Thursday, October 20, 2016

Reddit is distracting

Low post rate here lately has been because I've gotten sucked into reddit's r/spacex forum.

Partly for my own reference, here are links to some comments and submissions I've made:

Single-window round trip of the ITS ship
This was a trial run using Trajectory Optimization Tool to see if it was possible to send an ITS ship to Mars and return it within the same window. Provided you can refuel the ship in about a week and can handle a long (>200 days) return trip, it's definitely possible.

ITS system performance tables for near-Earth space
(public spreadsheet supporting tables)
A table of payload performance values for missions to various locations around the Earth and the Moon, with estimated costs.

ISRU system scaled to fuel one ITS ship per window, pt1
An extrapolation from an ISRU study that outlines the equipment needed to refuel an ITS ship (1950 tons of propellant) in one synod (~780 days). Includes mass estimates and is based on fairly good data. TL;DR is about 101 tons of gear and 20 tons of spares, with another 10-20 tons of spares each trip. The ISRU advantage is 16:1 for the first trip and about 98:1 for the next four trips. If the replacement cost is amortized over ten years then the advantage is 48.5 tons of propellant per ton of equipment.

Discussion of radiation shielding
This is an extension of my thoughts on shielding for large, permanent habitats that require Earthlike radiation levels. The conclusions do not apply for spacecraft in general because most spacecraft proposals cannot support several tons of shielding per square meter of surface area.


  1. Wow, that's quite the collection of crunchy data!
    I'll be reading through those posts for sure.

    On G+, there has been talk of stripping away the heatshield and aerodynamic habitat modules to turn the ITS into a more sensible solution for non-Mars destinations. Can you confirm if this is even possible?

    1. It would be a different ship, which would need to be simulated, built, tested independently.
      I don't think removing the heatshield material from a baseline hull would be feasible, but it should be possible. Removing the pressurized volume really would be impossible; that's a load-bearing component during atmospheric ascent.

      The approach depends on what kind of mission(s) you have in mind. If you want to do a one-off probe or something then use a baseline tanker and hack a payload deployer into the upper section.
      If you want to send multiple interplanetary probes a year then it would make sense to build a dedicated exploration version. This would probably use the same hull but with proper payload bay doors. Omit the heat shielding and sea-level Raptors, add extra insulation and cryocooling and add some deployable large-diameter dishes for communication. Dry mass would probably still be around 80-100 tons. (That would include a modest fission power source in an alternate reality where the word 'nuclear' doesn't make people's heads explode.)

      Suppose an exploration version is built, masses 120 tons dry, carries 2,400 tons of propellant and has about 500m³ of cargo volume. Further suppose that this is meant for probing the gas giants; it has 100 kW of fission power and 200 kW (@ 1 AU) of PV.
      This craft would have about 9.2km/s of dV with 100 tons of payload. 10 tons, 11.1km/s. 500 tons, 6km/s.

      Add a supplemental plasma thruster system with Isp of around 3000. Assume we reserve 10% of the propellant mass (240t) to be used in this thruster. (Ideally it would consume the existing methalox or a similarly dense cryogenic propellant.) That changes things a bit... the 100-ton payload case would have 6.5km/s of chemical dV for efficient departure and anywhere from 2.8km/s to 21.7km/s of electric dV. The exact amount for an orbit mission will be somewhere in the middle, since the thruster would be used after departure but before capture. A flyby would get the full benefit, a total mission dV of 28.2km/s.