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5.18 Why don't I just make my own model rocket motors? Shouldn't I be able to custom-make better, more powerful motors, at a cheaper price?




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This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.

5.18 Why don't I just make my own model rocket motors? Shouldn't I be able to custom-make better, more powerful motors, at a cheaper price?

This subject has been hotly debated on r.m.r. It is one of those
'emotional' subjects that find people either firmly for or against.
The following post from Lawrence Smith probably says it all best.

From: thesmiths@mv.mv.com (Lawrence Smith)
In our pre-made, factory-produced society there is a bit of nostalgia
for doing things the "old-fashioned" way - "home made" carries with
it the feeling of being somehow "better". It could be tastier, more
durable, easier to maintain, or cheaper, it is some quality that we
don't find in mass-produced items. It is therefore natural for fans
of rocketry to look at the prices of the ingredients of an engine and
think to themselves that such engines really can't be all that difficult
to make - and that "home made" might be not only cheaper, but
just as good as factory engines in other ways.

There is a grain of truth here - just a grain - but that is enough
to make the idea pretty dangerous. Most readers of this will not be
old enough to remember the "Basement Bomber" headlines from the late
fifties and early sixties. Making solid-fuel rocket engines is a
pretty dangerous proceeding, unless you do it right. And even if
you do it right, you must be consistant in following the safety
precautions. It really won't matter whether it is the first or the
fiftieth engine that blows your hand off in the long run. It is the
need to watch the safety precautions that is your first "hidden cost",
something that most people dreaming of home-made engines seldom add to
the calculation.

You need room, you can't do it in your basement, nor, indeed, in
your house at all. Nor in your apartment, nor in your school chem
lab, nor anywhere else there is something you don't want blown up.
You must assume the engine will blow up, and ask yourself where
it would be okay. Maybe an outbuilding on your property, maybe out
in some field. You need space to build engines. You either need
to own that space or have the permission of people who do, too.

In most places in the country, you need some sort of license.
Rocket engines are first cousins to pipe bombs, and there are
few municipalities that would care to have you building those. On
the other hand, there are many municipalities that don't care if you
reload spent shotgun shells, even without a license. You need to
know and understand the local regulations. You are not building a
class "C" toy propellant device. Even if you think you are. That
is a legal name, not a descriptive one.

You can't reuse Estes or other maker engine casings. Spent casings
have undergone considerable stress, they can no longer be guaranteed
to hold if reloaded and fired again. Yes, they have a fairly consider-
able safety margin. It isn't enough to reload them.

You can't use the same stuff Estes uses, nor can you easily obtain
composite fuel. Estes uses black powder - gunpowder - but they are
using a special formulation, not just mixing the usual ingredients.
In fact, it's pretty dumb to use any powder at all. Powder must be
rammed to make it solid enough for "solid fuel" - that means you have
to pour the powder into the casing and then take a stiff rod and ram
it up and down, like one of those old-time muskets. Ramming will
compress the powder - which is the objective - and will also heat it,
which will also make it more shock-sensitive, not to mention the heat
from the friction of the rod itself moving up and down in the casing.
It can also raise dust, which is also more easily ignited than a solid
slug. A fellow by the name of Jim Flis posted a commentary on creating
such engines, I recall. If anyone saved that post, I'd like to see to
see it inserted here.

Even if you manage to pull off all the above, the engine may not perform
like you expect. Your ingredients may not be up to snuff, or maybe you
didn't ram enough, or maybe something else got mixed in by mistake, or
maybe you forgot to add the final layer to the casing - whatever. There
is a real good chance any home-made engine is going to do serious damage
to your rocket, even if it didn't do any to you. One of the things you
pay for from Estes and Quest is qualitycontrol. And even with
professionals with years of experience with A, B, and C engines, Estes
has problems with D and E engines. Can you really do better?

Estes, Quest, and the others have time, they have equipment, they
have space, they have quality control and lastly, they have liability
insurance for when something happens with one of their engines. That
is what you are paying for, not just a couple teaspoons of black powder
and a bit of clay and cardboard. As you can see, there is more than meets
the eye in an Estes or Quest engine. The fact of the matter is this: a
premade, off-the-shelf engine is cheaper, more reliable, and far, far
less effort to obtain. There is really no reason to try to duplicate a
standard engine.

That said, there will remain a small core of people who still want to
roll their own. Maybe they need a non-standard size, or they just are
more interested in the engine than in the rocket. Before you proceed,
you will need a copy of "Rocket Manual for Amateurs" by Bertrand R.
Brinley, Capt. It's out of print, so you'll have to do some looking.
If you aren't patient enough to track down a copy then you aren't
patient enough to build engines. Take the hint.

Brinley gives a good overview of propellant technology up to the mid-
sixties or so, which is good enough for you, since the advances have
mostly been in the realm of "real" rockets with incredibly exotic
chemistry. He concludes that amateurs should stick to zinc and sulfur.
Zinc and sulfur can send rockets high enough to need an FAA waiver, so
this is really not limiting. Also, though zinc and sulfur can be ram-
med, they can also be mixed with acetone or alcohol to form a putty
that can be inserted into a casing and cast into shape. This he terms
"micrograin". I won't go into detail on proportions or other infor-
mation, you'll have to find the book. I'm not telling you how to
build an engine, I won't do it, and I don't think you should, but
if you are anyway I want you to be aware of what you need to do to
succeed, for if you fail the consequences will be horrible.

While Brinley is helpful for fuel, he is more valuable for the kinds
of safety precautions you need to take. You are going to need sand-
bags. Take the hint.

One thing Brinley assumes, though, is using metal cases, including CO2
cases for small engines. This will definitely make your rocket require
an FAA waiver, and the metal content will make it easy to spot your
rocket on radar, so you better not try to mickey-mouse it. Of course,
the FAA will not be forthcoming with a waiver for an experimental
free-flight rocket, unless you are launching from someplace in the
southwest desert area, and maybe not even then. In a way, this is an
advantage. A metal-encased engine will fragment when it explodes, and
is far more dangerous that one in fiberglass or carbon fiber. Also,
metal is far more likely to cause a spark somehow than are non-metallic
components, and so are safer to handle. This limits the size and
materials you can use.

You will need to improvise on Brinley and develop a fiberglass or
carbon-fiber casing and clay or other material nozzle. I have an
ulterior motive for mentioning this, and I admit it freely. Making
a casing like that is not trivial to begin with, and you will find
it's pretty expensive, both monetarily and in terms of time. If
that still does not deter, at least you will be encouraged to make
your engines small - which will make the process safer, for there
will be less to explode. I have no idea how thick the casing needs
to be, nor would I say if I did. Again, you need to do your home-
work.

If you take the advice in this post to heart, you will not make a
rocket engine. If you take only some of it, you will build a much
smaller engine, in a casing less likely to become deadly shrapnel
(not unlikely, just less likely), and with techniques less likely
to cause an explosion. You will be doing it with the advice of a
professional (Capt. Brinley, not me) and hopefully will succeed, and
then either give it up or go legit and start a career with Morton
Thiokol. Who are, of course, the makers of the shuttle SRB's used on the
last flight of the Challenger, just in case you thought being a
professional meant that you had everything figured out.

 

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previous page: 5.17 I've seen mention of all kinds of rocket motor types and sizes. Could you give a brief history and summary of the main marketing names for model rocket motors?
  
page up: Model Rockets FAQ
  
next page: 6.1 Rocketry: Cutting, Sealing, Attaching Fins