Jump to content


Photo
- - - - -

Flames have trouble propagating up fuel core


  • Please log in to reply
41 replies to this topic

#1 JMan

JMan

    Pyromaniac

  • Full Member
  • PipPipPip
  • 97 posts

Posted 16 April 2018 - 07:21 PM

Ok for any who do not know I use r-candy and hot cast it into the motor casing. I have a 20cm long rocket with a 3 mm diameter core and I cannot get the flame to travel up the core. It just burns the end. Any suggestions such as fuses or increased oxidizer sprinkled down the core? Maybe a bigger core? Has anyone had this problem or been able to fix it?

Excess info: the rocket is 2 cm diameter and I have gotten the flame to propagate down 4 of 20 attempts with astounding success.

#2 Baldor

Baldor

    Pyrotechnician

  • Donator
  • PipPipPipPip
  • 256 posts
  • Gender:Not Telling
  • Location:Barcelona

Posted 17 April 2018 - 12:05 AM

I don't know about sugar rockets, but.... Could you put a thin black match all along the core? Should be very thin to avoid clogging the core. The core will act as the sleeve in kick match.



#3 Arthur

Arthur

    Firebreather

  • HE Qualified
  • PipPipPipPipPip
  • 2,787 posts

Posted 17 April 2018 - 01:38 AM

With standard star primes the surface texture is important, sometimes a smooth star or prime doesn't take fire while a rough angular star does. Maybe look at the surface texture of the core.



#4 stix

stix

    Pyrotechnician

  • Donator
  • PipPipPipPip
  • 997 posts
  • Gender:Male
  • Location:Australia
  • Interests:Pyrotechnics, Model Rocketry, Programming, Photography

Posted 17 April 2018 - 04:46 AM

Ok for any who do not know I use r-candy and hot cast it into the motor casing. I have a 20cm long rocket with a 3 mm diameter core and I cannot get the flame to travel up the core. It just burns the end. Any suggestions such as fuses or increased oxidizer sprinkled down the core? Maybe a bigger core? Has anyone had this problem or been able to fix it?

Excess info: the rocket is 2 cm diameter and I have gotten the flame to propagate down 4 of 20 attempts with astounding success.

 

I always get confused when people describe which "end" of the motor they mean. Are you saying you are igniting from the nozzle end, or the bulkhead end?

 

It sounds like you are talking about the nozzle end. If so, then sugar fuel will take longer to propagate down the whole core than black powder. Perhaps some of the 4 out of 20 were slightly damp from exposure to air? therefore the inconsistency.

 

You could try to roughen the core by inserting a rolled piece of course sandpaper moving it in and out. This will also help you discover if it's damp. This is what I always do, even though I'm igniting from the bulkhead end way down in the core. Black match as suggested by Baldor is another good idea.

 

You might want to look at making some electronic igniters, that's what I use. I find that this way ensures you are igniting from within the core at the deepest end which ensures that the fuel is lit more evenly.

 

If you're talking about that you already ignite from the bulkhead end and it just "burns the end" and doesn't propagate up the whole length, then you must have a major manufacturing issue with your fuel.

 

I hope it's not that one :). In any case, we're all hear to help. Just need some further info.


Edited by stix, 17 April 2018 - 04:49 AM.

I just start the conversation - someone else has to question them.


#5 JMan

JMan

    Pyromaniac

  • Full Member
  • PipPipPip
  • 97 posts

Posted 17 April 2018 - 08:05 AM

Stix, yes Im lighting it from the nozzle end. Since I cast it directly into the casing it would be impossible for me to light the bulkhead end as its not exposed. Ive got very good electrical igniters (really simple actually just broken outdoors Christmas lightbulbs with a little fuel rolled around them). Problem is theyre about 5 mm and with how thin my rocket it is, making the core 5 mm would loose a lot of fuel. I need that small core for initial pressure.

I know the inside is not wet due to the way I cast it but perhaps I could slightly dampen the bottom or coat it with a little baking soda to inhibit it so the flame can actually go into the core?
Maybe this is a question for the pyro section but could making the fuel oxidizer rich (just by sprinkling oxidizer down the core and shaking it around on the walls) help pull the flame the length of the core?

Edited by JMan, 17 April 2018 - 08:06 AM.


#6 MrB

MrB

    Firebreather

  • HE Qualified
  • PipPipPipPipPip
  • 1,748 posts
  • Gender:Male

Posted 18 April 2018 - 04:10 AM

Igniting cored rockets from the top of the core, if you cant stick an igniter up there, is most easily done by folding a fuse over its own end, and sticking it up there.
Leaves you with either a progressive core ignition, if the core is allowed to take fire as the fuse burns in to the core, or, if the fuse is shielded from the core, it ignites at the top in one great go.
Leaving the fuse exposed, isn't a problem if you use fast fuse, but might be if your fuse is slow burning. YMMV, as they say.

#7 NeighborJ

NeighborJ

    Firebreather

  • Donator
  • PipPipPipPipPip
  • 1,118 posts
  • Gender:Male
  • Location:Pittsburgh pennsylvania
  • Interests:Rig welder, trecky, amateur rocketry

Posted 18 April 2018 - 05:57 AM

Jman, can we get a little more info? The core length is my concern. 3mm nozzle dia is more in line with what would be expected when making an endburner. I would expect that motor to Cato immediately if it was ever successfully ignited at the top.

Personally I would increase the core dia large enough to insert the igniter then continue to enlarge it with each new motor until it reliably ignites fully without a cato.

#8 JMan

JMan

    Pyromaniac

  • Full Member
  • PipPipPip
  • 97 posts

Posted 18 April 2018 - 06:06 PM

No a 3mm diameter core. I think I forgot to mention it is nozzless. It is 20 cm tall, 2 cm diameter, 3 mm core that goes the whole 20 cm up. I am using a 3/4 inch water hammer (if anyone knows what that is) for the case.

I appreciate the input mrB but I just cant get fuses that thin, I might have to just settle for a bigger core. Bummer.

#9 NeighborJ

NeighborJ

    Firebreather

  • Donator
  • PipPipPipPipPip
  • 1,118 posts
  • Gender:Male
  • Location:Pittsburgh pennsylvania
  • Interests:Rig welder, trecky, amateur rocketry

Posted 18 April 2018 - 07:13 PM

Thanks for the clarification. Nozzleless is much more forgiving. The larger core still might not be a bad thing depending on what you plan on using these motors for.

A super thin core may logicaly seem like it will give better performance but thats not always the case. The motor could have optimum case pressure at ignition but still not have enough thrust to lift a heavy rocket until the core opens up more. In this scenario all that extra fuel will be wasted on the launch pad. The problems are compounded when the motor starts to overcome the payload weight, these slow liftoffs can cause the rocket to tip over then shoot off horizontally in the wrong direction.

I suppose what I'm saying here is, that motor may have a high peak thrust but the payload should be matched to the motors minimum thrust. This limits the applications it can be used for.

On a side note: it is possible to make a very effective igniter suited for this application. I've made many using bell wire and nichrome resistance wire purchased from a local vape shop. It can then be dipped in your choice of thermitic prime, I bet they can be made to fit even smaller cores.
  • stix likes this

#10 JMan

JMan

    Pyromaniac

  • Full Member
  • PipPipPip
  • 97 posts

Posted 18 April 2018 - 08:54 PM

Neighborj I think you might be right. Perhaps just some nichrome wire on a very thin wire could be stuck far enough up the tube. Just a quick question, is there any household thing that might have some nichrome wire in it (old hair dryer or space heater etc.) because Id like to test it before I order a big spool.

Ps Im just making model rockets trying to make a nice easy motor but if this proves too difficult and no easy fix is found ima move on to a thicker rocket and thicker core.

#11 Baldor

Baldor

    Pyrotechnician

  • Donator
  • PipPipPipPip
  • 256 posts
  • Gender:Not Telling
  • Location:Barcelona

Posted 19 April 2018 - 12:24 AM

Almost any heating appliance you mentioned will have nicrome. If you can see wires glowing red, it will have nicrome. But the thickness will vary.

 

Alternatively, you can use a thin strand of copper instead of NiCr. You must make some tests to find right length and thickness.

 

If you have or can make some BP, it's very easy to make some black match with some cotton thread.


  • stix likes this

#12 stix

stix

    Pyrotechnician

  • Donator
  • PipPipPipPip
  • 997 posts
  • Gender:Male
  • Location:Australia
  • Interests:Pyrotechnics, Model Rocketry, Programming, Photography

Posted 19 April 2018 - 08:40 AM

Thanks for the clarification. Nozzleless is much more forgiving. The larger core still might not be a bad thing depending on what you plan on using these motors for.

A super thin core may logicaly seem like it will give better performance but thats not always the case. The motor could have optimum case pressure at ignition but still not have enough thrust to lift a heavy rocket until the core opens up more. In this scenario all that extra fuel will be wasted on the launch pad. The problems are compounded when the motor starts to overcome the payload weight, these slow liftoffs can cause the rocket to tip over then shoot off horizontally in the wrong direction.

I suppose what I'm saying here is, that motor may have a high peak thrust but the payload should be matched to the motors minimum thrust. This limits the applications it can be used for.

On a side note: it is possible to make a very effective igniter suited for this application. I've made many using bell wire and nichrome resistance wire purchased from a local vape shop. It can then be dipped in your choice of thermitic prime, I bet they can be made to fit even smaller cores.

 

Yep NJ, that's pretty much exactly what I was going to say. There is no point the rocket motor sitting there if it's not going anywhere - it's a waste of fuel. Therefore why not just start with a larger core to begin with.

 

@ JMan. So what type of r-candy fuel are you using? Are you adding Red Iron Oxide (or something else) to speed up the burn rate?. Nozzle-less, core burner r-candy motors are not very effective unless you have very fast burning fuel and not much weight. The best one I made was including rio - It had no casing at all except for an outer layer of aluminium tape. I stuck it to a thin meat skewer as the stick - it took off nicely. But unlikely that it would have taken off with much extra weight added (header etc.)

 

Nozzle-less sugar motors - I can't see them working well, if at all. BP Yes indeed, but r-candy - No.

 

Here's worked examples showing the burn profile of tests done almost 3yrs ago to this day. Not the best examples, but it does show a point. The tests were conducted to show the difference between the SAME weight of fuel, the SAME fuel grain and the SAME fuel type (r-candy) - using a nozzle, compared to not using a nozzle:

 

Nozzled (SKINT-37):

 

qpnbg3e.jpg

 

 

Nozzle-less (SKINT-38):

 

0aklx04.jpg

 

It's easy enough to see which one performs better.

 

Now, a few things to take into consideration. The fuel was not doped with RIO, if so then I'm sure the nozzle-less would have performed much better (and so would the nozzled version - if not a cato). Also the nozzled version (skint-37) was a bit lazy because I also used a new untested nozzle mix which eroded and accounts for the drop-off in the graph at the end.

 

However, with the nozzle-less version (skint-38) it can clearly be seen that after an initial thrust, then it tapers off and never increases in thrust again. Theoretically I think it should be a straight line? dunno. Nevertheless the difference is clear.

 

You might notice in each graph there is a vertical black segmented line. This is the cursor. If you look at the top of the graph you will see a panel that says "cursor info:". This panel shows at the cursor position: (sn) sample number, time, force value and G's (gravities), bad word I know. If we concentrate on the time and G's, then you will see in the nozzles-less (skint-38) 2.3 g's at 0.5 seconds. Meaning that 1 g's will lift it's own weight. Therefore 2.3 g's is just able to, and thereafter you can see that the force does not increase. Therefore this will barely take off.

 

However, the nozzled version (skint-37) shows 36.7 g's at the same point (0.5 secs). This would have taken off fast and would have been able to carry a header. The reason for the 0.5 seconds test period is a general rule: "A model rocket when launched will reach stability providing that after 0.5 seconds (or earlier) it has at least 5 g's). Skint-37 does, Skint-38 does not.

 

Yeah, lots to take in and I'm sure others will have differing opinions, but this is what I've tested and happy to discuss.

 

@JMan, I don't think that "nozzle-lesss" r-candy motors are viable. The fuel just does not burn fast enough - it's not well suited. With black powder, yeah it does burn fast enough and people use nozzle-less bp motors all the time. But those motors are for the purpose of a quick huge initial thrust and usually have a tapered core - the purpose is to carry a header. However, that design will always be regressive and waste more fuel, but that doesn't matter providing the header shell gets up there. It's also easier to make nozzle-less motors.

 

Maybe you could look into using a nozzle? but I don't know what it is you want to achieve. Hope this helps.

 

Cheers.


Edited by stix, 19 April 2018 - 09:19 AM.

I just start the conversation - someone else has to question them.


#13 NeighborJ

NeighborJ

    Firebreather

  • Donator
  • PipPipPipPipPip
  • 1,118 posts
  • Gender:Male
  • Location:Pittsburgh pennsylvania
  • Interests:Rig welder, trecky, amateur rocketry

Posted 19 April 2018 - 01:04 PM

The goal and purpose of this motor is still in question. I'm assuming the nozzleless design is for use as a header delivery platform?

If this is for pyro, then I must say that nozzleless sugar rockets do work for this purpose. I know someone here on the forum who uses them exclusively for lifting headers, heavy ones. A burn rate modifier must be used as Stix has suggested but they do not come without drawbacks.

The sugar fuel does burn much slower than BP, this does not quite help the goal of a quick intense burst of thrust needed to lift a shell to display height. The motors require a large core and thin fuel wall, to accomplish this as a nozzleless motor.

The theory of nozzleless motor mechanics is a bit different than with nozzles motors in that, when the motor first fully ignites, the case pressure far exceeds what the casing can withstand, then as the fuel burns, the case pressure drops to a level which is lower or at least meets the maximum pressure rating of the case.

Why doesn't it explode when exposed to the initial over pressurization? Well the fuel itself adds to the strength of its casing. It's their combined strength which allows the motor to withstand the pressure. Sugar motors [due to the burn rate] are more touchy than other fuels and require precise balance of wall thickness and core diameter to work properly.

I've played around a lot with the idea of these motors being made with a huge core but I couldn't escape the fact that the fuel thickness ads to the integrity of the motor. Big cores for me, were not reliable because the fuel cracks and exposed even more surface area which in turn leads to a spectacular Cato. The correct core diameter is the key to making these things work.

#14 JMan

JMan

    Pyromaniac

  • Full Member
  • PipPipPip
  • 97 posts

Posted 19 April 2018 - 08:52 PM

These are model rockets.

Im not using a burn rate modifier, and Im getting higher thrust with the smaller core size (due to over pressurazation).

I dont mean to offend any of you I always aprecieate help and insight but it was a simple question of how to get a flame to propagate down a core better, not weather this rocket is worth my time or able to lift what shells.

Im now experimenting with making super thin e matches.

#15 MinamotoKobayashi

MinamotoKobayashi

    Pyromaniac

  • Donator
  • PipPipPip
  • 75 posts
  • Gender:Male

Posted 19 April 2018 - 09:42 PM

Hello.

I have a lot of experience about r-candy rockets.

I have made in the time nozzle and nozzleless sugar rockets (enhanced with 1% red iron oxyde and titanium sponge for the tail) .. every type has pro and cons, but the most versatile and with less chances of catos are without doubt the nozzleless version.

For my 3" shells I use the nozzle version made with bentonite +1% graphite.

For my 4" shells I use nozzle version made with PET and sometimes nozzleless version.

For my 5" and 6" shells I use exclusively the nozzleless version.

All the versions are core burner.

 

In my modest opinion nozzle rockets are more scenic and spectacular, because they have a soft start, but when the weight to lift is much there is no other chance to use nozzleless.

 

Here is an example of a bentonite nozzle rocket with a 3" shell:

 

 

Here are two examples of a PET nozzle rocket with a 4" shell:

 

 

 

Here is an example of a nozzleless rocket with a 4" shell:

 

https://youtu.be/m7TNKlXZhMg

 

Here is an example of a supersonic nozzleless rocket:

 

https://youtu.be/aCp2VyHSCR0

 

Sorry no videos for the 5" and 6" version .. here is my new nozzleless with a 5" shell on the top with green and red stars inside. I hope to launch it tomorrow:

 

https://www.flickr.c...eposted-public/

 

Also, nozzleless version in much more versatile because it is possible to modify a working motor to vary the altitude of the shell ignition simply shortening its lenght without changing any other parameter (propellant, hole depth, hole diameter, and so on).


Edited by MinamotoKobayashi, 19 April 2018 - 09:49 PM.

  • NeighborJ likes this

#16 stix

stix

    Pyrotechnician

  • Donator
  • PipPipPipPip
  • 997 posts
  • Gender:Male
  • Location:Australia
  • Interests:Pyrotechnics, Model Rocketry, Programming, Photography

Posted 19 April 2018 - 10:25 PM

These are model rockets.

Im not using a burn rate modifier, and Im getting higher thrust with the smaller core size (due to over pressurazation).

I dont mean to offend any of you I always aprecieate help and insight but it was a simple question of how to get a flame to propagate down a core better, not weather this rocket is worth my time or able to lift what shells.

Im now experimenting with making super thin e matches.

 

Yeah, sorry about that JMan, I think I was the main offender, and I posted graphs and everything :blush:  I was obviously bored.

 

So, yes, best to shove the ignitor all the way into the core. I use Cat 5E/RJ45 network cables for the wire as it's pretty thin copper and already comes in twisted pairs. I think you need very thin nichrome otherwise it will be hard to get hot with a battery (if that's what you are doing.) You can coat the nichrome wire with a pyrogen mix - I make mine with bp+Al powder with nitrocellulose lacquer from cheap ping pong balls - these work great and spit out lots of hot material.

 

Sorry I can't post any pictures :P


Edited by stix, 19 April 2018 - 10:27 PM.

I just start the conversation - someone else has to question them.


#17 Baldor

Baldor

    Pyrotechnician

  • Donator
  • PipPipPipPip
  • 256 posts
  • Gender:Not Telling
  • Location:Barcelona

Posted 20 April 2018 - 12:22 AM

Minamoto... Doesn´t you have the terminology inverted? I always though nozzled motors have better peak thrust and better initial performance than nozzleless.



#18 MinamotoKobayashi

MinamotoKobayashi

    Pyromaniac

  • Donator
  • PipPipPip
  • 75 posts
  • Gender:Male

Posted 20 April 2018 - 01:00 AM

In my experience I have seen incredible initial performances with nozzleless.
In fact nozzleless motors permits a much longer hole into the compound, aka much more propellant ignited that burn in few seconds.
A nozzle created directly inside the compound will widen as the mixture burns inside the rocket, managing the pressure perfectly, unti it reaches a diameter
so big that the rocket will no longer push. This is my sweet point, aka the exact moment that my shell will be ignited.
I can tune the perfect moment of the ignition leaving 3cm or more r-candy mix between the end of the internal hole and the shell fuse (usually very fast).
Viceversa, my nozzle motors are made with PET because I need that the hole must be eroded (and so enlarged) in the meanwhile that the propellant burn inside
the rocket.
When the propellant hole inside the rocket is getting larger, more propellant is ignited. If the nozzle hole rest the same, at a certain point the hole can
no longer handle the internal pressure and there will be a CATO.
To found the sweet point between the various nozzle materials I have applied the well knowned procedure: try and error.
Nozzle rocket are also more sensitive to the small air bubbles that can form inside the r-candy propellant: a little pressure spike inside the rocket can transform
it in a firecracker.
 
This is my PET nozzle:
 
 
This is my nozzleless profile (note the flaring for a better thrust and elimination of exhaust gases):
 
 
Usually I use three different kinds of fuses: slow coloured "falling leaf" as initial fuse, fast visco fuse inside a straw for the core ignition and super fast fuse between the end of the r-candy and the shell.

Edited by MinamotoKobayashi, 20 April 2018 - 01:08 AM.

  • stix and Baldor like this

#19 NeighborJ

NeighborJ

    Firebreather

  • Donator
  • PipPipPipPipPip
  • 1,118 posts
  • Gender:Male
  • Location:Pittsburgh pennsylvania
  • Interests:Rig welder, trecky, amateur rocketry

Posted 20 April 2018 - 05:33 AM

Yeah, sorry Jman. I get a little bit carried away with talk of rockets. Guilty as charged. This place fuels my addiction.

I'll look thru my old sugar rocket stuff to see if I can find some home made unprimed igniters and post pics later. I believe I saw a tutorial on Richard Nakkas web page or Jacob's rocketry. Either way, they can be made to fit even a 1mm hole.

Minamoto, those nozzleless rockets are quite impressive, they make quite a lot of thrust for what they are.
  • stix and MinamotoKobayashi like this

#20 stix

stix

    Pyrotechnician

  • Donator
  • PipPipPipPip
  • 997 posts
  • Gender:Male
  • Location:Australia
  • Interests:Pyrotechnics, Model Rocketry, Programming, Photography

Posted 21 April 2018 - 08:26 AM

Minamoto... Doesn´t you have the terminology inverted? I always though nozzled motors have better peak thrust and better initial performance than nozzleless.

 

Minamoto is correct, or at least his actual tests agree with and confirm what I've always thought.

 

Nozzless motors have a huge surface area of fuel which is expelled out of a huge (pseudo) nozzle area. Therefore the initial thrust will be immense compared to a nozzled version. Perhaps a waste of fuel, but irrelevant if the job is done.

 

@Minamoto: Great to see that someone has actually launched shells using sugar rockets - and with a nice tail as well. I also like the idea of your PET nozzle which erodes whilst maintaining enough pressure. Simply brilliant!!!

 

I know, it's another distraction JMan, but I think Minamoto's tests are worthwhile thinking about and should be acknowledged.

 

@JMan - looking forward to hearing how get on making those thin ignitors and seeing some successful tests.


  • MinamotoKobayashi and Baldor like this

I just start the conversation - someone else has to question them.





0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users