NROL-61 initial results of observations

From: Ted Molczan via Seesat-l <seesat-l_at_satobs.org>
Date: Sat, 30 Jul 2016 00:39:32 -0400
NROL-61 was launched from CCAFS on an Atlas V-421 on 2016 July 28 at 12:37 UTC, which was the start of a 57 minute
window. Its payload is USA 269 (2016-047A / 41724), which I believe is the first of a new generation of SDS (aka Quasar)
communication relay. This report summarizes what has been learned from the initial observations by hobbyists.

1. Observations

We had excellent tracking of the initial orbit, thanks to Paul Camilleri, the newest member of our informal group, who
stayed up most of the night. Paul lives in Australia, which is ideally placed to observe many of the types of launches
that interest us.

Paul acquired the payload and its Centaur stage minutes after the de-orbit burn of the latter. He observed and imaged
both objects from about 13:48 UTC to 15:53 UTC. Cees Bassa reduced 11 positions of the payload and 34 positions of the
Centaur from Paul's imagery. Paul observed again on July 29 near 11:15 UTC; Cees reduced 2 positions from his imagery.

Scott Tilley reported 91 positions of the payload, reduced from imagery he took on July 29 between 06:05 UTC 07:10 UTC.

Kevin Fetter made available imagery of the payload. Has this been reduced yet?

2. Payload orbital elements in GTO

I fit the following elements to Paul and Scott's observations of the payload:

USA 269                                               1108 X 35806 km
1 41724U 16047A   16211.46979167  .00000000  00000-0  00000-0 0    09
2 41724  18.6749 325.0932 6985594 179.0324  19.1367  2.21873130    05
Arc 20160728.59-0729.47 WRMS resid 0.004 totl 0.002 xtrk

The GTO inclination is more than 5 deg greater than I had deduced based on the inclination implied by the position and
orientation of the Centaur's de-orbit impact zone, given by the NOTAM co-ordinates. Oddly, the orientation of the impact
zone proved to be somewhat askew the de-orbit trajectory.

3. Payload mass

Knowing the true inclination of the GTO enables estimating the approximate mass of the payload. As with the pre-launch
estimate, I based this on Table 2.6.2-1: Atlas V 401-431 Geo-transfer Orbit Performance, of the Atlas V Launch Services
User's Guide, Revision 11. The table covers the range between 18-30 deg inclination.

NROL-33, and before it NROL-38, launched on Atlas V-401 into a 20.7 deg inclined GTO. Performance to that orbit was
about 4320 kg. The remaining delta-V to reach GEO was 1545 m/s. Assuming Isp of 320 s, the maximum payload mass in GEO
at BOL (beginning of life) was 2640 kg.

NROL-61 employed the Atlas V-421, which has significantly greater performance to GTO. Some of this performance was used
to reduce the inclination of the GTO by 2 deg. The 421 could deliver up to about 5960 kg to this orbit. The next model
down - the 411 - could deliver 5130 kg. The actual GTO payload mass of NROL-61 is between those values.

The lower inclination of NROL-61's GTO reduced its delta-V to orbit to 1486 m/s. Assuming Isp of 320 s, the respective
BOL mass of the Atlas V-411 and 421 was between about 3190 kg and 3710 kg - about 550 kg (21%) to 1070 kg (41%) greater
than the maximum BOL mass of the NROL 33 and 38 payloads.

NROL-61's greater payload mass is consistent with its use of a longer payload fairing than that of NROL-33 and 38, as
reported by Spaceflight Now.

http://spaceflightnow.com/2016/07/19/secret-nro-satellite-mounted-atop-atlas-5-rocket-for-july-28-launch/

4. Parking orbit

I estimated the LEO parking orbit on the assumption that it intersected the GTO at its first descending node. Based on
the TLE of the GTO, this occurred on July 28 at 13:04:04 UTC, at an altitude of 1092 km. I assumed that this was the
apogee of the parking orbit, and that its perigee height was 190 km.

LEO parking orbit                                       190 X 1099 km
1 75401U 75401A   16210.54449074  .00000000  00000-0  00000-0 0    04
2 75401  28.7000 325.3960 0647000   0.0000 179.9000 14.76550000    05

5. Centaur de-orbit manoeuvre

The Centaur's MES3 burn at 13:42 UTC (MET 01:05) put it on a trajectory to de-orbit about 750 km south of Hawaii.
Delta-V was about 500 m/s.

Cees and I obtained similar orbits for the Centaur, from Paul's observations:

Centaur orbit (C. Bassa)                              -252 x 28014 km
1 75403U 16710A   16210.61864000  .00000000  00000-0  00000-0 0    02
2 75403  18.8996 323.3539 6976124 166.7514  81.1155  3.01131186    00
# 20160728.57-20160728.66, 34 measurements, 0.042 deg rms

Centaur orbit (T. Molczan)                            -226 X 28051 km
1 75403U 16047B   16210.57083346  .00000000  00000-0  00000-0 0    06
2 75403  18.8938 323.3747 6967986 166.7795  29.3374  3.00488254    01
Arc 20160728.58-0728.66 WRMS resid 0.006 totl 0.004 xtrk

The negative perigee height is consistent with the expectation that the Centaur was to re-enter on its first revolution,
revealed by the NOTAM. The re-entry NOTAM period began at 20:59 UTC, for launch at the start of the window. The MET was
08:22.

The re-entry impact zone was an approximately 160 x 600 km rectangular region, centred near 11.85 N, 154.65 W, about 750
km south of Hawaii.

I used GMAT 2014a (General Mission Analysis Tool) to propagate Cees's TLE and my TLE to re-entry, as depicted in the
following graphic:

http://satobs.org/seesat_ref/NROL-61/NROL-61_Centaur_re-entry.jpg

The re-entry trajectories enter the scene at about 100 km altitude. The alternating red and white line segments span 5 s
of flight.

The kmz file used to generate the graphic is here:

http://satobs.org/seesat_ref/NROL-61/NROL-61_Centaur_re-entry.kmz

The white rectangle is the NOTAM hazard zone. Hawaii is just visible on the horizon, 750 km to the north.

Both of our TLEs propagated to theoretical impact within the NOTAM boundaries, shortly after the start of the NOTAM
period, 20:59 UTC: Cees's at 21:05:30 UTC, mine at 21:06:40 UTC. Mine travelled about 120 km farther east.

At about 90 km altitude, the Centaur reached its maximum velocity - nearly 10.2 km/s inertial (not far below escape
velocity), 9.7 km/s relative the atmosphere. Maximum deceleration of about 47 G occurred at about 45 km altitude. Nearly
all horizontal velocity had been lost by 30 km altitude, following which any surviving debris would have fallen almost
vertically. The final vertical descent is clearly depicted in the above graphics.

The GMAT scripts used to propagate both TLEs, and the Excel ephemeris spreadsheets used to analyze the results and
produce the kml files, are here:

http://satobs.org/seesat_ref/NROL-61/NROL-61_Centaur_re-entry_analysis.zip

Ted Molczan


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Received on Fri Jul 29 2016 - 23:40:16 UTC

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