconds. However, if a layer of slow BP works for you, then of course stick with it.

[BigG]

I am making a rocket that carry a round shell. For my safety concern, my initial goal is to fly the rocket without the shell.

I make a rocket from a paper tube made by rolling a straw paper of 115 grams in 7 turns. with the length 100 mm, a nozzel 7mm ( 10 mm clay) ID 20 mm and a bore of 70 mm above the clay. Filled with meal BP (59% Pot. 32% of pine charcoal and 10% sulfur). There is no end plug, cause intended to attach the shell with a delay fuse.

Sometime the rocket took off very well to about 200 meter high, but some other rocket just bust off from the other end.
From the nozzel onward I fill it with the above BP (70 mm), and the rest of it I used different composition ( slower BP). I had it dry over night.

Could it be that the slower BP should add dextrin? Cause BP with dextrin would be very hard, when dry. It will hold the pressure and slow down the burning of the shell's fuse. I had try to ram this area very hard, but still the rocket burn through before the rocket reach to the height.

Thank you very much for any suggestion.

First, you have to use right terminology ? Meal bp is not 59% Pot. 32% of pine charcoal and 10% sulfur. Are you using Meal bp (very fine granulates of bp) or the composition you just gave ? 59% KNO3, 32%C and 10%S?

The problem you describe is quite common. There are a few possible reasons for it. The first possibility is a weak nozzle. 20mm ID rocket is a BIG rocket (between 1lb-2lb size rocket) and require a large nozzle. The hole can be 7mm, but the plug need to 20mm deep. If this is your problem then the bottom blow off first.
The second problem is that your powder is not compacted enough. You can hit your mallet 5-6 times and it will still not be compact enough. This will cause the fire to propagate very quickly between the grains, increase burn rate, and increase gas pressure, which will blow your powder through the top. Third problem can be your composition. 20mm is large bore ? pure bp will be too fast, generating too much gas and again ? blowing the rocket through the top. Fourth and last possible problem ? you mention that you roll your own tubes. Unfortunately, with very large bores this sometimes lids to problem. The tube must be very tight and not deformed in any way. In one of the tubes wall layers is loose, the fire can propagate through the space and ignite the rest of the rocket.

The last point I did not understand is your point about dextrin. Are you loading the rocket using wet composition? That can lead to deform in your tube well, and from there ? similar problem to point four above. Loading wet is not recommended practice.

To have success with rockets use standard sizes (for example for 1lb rockets the size are: 3/4inch ID, 1/8 (or ?) wall, 7.5 inch length.). For this type of rockets slow the BP by adding more charcoal, and use heavy mallet with many blows. Good 6-10 blows on each layer should do the trick. Each layer should be compacted to about the ID of the tube (so for 3/4inch id, make sure that each layer is about ? in length after compacted). Make sure you record your procedure so you can repeat it if it is successful. If you plan on participating in a competition in the rockets category, use a press for your trails ? it makes success much more of a guarantee but require a lot of work.

Hope this helps.

[Kembang Api]

Phoenix: The idea of having the top plug with clay would make the rocket weight heavier. I personally like to take the idea as my second choice. I had make a 2 inches shell that weight about 60 grams, so weight is my concern.

Reducing the core would be a better choice, will try and let you know the result. You are right about using slower BP, I didn?t think about it. Anyway I will try that too. Thank you for your advice.

[Kembang Api]

BigG: Sorry for not using the right terminology and thank you for correcting it.

The meal powder should be fine powder with the composition of 58% KNO3. 32% C, 10% S.

The bottom plug with 7 mm hole is alright, It never even one blow away. It's always the top. Point no. 2 on compaction you might be right, will try to work on it. Point no. 3 is true, I had try before using 70-15-10 bp and bad result, just like what you mention. Point no. 4 about the hand roll tube. I did notice few of them had deformation like a wrinkle mark of the outer body. Specially above the bottom plug. Maybe I should roll more turn instead of 7, in order to reduce this effect.

The rocket was loaded with dampen BP using a 70:30 alcohol and water mix, so it is not wet. This is to reduce the dust during ramming.

Dextrin mix composition was intended to load the top part of the rocket which is about 20 mm. Reason was to get a harder BP, so it can stand the pressure.

A standard rocket mention, would be very long (7.5 inches = 190.5 mm). Let me stay with my size and try to over come the problem as advice.

Thank you very much

[maxman]

I used to have lots of problems with top plugs blowing out even using slow burnig mixtures like 63/27/10 kno3 sugar sulphur. I found that the problem was my poor quality tubes! They had 1/8" walls but if you can squash them by pressing your two thumbs on the walls then they are prob too weak. I couldnt understand it at first thinking my core was too long or something. I think if the tube is soft and flexiable,when the preassure builds up the top expands and the plug becomes loose.

Just tried the big brother from my sucessfull 1/2"id rockets. These are still 80mm in length but 19mm id. I hope to soon make one that is about 2" longer (5")
I guess these are semi-end burners if there is such a thing. I'm working on the core size at the moment. 20mm seems about right (into the grain)with a 6mm throat. I guess when these work ok it should just be a matter of using the longer tube and packing more propellant in? I am using straight meal with 10%c for a tail.

I tried one last week using 31mm into the grain and it was that fast it was just plain silly But didn't blow up never the less.

I understand that core burners will lift a bigger payload, is this right? having said that my 12x80mm semi end burners will lift 75g no problem so I'm not sure that changing them would give mutch advantage. I wonder what these 3/4" x 5" will lift?

[sasman]

maxman how did you solve the problem with your tubes how do you
make them hard? i have just rolled a few and even if i make 1/2" thick walls i can still squash them ...
thanks
sasman

[BigG]

I used to have lots of problems with top plugs blowing out even using slow burnig mixtures like 63/27/10 kno3 sugar sulphur. I found that the problem was my poor quality tubes! They had 1/8" walls but if you can squash them by pressing your two thumbs on the walls then they are prob too weak. I couldnt understand it at first thinking my core was too long or something. I think if the tube is soft and flexiable,when the preassure builds up the top expands and the plug becomes loose.

Just tried the big brother from my sucessfull 1/2"id rockets. These are still 80mm in length but 19mm id. I hope to soon make one that is about 2" longer (5")
I guess these are semi-end burners if there is such a thing. I'm working on the core size at the moment. 20mm seems about right (into the grain)with a 6mm throat. I guess when these work ok it should just be a matter of using the longer tube and packing more propellant in? I am using straight meal with 10%c for a tail.

I tried one last week using 31mm into the grain and it was that fast it was just plain silly   But didn't blow up never the less.

I understand that core burners will lift a bigger payload, is this right? having said that my 12x80mm semi end burners will lift 75g no problem so I'm not sure that changing them would give mutch advantage. I wonder what these 3/4" x 5" will lift?

My 1lb Rockets (3/4id, 7.5 inch long) will lift 140g to 120 meters plus without much effort. I did push it to 160 with good results but didn't try more then that.

The more I read into members post the more I get convinced that the problem is loading pressure. Kembang Api mentioned that his rocket goes 200 meters up, and that is very high. It suggests that maybe there are allot of loose grains and therefore the fire propagate between them and create a lots of gas. If the rocket does not blow, then it will go to great highest.

Of course, the deeper the core then more area you expose for the fire to catch. Because of that the amount of gas generated for lift is very large and allow a higher lift. Higher loading pressure actually SLOW the rocket as it does not allow the fire to propagate as fast. pressures of 6000 PSI and above are not uncommon. In whistle rockets common pressure is 8500 PSI +...

My suggestion is to give every increment a few extra blows with your mallet. Good luck!

[pritch]

When you see those shells go off at professional displays when you see the weeping willow effects in the sky etc, can you make payloads for rockets with the same effect?

Is it seen as more difficult to have to lift shells using rockets so people prefer to use mortar racks?

[pritch]

Just out of curiosity. How big a shell do you think you would need to get something like this?

http://cc.oulu.fi/~kempmp/johan2dd.jpg

[italteen3]

Just out of curiosity. How big a shell do you think you would need to get something like this?

http://cc.oulu.fi/~kempmp/johan2dd.jpg

WOW. Looks like the diameter on that massive break is around 300 feet across atleast. So I am going to say a 16" shell atleast, but looks like it could have gone up a lot more. Nice shell though!

[BigG]

When you see those shells go off at professional displays when you see the weeping willow effects in the sky etc, can you make payloads for rockets with the same effect?

Is it seen as more difficult to have to lift shells using rockets so people prefer to use mortar racks?

Well ? that?s a good question Deserves a long answer.

Of course, both systems have advantages and disadvantages. Lets look at rocket first.
1) They provide an additional effect on the way up that a ?regular shell? does not provide.
2) It provides a ?gentle lift?. The shell does not have to withstand large pressure when lifted; therefore the shell itself can be built in a more flimsy manner.
3) They do no require specially build mortars.
4) The delay is built-in the rocket.

In many ways, for an amateur ? those provide the perfect start, as they are very easy to build. The shells attached to them do not require time fuse or spollettes ? and if you have an odd size shell then you don?t need someone to make a special mortar to it. The Spanish and German like to fire lots of them in quick secession, where their somewhat erratic nature (they fly up but not really in direct strait line) make a great spectacle. However, they have limitation:

1) Shells require less BP for lift and therefore cheaper to build.
2) They do not have size limit.
3) They are not self-propelled and therefore represent less of a risk.

It might come as a surprise, but rocket is a real BP waster. For example, a 1lb rocket uses between 50-75g of BP to be filled complete. It can theoretically lift about 180g safely. The same 75 grams can probably lift around a kilo of a shell fired from a mortar. As for the second point ? well, thoreticly you can lift any size shell with a rocket, but the size of stick required will grow to such size that it will become unpractical. Of course, you can fin stabilize your rocket, which means that the rocket stop being quick to build? but will carry much more. However, from a safety point of view, rockets are almost the most unsafe thing you can have. When you shoot a shell, all you have to make sure is that the mortar point away from the crowed. Even if something goes wrong and the shell does not fly high enough or far enough, it is not self propelled and can not change direction. That is not the case with rockets. When they go wrong, they can go horribly wrong.

I hope this is a helpful overview of the two techniques.

Edited by BigG, 04 May 2004 - 08:54 AM.

[Yugen-biki]

BigG

My 1lb Rockets (3/4id, 7.5 inch long) will lift 140g to 120 meters plus without much effort

Only 140g!

How did the name "1 pound" rocket come up? Is the wheight of the finished motor or the amount that it can lift or...?

http://www.angelfire...kets/index.html This guy manages to lift 1 pound of water with his "1 pound" rocket. I?m thinking of buying a rocket tool from fire fox but I?m unsurtain of the "wheight nameing system". Lifting a 3" shell would be nice.

[Phoenix]

sasman said,

maxman how did you solve the problem with your tubes how do you
make them hard? i have just rolled a few and even if i make 1/2" thick walls i can still squash them  ...
thanks
sasman

Was this a wet rolled tube, or just dry rolled? What was the ID?

[Kembang Api]

I used to have lots of problems with top plugs blowing out even using slow burnig mixtures like 63/27/10 kno3 sugar sulphur. I found that the problem was my poor quality tubes! They had 1/8" walls but if you can squash them by pressing your two thumbs on the walls then they are prob too weak. I couldn?t understand it at first thinking my core was too long or something. I think if the tube is soft and flexible, when the pressure builds up the top expands and the plug becomes loose.

maxman: If the top plug loose because of the tube weakness. I think the nozzel plug should loose first as the pressure build up from this end and the rocket would not lift-up at all.

Presently I am using 115 grams/m2 straw paper and build the tube by 7 full turns, which give me about 1.2 mm wall thickness. The nozzel plug can stand the pressure and if I plug the other end, the rocket give a very gentle lift. This is the test I made to assure that the rocket will lift off.

The reason I did not apply the end plug, because I was going to add the 2 inches shell. This is where the problem start. the rocket lift up to 6 to 7 meter than the remaining (unburn) propellant just blown away.

I have to agree with BigG The whole problem seem to be in loading pressure. Today I had make 15 rocket with different BP's mix for the upper portion. In a few day I will test them, while waiting for the BP to dry completely.