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“DESIGN OF A FOOD WARMER FOR A MARS TRANSIT MISSION”
PROJECT MID-TERM REPORT
For
Texas Space Grant Consortium
By
Brandon Henry
“Team Spacewalkers”
Team Members Allen Omardeen
Jack Ranken Tim Crispin
Brandon Henry
INDE/MECE 4334 Capstone Design IV
Spring, 08
iii
Table of Contents
Table of Contents............................................................................................................... iiiTable of Figures ................................................................................................................. iiiTeam Spacewalkers ............................................................................................................ 1Introduction and Background ............................................................................................. 2Statement of Goals.............................................................................................................. 4Progress Description ........................................................................................................... 5Cost Analysis .................................................................................................................... 14Scheduling ........................................................................................................................ 15Conclusion ........................................................................................................................ 16
Table of Figures Figure 1: Team Spacewalkers patch ................................................................................... 2Figure 2: Food pouches used by NASA (picture provided by NASA) .............................. 4Figure 3: Existing food warmer (picture provided by NASA) ........................................... 4Figure 4. Side view of Design 1 ......................................................................................... 6Figure 5. Top View of tray for Design 1 ............................................................................ 7Figure 6. Isometric and side view of Design 2 ................................................................... 8Figure 7. Wire racks for Design 2....................................................................................... 9Figure 8. Third design food warmer. All dimensions are in inches.................................. 11Figure 9. Concept Art of Design 3................................................................................... 13Figure 10: Updated Gantt chart of the project schedule ................................................... 15
1
Abstract
The current mid-term report was prepared by team Spacewalkers (Team 4), which
consists of Allen Omardeen, Jack Ranken, Tim Crispin and Brandon Henry. The
University Houston (UH) faculty advisor is Jagannatha Rao and the engineer-in-charge at
NASA is Dr. Michele Perchonok. This project is conducted as part of the Texas Space
Grant Consortium (TSGC) Design Challenge. The lead coordinator for TSGC is Debbie
Mullins. Developing a food warmer is crucial to mission success. The journey to Mars
will take up to 6 months for a one way trip. The food warmer is required to heat up to 12
packages per meal prior to consumption. In addition, specific constraints for weight, size
and temperature exist. The weight and size requirements of 13 pounds and 1 cubic foot
respectively were achieved by last semester’s design team, but the main challenge is to
attain a temperature of 155-175°F for heating the food packages in less than 25 minutes
using minimal power of only 280 watts.
The team has already developed several ideas to modify the current design and
this may include, but is not limited to, modifying heating source, redirecting airflow, and
a possible modification to the outer casing. The team is currently using COMSOL
software to analyze the system. Initial analysis for the food packet was generated using
COMSOL, 2D convection and conduction transient heat analysis. COMSOL models
verified that the Fall 2007 design was not able to meet the time requirements of 25
minutes. The team is on schedule to meet the deadlines for validation and verification
testing of the new design. Currently, there are no obstacles that will prevent meeting the
deadline for the final report on April 14th, 2008.
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Team Spacewalkers
Team Spacewalkers carefully choose a patch (Figure 3) that best symbolizes all
the elements of this year’s Design Challenge.
Figure 1: Team Spacewalkers patch
The school logo dawns in the center of the patch to show from where team Spacewalkers
come. The three acronyms in the middle show the three groups responsible for bringing
together the 2008 Design Challenge. The words “CHALLENGING THE FUTURE” at
the bottom of the patch represents the spirit of engineering. Progress is made by going
beyond the norm and challenging the notion that certain things just can’t be done.
Introduction and Background The current project is being conducted by conducted by team Spacewalkers
(Team 4), which consists of four team members: Allen Omardeen (Industrial
Engineering), Jack Ranken (Mechanical Engineering), Tim Crispin (Mechanical
Engineering), Brandon Henry (Mechanical Engineering). The faculty advisor is
3
Jagannatha Rao and the engineer-in-charge at NASA/Johnson Space Center is Michele
Perchonok. This project is being conducted as part of the Texas Space Grant Consortium
(TSGC) Design Challenge coordinated by Debbie Mullins and as a requirement for
graduation by the University of Houston (UH) Mechanical and Industrial Engineering
department’s Capstone Design course.
The Fall 2007 UH Capstone Design Team developed a 2nd generation food
warmer to be used during trips to and from Mars. The food warmer had to meet several
requirements set forth by NASA which included: it must be able to heat 12 food packets
to 155-175F within 25 minutes, it must have a maximum volume of 1 cubic foot, it
cannot weigh more than 13 pounds, and it can only use 280 Watts power. The Fall 2007
team was able to meet all of the requirements, except for the 25 minute time limit.
Spacewalkers objective is to take the Fall 2007's design and analyze it to determine its
deficiencies and recommend several modifications for improving the design. Material
cost and vibration analysis will be considered but will not be extensively covered.
The project is proposed by NASA’s Space Human Factors and Habitability
(SHFH) project at Johnson Space Center and is sponsored by the TSGC. During long
space flights, NASA uses a quad-laminate pouch made of polyolefin, aluminum foil,
polyamide, and polyester to store meals in as seen in Figure 1. The packets are stored at
4
Figure 2: Food pouches used by NASA (picture provided by NASA)
room temperature and during meal times are warmed and must be consumed within 60
minutes of heating in order to adhere to safety standards. The current version of the food
warmer as seen in Figure 2, takes about 40 minutes to warm the food packets and is
bulky. Also, the vehicle for which the food warmer is designed is smaller than the
Figure 3: Existing food warmer (picture provided by NASA) current shuttle. Microwave technology cannot be used because the microwaves can
interfere with communications and sensitive on-board equipment.
Statement of Goals
The main goal of this project is to develop a working prototype of NASA Food
Warmer. To achieve this goal, the following milestones are used to track the progress
throughout the project lifecycle:
Milestone 1: Analysis of current prototype
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• Meet with person involved in last years’ NASA Food Warmer design project
• Analyze pros and cons of previous design
• Determine needed modification to current design
Milestone 2: Analysis of heating system
• Test and collect data in lab on current heating method
• Investigate alternative power consumption methodology
• Re-visit internal size and shape requirements
• Develop improvements for current design
Milestone 3: Validation and verification testing
• Implement modifications to current prototype
• Construct re-designed prototype
• Verify prototype up to specification
• Test and collect data in lab on revised heating method
Milestone 4: Final report and presentation
• Generate final technical report and presentation
• Submit final technical report and presentation
Progress Description
Three major modifications were made to the Fall 2007 design team’s prototype
and are described below. All three designs will be constructed of an aromatic (aramid)
polymide fiber and carbon fiber as the inner and outer shell and supported by a thin
aluminum frame. Carbon fibers are strong and light weight and aramid fibers can
withstand high temperatures. Both materials, however, are expensive to purchase. A full
6
body carbon fiber frame is expensive and takes a long time for production and may be
considered at a later date if weight requirements cannot be met.
Design 1
The first design (Figure 3) will take advantage of conduction and forced
convection. The design consists of an outside chamber (case), heating element,
convective fan, insulation, an electrical circuit to control the fan, heating elements, and
thermostat. Concerning convection, the heat is fan-circulated from a dedicated
convection element in the rear and the side of the oven cavity to break the thin layer of
air, which breaks the “air insulation”. The fan motor will be placed in a separate section
to avoid overheating. The main heating element will be placed at the bottom and side of
the oven.
Figure 4. Side view of Design 1 By installing the heating element as part of the oven, conduction will play a significant
role in heating the food. This is one of the simple heating methods used in regular ovens.
7
The combination of conduction and forced convection methods make this design more
efficient. The following is a detail of the trays:
• Dimension of the 3 trays (see Figure 4) • Width 7” • Length 16” • Thickness 0.125” • Holes are 1” in diameter and 1” in between them with 1” at each end. The holes
were designed to assist in the air flow.
Figure 5. Top View of tray for Design 1
The fan will be installed at the backside of the oven. The thickness of the fan is 1”, which
is accounted for in the dimensions listed above.
An aluminum fan, usually used in regular microwave ovens, will be used.
Aluminum was selected due to the light weight of the device and the temperature inside.
The fan motor selected will have four-pole motors. The team is currently searching for
the best motor to fit the device. Due to power limitation, 4 cartridge heaters will be used.
Two heaters will be placed on the sides of the oven, as well as one at the bottom and one
facing the fan in the back of the oven. The heating source that will be used is a 1/8"
diameter by 1-1/2" long SunRod Split Sheath cartridge heater (24V, 60W, 9.6 ohms).
Design 2
The second design (Figure 5) is based on the idea of heating the food through
forced convection and conduction. The forced convection concept will be implemented
by using the fan motor. The conduction concept will be implemented by placing the
heating
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Figure 6. Isometric and side view of Design 2 element in contact with the oven racks. Convection ovens or fan ovens augment a
traditional oven by circulating heated air using a fan. The fan motor is in a separate
enclosure to prevent it from overheating. Food warms faster in a convection oven since
the moving air strips away the thin layer of air, which otherwise would surround and
insulate the food. By moving fast hot air past the food, convection ovens can operate at a
lower temperature than a standard conventional oven and yet can cook food more
quickly. The air circulation, or convection, tends to eliminate "hot spots" and thus food
may bake more evenly. The two fan motors and the heating elements are connected in
parallel with the power supply through connecting wires attached to the outer motor
casing and the inner side walls of the oven respectively.
Each heater will draw 60 Watts of power and the fan motors will draw 50 Watts
each, bringing the total power used to the 280 Watts limit for this conceptual design. The
power will be submitted to all elements of the circuit at the same time through a
9
connection at the side of the food warmer. The two motor fans are installed in the center
of the back side of the oven. This is done to force the airflow circulation in a horizontal
plane within the oven cavity to minimize the potential for airflow paths to be blocked by
the objects placed in the oven.
There are three wire racks (Figure 4) in the oven to hold 12 food pouches. Each
rack consists of 2 major parts: the bottom part is used to carry the food packets; the top
part holds the food packets in place (like a lid) due to zero gravity. A wire rack is used to
minimize weight and to allow for more heat exposure to the food pouches.
Figure 7. Wire racks for Design 2
The heating elements are custom built high-resistance wires. Nichrome is used to
make the resistance wires because it has a high resistivity and resistance to oxidation at
10
high temperatures. Two aluminum fan motors are used for airflow circulation.
Aluminum fans are used because they can withstand heat and are light in weight. There
are two return vents, which are located on the side walls of the food warmer. For internal
insulation, Fiberfrax will be utilized. Fiberfrax ceramic fiber products are manufactured
from alumina-silica materials and offer such characteristics as high-temperature stability,
low thermal conductivity, low-heat storage, excellent thermal shock resistance, light-
weight, and superior corrosion resistance. Fiberfrax ceramic fiber products exhibit
thermal stability at temperatures up to 1260°C (2300°F).
Design 3
The third design (Figure 5) entails the use of pure convection. This design uses
three rows of five thin aluminum fins with a heating coil embedded in the fins. Each fin
is slightly smaller than the approximate size of each food packet to reduce excess weight
and to assure that contact with the heating area is maximized. Durablanket insulation is
used to prevent heat escape. The exterior walls are made of carbon fiber, which is light
in weight and durable. The interior walls are lined with aluminum foil to take advantage
of (and contain) any radiation heat.
11
Figure 8. Third design food warmer. All dimensions are in inches.
Aluminum was used for the rack because it is light in weight, conducts heat well,
and is cheap as compared to other metals of similar densities. Aluminum is also readily
available. Fin theory controls how efficient heat can be transferred to and from the fins.
The fins need to be thick enough to encase the heating element and thin enough to
minimize the weight of the rack. The spacing between the racks will snugly hold the
food packets in place during heating. Since the fins are slightly smaller in stature than
the each food pouch, the edges of the pouches can be grabbed after cooking without
touching the aluminum rack.
A 0.125” diameter by 223” long Watlow cable heater will be used as a heating
source. It has a maximum wattage of 240 and can sustain temperatures of 630˚F for
extended periods and is capable of a wide range of voltages. Of course the maximum
wattage will not be used due to power needed for other components and a 280 Watt limit.
With an aluminum frame (1” trim), the weight of this design comes in at about 15.2
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pounds. Using a completely carbon fiber body will reduce the weight by 3 to 4 pounds.
The volume for the outer case comes in at 1691 in3 which is just under one cubic foot
(1728 in3). All of the specifications given by NASA are met by this design. Also, initial
calculations predict the warm time of about 27 minutes. This is slightly over the required
time, therefore a further evaluation will need to be conducted. Ideally a lab test would
confirm the results, but time may not permit that.
Final Design
Out of the three designs mentioned above, a simple analysis was performed using
a Pugh chart (see Table 1). None of the designs were able to meet the weight
Table 1: Pugh Chart. 0=Does not meet requirements, 1=Meets requirements, 2=Exceeds requirement
CriteriaImportanceWeight
CurrentFood
Warmer Design1 Design2 Design3
HeatingTime 0.26 0 1 2Weight 0.18 0 0 0
Size 0.18 1 1 1
RequiredPower 0.18 1 1 1
EaseofUse 0.15 1 2 2
AvailabilityofMaterials 0.05
DATUM
2 1 2
TotalPoints 0.61 0.97 1.28
requirements, which means some work will need to be done to lighten the food warmer.
Design 3 was chosen based on the Pugh score but in the following weeks some tweaks
will need to be made. For example, the weight will need to be reduced. Below is a
concept design of Design 3.
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Figure 9. Concept Art of Design 3.
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Cost Analysis
Table 2 shows the breakdown of cost to potentially build the third design from
scratch. Up to this point no money has been spent. In the next few weeks the team is
planning to start purchasing the materials needed to build the agreed upon design. The
decision to select the appropriate design should be accomplished no later than March 16,
2008. The total cost listed in Table 2 is a rough estimate based on core parts alone. Man
hours and cost of producing the model itself is not included. The cost for the other two
designs is not listed because the cost was much more than Design 3. Most components
were priced through Grainger [3] and Watlow [4].
Table 2: Design 3 cost analysis
Items Description Cost
Construction 6+ Man Hours and Cost of Construction $250.00
Aluminum 2 x 6’Aluminum Flashing, 1” trim, 0.125” Thick $65.00
Carbon Fiber 1 Yard US Composites Carbon Fiber, 5.7oz x 50" Width
$46.00
Axial Fan Motor Square AC Axial Fan, Air Flow 105 CFM, Voltage Rating 115 Volts, Width 4 11/16 Inches, Speed 2900 RPM, Current Rating 0.18 Amp, Bearing Sleeve
$24.90
Heating Element 2 xWatlow Cable-Coil heater, Length 30”, Voltage 120, Watts 175
$160.00
Insulation Auralex 2" Mineral Fiber Insulation - 24" x 48" x 2" Sound Absorption and Insulation Panels - 6 Pieces
$119.00
Miscellaneous + Project Reserve
Bolts, nuts, accessories, etc. $100.00
Total $764.90
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Scheduling
The team remains on schedule for the overall project. The project is expected to
be completed by the deadline of April 14, 2008. The updated Gantt chart is provided in
Figure 10: Updated Gantt chart of the project schedule
As can be seen from the Gantt chart, the team has decided on a final design. This
is expected to be completed by March 16, 2008, as indicated on the chart. Concurrently,
other tasks are being completed, because of the team’s structure and flexibility. The
project is expected to be completed by the deadline of April 14, 2008.
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Conclusion
The team remains confident about the successful completion of the project by the
deadline of April 14, 2008. Progress is being made in multiple areas of the project. Over
the past four weeks, the team has met with NASA enginner-in-charge Dr. Michele
Perchonok, and contacted UH faculty advisor Dr. Jagannatha Rao to discuss the project
parameters and to gather all necessary information and recommendations needed to
proceed. The team has gathered background information about the project. Additionally,
the team has studied and analyzed the current design that was developed by last
semester’s Capstone Design team. Due to the limited time that the team has, three
different designs were obtained to allow the team to select one or a combination of any of
the three designs to save time. Concurrently, other tasks were completed through the
team’s structure and flexibility. As of now, the team is expected to complete the new
design by the deadline of April 14, 2008.
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Appendix A
Budget Report
No funds have been spent toward this project at this point. There are plans to
build a prototype, but only if time permits.
Team Trip Report
The team has not made any field trips due to the tight schedules of each member.
Also, two of the members work at the facility of the engineer-in-charge. The project did
not facilitate ant field trips at this point.
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Reference
[1] Design of Food Warmer for a Mars Transit Final Technical report; D. Perez, I. Castro,
T. Stoner, E. Lopez; 2007
[2] A.F. Mills, Heat Transfer, Prentice Hall, Inc., Second edition, 1999.
[3] www.grainger.com
[4] www.watlow.com