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#26 2022-04-03 12:47:11

Registered: 2015-01-02
Posts: 7,607

Re: Large Ship Momentum (Mass) Management


I need to scale up the solar power array size (4.8MWe), the PPU (4.8MWe), and the input current into the MPD thruster (50kA vs 15kA).

The mass of ATK MegaFlex panels to output 4.8MWe is about 40,000kg.

The mass of the MPD thruster is no more than 50kg, tops.

The mass of the PPU is unknown to me.

I am aware of MPD thrusters being run up between 15,000 Amps to 20,000 Amps of input current.  I'm unaware of any that were operated at 50,000 Amps.  The MPD thruster's thrust output level is a function of anode radius to cathode radius and input current.

ra = radius anode
rc = radius cathode
μ = permeability of a hard vacuum / free space

F = ((μ * I^2)/4π)(ln(ra/rc)+0.75)
I ≈ sqrt(4π/(μ/(ln(ra/rc)+0.75)))

Anyway, for MPD thrusters, this is how current affects thrust:
10kA input current delivers ~20N of thrust
50kA input current delivers ~500N of thrust
100kA input current delivers ~2,000N of thrust (for my ship design, we need to be here)

I require about 100N of thrust to complete the transfer in 2 months.  More thrust would obviously be better, but I don't have to invoke any new and untested MPD thruster design to do this.  I don't have to go beyond existing MPD thruster performance using H2 propellant.  With the 4.8MWe solar array and ~100N of thrust, the spiral out will be completed in 2 months, and ΔV works out to about 8km/s.  With 500N of thrust, the spiral out will be completed much faster.  The cost of the higher thrust / faster spiral out will be rapid destruction of the MPD thruster's cathode.  At 50kA to 100kA, the cathode will be toast by the end of each firing sequence, and possibly before then.

Edit: Argh! Stupid misplaced zeros...

I will need 2kN of thrust for a 39 day spiral out, not 500N.

Last edited by kbd512 (2022-04-03 16:47:54)


#27 2022-04-03 16:09:40

From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 7,852

Re: Large Ship Momentum (Mass) Management

I am assuming several new technologies that make a significant difference, and are quite feasible. The best photovoltaic panels currently available from SpectroLabs are XTE. With standard fluence (light intensity) they provide 32.2% conversion to electricity Beginning-Of-Life, 27.9% End-Of-Life. This is triple junction, the top junction is gallium-indium-phosphate. However, remember the paper I have cited from year 2000: gallium-indium-nitride with 70.2% efficiency BOL with 8 junctions.

Any sort of electric propulsion has the problem of low thrust. High specific impulse, but low thrust. That means it takes significant time to develop speed for TMI. The ship should park in LEO, when at Earth, because that has low radiation. At Mars we have a couple choices: highly elliptical High Mars Orbit has advantage of minimal delta-V required for Mars orbit departure; disadvantage is high propellant required for a shuttle between Mars surface and the ship. Other option is Low Mars Orbit, which requires more propellant for Mars departure, but easier to access from Mars surface. Again I'm assuming one or more SpaceX Starships will be permanently parked on Mars to act as shuttles.

GW Johnson wrote:

And it is why I suggested getting gas core nuclear and the fission explosion technologies into serious development,  because they look even better!  That's what the numbers I have been running are really saying!


#28 2022-04-03 16:55:33

From: Northern England, UK
Registered: 2019-08-18
Posts: 3,572

Re: Large Ship Momentum (Mass) Management

The cathode of an MPD thruster appears to be a pin.  It has a very high current density and is therefore a region of high heat generation.  Could the cathode be a wire that is wound out like a welding electrode?  The other option is to bleed some propellant through pores in the electrode and cool the surface that way.  But I doubt that would work.  It seems better to design the engine such that the cathode is a sacrificial element.

I have been reading about IC Fusion for a while.  I have run calculations on how much fissile material is needed to heat a central hot spot containing 2% of Fusion fuel to 5KeV.  For a 1mg fuel mass, it works out to be 1E-4 pellet mass, yielding 1E-5 of total energy yield.  That should be sufficient to initiate a detonation wave within the pellet.  For a 2mm diameter pellet, there is sufficient fissile material in a single smoke alarm.  Fusion fission micro explosions are something that we know can be made to work.  It is more a question of what the minimum possible size is and what can be used as a satisfactory driver.  I feel that I have taken the work almost as far as I can.  I don't have sufficient knowledge of plasma physics to model this in more detail.  And learning would take more time than I have.  An experiment would be useful.  But few facility owners would want an experiment that generates fission products in their facility.  The purist attitude of Fusion researchers is an impediment to useful power generation.

Last edited by Calliban (2022-04-03 17:22:34)

"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."


#29 2022-04-03 17:55:59

From: New Hampshire
Registered: 2004-07-22
Posts: 29,310

Re: Large Ship Momentum (Mass) Management

Please consider the tunnels being brought out to the full hull width of 19 meters as it will give more space and structural force capability for moving it based on the internal ribs and stringers that will be used to help in the forming and welding of the shell.

The larger tunnel will allow for a bi-direction elevator that can now have one at each end of the tunnel and switch location when one is used so as to increase safety needs.


#30 2023-01-15 12:33:31

From: New Hampshire
Registered: 2004-07-22
Posts: 29,310

Re: Large Ship Momentum (Mass) Management

Of course, a space tug, departure and arrival contain the concepts of just what this title implies for its use.


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