MAE 1201: Introduction to Aerospace Engineering

Florida Institute of Technology

Dr. Daniel R. Kirk

Introduction to Propulsion Homework Assignment

Date Assigned:

Date Due:

Foremost, it was a pleasure to give the propulsion lecture to your Introduction to Aerospace Engineering class on Friday, November 17, 2006. I very much appreciated the overall enthusiasm, positive feedback, and the opportunity to meet many of you after class. Next semester, in MAE 1202 Aerospace Practicum, we will revisit many of the ideas covered briefly in Friday’s lecture, and we will continue to develop the governing physical principles that will allow you to understand and model propulsion systems in greater detail. Furthermore, you will work together in teams to apply many of these principles to design and build a rocket to meet a specific set of performance criteria.

This homework assignment is designed to be interesting (and fun?), as well as to help review some basic concepts of air-breathing and rocket propulsion, which were introduced in Lecture. To help you complete the assignment, read Sections 9.1, 9.4-9.6, 9.8 from John Anderson’s Introduction to Flight. If you have any questions on the assignment, propulsion, or aerospace engineering in general, please feel free to come to my office hours.

In answering the following questions use whatever means you feel best conveys your answer (i.e. drawings, equations, or words).

1.  If you were giving a lecture to a group of high school students, how would you describe how jet propulsion works, using just a few well thought out sentences?

2.  Explain the difference between a turbojet and turbofan engine. Why are the vast majority of commercial airplane engines turbofans?

3.  The following website, created by one of the world’s leading manufacturers of airplane engines, shows an excellent animations of the workings of an air-breathing airplane engine: http://www.geae.com/education/index.html

Using this website and the information provided in lecture (which can be found at: http://my.fit.edu/~dkirk/) summarize the purpose and function of the following major components of an air-breathing engine:

  1. Inlet
  2. Fan
  3. Compressor
  4. Combustor (also called a burner)
  5. Turbine
  6. Nozzle

4.  Example 9.3 in John Anderson’s Introduction to Flight, demonstrates a simple calculation of thrust for a turbojet engine. The calculation is performed at an altitude of 30,000 ft and at a flight velocity of 500 MPH, and the resultant thrust for this engine was shown to be about 4,000 lb. Repeat this calculation for when the airplane is sitting at the end of the runway about to take off, operating at the same exhaust pressure and velocity. How does your ‘static’ thrust value compare with the 4,000 lb ‘cruise’ result? What does this imply?

5.  In a few sentences and equations, explain what specific impulse is and why it is a relevant metric for assessing rocket performance. Give a typical value for (a) solid rockets, (b) LOX-LH2 liquid rockets, and (c) an ion thruster.

6.  Which is greater:

  1. The ratio of the propellant mass to total vehicle mass of a rocket, or
  2. The ratio of the fuel mass to total vehicle mass for an airplane

Briefly explain how you came to this conclusion.

7.  Consider two rockets, a single-stage and a two-stage rocket. The two rockets are designed so that they have identical propellant mass, the identical total mass, and the identical specific impulse. Assuming that both are launched vertically, gravity is constant, and drag is zero, which of the following is correct?

  1. The two rockets will go the same distance
  2. The single stage rocket will go farther
  3. The two-stage rocket will go farther

Briefly explain how you came to this conclusion.

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