16 replies to this topic

[deer]

I have a problem with my rockets that they seem to burn really slowly, causing slow pressure build up, sluggish lift-offs (sometimes they've even started falling back and then darting sideways) and worst of all, bulkhead burning trough really slowly and making it explode on ground. Something like this:

 

Should I simply reduce bulkhead / delay time to make it burst faster (OTOH with slow take-off that means risk of ground burst) or try increase rocket performance? IMHO in most cases I’ve just stored them for too long, thus moisture has accumulated in fuel. Still, even if I launched it fresh of the stove, would the improved power produce more vertical trajectory?

[MadMat]

Low initial thrust will cause instability at take-off. Are your rockets end burners or core burners? Do you add red iron oxide? If your rockets are end burners, you should really add a small amount of RIO to your mix. And yes, Rcandy is hygroscopic and will absorb moisture from the air, this will definitely cause problems. Or maybe you have to heavy of a payload and it's caused imbalance. How long (and heavy) are the sticks you are using for guidance?

Edited by MadMat, 03 October 2015 - 04:31 PM.

[stix]

I have a problem with my rockets that they seem to burn really slowly, causing slow pressure build up, sluggish lift-offs (sometimes they've even started falling back and then darting sideways) and worst of all, bulkhead burning trough really slowly and making it explode on ground....

 

Should I simply reduce bulkhead / delay time to make it burst faster (OTOH with slow take-off that means risk of ground burst) or try increase rocket performance? IMHO in most cases I’ve just stored them for too long, thus moisture has accumulated in fuel. Still, even if I launched it fresh of the stove, would the improved power produce more vertical trajectory?

 

Perhaps you're getting ahead of yourself? It might be a good idea to use a dummy header until you've sorted out the issues with the fuel, stability or the motor design itself - it could be dangerous. You really need to have consistent reliable motors before even considering attaching a pyrotechnic header.

 

Storing the motors/grains in a ziplock bag with a desiccant should ensure that the fuel remains dry. I've used this method and some grains that I have stored for over 2 years are still dry.

 

Some info on your fuel and motor design would be helpful.

[deer]

The motor was basically like this (seen at 12th second):

https://youtu.be/6S_qhtKiIe0?t=12s

 

Except for the booster BP, and reduced length at end by ~3cm (enough so that bulkhead is only as thick as walls). At top of it was approx. 100..200g cylinder shell. So, core-burner and no RIO.

 

Stick was quite long, like 1..1.5m, but was required to balance it.

 

From what you've written, I understand, the delay is fine, motor needs improvement?

 

I have somewhat conflicting opinion on dummy shells, because without mid-air burst the rocket comes down darting instead of just flailing to ground in pieces.

IMHO best approach would be static tests of both, shell and motor. But without proper measurement's motor on ground is just a dangerously overpowered smoke screen with run-time information. And doing plain ground shells would attract unwanted attention too. It's a large empty field, so only thing to worry about is getting behind a cover myself.

 

Zip-loc doesn't cut it in here. I need to vacuum them and the need to be at least dual lock. I've had better luck with simply glueing both ends of motor shut, but that doesn't work when there is a shell already in place.

 

Maybe I should start with mortars first, eh? But that's half the fun and I'm interested in especially in rockets and amateur rocketry too.

Edited by deer, 04 October 2015 - 01:10 AM.

[stix]

I had a look at the video - and the motor design looks similar to my own motors, except for the nozzle diameter. Also, I would take that video with a pinch of salt. In the end, we have to modify, experiment and come up with our own motors that suit our particular chems, materials, tools, methods etc. I'm an "EXPERT" when it comes to making "MY" motors .

 

There are so many important and inter-dependent relationships at play - ie. fuel type, repeat-ability of fuel making (consistency), fuel length, nozzle diameter, casing strength, etc., etc., etc. Therefore it's impossible to pinpoint a reason for an under-performing motor. It could be too much header weight, although my gut feeling is that it's a case of not quite enough pressure.

 

From my own experience, some areas to consider are:

• The fuel in the video uses "sugar syrup" (caro/glucose etc)

Whilst there is a certain degree of plasticity (which can be beneficial), I've found that this will contribute to the hygroscopic nature of the fuel, ie. it will readily attract moisture compared to a basic KNO3 65/35 SU mix.

• The issue of over-hygroscopic fuel is also a problem when using the "dissolved" method.

Although one can perform a "snap test", it's never accurate enough, and therefore the residual water content is not known. Any water in the fuel will slow down the burn rate, and also help attract moisture.

 

You could always abandon the dissolved method and buy yourself a coffee grinder to powder the KNO3 and use icing/confectioners sugar and then just mix and "melt" it - although you would likely have to change your methods.

 

Alternatively, if you are confident that you are making your fuel consistently, and want to stick with that method, then I suggest trying ONE of these modifications, all of which will increase pressure and speed up the burn rate:

• Drill the nozzle and core a slightly smaller diameter (around 1/3rd the ID of the casing)
• Extend the length of the fuel - and bore accordingly
• Add approx. 1% Red Iron Oxide

I stress that you only try ONE method at a time simply because if you try more than one, you won't know which method improved it (or not), and won't know which direction to go. Also a CATO could result.

 

btw. Please take what I've said with a teaspoon of cayenne pepper!!

 

Cheers.

Edited by stix, 04 October 2015 - 05:06 AM.

[Arthur]

First get the height right then get the header calculated well Then put them together.

[deer]

Since I've never had a CATO, I've always had the feeling that my nozzles were very far on the safe side.

Next time I'll try 8, 7 and 6mm opening, with some RIO for the 8mm one (since that's the nozzle diameter I already use).

 

Main advantage of that fuel method is safety. There is no need to use temperatures that can ignite the fuel, to keep it soft.

Also, for the most part of time, the fuel is too moist to ignite completely. Dry mixture can go off from tiny hot spot as soon as you apply heat.

IMHO to actually improve, I should make BP rockets, but before I get a mill, I'm stuck with sugar and this method.

 

On a side note, I assume it's not normal for a shell to blow off it's end like this, is it:

 full.jpg   140.82KB   1 downloads

 shell.jpg   187.24KB   1 downloads

Any advice?

 

P.S. Now that I look at the picture, it seems the poor burst pattern might be because of the shell absorbing impact from motor smashing into it, weakening that cap.

 

P.P.S. Since in most cases I recover all complete parts, how feasible would it be to refill the motor? Sometimes they are visibly heat damaged, but often most damage is just slight erosion of nozzle and wooden top plug pass-fire hole.

Edited by deer, 04 October 2015 - 10:45 AM.

[Ubehage]

 

Perhaps you're getting ahead of yourself? It might be a good idea to use a dummy header until you've sorted out the issues with the fuel, stability or the motor design itself - it could be dangerous. You really need to have consistent reliable motors before even considering attaching a pyrotechnic header.

Amen to that!

I learned that the hard way, along with some new words from my neighbours 

We were all lucky, though.

[Arthur]

My finding with what few rockets I've fired was that the ball head needs NO delay, The coasting time if the rocket, during which the bulkhead fuse burns IS all the delay that the shell needs. If you light a shell fuse (spollette?) at the end of the bulkhead fuse then the shell will fall to the ground and then ignite -which might not be your aim.

[deer]

OK, I'll try that after I dial in the pressure and get a proper lift-off. This time I used delay fuse with approx length for 3..3.5 seconds.

So you just stick a Visco trough hole to pass the fire? Where do you anchor your spiking then.. ah right, ball shell. I should read up on that...

 

P.S. are there any simple tricks to ruin rocket's trajectory at the end of bulkhead burn? Otherwise it just takes on a smooth curve and comes down like a missile.

I was thinking maybe I should drill an opening to the side, where the fuel ends, so the motor starts spinning when it's done burning.

[MadMat]

After watching you last video, I noticed it took some time after ignition for your rocket to actually take off. IMHO you have a problem building up pressure/ exhaust velocity. Your nozzles seem a bit on the large side, I would try a smaller diameter. I usually make my rockets with a short core in them (approx 15% the total length of the grain) and add a small percentage of RIO to the mix. Because of the initial core and RIO, my rockets "jump off the launch pad". This makes for a nice straight trajectory in flight. One note of caution, if you try this rocket configuration; it needs a STRONG walled tube.

Edited by MadMat, 04 October 2015 - 05:52 PM.

[stix]

Since I've never had a CATO, I've always had the feeling that my nozzles were very far on the safe side.

Next time I'll try 8, 7 and 6mm opening, with some RIO for the 8mm one (since that's the nozzle diameter I already use).

 

Main advantage of that fuel method is safety. There is no need to use temperatures that can ignite the fuel, to keep it soft.

Also, for the most part of time, the fuel is too moist to ignite completely. Dry mixture can go off from tiny hot spot as soon as you apply heat.

IMHO to actually improve, I should make BP rockets, but before I get a mill, I'm stuck with sugar and this method.

 

On a side note, I assume it's not normal for a shell to blow off it's end like this, is it:

full.jpg

shell.jpg

Any advice?

 

P.S. Now that I look at the picture, it seems the poor burst pattern might be because of the shell absorbing impact from motor smashing into it, weakening that cap.

 

P.P.S. Since in most cases I recover all complete parts, how feasible would it be to refill the motor? Sometimes they are visibly heat damaged, but often most damage is just slight erosion of nozzle and wooden top plug pass-fire hole.

 

Sounds like you have a good plan. Personally I think bp is far more likely to ignite from a stray spark or static than powdered sugar fuel will (if ever). If I had the unenviable choice, I'd rather be standing next to a kilo of powdered sugar fuel than bp when it went off . touch wood.

 

Most of what you learn from making sugar rockets (except the fuel) can be applied to bp rockets - so I wouldn't look at it like a waste of time, but useful experience gained.

 

As far as refilling the motor goes, I'm not sure with pvc tubing but with Al tubes I've done this quite a few times by soaking the tube in water for a few days to dissolve the nozzle (which are usually eroded), clean it out and ram a new nozzle.

 

... are there any simple tricks to ruin rocket's trajectory at the end of bulkhead burn? Otherwise it just takes on a smooth curve and comes down like a missile.

I was thinking maybe I should drill an opening to the side, where the fuel ends, so the motor starts spinning when it's done burning.

 

I've never really put much thought into this issue, but now that you bring it up, it's something to take into consideration. I did read on this forum someone attaching a salute to the stick where it attaches to the motor, which blew the stick off at the appropriate time, leaving the rest to tumble down instead of the missile dreaded lawn dart. Seems like a lot of work though.

 

After watching you last video, I noticed it took some time after ignition for your rocket to actually take off. IMHO you have a problem building up pressure/ exhaust velocity. Your nozzles seem a bit on the large side, I would try a smaller diameter. I usually make my rockets with a short core in them (approx 15% the total length of the grain) and add a small percentage of RIO to the mix. Because of the initial core and RIO, my rockets "jump off the launch pad". This makes for a nice straight trajectory in flight. One note of caution, if you try this rocket configuration; it needs a STRONG walled tube.

 

Interesting. After the initial burst of thrust it becomes an end burner. I did something similar once and re-filled a used Estes C6 casing with KNSU+RIO, drilled a small bore about a third the way down, and it took off like all hell. I still remember almost pissing myself laughing about it .

 

Amen to that!

I learned that the hard way, along with some new words from my neighbours 

We were all lucky, though.

 

Yep Ube's, been there. And to show that I'm not just some sort of wanker on this forum (oh, I'm so much more ), who dishes out advice without ever making mistakes or doing stupid things myself - some 30 odd years back, I made some pyrotechnic devices (fountains, roman candles etc.) and took them to a party.

 

As the night got dark and the party was getting into full swing, everyone went out to the back yard to watch the display - all went very well. There were cheers, yippees, claps etc... I felt pretty satisfied and good about my achievements. Then came out the finale, which was a bp rocket with a salute. I lit it, it went straight up, then curved at bit and came straight back down smashing into the next door neighbours roof, and as if that wasn't bad enough, 2 seconds later it exploded!!!

 

Most of the drunken crowd just laughed. The fact is that it could have gone anywhere!! and someone got hurt or worse - it certainly was a reality check. The neighbours weren't home so I never had to deal with any aftermath except for the fact that "I" knew what could have happened.

 

Occasionally I bump into people who still remember that party. The incident is inevitably brought up and they always give me a look of reverence mixed with... well, I'm not sure.

 

Cheers.

[deer]

I've heard the stick jettison method. IMHO it's a whole device on it's own, so in my case might just add failure points.

Also, unless I want it to be a fireball termination, I'd have to mess with FP which I'd like to avoid completely in anything experimental.

Edited by deer, 05 October 2015 - 11:37 AM.

[Arthur]

Dead simple, get each bit right one at a time, Your header should burst exactly at the top of the glide phase when all the propellant is gone and the rocket then has slowed BUT before it turns over and points to the ground.

 

Til you have it right you need to do tests in an open space, not near property.

[deer]

Usually I pick a spot with at least 500m of empty space around me. Still, it's difficult to survey such large area and I've had people pop up just 100..200m from me after lift-off. Sometimes someone just happens to be crouching while picking berries or I don’t know what..

 

Lately I've begun firing off fuel samples on ground just prior launch. Noise and smoke draws attention, so I can notice when people stand up to watch and they can pay attention to any potential danger. Can't wait until the winter. Then there will be less people around and the option to do tests in the middle of frozen lakes. That's when I’ll really test things out.

Edited by deer, 05 October 2015 - 12:17 PM.

[Col]

I agree with Arthur,  i dont use any kind of delay fuse on my rocket headers whether they`re ball or can shells. Just a few strands of blackmatch in contact with the top of the motor delay grain which passes fire up a 6mm bore paper tube into the center of the shell.

Last thing you need is a doodle-bug, especially at night when you cant see it once the motor has burnt out

Edited by Col, 05 October 2015 - 05:05 PM.

[MadMat]

IF you switch to cardboard tubes, there is a problem that can arise when filling/packing your melted composition. a small amount can stick to the walls of the tube all the way to the top. This will cause premature ignition of the header or blown out bulkheads as the mix burns around the delay/bulkhead. I always line my tubes with tape about 1" (25mm) into the tube. After packing the tube you remove the tape and presto! clean walls to receive the delay comp or bulkhead.

Edited by MadMat, 05 October 2015 - 02:42 PM.