High Power Rocketry ||
Designing Rocket Motors
What is your reason for wanting to build your own rocket motors? Is it
to save money because model rocket motors or high power motors are SO
EXPENSIVE? If that is your reason, you are wasting your time. If
it is because you like the challenge of building something yourself, and you
don't mind spending some money doing it, you might be on the right track.
You don't have to spend thousands of dollars to get started building rocket
motors but you might end up spending hundreds down the road because you want
to either make more efficient motors or more powerful motors. One
thing leads to another and you will soon be unsatisfied with what you are
doing and will want to do something more. Many people eventually (if
not sooner) decide they want a lathe so they can build their own reloadable
casings. Then they wish they had a mill for making tools and test
equipment and so on. Others have been content to build PVC motors with
sucrose or sorbitol and potassium nitrate propellants. Note: sucrose
propellant will crack from the pressure and stresses of accelerated rocket
flight in larger motors. Sorbitol propellant is not as susceptible to
cracking. Also, sucrose propellant motors are not allowed at Tripoli
Do you just want to build your own rocket motors and it isn't that important
that they are the absolute most efficient? Then all you need is PVC
motors and sucrose-KN propellant. Do you want the most efficient and
most powerful motor to try for records or for contests? Then you want
a metal casing and composite propellant. You can either build your own
metal casings or you can buy your casings and nozzles and just make your own
fuel loads. For now, I am going to focus on the simplest and easiest
-- single use PVC motor casings with concrete nozzles and caramel candy
For those in a hurry to try making some motors or who don't want to do the
design but still want to experiment, I have listed all the specifications
you need to know to build a variety of motors with one to six BATES grains
and from 1/2" PVC through 1-1/2". To date, only the 1/2" and 3/4" have
been tested. The larger motors may have a tendency to crack and CATO,
so extra caution should be used while experimenting. The tables are the
product of specific inputs to FPRE software. Click
here for the tables.
If you are going to design everything from scratch, then you need to know a
little engineering. You don't need to be an engineer, but you need to
be mechanically minded and good at math. You can calculate everything
by hand but since this is the age of computers, most everyone uses computer
programs for the more difficult of the calculations.
You can approach design from three directions.
Design the rocket first, its dimensions, weight and the altitude and/or
velocity you want it to achieve. From that you can calculate the total
impulse needed for your rocket and from the dimensions of the rocket, figure
out the maximum dimensions your motor can have. Finally, you have to
figure out what propellant, will give you that impulse within those
dimensions, and last, design the motor. This is a pretty complicated
Next option is to do the design in the opposite order. Decide on a
total impulse and a motor type, design the motor, then design a rocket for
The easiest and the more normal route for beginners is to just decide what
kind of motor you are going to build such as a PVC sugar propellant motor or
reloads for a commercial reloadable motor, build some and see what you get.
Actually, you can calculate everything about the motor and then see how
close your motor performs to your calculations. This is the approach I
The first two design approaches are actually re-iterative. You will go
through the calculations more than once, each time refining the design until
you get what you want. In the first approach you may find you can't
achieve what you want with the dimensions of the rocket and then you have to
go back and try again. On the second approach you may find you can't
build a rocket to attain the desired results with the motor you have
designed. With either method, you need to have a ball park idea of
what you can achieve. Obviously, you can't put a rocket into orbit
with an ounce of propellant.
What kind of propellant are you going to make?
What kind of casing will you use?
What will you use for the nozzle?
Will your motor have a time delay and ejection charge?
What type of grains will you use -- Bates, C-slot, Moon-burner, etc.?
What, if any, surfaces will be inhibited?
What is the maximum designed chamber pressure?
What is the grain configuration and dimensions -- OD, ID, length?
How many grains will you use?
What is the area required for the nozzle throat?
Will the propellant be case-bonded?
Will you use a liner?
How will you seal the nozzle and header from the hot gases?
Let's design a motor. Refer to the "What you need to know" and we'll
answer those questions.
Propellant. We are going to design a motor using sugar
and potassium nitrate. These are the easiest ingredients to
get normally and have been used for scores of years.
Casing. For the casing we will use standard schedule 40
Nozzle Material. The nozzle and header will use Rockite
anchor cement. It is very easy to work with, sets quick but
not too quick (20 minutes), gives a nice surface finish on the nozzle,
expands when it cures to make a better seal and is readily available.
All cements and clays erode and so we will also use one or more steel
washers so the throat diameter will remain fairly constant.
Time Delay & Ejection Charge. None. Since
we are just getting started and want to just see if we can make a motor that
works, we are not going to design in a time delay and ejection charge.
We can do that later. For high power rockets, people usually end up
getting altimeters or electronic timers anyway which both are much more
reliable and precise than time delays built into the motor.
Grain Type. The standard type of grain used is a Bates
grain and this is what we will use. (the name BATES is made
from the BA of "ballistic" and the TES from "test" in BAlistic evaluation
TESt motor). A bates grain is a cylindrical grain with a round core
running through its length. The outside is inhibited so that it will
not burn. Burning takes place on both ends and in the core. The
grain dimensions are designed such that the total thrust is relatively
constant. Actually, the beginning and ending thrust are about equal
but is a curve that increases a little and then decreases in an arc between
the two end points. The idea is to have the burn area as constant as
possible since the area of the grain that is burning is directly related to
the motor case pressure and so the thrust.
Inhibited Surfaces. Since a Bates grain by definition means
that only the OD is inhibited, that answers
Chamber Pressure. The higher the pressure, the more efficient
the motor will be (the higher the specific impulse will be). So we
want the pressure as high as the motor will stand without rupturing or
blowing out the nozzle or header. Looking at the pressures for 1" PVC
pipe in the Misc.
Tables, Schedule 40 PVC pipe, we
find a maximum working pressure of 450 psi, a required minimum burst
pressure of 1440 psi and a calculated burst pressure of 2220 psi. The
working pressure is the industry standard maximum pressure the pipe is
supposed to be subjected to at normal temperature. This is supposed to
give a healthy factor of safety so that if the pressure goes some over that
pressure, the pipe won't burst. The minimum required burst pressure is
the industry standard minimum value of pressure at which the pipe will
burst. Actually, this pressure is also lower than what the pipe will
normally burst at. The calculated burst pressure is the pressure that
has been calculated at which the pipe will actually burst under ideal
conditions and this pressure is likely to be too high. Also, at
elevated temperature, the strength goes down rapidly. Fortunately,
these motors burn less than one second and so there isn't time for the heat
to weaken the pipe very much. The larger the motor, the more affect
the heat will have and at a certain point, the pvc pipe must be insulated
with rubber to prevent the heat from weakening the pipe wall. At 1",
the pipe does not need insulation. Also, the fact that the grain is
inhibited and so won't burn on the outside helps to prevent the pipe from
getting too hot, although the hot gases can still seep between the grain and
motor wall. In the case of PVC motors and quick setting concrete
nozzles, the nozzles will actually fail before the pipe will burst and the
nozzle will be ejected. At higher pressures, the exhaust gasses can
also start leaking around the nozzle and header and the motor wall. A
good pressure to run in 1" PVC pipe is going to be around 350 to 400 psi and
it will still depend on how the nozzle and header are retained and how well
it is sealed against the pipe wall.
Grain Configuration. This is the point at which we actually
have to make some decisions and do a little calculation. As a first
motor, we are going to start with grains for a 1" PVC pipe motor. You
can make motors from 1/2" or 3/4" PVC pipe but the smaller they get, the
harder it is to load the propellant into them. A two grain 1" PVC pipe
motor will make a motor in the high F range which may be higher than what
you want for starters but is actually easier to make. A Bates grain
has the OD inhibited so we are going to make a glued paper inhibitor sleeve
and cast the propellant into that. For 1" PVC, you can use regular
typing paper but card stock is better and easier because it takes less
wraps. The larger the grains, the longer the burn time and the thicker
a paper inhibitor sleeve must be. 90# card stock is .005" thick.
Regular typing paper or printer paper is about .002 thick. The
inhibitor sleeve should be around 3%-5% the thickness of the web of the
grain (distance from the inside surface to the outside surface of the grain.
From Misc. Tables,
Schedule 40 PVC pipe we find the
ID of a 1" PVC pipe is 1.033". For a .25 It also depends
on the pressure. It is not worth doing all the calculating yourself.
You need a program that will calculate the pressure and thrust given a
certain geometry of grain. I use FPRED by CP Technologies (John Wickman's
Company) which you can get for $19.95 at
http://www.space-rockets.com/fpred.html. A free online
calculator is available from
I find the thrust and impulse results from his to be higher than what I
actually achieve with sucrose potassium nitrate motors. He doesn't
have a place to input efficiency and the efficiency he uses in his equations
seem to be high. See the links page for
more software sources. I have compiled
some information taken from this software that you can use without the
program or to get some starting values for the program (see
Motor Design Tables). Using the
program is an iterative process. You plug in values, click on
calculate and see what the pressure is for the parameters you put in.
If it is not what you want, you adjust the parameters and try again.
Normally, you will choose the number of grains, the OD of the grain, the
length of the grain and the core diameter. Then you will try a nozzle
throat diameter and calculate. You then adjust the nozzle diameter up
or down to change the pressure. If the throat diameter becomes larger
than the core diameter, you have to increase the core diameter. When
you have the right pressure, you plug in the optimal exit cone diameter that
the program calculates for you back into the exit cone diameter in the
Nozzle Properties section and do one last calculate. If you are doing
Bates grains, you ignore the C-Slot width & depth and Moon burner offset.
You also click on "No Ends" under "End Restrictions". Once you have
the OD and Bore Diameter, you can click on "Neutral Length" and it will
calculate the proper length of the core for you and put that number in the
"Bore Length" cell. I usually adjust this to an even 1/8" just to make
it easier to make and measure. The neutral length for a bates grain
(OD inhibited, full length core, no ends inhibited) is easily calculated.
It is 1.5D + 0.5d where "D" is the outside
diameter and "d" is the inside diameter. So for a 1" OD, .25 ID you
calculate 1.5 X 1 + .5 X .25 = 1.5 + .125 = 1.625" long.
To use this software, you need the following information:
|Gamma (or "k") ratio of specific heats
|Burn Rate Coeff. (in/sec)
|Burn Rate Exponent
|Comb Eff. (%)
||Throat Dia (In.)
||Experiment to get the right
|Exit cone Dia. (In.)
||Pick a number about 1.5 times
larger than your throat diameter and then when you have the pressure you
want, move the optimal calculated exit cone diameter to this field and
do one last calculation.
|Exit Cone Angle (Deg)
|Ambient Pressure (psia)
|Radial Throat Erosion (Mils/Sec)
||0 if you use washers, will vary
if you don't. Try 15 and then measure from experiments
|Exit Cone Eff. (%)
|| 90 is a good guess
||Outer Dia. (In.)
||The ID of the PVC pipe you are
using minus twice the inhibitor sleeve thickness. 90% of the ID
should be close.
|Bore Diameter (In.)
||A little larger than your
nozzle throat diameter -- pick a size that you can get material for,
usually 1/16" increments up to 3/8" and then 1/8" increments.
|Bore Length (In.)
||Calculate using formula or
"Neutral Length" button in software.
|Number of Cartridges
||1 to usually a max of 6 -- your
||Click on if your core
is close to your throat area. The rule is you have erosive
burning if your core cross sectional area is less than twice your throat
cross sectional area.
It doesn't actually change the calculated output much.
|Ignition Time (Sec)
||Use 0. If you put a value in,
the software will spread out the total burn time and will calculate a
lower pressure and thrust. You want to know the highest pressure
possible to make sure that is low enough to not burst your case.
Number of grains. Start testing with just one or two. As
you increase the number of grains, your nozzle throat diameter will increase
and you will also have to increase the core diameter. Alternatively,
if you plan on testing a maximum number of grains, say, six, then figure out
the minimum core size that will be larger than the throat with that number
of grains and then make all your grains with that core diameter. It is
surprising that you won't loose that much total impulse over the minimum
core diameter for just one grain.
Nozzle throat diameter. Get an idea of the throat diameter for
what you are designing by looking at the design tables page and then modify
that until the software calculates a pressure that is appropriate.
Case Bonding. Case bonding is a different style of grain and is
not a bates grain except if the overall length is the neutral length for one
Bates grain. Case bonding is when you don't use an inhibitor sleeve or
a liner. You cast your propellant directly into your motor case so
that the propellant sticks to the inside surface of your case. If the
grain is longer than the neutral length, your motor will have a progressive
burn which doesn't necessarily mean your rocket won't go as high. This
is not an uncommon practice and is much simpler because you don't have to
deal with liners. However, for a longer motor, it is more difficult to
get the propellant evenly distributed without voids, especially with smaller
Nozzle and Header sealing. Part of the experimenting is to see
if you can make a motor that doesn't leak exhaust gasses around the header
or nozzle. The higher the pressure, the more likely this will happen.
Just a little more pressure and you will usually eject the nozzle or header
or both. The most common method of sealing a PVC motor with concrete
nozzles and headers is to use RTV which is the high temperature red silicone
like sealant you can get at any auto parts store for use with gaskets.
This is a little tricky to apply (and messy). You have to get a bead
around the junction of the concrete with the PVC case on the inside.
This is done with a long stick. You have to be careful not to get it
up on the wall or you won't be able to get your grains all the way in.
After you put your grains in, you can put a piece of foil on top of the last
grain and then put a ring of seal around that and let it dry before you pour
in your header concrete.
High Power Rocketry ||