Amateur and Experimental Rocketry
of 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 fail under high pressure, temperature and thrust.
Some pictures of a 38mm G motor assembly
are here . Here is a close-up
of the forward bulkhead, snaprings, and O-rings. The nozzle shown here
was a test nozzle - I did this by hand before I got a lathe. It has seen
many firings.
Go here for reference information
.
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. Click here to see a drawing
of a G motor.
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).
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 few 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 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. 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.
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.
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-help 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.
A good ABC fire extinguisher. Keep it charged and handy.
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.
You will also want a large and clean workbench. Make sure
there is no clutter.
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
Before you start making your motors, please click
here . The Reaction Research Society has a propellant course that
will teach you everything you will need to know. Highly recommended.
To start with, put on latex or nitrile gloves. 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. However - 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.
(Note - I removed the propellant listing from this site. If you want to know
what to use, take the RRS course. I also removed the spreadsheet. Sorry.)
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".
By the way, a Bates grain is not neutral. It is within 5% neutral,
but we can figure out mathematically that it's not. Don't argue with me
- I'll show you the math.
After it is drilled, use a sharp knife cut the excess off the ends
so that they are square. Look for any voides or bubbles in the propellent.
If there are any, destroy the propellant slug. It may cause an explosion.
Do the above with the rest of the slugs. Burn off the shavings.
You can contact me at kurt@landfall.net
for more details.
Contact me at kurt@landfall.net
for questions, complaints, or more information.
TRA member 4323
RRS member since 1996
Kurt's Main Page
Last update 5/24/05