NASA Fires Up Powerful Lithium-Fed Thruster for Trips to Mars

[eppur_se_muova]

13. More from the paper cited by OKIsItJustMe ....

The thrust production mechanisms in self-field thrusters are very well-understood,14 and the total electromagnetic thrust, which is equal to the Lorentz force density integrated over the discharge volume, is approximated well by the Maecker formula^15

TSF = b*J^2 {the rest of the formula involves only the engine dimensions}

where {T-sub-SF is thrust, J is total current, and} μ0 is the permittivity of free space, ra is the anode radius, and rc is the cathode radius. Unlike other thruster types, the electromagnetic thrust is independent of the flow rate (dm/dt) and propellant properties{!!!}. The specific impulse is given by

Isp = b*J^2/(dm/dt) .

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The useful work in an MPD thruster is that done by the Lorentz force, and the thruster efficiency is determined by the magnitude of the thrust power compared to loss mechanisms such as frozen flow losses and power dissipated at the electrodes. The efficiency with inert gas propellants is generally low, not exceeding 20-30%. With hydrogen the efficiency can exceed 50% at over 10000s and very high power levels (over 10 MWe per engine). Lithium appears to be the best option for powers up to several MWe per engine and more moderate Isp, having demonstrated 50-60% efficiency at 4000-5000 s. This is in part because of low frozen flow losses. The first ionization potential ... {see OKIIJM's post for the rest}

https://alfven.princeton.edu/publications/pdf/polk-iepc-2024.pdf

So my remarks about low MW of the exhaust atoms -- so important in chemical rockets -- were barking up the wrong tree. The use of high MW ejecta in electrostatic ion drives is particular to that particular mode of propulsion. MPD/MHD is all magnetism*, baby, and the equations reflect that -- they are profoundly different from the derivation of SI for chemical rockets. So no need to look for low or high MW (bye-bye Li-6, which seemed like a neat tweak), and other factors dominate the choice of propellant.

It must have been a real pleasure to work out the mathematics of MPD for the first time and realize that by a truly remarkable cancellation of terms, suddenly MW didn't matter -- nor did mass flow ! It would have been such an astonishing result -- almost an apostasy -- that it looked like it was more likely to be a mistake. As the equations above show, once a particular MPD engine is designed, varying thrust is as simple as varying current. Turn a knob, get more acceleration. And with the advantages of lithium spelled out, it's hard to see how anything better is likely to be found -- but then, it's not obvious why Li is a better choice at some energies and H2 better at others -- all the advantages of a Li+/e- plasma seem to apply to an H+/e- plasma as well. It seems to be a matter of some fairly subtle tweaks -- just the thing to be worked out by dogged engineering, not just shopping the Periodic Table.





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