19 replies to this topic

[BlackCat]

Reading through some of the more recent posts on stars reminded me that a star gun is essential for evaluating compositions and I need to put one on my shopping list.

 

From what I've seen online, there seems to be three distinct types out there and offered by reputable suppliers of pyro materials.

 

The first consists of a number of different diameter tubes welded on end to a plate and states that the length of the tube is 9 times the diameter so, for a 3/4 inch star, that would be a 71/4" long tube. This seems excessive to me just to fire one star.

 

The second is of the same general type but the tubes are all the same length regardless of diameter. Looking at other items in the photograph, I'd estimate that the tubes are something like 4 - 5 inches in length.

 

The third type uses round aluminium bar with holes drilled down the length of the bar. No dimensions are given but looking at where the fuse holes are, it looks as if the holes get shallower as the diameter decreases. This would be in keeping with the sizes of drill bits which also get shorter as they get thinner.

 

As I do have some limited metal working facilities and skills, I feel confident that I could make one of the later type so t is of particular interest.

 

I would be interested in other peoples views of the above and, in particular any thoughts on the safety and effectiveness of the three types. Perhaps those of you who already have a gun would be kind enough to let me know what length the tubes are for 3/8", 1/2" and 3/4" diameter tubes on your device as these are the only sizes that I'm interested in at the moment.

[samboradford]

I was given some useful advice when I came to putting together a star gun.  The guide I was given was 7 - 10 times the ID for the length.  I use 10 x to be certain, so my 3/4 ID tubes are obviously 7.5 inches long.  In fact, I found a single star gun to not be all that useful as often you want to test several stars for consistency or variation in one go and I don't like to re-use the tube unless of course it was thoroughly washed out afterwards.  Blue MDPE water pipe happens to be the perfect size for 3/4 inch and 1 inch stars or comets, i'd imagine it would work well for other sizes to. So I have several lengths cut with wooden plugs cut from dowel pipe epoxied to the bottom.  

 

If you're going to continue down the all-metal path I'm afraid I don't know what would be the best design or the safest.  

 

I can never seem to find enough tubes of the right diameter as there is always something that causes a snag, such as trying to test a hummer or a crossette with a paste wrap or a married comet or whatever and I always end up finding i'm a mm or a few too large for the gun and have to find a broom handle or a piece of dowel to roll a tube, ram a clay plug and test that way.

 

Sam

[rocketpro]

This may be of some help, but I only use one star gun which is for 2.3/4" x 3/4" Bombettes.

 

After a few length adjustments I ended up with an optimised tube length of 4.25" (excluding bung)

 

Incidentally, each one only needs .7g of fast lift to reach the correct height.

Edited by rocketpro, 04 November 2017 - 09:36 AM.

[BlackCat]

Thanks guys, both of your replies are very helpful.

 

I particularly like the idea of using MDPE pipe Sam, I guess you use the 20mm ID pipe for a 3/4 inch star, and I can see that you can make several of them up quite cheaply to use in succession. It does, however, still leave me with the problem of testing smaller diameter stars.

 

Maybe there's a few other comments out there that'll help with the smaller ones.

[Tinderbox]

I use cardboard/paper tubes with a clay bung. Cheap but only last about 8-10 tests max.

And, If I want to test a hard lift, the tube will normally fail. But good enough for most stars.

Metal is probably best for the more dedicated enthusiast.

[Tinderbox]

A

Edited by Tinderbox, 05 November 2017 - 12:18 AM.

[Tinderbox]

Aluminium or one of it's alloys would be preferable to ferrous metals. BP corrodes the crap out of iron after ignition. Most likely due to sulphur salts.

[Rob.L.]

I have been thinking about star guns myself recently, now I know!!

 

Thanks folks.

 

I too like the idea of MDPE water pipe as it is really safe. I need to find a 10 mm version for my tiny stars.

 

I may try drilling out some PTFE rod that I have, It is heat and chemical resistant. PTFE is so awsome.

 

More greens for me!

[BlackCat]

Thanks a lot guys, all of your replies are extremely helpful and have given me a lot to think about.

 

I don't think that I'll be making a huge number of stars, so perhaps working with cardboard tubes or MDPE for the 3/4" stars is the most cost effective and safest way to go. I also like Rob's idea of drilling out PTFE rod and I'll take a look at that option too.

[Arthur Brown]

My local B&Q has a rack of steel tubes of various sizes, maybe there will be a few to meet your needs. Yes the sulphur in comps does cause barrel wear. 

[BlackCat]

Thanks Arthur, I hadn't considered them as my local B&Q is very small, as B&Q's go, however, the one about 8 miles away is massive so would be a better bet for this kind of thing. I'll have a look at their website.

[BlackCat]

Thinking about Rob's suggestion of using PTFE reminded me that there is a slight risk here that needs to be considered before making such a gun and, in particular, where it's tested.

 

All plastics have a degree of sensitivity to strain rates in terms of their measured mechanical properties. In other words, the properties you measure are dependant to some degree on the test conditions. Strain rate simply means the speed at which a load is applied to the material. Imagine hooking your thumbs through an elastic band then separating your hands slowly. This is a low strain rate as far as the elastic band is concerned. If you jerk your hands apart as fast as you can, this is a faster strain rate condition but, the effect of this on the properties would be minimal in this case because although it's a faster strain rate, it isn't that fast really.

 

One of the properties most affected by strain rate is the materials modulus which relates to it's rigidity. You can think of it in terms of stiffness i.e. the higher the modulus, the stiffer the material, however, in Materials Science terms, stiffness is somewhat more complicated but it does involve modulus. So, as the strain rate increases, the material behaves in a more rigid way and can reach a point where the material breaks in a brittle way rather than the ductile way normally seen with plastics. In other words, it explodes with the production of shards.

 

In the case of using it in a star gun, the deflagration of the star lifting charge is obviously a pretty extreme case of high strain rate being applied so some care needs to be taken in testing or using such a device. Although data is available for many plastics at different strain rates, they aren't available for such extreme conditions.

 

If anyone wants to pursue this as an option, I would suggest a very thick wall and to test fire in suitably isolated situation. A barrier between the gun and any people around would also be a good idea.

[Arthur Brown]

The issue with PTFE is that when machined and when very hot it can degrade into HF based chemicals which tend to be toxic. The standing instruction used to be that smoking was forbidden whilst machining PTFE so that no-one got PTFE dust on the cigarette and inhaled the fumes.

[BlackCat]

Yes, of course any organofluorine material will degrade into HF just as any organochlorine will degrade to HCl and I do take your point about dust from machining PTFE. However, with care, the dust issues can be minimised. The issue of thermal degradation is an interesting one that hinges on whether or not the time taken for the lifting charge to burn is long enough to transfer sufficient heat to the PTFE in order to raise it's temperature to the point that it begins to degrade and hold it there long enough for a significant amount of degradation to occur.

 

The published data show that the main decomposition product at temperatures up to 825 dg C is the monomer, tetrafluoroethene, at 525 dg C the initial rate of decomposition in air is 4% per minute and at 425 dg C it's just 0.07% per hour. So, as always you pays your money and takes your choice.

[samboradford]

Personally I'm happy with blue pipe ( MDPE ) or any HDPE if you can get it, but not much else in terms of plastics. Certainly not PVC, unless you're testing very small stars with tiny amounts of BP lift, for obvious reasons.

 

I don't really test small stars. I just test at 3/4 and 1 inch or if I've rolled something smaller I just put a handful in a 3/4 inch gun with some BP and evaluate as a mine.