Getting a Manned Mission to Mars - Part 1: Propulsion and Consumables

In 2004, President George W. Bush gave NASA a direction that, if continued, will take them back to the Moon and from there to Mars. That’s the plan, anyway, but it’s a lot easier to say than to accomplish. In its own way, the demands of a trip to Mars will be every bit as demanding as those in far earlier stages of our species’ exploratory history. Numerous challenges will face us along the way, but three of the most serious are logistics (in terms of both fuel and crew consumables), mass, and crew interaction.

Tsiolkovsky’s rocket equation can be used to determine the delta-v or change in velocity attainable for a given mass fraction, (m sub initial)/(m sub final), and the figure at the bottom of this page displays the required delta-v for points in the Earth-Moon and Mars-Phobos-Deimos systems. Given the performance specifications for the Shuttle’s engines, the most efficient main rocket engines currently in production, the total required delta-v for a one-way trip from Earth’s surface to Mars is about 18.7 kilometers per second (kps); for reference, the delta-v to get into low-Earth orbit (LEO) is around 11 kps. Plugging this number into our rocket equation, we get a truly prohibitive mass fraction: 67.2! To turn that number on its head, that’s the same as saying that only 1.5% of the total mass is the space vehicle, the remainder being fuel. From an engineering standpoint, that’s totally unrealistic, even assuming dramatic materials science breakthroughs. Mass fraction is the reason why most rockets have stages; staging enables a space vehicle to shed dead weight and improve its performance.

In this particular case, the solution was to launch pieces into LEO individually and assemble them there; that way, the largest portion of the work was already done and the fuel load of the Mars-bound vessel was reduced. Our mass fraction using this method is far lower, around 11.1 (~90% fuel). Of course, the fuel still needs to be expended to get the vehicle into orbit (and the total fuel required to get the parts to orbit is higher, since the total empty mass of all the launch vehicles is also higher), but the delta-v when launching the mission from that point is much more reasonable. Total fuel expenditure could be dramatically reduced by relying on an aerocapture maneuver on arrival at Mars rather than using a braking maneuver (total delta-v would be 3.8 kps, and mass fraction would be 2.4). For much the same reasons as aerocapture on return to Earth, making the atmosphere do the work is a much more fuel-efficient strategy. For the aerocapture profile, the return trip to Earth would require 6.9 kps delta-v for a round-trip total of 10.72 kps and mass fraction of 11.45, numbers definitely achievable through the use of staging. One way to decrease the overall delta-v required even further would be to establish a foothold on the Moon and assemble/launch the vehicle from there, since the delta-v to achieve escape velocity from the Moon is 2.2 kps compared to 13.22 from the Earth’s surface. Considering that the delta-v required to achieve escape velocity from Eath’s gravitational well is greater than that required for the mission from that point on, such a change would mean very significant savings in terms of fuel required. Gravitational assists can also reduce the required velocity, usually at a cost in terms of transit time and trajectory complexity.

To put the crew logistics issue in perspective, the mission as outlined in the book The Mars One Crew Manual, a 1985 book outlining a fictitious 1996 Mars mission based on then-existing technology, scheduled 350 days from Earth to Mars, 30 days on Mars, and 290 days from Mars to Earth, for a grand total of 670 days or the better part of two years. Estimated life support requirements were 20 lbs. of oxygen, 25 lbs. of food, and 102 lbs. of water per day for an 11-person crew. Even recycling water from fuel cells (a convenient by-product of their power generation), exhalation, and other sources (don’t ask where the drinking water comes from!), recovering oxygen, and minimizing losses to space, that would require eight and a half tons of food alone, as well as significant amounts of hydrogen and oxygen. Also keep in mind that all those resources, (not to mention the mass of the space vehicle and all its components, as well as the crew), have to be put into Earth orbit before the mission can depart, then be boosted out of Earth’s gravity well entirely. You might ask, "why not use solar for power?" While it's a definite possibility, whether it's truly practical depends on the power requirements, the ability to maintain and repair panels, and the effect of the increasing distance from the Sun on power generation.

While a mission to Mars is definitely possible from a technological standpoint, the challenges are substantial. Not only will it require a vehicle superior in both design and size to anything we have built to date; it will also necessitate far greater reliability and redundancy. After all, whatever may befall that first mission to Mars, they will be a very long way from home…