Making Rockets That Really Fly
by Richard J. Kinch
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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.
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