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Archive for July, 2011

Elevator Pitch

Imagine you find yourself in an elevator with someone you know has the money and time to help start your new business.  You’ve only seconds to make your pitch, hoping to pique their interest enough to get a real presentation opportunity.  This is the “elevator pitch” concept.  People spend months refining their speeches and there are even high value competitions for these short bits of business-poetry.  What follows isn’t really poetry or even short, but was meant to start that journey.  Hopefully it will help explain our project.

Short Description
We’re build and flying a TubeSat-style satellite to test new effects on ion engine performance and power requirements.

Introduction
Ion engines are fantastically fuel efficient but their thrust is too low for general use.  By finding ways to increase thrust and lower power consumption, ion engines can be used for more mission profiles.  This is especially important when fueling a craft from expensive local resources such as on a lunar base.

Three new effects on ion engine performance will be tested by FRETS1 – my TubeSat-style satellite flying in early 2012.

The experiment tests three techniques:  one to increase thrust density beyond the traditional Child-Langmuir limit, another to reduce plasma formation power requirements, and one to reduce exhaust neutralization power requirements.

Supporting the experiment is an adaptive machine learning algorithm that maximizes the impact of the effects and adjusts alignment of internal parts to compensate for thermal effects and unwanted internal ablation.

A few details
Extracting charge from a plasma is governed by space-charge limits. In a low density plasma, the limit is described by the Child-Langmuir law:

where:

  • j is the current density (A/m^2)
  • e0 is the permittivity of free space (F/m)
  • e is the charge of a single electron or singly charged ion (C)
  • M is the mass of a single charge carrying electron or ion (kg)
  • V is the voltage drop across the extraction region (Volts)
  • d is the distance across the extraction region (m)

An ion engine’s Isp is also set by its voltage:

where:

  • Isp is the specific impulse (seconds)
  • v is the exhaust velocity (m/s)
  • g is earth gravitational acceleration (9.81 m/s/s)
  • e is the charge of a single electron or singly charged ion (C)
  • M is the mass of a single charge carrying electron or ion (kg)
  • V is the voltage drop across the extraction region (Volts)

Work by others on laser/plasma fusion has shed light on how to decouple these, raising the current density, and therefore the thrust density, without altering Isp.  Researchers have shown 1,000x the Child-Langmuir flow rates in lab conditions.

Adapting this work to ion propulsion and scaling it to realistic power requirements is a goal of this project.

Project Steps
The major steps needed, some of which are already well underway, are:

  • Initial concepts
  • Project feasibility
  • Market feasibility
  • R&D of Effects and Systems
  • 2 near-space balloon launches to test equipment
  • Flight and Ground operations

Challenges Ahead
Power and mass.  Both are extremely limited on the TubeSat platform.  Mission obstacles stem from these two:

  • High power is needed to affect the engine’s plasma.  TubeSats have extremely little power available.  Possible solution: rapid discharge capacitors to produce pulses.
  • Holding mechanical alignment over temperature extremes and launch vibration could require massive supports.  Possible solution: piezo-electric positioning tied to the core learning algorithm.
  • Fuel consists of compressed gas.  Managing the flow of the expanding gas requires massive parts.  Lowering the gas pressure makes the issue easier but reduces the amount of fuel available.  Further research is needed, likely by continuing work with the model engineering community, especially those building working miniature steam engines.

More Help Needed
Advice and information is needed in at least these areas:

  • Market information on forecasted use of low thrust engines.
  • Thrust stand design.
  • Financial sources for a second, larger version.
  • Vibration dampening and testing.
  • Resources for micro-machining of gas flow channels.
  • Data on response of various sensors in the LEO environment.
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We’ve done some calculations to estimate the amount of solar power available to our satellite.  As you can imagine, it varies greatly with orientation.

There are 8 solar panels, each with 6 solar cells.  Each solar cell is 2.277 cm^2 and converts the 1,366 W/m^2 of solar radiation with 27% efficiency.  With perfect alignment to the sun, that gives 0.49 W for one of the 8 solar panels.  However, perfect alignment is rare and staying perfectly aligned would probably melt the solder holding the solar cells in place.  So, we consider several possible orientations in orbit and calculate the second-by-second orientation to the sun.

We consider first a “flat” orientation (aka “bullet” orientation) where the satellite’s nose is toward the sun at one pole, tail to the sun at the other pole, broadside to the sun at the equator, with the satellite long axis aligned to the orbit direction.  This orientation takes in 2,042 Joules during its half-orbit facing the sun, for an average of 0.75 W during the half-orbit.  Most power comes in near the equator diminishing near the poles.

Next we consider a “radial” orientation where the broadside is to the sun at each pole and the nose to the sun at the equator, always keeping a narrow end pointed toward Earth.  This orientation takes in 2,047 J for an average of 0.75 W.  No power comes in near the equator with most coming in near the poles.

Last, we consider a “sun seeker” orientation where the broadside is always facing the sun, likely combined with a “BBQ” roll along the long axis for cooling.  This orientation brings in 3,208 J, averaging 1.18 W, during its half-orbit.  Power is constant throughout.

Conversion of any light reflected from Earth will only increase these numbers.  We’ll likely revisit these as we do thermal calculations which must consider Earth visible and IR reflection.

What do these numbers mean for the mission?  Basically, with communications and CPU drawing a few watts, it means we can’t run everything at once.  We’ll have to run communications infrequently anyway according to international law – 10% duty cycle for transmissions.  This takes the 1 W of communication to 0.1 W on average, needing 0.2 W during the sun-side to ensure we’ve banked power for use on the dark side.  That leaves 0.15-0.25 W on average available for everything else.  We’ll have to design the system to bank energy, waking up for a second or two to check conditions and running experiments only when energy is available.  My best estimate right now is 5 minutes of engine operation every 9 hours.  We’ll know for sure after we build the onboard power supply.

The full calculations are here: TubeSat Power Estimates

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We’re less than a week away from Maker Faire Detroit.  Tick tock!

Today, we put together two satellites to show and finished planning the display.  Check out the Gallery tab above for pictures.

We’re trying to bring lessons learned from our great experience at the Ann Arbor Mini Maker Faire where we learned a lot about talking to people about satellites.  The first thing we learned was that satellite conversations are longer than we saw last year with our 3D display.  So much longer in fact that many people just reached in for a business card and wandered off.  We’re adding more self-serve information and a more informative take away piece.  That, and well, getting better at bringing people into a conversation already underway.

The second thing we learned was that most people don’t actually know what an ion engine is and that, to most people, “plasma” is a component of blood.  Many children claimed to know what plasma was, but got very confused when we told them we’re making plasma from air and electricity.  Very confused.  Especially confused when they hear plasma is a rocket fuel.  It’s probably best to just tell people what plasma is (in our context) rather than ask if they know already.  It made for some funny moments in hind sight but still, I offer my [grinning] apology to the parents that had awkward conversations afterward.

We hope to see everyone at Maker Faire Detroit where we’ll learn something new!
See us at Maker Faire!

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Hello world!

What is it?
FRETS1 is a TubeSat-style picosatellite provided by and launched by Interorbital Systems. It will orbit the Earth at 310 kilometers in a sun-synchronous polar orbit. Its primary mission is to test a new kind of ion engine.

Who’s doing this project?
FRETS1 is being funded by Fluid & Reason, LLC. Thus the name: Fluid & Reason Engine Test Satellite 1.
Working on the project are Wes Faler, Don Smith, Ed Campbell, and anyone else with something to contribute.
Special thanks go to the Part Time Scientists GLXP team for their to-be-announced contributions and to Alex “Sandy” Antunes at Project Calliope for paving the way.

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