Hybrid Rocket Definition & Current Design Materials
Hybrid rocket engines are classified as those that
utilize a liquid oxidizer and solid propellant to achieve
thrust. This team is currently using Hydroxyl Terminated
Poly-Butadiene, or otherwise known as HTPB, as the solid
fuel. This material is essentially the same form of rubber
that is in everyday car tires, minus a couple of elements.
The liquid oxidizer in this system is Nitrous Oxide. These
two components together in a hybrid rocket system are not
necassarily the most efficient. However, it is considered
to be one of the safest combinations and considering that
this is a school project, safety is of the utmost concern
for all of the parties involved.
Overview of Components of the Current System
To describe the system, it is best to start from the
beginning, the feed system, to the end, the snap ring that
holds everything in place. Because the goals of this
project are dependent upon testing a hybrid rocket engine,
safety is a primary concern. As you will see, every
component keeps this in mind.
The feed system is comprised of two tanks: a Nitrous
Oxide tank that deliivers the liquid oxidizer to the rocket
chamber during a fire and a Nitrogen tank that is utilized
to purge the entire system after a fire. Nitrogen is an
inert gas so it will extinguish any flames that are left
over after the NOX is turned off. Seperate solenoid valves
are utilized to open and close the appropriate feed lines.
All that these need are the necassary amount of voltage to
open so this means they can be operated from a safe
distance away. Other components include filters, reducer
couplings, pressure relief valves, and a cylinder warmer.
The cylinder warmer is used to heat up the Nitrous Oxide
Tank, therefor increasing its available pressure.
Feed System Setup
Above is an example of how this set up looks during a
round of testing. You will note that the tanks are on the
outside of a large concrete "bunker" that serves as the
housing for the rocket chamber. It serves as a means of
protection to the environment and surrounding people in
case an emergency occurs.
The chamber includes all of the major components of
the hybrid rocket engine. Reinforcing the safety concern,
it is a half-inch thick peice of 304 Stainless Steel tube,
hence the name "Steel" Rocket Team. The final set-up, prior
to testing is pictured On this
. A graphic is pictured below. Overall, the chambe
encompasses an injector plate, pre-combustion chamber,
igniter ring, fuel grain, nozzle, and the snap ring that
secures all components.
Hybrid Rocket Chamber
The injector plate serves as the interface between the
feed system and the engine itself. It consists of small
holes that particulate the liquid nitrous oxide. This
increases the overall surface area of the oxidizer,
allowing it to combust more efficiently. A lot of
properties about the injector plate can affect this
process; including but not limited to the total inlet area,
the number of holes, the diameter of the individual holes,
the orientation of the holes, etc...
The pre-combustion chamber gives the liquid nitrous
oxide time to separate before it reaches the solid fuel.
Too short and it will not have separated enough. Too long
and the NOX will lose momentum, decreasing mass flow and
thus efficiency of the engine. The current material for
this chamber is a ceramic known as Garalite.
The igniter ring is what initiates a rocket burn.
Nitrous oxide will not ignite until approximately 575
degrees Celsius. The job of the igniter is to get
temperatures above and beyond this point in a vaccuum
atmosphere. It consists of several components. The main
portion of the igniter is a mixture of HTPB and Amonium
Perchlorate powder. What initiates the flame is a
wire/black powder/AP set-up. The wire heats up enough to
ignite the black powder when current is supplied to it.
From here, the AP and HTPB begin to burn. After a few
seconds, the Nitrous Oxide is supplied to the system,
initiating the rocket burn.
The HTPB fuel grain is what makes up the majority of
the rocket. The goal for an HTPB/NOX Hybrid Rocket is to
have an Oxidizer to Fuel (O/F) Ratio of between 6/1 and
8/1. Studies have shown that this range is where this form
of engine is the most efficient.
The post-combustion chamber allows for the combustion
process between the HTPB and NOX to complete. It is very
similar to the pre-combustion chamber in that it can't be
too short, and it can't be too long. Too short and not
enough time will be given for combustion to take place. Too
long and the flow could potentially lose momentum. Because
garalite does not survive the high temperatures seen at
this end of the rocket, graphite is being used to provide
The Rocket Nozzle is regarded as the most difficult
portion of the design. It is also the most important,
having up to a 30% affect on the thrust capabilities of the
engine. Prior to optimization of the nozzle, certain
temperatures and pressures within the system are needed.
Here is a sample of a
and an example of the Nozzle
spreadsheet being used for design.