8 replies to this topic

[BlackCat]

In my meanderings around the web, I came across the following formula for a "Gerb, Blue":

 

Ammonium perchlorate               38%

Copper (II) oxide                         16%

Parlon                                          12%

Aluminium flake, 100-325 mesh  10%

Hexamine                                      8%

Charcoal, airfloat                           3%

 

On paper, this looks to me like it would be a very nice effect, however, I have very little experience of working with ammonium perchlorate, just a few coloured flare type compositions.

 

The formulation came with no information about mixing or charging to a tube, however, I think that the mixing should be quite straightforward, pretty much the same as anything else really with a few extra safety precautions.

 

My main concern is the best way of charging it to a tube. I've read that perchlorate compositions and, indeed, any composition containing metals should never be rammed with a mallet and drift so that seems to be out especially with the "enthusiasm" of amm. perchlorate. Lightly tamping by hand pressure with a wooden drift would seem possible but always runs the risk of leaving a void in the packed tube with unpleasant consequences. So, that leaves pressing the composition which I can do if necessary. Is this the best or safest way to proceed?

 

So, does anybody have an opinion on the best way to fill a tube with such a composition? has anyone made something like this and would share their views on the best band safest way of charging a tube?

 

I was also very interested to come across this formulation as, at the back of my mind, I've been thinking about amm. perchlorate compositions as drivers for wheels etc. The fact that a stoichiometric mixture with something li8ke red gum would give all gaseous products would seem to me to make it a good candidate for such an application. However, it may be a bit too energetic unless you want a very fast wheel!

 

Has anyone tried such a composition as a driver?

[samboradford]

I haven't seen this particular formula but I'd imagine you'd encounter some of the same issues as with the blue steel gerb.  I personally find it rather underwhelming and all AP gerbs have a tendency to blow up so getting the choke right can be a challenge and even then they seem to blow up ( quite spectacularly ) when you're unawares. Pressing is the only way to go if that helps.  I'd be very interested to see how you get on.

Sam.

[BlackCat]

Thanks for that Sam, very helpful. I obviously need to give it some more thought especially with your warnings about them blowing up and with neighbours nearby.

[icarus]

you are welcome to experiment in nwleics be good to see you again . we cannot store, so any composition must be burnt or deactivated on the day of experimental work. rocket motor /driver  and gerb research welcome ,  

[BlackCat]

Thanks for the offer Icarus and it would indeed be good to see you guys again.

 

However, whilst I appreciate the "knowledge for knowledge's sake" aspects of development work, I'm not sure that I want to work on something that I'll never be able to use in my garden. There's also the possibility that, if I don't like the blue, I could always look at other colours so it may turn out to be a significant investment in time.

 

My first thoughts on the variability of such gerbs echoes Sambo's comments as I can easily imagine that, with such a high volume of gas being produced, small variations in the choke diameter can lead to very large changes in the internal pressure of the gerb and maybe that's what's responsible for the tendency of them to blow up unexpectedly. Lets face it, the usual method of creating a choke with bentonite isn't terribly accurate in terms of reproducibility so, it's about getting an accurate and reproducible choke as well as, as Sambo says, getting a choke of the right diameter.

 

I know that some people put a solid plug of bentonite in the tube and then drill a hole in the clay after filling the tube. Personally, I think that the safety of this process is questionable and I certainly wouldn't want to try it with AP compositions. So, I need to think of a way of making a consistent diameter choke.

 

There's also the possibility of using a somewhat less vigorous composition by replacing the AP with a stoichiometric equivalent of KP. Again, I've never tried KP compositions in a gerb so if anyone has some experience of such things I'd appreciate some comments.

[samboradford]

The most effective way to make the choke would be with compression, insomuch as you compress the top of of the tube circumferentially to your required diameter.  This gives you a choke that is more forgiving as it is simply made from the cardboard tube and not a hard rammed clay plug.  You also get some burn through on the choke which slightly widens it as the gerb burns and reduces the buildup of internal pressure and hence the liability to blow up.  That might help you get closer to where you want to be ?

Sam.

[BlackCat]

Thanks Sam, I can see where you're coming from. I'd forgotten about crimping as a possible way forward although I have read about it and seen pictures. Unfortunately, I don't have the necessary tooling to form a choke in that way. I will look into it though.

[samboradford]

It can be done easily if you roll your own tube and crimp it whilst it's wet.  It's not so easy if you crimp a commercial tube - in that case you need a clever machine.

[BlackCat]

Thanks again for that Sambo, I do roll some of my own tubes but, to be honest, a bit more practice wouldn't hurt! Anyway, I'll certainly look into that possibility and see what I can do.