From: gerry@cmu.edu (Gerry Roston)
Newsgroups: comp.robotics
Subject: Re: How to Explore Mars
Date: 12 Jan 93 04:17:54 GMT
Article-I.D.: onion.GERRY.93Jan11231754
Reply-To: gerry@cmu.edu (Gerry Roston)
Organization: Field Robotics Center, CMU

[As Erann and Gerry cross verbal swords...]

In his post, Erann brought up one important fact: what are you really
trying to accomplish with a mission.  If all you are trying to to is
to determine if there is water at the surface at some point
(literally) on the surface of Mars, the cornet cube concept may be the
appropriate solution.  However, this is a fairly meaningless mission.

Let's backtrak.  IMHO, the reason for exploring the moon and mars is
two-fold: first to gather scientific data for two reasons: pure
science and the economic feasilbility of exploiting resources, for
both in-situ and exportation; and to determine the feasibility of
establishing a permanent manned habitat.   The question being
addressed is whether or not a "micro" rover or a "maxi" rover is more
appropriate to carry out this mission.

The "common wisdom" says this: a "maxi" rover is more capable but the
"micro" rover provides greater reliability.  Is this true? 

Those who have been following this thread know that I am not a
proponent of "micro" rovers.  I will explain some of the difficulties
with these machines, then I will show why they are not a good idea in
reality. 

The biggest drawback with "micro" rovers is their inability to
accomodate components that do not scale in size, for instance
scientific instruments and telemetry systems.  For example, SDIO has
developed a number of imaging sensors.  These are (to the best of my
unclassified world knowledge) the smallest, space qualified imaging
sensors in existance.  For example, a wide field of view camera for
350 grams and about 5 watts of power.  However, when Erann speaks of
"micro rovers" (and Erann, PLEASE correct my impresion if I'm wrong),
he is thinking in terms of machines that mass less than 5 kg.  Thus,
to have only one imaging device consumes 7% of the total available
landed mass!  Now, to do a survey of the type I identified, a suite of
instruments are required.  Okay, I hear the chourus shouting, send
many "micro" rovers, each with one instrument in to the same area.
Bzzt!  How do you register the data returned by one rover with the
data returned by another?  This is a very difficult problem. 

Other difficulties with "micro" rovers is the small surface area
presented (needed for solar power - an assumption, RTG type sources
could be used, but at 1 thermal watt per 30 grams and a 5% conversion
efficiency, this would consume too much landed mass to produce any
reasonable amount of power.  Of course, you could store the energy in a
battery/capacitor to get higher power, but this again consumes
mass...)  Of course, the biggest problem with "micro" rovers is this:
Because of the lack of scaling, the mass fraction of a micro rover
that can be used for scientific purposes will be quite limited.  Let
us say, giving them the benefit of the doubt, that 25% can be thus
allocated.  Having many micro rovers does not change this mass
fraction.  We (FRC) are currently working on the Ambler's successor, a
combined lander/rover that can deliver approximately 50% of the landed
mass as scientific payload.  

Now, let me lastly address one of Erann's comments, specifically,
using a relay satellite.  This solution has two difficulties: first,
the satellite will not be able to continuously communicate between any
given "micro" rover and the earth.  This necessitates, therefor, the
ability of "micro" rover to store and forward collected data, or to
sit idle.  This will have the overall effect of decreasing the amount
of data returned to the earth.  The second problem is that the
satellite is free: it costs additonal dollars and consumes deliverable
mass on launch vehicle, further decreasing the deliverable scientific
mass to the planetary surface.  

As a final note about the corner cube concept.  Let us assume that a
given launch vehicle (say a Delta II) can deliver 500 kg to the
Martain surface.  (It is capable of launching a vehicle that can soft
land 450 kg on the lunar surface, so this seemed like a reasonable
estimate.)  The "micro" rovers to be delivered each have a mass of 1
grams and can determine if there is water underneath them.  Due to
their low mass, these rovers are simply seeded over the surface.  So,
we have 500,000 rovers being delivered.  Now, assuming they all
function and all report a positive finding, this is a mere 500,000
bits of information, the same as only two video images.  So, this may
seem like a neat idea, but in the analysis, it doesn't seem too
worthwhile. 

Finally, lets deal with reality.  There are only a fixed number of
launch vehicles available in the world.  Since this represents the
single largest cost item in a mission, you want to maximize
utilization of this resource, both its mass and volume.  LV payload
fairing are basically cylindrical in shape.  To maximize utilization
of this volume, the "best" shape is a cylinder that fits snuggly
within this volume.  By using multiple, smaller objects, volume
utilization will fall far short of optimal.  Also, due to launch
loads, etc, the multiple smaller payloads will have to be fixed in
place by some sort of bracing structure, yet again additonal mass that
reduces deliverable scientific payload to the surface.

To directly address the question of big versus small, let me make some
comments about the Ambler.  The Ambler's critiques frequently point to
its large size as being its most serious drawback, but this is not the
case.  The Ambler, as designed, not as built, could have been
accomodated by a Titan IV launch vehicle.  But then again, this was
not one of hte driving design issues at the time the Ambler was built.
Its most serious drawback was its immense complexity, however, this
complexity allowed the Ambler an unprecedented range of mobility, a
range that no "micro" rover could ever hope to mimick.  

Now, in conclusion (finally!), am I saying that making robots the
size of the Ambler is appropriate?  Probably not, though it depends on
the goal of the specific mission.  By the same token, however, I will
say that there appear to be no missions for which a "micro"rover is
appropriate.  It seems to me that the "best" sized rover is one that
will completely consume the available payload of a cost-efficient
launch vehicle, and use this "large" landed mass for carying a
meaningful suite of scientific instruments, along with the capability
to deliver these instruments to a wide variety of locations with a
high degree of competency.

--
Gerry Roston (gerry@cmu.edu)    | We hold these truths to be self-evident, that
Field Robotics Center,          | all men are created equal, that they are
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position of CMU, FRC, RedZone,  | That whenever any Form of Government becomes
or any other organization.      | destructive of these ends, it is the Right of
                                | the People to alter or to abolish it...
                                | Thomas Jefferson