Preliminary Radioisotope Powered Engine Analysis for Mars Flight Applications

1
Preliminary Radioisotope Powered Engine Analysis
for Mars Flight Applications

• Chris Miller, John Joyce, Ben Schreib, Holly
  Szumila
• Center for Space Nuclear Research
• August 10, 2007

2
Contents

• Advantages of radioisotope power for Mars flight
• Engines selected for analysis
• Details and analysis of each engine
• Initial findings

3
Radioisotope Powered Flight

• Offers potential for months or years of
  continuous aircraft operation
• Utilization of radioisotope decay heat instead of
  combustion allows operation in oxygen-free
  environment
• Attractive for unmanned aircraft exploration
  missions in Martian atmosphere

4
Engines Considered

• Turboprop
• Used in low power, low speed Earth aircraft
• Most (90) of thrust from propeller
• Propeller efficiency decreases at high flight
  velocities
• Turbojet
• Used in high speed Earth aircraft
• Thrust generated from expansion of high pressure,
  high temperature exhaust gases in nozzle
• Ramjet
• Used in supersonic Earth aircraft, missiles
• No moving parts
• Thrust generated from expansion of high pressure,
  high temperature exhaust gases in nozzle

5
Turboprop

• Rise in stagnation pressure, velocity across
  propeller, generates thrust (1-2)
• Rise in static pressure in compressor (2-3)
• Energy (heat) added to gas in heat exchanger
  (3-4)
• Work extracted from gas in turbine to drive
  compressor and propeller (4-5)
• Gas expanded to ambient pressure in nozzle,
  generates thrust (5-6)

6
Turbojet

• Flow velocity reduced to 0 in diffuser (1-2)
• Rise in static pressure in compressor (2-3)
• Energy (heat) added to gas in heat exchanger
  (3-4)
• Work extracted from gas in turbine to drive
  compressor (4-5)
• Gas expands to ambient pressure in nozzle,
  generates thrust (5-6)

7
Ramjet

• Velocity reduced to 0 in diffuser, stagnation
  pressure recovered from high dynamic pressure
• Energy (heat) added to high pressure gas in
  burner
• Gas expanded to ambient pressure in nozzle,
  generates thrust

8
Mars Mission Parameters
9
Engine Specifications and Assumptions

• Turboprop
• 2 m propeller diameter
• 90 of thrust from propeller, 10 from nozzle
• 80 propeller efficiency, all other devices
  isentropic
• Adiabatic everywhere except heat exchanger
• Engine inlet diameters 0.25 m, 0.5 m, 0.75 m
• Compression ratio 10
• Turbojet
• Isentropic everywhere
• Adiabatic everywhere except heat exchanger
• Engine inlet diameters 0.5 m, 0.75 m, 1.0 m, 1.25
  m
• Compression ratio 10
• Ramjet
• Isentropic everywhere
• Adiabatic everywhere except heat exchanger
• Engine inlet diameters 0.5 m, 0.75 m, 1.0 m, 1.5
  m
• All engines
• Atmosphere behaves as ideal gas
• Exhaust gases perfectly expanded in nozzle

10
Required Turboprop Heat Input
11
Required Turboprop Heat Exchanger Temperature Rise
12
Required Turbojet Heat Input
13
Required Turbojet Heat Exchanger Temperature Rise
14
Required Ramjet Heat Input
15
Required Ramjet Heat Exchanger Temperature Rise
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Required Heat Exchanger Power Input
17
Required Temperature Rise
18
Future Work

• Integrate overall aircraft design to provide more
  accurate overall mass
• Perform engine analysis incorporating reasonable
  efficiencies for all stages
• Evaluate engine designs on mass basis
• Analyze flow through heat exchanger to form basis
  for heat exchanger design

19
Conclusions

• 1.5 m diameter ramjet engine best choice based
  solely on power and temperature rise requirements
  at all but lowest flight velocities
• Ramjet attractive relative to turbomachinery
  options on basis of no moving parts
• To reduce amount of radioisotope material
  required in each engine, two engines (one per
  wing) could be an option
• Two engines may be advantageous if turbomachinery
  is used to balance aircraft

20
Questions?
21
The Human Factor in UAV assembly and Operation

• M. David Keller, MA
• Center for Space Nuclear Research

8/10/07
22
Conceptualizing Error

• Humans are natural error emitters
• On average we make around 5-6 errors every hour
• Under stress and fatigue that rate can increase
  dramatically
• Most errors are inconsequential or mitigated
• No consequences or impact from many mistakes made
• Where there may be consequences, many times
  defenses and recovery mechanisms prevent serious
  accidents

23
Probably wont cause any accidents
24
Consequences of error can be much greater
25
The Human Factor

• The human plays a major role during UAV assembly
  and operations (even if the system is fully
  automated).
• E.g. Supervisory Control
• In order to reduce the risk of human error it is
  necessary to understand how humans may perform
  given the unique set of challenges of this type
  of mission.

26
Limited Communication

• Due to the long distance, communication becomes
  difficult
• Time Delay
• Limited Bandwidth

27
New Systems

• A new system will require the development of new
  control interfaces with new Procedures and
  Training

?
28
Challenging Materials

• Materials for the power source will be
  radioactive making tasks such as assembly of
  engine difficult.

29
Designing for the Human

• Safe and reliable human-machine interaction
  through good design
• Reduce Risk due to Human error
• To the Human
• To the System

30
Human-Machine Interface

• Ways to optimize the human-machine interface
• Improving system feedback
• Designing displays and controls to ergonomic
  standards
• Improving clarity and accuracy of procedures
• Examples
• automation, streamlining communications,
  labeling, alarm presentation, alarm filtering,
  and administrative controls (Gertman Blackman,
  1994).
• Affects of optimization on human performance must
  first be understood in the appropriate situation.

31

• The fuel lights on, Frank! Were all going to
  die!...Were all going to die!..Wait, wait...Oh,
  my mistake - thats the intercom light.

32
Procedures and Training

• Well Defined
• Requiring minimal interpretation by the human
• Highly Practiced
• Operators should become very familiar with every
  task to perform
• Minimum number of tasks at one time
• To avoid high workload and mental strains
• Applied to normal assembly and operations but to
  emergency cases as well.

33
Procedures and Training

• Context Specific Example
• Manual Assembly of the Engine
• Material already radioactive
• Procedures must be practical for humans given the
  safety requirements (working with shielding and
  visual impairments)
• Must include the use of specialized tools

34
Predicting Risk from Human Error

• Predicting risk in these new situations, based on
  existing methods and design principles is
  difficult.
• Validating these new human-machine system designs
  is important.
• Validating can be done by way of usability
  studies and applying Human Reliability Analysis
  (HRA) methods.

35
Usability Studies

• Means for measuring how well people can use some
  human-made object for its intended purposes.
• Given that performing these studies in the real
  environment is practically impossible until the
  real mission is underway, all effort should be
  made to simulate environmental conditions as
  accurately as possible and prototype realistic
  interfaces.

36
Human Reliability Analysis

• HRA attempts to
• Render a description of human contribution to
  risk
• To identify ways to reduce that risk

37
Specifically, HRA Can Be Used To

• Compare alternate design configurations
• Predict the relative human performance expected
  of a system
• Diagnose factors in the system leading to
  undesirable human performance
• Identify improvements in human performance
  resulting from design changes or proposed
  tradeoffs
• These are accomplished by considering performance
  shaping factors (PSF) that influence human
  performance and consequently risk, given a
  specific task.

38
Performance Shaping Factors

• PSFs can include topics such as
• Available Time
• Stress/Stressors
• Complexity
• Experience/Training
• Procedures
• Ergonomics/HMI
• Fitness for Duty
• Work Processes
• Each PSF should be considered carefully in the
  context of the proposed task in order to
  accurately determine and reduce risk.

39
Conclusion

• Flying UAVs on other planets presents new
  challenges for human operators
• The design of these new systems must take into
  account these challenges and how they will affect
  human performance.
• In order to identify the risks using in proposed
  designs, usability studies and HRA methods should
  be employed.

40
Questions?
41
Assembly of Remote Piloted Vehicle

• Brian Gross, John Joyce,
• Jeff Perkins and Joel Sasser
• Center for Space Nuclear Research
• August 10, 2007

42
Outline

• Objectives
• Launch Vehicle Selection
• Fabrication Process
• Cooling
• Radiation Shielding
• Conclusions

43
Objectives

• Constrain RPV to most economical launch vehicle
• Obtain optimum fuel element arrangement
• Limit exposure to fabrication personnel
• Develop cooling system for in-transit heat
  rejection
• Maintain radiation containment upon launch abort

44
Launch Vehicle Selection
Current US Launch Vehicles

• Athena
• Atlas
• Delta
• Space Shuttle
• Taurus

http//www.nasa.gov/centers/kennedy/launchingrocke
ts/vehicles.html http//www.lockheedmartin.com/dat
a/assets/13431.pdf
45
Fabrication Process

• Engine Parts Fabrication
• Can fabricate majority of parts outside of hot
  cell ()
• Assembled engine testing must be performed in hot
  cell or with shielding in place
• Airframe Fabrication
• Use carbon fiber to minimize mass
• Use biplane wing design to minimize overall
  volume
• Fold front wings backward and rear wings forward
  to minimize length

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Fabricate fuel elements
Fabricate jet inlet, nozzle, and sheath
Load fuel inside of hot cell
Assemble jet engine inside of hot cell
Place jet in transportation cask
Transport jet to KSC
Qualify jet for Use in space
Fabricate airframe
Final assembly of engine to airframe
Place in Delta II payload bay
Radioactive Materials Present
Non-Rad Work
Launch to Mars
47
Fabrication Process

• Radioisotope Fuel Elements
• Geometry of fuel elements should be optimized for
  ease of handling, durability, and heat transfer
• Teardrop shaped fuel elements
• Cylindrical grate design
• Ferris wheel design
• Bicycle wheel and spoke design
• Plate fuel elements
• Finned fuel elements

48
Fabrication Process

• Cylindrical Grate Design
• Multiple rods connecting at two points of the jet
  interior
• Disadvantages
• Longer rods more prone to bending
• Fretting issues from vibration

49
Radial Rod Design
Front view Conceptual radial rod design
Side view (a) Bicycle wheel design (b) Ferris
wheel design
50
Airflow Direction
(a)
(b)
(d)
(c)

• Air inlet
• Center shaft or turbine shaft
• Heating chamber with spoke fuel elements
• Exit nozzle

51
Cooling

• Need to remove 10KWt from inside the heat shield
  due to running engine
• Proposed Concept
• Active Cooling System employing gaseous He
• Pipes will be attached to plenums at each end of
  the RPV engine and routed through the heat shield
  
• Heat removed from engine is rejected by a
  radiator

52
Cooling
Heat Shield

• Orbit Insertion Procedure
• Disconnect piping
• Re-establish heat shield integrity
• Begin orbit insertion
• Open heat shield after successful reentry
• Jettison plenums and vent He into atmosphere

Engine
Radiator
Plenums
Pump
53
Radiation Shielding

• Gamma stopped by high Z, high electron density
• Tungsten
• Acceleration of charged particles produces
  Bremsstrahlung radiation
• High Z creates more due to electron density
• High scattering cross section attenuates neutrons
  
• Lithium hydride
• Thermal neutrons absorbed by large thermal
  neutron cross section materials
• Gadolinium-157, Boron-10

54
Radiation Shielding

• 16.5 kg Cm2O3 required for 40 kWt
• Unshielded dose 8.6 rem/hr
• 300 kg LiH 50 mrem/hr
• 90 kg LiH and 190 kg of a Gd/U mixture 100
  mrem/hr
• Pure LiH may be more attractive than including
  Gd/U if volume is not important

Above RPV source model (MCNP VisEd)
dose cylinder and isotope shielding Left Dose
vs. Mass of pure LiH shielding
55

• Delta II should be capable of launching RPV
  mission
• Bicycle wheel seems to be optimum fuel
  arrangement
• Active Helium Cooling system is required due to
  continuously running engine
• RPV packaged into reliable rocket and fuel
  surrounded by robust cladding should mitigate
  contamination upon a launch abort

56
Questions?
57
Materials and Systems Integration

• Jeff Perkins, Joel Sasser, Brian Gross,
• Jeff Katalenich, Chris Miller, Jon Webb
• Center for Space Nuclear Research
• August 10, 2007

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Contents

• Aerospace, nuclear and safety requirements
• GPHS for RPV
• New radioisotope fuel element designs needed
• Radiation considerations
• Environmental and materials considerations
• Materials processing methods

59
Engineering Requirements

• Bottom line AE want current UAV design, NE want
  GPHS design, SE want survivability

60
GPHS Shortcomings

• Designed for long-term radiation of heat, not
  convection
• GIS shell excellent for accident scenario, but
  GPHS not aerodynamically suited for RPV
• Only designed for Pu-238
• Other radioisotopes require shielding other than
  Ir clad and graphite
• Power output relative to size is unacceptable for
  RPV

61
New Fuel Element Geometry Needed

• Maximize surface area for heat transfer to
  atmosphere
• Radiation shielding of fuel
• Robust design to survive accident

62
Radiation Considerations

• Beta emitters must be shielded by low-Z material
  encased in high-Z material
• ?- high-Z Bremsstrahlung radiation
• Alpha emitters must be encased in porous material
  to allow helium to escape
• Neutrons must be captured (B, Gd, Hf) or
  reflected back to radioisotope (Be)
• Gamma radiation can be mitigated by high-Z
  materials
• Material resistant to radiation-induced
  dislocations

63
Radiation Dose From RPV Fuel

• 16.5 kg Cm2O3 required for 40 kW (thermal)
• Unshielded dose 8.6 rem/hr
• 300 kg LiH 50 mrem/hr
• 90 kg LiH and 190 kg of a Gd/U mixture 100
  mrem/hr
• Pure LiH may be more attractive than including
  Gd/U if volume is not important

Above RPV source model (MCNP VisEd)
dose cylinder and isotope shielding Left Dose
vs. Mass of pure LiH shielding
64
Environmental and Material Considerations

• Corrosion resistant to air and Martian atmosphere
• Sweet corrosion of metals by carbonic acid
• CO2 H2O ? H2CO3
• Requires gt50 ppm of H2O, Mars 300 ppm H2O
• Erosion resistant to particulates in Martian
  atmosphere
• Material at constant high T
• T gradient between centerline and surface of fuel
• Recrystallization, grain growth, crystal
  structure
• Ficks Law Diffusion is proportional to T

65
Materials Requirements
66
Materials Fabrication Techniques For RPV Fuel

• Heavy reliance on newer methods of fabrication
• Non-equilibrium alloys must be formed
• Ex. Cu-W-LiH-Stainless w/ ceramic composite
• Refractory metals (W, Ta, Re, Nb) and ceramics
  cannot be casted
• Newer techniques allow shorter exposure time for
  radiation workers
• Design tolerances met by better engineering
• Older methods still very useful for
  post-processing
• Surface treatments of radioisotope fuel elements
• Integration of radioisotopes into engine design

67
New Materials Systems and Technology

• State-of-the-art technology benefited by
  production of RPV radioisotope fuel elements
• Understanding of new materials systems
• Materials systems for lunar surface power
• Applications to terrestrial systems
• Better engineered materials
• RPV will rely on new fabrication technology
• Allows study of Mars, and potentially Venus,
  Jupiter, Saturn, Uranus, Neptune

68
Questions?