Rocketdoc Notes – Week of October 11, 2020

I “attended” the Mars Society Virtual Convention this week. It was largely educational and interesting so I thought I would translate what I learned to you on these pages. The Mars Society is all about human colonization of Mars. I fully support that effort, but as a retired rocket scientist I know how difficult and incredibly expensive that is going to be. I will try and translate what I learned into the real possibilities and future problems we need to solve. There are two major problems that need to be solved. The first is cost-effective transportation to Mars, and the Starship design being built by Space-X might possibly solve that problem. The second problem is the economics of the future colony, and that problem might be unsolvable with what we know today. I’ll cover each of these problems in depth below.

Elon Musk was a guest this year and he gave a very interesting talk on his motivation for funding Starship and what he hopes it will accomplish. Elon’s dream is human colonization of Mars and the Space-X Starship Program is his answer to solving the transportation side of the problem. Elon and his engineers have come up with a very clever solution to reducing space transportation costs. To illustrate their solution, I have included a Mars/Moon/ Earth Delta-Vs chart as figure 1 on the right. Using this chart you can trace that the Delta-V to get from Low Earth Orbit (LEO) to the lunar surface is 6.4 km/sec, the Delta-V to get from LEO to Mars Transfer Orbit is 3.8 km/sec, and the Delta-V to get from the Surface of Mars to Earth flyby is 7.0 km/sec. These number include no losses, so they are a bit optimistic. What Elon and company figured out was that if their Starship upper stage was designed to provide a Delta-V of about 6.1 km/sec with 160 tons of payload, which is sufficient to go ETO on top of their booster; that same Starship, if refueled in an elliptical LEO, could deliver 100 mT to the lunar surface and return to Earth empty. That same basic Starship configuration, if refueled in LEO, could boost to Mars Transfer orbit (during the Mars Launch Window) and could aerobrake and land propulsively on the surface of Mars, and if refueled on Mars could boost back to Earth with about 95 mT of cargo.

Figure 1 Mars/Moon/Earth Delta-Vs chart

Therefore, Space-X is proposing a basic two-stage reusable rocket configuration that with relatively minor modifications can provide low-cost access to LEO, to the lunar surface, and to Mars. Needless to say, I’m very impressed and wish I had thought of it first. There are issues with the concept, especially with respect on-orbit refueling and a Thermal Protection System (TPS) that can stand reentries at Mars and at Earth return coming back from Mars, but the overall concept is very interesting. Let’s assume that the Starship can be made to work as advertised and look at the economics.

Elon stated that his estimate is that a self-sufficient Mars Colony would require one million tons of cargo on Mars. At 100 tons per spacecraft that is 10,000 flights plus another 2000 for the colonists and their belongings. Is this reasonable? Probably not when you realize that the launch window for Mars occurs every 26 months (~ 750 days) and the total mission duration for a conjunction class Mars Mission is approximately 900+ days. This means no starship is available to launch on successive launch opportunities. Hence, assuming we build up our Mars Colony on three successive launch windows (6.5 years) we will need 24,000 operational Starships and 12,000 of them must be launched and refueled on orbit within a 60 days launch window. Note that refueling requires five more Starship launches for each Mars-bound Starship. Do you see the problem? I count 1200 launches a day for 60 days.

Elon has set his flight rates so high because he fears that Earth is headed for a catastrophe within the next twenty years (there is a lot of that going around), so he wants a self-sufficient Mars civilization before we lose the capability to colonize Mars. I understand his fear, but I believe that 500 people is a viable Mars colony, at least short term and it gets more viable with each additional launch window. If you limited the initial colony to 500 people (five Starships) and 2000 tons of infrastructure (20 Starships) launched each launch window, now we’re talking 50 starships dedicated to Mars Colonization, out of a fleet of hundreds. These colonists will have to make do with primitive infrastructure, do without many modern connivences, and will definitely have to live off the land, but I believe it is doable unless Earth-Mars transportation fails (Elon’s fear). I realize the first settlers are taking a chance, but with this plan the colony would add roughly new 2300 colonists every ten years and as always, the first settlers get their pick of the prime real estate. The Mars Society Conference showed many Mars Colony Configurations with sizes as low as 500 people.

Next question, is it affordable? Space-X advertises a launch cost of $2M per flight for Starship which I calculate to be the flight propellant cost. If we assume 15% propellant losses during chill down, a launch support crew of 2000 supporting 500 launches per year, and a useful life of 100 missions (pretty standard for reusable launch systems); I can calculate a cost/flight of about $11.5 M (includes Direct and Indirect Operating costs using an industry accepted model). This means our first launch campaign to put our 500 people on Mars will require 100 launches (three refueling launches for each starship to Mars) and cost about $ 1.15 B for ETO transportation alone. Now add the Mars infrastructure costs (~ $1B for the power and propellant generation plant plus at least one billion dollars for habitation systems and we’re looking at a minimum of $600,000 per person. It is a lot less than I would have guessed, but it appears Mars is for millionaires and billionaires.

This is my quick and dirty explanation for what I learned at the Mars Society Convention. I think they are doing good work, but occasionally they need someone like me to check their numbers.

Thanks for reading.

Dana Andrews


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