Proposal of a new architecture for the first manned flight to Mars
At EMC25 (the last congress of the European Mars Societies), Robert Zubrin, founder of the Mars Society in the United States, author of ‘The Case for Mars’ in 1995, and Elon Musk’s astronautics mentor in the early 2000s, suggested that, for the first manned missions, the starship’s descent to the Martian surface could pose a problem. The reason why is the amount of propellant that would be needed to be produced and stored on Mars during the 18-month stay (the compelling duration for a mission to be completed for a return to Earth under acceptable conditions). As a solution, he suggested leaving the starship in Martian orbit and using a shuttle to descend to the surface.
I seize this opportunity to propose the mission architecture described below. Note: I used the artificial intelligence claude.ai for the calculations and visual representation.
The steps are as follows:
(1) Launching simultaneously from Earth orbit of a Starship and a Falcon Heavy.
(2) Immediately after interplanetary injection, connecting the two spacecrafts with three 200-meter-long tethers, with the attachment points located around the nose of the Starship. Note that the Falcon Heavy’s “upper stage” (i.e. its second stage) will not return to Earth but keep flying towards Mars, carrying a third-stage module (the mass of which is, of course, dependent on its carrying capacity).
(3) Rotating the two spacecrafts. Given their respective masses (in a ratio of approximately 1.8/1), the system’s center of gravity (after rebalancing as described below) will be located between the tethers (each 150 cm apart of the others), approximately 89 meters from the Starship and 111 meters from the Falcon Heavy. This will generate, with a rotation speed of 2.2 revolutions per minute (rpm) of the system, a gravity of 0.29 to 0.41 g within the habitat of the Starship (under the cable and cargo bays) and a gravity of 0.62 g in the Falcon Heavy (near its nose, ahead of the cargo bay). During the journey (approximately 6 months), the passengers (likely 6 for the first mission) will live in the Starship, as its habitable space is more spacious (at least 1000 m³, including 850 m³ excluding the cable and cargo bays) than that of the Falcon Heavy (at most 140 m³). The Starship will be able to accommodate more life-support and will therefore be more comfortable. Of course, the passengers will travel with the equipment necessary for their survival on Mars (30 to 40 tons). NB: this equipment will be different from that used aboard the Starship during the trip. But it will have been transferred from the Starship to the Falcon Heavy, using the tethers, immediately after interplanetary injection and starting rotating the spacecrafts, in order to reduce the mass difference (and alleviate the tension at the ends of the tethers). It will include two rovers capable of carrying six passengers with their luggage and equipment, in particular a power generator, a life support kit, and a telecommunications system. It will be designed to allow the crew to survive after landing and reach the base prepared by the robots, in case the landing could not be made close to the base.
(4) Upon approaching Mars, the passengers will don their spacesuits and be transferred (along with the rest of their personal luggage) from the Starship to the Falcon Heavy, using the same tethers.
(5) Then, the tethers will be disconnected from the Falcon Heavy and rewound within the nose of the Starship.
(6) The two spacecraft will correct their attitude since they will have to follow separate trajectories.
(7) The Starship will continue in orbit around Mars at an altitude where it can remain for 18 months without altitude correction, while the Falcon Heavy descends to Mars.
Notes:
Before the transportation of humans, there will have been a transport of various equipment (by ‘cargo flights’) to start building a base on Mars and also to produce and store the propellants necessary for the return of humans to Earth. This transport will have been carried out by two preparatory robotic missions (hopefully in 2029 and 2031), using Starships that will remain on the Martian surface. There will also be a cargo launch during the window used by the crewed flight (in 2033). In order to benefit of the most favorable energy-to-mass ratio, the speed of these cargo flights will be lower than the speed of the crewed flight (pure Hohmann transfer orbit). Upon their arrival, the astronauts will have immediate access to the equipment that had left with the two preparatory robotic missions, but they will have to wait two or three months for the equipment that will have departed Earth at the time of their own departure. Once completely unloaded (most of the equipment needed by the robots, including several Optimus humanoids, to prepare the landing site and the inhabited base, will have been unloaded), these starships could be used as shelters or workshops, pending the time and action needed to make the habitat fully viable. They could also be used as an alternative to the habitat as prepared by the robots if it does not meet all the necessary criteria for living in.
This architecture offers several advantages:
(1) During the flight, the crew’s living quarters within the Starship will benefit from a minimal gravity, similar to that of Mars (from 0.29g to 0.41g depending on the habitat level). This will allow the crew to get ready before arrival for the 0.38g of the Martian surface and, most importantly, avoid staying six months in microgravity. In any case, we are constrained by the length of the cables. 200 meters is a maximum to avoid problems with unwinding and winding. And we must manage the gravity differential inside the two spacecrafts as efficiently as possible in spite of the difference in their masses. On the one hand, the Starship will have a total mass of 280 tons, including 120 tons for the empty mass, plus 60 tons of payload and some 100 tons propellant. On the other hand, the Falcon Heavy will have a 152 tons total mass, including 12 tons for its empty mass, 100 tons of propellant, and 40 tons of payload. While the average gravity inside the Starship’s habitat could be 0.38g, it must not be too much higher in the Falcon Heavy. The reason is to avoid a too large difference in tension between the ends of the cables, as already mentioned, and also the need for crew or robots to move around within the Falcon Heavy’s environment. In any case, the mass transfers will have to be made twice. Once at least just after the departure from Earth orbit (transfer of equipment from the spacecraft to the Falcon Heavy) and then during the approach to Mars (transfer of passengers and luggage from the Starship to the Falcon Heavy). Furthermore, at 103.50 meters (89+2+5+2.50+2.50+2.50) from the center of gravity (that is the floor furthest from the nose in the spacecraft’s habitat) and at a rotation speed of 2.20 rpm, the gravity differential in the spacecraft between the head and feet of a 1.80 m tall passenger will be only 2.9%, i.e. significantly less than the 10% considered a medically acceptable maximum.
2) Landing a Falcon Heavy on Mars on unprepared ground will be less difficult than landing a Starship, since the Falcon Heavy is less massive, the rocket is not as tall, and its center of gravity is lower. The inherent risk of a Starship landing might be acceptable for equipment that will be duplicated anyway, but much less so for human beings. Also, the propellant requirements will be lower if a Falcon Heavy is used for returning to Mars orbit to meet the Starship waiting for it with some propellant left in, rather than if a Starship takes off from Mars surface and flies directly to Earth. The propellant needed for the Falcon Heavy can be more easily produced and stored on Mars during the 18 months of the stay (in case the propellants produced before is not available).
3) The Starship will always be up there in the sky. And if, for some unforeseen reason, life on the Martian surface turns to be impossible, the crew could always return to the Starship and wait for the next return window to open, and then head back to Earth (provided the propellants needed for the Falcon Heavy would have been produced beforehand, starting from the arrival of the previous robotic mission).
The difficulties are several:
(1) Reaching the Starship in orbit. However, humans have successfully made this connection several times around Earth, even with unmanned satellites.
(2) Returning the Starship to Earth without refueling. But the main effort (and therefore the highest propellant consumption) is required to reach orbit and then to decelerate before landing. In this case, the Starship will have been refueled in Earth orbit and will not descend onto Mars. Therefore, the ‘only’ fuel consumption will come from the interplanetary injection to reach Mars orbit from Earth orbit and later, the interplanetary injection to return to Earth orbit from Mars orbit. The optimal parking orbit must be chosen to minimize the required effort. This could be a geostationary orbit (which would also allow the Starship to be monitored from the ground of Mars). The Falcon Heavy could potentially deliver additional propellant produced on Mars to the Starship. It should be noted that for the Falcon Heavy, the ground-to-orbit flight and then the Mars-orbit to Earth-orbit flight, the mass to be taken from Mars will be minimal, since only the crew and a few Martian soil samples will be returned to Earth for more in-depth study (geological or exobiological) than could be conducted on-site.
(3) Having a Starship that will be fully operational despite being unused and unmanned for 18 months. However, it is worth noting that even the most sophisticated satellites sent into Mars orbit have survived longer periods. The advantage of positioning it in geostationary orbit above the inhabited base would be an ability to maintain constant telecommunications with the onboard equipment. This could allow for continuous monitoring and interventions, including from humanoid robots inside the Starship (which would enable visualizations and manipulations). These robots, like others, could be powered by solar panels extended through the airlock door by the crew as they exit the Starship.
During the return flight, the Falcon Heavy will be reconnected to the Starship via its tether system, and gravity will be restored inside the Starship, as it was on the outbound journey (NB: reactivating this system would be facilitated by the Starship’s high altitude (less pull from Martian gravity); geostationary altitude would therefore be an advantage. NB: the gravity center of the system will be different as the payload inside the Falcon Heavy will be different. The two spacecrafts will return not to Earth’s surface but to Earth orbit, where they will dock with the ISS (or its successor). At this orbital station, the crew will undergo a thorough medical examination (planetary protection) before being allowed to return to Earth. After this medical examination, the Starship and the Falcon Heavy, if found to be free of contaminants, will be able to return to Earth (if there is any doubt, they will be destroyed).
From these principles, we may look at the ‘details’.
The formula for calculating the center of gravity between two rotating masses is as follows: If we have two masses m₁ (Starship) and m₂ (Falcon Heavy upper stage and 3rd stage), connected by a cable of length L, the center of gravity is located at a distance d₁ from the Starship such that: d₁ = L × m₂/(m₁ + m₂). The mass of the Starship spacecraft is, in my hypothesis, at a ratio of 1.8/1 to that of the Falcon Heavy, but of course, this can be discussed. The imbalance between the two masses will undoubtedly be much more pronounced during the return flight. The rotation can be slowed to avoid a too strong gradient between head and feet. This will result in lower gravity. It’s possible to go as low as lunar gravity (0.16g) at the nose, which is much less comfortable (and will induce spending more time at the level furthest from the system’s center of gravity). But this lower gravity will be better than nothing, and will not be a problem since all the necessary support services will be available in Earth orbit and, more generally, on Earth itself, to help astronauts readjust to Earth’s gravity.
The artificial gravity inside the Starship depends on the distance from the center of rotation according to the formula: g = ω² × r, where ω is the angular velocity (in radians per second) and r is the distance from the center of rotation.
Important: The coupling of the spacecrafts and the mass imbalance do not affect the overall trajectory to Mars. Indeed, due to the conservation of the angular momentum, the rotating system maintains its common center of mass, which continues to follow the planned ballistic trajectory. Furthermore, no external force can act on the system other than solar gravity (which affects the entire system uniformly).
Nevertheless, the more asymmetrical the masses, the more the tethers experience different stresses on each side. Therefore, the tethers must be very strong. The tethers could have the following characteristics (recommendation claude.ai): material: Spectra 2000 (UHMWPE – Ultra High Molecular Weight Polyethylene), diameter: 20 mm, breaking strength: 180 kN, linear density: 0.15 kg/m, total mass (for 200 m): 30 kg. They will be held by a titanium frame 2.5 m in diameter and 0.5 m thick, integrated into the primary structure of the Starship’s nose cone. The entire set would have a mass of 520 kg (including drums and motors) and would fit within a volume of 2.5 m³ (inside the Starship nose). On the Falcon Heavy side, there would only be a guidance and attachment system.
Of course, the Falcon Heavy would need to be modified to perform the mission (without changing the maximum mass and thrust). The fairing module at the top of the rocket (which is the same as that of the Falcon 9) would need to be transformed into a true habitable module with a cargo bay (corresponding to the maximum mass the Falcon Heavy can carry). The upper stage (currently not designed for ground landing) must then be modified into a vehicle capable of landing on Mars, thus requiring retractable landing legs. Finally, the upper stage currently has one engine only. It would be safer to add a backup engine that would ignite in case of failure of the primary engine. The belly of the upper stage and the third stage would also need thermal protection, similar to that of the Starship, as it will be needed for crossing the Martian atmosphere and, upon return, for crossing the Earth’s atmosphere.
Conclusion: This proposal does not preclude the possibility of a successful first crewed flight using only Starships. However, it provides an interesting alternative in terms of passengers’ health (minimum gravity) and safety (smaller size).
Title illustration: the proposed system, visualized by claude.ai. The initial mass and rotational speed values have been changed along the study, but the principles remain the same throughout, and that’s what matters. Let’s say this diagram represents a « worst case ».
Copyright Pierre Brisson
