I read on https://arstechnica.com/information-technology/2020/01/directv-races-to-decommission-broken-boeing-satellite-before-it-explodes/ (mirror):

In the time available, DirecTV said "it will be able to deplete only a nominal portion of the approximately 73kg of bipropellant remaining onboard Spaceway-1." Fully depleting the bipropellant would take two or three months.

Why does it take Spaceway 1 two or three months to dump 73 kg of bipropellant? That seems longer than what I would have guessed.

  • $\begingroup$ From the link "The 15-year-old Boeing 702HP satellite is in a geostationary orbit." "The risk of a catastrophic battery failure makes it urgent that Spaceway-1 be fully de-orbited and decommissioned prior to the February 25th start of eclipse season." But geostationary satellites are not deorbited, they are sent to a higher graveyard orbit. Deorbiting them requires much more fuel than available. $\endgroup$ – Uwe Jan 25 '20 at 20:49
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    $\begingroup$ I’m guessing there’s some limiting factor on the fuel dump rate, possibly either: - reducing orbit disturbances due to venting - reducing “crop dusting” effects of fuel on nearby satellites - small valves not allowing for much flow rate $\endgroup$ – CourageousPotato Jan 25 '20 at 21:26
  • $\begingroup$ Do they have real dump valves, or are they just burning it? $\endgroup$ – ikrase Jan 27 '20 at 10:59
  • $\begingroup$ "The procedure from the manufacturer limits the effective rate at which bipropellant can be vented to ensure builtup momentum isunloaded before subsequent operations occur" $\endgroup$ – user20636 Jan 27 '20 at 12:52
  • $\begingroup$ Why can't they just burn half the fuel prograde, then in half an orbit burn the rest to circularize? $\endgroup$ – Loren Pechtel Jan 27 '20 at 15:45

From the Waiver Request linked from the article in the question:

The Boeing 702HPspacecraft design features a reaction wheel pointing control system. As all venting must occur within the capabilities of this pointing control system, the firing of thrusters for venting and to address momentum buildup must be of more limited duration and additional time is needed for the pointing control system to respond. Other spacecraft of similar design have required two to three months of continuous operations to fully deplete their bipropellant systems.

and in fact, very little time may be available for venting:

In the absence of additional ground station solutions, Spaceway-1 will first need to increase its eastward drift before turning around and completing a near-continuous burn until it reaches its disposal orbit. This sequence will take approximately 21 days,leaving at least seven days for venting operations before the spacecraft is to be decommissioned.

In addition some of the propellant can't be vented:

Spaceway-1 is a Boeing 702HP that is not designed to vent the pressurant used during orbit raising, which was permanently isolated from the propulsion system by firing a pyrotechnic valve at beginning of in-orbit life. Information regarding the approximate amount of residual helium and xenon gas that cannot be vented at the end of operational life is as follows: HE1: 0.5096 kg, HE2: 0.5096 kg, XE1: 5.05 kg, XE2: 5.05kg

But the whole thing seems a little confusing as the satellite has two propulsion systems

  • Moog Columbium 22N MMH/MON thrusters, used for orbital insertion
  • Boeing XIPS-25 165mN xenon ion thrusters used for station keeping

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