The concept of a quantitative figure-of-merit (FOM) to evaluate different and competing options for space missions is further developed. Over six hundred individual factors are considered. These range from mission orbital mechanics to in-situ resource utilization (ISRU/ISMU) plants. The program utilizes a commercial software package for synthesis and visual display; the details are completely developed in-house. Historical FOM's are derived for successful space missions such as the Surveyor, Voyager, Apollo, etc. A cost FOM is also mentioned. The bulk of this work is devoted to one specific example of Mars Sample Return (MSR). The program is flexible enough to accommodate a variety of evolving technologies. Initial results show that the FOM for sample return is a function of the mass returned to LEO, and that missions utilizing ISRU/ISMU are far more cost effective than those that rely on all earth-transported resources

Pan, Thomas

Preiss, Bruce

Ramohalli, Kumar

English

NASA Technical Reports Server

6 N91-243 
Development Of A Figure-of-Merit For Space Missions
Bruce Preiss, Thomas Pan, and Kumar RamohaUi
Aerospace and Mechanical Engineering
University of Arizona
(."
ABSTRACT,
The concept of a quantitative figure-of-merit (F-o-M) to evaluate different and competing
options for space missions, is further developed. Over six hundred individual factors are considered
at the time of this report. These range from mission orbital mechanics to In-Situ Resource Utilization
(ISRU/ISMU) plants. The program utilizes a commercial software package for synthesis and visual
display; the details are completely developed in-house. Historical FoM's are derived for successful
space missions such as the Surveyor, Voyager, Apollo .... A cost FoM is also mentioned and will
be further developed. The bulk of this work is devoted to one specific example of Mars Sample
Return (MSR). The program is flexible enough to accommodate a variety of evolving technologies.
Initial results show that the FoM for sample return is a function of the mass returned to LEO, and
that missions utilizing ISRU/ISMU are far more cost effective than those that rely on all earth-
transported resources.
INTRODUCTION
The variables in a space mission are so numerous and their interactions so complex that
it is not easy to visualize the overall mission impact of many components. Frequently, we are faced
with difficult choices that must consider competing technologies, anticipated advances, expected
improvements in reliability, and the overall life-cycle costs that could easily run into hundreds of
millions of dollars. For initial screening purposes, it was felt highly desirable to develop an overall
figure-of-merit (FoM) for the mission that could be used for ranking these competing candidates.
When one recognizes that the parameters vary from orbital mechanics to rocketry, and chemical
plant masses to the power source reliability, the difficulty of accurately combining these becomes
apparent.
Work was initiated last year on this difficult task. The input parameters were divided into
four major categories: the mission, transportation, ISRU/ISMU plants, and the support systems. The
feasibility of developing a meaningful FoM was proven. The quantitative examples included various
MSR missions that cleady showed the advantages of ISRU/ISMU [reference 1]. It was felt desirable
to further develop this highly useful concept. One of the main aims this year was to tie this concept
to the current SERC work on the oxygen plant design. In addition, the FoM's of various successful
https://ntrs.nasa.gov/search.jsp?R=19910015074 2020-03-19T18:25:02+00:00Z
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missions were examined within the framework of the software.
Section II describes the input parameters. Section III describes the software development.
Section IV presents the principal results. This report concludes with a brief outline of future work
and applications.
THE INPUT PARAMETERS
The FoM concept organizes the inputs into four major categories and then integrates the
dependent variables through the appropriate governing equations. The four categories are loosely
described as mission details, rocket design, in-situ resource utilization (ISRU), and support
components. The bulk of the Inputs pertain to the joint spacecraft and rocket design. This is where
most of the flexibility will occur in any given design. The governing equations have been modified
to accurately reflect the dependencies of many detailed spacecraft parameters. The categories have
been developed and defined as much as possible, but the technology is constantly changing and
innovations are always being introduced. It should be stressed here that the FoM is a dynamic
property since it directly reflects even minor changes in the mission planning.
SOFTWARE DEVELOPMENT
The FoM itself is calculated within a commercial spreadsheet software package. The
spreadsheet option was chosen over a programming language due to its ability to integrate either
internal or external data and then present the results in simple color graphics. Ease of use is also
a major factor, since no knowledge of any programming language is necessary. Only an
understanding of the basic equations is required. The spreadsheet structure was completely
reorganized to simplify the input process and take full advantage of the three-dimensional
capabilities of the software. The spreadsheet equations and structure have been generalized
wherever possible to maximize the flexibility of the approach.
Two key emerging technologies have been included inthe spreadsheet program. The ISRU
concept and the use of modular engines are options now available for the mission planner. Both
technologies are innovations that may provide major improvements in mission capabilities. The
quantitative effect of either on mission planning can now be calculated In a relative manner.
The ISRU idea is to use local resources whenever possible to help fulfill the mission goals.
Exploration of the Earth has historically utilized local resources to succeed, and the natural and
logical progression is to apply this concept to planetary exploration also. A proof-of-concept
oxygen production plant is being built at the Center for the Utilization of Local Planetary Resources.
It is designed to use an electrocatalytic process to convert gaseous carbon dioxide into molecular
oxygen. The available data from the Center has been included in the spreadsheet to provide
accurate figures for the masses of each component. The projected production rate and total mass
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are,therefore,all based on current research directly related to the manufacture of the plant. As the
technology matures, the spreadsheet can easily be updated to reflect the improvements.
The use of modular engines will not reduce the initial total mass of the spacecraft as the
use of ISRU will. However, the other direct benefits will have an overall effect of increasing the FoM.
The benefits to be gained from modular engines are increased reliability, reduced risks, simpler
reparability. There is even the Important added benefits of a reduction in both design and
production costs. The spreadsheet has been configured for the possible use of modular engines
for the return voyage. The size of the engine is dictated by the thrust required for the last staging
maneuver. The number of engines needed in a cluster for all previous stages is based on this one
engine. All the calculations are done automatically. The current limit imposed on the modular
engine design is the restricted use of propellants. Only fuels and oxidizers that do not need
refrigeration for the return voyage in space can be utilized.
A propellant database has also been incorporated into the overall scheme for the calculation
of the FoM. The database has been constructed in another commercial software package with
which the spreadsheet can form a direct link. All of the pertinent performance parameters have
been input for a total of twenty-six different propellant combinations. The parameters that have
been vaded for each database record are the chamber pressure, oxidizer to fuel ratio, and the
nozzle area ratio. For each of these various combinations, the chamber temperature, characteristic
velocity, specific impulse, pressure ratio, and vacuum specific impulse have also been input.
Special thermodynamic data is being included in the spreadsheet for each propellant in the
database so the storage parameters can be automatically calculated. Only by knowing the specific
volume of both the fuel and oxidizer can the tanks be sized accurately. The database, of course,
could be used for other projects, but its sole use until now is for the FoM spreadsheet.
RESULTS
The FoM definition is allowed flexibility in order to apply to almost any mission architecture.
A set of definitions has been devised, and a few of them have been applied to historical U.S. space
missions. To date, only technical definitions have been utilized. Cost definitions have been
identified and would be very useful, but the difficulty in gathering accurate accounting data has
caused many problems. The application to historical missions has identified the usefulness of using
different definitions depending on the type of mission. For example, the definition used in a sample
return mission is defined as the mass of the payload retumed to low-Earth orbit (LEO) divided by
the initial spacecraft mass starting in LEO. It was important to factor in the mass of the sample for
this type of mission, but of course, this parameter did not apply to previous orbiter missions. In
essence, the flexibility of the FoM concept was tested within a historical context and given a certain
validity that will hopefully also apply to any future missions. In an effort to quantify all important
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aspectsof a plannedmission,allthedefinitionsfactor in a multiplier to account for variations in
reliability, reparability, and risk.
The spreadsheet has been developed sufficiently to handle a rigorous analysis of a Mars
Sample Return (MSR) mission. The following graphs show some preliminary findings inthe analysis.
The FoM is defined in both graphs as
FoM Sample Mass x[R_]xlO 4
Initial Mass in LEO
where
R_ - (Inverse Risk)x(Re#ability) x(Reparabil/ty)
The first graph presents the FOM for a range of payload masses to be returned to LEO. The small
masses represent a typical sample mission with the larger masses possibly representing an
evolution of manned missions. A series of different propellant combinations is shown with some
including the use of ISRU. The important thing to note is that the maximum FoM within each mass
category is always accomplished by using ISRUo The second plot shows the typical variation in
mission strategies for a MSR mission. The relative gains realized through the use of modular engine
configurations and with ISRU, both separately and together, are presented. Two propellant
combinations are shown for comparison. The benefits attained using both technologies may not
seem greatly significant on the surface, but in terms of the sample mass, even small improvements
are very important.
FUTURE WORK
The FoM methodology is fairly unique since it identifies promising mission architecture from
the initial planning stages before focusing on more detailed design. The spreadsheet is configured
for a MSR mission, and since a mission of this type is only in the planning stages at this point in
time, it would be ideal to analyze any preliminary results the FoM can provide. The previous results
are interesting in their own dght, but a much more detailed analysis is still to be accomplished. All
the propellants in the database need to be analyzed to determine possible hidden advantages in
an overall framework. In addition, many of the spacecraft design variables should be varied to help
focus on a more beneficial design. Both strong and weak correlations need to be identified between
related input parameters, and further overall optimization needs to be accomplished.
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REFERENCES
1. K.N.R. Ramohalli, B. Preiss, and T. Kirsch, "A Figure-of-Merit Approach to Extraterrestrial
Resource UtUization," paper no. IAA 89-716 at 40th Congress of the International Astronautical
Federation, 1989, also accepted in AIAA Journal of Propulsion and Power, 1990.
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FoM
MARS MISSION VARIATION3
Figure-of-Merit vs. Returned Mass
8
6
4
2 _- Io _
2 loo looo loooo
Mass returned to LEO, kg
H2÷O2 (All-Earth Carried) _ H2÷O2 (ISRU 02)
r--q CH4,O2 (ISRU 02) _f"//-J'/_ H2.O2 out/CH4÷O2 in (ISRU O2)
LEO • 500 km altitude
ISRU • In-sltu resource utilization
I FoM - [(M sample)/(M LEO)] • R lactor • 104 [
I
R lactor • Inverse Risk • Reliability • Reparability I
MARS SAMPLE RETURN VARIATIONS
Sample Mass • 4 kg
FoM
0.7
0.6
0.5 i
0.4
0.3
.2
0.1
0
7
I
MOD
ISRU
ISRU MOO ISRU/MOD
Mission configurations
H2.O2 outbound/CH4*O2 Inbound _ CH4.O2 (All Stages)
• Moduler engine use
• In-sltu resource utilization


Development of a figure-of-merit for space missions

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