MAE 4316: Propulsion

An Introduction to RocketPropulsion Systems

Lesson Topics

Some math– Thrust, Impulse, Isp

Types of launch vehicle propulsion systems

Design considerationsLaunch TrajectoriesThe Big Picture

Launch Trajectory

Rocket Propulsion

MATH Take notes…

Liquid Rocket Engines

A liquid rocket engine is a system that uses completely liquid propellants to produce thrust

Advantages– High Isp– Throttle-able– Stop and restart– Reusable– High density Isp (compared to gas storage)

Disadvantages– Low Thrust (compared to solids)– Complex– Cryogenic storage of propellants– Safety

Liquid Rocket Engines

Liquid rocket engines are used for– Launch vehicles

» Bipropellant systems• Fuel• Oxidizer

» Generates enough thrust» Relatively high storage

density» High Isp allows reasonable

propellant mass for ΔV required

Liquid Rocket Components

Solid Rocket Motors

A solid rocket motor is a system that uses solid propellants to produce thrust

Advantages– High thrust– Simple– Storability– High density Isp

Disadvantages– Low Isp (compared to liquids)– Complex throttling– Difficult to stop and restart– Safety

Solid Rocket Motors

Solid rocket motors are used for– Launch vehicles

» High thrust (high F/W ratio)» High storage density

– Ballistic Missiles» Propellant storability» Excellent aging» Quick response

• storability• high F/W ratio)

Solid Rocket Motor Components

Hybrid Propulsion Systems

A hybrid propulsion system is one in which one propellant is stored in liquid (or gaseous) state while the other is stored in solid phase.– Solid Propellant / Liquid (or

gas) Oxidizer» Most Common

– Solid Oxidizer / Liquid Propellant

» Less Common

Hybrid Advantages

SAFETY: Literally no possibility of explosion

Controllable– Throttle– Stop / Re-start

Safe exhaust products Higher Isp than solids Higher density Isp than liquids Lower complexity than liquids Lower inert mass fraction than

liquids

Hybrid Disadvantages

More complex than solidsLower Isp than liquidsLower density Isp than solidsLower combustion efficiency than

either liquids or solidsO/F variabilityPoor propellant utilizationHigher inert mass fraction than

solids

Hybrid Schematic

Nuclear Propulsion

Nuclear Thermal Propulsion (NTP)– System that utilizes a nuclear fission reactor– Energy released from controlled fission of material is

transferred to a propellant gas– Fission

» Absorption of neutrons in a fuel material» Excitation of nucleus causes fuel atoms to split

• Two new nulcei on average (Fission Fragments)– High KE from release of nuclear binding energy– Usually radioactive

• 1 to 3 free neutrons– Necessary to keep reaction going– Critical if each fission events leads to another– Can be absorbed by reactor material or leak from reactor

Nuclear Propulsion

ADVANTAGES– High Isp (2-10x that of chemical

systems)– Low Specific Mass (kg/kW)– High Power Allows High Thrust– High F/W– Use of Any Propellant– Safety– Reduced Radiation for Some

Missions

A Nuclear/Chemical Comparison

One gram of U-235 can release enough energy during fission to raise the temperature of 66 million gallons of water from 25oC to 100oC.

By contrast, to accomplish the same sort of feat by burning pure octane, it would require 1.65 million gallons of the fuel

Nuclear Propulsion

DISADVANTAGES:– Political Issues– Social Issues– Low Technology Readiness Level

(Maturity)– Radiation issues (Shielding)– High Inert Mass

Nuclear Propulsion Schematic

Propellant Tank: Similar to tanks discussed for liquid propulsion systems. Tank can also be used as a radiation shield.

Turbopump: Provides high pressure propellants to the heat exchange region of the propulsion system. Warm gas from regeneratively cooled nozzle drives the turbines.

Radiation Shield: Protects the payload from radiation from the reactor by absorbing or reflecting neutrons and gamma rays.

Design Considerations

Why does the Space Shuttle look the way it does?

Launch Trajectory

Launch Trajectory

The Big Picture

Ballistic missiles and launch vehicles need large amounts of ΔV

As inefficient as rockets are (low Isp), they are currently the only means by which high velocities (hypersonic) can be easily obtained