MUOS 5: estimated remaining fuel

From: Ted Molczan via Seesat-l <seesat-l_at_satobs.org>
Date: Mon, 24 Oct 2016 19:05:06 -0400
Brad Young's observations of MUOS 5 (16041A / 41622) on 2016 Oct 19, 20 and 23 UTC reveal it nearing a 9.7 deg,
34902 X 36672 km geosynchronous orbit. It was at the Western stable point, 105 W. Synchronous spacecraft located there
experience minimal longitudinal drift, greatly reducing, if not eliminating the need to perform east-west station-
keeping manoeuvres. It would make sense to permanently station MUOS 5 at the Western or Eastern stable point (the
latter is at 75 E), since much of its fuel reserve for use in GEO was consumed in recovering from the failure of the
LAE (liquid apogee engine) that left it stranded in GTO in late June.

In the hope of gaining some insight into MUOS 5's fuel supply situation, I have made estimates for three different
mission salvage scenarios, summarized in the following file:

http://satobs.org/seesat_ref/misc/MUOS_5_fuel_estimates.pdf

The spreadsheet used to generate the file is here:

http://satobs.org/seesat_ref/misc/MUOS_5_fuel_estimates.xlsx

Scenario 1

To reach the nominal 5 deg inclined GEO orbit, required that the LAE impart a total delta-V of 1295 m/s. The REA
(reaction engine assembly) would have had a maximum of 671 kg (367 m/s delta-V) of hydrazine available for orbit
manoeuvres, including east-west station-keeping and end of life disposal.

The failure of the LAE after achieving only 702 m/s of the required delta-V, left 474 kg of unspent hydrazine and
452 kg of unspent oxidizer. The latter was dead-weight, but the REA could burn the hydrazine, increasing its available
delta-V to 543 m/s. However, with the lower Isp of the REA, this represented a shortfall of 98 kg (53 m/s) to reach the
planned orbit. It would have been impossible to reach synchronous altitude at any inclination.

Scenario 2

Venting the unspent oxidizer would eliminate 452 kg of dead-weight, and generate a modest delta-V. The series of small
orbit manoeuvres in late July and early August 2016 probably resulted from venting the unspent oxidizer. The observed
increase in perigee height required a delta-V of about 20 m/s. Assuming this was due to venting the 452 kg of oxidizer,
then the implied Isp was about 23 s. The reduced spacecraft mass increased the available delta-V of the REA by 53 m/s.
The net effect would have barely enabled reaching the planned orbit, with just 34 kg (20 m/s delta-V) of hydrazine
remaining.

Scenario 3

Accepting the present inclination of 9.7 deg reduced the required total delta-V by about 50 m/s, leaving about 120 kg
(70 m/s delta-V) of hydrazine upon arrival at GEO. That is far less than the estimated nominal values of 671 kg and
367 m/s delta-V. The maximum annual delta-V for east-west station-keeping is about 1.7 m/s, which the estimated
remaining fuel could accommodate for the planned 15 year service life. As mentioned earlier, operating at one of the
stable points would all but eliminate the need for E-W station-keeping manoeuvres, which appears to be the plan. That
would seem to bode well; however, the RCS may also be required to de-saturate momentum wheels. I am unable to estimate
the fuel mass requirement, which may be significant.

The greater than normal eccentricity greatly reduces intrusion into the geostationary protected zone, which could
provide justification for not manoeuvring to a graveyard orbit at end of life, saving more than 10 m/s (based on
performing the manoeuvre from a circular GEO orbit).

I am interested to know of any errors in need of correction, better input data, and alternative analyses.

Comment

Judging by the above estimates, this was a very close call in terms of losing the mission. Had the LAE failed sooner,
before achieving at least ~580 m/s of the delta-V to reach GEO, synchronous orbit would have been out of reach. The
story of the salvaging of the MUOS 5 mission no doubt would make for interesting reading. I hope that a paper will be
published.

Technical references

I found little authoritative information on spacecraft and fuel mass on the web. I based my estimates on the
information on p.11 of the following file:

http://satobs.org/seesat_ref/misc/2.8.12_MUOS_Kit_II.pdf

I estimated the mass of hydrazine and oxidizer for a nominal LAE mission, which I subtracted from the total fuel mass
in the above file, to estimate the hydrazine mass of the REA. I guessed the mass of hydrazine used by the REA for
attitude control in GTO.

Performance data on the LAE is as published by the manufacturer:

Nominal Thrust 450 N
Nominal Mixture Ratio 0.92 ± 0.015
Fuel (Hydrazine) 245 psia
Oxidizer (MON3) 235 psia
Isp (nominal) 329 seconds

For the REA Isp, I used a nominal value of 232 s, that falls within the range reported here:

http://arc.aiaa.org/doi/abs/10.2514/6.2005-3954

Ted Molczan



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Received on Mon Oct 24 2016 - 18:05:45 UTC

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