Anik E Spacecraft Life Extension

1
Anik E Spacecraft Life Extension

• Presented by Alex Yip, Telesat Canada, Satellite
  Control Division
• 2004 Space Ops Conference, Montreal QC

2
Anik E End of Life Determination

• The baseline method of determining End of Life
  (EOL) on the Anik Es at launch was the
  book-keeping method.
• Newly improved thermal propellant gauging system
  (PGS) approach provides significant improvement
  in mass estimates at the EOL.
• Thermal PGS uses response of tank temperatures to
  known heat input to gauge fuel remaining (tank
  thermal capacitance).

3
Anik E EOL Determination

• Thermal gauging accuracy improves as EOL
  approaches but requires several data samples.
• Book-keeping accuracy decreases at EOL.

4
Anik E Propulsion System

• Four tank N2H4 system (heritage S5000 design).
• No isolation valves between tanks

5
Fuel Distribution Between Tanks

• The first propellant gauging tests revealed a
  large fuel imbalance between tanks on both
  satellites.
• A total of 21 PGS tests (9 on E1 and 12 on E2)
  were performed over a 24 month period in order to
  keep track of fuel consumption and fuel
  distribution.
• One fuel tank on both Anik E1 and E2 was getting
  close to being empty.
• Unchecked this would lead to early depletion
• Maneuvers would then be no longer possible
  leading to an unacceptable early end of mission
  life.

6
Thermal PGS Approach

• Factors to consider in implementation
• Environmental temperature of each tank
• Thermal transport from tank (radiative
  conductive)
• Empty tank data improves accuracy of PGS
• Properties can be derived form cool down curve
• Thermal transport within the tank (fluid and
  wall) due to convection and conduction
• Heat input characterization (including voltage
  losses)
• Temperature sensor errors
• Temperature limits of components (tanks heaters,
  etc.)
• Fluid transport during PGS will affect results
  significantly

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Thermal PGS Approach

• Tank internal heat transfer
• Detailed SINDA/FLUINT model created to model
  these processes.
• Fluid distribution based on static 3-D fluid
  interface model (Surface Evolver)

8
Thermal PGS Approach

• Fluid transport
• Fluid movement due to differential heating of
  tanks can affect measurement.
• Telesat developed tank heating software that
  cycled tank heaters to match the temperature rise
  and fall of the slowest tank (i.e. tank with
  greatest propellant mass).
• Temperature differentials were also used to
  rebalance tank propellant mass
• Results of PGS used to determine required
  temperature differential to tank with most
  propellant.

9
Implementation

• Differential heating will control a tank
  temperature to XC above the reference tank
  temperature.
• The X value will be different from tank to tank
  to accommodate the fuel mass differences between
  tanks.
• The X value will change slowly from season to
  season, as the reference tank cools down and
  warms up, so the tank qualification temperature
  is not exceeded.

10
Implementation

• There is a dead band placed around the
  differential set point to reduce the duty cycle
  of the heaters. The nominal dead band is 1C.

(x)
11
Implementation

• Original PGS New PGS (matched Ts)

12
PGS Analysis

• Each tank thermal response was modeled to
  determine propellant mass
• Iterative approach to solution for each tank

13
PGS Results

• PGS results compared very well to book-keeping.
• Results of PGS used to establish thermal
  differentials that nearly balanced propellant
  load in tanks.

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Summary

• PGS testing provides an alternate method to
  book-keeping for fuel measurements with the
  advantage of providing fuel distribution between
  tanks.
• Multiple fuel tanks system are difficult to
  balance especially toward the end of life.
• Differential heating was successfully used to
  achieve the planned satellite mission life.
• Fuel life calculations improved with the number
  of tests and a good correlation with book-keeping
  results was achieved.