4 ms·

Not sure what was meant in the article, but Isp is not the same as efficiency. Also, high Isp is not desirable for a power-limited spacecraft such as cubesats b

Not sure what was meant in the article, but Isp is not the same as efficiency. Also, high Isp is not desirable for a power-limited spacecraft such as cubesats because the input power scales with the square of Isp. The efficiency of an electric thruster is a measure of the losses related to ionization energy, beam divergence, beam temperature etc. In theory Isp and efficiency are therefore orthogonal, though Low-Isp devices indeed tend to have a lower efficiency due to higher ionization-to-kinetic energy ratio.

nine_k 6y ago
Input power can be plentiful with modern solar cells, and Sun just keeps on providing 1 kW / m² in optical emission. Propellant, OTOH, is finite; once you use it up, you cannot maneuver any more. Solar cells may degrade, but keep providing power for decades, if not centuries. So a long-lasting satellite which just needs to keep its orbit may be better off with a low-thrust engine not ejecting any reaction mass. A long-range mission with a finite travel time towards the outer reaches of the Solar system may be better off with a high-Isp jet engine.
dr_orpheus 6y ago
Isp is an efficiency, but it is specifically efficiency with respect to fuel usage. As this has commonly been the main measure of non-electric propulsion it is still valid to invite the comparison as part of the goal is still reducing mass by reducing fuel usage. It does seem the efficiency they are referring to is efficiency in ionization. And I agree it is a bit odd that they bring up the claim of "superior efficiency" and quote the power usage of the thruster (50W) without any notes about the actual thrust capability that it provides.
Symmetry 6y ago
So efficiency is always going to be some A divided by some B. Traditionall with rocket engines it means impulse divided by propellant mass expended. But given that there are both electric thrusters with much higher ISP and electric thrusters with much lower ISP (e.g. resistojets) I'm not seeing any reasonable efficiency metric under which I think the claim is plausible. Even in terms of losses electric thrusters that don't have to ionize their propellant have a pretty big advantage.
- binarycoffee 6y ago
  I am not sure I understand the objection (or maybe there wasn't one?) but since there seems to be some confusion in this thread on this point, the electrical efficiency of electric thrusters is very well defined, it is: η = ½Q·(Isp·g₀)² / P where Q is the mass flow rate (kg/s) and P is the input electrical power. Hall thrusters have efficiencies typically ranging from 30% (small HTs) to 60% (large HTs). Gridded ion thrusters have slightly better efficiencies (add 5-10%), mainly because they operate at higher Isp (they would actually have worse efficiency than Hall thrusters when operating in the 1000-1500s range). Why I object to the (admittedly common) characterization of Isp as an efficiency: it is always better to have a high efficiency, whereas Isp is always a trade-off between mass budget and power budget so optimal Isp is always mission-dependent.

nine_k 6y ago

Input power can be plentiful with modern solar cells, and Sun just keeps on providing 1 kW / m² in optical emission. Propellant, OTOH, is finite; once you use it up, you cannot maneuver any more. Solar cells may degrade, but keep providing power for decades, if not centuries. So a long-lasting satellite which just needs to keep its orbit may be better off with a low-thrust engine not ejecting any reaction mass. A long-range mission with a finite travel time towards the outer reaches of the Solar system may be better off with a high-Isp jet engine.

dr_orpheus 6y ago

Isp is an efficiency, but it is specifically efficiency with respect to fuel usage. As this has commonly been the main measure of non-electric propulsion it is still valid to invite the comparison as part of the goal is still reducing mass by reducing fuel usage. It does seem the efficiency they are referring to is efficiency in ionization. And I agree it is a bit odd that they bring up the claim of "superior efficiency" and quote the power usage of the thruster (50W) without any notes about the actual thrust capability that it provides.

Symmetry 6y ago

So efficiency is always going to be some A divided by some B. Traditionall with rocket engines it means impulse divided by propellant mass expended. But given that there are both electric thrusters with much higher ISP and electric thrusters with much lower ISP (e.g. resistojets) I'm not seeing any reasonable efficiency metric under which I think the claim is plausible. Even in terms of losses electric thrusters that don't have to ionize their propellant have a pretty big advantage.

binarycoffee 6y ago
I am not sure I understand the objection (or maybe there wasn't one?) but since there seems to be some confusion in this thread on this point, the electrical efficiency of electric thrusters is very well defined, it is: η = ½Q·(Isp·g₀)² / P where Q is the mass flow rate (kg/s) and P is the input electrical power. Hall thrusters have efficiencies typically ranging from 30% (small HTs) to 60% (large HTs). Gridded ion thrusters have slightly better efficiencies (add 5-10%), mainly because they operate at higher Isp (they would actually have worse efficiency than Hall thrusters when operating in the 1000-1500s range). Why I object to the (admittedly common) characterization of Isp as an efficiency: it is always better to have a high efficiency, whereas Isp is always a trade-off between mass budget and power budget so optimal Isp is always mission-dependent.