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
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
- 20N Monopropellant Hydrazine Thruster Isp: 222 to 230 seconds
- 1N Monopropellant Hydrazine Thruster Isp: 200 - 223 seconds
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?