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A discussion below How much of a deep space spacecraft's structural mass is useless dead weight after launch? Any plans to shed it in the future? has lead to https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2006-001A which contains basic data on the New Horizons spacecraft, including

The 465 kg launch mass includes 80 kg of propellant

and

a delta-V capability of 290 m/s will be available after launch

The average mass 465-80/2 kg times the delta v 290 m/s divided by the propellant mass 80 kg suggests an exhaust velocity of 1540 m/s, or an Isp of 157 seconds.

The first hits in google I received for "isp of hydrazine thruster" were

Question: Are my estimates representative of New Horizons' thrusters roughly correct? Are they somewhat on the low side? If so, is it because there were tradeoffs between performance parameters?

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    $\begingroup$ Maybe. They tell after subtracting the attitude control requirement throughout the life of the spacecraft. $\endgroup$
    – zephyr0110
    Feb 6, 2021 at 10:06
  • $\begingroup$ @Prakhar good thinking, indeed that seems to be the case. $\endgroup$
    – uhoh
    Feb 6, 2021 at 13:38
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    $\begingroup$ Nit picking: Perhaps the Isp characteristic should be addressed in terms of efficiency, not "strength." Note that some of the highest Isp's you'll find come from very "weak" (low thrust) engines...e.g. I would replace "weak" in the title with "Low Efficiency". Same for the reference to "weak" in the body of the question. $\endgroup$
    – Digger
    Feb 7, 2021 at 18:24
  • $\begingroup$ @Digger I agree, I remember struggling with that. I've made a change but it's still not optimal. Please feel free to edit any way you see fit. Thanks! $\endgroup$
    – uhoh
    Feb 7, 2021 at 23:52

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Several sources (close to the end) show a table with a rough breakdown of the fuel use.

The total amount of fuel of 77 kg was used both for trajectory change and attitude control. The Δv = 242 m/s number does not include the fuel used for the latter.

There are 29.3 kg of fuel reserved for attitude control, leaving 47.5 kg for propulsion. Making the same simple assumption for average weight as stated in the question, this gives an Isp = 220 s, right in the same ballpark as the two thrusters mentioned.

There's a second source to estimate the thruster performance: Guo and Farquhar, New Horizons Mission Design:

Furthermore, it is estimated there will be as much as 47 kg propellant remaining after the Pluto flyby for the Kuiper Belt mission, corresponding to a ΔV capacity of 230 m/s.

This conversion gives us a second Isp calculation of about 205 m/s, confirming an average performance of New Horizons thrusters.

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