Rocket Engine Physics

and Design

By: Bryan Patricca

Basic Physics • 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.

Rocket Engines• 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. The combustion 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 rocket upward.

Components• 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.

Propellant • 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 combinations are:

• 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 missions

Specific Impulse • 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

• 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

Thrust Vectoring• 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 chamber gimbled.

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 Technology• 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 sailing.

New Insight• 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.