|Many Launchers for 1 Mission|
When talking about reusable assets, a tug to take payloads from Earth orbit to Mars orbit and a reusable Mars lander/launcher come to mind. For Mars, these assets have to be very sophisticated to include transferring of fuel and other consumables. They would have to be able to be controlled remotely with some automation and manually by any crew.
I've talked about a reusable tug before. For Mars, having a large tug that can travel many times between the planets could provide the means to take multiple missions on each trip. Distributing the cost of the trip among several customers could make the trip financially possible. I like to imagine such a tug as being based on the concept of the Saturn S-IVB. It would have to be much larger. I could have duo-propulsion: traditional chemical booster; electric propulsion such as ion drive or plasma drive. Space.com has a great article on electric propulsion called How Electric Spacecraft Could Fly NASA To Mars. A robitic arm such as the Canadarm 2 could provide a means of capturing and docking many different types of space modules. Also it should be able to carry many modules to include unmanned and manned modules simultaneously. Our module technology has become quite sophisticated so I don't think that would be a problem. One of the biggest issues of such a craft would be electric power. Solar panels are nice, but large ones would be needed to power the electric propulsion. They also should be able to retract and deploy on command and often. I expect that aerobraking maneuver would help reduce the amount of fuel needed for each mission. Solar panels would need to retract for that maneuver lest they are damaged. These panels would have to last a long time. Another option, though less popular, would be giving the craft its own nuclear power reactor. Of course, responsible handling of the nuclear material is a must; including a disposal plan for the depleted nuclear material.
A reusable lander/launcher is necessary. The idea of landing on an atmospheric planet is enticing and yet hard. If you going to reuse the lander, it has to launch from the planet as well. We've never created a craft like that before. It would need a heat shield that can pop off and be replaced easily (by robotic means), a reliable engine, and large tanks for fuel. After every launch back into orbit, it would need to be serviced and refueled for the next landing. This craft is important because for one manned mission, NASA would use 3 landers and 1 launcher. That's 4 vehicles. Consolidating hardware complexity and weight into one vehicle should save on cost. Using this vehicle for many missions should be a savings multiplier.
What do we really want to do on Mars? Do we want to just plant a flag and walk around a go home? No, many folks want to do many things. From geology to colonization to terraforming, Mars has inspired many possible missions. If we go with throw away assets, we won't be able to afford getting there but once in a generation if at all. Mars should also only be the first step to exploring and exploiting the inner solar system. Therefore, reusable assets are a must. We must build an infrastructure.
Imagine multiple unmanned mission accompanied by one manned mission packed on the reusable tug that is about to depart Earth orbit. These mission could be from all types: orbital, landers, rovers and sample returns. The landers, rovers, and sample return missions can get loaded up on the reusable lander/launcher to get down to the surface along with the assests of the manned mission. It seems they would all land in one spot. Well, that could create the opportunity to have some planetary transportation services such as fast rovers and aircraft. You can quickly imagine a multitude of missions and activity around and on Mars.
Spreading the cost for each trip and back from Mars among many customers seems reasonable. Using reusable assets such as a reusable transportation tug and a reusable lander/launcher goes hand in hand with multiple manifests. Such an effort could easily be a commercial venture. Such an infrastructure could have the effect of researching the red planet in a much faster fashion than what we do now. If regular trips to Mars every 3 years is sustainable then we would have established a permanent link worthy of colonization.