Inspired by Bert, I create this thread to discuss primes and priming techniques.

My original question relates to a general problem where slurry primes tend to form smooth surfaces after drying so that the speed of the flame surface propagation is affected negatively.

In the case where a star, comet or rocket grain is primed in two steps, with wet and dry prime, the final superficial layer of the prime is quite coarse. This makes the surface area bigger which in turn makes it easier to ignite and also speeds up the flame propagation over the surface.

I asked what could be done to manipulate a slurry prime to maximise its surface area after drying.

Tuatara proposed the idea to add coarser particles, inert substances like sawdust or active substances like bp granules together with a binder/solvent system that won’t dissolve the granules.

In my case, where I use an ordinary meal powder/gum arabic/water system, I believe adding very fine particles of bp granulated with NC solution could be effective. The problem though, is that you have to prepare these granules separately which adds time and costs to the work.

I have been thinking in the lines of adding extra solvents, or something that evaporates easily, to create pores or small cracks in the superficial layer of the prime after drying.

Do you have any additional ideas?

Well In rocketry if a grain is hard to ignite we rough the surface up with an abrasive, like sandpaper, a coping saw blade or a scapel, this works for rubber bound fuel but may be a bit harder on a solid prime. Appart from that you could layer the primes, like for a very hard to ignite star, but have a small layer of a very vigerous prime coating a more substantial main prime.

Hope that helps

Thanks for your effort to contribute Blackhawk, but I don't want to add any extra steps in the production. Also, scraping or brushing the dried prime surface could cause ignition through friction!

Yes that would always be a problem. Well if you simply want to modify the prime base rather than layering it you could go one of two ways, Make it easier to ignite or Increase it's surface area (Or lower it's density to facilatate burning) For making it easier to ignite, it really depends on the composition, but the surface area things have already been discussed (adding chunks to the slurry) How do you add the prime? If you spread it on (Like making a sandwich) then you could rough-up the surface with a thin stick or a plastic knife, this of course depends on the thickness of the slurry. If you want to be more consistant you could make like a 'stamp' out of a piece of wood with some raised lines in a grid shape on it, if you press this onto the wet slurry-coated-star it should increase the surface area.

If you want to make the composition crack up you would just use an excess of a particularly volatile solvent such as xylene to a mixture of fine particles. As far as voids go you could possiblly add some chemicals to the mix that would produce a small amount of gas (fizzing the slurry as it were) although it would take some trickyness to get the right chemicals and amounts in the mix to make the bubles without affecting the slurry in a negative way.

I think making a prime that shrinks so much it cracks would be a mistake. The same shrinkage would likely make the prime flake away from the substrate. Not only that, but flame propagation into the cracks would tend to blow the prime away from the substrate, I surmise.

I did have another idea though. With your usual slurry prime, or one with extra chunky bits, prime as normal, allow to dry completely, then wash briefly with pure solvent. Done right, this would remove the binder from the surface only, leaving particles exposed on the surface, and giving you the 'rough' surface you're after. This would be a real easy one for you to try, CrazySwede!

Even though my question was general, my specific problem sets some limits. The most important one is that the prime layer must be very thin and burn quite fast. If I make the prime too thick there is a risk of blowing the nozzle. If I make the prime too smooth, by wetting it too much, the start-up time for the motor will be too long.

The idea from Tuatara about washing away the superficial layer of binder from a smooth prime surface would only be possible if the solvent didn’t also dissolve the oxidiser. The binder would have to leach out very quickly and I also guess that the dried prime layer would have to be very solid and pore-free to prevent the solvent from washing it away. Unfortunately, none of my binders dissolve very rapidly after having being moist and dried once. Also, I wouldn’t want to use flammable solvents in this last stage of production.

Anyway, here are two ideas, inspired by the answers so far:

A possible solution would be to dip or paint the rocket motor grain (in my case, a composite of potassium perchlorate and phenolic resin) using wet slurry of meal powder, gum arabic and water. Before drying, the prime area could be increased by using a stiff brush to roughen up the surface.

If a small amount of washing-up liquid, or some other foam-inducing agent, could be added to the slurry, it might be possible to whip in enough air so that the prime layer contains lots of bubbles. Hopefully, these bubbles would enlarge the surface area and improve ignitability.

Your first idea sounds the best IMO. Adding a soap (washing up liquid) may retard the burn rate, or it could just create a gooey mess that may not dry fully. But then again, there is only one way to find out isn't there :p The idea that Tuatara put forward (using moderately fine sawdust in the prime mix) sounds like a good idea as well.

In my experience (limited at the present time) I have found that my re-invention (:p) of 'step priming' is really good for the hard to light compositions, including thermite.

Does anyone know if any of the single component polyurethane glues ("gorilla glue") or similar are flammable? These foam as they cure... I might have to do a test! Excel One? (http://www.excelglue.com/EXCEL%20One/excel%20one.html)

I am unsure about the glue, but I would also agree with using sawdust. Possibly bits of polystyrene? Although it may be difficult to get the styrofoam to a desireable size. I recall grinding up papertowels or paper in a coffee grinder. It resulted in almost a cotton like fiber. Its texture was quite unique, and it burned quickly. Unlike cotton, this cellulose pulled apart easily, and might work in place of the sawdust or what have you.

Errr, I can't stand how long I must wait too see my posts, since I only have a few.

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That's necessary to minimize the number of kewl posts.

Rhadon