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Comment by jsemrau | Hacker News Reader

Interesting that they still use cold gas thrusters. Electric propulsion (FEEP) is much more efficient for this kind of maneuver.

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From what I can see on wikipedia [0] the first FEEP to be used in space was in 2018. Proba-3's implementation phase started in 2014 [1]. The choice of thruster technology was probably made well before that. They wouldn't use a technology that hasn't flown yet, unless it was crucial for the mission.

[0] https://en.wikipedia.org/wiki/Field-emission_electric_propul... [1] https://www.esa.int/Enabling_Support/Space_Engineering_Techn...

That explains a lot actually

Everything has tradeoffs, and everything depends on use case.

The FEEP thrusters I've seen had a hefty power penalty to keep it on standby, and a long warmup time. Cold gas uses more propellant mass per unit impulse, but it can still come out ahead in total size/weight/power.

Yep, definitely power is an issue and a trade-off even with low levels of power. The other item is cost. Cold gas thrusters are super cheap and reliable. So the savings in amount of propellant in going to an electric propulsion system is not going to be worth it in the overall program. This is also because the total amount of maneuvering they are doing (in terms of deltaV) is probably not very large. Lots of little maneuvers. This is compared to something where you have large delta V for big maneuvers or continuously overcoming drag in low LEO orbits where the amount of propellant will become a huge design driver.

Is the power penalty really such a big issue anymore? The Enpulsion Nano for example uses 10-15 Watt only on hot standby.

From the article, Proba-3 has 10 thrusters, so that would bring the total to 150 Watt.

I don't know the power budget for the satellite, but we can make an educated guess : the occulter is 1.4 meter diameter, or a surface area of 1.54 square meter. That's an upper bound for the surface area of the solar panels - there's nothing sticking out beyond the disk, that would interfere with observations.

Solar power at earth is about 1.3kW/sqm, solar panels are maybe 20% to 30% efficient. That puts you in the ballpark of 400 to 600W.

150W on standby would be a pretty big bite out of your power budget.

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