Mars Colonization Transport : latest speculations before revelations

Richard Heidmann translation: Etienne Martinache In our analysis issued at the end of 2015 (see also its summary in English) we tried to make out what the SpaceX project called Mars Colonization Transport (MCT) would look like. Let us bear in mind that Elon Musk should disclose the details of this project on 27th September 2016 at the occasion of the next International Congress of Astronautics. This guesswork exercise took into account then the sparse amount of clues gleaned among the various statements made by the famous space entrepreneur, some of which seemed to contradict previous information (for instance: dropping the multi-body launcher option, Falcon Heavy type) or, on the contrary, seemed to confirm basic options such as refueling on earth parking orbit by a second launcher, total re-use and descent on Mars of the whole interplanetary shuttle (« landing the whole thing »). Since then, other clues from various sources showed up on the internet discussion groups of space enthusiasts. Let us have a look at their possible meaning. A 15 m diameter launcher In order to meet the required performance (landing a 100T payload on Mars), choosing a single body and re-usable launcher led us to increase the diameter of the propulsion bay to 15m, even if it looked like keeping the diameter of the stage proper to 12.5m was sufficient (for the record, the diameter of first two stages of the Saturn 5 lunar rocket was 10m). This configuration made it possible to accommodate a maximum quantity of 31 motors of 300T of thrust each and reach a total weight before liftoff (Gross Lift Off Weight or GLOW) of 7750T. Unfortunately, the flared shape of the bottom part of the first stage of the rocket, while favorable to its stability during the ascent phase of the flight, is very unfavorable during the re-entry phase (reverse flight), all the more so as the means to stabilize the trajectory are then much less efficient during this phase. Since it is not reasonable to reduce the GLOW and consequently the liftoff thrust, the solution consists in bringing the diameter of the whole stage up to 15 m. Now, one of the rumors spread across the « MCT-geeks » discussion groups deals precisely with that option; SpaceX is said to have sent requirements to tool suppliers for this same diameter. A Gross Lift Off Weight (GLOW) of almost 10,000T At that time, the motor type mentioned was a 250T thrust Raptor (optimal size per mass according to SpaceX). While not increasing too much the quantity of motors (considering what is usually accepted by Falcon Heavy specialists), we had deduced that the motor thrust should be brought up to 300T, which makes it still possible to accommodate them within a 15m bay diameter. The problem is that even with such a high power level, the required performance is still not met (unless we consider that our assumptions regarding the shuttle structure weight are too pessimistic); indeed this configuration leads to a possible payload on Mars of 90T (while the desired workload is 100T), moreover the calculation was made taking into account the most favorable conditions: Mars at the perihelion and long duration transfer orbit (Hohmann transfer orbit). Now, another rumor going around is that SpaceX is considering a 700T thrust Raptor! This amount of unit thrust would make it possible to increase significantly the liftoff capacity and overpass the thrust requirement, which can then be reached with 13 such motors only, which are easy to accommodate within the bay. Considering the performance reduction, we may speculate that, once the decision has been made to go for a … Lire la suite de Mars Colonization Transport : latest speculations before revelations