Making Rockets That Really Fly

by Richard J. Kinch

INTRODUCTION

You pull down your face shield and examine your handiwork. Sent on its journey skyward, your creation may disappear beyond your tracking and recovery perimeter, and this may be the last time you hold this item of your own craftsmanship. Placing the rocket in its cradle, you quickly scan the edge of the field, looking for interlopers unwittingly invading your exclusion zone. Seeing none, you signal your fireman with the words, "Standby for launch," spoken with calm but firm authority, and he drops his eye protection down across the bridge of his nose. As you commit to launch with the descent of your hand to the firing apparatus, you fight off a tremor of excitement to speak the word, "Ignition." A thin curl of smoke appears at the nozzle, and time freezes.

For a moment you question all the theory and technique that brought you to this moment, as you watch the exhaust grow to a puff of translucent smoke. You have seen many failures--"rockets" that were just jumpy smoke generators, or that rebuked your self-guided, amateur attempts with a sharp report at eye-level and a rain of debris. You have felt the same excitement before, the same trial and the same worry, and you wonder if the same disappointment will follow in the same way. You examined and analyzed each failure, and considered the balance of chemistry and mechanics carefully, and put the sum of your knowledge into this one last attempt. You know that a failure now will discourage you from trying further. The quiet puff transforms into a hissing, angry jet. The smoke rolls and flattens, hugging the ground and spreading an aroma of brimstone across the moist turf. To some this odor may be a stench, but to your nostrils it is the promise of a fleeting moment of gravity defied. The object of your labor leaves the ground with a lusty, spattering whoosh. It climbs on a tower of smoke, spiking high into the zenith where it vanishes up yonder.

Up yonder, indeed. You have succeeded, and all too well. Somewhere in a few acres of trees and brush, lies your first homemade rocket. It will lie there for years, a cold relic, damp and musty in the carpet of the woods. Perhaps a child will find it, and wonder over this strange, spent device that has none of the usual Asian glyphs or bright tissue-paper wrapping. Thinking it a crude object, he will cast it aside.

Had you recovered it, you would have eagerly returned it to your workbench. With ghoulish glee you would have vivisected it with a razor down the ventral line, and examined the cross-section for the reasons for your success. How did the casing stand up? Did the nozzle clog or ablate? Was the fuel completely ejected, or was a residue retained?

Yours is not a hobby concerned with the niceness of a faerie contraption of balsa and tissue. No, you think in terms of ramrods, and powders that ignite, and milling with steel. You want thrust and acceleration and altitude for its own sake, not just to carry a toy image of something bigger. No store-boughten, machine-made, drop-in motor will do. You build your own rockets from what the ancients had at hand: from nitre, brimstone, char, clay, coarse paper and primitive glue.

And having seen but once the power of this fire turned to motion, you know that you must persevere and perfect this art to the limit of your simple tools, your ability, and your courage and perseverance.

You are a home-made rocket builder.

This is a summary of home-made small rocket craft as practised by me during the summer of 1993. Like every modern publication on the art of pyrotechnics, I will appropriate freely the recipes and techniques promulgated by earlier authors. But unlike many authors, I will mostly be telling you about what I know works, because I did it, instead of just theorizing about and reorganizing what someone else thought about the subject.

There is a popular hobby of "model rocketry" that involves you in building models into which you place pre-fabricated motors for which you pay cash money at retail, with boring code names like "D5-3". The advocates of this "rocketry" are very fussy about their electrical ignition source; they abhor black-powder fuses and butane lighters. It is a mortal sin for them to merely crave the forbidden act of reloading a motor casing. They even write all these precepts down on tiny slips of paper, and carry these mazuzahs about with them, and recite them with every purchase of motors. They must enjoy the pure, rigorous discipline of the strictures, because surely there is no joy in spending hours building something that just arcs over into the wilderness the first time you put any real energy into its keister.

This is not at all a description of that hobby. I like to make the motors (instead of buy them), and glue them on a stick, and watch them go. If I were to give them code names, it would be "Cloud Tickler" or "Neighbor's Cat Chaser" or something poetic in the Far Eastern tradition. I put fuses on them, summon my elan, and nonchalantly light them from the flame of a Bic lighter. If I smoked cigars (routinely) I would use one as a zero-th fire, no doubt.

What I make are not models. They are small, but very real, rockets. They are exactly what real rockets were before the industrial age, which is appropriate since pre-industrial tools and materials are the ones which present little trouble for the amateur to obtain.

There are some quaint aphorisms about amateur pyrotechnicians, like, "most people classify them with amateur brain surgeons [Lancaster]", or, "evolution in action [Nelson]," or, "it is illegal [whether or not it is] for good reason." If you look at the whole of pyrotechnics, the prudent amateur will consider most of it beyond his experience or skill. But there are certain pyrotechnic endeavors, dealing with modest sizes, low-order propellants, and fail-safe designs, that may be safely executed with due attention to face, eye, ear, and fire protection. It is not without risk, but the risk is manageable in the sense that the materials and processes are under your control, and even if you should lose control, you may beforehand take precautions which protect your body and your property from the effects of worst-case failures.

There are two aspects to succeeding at this craft. The first is to learn all you can from what others can authoritatively teach you, and the second is to practice the techniques skillfully and learn gradually, without exceeding the limits of your present experience.

To share my knowledge with you I will be quoting or paraphrasing from a number of famous pyrotechnical books and a few less-widely published documents. One of the latter is a short paper sold by the Teleflite Corporation, a document called, "The Incredible Five Cent Sugar Rocket". If you make enough of these they really do cost "just pennies apiece" as they advertise. And they work, subject to your care and skill at a labor-intensive process. But for the hobbyist this is a fun project and, as pyrotechnics go, relatively safe.

If you're wanting to do this, I would suggest you order their 14 pages of illustrated instructions; I believe this costs $2.00 from: The Teleflite Corporation, 11620 Kitching, Moreno Valley, CA 92387; check the back of Popular Science for their current advertisement. Experienced or quick-learning experimenters might have enough to go on in this note to get results; however I am leaving out all the details that Teleflite explains clearly, like how to weigh and what size batches to make, etc.

MIXING FUEL

By weight mix:

63% Potassium nitrate (KNO3, saltpetre, niter). I recommend drying prills or crystals in a warm (250 deg F) oven for an hour, thinly spread on a cookie sheet. Then cool and grind immediately (before hygroscopic action absorbs water) to talcum powder consistency with mortar & pestle, or, ideally, a ball mill. I got my mortar and pestle at the local gourmet kitchen retail shop. My ball mill is a "Tumble Stones" toy made for lapidary work that I got at a yard sale for 25 cents.

Getting the raw KNO3 can be difficult. I got mine at the local agricultural feed and fertilizer store, which sells the prilled form during the growing season ... in 50 lb. bags. This is a lifetime supply, at least, and cheap. The purity is acceptable for rockets, or you can recrystallize by cooling a hot, saturated solution, if you want to be neurotic about purity. The salesman asked me if I was making a bomb and laughed sarcastically, hahahah. I think he had it confused with ammonium nitrate, which can act as a high explosive in the right form and with the right initiator. This was when the World Trade Center bombing was a recent event.

Before that I got a pharmacist to special-order some from a catalog. This was a friend who trusted me, however. It was one pound, very pure and very expensive, suitable for food (curing hams!) or pharmaceutical use (?). Other pharmacists gave me the hairy eyeball and said that if I wanted to make black powder I should to the fertilizer place, like I had uttered some secret code. I think they teach them to say this in pharmacy college.

27% Powdered sugar (C12H22O11, sucrose). This is the carbon source which reduces with the KNO3 oxidizer; if you take the 12 C's out of the chemical formula you have 11 H2O's remaining, which is to say, "steam" when this fuel burns. This "dead weight" in the reaction moderates the temperature and thus the impulse available; this is definitely a low-impulse fuel. You can oven-dry this like the KNO3, although it's only about 1% w/w hygroscopic and shouldn't be necessary. The slight amounts of cornstarch sometimes added to the supermarket variety to keep it free-flowing will not affect its use as a fuel.

One could instead of sugar use finely ground charcoal, say at 15% by weight, in which case you would have the classic 75%-15%-10% meal powder (black powder before processing) formula. You can get more thrust this way, if you are serious about such things. You also get pretty orange-gold sparks that sugar just doesn't make. Forget lampblack (you can make a small amount easily by holding a metal plate in a candle or propane flame), it is relatively pure carbon but physically unsuitable for rocket fuel. Serious pyro makers treat their charcoal like wine tasters: willow and grapevines are good, hardwoods bad, etc.

10% Sulphur (S, brimstone). This is another reducer (fuel) that helps the burning characteristics. It also gives your product a gloriously authentic smell of fireworks. Mmmmmm. I got mine as dusting sulphur from a garden shop.

The fuel mixture is moderately hygroscopic, so if you don't keep it well-sealed (and perhaps protected inside the container with a dessicant like you can get at a hardware store) after drying you won't get quite as good results. The easiest thing to do (unless you live where it is very dry) is to just make a small batch at a time and load it all into casings before many hours have passed. Oven-drying the mixture requires much care, since any contact or spillage onto the heating element or burner can ignite the batch; it won't explode but depending on the size you have anything from a mess to a structure fire to burns. Your mom or your wife will be displeased.

MAKING CASINGS

Casings are very important to rockets; don't be deceived by the simplicity of store-bought bottle rockets and skyrockets. The technique here is to cut an 8" strip of 2"-wide gummed unreinforced kraft paper tape (old-fashioned tape used to seal packages), wet the whole ungummed side and all but the first inch of the gummed side, and tightly roll it around a 1/4" hardwood dowel overlaid with several layers of plastic mending (e.g., Scotch(tm) brand) tape; the mending tape layers keep the product from sticking to the dowel so you can slip it off, and also make an inside diameter of the tube that allows the same size dowel to be used as a tamper. This technique takes some practice and spoiled work before you get tubes that spiral less than about 1/8" off a perfect roll-up. After a day to dry you will have an amazingly strong and light tube. When dry, you should trim the ends square with a new single-edge razor blade.

Rocket casings from pyro suppliers are a lot more convenient if you can get them. These are just convolute tubes, strong and thick, with inside diameters of 3/8 inch or 1/2 inch and about 7 times that length. Being non-hazardous by themselves you don't have to worry about ordering or shipping restrictions. You might find a thin, strong tube used as packaging in some commodity you use, like ballpoint pen refills used to come in; if you can get a quantity of these it might be worth some experimenting to see if they can stand up to the stresses of a new ontology of Newtonian impulse generation.

MAKING NOZZLES

Nozzles are likewise deceptively simple but critical to the design. The plug material must be light, non-burning, strong, non-shrinking, and machinable with the tools at hand. Teleflite suggests "Durham's Rock Hard Water Putty" as the best commonly available substance; it cures by air-drying and does not dull steel drill bits. Indeed, this substance is peculiarly suitable. Don't confuse this with plumber's putty or Silly Putty (tm) that stays soft forever; this stuff is some kind of (quicklime?) plaster that gets literally hard as a rock (well, a rehydrated limestone rock, anyway).

Insert a length of dowel taped for a 1-3/4" stop into the casing to make a form for the wet plug, pack the wet putty plug material into the remaining 1/4", and then slowly withdraw the dowel so as to not slurp the gooey stuff back down into the tube.

When the plug is cured (a day or more), drill a 7/64" nozzle in the center. The diameter of this nozzle is crucial to optimizing the internal pressure and thrust versus the bursting strength of the casing and nozzle. It also is related to the mandrel used to make a hollow-core fuel load.

Better nozzles can be made quicker and easier with the clays known as kaolin or Bentonite. Kaolin might be available locally at an art-supply retailer, as it is used to make some types of pottery. Bentonite is a pyrotechnic specialty item, as far as I have found. For rocket nozzles, you use it in the powder form it comes in, and pound it into the tube over the mandrel. There is no need to fire-harden or even dry the clay, as it is strong enough when just pounded. This surprised me, as it goes against one's intuition. But it works. Its just harder to find this stuff as compared to the water putty.

I've had some success using hot-melt glue to plug the nozzle end of the tube, instead of water putty or clay, and then drilling a nozzle. This makes a nozzle that ablates during burning, so the thrust decreases as the burn progresses. The burn is short enough that the nozzle will not melt completely. For early experiments this is more likely to produce a working rocket that doesn't stress the case as much. It also is ready in a few minutes, instead of waiting overnight for putty to dry. This glue costs more than putty or clay, so I consider it a prototyping medium.

LOADING FUEL

Load the fuel in so as to produce a hollow core as follows. The hollow core allows the full length of fuel to ignite, which is important for this low-impulse formula. A solid propellant grain that just burns at the end will not develop much thrust with this fuel.

For a mandrel, put a 6d finishing nail with head removed, 3/32" diameter piano wire, or other item of similar diameter upright in a wooden block, so that a 1-1/2" length projects above the surface. Make a wooden (or better yet brass or aluminum, but certainly not steel) tamp by drilling a slightly wider and deeper hole down the center of a 1/4" wood dowel or metal rod--not an easy task with a hand tool, but possible after several careful tries or a visit to a machine shop. Some slop in the fit of this tamp to the tubes and mandrel is OK, such as if you are machining and drilling with hand tools; you'll just have to be more careful in packing in the fuel.

Place a casing with nozzle over the mandrel, nozzle down, and funnel a bit of fuel in, then tamp down with the drilled dowel and mallet. Fill up to the last 1/2" of the casing, which should be the top of the mandrel. Top off the fuel with a wad of tissue or circle of paper and seal the top with clay, fast-set epoxy, or even hot-melt glue.

The fuel burns faster under pressure inside the casing. That's why your rocket will fly. If you just light a bit of it loosely piled on the ground, the burning will seem slow. The pressure from the reaction will accelerate the reaction and add to the pressure, and so on. You must have a casing and nozzle able to take this violence.

If your fuel is packed too loosely or the casing is not rigid enough, the fire will leak up the side or through the corpus of the fuel, and the fuel will all ignite too quickly. This will burst the case or eject the nozzle or both. This is known in the trade as "exploding." This can also happen if the nozzle opening is too small.

FUSING

Applying a fuse is the culmination of the rocket construction. Unfortunately the fuses themselves are the least amenable to home construction. I admit that I cheated and bought mine.

I like to use the green Visco fuse that is sold at black-powder hobby arms shops for lighting ceremonial or model cannons. I just stick a 2 inch length of it a short ways into the nozzle. The burn rate is reliable so that you can get out of the way.

Cannibalizing firecracker fuses is likely to get you a rocket climbing up the side your head, because the tissue-paper fuses tend to turn into miniature quickmatch tubes when you don't expect it. That is, they burn almost instantaneously up their whole length, instead of slowly and evenly, which is what a fuse must do.

Blackmatch, which is a thin cotton rope thoroughly impregnated with a wetted meal powder and glue, would make an acceptable fuse in a suitable length. Making good blackmatch is possible with simple ingredients and apparatus, but is beyond the scope of what I can present here.

In a jam I might want to try soaking cigarette paper in saturated KNO3, drying it, and twisting it to make a very slow touch-paper fuse. I haven't tried this but as a last resort in the absence of any other supply of fuse it might work in an slow, less-than-reliable way.

PRIMING (optional)

You need not worry about priming while you try to get your first couple of designs perfected, but once you do, you can make this optimization.

For maximal impulse at launch you need to ignite the whole core quickly, and just lighting the bottom doesn't do that. You can omit the primer, but you will get a slower lift-off and less altitude due to the flattened thrust curve.

To make a good primer one must depart from the home-made realm and locate a small amount of commercial ingredients.

If you can cannibalize the very thin fuse used in flashlight-cracker firecrackers, which is ideal when inserted up the length of the core and wedged with a twist of paper or sliver of wood. It might take two, one in deep and one after that sticks out for lighting, to get enough length from these type fuses. Inside the core the fuse will burn very quickly, almost like the piping of blackmatch makes a very fast "quickmatch."

At this point we get into materials I haven't tried; they scare me because they are so energetic. You can allegedly use Pyrodex (tm) brand "P" synthetic black powder equivalent or commercial black powder, either in the FFFg or FFFFg grain size, as a primer material. In the US gun shops often deal in black-powder arms and supplies, which are a popular hobby. Unlike smokeless powder, black powder must be handled according to some safety rules which restrict its sales to the smaller, "mom-and-pop" type shops. Don't try to use the easier-to-find smokeless (nitrocellulose) powder for this as the unconfined burning speed is far too slow to be effective; if anything you will just slow down the unprimed ignition speed by clogging up the core with a slow-burning material.

To prime with black powder, I am told but have not personally verified, invert the fueled casing and trickle the loose powder into the core; tap to settle the powder in but do not tamp. Cap the loose powder through the nozzle with a paste of the same powder wetted with some water; this is supposed to dry quickly. This sounds like a ``first fire'' technique similar to that used in sophisticated military pyrotechnic devices.

MAKING IT FLY UP AND AWAY

Bamboo skewers (12" or longer, the common 8" size is too small) work well as guide sticks, glued to the side of your engine. If money is not a problem, you can use very thin dowels from the hardware store. They will wonder why the stock disappeared so fast.

For a launch pad one can use the classic bottle, a pipe in the ground or lashed to a ladder, or a trough.

Remember that a flawed rocket can (1) develop a small thrust to lift it off the pad, (2) sputter and tip over, and (3) regain thrust and head off in a less than vertical direction, that is, at you or other persons or property nearby. One must choose a suitably isolated site.

ADVANCED DESIGNS

The Teleflite instructions give you more information on delayed and undelayed parachute-ejection charges, two-stage designs, and adding report charges for apogee detonation.

Smaller sizes are worth experimenting with. They tend to be less demanding on the structural materials than larger sizes.

Scaling up to bigger sizes requires that you arrive at new values for nozzle opening, case thickness, etc. As we used to say in engineering school, one does this ``empirically'' and not ``axiomatically.'' In other words, you can't just double everything, you have to fiddle with it until it works.

CAUTIONS

Do this where it is legal: observe local, state, and federal laws, and fire and storage regulations. Consider your neighbors' reaction before you use your back yard as a launch site, especially when you choose to violate the previous caution. Wear eye protection during manufacture and while launching, and the prospect of an rocket exploding on the ground should suggest ear protection. Don't handle duds without soaking them in water or waiting a long time. Don't ever sell or give away what you make. Stick to the US Class C fireworks propellant limit (150 mg) and you will be limiting the hazards.

Hey, you must be serious about this if you read this far. You are also maybe an underage boy who yearns to do this without your parents' permission. I recommend you try to interest an older and wiser person to help you, like your Dad or your Grandpa or whoever thinks this stuff is fun. I have an extra dimple on my face where I learned at the age of fourteen that there are hazards to pyrotechnics that the imprudence of youth will neglect. Twenty-five years later, I can still have fun, but acting like an adult makes it so much less painful.

Copyright (C) 1994 Richard J. Kinch, Ph.D.

All rights reserved, except that publication not for profit on electronic networks granted, provided that the above copyright notice is retained exactly.