32 replies to this topic

[Sparky]

Hi all

 

I've been trying to improve the reliability of my 1lb core burners and also get as much kick out of them as possible. Currently I work with a formula which is as follows:

 

KNO3: 55%

C (Willow AF): 31%

S: 14%

 

The KNO3 is ground to a fine powder and I simply screen everything together. When I ram this into my purchased spiral wound tubes they work pretty well but not quite as powerful as I think they could be. I know many people use 60:30:10 so I know my mixture could be hotter.

 

I've had some CATO's in the past using the hotter ratios so I went back to my tried and tested 55:31:14. Now tTo reduce dust and to see if I get a performance boost I wet the green mix like I do when I granulate BP. I then used a fine grater to create the granules, let it dry and rammed another engine. I don't add any dextrin and the granules are fairly weak which is no problem, I only need to keep them for a few hours anyway.

 

This motor CATO's even though I was very careful to ram it just as well. The plug and nozzle held, it simply split the tube open and made a fairly decent bang! I was also careful not to light the core all the way up, I specifically only ignited maybe the first 30% using blackmatch after a visco fuse. This was exactly how I fused the successful motor.

 

I've read a few places that the wetting process does increase the strength of the mixture but this seems excessive. Any other thoughts on what it could be? Does anyone here recommend convoluted paper tubes and know where to get them? I presume I could work with hotter mixes if I got stronger tubes.

 

 

[maxman]

Spiral tubes are not strong enough for rockets. I would say this is your first problem. 55.31.14 is not a strong mix so shouldn't blow your tubes. Using a press will also slow the reaction down so you can get away with a hotter mix but you WILL need a parallel tube and a sleeve to stop it splitting. Adding 1% dextrin will make the grain harder when pressed.

 

I'm sure cooperman could sort you out for tubes or maybe Oliver brown

 

Rod

[Sparky]

Hi Rod

 

These are fairly solid spiral wound tubes which seemed ideal for the 1lb tooling. I'm not using a press as yet but that is mainly due to lack of space until I extend my workshop.

 

Thanks for the advice, I think I will need to try Phil as I don't think Alex does parallel wound tubes.

[digger]

Why not roll your own?

 

Not wishing to stop a sale to whomever. However you can roll extremely strong tubes yourself on a simple mandrel with a rolling board.

 

Do a search on here for techniques.

 

I used to hand roll inch ID tubes with a 3mm wall that would hold a 75:15:10 core burner without CATOing (pressed in a clam shell casing).

 

If you are not doing many rockets it is not a big deal rolling a few, and you will have total control over the quality of tubes. I rolled about 200 tubes in one night.

 

G

[dr thrust]

Tell us about your tooling sparky, home made or commercial ?
Your choice of charcoal maybe the problem it's not the first choice for cored rockets, try ordinary garden BBQ stuff, or pine, it's the best for sparks ,sparky! I reserve willow for, "out of the tube like a stabbed rat" nozzle- less motors

[maxman]

If the tube is spiral, fire could have passed up the spiral winding causing the cato or the tube was just to weak. I can't imagine that the fuel would be too hot at those ratios. You need a strong tube and ram or press hard. Use a sleeve if you can. Or why not try a nozelless motor? Same tooling and use 75.15.10 hot fuel with no clay plug like the good Dr suggests.

 

Rod

[maxman]

Just seen these  http://www.oliverbro.../copy_of_1.html

 

By the way I prefer 4oz core burners as they take much less comp  (about 16 grams) they still lift a nice header and are much lighter in terms of fallout.

 

Rod

[Vic]

On core burners using standard tooling I settled on 58/33/9 using apple wood charcoal, I guess spruce wood be ok on those ratios as well. Screen the mix or ballmill for 5mins depending on the thrust required, myself preferring a slower lift off for the more dramatic effect. It will all need fine tuning with variable weights of the headers to be taken into account. I used to make all my own tubes and every inch as good as NEPT.

 

[Sparky]

Thanks everyone for the advice. I think I've got some things to go away and look at.

 

• Tube construction, I would like to see how parallel wound tubes work. I may try and make my own, sounds interesting.
• Charcoal - yes it may be a bit frisky for this application. Slower my be what is required.
• My tooling is commercial and should be fine. Tapered aluminium design from Oliver Brown.
• I thought nozzleless motors need a press? I would try hot fuel if it will work with rammed?

[maxman]

Sparky, Its best to change only one thing at a time then you know what makes the difference. I suggest using the same comp and tooling and making in the same way but use a purchased tube to start with. If you make your own tube now and you have problems then where to go next?? If they work with a bought tube you could then up the fuel speed a bit and see how they perform. Then if all is well make a tube if you want and use the same fuel batch etc and if they pop you know your tube isnt good enough. You wont get far by failures unless you have some idea what is causing them. If the plug and nozzle held but your spiral tube burst then its a fair bet that your tube is too weak.

 

Years ago I had problems with nozzles and plugs blowing out which was due to the homemade tube expanding under pressure which was cured by using good parallel wound tubes.

 

Rod

[dr thrust]

Excellent advice rod, sparky I notice you had a similar problem on a earlier thread with 60,30,10 is this the reason you've gone for the cooler propellant , could be a ramming problem how bigs your hammer lol,
Sorry sparky, your correct.. nozzle less motors need pressing but I remembered that willow made them insane! So concluded that it could of been a contributory factor.
Also ask a hundred rocket guys the same question and get a hundred different answers, that's rockets for ya

Edited by dr thrust, 26 June 2013 - 11:02 PM.

[Sparky]

Thanks for the advice all. I'm definitely going to be trying one thing at a time and see how I get on. I quite like doing this anyway as I like the sense of achievement.

 

Yes the reason I backed off from the 6.3.1 ratio was due to them exploding previously. The thing is at my new lower power ratio they work brilliantly so I may just stick with that for now then. I may order some parallel wound extra strong tubes to see if I can move back up to a hotter mix.

 

I think my rammer is big enough :-) I am ramming just hard enough so I don't split the casing and adding about a teaspoon of comp for each increment. The only thing about this is all I changed this time was wetting and granulating the comp. So either this made it more vigorous or maybe I am not consolidating the grain as well with a granulated mix.

[starseeker]

Thanks for the advice all. I'm definitely going to be trying one thing at a time and see how I get on. I quite like doing this anyway as I like the sense of achievement.

 

Yes the reason I backed off from the 6.3.1 ratio was due to them exploding previously. The thing is at my new lower power ratio they work brilliantly so I may just stick with that for now then. I may order some parallel wound extra strong tubes to see if I can move back up to a hotter mix.

 

I think my rammer is big enough :-) I am ramming just hard enough so I don't split the casing and adding about a teaspoon of comp for each increment. The only thing about this is all I changed this time was wetting and granulating the comp. So either this made it more vigorous or maybe I am not consolidating the grain as well with a granulated mix.

 

 

I think you may have found your problem, you say you ram hard enough so you do not split the tube ?

 

That would suggest that you are not using a sleeve which means that you are not ramming hard enough,

 

If you do not have a sleeve ,i have one that i made out of 28mm copper pipe which i have spare as i now have a clamshell.All you need to add is some jubilee clips,if you need it ,if you p.m me your details i will pop it in the post to you

[Sparky]

Starseeker - I've seen very little about using sleeves when ramming. I know they are used a lot when pressing as it is vital but less so for ramming.

[maxman]

Them problem with using a sleeve for ramming comes from a potential accidental ignition of the motor as you will most likely be holding the motor in the sleeve as you ram. This could potentially be more dangerous than just holding the paper tube should an accident occur. Of course if you are using a hydraulic press  you wont be holding the motor and you should be behind a blast sheild so much safer.

 

Rod