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One of the proposals for Truax's original Sea dragon design was to refill the upper stage with oxygen gathered from the rarified atmosphere in orbit. This would allow a powered descent, presumably eliminating the need for a reentry shield.
Ion propulsion would be needed to counter atmospheric drag for such a plan to work, and it would obviously be most useful if a portion of the collected gases could be used as propellant, rather than having to ship up propellant especially.
An expansion of the on orbit refuelling idea, would be to gather both oxygen and nitrogen and manufacture ammonia/LOX bipropellant in orbit, using hydrogen brought up from Earth. This would make the propellant much more easily storable in orbit.
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I think this was the other topic that is also one which ignites flames within the nuke versus solar realm....
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SpaceNut,
I'm more interested in Iodine than Xenon as the next propellant for electric propulsion. Argon and Xenon have to be kept cryogenically cold to achieve sufficient bulk density to minimize the mass of the propellant tanks, whereas Iodine is solid at STP and has a bulk density of 4,933kg/m^3.
Iodine as an Alternative Fuel for Electric Propulsion
Look at the thrust graphs on pages 19 and 20 from the link immediately above. There's not a dime's worth of difference in terms of thrust per unit of input power when Iodine is compared to Xenon.
It's going to take more work to make this propellant a practical alternative, but the density impulse, ease of storage, and cost advantages offered are pretty significant for exploration missions. Iodine is $12,000/t, Argon is $5,000/t, and Xenon is $850,000/t. There's not really enough Iodine or Xenon for colonization campaigns, but there's certainly no shortage of Iodine that would preclude its use for space exploration. In terms of propellant cost, Iodine offers a 70 to 1 advantage over Xenon. The cost advantage of the smaller and lighter propellant tanks is likely to be much greater. Argon is the most likely propellant for colonization since there's plenty of it in the atmosphere of Earth and Mars to sustain such campaigns. In any event, the cost of Iodine vs Argon propellant, when compared to the cost of other components in the vehicle's propulsion system, is just noise.
It may turn out that propellant cost ultimately wins the argument for colonization, in which case O2 will be pretty hard to beat. As long as we're using electromagnets (VASIMR) that don't allow hot O2 to touch the walls of the engine, there's a case to be made for paying the piper for the added cost and complexity of actively cooling a cryogenic liquid using some combination of tank insulation and larger solar panels / power processing equipment. We have telescopes and other scientific experiments in space with cryocoolers that have been in continuous operation for more than 10 years, so it's not as if we don't know how to do this.
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The topic of Iodine Ion Thruster which is just an amount of push difference.
Energy, costs and mass seem to be defeat man thus far from getting off from this rock....
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Mini space thruster that runs on water
https://www.spacedaily.com/reports/Mini … r_999.html
Designs
https://www1.grc.nasa.gov/facilities/co … s-by-type/
Coil-On-Plug Ignition for LOX/Methane Liquid Rocket Engines in Thermal Vacuum Environments
https://ntrs.nasa.gov/citations/20170004966
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