Leaving the surface of Mars
In “Going Down to Mars” and “Staying on Mars,” we described the descent and then life on the ground: the site, the habitat, energy, the people and their robots. But in our architecture, the surface is not a terminus. Sojourns there are short because of the weak Martian gravity, and every ship that goes down must go back up. It therefore remains to be shown that leaving the surface of Mars is surprisingly easy for anyone who compares it with departing from Earth.
Il s’agit de retourner dans les habitats que constituent les Eagles (chapelet de stations rotatives en orbite aréostationnaire), c’est-à-dire d’effectuer les voyages devenus « de routine », une fois la couronne des Eagles en place. Le départ jusqu’à la Terre depuis la surface de Mars et non depuis Deimos, ne se fera qu’à la fin des tout premiers séjours dans le monde martien, avant que la station Deimos-II soit construite et ancrée dans le sol de Deimos.
Let us make clear from the outset that the point is to return to the habitats formed by the Eagles (a string of rotating stations in areostationary orbit), this is to say, to undertake the trips that have become « routine », once the Eagles’ collar is in place (the very first departures will be the subject of a separate article). Departure from the surface of Mars for Earth and not from Deimos, will happen only at the end of the very first sojourns in the Martian world, before the Deimos-II station is built and anchored in the soil of Deimos.
Once that stage is passed, one will leave the surface of Mars only for Phobos, where one will change carrier, moving from a Starship (the equivalent of a “first stage”) to a Lyoba* (the equivalent of a “second stage”). Heavy freight intended for all the Eagles, including the propellants for the journeys to Earth, will go to Deimos. The people will go directly to their Eagle with the loads that this one station will need, unless they have to take part in a meeting on Deimos.
*high altitude conveyor designed by the Swiss startup, Pave Space.
Once “home,” (in the Eagle of the passenger) there will be a debriefing meeting, the reunion with the family, and in the evening stories to tell the children about Mars, its landscapes, its beauty, its dangers.
Illustration: back up to Phobos, then onward journey to an Eagle or to Deimos
Climbing to Phobos: propulsion does not limit the payload
The velocity budget from our equatorial site to the rendezvous with Phobos breaks down as follows: about 4.1 km/s for the ascent to low orbit (gravity and drag losses included, far smaller than on Earth — the planet’s rotation even offering a bonus of about 240 m/s when lifting off toward the east — in the Martian sky, Phobos rises in the West and sets in the East), 0.70 km/s for injection onto the transfer ellipse to Phobos’ altitude (5,980 km), 0.55 km/s for circularization and rendezvous, plus a margin — a total of about 5.3 to 5.5 km/s, to be compared with the 9.3 km/s of a mere insertion into low Earth orbit. That is the whole difference between the two worlds: what Earth forbids, Mars allows.
With a starship-type vessel (dry mass ~120 t, capacity of 1,200 t of propellants, specific impulse of 375 s for the six vacuum engines), the Tsiolkovsky equation gives the following results (depending on the payload carried):
| Payload lifted to Phobos | Propellants required (Δv = 5.35 km/s) |
| 0 t (empty vessel) | ≈ 395 t |
| 100 t | ≈ 720 t |
| 150 t | ≈ 885 t |
| ≈ 245 t (theoretical maximum) | 1,200 t (full tanks) |
The verdict is unambiguous: propulsion places no limit on the useful mass. The theoretical maximum (~245 t) even exceeds what the vessel’s structure can plausibly carry (100 to 150 t, the limit coming from the payload mounting, not from the engines), and the thrust-to-weight ratio at lift-off is so comfortable (greater than 3) that thrust will have to be reduced. A launcher is therefore genuinely unnecessary — and that is a decisive advantage of the Mars–Phobos pair over the Earth–low-orbit pair.
The constraint actually shifts from propulsion to logistics: each rotation to Phobos with 100 to 150 t of payload consumes 700 to 900 t of methalox produced on site. For ~800 t of propellants, the net reaction — the net balance of the Sabatier-electrolysis chain (CO2 + 2H2O → CH4 + 2O2) requires extracting about 400 t of water ice — which reinforces the first criterion for choosing the site of the ground base (presence of a water ice deposit) — and releases along the way a surplus of oxygen recoverable for life support. The energy cost of these tonnages and the duration of their production are quantified in the article “Propellants: needs, ground production and storage”; let us retain here the essential point: the pace of the surface–Phobos flights is directly proportional to the installed nuclear power (used to produce propellants) — that is the structuring link between the reactors and the base.
Moreover, the climb back up will never be made empty: no one will ever make a journey “for nothing,” and the residents of the Eagles will constantly need the products obtained on the Martian ground — water, propellants, the various products of the industry established there. One must therefore reckon that a Starship’s carrying capacity, i.e. 150 tonnes, will practically always be used in full. This is, moreover, the discipline laid down in the article on propellants — no Starship climbs back up empty — which, by entrusting propellant deliveries to mission rotations, brings the cost of a tonne lifted down from ~9 to ~4.3 surface-tonnes.
* Why this drop from ~9 to ~4.3? The two figures do not measure the same thing. Delivered by a dedicated tanker flight, a tonne of propellant must bear all the costs of the flight: the ascent of the empty vessel (~450 t of propellants, spread over the 150 t of payload), the ~3.3 t of propellants that lifting each tonne of payload costs, and the propellants for the descent — about ~9 surface-tonnes in all. When the same tonne rides up in the free hold of a mission rotation that had to fly anyway, those fixed costs are charged to the mission; the tonne of propellant then bears only its own weight and its marginal cost of ascent, i.e. 1 + 3.3 ≈ 4.3 surface-tonnes. The physics does not change: it is the elimination of dedicated flights that makes all the difference.
Two clarifications complete the picture. First, the descent from Phobos (deorbit, atmospheric braking, landing burn) costs about 1.1 to 1.3 km/s of propulsion, i.e. some one hundred tonnes of propellants to be loaded at the Stickney depot before going down — propellants themselves lifted from Mars during previous rotations: the loop is closed. Delivering one net tonne to the depot costs about nine produced at the surface — the detailed accounting is in the article on propellants — a ratio that remains far better than any shipping from Earth. Second, the orbitography is accommodating: from an equatorial site, a launch window toward Phobos opens about every 11 hours (Phobos’ synodic period as seen from the ground), and Phobos’ near-zero inclination (~1°) to the Martian equator spares any plane change — a further validation of the choice of site.
Resupplying the interplanetary fleet
One item of expenditure remains to be mentioned, the heaviest in the whole system: every 26 months, at each synodic window, a fleet of Starships arrives from Earth — the cargo ships at Phobos (where they will unload in a place protected from dust), the crewed vessels at Deimos. The latter will all have to leave again, for Earth or at least for the L1-TL station (the L1 Lagrange point of the Earth–Moon system). The return of the cargo ships will be less useful, since Mars will be more an importer than an exporter of massive materials. But rather than sending ships back empty, a use will be found for them: either renewing the fleet of surface–Phobos shuttles, or nevertheless carrying to Earth costly Martian products and/or ones available only in certain countries (such as rare earths), or industrial products generating large masses of CO2 (we do not pollute Mars with CO2!)
Cost of the return to Earth:
Arrivals cost almost nothing: a Martian aerocapture followed by circularization at Phobos’ altitude requires only ~0.55 km/s, i.e. 40 to 50 t of propellants taken from the journey’s margins. Departures are the real subject, and the good news is that leaving from the Martian moons is remarkably cheap: from Phobos’ orbit, injection toward Earth costs only about 1.9 km/s (2 to 2.5 km/s with margins and fast transits) — against ~3.8 km/s and 1,200 t of propellants to leave from low Earth orbit. This is where the “Tsiolkovsky reset*” argument plays in full. Per departing vessel: ~110 t of methalox for a cargo ship leaving empty, ~130 t for a crewed vessel with 20 to 30 t of payload, ~180 t for a cargo ship lifting 100 t back to Earth. An elegant orbital coincidence: departure costs almost exactly the same from Deimos, which loses in Oberth effect what it gains in potential — no penalty for the crewed vessels.
* The “Tsiolkovsky reset”: the Tsiolkovsky equation being exponential, each additional km/s asked of the same vessel costs more and more in propellants. Refueling at an intermediate stage — here Phobos — resets that exponential counter to zero: two modest legs cost, together, far less than one single large Δv crossed in one go. See “Phobos, the Marshalling Yard.”
For the crewed vessels docked at Deimos, propellants will climb from Stickney by tanker-Lyobas on the Phobos–Deimos link (~0.75 km/s), with a 30 to 50% surcharge. What this resupply demands of the ground nuclear park — and the sizing rule that follows from it, the size of the park being set by the departing fleet — is quantified in the article on propellants.
Thus the system’s economic loop closes: Earth supplies the vessels and the reactors, Mars supplies the energy and the reaction mass, and reuse turns a recurring cost into a one-time investment.
Note: one day, spacecrafts traveling from Mars to Earth will use nuclear energy. But that’s another story.
It will have been noticed: at every step of this demonstration — producing the propellants, sizing the park, setting the pace of the flights — it is energy that commands. It is the key to this text as to the previous ones, from Phobos to the Eagles. That is why the whole study will end, after the progressive development of Deimos and Phobos and the question “Deimos or Phobos?”, with the articles devoted to propellants: “Propellants: needs; Propellants: ground production and storage.”
Title illustration: Going back to the orbit from the surface. Graph made by claude.ai, upon request of Pierre Brisson. Credit Pierre Brisson
Copyright Pierre Brisson
xxxx
To find another post in this blog which could be of interest for you, click on:
xxxx
And, if you enjoy this blog, subscribe (it’s free)!
