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Astronatix's Intelsat 39 lists the two engines

  • SPT-100 Stechkin 1.35 kW Hall thruster.
  • R-4D Marquardt N2O4/MMH rocket engine. 490 N.

Question: Why is this combination on Intelsat 39 called "hybrid" exactly? Do they operate together in some way? How are they operated (in combination or separately) to raise the spacecraft's orbit to GEO? Since all-electric is in vogue now, why have the R-4D at all?

Stechkin 1.35 kW Hall thruster

R-4D Marquardt N2O4/MMH rocket engine

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  • $\begingroup$ companion question: How does a Hall effect ion thruster have a sea-level Isp of 800? Operates at 1 atmosphere? $\endgroup$
    – uhoh
    Aug 8, 2019 at 12:24
  • $\begingroup$ I am slightly confused about the Hall Thruster's supposed thrust, which is declared as a whopping 196N by astronautix (the link you provided) while Wikipedia, referencing the tech report paper, puts it at a more realistic 80mN. Most likely a mistake by astronautix - electrical thrusters have VERY little propulsion, they'd quite literally do nothing in non-theoretical atmospheric conditions. This is also why there are two thrusters: An electrical one for constant raising of the orbit (spiraling up) and a conventional thruster when you need actual power. $\endgroup$
    – Infrisios
    Aug 8, 2019 at 13:27
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    $\begingroup$ @Infrisios I believe the hall thruster is used more for station-keeping as opposed to orbit-raising. If I were to guess, the R-4D would be used for initial orbit raising when the satellite is first deployed. $\endgroup$
    – aaastro
    Aug 8, 2019 at 13:37
  • $\begingroup$ @Infrisios if you click the companion question in the comment above you can see the discussion about the engine specs. There's likely been an error editing that page. However, as mentioned in the question there are all-electric communications satellites and they can take moths to go from GTO to GEO by ion propulsion. (1 2, 3, 4 and en.wikipedia.org/wiki/Boeing_702#702SP) $\endgroup$
    – uhoh
    Aug 8, 2019 at 14:27
  • $\begingroup$ I've deleted the 196 N, that's got to be a mistake on the site. $\endgroup$
    – uhoh
    Aug 8, 2019 at 15:33

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I think the term is used like it is with automobiles. According to Wikipedia's hybrid electric vehicle page they don't have to operate together to be a hybrid (at least for autos). I would think that keeping the trusty R-4D, see Wikipedia's "Used In", adds the some all important reliability to the system.

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