Beginner questions for first nozzleless BP rocket

[lloyd]

Heh! Don! You don't top-match a cored rocket with piped match! You insert several strands of bare match all the way up, and ignite with a fuse!

 

It will (umm...) "quickmatch" all the way to the top in an instant. Consider that the powder core offers the same sort of containment that a paper tube would. <G>

 

Some folks just coat visco with PVC cement or rubber glue, or wrap a turn of masking tape over it, if they don't want to use match. Stuffing a hunk of shrink tubing up there MIGHT have contributed to the CATO, but I actually doubt it. It would have been instantaneously ejected, LONG before the pressure could have increased enough to cause a CATO.

 

We light 'em all the time with an ematch stuffed up to the top, and that rigid wire must be ejected before they launch. I think you have "expansion" problems, where the pressure is cracking the grain.

 

 

Lloyd

[DonCopal]

Oh, that makes sense to me now. Next time, I'll put in 3 strands of blackmatch and I'll also use a tube support for ramming because the tube was deformed a littlebit, that might have caused a crack. So, you don't think I'll have to change the mallet? Also, do you have any other tips? Do you really think I need to wax the tube, if yes, how?

[MrB]

I'll also use a tube support for ramming because the tube was deformed a littlebit, that might have caused a crack. So, you don't think I'll have to change the mallet?

 

If your tube is deforming then you need less, not more hammer... But yeah, that might cause cracks, or just have weakened the tube to the point where it will burst. Try, try again.

Good luck.

B!

[DonCopal]

Well, the Deformation was just barely visible, nothing significant.

[lloyd]

Don,

If your tube deformed "a little", "how little" counts! If any inner turns were split, it will CATO. A tube support is pretty important for making 'hot' rockets.

 

I know it's not your wish, but you'd be much better off buying a firm-faced dead-blow mallet or a leather mallet.

 

As far as waxing the tube -- it isn't necessary, but vastly improves the CATO ratio. It's done, basically, by melting some paraffin wax HOT (wax, not the 'paraffin' you Brits call "kerosene" <G>), then pouring it into a plugged tube, and drain it out (rather quickly, so it doesn't coat too thickly).

 

Dave Forster published a document describing the method. I don't have a link to it right now, or I'd put it up. But if you search on "tube waxing" and-ed with "Dave Forster", you ought to find it.

 

Lloyd

[DonCopal]

Hahaha I'm German not British but here cheap candles are made from paraffin wax, so that isn't a problem. I'll try again and report here.

[Mumbles]

I'm not entirely sure what is the most up-to-date source on this at the moment. Hopefully Dave will swing by and fill us in. Here are a few articles and a video. From comparing them, the one from pyrobin looks to be the most recent of the set.

 

https://www.wolterpyrotools.com/sheets/120.pdf

http://www.pyrobin.com/files/Reducing%20Catos%20in%20Pyrotechnic%20rockets-V2.pdf

[Col]

Oh, that makes sense to me now. Next time, I'll put in 3 strands of blackmatch and I'll also use a tube support for ramming because the tube was deformed a littlebit, that might have caused a crack. So, you don't think I'll have to change the mallet? Also, do you have any other tips? Do you really think I need to wax the tube, if yes, how?

I use piped match so fire propagates down the core. 3 bare strands will propagate fire up the core.Either way it happens in a split second It doesnt take much to damage a tube, if the od changes by 0.5mm, every turn of paper in the wall will need to stretch by upto 7mm

[DonCopal]

So, I'll soon try again with waxed tubes, tube support and 3 strands of blackmatch as a fuse. I'll report how it worked then.

[Col]

Make two and try both fusing methods.

Top fused 19mm nozzleless, pressed to 6,500psi

 

https://youtu.be/pVHSTUBgtTk

[FlaMtnBkr]

You should try knocking out a couple more and you will get the hang of it. I messed up the first couple too but then got a feel for how hard to ram and a consistent increment size.

 

Tons of rockets have been hand rammed without the need for waxed tubes, tube supports, or worrying about the fuse placement. Yes, the first two can make them more reliable, and a good idea when trying to hand ram bigger rockets, but with a smaller BP nozzle-less rockets you can make reliable rockets with a little practice and work on your technique.

 

You don't want to ram them as hard as you can. I whack the rammer about like if I was driving a good size nail into an untreated 2x4 10 times for each increment. Just count them out, add an increment of BP that raises the grain 1/2 the ID of the tube when compacted, and repeat. The size you're making shouldn't take long 5, maybe 10 minutes depending on your pace and you should be done. Tape a stick on, fuse it with visco at the bottom and put a piece of making tape over the nozzle to hold it in place and keep any stray sparks out, and go light it. With a nozzle-less rocket with fast BP it's going to light and burn up the core section in a fraction of a second and take off like a bullet. Top fusing it won't make a huge difference in lifting capability and you aren't going to be lifting anything huge anyways. If you were making a rocket with a clay nozzle and slow BP then top fusing will make a bigger impact because the fuel has been slowed down and you can get the whole core lit about once lighting at the top, but in the case of a nozzle-less rocket just bottom fuse it.

 

When you get them reliable and get ready to put a shell on it you should be starting with a dummy shell to see how much weight you can lift to an appropriate display height. You can likely lift a 2" cylinder shell or a 3" ball shell fairly easy and likely a size bigger if your BP is really hot and you have the motor dialed in. If for some reason it's not enough then try top fusing it to get that little extra uumpff. But for now just bottom fuse it and make a bunch so you get some practice until they are reliable.

 

Then maybe add a gram of flash for a report and once you're sure at least 9 out of 10 will fly then start thinking about strapping your hard work to the top. You also should have a water filled fire extinguisher or a bucket of water handy when you put a shell on top and have plenty of distance or something you can duck behind because eventually you will have stars flying past you and need to be prepared to put out any fires that may cause.

 

At least that is how I would do it. Good luck with them!

[DonCopal]

Another question: I don't have to add a clay bulkhead for nozzleless rockets, do I?

[lloyd]

Unless you have a LONG delay charge above the main fuel, you need one ABOVE the fuel grain, yes, or it will rapidly burn through the core and begin to exit at both ends.

 

Many makers will use the delay (slow-burning powder) AS the top bulkhead, and simply allow that delay charge to spit fire into the heading.

 

I'm not partial to that method, preferring a bulkhead, regardless, and putting a small 'fuse hole' in the bulkhead to convey fire to the heading.

 

Lloyd

Edited December 22, 2015 by lloyd

[DonCopal]

Oh.... On the skylighter website it said I didn't have to. Last time, I had one, but today I've build a new rocket with support and waxed tubes, but I didn't add a bulkhead to it. There is still some space above the grain, so could I just put it on the tool again and ram some clay in?

[lloyd]

I would.

 

Like I said, Don... some don't. I always do. I get more consistent results, and totally eliminate the possibility that it will 'jet' out the wrong end with enough thrust to mess up the flight.

 

It's certainly no trouble to do!

 

Lloyd

Edited December 22, 2015 by lloyd

[DonCopal]

It thought that when I put it back on the spindle, it might not perfectly line up with it anymore, so when I ram, I might make the core deform just a very little bit so it get crumbly or cracks or so. Are you sure it's not a problem? I could also fill that space above the grain with hot glue

[lloyd]

I wouldn't "re-insert" a spindle in the core, no. It won't fit, anyway, no matter how you turn it. You must make the bulkhead while everything's in place on the first go.

 

You can fill that void with hot-melt. It's better than nothing.

 

Lloyd

[DonCopal]

Ok, I'll do that.

[Col]

If you`ve got a reasonable amount of grain above the spindle it should hold the pressure without the clay..

[lloyd]

Not if the "above grain' is main charge powder. It'll burn through very rapidly. IF it's 'delay charge' of much slower powder, it will serve (as I wrote earlier).

 

Now, I'm not trying to be ridiculous: If you have more than (say) a tube-diameter-and-a-half's grain above the core, it probably WILL last; but why chance it? It's an easy problem to solve without costing you anything but a few more whacks with the mallet.

 

Besides... do you want your header going off the instant (or so) after the main thrust stops, or do you want it to delay long enough to reach somewhere near apogee?

 

Lloyd

[DonCopal]

I want a delay in the final rocket, and I'm familiar with how it works and I always use them on my whistle rockets and endburners. But since this rocket is only for test, I didn't add a delay and I also forgot to add the bulkhead, but I'm pretty sure that the hot glue will do the job as well since it's very strong.

[Col]

The core provides the thrust, anything above the core is delay. A couple of increments of the same fuel is still going to burn a lot slower than the core.

Don, how many increments of fuel did you ram above the spindle?

Edited December 22, 2015 by Col

[OldMarine]

How do you determine how much delay? Do certain comps burn at certain speeds?

[DonCopal]

I think it were like 4.

 

@OldMarine: I use some Veline Magenta comp that I have still left over. I use two increments of that for a good delay, I just tried it out.

Edited December 22, 2015 by DonCopal

[lloyd]

"A couple of increments of the same fuel is still going to burn a lot slower than the core."

------------

It's going to burn at the same linear rate as the sides beside the core.

 

But you're correct that a couple of diameters above the core would provide some 'delay'... just not enough for anything pleasing.

 

Think about it -- a cored nozzle-less rocket consumes almost all of its fuel in a second or two. Do you really want the heading going off while the rocket is still at high velocity? Or do you want it to display when the rocket is almost still, at the top?

 

<shrug>

Lloyd