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Upon news of RocketLab's successful flight of the Electron launch vehicle, it was announced that not only had they deployed the three onboard payloads into the desired orbit, they had done so using a newly developed kick stage powered by an engine called Curie:

The successful launch of Rocket Lab’s Electron rocket Jan. 20 also tested a kick stage designed to circularize the orbits of its satellite payloads. [...] The heart of the kick stage is Curie, an engine developed by Rocket Lab.

From SpaceNews' article: Rocket Lab launch also tested new kick stage.

An image of the Curie kick stage can be seen in this photo supplied by RocketLab:

enter image description here

However, the kicker is that this doesn't appear to be using the typical monopropellant of Hydrazine:

The heart of the kick stage is Curie, an engine developed by Rocket Lab. Beck described the engine, capable of performing multiple burns, as 3D-printed and using an unspecified “green” monopropellant. [Emphasis mine]

What is this green monopropellant? Is it similar to AF-M315E, LMP-103S, or something else entirely?

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    $\begingroup$ Reddit chatter suggests (with no citations) either AF-M315E or RocketLabs' patented "viscous liquid monopropellant (VLM)", a thixotropic goo: google.com/patents/US20120234196 -- personally, though, and also without citations, I think if it were anything more exciting than peroxide they'd be bragging about it. $\endgroup$
    – Russell Borogove
    Jan 24, 2018 at 21:29
  • $\begingroup$ @RussellBorogove it's clear from that patent that the oxidizer is some kind of organic nitrate, not peroxide. Better specific impulse than peroxide but not particularly exciting. It's also clear that they're premixing fuel and oxidiser, which has some safety implications. $\endgroup$
    – Level River St
    Jan 24, 2018 at 22:25
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    $\begingroup$ I'm saying I suspect Curie's propellant is something old-school like peroxide monoprop rather than VLM, because if they were using their patented goo for the kick motor they'd want to say so, not that peroxide is part of VLM. $\endgroup$
    – Russell Borogove
    Jan 24, 2018 at 23:00
  • $\begingroup$ JAXA SHP163 might be another interesting candidate, which is HAN-based, like AF-M315E. However, it is unlikely Curie is running either on HAN or ADN based propellants as these have not been scaled up that big $\endgroup$
    – kert
    May 8, 2019 at 15:55
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    $\begingroup$ Also, Curie does NOT use VLM as per Peter Beck: reddit.com/r/space/comments/8a1wwy/… $\endgroup$
    – kert
    May 8, 2019 at 16:16

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Unfortunately, the composition of the monopropellant is a trade secret per Wikipedia's Curie (rocket engine):

While Rocket Lab is not known to have specified the monopropellant used by Curie, in 2012 Rocket Lab demonstrated the use of a non-toxic Viscous Liquid Monopropellant (VLM) that it had developed.

So I don't believe that we can actually know. We do know from Peter Beck that Curie runs on some hypergolic fuel, but that is all.

Additionally, in the years since Electron's first launch, RocketLab have developed a bipropellant version of the engine - see Electron's Payload User Guide (PUG). By inspection of the same PUG, they may even have phased out the use of the original monopropellant engine. However, the composition of the bipropellant is also unknown.

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    $\begingroup$ @Infinite_Maelstrom as Russell Borogove pointed out in comments on the question, the Electron user's guide only indicates a biprop kick stage (page 10). I think that's an indication that the monoprop version is no longer in use, though admittedly a weak one. Your point that the OP was asking about the monoprop engine is 100% valid though. $\endgroup$
    – Erin Anne
    Mar 23 at 0:33
  • $\begingroup$ I will keep the monoprop as Infinte Maelstrom is correct. The purpose of answers is to answer the OPs question @ErinAnne $\endgroup$
    – Starship - On Strike
    Mar 23 at 0:34

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