**[Learning Unit - Systems Engineering Design Methodology]{.underline}**

**[with examples utilizing Advanced Vehicles for Space
Transportation]{.underline}**

Joseph S. Miles and Siva Thangam

New Jersey Space Grant, Stevens Institute of Technology, Hoboken, NJ

> Target Audience: Students in Grades 11 -- 12

### 

### Summary: 

### Engineering design is a more advanced version of a problem solving technique that many people use routinely. The general procedure for solving real everyday problems is straightforward: A problem is encountered, information about the problem is obtained, alternate solutions are formulated, and the best alternative is adopted. Some problems are so straightforward and solutions so obvious that people solve those problems without being consciously aware of the specific steps in the process. We have organized this Learning unit into four modules, starting with an introductory segment followed by examples.

**Learning Unit Module 1: Introduction to Engineering Design at a System
Level**

Engineering design is the process of devising a system, component, or
process to meet desired needs. It is a decision-making process (often
iterative), in which the basic sciences, mathematics and the engineering
sciences are applied to convert resources optimally to meet a stated
objective. Among the fundamental elements of the design process are the
establishment of objectives and criteria, synthesis, analysis,
construction, testing, and evaluation.

### The Engineering Design Process can be described as a seven-step process whereby the needs of the user or the marketplace are transformed into a product satisfying those needs. Design is usually conducted by an engineer or designer but requires consultation with other people in the organization and is essentially an exercise in problem solving. During this learning unit, we will endeavor to describe the product development life cycle from an engineering point of view (rather than a financial point of view). Typically, the design of a new product consists of the seven stages shown on the following page. It may also require the development of a prototype as a "proof of concept" to demonstrate that the new technologies work before committing resources to full-scale manufacture.

###  The traditional view of the design to manufacture process is that it is a sequential process; the outcome of one stage is passed on to the next stage. This tends to lead to iteration in the design (i.e. having to go back to an earlier stage to correct mistakes.) This can make products more expensive and delivered to the marketplace later than would otherwise be necessary. A better approach is for the designer to consider the stages following design to try to eliminate any potential problems. This means that the designer requires collaboration from the other experts in the company, for example the manufacturing expert to ensure that any designs the designer proposes can be made. Embodied in the 21^st^ Century Engineering Design process is the process of "Total Design" which clarifies that the engineering disciplines cannot survive on their own merit in today's industrial world. Rather they must be integrated, not only in their own right, as engineering disciplines (such as mechanical, electrical, chemical, civil, and computer engineering) but also with marketing, finance and sales in order to bring a product into existence.

Finally, we will conclude the learning unit with an example of the
techniques applied to "An 'Earth to Orbit' and 'In Space' Propulsion
System with Vehicle Design".

**[Objectives:]{.underline}** The learning unit will familiarize the
reader with the following objectives:

-   The Engineering Design Process applied to the product development
    life cycle.

```{=html}
<!-- -->
```
-   Within the Engineering Design Process, the concepts of:

    -   product need or demand

    -   user requirements analysis

    -   product stakeholders (both direct and indirect)

```{=html}
<!-- -->
```
-   Conceptual design and the methodology for evaluating the various
    design alternatives via a quantifiable matrix.

```{=html}
<!-- -->
```
-   The product development life cycle in the framework of "Total
    Systems Design" demonstrated through step 3 of 7, of the Engineering
    Design Process.

```{=html}
<!-- -->
```
-   A demonstration of the above principles of systems design applied to
    "Earth to Orbit and 'In Space' Propulsion Systems".

**[Background Concepts:]{.underline}**

-   All design must begin with a customer need or an opportunity, which
    can be characterized by a demand or opportunity statement.

-   From the opportunity or demand statement, a design specification
    must be formulated -- the specification of the product, process, or
    service to be designed. The specifications act as a mantle or cloak
    that envelope all the subsequent stages in the design process. It is
    the "control" for the total design activity.

-   Conceptual design, preliminary design and detail design are carried
    out within the envelope of the design specification.

-   Detail design and manufacturing are part of the engineering design
    enterprise.

-   "Total Design" is the systematic activity necessary from the
    identification of a market/user need, to the selling of the
    successful product/process/service to satisfy that need -- an
    activity that encompasses product, process, people and organization.

-   Historical engineering education is, by necessity, mostly concerned
    with the analytical techniques and skills in engineering within a
    specific discipline or domain (e.g. mechanical, electrical, etc.).
    The rigorous application of such skills and knowledge to engineering
    elements is [partial design.]{.underline} During the twenty-first
    century, industry is concerned with [total design]{.underline}: the
    integration of numerous technical and non-technical disciplines
    toward a new product/system throughout its useful life.

-   Misdirected engineering rigor, i.e. too much emphasis on focused
    engineering solutions, will nearly always give rise to bad total
    design. Therefore, design or product development teams should always
    include non-engineers.

**[What is a System?]{.underline}**

Below are several definitions of a system, all which define what a
system is from various perspectives:

-   A dynamic and complex whole interacting as a structure.

-   A set of interdependent interacting parts which are generally
    systems themselves.

-   A product or process that is well integrated into the rest of it's
    environment.

-   A whole with component parts that are well integrated.

> Systems Theory focuses on organization and interdependence of
> relationships. A system is composed of regularly interacting or
> interdependent groups of activities/parts the emergent relationships
> of which are a whole.

-   A system is a set of individual components (subsystems, segments)
    acting together to achieve a set of common objectives. It is a whole
    that cannot be divided into independent parts without losing its
    essential characteristics as a whole.

```{=html}
<!-- -->
```
-   It follows from this definition that, a system's essential defining
    properties are the product of the *[interactions]{.underline}* of
    its parts, not the actions of the parts considered separately.
    Therefore, when a system is taken apart, or its parts are considered
    independently of each other, the system loses its *[essential
    properties]{.underline}*.

-   Furthermore, when performance of each part taken separately is
    improved, the [performance of the system as a whole may not be
    improved.]{.underline} The incremental improvement in a part may be
    lost when the whole system is considered, or the integrity of the
    system may be compromised.

**[Market Need:]{.underline}**

In many projects, the "need" is identified by an organization other than
the one that will eventually accomplish the effort. This is true by most
projects sponsored by government organizations, and, in these
situations, the feasibility study and conceptualization will normally be
completed prior to the market call for the design, development and
production phases. The NASA space shuttle was an example of this type of
arrangement.

**[Requirements and Needs Analysis:]{.underline}**

The functional need or deficiency of the product must somehow be
translated into a more specific set of qualitative or quantitative
customer requirements (Not Design Requirements, which come later).
Inadequate resolution or improper emphasis will likely cause a
misalignment between the design and the "real" need of the customer(s).
Many companies use a variation of a process called "Voice of the
Customer" or "Concept Engineering" to capture customer requirements.
They begin by identifying both the direct and indirect stakeholders for
the product and then matrix each of these stakeholders with a
requirement of the product. Some requirements may be the same, similar
or exclusive.

-   "Direct" Stakeholders can be Individuals, Entities, Other Systems,
    that will actively **interact with** the "system" once it is
    operational and in use

-   "Indirect" Stakeholders" can be Individuals, Entities, Other
    Systems, Standards, Protocols, Procedures, Regulations that will
    also **influence** the "success" of the system

-   Customer(s) can be Direct or Indirect

Successful system design must start with an understanding of all of the
STAKEHOLDERS (direct and indirect) in the system, and what are their
requirements. Stakeholder requirements never specify a solution or
select a particular design alternative. They are top-level system
requirements that address a NEED. Properly written requirements will
quantify a need in measurable terms. That would conclude the problem
definition phase of the product development life cycle.

**Learning Unit Module 2: An Example to Identify Market Need including
Stakeholders & Requirements and Design Activities**

-   [Market Need, Opportunity, or Requirement]{.underline}

    -   Findings of IDC, a market research firm in September 2004

        -   \$58 billion market for portable MP3 player industry over
            next 5 years

```{=html}
<!-- -->
```
-   Portable flash players will overshadow hard drive based players

    -   Hard drive based players are larger and heavier

    -   Hard drive based players are not conducive to shock as they have
        moving parts

    -   Falling prices in flash memory and technological breakthroughs
        of higher capacity flash cards

    ```{=html}
    <!-- -->
    ```
    -   12.5 million flash players sold in 2003

    -   50 million units in 2008

```{=html}
<!-- -->
```
-   Design a portable audio player that enables flawless playback in
    "extreme environments" for active individuals

-   Portable audio player for active individuals

    -   Multi-format

    -   "Rugged"

    -   Water Resistant

    -   Small

    -   Affordable

    -   Enhanced Connectivity Options

    -   Recording option

    -   Data Storage Capability

```{=html}
<!-- -->
```
-   [Identify the Stakeholders - Portable Audio Player]{.underline}

+-----------------------------------+----------------------------------+
| **DIRECT**                        | **INDIRECT**                     |
+-----------------------------------+----------------------------------+
| **Users**: Swimmers, Runners,     | **FCC**                          |
| Hikers, Surfers etc               |                                  |
+-----------------------------------+----------------------------------+
| **Company:** Engineering,         | **Regulating Bodies**            |
| Manufacturing, Marketing, Sales,  |                                  |
| Quality Control                   |                                  |
+-----------------------------------+----------------------------------+
| **Retailers**                     | **Company Shareholders**         |
+-----------------------------------+----------------------------------+
| **3rd Party Vendors**             | **RIAA** (Recording Industry     |
|                                   | Association of America)          |
+-----------------------------------+----------------------------------+
| **Subcontractors**                | **Recording Artists**            |
+-----------------------------------+----------------------------------+
| **Intellectual Property Rights    |                                  |
| Holders:**                        |                                  |
|                                   |                                  |
| MP3, WMA, WAV, AAC, etc.          |                                  |
|                                   |                                  |
| Hardware & Software               |                                  |
+-----------------------------------+----------------------------------+
| **Music Service Providers**       |                                  |
+-----------------------------------+----------------------------------+

**[Portable Audio Player -- Direct Stakeholder Requirements (by
Stakeholder Category)]{.underline}**

-   **Users**

    -   Work under water (Able to withstand submersion to 5 feet)

    -   Temp Specs (Operate from 0 to 120 degrees F)

    -   Shock environment (Operate during shock created by jogger)

    -   Play multiple existing formats and should be upgradeable

    -   Fast/Easy Connection to a PC (connect within 5 seconds)

    -   Easy to use GUI application s/w that can be learned in 10
        minutes

    -   Capable of "data" storage, other than audio

    -   Reliable (Mean time between failures greater than 10,000 hours)

    -   Size should be equal to or smaller than a pack of cigarettes

    -   Must be able to attach to multiple locations on the user

    -   Battery life (up to 8 hours of continuous play per charge)

```{=html}
<!-- -->
```
-   **Company**

    -   Appeal to Market Demographic

    -   Meets schedule

    -   Meets budget

    -   Supply enough product to meet the demand

    -   Profitable (Quantify)

    -   Expand market segment

    -   Increase market share

    -   Reliable (high Mean Time to Failure -- MTTF)

```{=html}
<!-- -->
```
-   **Retailers**

    -   Supply enough product to meet the demand

    ```{=html}
    <!-- -->
    ```
    -   Profitable

    -   Reliable

    -   Aesthetically pleasing

    -   Size should be equal to or smaller than a pack of cigarettes

```{=html}
<!-- -->
```
-   **Third Party Vendors**

    -   Standard Interfaces

```{=html}
<!-- -->
```
-   **Subcontractors**

    -   Profitable

    -   Producible

    -   Manufacturability

```{=html}
<!-- -->
```
-   **Intellectual Property Rights Holders**

    -   Licensing

    -   Copy Protection

```{=html}
<!-- -->
```
-   **Retailers**

    -   Supply enough product to meet the demand

    -   Profitable

    -   Reliable

    -   Aesthetically pleasing

    -   Size should be equal to or smaller than a pack of cigarettes

```{=html}
<!-- -->
```
-   **Third Party Vendors**

    -   Standard Interfaces

```{=html}
<!-- -->
```
-   **Subcontractors**

    -   Profitable

    -   Producible

    -   Manufacturability

```{=html}
<!-- -->
```
-   **Intellectual Property Rights Holders**

    -   Licensing

    -   Copy Protection

**[Portable Audio Player -- Indirect Stakeholder Requirements (by
Stakeholder Category)]{.underline}**

-   **FCC & other regulating bodies**

    -   Adhere to applicable standards

```{=html}
<!-- -->
```
-   **Company Shareholders**

    -   Profit Impact on Earning per Share

```{=html}
<!-- -->
```
-   **RIAA\\Recording Artists\\Music Service Providers**

    -   Uphold copyright laws

    -   Licensing

    -   Downloads

**[Activity #1 -- Identify the Need/Market Opportunity for an Electric
Screwdriver]{.underline}** A typical screwdriver can be the Black &
Decker Model VP750 that can be seen at the following link. (45 minutes)
<http://www.blackanddecker.com/ProductGuide/Product-Details.aspx?ProductID=2325>
(A shell of the solution is provided.)

-   Design an electric screwdriver for easy insertion of screws and
    screw removal

```{=html}
<!-- -->
```
-   There is a need for a light duty electric screwdriver for self
    installation and maintenance jobs in households and offices

```{=html}
<!-- -->
```
-   Some of the applications will include assembly such as the
    following, but there are many more:

    -   Installing mini blinds

    -   Hanging curtain rods

    -   Hanging pictures

    -   Installing Light Switches

    -   Installing hinges

**[Activity #2 -- Identify a list of Direct and Indirect Stakeholders
for the Electric Screwdriver]{.underline}** (45 minutes)

-   The following are typical stakeholder categories for both direct and
    indirect stakeholders:

    -   Users Types (10)

    -   Buyers Types (5)

    -   Regulatory Bodies (3)

    -   Standards Bodies (3)

    -   Sales Establishments (3)

**[Activity #3 -- Integrate the Need/Market Opportunity with each of the
stakeholders (both direct and indirect)]{.underline} (45** minutes)

-   From the Need/Market Opportunities identified in Activity #1 and the
    Direct and Indirect Stakeholder identified in Activity #2, integrate
    the market needs/opportunities within the stakeholder lists as shown
    in the portable audio player example.

**Learning Unit Module 3: Engineering Conceptual Design**

After the problem has been completely defined, viable solutions need to
be identified from which a feasible approach can be selected.
Brainstorming, whereby a group of participants suggests as many ideas
for solutions to a problem as possible, in a specified period of time,
is an efficient way to identify alternative solutions to problems. A
synergy can be created when several people are involved in the process.
The ideas suggested by one person may trigger ideas for a better
solution on the part of another person.

The conceptual design process is an iterative process whereby
participants generate solutions to meet a stated need, evaluate and
refine the solutions generating new solutions. The conceptual design
life-cycle is continued until all concepts are exhausted and the concept
evaluation process begins. The evaluation matrix generally employs a
Pugh matrix where the solutions are evaluated in order to select the one
that is the most suited to matching the requirements.

**As an example of the conceptual design evaluation process, consider
the problem of identifying an automobile power source. At least four
power sources can be considered with minimal thought: Hydrogen Fuel Cell
Power, Hybrid Gas-Electric Power, Gasoline Internal Combustion Engine,
and a Diesel Engine. Now establish various acceptance criteria and their
importance weighting to the project as a whole: Overall vehicle weight,
Operating Cost, Purchase Cost, Reliability, Emissions, and Fuel Access.
Finally, assign a raw score for each type of engine for each of the
acceptance criteria, tabulate the weighted score and then sum the
weighted score to reach a final score for each power source.**

  -------------------------- -------------------------- ------- ------------ ------------ -------------- ------------ -------------- ------------ ------------ ------------
  **[Car Power Source                                                                                                                                          
  Selection]{.underline}**                                                                                                                                     

                                                        ** **   **Hydrogen                **Hybrid                    **Gasoline                  **Diesel**   
                                                                Fuel Cell                 gas-electric                Internal                                 
                                                                Power**                   Power**                     Combustion**                             

                                                        ** **   **Raw        **Weighted   **Raw Score**  **Weighted   **Raw Score**  **Weighted   **Raw        **Weighted
                                                                Score**      Score**                     Score**                     Score**      Score**      Score**

  ** **                      ** **                      ** **                                                                                                   

  **[Acceptance              **[Weight]{.underline}**   ** **                                                                                                  
  Criteria]{.underline}**                                                                                                                                      

  Vehicle Weight             15                         ** **   5            75           5              75           5              75           5            75

  Operating Cost             15                         ** **   7            105          6              90           7              105          7            105

  Purchase Cost              10                         ** **   7            70           6              60           4              60           7            105

  Reliability                15                         ** **   9            135          10             150          9              135          5            75

  Emissions                  20                         ** **   8            160          8              160          8              120          8            120

  Fuel Access                25                         ** **   9            225          10             250          9              135          9            135

                             ======                     ** **                ========                    ========                    ========                  ========

  Final Score                100                        ** **                770                         785                         630                       615
  -------------------------- -------------------------- ------- ------------ ------------ -------------- ------------ -------------- ------------ ------------ ------------

**[Activity -- Conceptual Design -- Identification of A Robot Obstacle
Avoidance Subsystem]{.underline}** (60 minutes)

Refer to the robot in the learning unit entitled "**Learning Unit-****A
Robot Construction and Competition Project"** for complete details. That
learning unit identifies the creation of two modules, a "Bumper Sensor
Module" and a "Navigation Sensor Module" as shown below:

[Bumper Sensor Module]{.underline}

The robot should be equipped with two bumpers, mounted across the front
of the robot. No matter what angle from which an obstacle was hit by the
robot, one of the two switches behind the bumpers would be triggered and
the robot advised to take evasive action. The two bumper arms should
directly come in contact with each of the two contact switches.

[Navigation Sensors]{.underline}

A light sensor for the navigation light and a light sensor for the
target lights were placed on the robot. The light sensor for the
navigation light was placed on the front of the robot facing upwards at
about a 45-degree angle to try to detect a more direct light. Shielding
was used to isolate the different light angles in respect to the
navigation light and its associated light sensor.

The target light sensor should be placed directly in front of the robot,
to be in the line of sight with the target lights. Shielding can also be
used to improve its directionality.

The results of the conceptual design process in "**The Robot
Construction and Competition Project"** chose two bumper switches, one
target light sensor and one beacon light sensor.

For the **"Systems Engineering Design Methodology"** learning unit,
develop a conceptual design matrix utilizing both a 2 bumper robot and a
3 bumper robot design. For example, there can be two side bumpers and
one front bumper, or there can be two front bumpers and a rear bumper.)

In conjunction with the bumper design, identify the number of target
light sensors and the number of beacon light sensors. There can be a
left and right beacon light sensor or a front and back beacon light
sensor. Additionally, there can be various scenarios for target light
sensors.

The trade offs being that the more sensors and bumpers you utilize, the
more accurate control of the robot that is maintained, but the more
difficult the software development process becomes. That was the reason
that "**The Robot Construction and Competition Project"** uses a minimum
number of bumper and light detecting sensors. A typical conceptual
design matrix might look like the following:

  -------------------------- -------------------------- ----------------- ----------------------------- ---------- ------------------------- ---------- ------------------------- ---------- ------------------------- ----------
  **[Enemy Target Light                                                                                                                                                                                                
  Locating                                                                                                                                                                                                             
  Subsystem]{.underline}**                                                                                                                                                                                             

                                                        [ ]{.underline}   **[All                                                                                                                                       
                                                                          Combinations]{.underline}**                                                                                                                  

                                                                          2 Bumpers                     Weighted   2 Bumpers                 Weighted   3 Bumpers                 Weighted   3 Bumpers                 Weighted
                                                                                                        Score                                Score                                Score                                Score

  Number of Target Light                                                  1                                        1                                    1                                    1                         
  Sensors-Front/Rear                                                                                                                                                                                                   

  Number of Target Light                                                  2                                        2                                    2                                    2                         
  Sensors-Sides                                                                                                                                                                                                        

  Number of Beacon Light                                                  1                                        2                                    1                                    2                         
  Sensors                                                                                                                                                                                                              

                                                                                                                                                                                                                       

  **[Acceptance              **[Weight]{.underline}**                     **[Score]{.underline}**                  **[Score]{.underline}**              **[Score]{.underline}**              **[Score]{.underline}**   
  Criteria]{.underline}**                                                                                                                                                                                              

  Criteria 1                                                                                                                                                                                                           

  Criteria 2                                                                                                                                                                                                           

  Criteria 3                                                                                                                                                                                                           

  Criteria 4                                                                                                                                                                                                           

  Criteria 5                                                                                                                                                                                                           

  Criteria 6                                                                                                                                                                                                           

  Criteria 7                                                                                                                                                                                                           

                             ======                                                                     ======                               =====                                =====                                =====

                             100                                                                                                                                                                                       

                                                                          **Highest weighted score is                                                                                                                  
                                                                          the winning Conceptual                                                                                                                       
                                                                          Design**                                                                                                                                     
  -------------------------- -------------------------- ----------------- ----------------------------- ---------- ------------------------- ---------- ------------------------- ---------- ------------------------- ----------

**Learning Unit Module 4: Example of Development of Earth-to-Orbit Space
Craft by System Level Analysis**

![](media/image2.jpeg){width="3.125in" height="2.6458333333333335in"}The
development of efficient Earth-to-Orbit (*ETO*) vehicles has been in the
forefront of NASA research since the dawn of the space age. Among the
major stumbling blocks is the development of an efficient propulsion
system that is reusable, safe and cost-effective. Although NASA uses
rockets or thrusters powered by hydro-carbon based fuel, it is well
known that conventional chemical rockets, whether liquid or solid,
monopropellant or bipropellant, are fundamentally limited by their
available combustion reaction energies and heat transfer tolerances to
exhaust speeds of a few thousand meters per second. Effective space
missions entail exhaust velocities of at least an order of magnitude
higher.

This exercise involves a conceptual design of a cost-effective *ETO*
Space Craft for NASA (the customer) that is both reusable and capable of
handling a larger payload. NASA's current plans call for the development
of space colonies on the moon by 2024. Exploration of Mars for
colonization is to follow. Since a comprehensive development of such a
spacecraft is beyond the scope of the current learning unit, this
exercise will only consider *ETO* and space propulsion systems for
spacecrafts.

![](media/image3.jpeg){width="4.559722222222222in" height="2.4625in"}The
current propulsion systems in use by NASA for *ETO* applications
involving the Space Station are hydrocarbon based and they are to be
phased out by 2010 to ensure that future missions are cost effective.
The available alternatives include: electric propulsion systems, the
space elevator, as well as hybrids of these systems. The generic
chemical thrusters utilize hydrocarbon fuels and involve a nozzle to
accelerate the combustion products to provide the thrust.

![](media/image4.jpeg){width="2.65in" height="4.545138888888889in"}*EP*)
encompass a broad variety of strategies for achieving very high exhaust
velocities in order to ![](media/image5.png){width="4.320138888888889in"
height="2.2305555555555556in"}reduce the total propellant burden and
corresponding launch mass of present and future space transportation
systems. These techniques group broadly into three categories:
electro-thermal propulsion, wherein the propellant is electrically
heated, then expanded thermodynamically through a nozzle; electrostatic
propulsion, wherein ionized propellant particles are accelerated through
an electric field; and electromagnetic propulsion, wherein current
driven through a propellant plasma interacts with an internal or
external magnetic field to provide a stream-wise body force. Such
systems can produce a range of exhaust velocities and payload mass
fractions an order of magnitude higher than that of the most advanced
chemical rockets, which can thereby enable or substantially enhance many
attractive space missions. The attainable thrust densities (thrust per
unit exhaust area) of these systems are much lower, however, which
predicates longer flight times and more complex mission trajectories. In
addition, these systems require space-borne electric power supplies of
low specific mass and high reliability, interfaced with suitable power
processing equipment.

The Space Elevator works by having a very strong nanocarbon-based cable
that is tethered to the Earth. The elevator would travel to a
geostationary location above the Earth from where it would be released
or transferred to a spacecraft for travel to the moon or other locations
in space. Because of the fact that it is tethered to the Earth however,
there is no minimum escape velocity needed and much less energy will be
used.

Optimization of *EP* systems thus involves multidimensional trade-offs
among mission objectives, propellant and power plant mass, trip time,
internal and external environmental factors, and overall system
reliability. Meanwhile, yet more advanced concepts are currently being
pursued and when matured provide high credibility for future mission
applications.

From the point of overall metrics (customer-based) the following are
selected for the preliminary design.

1)  Fuel usage: This is typically expressed in terms of fuel usage per
    unit payload

2)  Velocity: For *ETO* applications, exit velocities greater than 10
    *km/s* are desirable.

3)  Weight: Overall weight of the spacecraft (which includes the fuel
    weight)

4)  Size: Physical size of the spacecraft (which includes the propulsion
    system)

5)  Safety: Overall safety of the enterprise both during *ETO* and space
    travel.

A qualitative ranking scheme can be used with an ascending scale (1 for
worst and 5 for best) to evaluate the concepts.

  --------------------- ------ -------- ------ ------- ------ -------- ---------
                        Fuel   Weight   Size   Speed   Cost   Safety   Overall

  Chemical              1      1        2      2       1      3        10

  EP-Electrothermal     3      3        4      3       4      4        21

  EP-Electrostatic      3      3        3      3       4      4        20

  EP-Electromagnetic    4      4        4      4       4      4        24

  Hybrid (Hall          4      4        3      4       4      4        23
  Thruster)                                                            

  Space Elevator        4      4        3      1       2      4        18
  --------------------- ------ -------- ------ ------- ------ -------- ---------

The Electromagnetic propulsion or its hybrid, currently offers the best
option for thrusters with sustained power levels of the order of 1 *MWe*
and cargo capacities of the order of 90 metric tons and a mission life
time of about 4000 hours (and at least 8000 hours for a Mars mission). A
brief discussion of the possible candidate systems are given next.

Thus, if we restrict our selection to thrusters that have the ability to
process hundreds of kilowatts to megawatts of power at reasonably high
efficiencies (based on direct measurements) and those with demonstrated
potential for attaining a significant lifetime (order of several
thousand hours), only the thermal arcjet thruster, the Hall thruster and
the magnetoplasmadynamic thrusters (*MPDT*) meet the mission
requirements.

In a typical arcjet system, an electric arc is used to add enthalpy to
the propellant. A kin to a chemical thruster, part of the enthalpy in
the flow is converted to directed kinetic energy using a nozzle. As
shown in the figure, a tightly constricted electric arc, carrying
currents up to 100*A*, heats the core of the propellant stream to
temperatures up to 10,000*K*, while the walls of the thruster are
maintained at much lower temperatures (\< 3000*K*) to prevent melting.
Because of the higher temperatures in the core, and consequently, higher
specific enthalpy, the exhaust velocity of an arcjet can reach, or even
exceed, 10 *km/s*, as opposed to only 4 *km/s* for a chemical thruster.
Its simple design and its high thrust density are some of the attractive
features of the arcjet.

![](media/image6.wmf){width="2.859722222222222in"
height="1.7826388888888889in"}

![](media/image7.wmf){width="3.3in" height="2.051388888888889in"}Hall
thrusters derive their thrust by accelerating heavy ions, using an
electric field, to high exhaust velocities. Under high magnetic fields,
and low densities, the current in the direction perpendicular to the
electric field (commonly called the Hall current) can exceed the current
along the electric field. As implied by their name, Hall thrusters
utilize this Hall current to lock electrons into a nearly collision-less
cross-stream drift, leaving the positive ions free to be accelerated by
the applied electric field (cf. figure). In a sense, these devices are
hybrid electrostatic-electromagnetic accelerators with space-charge
neutralization automatically provided by the background of drifting
electrons. Because of this, the Hall thrusters are not affected by
space-charge limitations. Therefore, Hall thrusters produce higher
thrust densities than space charge-limited devices, such as ion
thrusters. Two types of Hall thrusters that have high efficiencies and
show promise are those that use Bismuth and Xenon as propellants.

The magneto -- plasma - dynamic thruster (*MPDT*) involves the
application of a voltage across concentric electrodes to break down a
propellant gas, creating a quasi-neutral plasma within the thruster
chamber. A high current (\~100*A*) carried by the plasma to the
electrodes induces an azimuthal magnetic field, causing a Lorentz force
to accelerate the plasma out of the thruster at velocities of the order
of 10 *km/s* in the direction perpendicular to both the electric and the
magnetic fields (see, figure). The *MPDT* has a unique place among
electric thrusters in its ability to process megawatts of electrical
power in a small, simple, compact device and produce thrust densities
(thrust per unit exhaust area) of the order of 10^5^ *kN/m^2^*. In the
conventional *MPDT* using inert gas propellants (such as, argon, helium)
high efficiencies (\> 30%) are only reached at high power levels (\> 200
*kW*).

![](media/image8.wmf){width="3.109722222222222in" height="1.91875in"}In
order to overcome this, lithium vapor based propellant (that requires
low ionization energy) with multiple electrodes tightly packed in a tube
is used in the lithium Lorentz Force Accelerator (*LiLFA*) *MPDT*.
Propellant flow in *LiLFA-MPDT* (see figure) is through the channels in
the cathode that are created in between these smaller rods, rather than
from the electrode base, as in the conventional *MPDT*. The performance
of *LiLFA-MPDT* is further enhanced by the application of an additional
magnetic field, leading to the current version, Applied Field Lithium
Lorentz Force Accelerator (*AF-LFA*) *MPDT* that has high efficiencies
compact and cost effective.

At the power levels considered for cargo missions, the applied field
lithium Lorentz force accelerator, jointly developed by *NASA-JPL* and
the Moscow Aviation Institute is a suitable candidate for consideration.
The conclusion of a 5-year *AF-LiLFA* research program was the design of
a 48% efficient thruster operating at 185 *kW* with 4200*s* and 4.5*N*.
Currently, a collaborative effort by *NASA-JPL* and Princeton University
is aimed at testing the *AF-LFA MPDT* at power levels of the order of
100 *kW* has been initiated for finalizing the design of the next
generation thrusters for *ETO* and Space missions.