Mars Musings Pt 1
Insomnia lately has led me to dredge up some old thoughts on the subject of a manned mission to Mars. I’m sure this is due in part to recent talk about VASIMR and its promised 39 day trip to Mars circulating online. For those unfamiliar with VASIMR, have a look here. To appease my subconscious so that I can possibly get some sleep, I have decided to write about some of my thoughts, ideas, designs, etc. related to a manned Mars mission. This will be the first in a short series of posts related to a hypothetical manned mission to Mars.
In order to design our hypothetical spacecraft, we need to start with a realistic set of requirements. The requirements can start broad, and be refined as individual aspects of the design are considered. So, what is the minimum set of requirements for our mission? Since the mission is manned, the vehicle will need to carry a handful of people to Mars and back. Since we like our astronauts, our design must return them all alive. When we get to Mars, we want to do some science stuff, which may or may not require landing. That means carrying some cargo and some way to land and return to Mars orbit. For this discussion I will assume somebody has a good way to get from Mars orbit to the Martian surface and back to orbit. Go talk to Elon Musk about his pet Dragon for doing this.
That’s about it for the absolute requirements, but adding a few goals make the mission more politically feasible. For instance, setting a goal of going to Mars on a reasonable budget will not help with the main mission requirements, but is necessary if Congress is going to fund it. To this end, we should also set a goal of making large portions of our spacecraft reusable. After all, we probably want to go to Mars more than once, right? If we can shave a significant amount of cost off the second trip, that trip is more likely to be funded. We should also set a goal of taking more than just two or three astronauts to Mars. We do want to do science when we get there, so taking several scientists is probably a good idea.
Fast or Slow
The first decision we need to make in our design process is whether we want to drive the Ferrari or the Chevy Van. In order to get to Mars our spacecraft needs to change its solar orbit from one that coincides with Earth, to one that coincides with Mars. To do this efficiently, spacecraft use Hohmann transfer orbits which require the spacecraft to speed up along the orbital path, which causes the vehicle to enter an elliptical solar orbit. When the spacecraft is far enough from the sun, it accelerates some more to circularize its orbit at the new distance. This is, of course, a great simplification of real transfer orbits, but gives us an idea of what is involved. We can talk about orbital transfer more when we discuss propulsion at a later date. The important part, which we have to decide now, is do we want to go to Mars fast or slow. Going fast takes more fuel which means we can haul less cargo. This is offset a little by the decrease in crew needs (food, water, etc) that the shorter trip allows, but it’s not much. I’m not going to Math this out tonight, but we are talking something on the order of doubling fuel use for a 10% decrease in trip time. Carrying all of that fuel means we have to take less stuff for a given total mass. Since our goals are to take along several people instead of two or three, and to do significant science when we get to Mars, it seems like we are going to need the cargo hauling ability. So no space Ferrari for us, looks like we are taking the interplanetary Chevy Astro van.
Look at that, our first design decision! With that accomplishment, I will sign off for tonight. Stay tuned for more.