Inconsistent fuel burning with BP

[blindreeper]

Tonight after making my rocket spindal with a dowel and a lathe I decidied to try it out. It made the perfect nozzle and right size hole. I rammed BP (CIA method) The tube has 2mm wall thickness 10cm length and a 12.5 ID home rolled dextrin tubes. After ramming the BP I went to send it on it's way. I had a 3mm dowel 40 cm long taped to the rocket with the stick in a wine bottle. I used a sparkler fuse and it had a core of about 5mm (I had a bad experience with long cores ;)). Upon ignightion it jumped a few times and then jumped out of the bottle and shot across the ground. You may say it just didn't have the thrust but the fuel burnt in little bursts. Like some sparks would come out for a second then just a hissing noise and it repeated this until it was burnt out. I tried another rocket with the same dimensions but with a core of 1cm and fuel from the same batch of BP.

I thought the inconsistent burning was from lack of ramming so I proceded to ram and did little bits at a time so I added more BP and rammed 10 times. I thought this would be consitent but it did that same inconsitent burn but it shot up about 3m and then the first 'burst' of fuel stopped tillting it side warda and shot off doing the same thing 30m arcoss my yard.

I have no idea why it burns like this and it is not the proppellent because yesterday I did another rocket same dimensions but the nozzle was just drilled and a 2cm core and I did it static and it made the beautiful WHOOOSHHH!!! sound like with eastes rockets. The fuel was from the same batch of BP.

There are only 2 reasons that it could have failed I thought up of after much pondering.

1. When I rammed it the tube buckled a bit because it had no sleeve so this may have made the already rammed fuel not packed so tight thus givin inconsistent burn rate.

2. When I did the rocket yesterday the BP was slightly damp because I made a 500g batch of BP and the acetone smell was ever so faint and I thought what the hell it'll dry.

So can anyone explain why it failed apart frokm the CIA BP because I am int he process of resurecting my ball mill and are just going to make BP in a mill.

Sorry for long thread but detail is good in a situation like this right?

[Arkangel]

It might be that your BP needs a certain pressure to burn effectively. If your nozzle is too big for the fuel, that might cause what you're seeing. To be honest that's low on the scale of probabilities, but you might try it with a smaller nozzle if possible - or a bigger initial core

[kingspaz]

'Like some sparks would come out for a second'

to be honest it sounds like you have shit BP. my BP does that and i made it via the CIA method and only ground the ingredients with a mortar and pestle. its shit but works for fuses. how does yours burn when in a card tube? does it burn in a split second when a very small heap is ignited?

[blindreeper]

I made my own charcoal with pine and put that into a big coffee can with lead blass and rolled it around for 1/2 hour and had very fine charcoal. The BP is not gritty between the fingers. But it burns very fast when a small amount is ignighted with a puff of smoke. I will metion that I rammed with with a hammer. I will try again tonight and I may be able to get pics of my bros gf comes over. I'll try the bigger core. If that doesn't work smaller nozle and if that doesn't work well I need a ball mill.

[Mr Cool]

"I may be able to get pics of my bros gf"

Why, is she nice? :D;) Hehehe...

To me it does sound like your nozzle is too big/core too small or a bit of both, like Arkangel said. There aren't many other possible reasons that I can see.

[blindreeper]

She is alright ;)

But I have some pics and a crappy movie of one of my 3 rockets, the first one went perfect with a 2cm core and the other 2 went all over the show with a 1cm core and they are identical.

http://www.geocities.com/blindreeper_chemistry/rockets.html

And fo you kingspaz I got a video of 5g of my BP burning for you ;)

[Arkangel]

Someone posted a link to a rocketry site run by James Yawn (http://www.jamesyawn.com/) , and it's a really great site. One of his videos shows a rocket taking off and chuffing badly like that. I had an email discussion with him, during which he said the following:

The "chuffing" of my motor in the Shorty movie is due, no doubt, to more factors than I can imagine. But it was corrected by increasing the Kn ratio - apparently my nozzle throat was a bit too large on that motor, and my propellant does not burn very stably at low pressure. A Kn of about 150 seems the minimum to ensure a stable burn. You might note that there is little chuffing, if any, in the Dr. Rocket casings, where the Kn ratio is much higher.

Your nozzle looks on the big side to me, and your core pretty short. I'd be inclined to make a longer, slightly narrower spindle, and make sure you light the motor from the top first - quickmatch being a great way to do it - far better than trying to fill the core with BP. James Yawn makes his own quickmatch - with a variation "sparklematch", which has titanium flakes in it. These burn more intensely, and make sure he's lit along the full length of the core

[blindreeper]

Thanks Arkangel, I will make the spindal from aluminium on a lathe and I'll make the core longer and will give results. I light the rockets with a sparkler but the time I have stuck the ignighter (nichrome) all the way to the end of a full length core on one of these sized rockets I have been put off long cores :p But I suppose that was a full length so I will be fine. Thanks for the help guys!

EDIT: Ark; I couldn't find his sparkmatch page, is there one or is it some thing he talked to you about? And with quickmatch how am I going to make paper tubes 3mm to fin inside my core necause 3mm is pretty thing and I'd have to have 1mm or so blackmatch for the quickmatch effect to come in? Am I missing somthing?

[Crazy Swede]

First I must confess that I haven't read the thread thoroughly, so I apologise if my information already have been covered above.

Anyway, to my experience black powder rocket motors never chuff unless cracks have developed in the grain or the black powder have separated during loading (home made bp should always be granulated before loading to prevent separation of the ingredients).

To see if cracks is your problem, ram a motor like you did before and carefully unwind or cut the tube open to examine the grain. Quite often the problem is that the inner layer of paper is not glued well and wrinkles during ramming. This creates air pockets and cracks that will disturb the even burning of the rocket motor.

The lenght and diameter of the core shouldn't affect the smoothness of the burning, but I highly suggest having a core of at least 3/4 of the grain lenght.

[Arkangel]

"sparklematch" is something he talked about by email.

I also made some "sparklematch" where I just sprinkled some magnesium flakes over the match before it dried. I use magnesium turnings in my ignitors (electric match, 1/2g homemade black powder, pinch of magnesium flakes wrapped in masking tape) to create "hot spots." I have found ignition to be much quicker and more reliable with the magnesium, perhaps because my candy propellant gets a bit damp in this climate

We were discussing the fact that quickmatch can be used for some very interesting things in rocketry - multi engine ignitions, multi stage ignitions, ripple firings etc.

If you read through the thread on spin stabilised model rockets, we talk a bit about ignition, especially using quickmatch. The thing is, the paper tube fits loosely around blackmatch to make the quickmatch. pinching it all together actually makes it burn a little slower. What I have been doing with excellent results, is to strip away the sleeve on my quickmatch for as long a section as my motor core. So if you can imagine, I have a length of visco, taped into the end of my quickmatch, and after a couple of inches the quickmatch has the sleeve removed, before being inserted and fastened into the core. When it's inside the motor, the core acts the same way as the paper sleeve would have done, containing the flame, and allowing it to travel the full length of the "blackmatch" in a fraction of a second. Since I've started doing this I've had flawless ignitions every time.