Rocket Engine Physics and Design
Rocket Engine Physics and Design
Rocket Engine Physics
By: Bryan Patricca
• Rocket engine propulsion is based on two laws of physics.
• Newtons third law: For every reaction there is an equal and
opposite reaction. This law describes the rockets forward
propulsion. Since there is nothing to push against in space
this law provides an example for how rocket engines work.
• The law of conservation of momentum: momentum equals
mass times velocity. A rocket on a launch pad has zero
velocity and total momentum.
• Liquid rocket engines work by pressurizing fuel through a series
of turbo pumps so the fuel moves at a high speed. (On the
space shuttle pressures reach 420psi.) Fuel then enters a series
of feed lines that branch out to each engine. The oxidizer and
fuel pre burners are welded to the hot gas manifold. Fuel and
oxidizer are then mixed so that combustion can occur.
chamber receives fuel
rich hot gas from the hot
gas manifold. The dual
redundant igniters are
used to ignite the fuel
and force it through a
nozzle. This provides the
thrust to propel the
• Fuel pumps – Rockets contain many high pressure and low
pressure pumps to move fuel to the combustion chamber and the
nozzle. The pumps on the space shuttle are so strong that they
move 120 gallons per/sec.
• Combustion chamber- The combustion chambers are typically
shaped in cylinder form and reach very high temperatures. The
fuels mix in the combustion chamber and burn.
• Nozzles – The fuel leaves the combustion chamber through an
opening( throat of the nozzle). If there is enough pressure in the
nozzle, it chokes and a jet is formed. The thermal energy is then
transferred into kinetic energy.
• There are two different components that
make up a liquid rocket engine propellant.
They are usually a chemical liquid and an
oxidizer. Some of the more commonly used
• liquid oxygen and liquid hydrogen - used in
the space shuttle
• liquid hydrogen and kerosene - commonly
used in Russian and Chinese rockets
• nitrogen tetroxide and hydrazine - used in
military operations, and for deep space
• Usually abbreviated isp- Specific impulse describes the
efficiency of a rocket engine. It represents the change in
momentum of the propellent used. The higher the specific
impulse the less propellent needed and the more efficient the
engine becomes. Specific impulse is the same as miles per
gallon for your car. The space shuttle main engines have a
specific impulse of 453s.
• Equation is given by F thrust = Isp * dm/dt *g
• Isp= specific impulse
• dm/dt= mass flow rate
• g= acceleration at surface of the earth
• Thrust is the reaction force
described in Newton’s third law.
When a system accelerates in one
direction this will cause the same
amount of force to be expelled in
the opposite direction. For a
rocket to be launched off the
ground the initial thrust must be
more then the weight. For a
simple system thrust can be
determined by T= dm/dt * v.
• T is thrust,
• dm/dt is the rate of change of the
mass with respect to time.
• V is velocity
• Most engines need a way to
steer the rocket on its
designated flight path.
Engineers achieve this by
this by mounting the engine
on a gimbal which gives the
engine a specified range of
motion that it can pivot on.
Some engines have the
nozzle and the combustion
In Space Applications
• When a rocket exits the earth’s
atmosphere most manned vehicles do
not fire their engines in space. This is
explained by Newton’s first law: Every
body remains in a state of rest or
uniform motion unless it is acted on by
an external unbalanced force.
• To make little adjustments and to
position for reentry, rockets have
smaller engines called RCS thrusters(
reaction control systems ). These
thrusters allow astronauts to change
altitude in any desired direction. They
also can control rotation.
• New propulsion technology is being developed to take
us farther and faster then ever before. This technology
will be more efficient and reliable then the liquid and
solid rockets we use today. Some of the new
technologies being developed are nuclear propulsion,
ion propulsion, electromagnetic propulsion, and solar
• I have always loved learning about spaceflight and flight in
general. This Project really helped me understand how rockets
work when applying Newton’s Laws. When doing my research
I learned interesting facts that I would have not otherwise
found. One interesting fact that I found that I don’t think is true,
is that the pumps in the space shuttle can launch liquid 36
miles in the air. If you do the math it seems pretty impossible to
me. Learning about the new technologies was intriguing to me
and I cant wait till we design and build the new spacecraft that
visits mars and beyond.