Mars Cycler
(http://www.digitalspaceart.com/marsvisions/myers/wmyers_mars_cycler_intro.jpg)
(http://foundconnections.files.wordpress.com/2009/08/mars_cycler.jpg?w=610&h=292)
A "Cycler" is a type of a spaceship that regulary flyes between two bodies (like planets) using there gravity as a means of propulsion.
As a result - they use none fuel to travel between those bodies...and they do it indefinitely.
Once put in motion a Mars-Cycler can transfer hundreds of tons of materials to-and-from Earth/Mars every cycle.
In order to maintain a constant flows of materials (and crew!) we need two mars-cyclers that will make the trip (one flyes TO the Mars, and the 2nd one goes FROM it to Earth).
---> http://buzzaldrin.com/files/pdf/2001.12.15.REPORT_FOR_NASA-JPL.Evolutionary_Space_Transportation_Plan_for_Mars_Cycling_Concepts.pdf (http://buzzaldrin.com/files/pdf/2001.12.15.REPORT_FOR_NASA-JPL.Evolutionary_Space_Transportation_Plan_for_Mars_Cycling_Concepts.pdf)
A "Taxi" is a type of a spaceship that recovers/sends to the cycler the materials/crew while it's nearest to the planet.
(http://www.damninteresting.net/content/aerobraking.jpg)
A "Semi-Cycler" is a type of a spaceship that uses grawity assists just like a normal cycler, but with the difference that it can stop at the planet for a period of time. It uses some fuel to do that manouver, but you don't need a "Taxi", plus you have more time to transfer materials/crew. As a negative aspect: it travels longer than a normal cycler, and you would need at least 3 of those for a regular activity.
---> http://web.mit.edu/aeroastro/www/people/dnewman/abstracts/aas_00255.pdf (http://web.mit.edu/aeroastro/www/people/dnewman/abstracts/aas_00255.pdf)
This would make a great aspect for the simulation. You wait for a cycler to pass-by for some missions and equipement, as well as to sent it resources. On the other hand - it creates a backstory on how wide-mars colonization started.
<Animation on how the Mars Cycler works>
--->http://www.youtube.com/watch?v=qCVfUlFZQ4U&list=FL7QJec1ANd6cIHJxzpE6zBA&index=12&feature=plpp (http://www.youtube.com/watch?v=qCVfUlFZQ4U&list=FL7QJec1ANd6cIHJxzpE6zBA&index=12&feature=plpp)
(http://clowder.net/hop/TMI/3VehiclesToMars.jpg)