🚀 go-pugleaf

All of this talk about small rovers strikes me as being 
quite silly.  Let's get real folks! small rovers can not
accomplish meaningful scientific experiments.  

Let's consider a basic issue, power.  One person suggested
the use of solar panels. The most efficient solar panels
avaiable for commercial use, can deliver approximately 
150 W/m^2.  If we define a small rover to have an area of
1/4 m^2, we can get ~40W of power HERE ON THE EARTH.  If
we now move this solar panel to Mars, the amount of power
that can be extracted will be approximately 43% of the 
power on the Earth.  Now, let's allow for wiz-bang solar
panels which are twice as efficient as current ones, and
we arrive at a total energy of about 34W.  These same panels
have a mass of about 15 kg/m^2.  For our rover, this amount
to a mass of 3.75 kg for the solar panels ALONE.

Since this is to be a simple rover, we will assume minimal
computing requirements, say the equivalent of a single
MC68020 processor.  To get ball park figures, we can look
up the power consupmtion for a single board computer.
A typical number is about 25 W.  Again, let's assume that 
advanced technologies make this available at 13 W and a weight
of 1 kg.  To continue the analysis, let's assume that we 
wish for the computers to remain active during the night.  If
the vehicle is stationary, then we must simply supply 13 W times
13 hours.  However, we must also allow for dust storms which will
block the light, so let's up the 13 hours to 25 hours.  So, we need
a capacity of 325 watt hours of power.  Using silver-zinc batteries,
which are among the most efficient, we find that we need about
22 Kg of batteries, using about 0.01 m^3 of volume.  Now, if the
batteries can be slowly charged over the 13 hours of daylight,
we must store (assuming 100% efficiency and no leakage) about
13 W/hr.

Keeping with the notion of a small rover, let's assume that the
rover is 1/2 meter tall, giving a total volume of 0.125 m^3.

So, where are we now?  We have a rover with solar cells, batteries
and a computer.  We can supply 34 W/hr of power and we are using
26 W/hr.  We also have a mass of 26.75 Kg and have consumed almost
10% of the available space; AND WE STILL HAVE NO LOCOMOTIVE SYSTEM
OR EXTERNAL SENSORS!

If we assume that half of the available space in our rover if filled
such that the average density is that of water, the total vehicle mass
would be in excess of 60 kg.  If interesting items do indeed
occur in areas of rough terrain, the ability of the rover to
scale slopes with 30 degree inclines is important. Given the
power avaiable for locomotion and assuming to losses during
movement, the vehicle could maintain a steady speed of 0.07 m/s
going up the incline.  

I think that this back-of-the-envelope discussion should be 
sufficient to pursuade folks that the idea of solar powered
micro-rovers is a pipe fantasy.  This is not to say that they
can not be made, but rather that they would be incapable of
doing anything meaningful.  The size of JPL's Robby and the 
CMU Ambler reflect this reality.  Both vehicles are sized as they
are so they can carry out meaningful scientific experiments,
overpower the terrain they are in, and carry with them a power
supply such as an RTG (radio-isotope, thermo-electric generator).



-- 
gerry roston, field robotics center
robotics institute, carnegie mellon university
pittsburgh, pennsylvania, 15213  (412) 268-6557
gerry@cive.ri.cmu.edu