routemap

Table 1: Direct Access to the Sub-Disciplines of Aerospace Engineering

DESIGN-CENTERED INTRODUCTION TO AEROSPACE ENGINEERING

3. Aerospace Design: A Route Map of Disciplines

Aerospace Engineering involves many "disciplines": each might warrant a separate division in a major company, with dedicated experts who spend decades specializing in it. Here we take a quick look at some of these disciplines which you will encounter in this course. To become an expert in each of these disciplines, one should pay careful attention to the basic courses in school which don't always seem at first sight to be very relevant to aerospace engineering.

ROUTE MAP OF DISCIPLINES
(or, classes where I might have stayed awake if I had been smarter )

Aspect of Aerospace Engineering	Basic disciplines / courses needed from the 1st 2 years of engineering school
Mission Specification	Technology forecasting, market surveys, vehicle performance, economics, social sciences, political science
Weight Estimation	Statistics, technology forecasting
Aerodynamics	Physics, calculus, computer science, optics, lasers, signal processing, image processing, acoustics, thermodynamics
Propulsion	Physics, thermodynamics, chemistry, lasers, optics, environmental sciences, acoustics
Performance	Physics, Statics and Dynamics, calculus; flight mechanics
Structures	Materials, Statics, Dynamics, Strength of Materials.
Layout and detail design	Engg. graphics, psychology, economics, ergonomics
Stability	Statics, calculus
Controls	Laplace transforms, differential equations, electrical engg., computer science
Instrumentation & communications	Optics, electronics, magnetism, signal processing, computer science
Space propulsion	Electricity, magnetism, nuclear engg., chemistry, physics, dynamics, thermodynamics
Trajectories & space mission design	dynamics, astronomy, modern physics
Spacecraft design	heat transfer, materials, photoelectricity, thermodynamics, chemistry, physics, physiology.
Flight Simulation	Flight mechanics, image processing, engg. graphics, computer science, control theory.
Ground and flight testing and experimentation	All aerospace engg. disciplines, physics, chemistry, mechanical design, electronics, signal processing, image processing, computer science.
Lifecycle cost	Manufacturing, Systems Engg., Optimization, Economics, Political and Legal Issues.

And perhaps most important of all, the general knowledge and common sense acquired by reading newspapers and magazines, watching TV, talking to people and thinking quietly.

4. Conceptual Design of a Flight Vehicle

4. (a) Mission Specification

Where does one start, to go about designing one of these grand contraptions? The answer is quite easy when one stops to think about it. First, we have to decide what we want the contraption to do. We will write out a wish-list, then think about it and perhaps constrain those wishes just a little. Then we will think of what a "typical mission profile" might be. For example, we will consider the design of a large airliner, one which is slightly bigger and faster and can go further in greater comfort, and cheaper, than the best of today's airliners. Our aircraft is to carry 400 passengers, non-stop, 10,000 miles, and do this with the comfort-level of today's Business Class for everyone. As we write out the mission profile, various other requirements occur to us. The aircraft must be able to take off from any large city airport, in hot weather. Such as Denver or Mexico City (5000+ feet above sea-level), where the temperature may be 100 deg. F and very humid (well, at least in Mexico), and still fly the full range and payload. And be able to take off, no problem, even if one engine quits just as the aircraft is lifting off the runway. And land safely even if one engine quits when the aircraft is as far as it can be from any airport. And have enough fuel left at the destination to be able to fly another 500 miles, or loiter for 1 hour, because the weather may be bad at the destination... And....And...
The list gets much, much longer as we think about the detailed design, later. Aerospace engineers think about everything that can possibly go wrong, and many things beyond that. And then they worry, and plan, and check their calculations, and talk to other people about how to improve their estimates and calculation procedures. They develop simulators to test out every eventuality. Nothing is left to chance. And yet, we know that things still go terribly wrong sometimes, so there's always more to think about...

Table 1: Simplified Design sequence

Step Issues

Define the mission What must the vehicle do?

Survey past designs What has been shown to be possible? (don't worry about WHY yet)

Weight estimation How much will it weigh, approximately?

Aerodynamics Wing size, speed, altitude, drag

Propulsion and engine selection How much thrust or power is needed? How many engines? How heavy? How much fuel will they consume?

Performance Fuel weight, take off distance, speed/altitude boundaries

Configuration How should it look? Designerís decisions needed!

Stability & Control Locate & size the tail, flaps, elevators, ailerons etc. Fuel distribution.

Structure Strength of each part, material, weight reduction, life prediction.

Manufacturing: concurrent engineering Design each part, see how everything fits, and plan how to build and maintain the vehicle. Break this down into steps involved in manufacturing.

Life-cycle cost Minimize cost of owning the vehicle over its entire lifetime.

Iteration Are all the assumptions satisfied? Refine the weight and the design.

Flight Simulation Describe the vehicle using mathematics. Check the "flight envelope".

Testing Build models and measure their characteristics, verifying the predictions. Explore uncertain regions. Build & test first prototype.

Iteration and refinement Keep improving, reducing cost and complexity, and extending performance, safety and reliability.

4 (b) Weight Estimation

One simple way to start the conceptual design is to realize that we are designing something that must lift some weight and carry it a certain distance. The mass to be carried is the "payload": the load which we (hopefully) get paid to carry. Once the payload is determined (as simple as figuring out how much the passengers, their bags, food, etc. will weigh), we ask: "Haven't others tried to do something similar or close to this? How much did their aircraft weigh? We know we are smarter than anyone else, but maybe they too thought carefully, and maybe we can learn something from the results that they got". This is called "benchmarking". From this, we can get a rough idea of the weight fractions of the various systems involved. For example, it is a rough "rule of thumb" that the fuel weight may be as high as 50% of the take-off weight of a large airliner which is to fly a very long distance. This applies also to birds flying across oceans (Ref: Tennekes): they eat until they can barely get off the ground even with a long takeoff run on the beach, running into the wind to increase airspeed.

Table 3: How the Take-off Gross Weight (TOW) of an Aircraft is broken out among the systems

Component Fraction of TOW

Payload Fraction: passengers+ crew, baggage, food&water (including peanuts& pretzels), cargo Wpl/Wto

Propulsion Fraction: Engines, engine control systems, nacelles, fuel lines, fuel pumps, fuel tanks We/Wto

Structure and Controls: Everything else fixed to the aircraft: wings, fuselage, control surfaces, instruments, landing gear, hydraulic systems, servo motors, airconditioning system, ducting, lights, interior furnishings, movie screens... Ws/Wto

Fuel: Wf/Wto

Total: 1.0

Now, if we can figure out the payload weight (which only we know, based on the intended mission specification), and we can get the payload fraction from somewhere, (maybe by taking an average of existing designs and being slightly more ambitious) the Takeoff Gross Weight is simply the Payload divided by the Payload fraction.

For example, if the Payload is 30,000lbs, and the Payload fraction is 0.15, then the TOW is 30,000 / 0.15 = 200,000 lbs.

This is of course an estimate. The rest of the design is to make sure we come in under this estimate, when we calculate everything else. When we have a rough calculation of all the other things, we'll go back and "iterate", many times: refine our estimates, so that the whole vehicle gets better and better.

4 (c) Benchmarking

Hunting through the available data on various aircraft, we find that there is a wide range of answers to our question on the payload fraction. Some craft weigh only 5 times their payload; others weigh 90 times the payload. As we look closer, we see that there is some similarity between these "payload fractions" for aircraft which have similar "missions" and payloads. So in our case, we ignore the fighter designs and the space launcher designs and the helicopters, and the birds, and focus on large airliners, like the Boeing 747, 767, 777, Airbus A340, A320, McDonnell-Douglas MD11, MD90, and the Lockheed L-1011. These are all aircraft meant to carry large numbers of passengers (100 to 500) over long distances (upto 8000 miles). We find, however, that no one has quite designed an aircraft which can do all the things that we want our aircraft to do. So we are on our own in that respect, going out into the unknown.

The specifications of several large aircraft and aircraft engines are given below.

Aircraft and Engine Specifications: Examples

Aircraft Engine Specifications from United Technologies Pratt & Whitney Aircraft Engines

* The aircraft and engine characteristics above were provided by:
Pratt & Whitney
Marketing Operations and Support
January 1997

Proceed to the Next Section: Forces and Moments in Flight

Return to the Previous Section: Today's Dreams in Various Speed Ranges "#CC0000">Return to the Previous Section: Today's Dreams in Various Speed Ranges

Step	Issues
Define the mission	What must the vehicle do?
Survey past designs	What has been shown to be possible? (don't worry about WHY yet)
Weight estimation	How much will it weigh, approximately?
Aerodynamics	Wing size, speed, altitude, drag
Propulsion and engine selection	How much thrust or power is needed? How many engines? How heavy? How much fuel will they consume?
Performance	Fuel weight, take off distance, speed/altitude boundaries
Configuration	How should it look? Designerís decisions needed!
Stability & Control	Locate & size the tail, flaps, elevators, ailerons etc. Fuel distribution.
Structure	Strength of each part, material, weight reduction, life prediction.
Manufacturing: concurrent engineering	Design each part, see how everything fits, and plan how to build and maintain the vehicle. Break this down into steps involved in manufacturing.
Life-cycle cost	Minimize cost of owning the vehicle over its entire lifetime.
Iteration	Are all the assumptions satisfied? Refine the weight and the design.
Flight Simulation	Describe the vehicle using mathematics. Check the "flight envelope".
Testing	Build models and measure their characteristics, verifying the predictions. Explore uncertain regions. Build & test first prototype.
Iteration and refinement	Keep improving, reducing cost and complexity, and extending performance, safety and reliability.

Component	Fraction of TOW
Payload Fraction: passengers+ crew, baggage, food&water (including peanuts& pretzels), cargo	Wpl/Wto
Propulsion Fraction: Engines, engine control systems, nacelles, fuel lines, fuel pumps, fuel tanks	We/Wto
Structure and Controls: Everything else fixed to the aircraft: wings, fuselage, control surfaces, instruments, landing gear, hydraulic systems, servo motors, airconditioning system, ducting, lights, interior furnishings, movie screens...	Ws/Wto
Fuel:	Wf/Wto
Total:	1.0