Packing algorithms have always been a hot topic in computer science, inventory management, and many industries like airline travel. The fundamental question is often, "How can we pack more stuff into a limited-volume container?"
For a Mars mission, packing algorithms are critical. A limited volume is available for transporting equipment, supplies and people to Mars. Far more important, mass (weight) severely limits nearly every aspect of the mission architecture. Safety considerations are perhaps most important of all, since certain pieces of hardware need to be redundant. Everything needs to be in the right place (or on the right launch vehicle) at the right time, able to perform its function (i.e. all its dependencies need to be present as well). Packing is a mission planner's worst nightmare - as well as their best opportunity to shine.
Where can we start to disect this beast? Let's begin by assuming that we have a heavy lift vehicle (HLV) that can throw 100 metric tonnes into Low Earth Orbit. Refer to the Heavy Lift Vehicle discussion for the reasons behind this assumption.
Let us also assume that such our HLV can ultimately land 18.2 tonnes of Mass on Mars, using a six-month, crewed transfer orbit with free-return capability, or 20.9 tonnes on a slower, optimal Hohmann Transfer Orbit. The exact figures can be debated, depending upon rockets and fuels used, but these figures should be very close and reasonable. Furthermore, we'll assume that our mission profile allows two crewed/fast launches and four slow/cargo/ERV launches.
For the 18.2 tonnes transferred with each crew module, here's a possible breakdown of masses for a somewhat-traditional mission (roughly similar to Mars Direct), but with a crew of three (total mission crew size of six):
| HABITAT ITEM | MASS (tonnes) |
| Habitat Structure | 3.5 |
| Life Support System | 3.0 |
| Consumables (food, water) | 4.5 |
| Power (15kw solar array) | 2.0 |
| Reaction Control System | 0.4 |
| Communications, Computers | 0.2 |
| Lab/Medical Equipment | 0.1 |
| Crew | 0.2 |
| EVA Suits | 0.4 |
| Interior Furniture | 0.4 |
| 1 ATV + Fuel | 0.4 |
| Field Science Equipment | 0.2 |
| Robots (2+) | 0.4 |
| Spares and Margin | 2.5 |
| TOTAL | 18.2 |
Note that the mass of each habitat is much less than the 5.0 tonnes allocated for the habitat in the four-person Mars Direct plan, let alone the six-person behemoths in the latest NASA plans. This is a critical, though difficult, design constraint because if it's light enough, the same habitat can be used for the return trip back to Earth. That's an important goal. The advantage of this approach is a sharp increase in available mass for solar power cells in the ERV. Two big disadvantages are a lack of redundancy in crew quarters, as well as the failure to leave a safe habitat behind on Mars for future exploration teams to use. Both of these issues are studied within the pages of Shadows of Medusa, along with many other habitat mission design considerations.
What's most important to take away from the table above is the fact that a cramped, light habitat could support a crew of three, with a minimal amount of supplies and consumables. Launching two of these vehicles allows a total mission crew size of six, with redundancy of the most important mission component: the crew.
With more-capable launch vehicles, each additional tonne sent to Mars can add to the habitat structural integrity or supply margins and vastly improve the safety of the mission... however, the true, limiting factor is how much mass can be launched back to Earth on the ERV. In a traditional mission, a heavier cabin structure requires more fuel to be generated on Mars, in-situ. Ponder these facts, and you might appreciate the true meaning and beauty of the ending in Shadows of Medusa, from an engineering point of view.
What would each ERV look like? Here are some possible cargo items and masses that might be sent to Mars and then back to Earth again:
| ERV ITEM | MASS TO MARS (tonnes) | MASS TO EARTH (tonnes) |
| Habitat Structure | 0 | 3.5 |
| Life Support System | 0 | 1.0 |
| Consumables (food, water) | 0.2 | 3.4 |
| Power (solar arrays +) | 7.1 | 1.0 |
| Reaction Control System | 0.4 | 0.5 |
| Communications, Computers | 0 | 0.2 |
| Lab/Medical Equipment | 0 | 0.1 |
| Crew | 0 | 0.2 |
| EVA Suits | 0 | 0.4 |
| Interior Furniture | 0 | 0.2 |
| ATV (for solar deployment) | 0.2 | 0 |
| Lift Thrusters | 2.5 | 2.5 |
| Fuel Tanks | 2.0 | 2.0 |
| Aeroshell | 1.5 | 1.5 |
| Tethers | 1.0 | 1.0 |
| Batteries | 1.5 | 0.1 |
| Fuel Production Plants | 0.9 | 0 |
| Hydrogen Feedstock | 3.5 | 0 |
| CO2 Air Pump | 0.1 | 0 |
| Samples, Spares | 0 | 1.3 |
| TOTAL | 20.9 | 18.9 |
| ISRU Launch Fuel | 0 | 150 |
Note that over nine tonnes of to-Mars mass is saved by re-using the habitat for the return trip to Earth. Some of the numbers above are very soft, since we don't know exactly how to build, land, and launch an ERV yet! Fortunately, we do have some margin for spare parts and return samples, depending upon just how much in-situ fuel is generated.
Creating 150 tonnes of fuel via solar-power is a challenging prospect... hence another important paradigm shift - the crew might need to stay an additional two years on Mars! Alternatively, additional cargo, solar cells, and batteries could be sent from Earth... which is probably a good idea anyway. Cargo launches would be cheap, and at least one extra launch (for a total of three) would be useful. In fact, why not throw in additional ERV and habitat launches, too, at the two-year mark?
Cargo contents are flexible, depending upon the needs of the mission. Here's the advertised cargo manifest in Shadows of Medusa:
| CARGO ITEM | MASS (tonnes) |
| Consumables | 4.5 |
| Lab/Science Equipment | 1.5 |
| Extra EVA Suits | 0.3 |
| Furniture | 0.5 |
| Pressurized Rover | 2.0 |
| Cart | 0.1 |
| Garage/Workshop Shell | 1.1 |
| Tires, Rims | 0.6 |
| CO2 Pumps | 0.2 |
| Greenhouse Pods | 1.0 |
| Inflatable Greenhouses | 3.0 |
| Digging Equipment | 3.3 |
| Misc Spares | 1.8 |
| Water Extraction Unit | 0.2 |
| Robot | 0.1 |
| Brick Kiln | 0.2 |
| Gravel Crusher | 0.3 |
| Communications, Computers | 0.2 |
| TOTAL | 20.9 |
Again, don't get too hung up on the exact numbers and items of equipment above. What's important is the use of smaller, cheaper "modules" (cargo/ERV/habitat) to improve the flexibility and capabilities of the mission. Such modules are a reasonable, safe way to increase the crew size over the Mars Direct plan, while increasing redundancy and reducing risk.
Economies of scale would dictate that our overall goal should be to increase the number of module launches. Traditional assembly-line techniques can improve quality and drive down the per-unit costs, eventually resulting in vastly cheaper and safer missions.
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"Someday, I hope to be quite mortified when my representation of Mars becomes naively antiquated. Our knowledge of the vast resources in our solar system is expanding every day, and the revelations should continue for some time to come. A similar fate has befallen several sci-fi classics over the years. Reality always has the last word, and our preconceived notions must continue to adapt. It's all part of the learning process, as humanity grows up. A bit of personal mortification is a small price that I'm most willing to pay to be involved. "
Preface