How to get to Mars

Another excerpt from Robert Zubrin's The Mars Dream is Back

The SpaceX Plan

While there should be fair competition for all mission hardware used by the human Mars exploration program, it is a foregone conclusion at this time that the best launch system for the effort will be the SpaceX Starship. This soon-to-be operational system will offer comparable lift capacity to the SLS but with at least twenty times the launch rate and two orders of magnitude lower cost. We therefore assume that Starship will be selected as the program launch booster.

After payload delivery to low Earth orbit (LEO), however, there are a number of ways that the mission could proceed. SpaceX’s own proposed mission plan would be to fly the Starship to LEO along with 100 tons of cargo, and then refuel it with 600 tons of methane/oxygen bipropellant delivered to orbit by six tanker Starships. This would provide it with sufficient propellant to fly to Mars on a six-month Conjunction-class trajectory, aerobrake into Mars orbit, and then land on Mars. After unloading its cargo, the Starship could serve as the home for a very substantial crew for a year and a half, during which time it would be refueled with some 600 tons of methane/oxygen bipropellant produced from Martian carbon dioxide and water. This would be enough to fly back to Earth on a six-month trajectory carrying the crew and ten tons of cargo.

This mission plan offers a number of advantages. First and foremost, it requires use of only a single flight system that is already in an advanced stage of development and scheduled for use as part of the Artemis Moon program as well. Thus, the same team and infrastructure used to operate Artemis could support the Mars program simultaneously, offering both programs large cost savings. Second, the payload delivered to the surface of Mars is enormous relative to competing approaches, and so is the potential crew size. Elon Musk advertises Starship as a transport capable of delivering 100 colonists to Mars at a time. Such a large number would neither be necessary nor desirable for an exploration mission, but a crew of twenty or so might be readily accommodated. This would be around four times the size of the crew proposed in most other credible Mars mission plans. Moreover, the entire crew would be landed on Mars, where they all would be available to support the field exploration effort, and where they all could avail themselves of natural gravity and substantial radiation protection offered by the Martian environment. Unlike typical NASA mission designs, no one would be left on an orbiting mothership doing nothing useful except for minding the store, while undergoing extensive deconditioning from extended exposure to zero gravity and soaking up cosmic rays. Furthermore, there would be no mission-critical Mars orbit rendezvous on the return leg of the mission.

There are difficulties with this plan, however, which stem from the same source as the problem with SpaceX’s lunar mission architecture: the Starship is way too heavy to serve as an optimal ascent vehicle. By a rough estimate, to make the 600 metric tons of propellant required to refuel the Starship once on Mars within a year and a half would require a power source with an average round-the-clock output of 600 kilowatts. A solar array that could do that would cover 60,000 square meters — that’s over 13 football fields in size — and weigh about 240 metric tons. It would require three Starship flights just to deliver such a solar array to Mars, and it would then be a major burden to deploy and maintain. A more practical alternative would be to use nuclear power. We could imagine a plausible reactor design at this power level with a mass of about ten tons. (See Endnote 2.)

From a technical point of view, nuclear is the far superior alternative to supply the required surface power. However, to achieve the necessary compact size and weight, space nuclear reactors require the use of either plutonium or highly enriched uranium, which are both controlled substances. Thus the government will need to be involved. This poses issues, because the Department of Energy is afflicted by all the same bureaucratic pathologies as NASA, if not more so. A reactor development program done in-house at the modern DOE would never produce a working system on the timeline required for a human Mars mission program. Instead, it would have to be a commercially-led effort with the DOE playing a supporting role.

 

The Starboat Plan

There is another way to mitigate the energy production problem. We could achieve a very large reduction in the amount of propellant needed by introducing an additional flight element, which I call a Starboat. This could be a vehicle of similar type to the current SpaceX Starship but scaled down by about a factor of five in mass. This could play numerous roles that would correct the weaknesses in the SpaceX plan. For example, it could do a direct return from the Mars surface to Earth using 120 tons of propellant or perform a low-Mars-orbit rendezvous using just 50 tons of propellent, with a single tanker in low Mars orbit being able to support five such return flights. It could also be lifted to Earth orbit fully fueled by a single Starship and sent directly to Mars with five tons of cargo without any Earth-orbit refueling, or 25 tons of cargo with a single tanker refueling. This would eliminate the problem of needing to launch seven Starships (the mission vehicle plus six tankers) within a single launch window as is required by the SpaceX plan. If, as assumed in these examples, the Starboat is used as the interplanetary flight vehicle, the crew size would have to be reduced from twenty to four or five, but that might well be appropriate for initial missions that will need to be conducted before all the base infrastructure is up and running.

Alternatively, instead of putting a tanker in low Mars orbit, a Starship fully fitted out for crew could be stationed there, and the Starboat only employed as a reusable shuttle between the surface and orbit. In that case, the plan could retain the ability to employ twenty-person crews, as they could ride out and land Mars along with 100 tons of freight on a standard Starship, only needing to accept the closer quarters on the smaller vehicle during a short Mars-to-orbit flight on the return leg.

The development of the Starboat would also fix the excessive launch problem with the SpaceX Artemis mission plan. The current plan requires 200 tons of propellant to be delivered to low lunar orbit to fuel the Starship on a roundtrip sortie to the lunar surface. At one fifth the size, a Starboat could make the same trip with only 40 tons of fuel. Similarly, the propellant requirement for a round trip from the Gateway to the lunar surface would be reduced from 400 tons to 80. And this could be further reduced by another factor of four when and if lunar oxygen production becomes operational. (See Endnote 3.)

The Starboat could also serve as the upper stage of a reusable first-stage booster in the same class as the Falcon-9, Neutron, and New Glenn boosters, thereby creating a fully reusable medium lift system capable of performing many important supporting mission roles. With a payload delivery capability to Mars of up to 25 tons, about twenty times as much as the landing system used to support the Curiosity and Perseverance missions, it could also deliver large scale robotic exploration missions to the Red Planet, as we shall discuss below.

Finally, and critically, the Starboat would endow the Mars base crew with global mobility. Mars is a planet with a surface area equal to all the continents of the Earth put together. It cannot be explored from a single base using slow moving ground vehicles with limited range. To explore Mars competently, we need worldwide access and the ability to travel rapidly across distances of continental scale. With 50, or better yet, 100 tons of propellant, the Starboat could give us this capability in spades. (See Endnote 4.)

Without Starboat, Mars base explorers would be limited to a region about the size of Brooklyn. With Starboat, they would have the freedom to roam over an expanse nearly double the size of the continental United States.

If additional Starships were landed to establish refueling bases scattered at long distances across the planet, more such explorable regions could be opened up. Nine such refueling stations would provide coverage of the entire world.

The Starboat would add enormously to both Artemis and Mars mission effectiveness, and make the two programs coherent with each other. It should therefore be developed as an essential program element. (See Endnote 5.)

Musk said when he first proposed Starship that it would be better to concentrate all development and research on Starship and its booster, because making a successful re-usable rocket would be so difficult.  But once SpaceX has made Starship work, there is no particular reason why "Starboat" can't be made. 

The last couple of months have however raised another issue.  Is Musk still capable of running any business?  He tweets all day; he's obsessed with spreading right-wing tropes and memes; he's obviously neglecting Tesla.  Gwynne Shotwell runs SpaceX, so it might be OK. 

One of the saddest things about Musk disappearing down his rabid-right rabbit-hole is that we may not get to Mars for another 30 years.  This would have been an extraordinary achievement for him.   Instead he will be remembered for the DODGY disaster.

DreamChaser, Sierra Nevada's re-usable "boat".