Space for the People

Optical Beacon

Xenon flash tubes are often used in photographic strobes and as warning beacons on the wing tips of airplanes. Our xenon flash tube meets the basic power, size, and cost requirements required for the cubesat. It has a high temporal energy density, i.e. large amounts of energy emitted in short bursts, with a flash brightness that corresponds to an apparent magnitude of 2 (similar to a bright star).

The operation of an XFT is dependent on the ionization of low-pressure Xenon gas within the flash tube followed by the cascade of energy across the bulb via excitation of the Xenon ions. First, a trigger voltage is applied to a small wire coil surrounding the bulb. This pulse is relatively low energy (several kV at ~.2 μF), but the high voltage allows the bulb itself to act as a capacitor. As charge accumulates on the inner wall of the tube, a small number of Xenon atoms are ionized. Now, a high-energy pulse is provided to the bulb anode via the main discharge capacitor, and uses the ionized Xenon as a medium to pass from the anode to the cathode. The large current excites the ionized Xenon and causes them to emit light. Some of this emitted light is then absorbed and ionizes the rest of the Xenon in the tube, thus creating even more conductive medium and lowering the overall resistance. As current continues to flow a large amount of light is emitted from the excited Xenon ions.

During each flash, power use briefly peaks at tens of thousands of watts, but for most of the life of the satellite is zero. The Xenon flash power budget therefore is analyzed from a total energy use standpoint rather than a power use analysis. Feasibility of a given design is assessed by a Joules in minus Joules out calculation for a given “cycle” scenario. In our case, one “cycle” is defined as two orbital periods in which flashing only occurs during the orbital “night” of the second period. This allows enough energy to be collected and stored in order to meet visibility requirements

Requirements:

  • Bright enough to be seen at 300 km by the unaided eye
  • Flash at least 10 times per overhead pass (126 seconds at a maximum 60 degree viewing angle and 300 km orbital height)
  • Must coincide with the power budget of the satellite
  • Components should be low-cost and off-the-shelf

Specifications:

  • Energy per flash: 30.9 J
  • 90% of the flash energy is emitted in 1/1100 seconds
  • 7 seconds between flashes (or 18 flashes per overhead flash)
  • currently designed using a 14mm Paul C. Buff replacement bulb

Requirements:

  • Robust enough to protect the fragile bulb, while still meeting the mass budget of the satellite
  • Extensible to be able to accomodate both linear and annular bulbs
  • Coefficient of thermal expansion compatible with the chassis to prevent undue stress during operation
  • Provide high reflectivity on the back side of the flash tube
  • Must fall within manufacturing guidelines of the project (no parabolic reflectors)

Specifications:

  • Machined from 6061 Al to ensure compatible thermal properties
  • Hemispherical reflector easy to machine and maximizes flash tube structural support (fasteners seen upper right)
  • Silicon O-rings decrease flash tube loads and prevent shorts in the trigger coil (mold seen lower left)
  • Integrated antenna deployment system
  • Provides thermal coupling and decreases thermal stresses in the bulb