[Server-sky] Drag and LEO thinsat experiments

Keith Lofstrom keithl at gate.kl-ic.com
Tue Jul 2 01:57:01 UTC 2013

Some thoughts about microgravity thinsat tests.

Take a look at http://server-sky.com/Drag .  The air drag at ISS
altitudes (350-400km) and below is way too high to maneuver 
thinsats - they are tissue paper in a hurricane.  Once thinsats
drop below 1000km altitude, they will re-enter in weeks;  below
400km, re-enter in hours, unless they are shielded from air drag.

Thinsats in light pressure accelerate away from the sun at
around 20 μm/s².  At ISS altitudes, the orbital drag is 200
times higher, -4000 μm/s², rapidly slowing down and losing
altitude.  That is 15 m/s per hour, dropping orbit altitude
by 30 kilometers, which doubles the drag in an hour, which
doubles the drag in half an hour, ...

A 1U cubesat has an area of 0.01m²and weighs 1kg, 200 times the
mass and 0.4 times the area of a thinsat, so the drag acceleration
is 500 times lower, about -8μm/s².  

ISS accelerates at -0.2μm/s² because of drag.  Small, but it adds
to drop the orbit by 10s of kilometers over months - putting ISS
in denser atmosphere, decreasing the decay rate and requiring
frequent rocket reboost.  ISS flexes, turns, outgasses - certainly
not perfect vacuum and zero gravity.

Imagine testing a thinsat inside of a transparent container to
protect it from air drag.  If the container is in the portion of 
its orbit moving towards the sun, and slowing at 20μm/s², it will 
track the light pressure acceleration of the thinsat and the 
thinsat can maneuver inside the container.  That is way more
acceleration than ISS, a bit more than a slowing cubesat. 

But if the cubesat deploys a larger transparent plastic bag,
that could add enough drag to match the thinsat's 20μm/s²
light pressure acceleration for a small portion of a 92
minute ISS-altitude orbit.  A full size 20cm thinsat won't
fit in a small bag, but for testing a 5cm thinsat will do
fine.  A small thinsat turns four times faster, facilitating
short duration experiments.

A proposed experiment:  

Make a cubesat with small TV cameras, an S-band transmitter,
a bluetooth transmitter, and a small deployable tent with a
5cm baby thinsat inside.   The baby thinsat has InP solar
cells and electrochromic thrusters controlled by a single
chip bluetooth receiver/CPU.  The cubesat commands the captive
baby thinsat to manuever, the behavior is captured by the
cameras and flash memory, to be slowly transmitted to the
software radios on ISS. 

As the cubesat drags and slows down, the orbit will drop
and move forwards in orbit, eventually falling out of range
of ISS and re-enter.  If the drag tent stays deployed,
the cubesat will come down in less than two weeks.  That
will result in a few dozen experimental passes.  

Also, developing a low-mass deployable drag tent from a
cubesat might be a useful technology for other cubesats.
If their orbit takes them near some other LEO asset, it
slowing down and missing a collision could save $$$$$ in

A lot of handwaving, but a concept for an experiment that might
refine into something practical.  CHECK MY NUMBERS please!


Keith Lofstrom          keithl at keithl.com         Voice (503)-520-1993

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