I found the following document to be quite enlightening. It presents the deltaV requirements, transit times, and stay times for a variety of transfer orbits for long duration missions to Mars (these are probably the trajectories Musk references when he says “3 to 5 months” transit time). The differences in deltaV and entry velocities are also significant enough to imply large changes to how much payload can be delivered depending on the year of the launch. As such, the paper gives a “lay of the land”, showing how rough the “terrain” of interplanetary travel is. I figure the TMRO:Space crowd would appreciate having a better understanding of the challenges Mars presents.
Wooster et al. Trajectory Options for Human Mars Missions. American Institute of Aeronautics and Astronautics. 2006. ( https://smartech.gatech.edu/bitstream/handle/1853/14747/HumanMarsMissionTrajectories.pdf?sequence=1&isAllowed=y )
The document is great you should read it, but, in case you didn’t, here at the basics:
- There are 7 different conjunctions between Earth and Mars. They reoccur in a cycle every 15ish years.
- At conjunction, the deltaV required to reach Mars is at a minimum. Less deltaV means less fuel and more payload.
- Depending on how much extra deltaV you use, you can get to Mars faster: these options are the “propulsive abort” trajectories. For these, if you decide not to stop at Mars, you can spend fuel to abort and instead return to Earth in 1.5 years.
- There are 1 or 2 special trajectory which will return you to Earth if you decide not to stop at Mars without spending any fuel, these are the “free return” trajectories.
- Most of these trajectories will get you to Mars in about 5 months, but there is room for change here, the lowest of these being a “free return” with about 3.5 months of travel time.
- The Mars to Earth return trip determines the size of the heat shield, with entry velocities of nearly 13 km/s. Mars entry velocities can be as low as 5 km/s or as high as 12.5 km/s.
- Each conjunction has different requirements for deltaV and entry velocity, the overall worst of these is 2030 and the best is 2033, with 2035 being a close second.
Note: Because of the cyclical nature of the conjunctions, 2033 is equivalent to 2018! Thus, this year’s conjunction was one of the best opportunities to get to Mars, and the next conjunction in 2020 will be good as well.