Watching the video of the 1 May NROL launch, I was surprised at the frequency and apparent size of the cold gas thruster firings. Does anyone know approximately how many kilograms of N2 are required to be loaded to fuel these firings? Is the loading a standard quantity or are there different requirements for a sea or land recovery?


1 Answer 1


Pretty sure only SpaceX knows.

Assuming ~2 m/s of delta-v, an ISP of say 50 s and we get 0.4% of the mass of the rocket in nitrogen. Now the mass of the rocket is somewhere between its dry mass and its wet mass, which again is unknown but somewhere between ~40 tons and ~400 tons, that would mean 160 kg to 1600 kg of nitrogen gas, probably much closer to the former as the thrusters are only used when the rocket is nearly empty.

If your asking your self why don't they use more efficient thrusters like bi-propellant N2O4+MMH well run the numbers: A system with an ISP of say 300 s would weigh 0.07% the mass of the rocket or 27 - 270 kg of fuel, but then you need the mass of the fuel and oxidizer tanks, mass of double the piping, mass of the pressurizing gas, mass of pressurizing gas tanks, mass of control system. The simplicity of cold gas thrusters is probably worthwhile in weight alone when you only need a few delta-v for a few minutes of attitude control consisting of one back-flip and some aiming.

  • $\begingroup$ @ bendy. Thanks for the answer. Do you know if the ISP decreases significantly from first use value as the pressure of the stored N2 decreases with use? $\endgroup$
    – Mike H
    May 4, 2017 at 5:13
  • $\begingroup$ The Isp is much much less affected by decreasing pressure than the thrust. The thrust would tail off to zero at an empty tank. Using a regulator would even it out, i.e. allow a lower thrust level for longer, but wouldn't improve the total impulse delivered. $\endgroup$
    – Puffin
    May 4, 2017 at 9:46

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