News hit the internet this month of SpaceX's acquisition of a pair of mothballed Oil-rigs, renamed on-brand to moons of Mars - Phobos and Demios. Reported by a few mainstream publications here, and here - even on the wikipedia page.
Shot of the Heli-deck on the recently renamed Deimos - Martian Moon - oil platform. Credit: Jack Beyer - Nasaspaceflight.com
SpaceX fans and shutterbugs alike are also snapping necks to get a glimpse of progress on these new behemoth additions to the SpaceX fleet. The community can look forward to some excellent renderings and professional quality home-made cutaway animations, launch, landing and fun renders.
I'm taking a moment here to review some key safety and design philosophy requirements that will be essential to minimised safety risks as SpaceX leads the trans-planetary travel effort.
Addition of fire and gas detection systems. Being powered by Liquid Natural Gas (LNG), there are particular precautions to take. First we look at philisophy and the characteristics of LNG.
Consider that, being super cold, and in liquid form, LNG, will tend to spill and spread like a conventional gas or diesel spill. This means that there needs to be some such consideration in starship bunkering fuel supply design concept.
Let's take a moment to consider this rocket fuel itself: LNG itself is too cold and too high in 'richness' of stoichiometry to burn. It does not burn. CH4 vapor does burn - but only during the range of 5 - 15% volume in air. However, LNG vapor will explode, like any other flammable gas, if it accumulates in a confined space, or space with a lot of obstacles and equipment in the way - Think Buncefield - conflagration, air mixing leading to detonation.
In this way, SpaceX can borrow from some concept in the land-based truck and ship refuelling sector, here. Here, detection of leak or spill is a key piece. There are procedures around bunkering - and BOC procedures do a lot to ensure that particles - dust, dirt - don't enter or get associated with the fuelling infrastructure - nitrogen purging of lines, etc. This removes point defects which could disturb the fuel, or damage the fuelling infrastructure.
Looking again at the mobile launch platform model - these platforms will be based on the water. Another hazard to look out for is the rapid phase transition that occurs when super-cold LNG (-161 degC, or -259 Fahrenheit) contacts with water (in the Mexican Gulf climate zone - ranging from 12 to 20 deg C). When this event happens, the LNG 'flashes' and expands in volume - changing phase from liquid to gas phase. In this process, the vapor cloud expands to 600 x original volume - creating a huge cloud of expanding, cooling vapor.
Given that a starship/ superheavy configuration can accomodate 1200 t, and 3600 t respectively, this means that SpaceX needs to have approximately 4800 t of LNG on hand for each launch.
Given a relative density of (at the upper end estimate - LNG is a mix of hydrocarbons including majority of CH4) LNG - at 470 kg / m3, this means that a volume of approximately 10 k m3 is needed for each launch - see here for physical size of a 10 k m3 tank.
SpaceX will need to take the call as to whether they have storage on the platform, taking regular deliveries from a barge or some such bunkering arrangement - or pipe it in from the shore for "Just-in-time" refuelling. If they go with on-platform storage of LNG - they will adhere to standards similar to "International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code)", with cryogenic storage, and ship-to-ship refuelling infrastructure.
Render of proposed SpaceX future Sea Launch platform. Note the storage spheres for liquid oxygen and LNG propellant at either ends of the platform. Credit : SpaceX via Forbes
I initally thought, seeing the size of a 10k m3 tank, that the large volumes of LNG required would drive SpaceX to pipe in the fuel as needed from shore. However, considering this render from HumanMars.com, the starship is itself dwarfed by the mockup rig. This means that - the volume of all the fuel and Liquid Oxygen (LOX) required for each launch will fit inside the footprint of the platform quite comfortably, and indeed enabling the designers to give adequate separation distances between critical parts of the vessel.
Ultimately, the risks and hazards can be designed, controlled and operated to a minimum level, to enable operations to reach an ALARP risk level. I am confident that the team at SpaceX will do a great job from a technical perspective, and also in compliance with design standards and disclosure to the public well ahead when launches start departing from these facilities.
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