Amateur and
Experiemental (EX) Rocket Propulsion and Motor/Engine Construction by
Kurt
Theis
I have been making solid fuel composite motors since 1996. The thrust
from these range from 20 pounds to over 450 pounds. Using HPR (High
Power Rocket) designations, they range from an F engine to L
class. I have long burn motors (7 seconds) and short burn motors
(.5 second
burn).
All of my rocket motors to date are composite based solid fuel motors
using HTPB ( hydroxyl terminated polybutadene)
and Ammonium Perchlorate
(AP) as an oxidizer.
I am currently making casings for 38mm, 54mm, 75mm motors. I am also
making hardware and propellant slugs for 2 1/2 inch motors (62mm) in
the L class. Here is a batch of casings
for an I motor.
Here is a 2 1/2"
propellant slug (uncored) for a K motor. And a comparison between 54mm propellant slug
and 38mm propellant slugs.
I am using an a 70% - 83% solids loading using AP (90 and 400
micron) and 325 mesh Aluminum, with the remainder of the binder
being HTPB, Isonate, DOA/other plasticizer and HX878.
To date, with a 38mm motor, I have G, H and I motors ranging from 6
inches long to 18 inches long. The J and K motors are 54mm and the
larger K and L motors are 75mm. I also make propellant for
pre-manufactured casings (Kosdon, Animal Motor Works) and 2 1/2 inch
dia. motors used
by the RRS . My motors use
graphite nozzles - the others made out of concrete, plastic and other
metals I found fail under high pressure, temperature and thrust.
Go here for reference
information on materiels and supplies.
WARNING
Rocketry is inherantly dangerious. I am not
responsible for your mistakes and errors. Anything you do to harm
yourself or others is your own fault. Don't blame me.
Details
The motor casing is 6061-T6 Aluminum tubing with a .125 inch wall. The
forward closure is milled from 6061-T6 Al. My motors are different from
Kosdon or Aerotech in that the forward and aft closures are not screwed
or snap-ringed in place. Screws are used to hold the closures to the
motor case. This gives increased strength. I do make some motor casings
with snap rings, but I prefer the more rugged casings held together
with screw fasteners.
I use PVC pipe turned down on a lathe for the propellant liner since it
is easy to machine and acts as an insulator for the metal tubing. For
38mm motors, I use 1 inch schedule 40 white PVC pipe. A couple of
passes on the lathe turns it down to size (about .037 inches less in
radius).
Since I use a Bates grain design, I have multiple propellant slugs. The
38mm propellant slugs are 2 inches long with a central core of .61
through .675 inches. I use different core diameters with different
solids loadings and to achive different pressures. More pressure gives
a greater overall thrust. I am experimenting with end core and D core
motors. I'll post results here after I get more data.
I've made some star motor cores, but for smaller motors the grain
doesn't hold up and tends to collapse in on itself. Stick with the
simpler cores.
The G motors have 2 propellant slugs. The H motors have 3-5 and the I
motors have 6- 8 slugs. (These are for 38mm motors). For L motors
typically I use 5 slugs. I tried longer-burning motors with 6
propelland slugs and a slower burn rate.
One difference between my motors and some others is that I use an
igniter on the top of the motor instead of through the nozzle in the
bottom. This keeps the nozzle un-obscructed and prevents blow-ups. I
learned this from the Reaction Research
Society's solid propellant course a dozen years ago. Typical
igniters
are derivitives of Davy Fire style igniters.
Supplies
Many web sites tell how hard it is to get propellant making materials.
I never had a problem. If you have basic machining tools like a metal
lathe, bench grinder, vise, band saw (or hack saw and a lot of
patience) and a drill press, you can form the casings yourself. If you
don't have a lathe, your best bet is to buy the casing parts and
propellant liners and concentrate on the propellant manufacturing.
Here is a listing of the Propellant Chemicals
needed to make composite motors.
Motor Hardware Suppliers
Online metals has
a good selection of aluminum tubing and aluminum stock. Their prices
are reasonable and they ship pretty quickly.
Metal Mart also has good prices
and ships quickly, but their web site sometimes makes it hard
to order.
McMaster-Carr has everything
you'll need for the rest of the motor. Snap rings, silicone O-rings,
graphite rod, etc. A little pricy, but they have it all.
Machine
Tools
When you make your own hardware, your limitations imposed by others
pre-manufactured cases dissapears. Machine shops do excellant work with
your raw stock, but for the price of 3-4 visits to a machine shop to
make your casings, you can buy your own metal lathe.
You don't need a large lathe to make 29mm, 38mm and 54mm motor
hardware. There is a class of smaller lathes called mini-lathes. Here is a picture of
one . They have enough accuracy and capacity for these smaller
motors. Most of these lathes are made in China and cost from $300 to
$800. All of these Chinese made lathes are basically the same.
The cheapest is sold by Homier .
I have one and recommend it. In fact, I found out that until recently
all Chines-made lathes were made in the same plant. They just used
different plans.
Others swear by those made by
Sherline . Also expect to spend another $100 to $300 for measuring
tools, tool bits and accessories. A good source for lathe and mill
accessories is
LittleMachineShop.com . They have good prices and ship the same or
next day. More information is available on mini-lathes at www.mini-lathe.com . Don't get
the smaller 7x10 lathes - they don't have the capacity. Get a 7x12 or
7x14. These have longer beds and let you turn longer pieces.
You will also want a drill press although you can drill precision holes
with your lathe. You will also need a bench grinder to make/regrind
your tool bits. You can get a good (Delta) model for about $40.
I have a milling machine. I (again) got mine from Homier. I've had mine
for about 7 years and just love it. between having a lathe and a mill,
there is nothing you can't make. You can even make lathes and mills!
Misc.
You will need a few more things before you start. One is an accurate
scale. You will be measuring propellants with an accuracy of 0.1
gram. You will be weighing things from 5 grams to over 500 grams or
more. Digital is the best. Some of the cheaper ones used to weigh
kitchen food work pretty well, and they are becomming cheap.
Also get a set of glass mixing bowls. These are used for
hand mixing. Also get some wooden, plastic and metal spoons.
You
can find these in the cooking section of any grocery store.
Hand mixing is nowhere as good as using a mixer. Get a Kitchen Aid
. They are heavy duty enough for your needs. You won't be happy with a
hand-held blender. Not enough power for those thick propellants. Also,
since the motor is usually not fully enclosed, you may get sparks near
your propellant mix. You may be killed because of this. I've used a
large pizza-style Kitchen-Aid blender and it was wonderful.
A good ABC fire extinguisher. Keep it charged and handy. Two
are better.
Keep a good log book. Record everything including
temperature/humidity when you make propellant. Record all firing data.
Keep video records
of your tests. Take lots of pictures.
Sandbags. When you test fire, pack your test stand with the
sandbags. I once had a motor explode and throw shrapnel 100 feet. I was
in a concrete blockhouse 50 feet away and 1 mile from the nearest
inhabated building. It was louder than a stick of dynamite. It
destroyed my test stand and parts of the motor were never recovered. I
also put a pretty good dent in the tower leg the motor was attached to.
You will also want a large and clean workbench. Make sure there
is no clutter. Good ventilation is a must. Some of the fumes can
destroy your lungs. Also, over time you may develop allergic reactions
to the chemicals.
To cure the propellant you will want a small oven. You can make
one with a heavy cardboard box, some insulation and a light bulb. Don't
use your home oven - the chemicals stay on the metal shell and can
poison you in time. Make sure there's no exposed wiring.
Making the Motors
I used to have a link to the Reaction Research Society's web-page that
delt with a propellant course. It looks like this is discontinued. Too
bad. However, many of the members are very
knowledgeable and have years of experience. Joining the RRS would be a
good investment.
To start with, put on nitrile gloves. many people find they have
allergies to Latex. I also found out that Latex tends to fall apart.
Nitrile seems the way to go. Keep them on during the entire process.
Keep these chemicals off of your skin. Next, put on your breathing mask
and goggles. Make sure your area is well ventilated. Have your fire
extinguisher handy.
Next, place some wax paper onto your cookie sheet or what ever your
propellant liners will be placed on. Take some coarse (60 grit)
sandpaper and rough up the inside of the liner beforehand. This makes
the propellant stick better than to the shiny surface of the PCV. Put
the propellant
liners on your sheet.
I thought long and hard on listing the ingrediants and chemical
percentages. I searched the 'net and found essentially what I used
here. Since I'm duplicating what others have on their web sites, I
don't feel so bad.
Since Sept 2001 I feel that it would be a bad idea to list specific
masses of the chemicals. You can e-mail me (see bottom of this page for
my e-mail address) and after some dialog I'll give you more
information. While I don't think any terrorist would try to make small
rocket motors, I don't want to help them. Besides, I'm sure they can
get completed motors easier than the chemicals needed to make them.
Before we start, I'll repeat what I said earlier:
WARNING
Rocketry is inherantly dangerious. I am not
responsible for your mistakes and errors. Anything you do to harm
yourself or others is your own fault. Don't blame me.
Start with the HTPB. Pour in the required amount into
your mixing bowl
. Next, add the HX-878. Only a small amount will be
needed. Use a small plastic spoon for small amounts or a large wooden
spoon for larger amounts. Mix for about 5 minutes (either hand mix or
machine mix). Next add the DOA or 2-Ethylhexyl
Acrylate . This will make the mixture a lot thinner. This is
needed later when you add the solids. Add between 2 and 5 drops of the Silicon
Oil . Use 1 drop for every 50 grams of HTPB up to a maximum of
5
drops. This will help to eliminate the bubbles created.
Now mix these for another 5 minutes then let sit for at least 1/2 hour.
The bubbles should dissapear in this time. If not, slowly mix
the mixture again for a few seconds and let sit another 1/2 hour.
The purpose is to eliminate all bubbles in the liquid. Bubbles cause
voids
in your propellant. Voids make the propellant burn faster. Faster burn
rates cause increased pressure which causes explosions. We're not
making
bombs here.
The best way to remove these bubbles is by vacume processing. By mixing
the whole thing under vacume, the gasses created and mixed in will
mostly be removed.
While the liquid is sitting, start mixing your solids. Measure out the
AP (ammonium perclorate) and Aluminum . I keep
the mixture in a large wax-coated drink cup. Don't use plastic! Static
charges will build up and cause a fire hazard. Use a wooden spoon or a
metal spoon for this and all further mixing.
After the bubbles leave the liquid mix, pour in the Isonate
. As soon as you do, the clock starts. You propellant will start to
harden and you have a limited time before you need to finish. About 1
hour.
Mix the liquid with the Isonate for a minute or so. Then slowly
pour in a little of the solids mixture while mixing the liquids. Keep
mixing and pouring in the solids letting the liquid completly mix with
the
solids. After all of the solids are added, mix for about 20 minutes.
Scrape
the sides of the bowl a couple of times during the process.
After the mixing, you should have a grey paste the consistancy of
modeling clay or cookie dough. You don't want to see any white AP - it
should all be mixed in.
Use a spoon and cut out a small ball of the propellant mix about the
diameter of the liner. Put this in one of the liners and push it down
with your fingers. Keep putting in propellant until you fill the liner.
Now, with the edge of your hand, hit the propellant packing it in and
removing all of the viods. This takes about one minute per liner.
Complete until all of your liners are full.
Place the liners in your oven and cure at appx. 120 degrees F for 8 to
16 hours.
Scrape the mixing bowls and spoons and collect all unused propellant in
one place. This includes your gloves. The only safe way to dispose of
this is to burn it off. Burn off the propellant in a small pit. Make
sure there are no flammibles near by. Have a water hose near for any
secondary fires. When lighting off the propellant, I wrap it in a sheet
of newspaper with the propellant in one end. I place the paper in the
pit and light the end without the propellant and stand back. Don't do
this when it's windy outside . Don't do this inside in your fireplace.
Don't let any kids near by and keep stupid people away.
Preparing the Motor Cores
After the propellant has cured, you will need to drill out the center.
I put a propellant slug in the lathe chuck and slowly (45-90
RPM) turn it while using a small diameter drill bit (1/4") to drill
through it. I then change the drill bit to the final size and re-drill
the slug. Immediately remove the propellant shavings and place them in
a bowl for later burnoff.
You may ask how big a center hole do I need? For a Bates grain (what we
are doing here) we need to figure a few things about Bates grains first.
For a neutral burning core (thrust is constant from start to finish) we
start with the propellant diameter. Since we are using 1" sched 40 PVC
pipe, the interior diameter I measured is 1.03". So the propellant
radius is .515". Next, we use a K value of .6 (Bates neutral
burning). We can now figure out the port radius of propellant radius
times K value. So, .515 * .6 = .309. Multiply by 2 to get the port
diameter. .309 * 2 = .618. This is the port diameter that needs to be
drilled out. Now we need to figure out the length of the propellant
slug. This is the propellant radius
(.515) times 3 + port radius, or .515 * 3 + .309 = 1.854. We can safely
round up to 2". This seems like a lot of propellant to remove. It is.
However, if the central core (hollowed out section) is too small, the
motor develops too much pressure and fails.
By the way, a Bates grain is not neutral. It is within 5% neutral, but
we can figure out mathematically that it's not. It is also the easiest
core to make. It can be done after the propelland has cures. Otherwise
a mandrel is needed. Difficult to remove. Probably not worth the time
for smaller motors.
After it is drilled, use a sharp knife cut the excess off the ends so
that they are square. Look for any voids or bubbles in the propellent.
If there are any, destroy the propellant slug. It may cause an
explosion. (Bubbles and voids increase the burn-rate which increases
the pressure. An un-wanted increase in pressure can destroy the motor).
Do the above with the rest of the slugs. Burn off the shavings.
A Few
Notes
I made some small G size motors and tested them in a test stand in the
back yard of my old house. The noise was pretty loud as the neighbors
came running outside. They were inside their house 300 feet away. I had
my test stand in the middle of a 3 acre parcel.
Once I had a motor nozzle come out of the motor casing. The motor was
firing upwards, and the nozzle shot up about 30 feet. It was somewhere
between yellow and white hot when it popped out, but cooled pretty
quickly. Watch out for flammible material when testing motors.
Epoxy cannot be used to hold anything together. The pressure will cause
it to fail.
White PVC that has been exposed to UV (in the sun, for example) will
fail. The UV causes the molecular bonds to weaken.
When testing motors, I use a cinder-block bunk-house. I make sure
nothing is above the test stand and that no one is near the area
unprotected.
I'll be adding some pictures as time permits.
Contact me at
[ktheis] at [landfall] [dot] [net]
for questions, complaints, or more information.
TRA member 4323
RRS member since 1996
Kurt's Main Page
Last update 12/29/2011