Required for Specialized Missions

Some Might Call them 'Bots

The US Air Force has retiterated pushing development of a plan for the use of arrays of small satellites in orbit, that when formed together can be fleets performing tasks in concert for a variety of crucial tasks.

These missions include: searching for distant potential systems, or scanning gravity waves. They can be used for maneuvering around damaged satellites, with other specialized types deployed for the repair. On the visited satellites these smaller cousins can upgrade electronics and even refill propellant tanks. The only thing needed to fulfill this dream -- is the successful completion of the technology providing micropropulsion.

This new fleet of 25 kg-class spacecraft will need the propulsion systems that provide exact 10 micro-Newton range of impulse bits. The Propulsion Directorate is meeting that demand in their 100 gram miniaturized propulsion systems.

Micropulsed Plasma Thruster (MicroPPt)

This is the invention of Directorate scientists: an uncomplicated device for propulsive attitude control (there are a bunch here that could use that) . They would like to use about four of these on their 100 k. class orbitors. It has two desirable attributes:

Teflon® is not just for Cooking

The use of the solid inert propellant, Teflon® , should give high specific impulse when mingled with sped-up electromagnetics. It eliminates the need for propellant tanks and valves typical of previous gaseous and liquid fuels.

Composition of Lightweight Flight-tested Electronics

This MicroPPT is about the size of a PC mouse and uses self- consuming circuitry that starts with the electrical converter charging approximately 3000 Vattached to a cap and switch. This is connected to an electrode shell, with the rest of the circuit going to the propellant face. The excess of surface breakdown from pulsed voltage combines thermal and electromagnetic push with the ionized fuel to create movement by thrust.

The self consuming element of this ablating propellant can either leave the electrodes, or have them burn up, but only a shell, one way or the other, is left, reducing mass for the satellites' lifetimes. Once the rigid structures survive the violent take-off, they can be consumed in the less gravitational harshness of orbit.

Cable Gun Guy as Father

The heritage of devices as plasma source is traced back to cable guns, although similar in design to what is needed in the mini satellites, they are several times larger. They have a graphite coating on their Teflon® insulator face that acts as the plasma's material source that erodes, causing irregular behavior unless replaced. Several things have been borrowed, and other things abandoned, however:

  • Teflon insulator needs to be solely used for ablative material; space work prohibits graphite replacement.
  • 25 kilovoltage must be reduced to less than 8000 Volts.
  • Substitute spark gap switch with one that meets flight qualifications.
  • Reduce discharge energy from 188 J to 1-10.
  • Increase reliablity of discharges from 20 to over a million.
R & D Results

Commercial Off-the-Shelf (COTS) Sources

Directorate scientists have been focusing on design of propellant assemblies compatible with COTS electronics. They have experimental data that reveals fuel surface area and discharge energy parameters have to remain in a necessary window. Errors result either in too severe temperatures, or the making of a char layer -- and incomplete Teflon® decomposition and vaporization.

University of Michigan

The University of Michigan (there is more to life than sports) is tweaking the physics in this balance. Modeling and analysis have provided the one million discharges without failure demonstrations desired, but the hang-up is the quantity of the propellant source.

AFRL TechSat21 Flight

A relatively less complex version of the microPPT is aboard the Air Forces' TechSat21 flight where engineering and qualifications can be observed in closer to authentic situations.

Ensuing Advanced Laboratory Models

  • Less than 60 gram system dry mass.
  • 10 times increased propellant throughtput while utilizing less voltage initiating charge.
  • Bundled assemblies to increase propellant throughput, Same electrical components used on next assembly in chain.
  • Using same electronics package to fire different propellant assemblies especially for atttude control system (ACS) and different thrust vectors.

Overall Goals

  • Testing on ground and in flight showing 500 % improvement total impulse via thruster wet mass -- over the current best PPTs
  • Make or Beat Phase II's Integrated High-Payoff Rocket Propulsion technology program in the Electromagnetic Spacecraft Propulsion area.
  • See a reward of this endeavor for the ninety percent reduction in ACS mass for future and present lilluputian orbiting platforms.
Source: AFRL Technology Horizons, December 2001

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