Frank

"Spiral" was not the right word, still learning the nomenclature, I just meant to say get to EML-1 efficiently and so, slowly, using solar engines. I guess I'll try and lay out a first draft scenario... Main ship assembled and fuelled at LEO, efficiently, slowly, using solar energy, reach EML-1. Mate the upper stage with crew capsule of a Falcon Heavy fitted with a Raptor engine to the main ship which has all the habitat module, fuel tanks and solar thermal engines/concentrators. Accelerate the ship with the Raptor engine. Use the solar thermal engine to accelerate and decelerate to capture orbit at Mars. Separate the upper stage and crew capsule for a Mars aerobraking landing. Previous drops have fuel and life-support , habitat etc.. Orbital refuelling of the joined ship and similar return journey to Earth. Separated main ship parks at EML-1 until the next mission. Following the Musk plan for colonization, more ships would be added for each launch window. No numbers yet or even a feasibility inkling... I agree about hydrogen, I was thinking methane if long term storage is needed or hydrogen if usable quickly. Boil-off seems to be an issue. SpaceX numbers? Don't know either way. They may have a secret/alternate/improved/yet unknown plan that differs from the original, would not surprise me. I'm not keen on the drop 100 people off for 2+ years on the first try without even knowing if 38% gravity is enough to sustain life that long... Why EML-1? The idea of getting LOX from the lunar surface is appealing, if not in the near term, in the long term since getting Oxygen from the Lunar surface is less difficult than from Earth and can reuse upper stages of rockets as vehicles. Lunar LOX could also be dropped at some Mars orbit and on Mars for refuelling. This all assumes it is cheaper to get Oxygen from the moon than the extra cost of lifting it from Earth to space. Also, a mass driver might ballistically launch fuel from Earth or Luna to EML-1 without much rocket Delta-V, basically just manoeuvring thrusters. Raptor Upper stage? That means 2..3.5 MN of thrust, perhaps convertible/augmentable with hydrogen instead of methane (it can be done in IC engines): “Nevertheless, Raptor itself is clearly well on the way to full production, partly thanks to the US Air Force. Yup, SpaceX isn't the only organisation interested in this technology. From 2009 to 2015 Raptor development was funded solely by SpaceX, but in January 2016 SpaceX pocketed $33.6 million from the US Defense Department to develop a prototype version of an upper-stage variant of the Raptor designed to be used on the upper stage of a Falcon 9 and a Falcon Heavy.” https://motherboard.vice.com/en_us/article/bmvpyw/how-spacexs-new-raptor-engine-will-get-us-to-mars

Enthalpy's threads say it can be done: Solar Thermal Rocket Manned Mars Mission Non-Hohmann to Mars Here's a Delta-V map to get an idea of the scale of things: https://upload.wikimedia.org/wikipedia/commons/8/8f/Delta_V_Earth_Moon_Mars.png I like Enthalpy's idea of using STR (Solar Thermal Rocket) to spiral up from LEO. Maybe it was implied, but the manned capsule would not be aboard for the slow trip, it would rendez-vous with it at, say EML-1 where the ships dock and maybe refuel. Doing this would save a lot of launches compared to sending everything directly to EML-1. It seems high delta-V is key for fast transit which is required to keep Radiation exposure minimal since shielding so far requires very large mass which is difficult to move. This is where a STR or Nuclear Rocket or Electric Drive is important, though Musk's ITS also promises 80 to 150 days, but for a 2+ year stay using a Methane-Oxygen Rocket with 382s Isp. . "The transport capacity of the 2016 spaceship from low Earth orbit to a Mars trajectory—with a trans-Mars trajectory insertion energy gain of 6 km/s (3.7 mi/s) and full propellant tanks—was projected to be 450 tonnes (500 tons) to Mars orbit, or 300 tonnes (330 tons) landed on the surface with retropropulsive landing.[32] SpaceX estimated Earth-Mars transit times to vary between 80–150 days, depending on particular planetary alignments during the nine discrete 2020–2037 mission opportunities, assuming 6 km/s delta-v added at trans-Mars injection.[32]" https://en.wikipedia.org/wiki/Interplanetary_Transport_System#Passenger_spaceship From reference 32 above, SpaceX expects to burn off 8.5 km/s in atmosphere at Mars and 12.5 km/s at Earth, so aerobraking instead of deceleration. Given a hydrogen rocket Isp of 450s, higher speeds could be reached, and would require some deceleration, perhaps offset by the reduced mass of hydrogen? Maybe ~15% faster? Compare to the Nuclear mission in the previous post, SpaceX would take 90 days instead of 124 there and 206 back, though the nuclear short-stay mission is a greater distance... Hmmm.

One year roundtrip, 30 day stay to Mars. Is it possible with solar thermal engines instead of nuclear thermal? https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140009587.pdf We can assume Earth and Mars capture orbits for the solar thermal part and chemical rocket to and from capture orbits. Perhaps with slower unmanned propellant drop missions ahead of mission if necessary.