Hybrid Rocket Motor Research Project

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					Hybrid Rocket Motor Research Project
Team Members:
        David Hohn – 2nd year ME
        Daniel Grieb – 2nd year ME

Executive Summary:
This report analyzes the developments and progresses of the hybrid rocket motor research
project during the spring quarter 2007, as an extension of our research from winter
quarter 2007. The report discusses in detail the background of the project, and the
direction that our group is striving for. It will describe the theory and experimentation
behind our work. Essentially, this report will provide a summary of our work over the last
two quarters.

Our work with the hybrid rocket motor research project involves an in depth analysis of
the regression rates of hybrid rockets. Hybrid rockets rely on a combination of a solid
fuel and a liquid/gas oxidizer. The regression rate is the rate at which the solid fuel is
consumed inside the combustion chamber of the rocket. Also, we are working to develop
a new, more efficient rocket nozzle to add to our hybrid rocket. Conventional rockets use
a simple converging/diverging nozzle. These nozzles are cheap to produce; however, they
are slightly inefficient because not
all the thrust is directed straight
down. Our nozzle is an aero-spike.
The principle behind this nozzle is
that it directs the thrust down to
the spike. Then the spike re-directs
the flow straight down in a
uniform manner. This provides the
rocket with the most thrust.
                                         Some of the different aero-spike nozzles we are working
Hybrid rockets began to interest experts at NASA in the mid-1960s. These rockets have
many advantages over both solid and liquid rockets. Hybrid rockets are safe to produce,
cheap to manufacture, and they are inherently different from their predecessors. This
characteristic means that there is a broad spectrum of research to be done on the subject.

Safety is always a concern when working with rockets. It only takes a slight error to turn
a rocket into a bomb. Essentially, rockets are driven by a controlled explosion. Therefore,
the dangers are extreme. Solid rockets are fueled by extremely explosive materials.
Intense caution is required when working with these fuels. We are experimenting with
two different solid fuels: Plexiglas and HTPB. HTPB is a high grade rubber, similar to
the material in car tires. Neither material is dangerous. They are easy to store,
manufacture, and handle. No special care is required with these materials. This alone, is a
major advantage with performing laboratory tests on rockets.

Money is a constraint when working with expensive materials and equipment. The fuel in
solid and liquid rockets tends to be extremely expensive and hard to come by. Few
companies deal with such volatile materials. The materials also need to be stored with
caution. Proper temperature and pressure are crucial to prevent the materials from
exploding. Just building the proper facilities to handle the needs of these fuels would cost
much more money than the funding Cal Poly has received for this project. However, our
fuels are cheap and very easy to purchase. HTPB is around $10 a pound and Plexiglas is
around $40. This is considerably cheaper than the solid and liquid alternatives.
Machining these fuels is also very easy. The cheap cost of materials makes this project

The uniqueness of hybrid rockets provides us with many different angles to study. Our
research focuses on the regression rate of hybrid rockets, while several graduate students
are working on the design and testing of the aero-spike design. We originally planned to
determine the fuel consumption rate by varying chamber pressures and flow rates.
However, we found that it is difficult to vary the oxidizer flow rate without directly
changing the regression rate.

We have found research published at Pennsylvania State University that deals with
controlling the regression rate by adding various additives to the solid fuel grain. By
adding metal particles, such as aluminum flakes, to the solid fuel, the regression rate can
be increased due to increased combustion rate as the metal assists in heat transfer into the
solid fuel. Researchers found that up to 13 weight percent aluminum flakes can be added
to the increase the regression rate without affecting the stability of the burn.

We are working on changing the design of our
rocket set-up to better suit the needs of our
research. Currently, the rocket design involves
a single grain of fuel. We are working towards
having an exterior grain with an interior core
grain. Hopefully, this will increase the
regression rate and provide more thrust. Also,
we have just changed the number of port
nozzles from which the nitrous oxide is
introduced. We increased the number of ports
from 4 to 12 in order to provide a more even
regression rate. Finally, we are working to
upgrade our current converging/diverging
rocket nozzle with a more advanced aero-spike
nozzle. Right now, we are developing an              Left: Current set up with conical nozzle and
                                                     single grain.
intermediate motor design, with a dual grain         Right: Desired set up with aero-spike nozzle
                                                     and dual grain
fuel configuration, but still maintaining the
converging/diverging nozzle design. This design will be used as the aero-spike nozzle is
designed to replace the converging/diverging nozzle.
During winter quarter we performed extensive research regarding hybrid rockets and
regression rates. We tried to develop an equation to predict the regression rates for our
data based on the oxidizer flow rate. We conducted hot firings and collected data from
them. One of our goals is to achieve a constant chamber pressure of 300 psi in order to
calculate the regression rate. One problem we consistently encounter is erosion in the
nozzle of the rocket. Due to the high
                                                                            Pressure & Temperature vs. Time
temperatures and pressures, the nozzle                        300                                                       1400
deteriorates throughout the course of                         250                                                       1200

the burn. This causes the chamber                             200
                                            Pressure [psia]

pressure and thrust to fall off as the                        150
rocket burns. Because the regression                          100
rate is directly tied to the chamber
                                                               50                                                       200
pressure, it also varies with time. This
                                                                0                                                       0
makes it very hard to calculate because                             0   2      4       6          8      10   12   14

                                                                                      Time [sec]
it is constantly changing. We are
                                                                Pressure vs. Time plot for hot firing with Plexiglas
looking into ways to prevent this
nozzle throat erosion. We plan to try a higher quality carbon graphite to make out nozzle.
Eventually we will attempt to coat the problematic area with a thin layer of carbon silicon
carbide in hopes that this will prevent the throat erosion and allow us to more accurately
calculate the regression rate.

A similar problem occurs with the aero-spike nozzle. Because all the flow is focused on
the spike, it has to endure extreme temperatures and pressures. We do not have a material
right now that is capable of dealing with this turbulent environment. We are currently
talking with a company to manufacture the spike.

We are planning a series of experiments to study the effect that aluminum flakes will
have on regression rate is we add them to HTPB. We will focus our study on the effect
that metallic additives have on the regression rate of the dual grain design, which has not
been previously studied at other universities. We are currently finding a supplier for
aluminum flakes with a thickness between 100 and 150 nanometers. After the dual fuel
grain testing setup is complete we plan to test the regression rate of HTPB with various
concentrations of aluminum and try to develop an equation for the regression as a
function of aluminum concentration. This research will be performed over the summer or
in the fall of 2007.

We have continued to make advances on hybrid rocket research this quarter. We have
learned a lot more about the principles behind hybrid rockets. We developed an
understanding about the advantages of hybrids and possible implications. We continuing
to improve the design of our testing set-up. Also, we have made significant progress
towards attaining the materials to produce more accurate data. We have found a
technique for changing the regression rate with aluminum additives in order to perform
an effective regression rate study. We are closing in on performing a hot firing with the
new aero-spike nozzle and conduction regression rate studies based on constant chamber
pressures. As our capabilities continue to increase, we will soon be able to expand our
research considerably and “the guys at Lockheed will love us”. (Dr. Thomas Carpenter)

        Bowers, Albion, Optimal Thrust Vectoring for an Annular Aerospike Nozzle,
        STTR Phase II-Progress Report #4, January 2007

        Platzek, H., Preliminary Solid Rocket Motor Design Techniques, Naval Weapons
        Center, China Lake, CA, December 1975.

        Risha, G.A., Metals, Energetic Additives, and Special Binders Used in Solid
        Fuels for Hybrid Rockets, Pennsylvania State University, Altoona, PA, 2007.

        Zilliac, Greg, Hybrid Rocket Fuel Regression Rate Data and Modeling, NASA
        Ames Research Center, Moffet Field, Mountain View, CA July 2006

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