NROL-61 pre-launch estimated orbital elements

From: Ted Molczan via Seesat-l <>
Date: Tue, 26 Jul 2016 12:08:00 -0400
NROL-61 is scheduled for launch from CCAFS on an Atlas V-421 on 2016 July 28; the launch window is from 12:37 to 13:34

I believe that the payload is the first of a new generation of SDS (aka Quasar) communication relay for LEO IMINT
(imagery intelligence satellites). Its design may be optimized to support the next generation heavy electro-optical
IMINT satellites of KH-11 lineage. I believe that there have been four KH-11 versions (aka blocks) since 1976. The first
two launches of what I call Block 5 KH-11 are planned for Sep 2018 and Jul 2020.

Spaceflight Now will provide live coverage of the launch of NROL-61:

Estimated orbital elements

I have estimated the following TLEs, based on launch at window-open. Accuracy is no better than several degrees in plane
and several minutes in time.

MECO 1 orbit at MET 00:28:20                             186 X 834 km
1 75401U 75401A   16210.54537037  .00000000  00000-0  00000-0 0    01
2 75401  28.7000 318.5343 0470000 359.9443 180.1334 15.20000000    09

MECO 2 orbit at MET 00:28:20                           844 X 35743 km
1 75402U 75402A   16210.54537038  .00000000  00000-0  00000-0 0    04
2 75402  13.4000 318.5343 7072816 177.1827   0.2500  2.24094669    08

The MECO 1 and 2 TLEs are stated for the epoch of MECO 2, which is assumed to occur at the first descending node. The
MECO 2 orbit has been estimated based on the simplifying assumption of an instantaneous change of elements.

MECO 3 at MET 01:05:00                                 -53 X 28104 km
1 75403U 75403A   16210.57083345  .00000000  00000-0  00000-0 0    06
2 75403  13.9200 346.0000 6900000 140.2500  28.5500  2.98000000    05

Centaur de-orbit

MECO 3 is the de-orbit burn of the Centaur. The orbit has been estimated based on an instantaneous change of elements.
The MET of 01:05:00 is approximate. Blowdown of remaining propellants will occur shortly after MECO 3, which likely will
take less than 15 minutes to complete.

Centaur re-entry occurs at about MET 08:22, on the first passage through perigee after the de-orbit burn. The
approximate centre of the disposal area is 11.85 N, 154.65 W, about 750 km south of Hawaii.

This will be the third SDS launch on which the Centaur was to be de-orbited. The first two were NROL-38 (USA 236) in
June 2012 and NROL-33 (USA 252) in May 2014, both of which were launched on the Atlas V-401.

The disposal orbits of the NROL-38 and 33 Centaurs were inclined about 21 deg, slightly greater than the 20.7 deg of
their payload. They performed their de-orbit burns about an hour after launch. The manoeuvres were nearly coplanar,
involving a reduction in semi-major axis and eccentricity, and a rotation of the line of apsides (the major axis of an
elliptical orbit). The NROL-61 Centaur's de-orbit manoeuvre will be similar, but with a significant out-of-plane
component, mainly in the RAAN (right-ascension of ascending node). There will be a greater reduction in semi-major axis
and magnitude of apsidal rotation. Total delta-V is about 630 m/s, compared with about 500 m/s for NROL-38 and 33.

Reason for use of Atlas V-421

NROL-61 is the first NRO spacecraft to fly on the Atlas V-421. Comparing the NOTAM co-ordinates of its Centaur's
disposal area with those of its predecessors yielded insights into the reason for the use of the higher performing
launch vehicle.

The NROL-61 Centaur's disposal area is close to those of NROL-38 and 33, but it is orientated differently, analysis of
which reveals an inclination of about 13.9 deg. The inclination of the payload's initial GTO almost certainly is
similar. I estimate 13.4 deg, based on the difference between the inclination of the NROL-33 payload and Centaur. This
finding informed the MECO 2 and 3 orbit estimates.

Reducing the GTO inclination from 20.7 deg to 13.4 deg would reduce the payload's delta-V to reach the expected
approximately 4.9 deg inclined GEO orbit by about 122 m/s. This would reduce the payload's fuel expenditure to reach
GEO, enabling a greater mass to reach GEO, resulting in some combination of greater payload mass and greater fuel mass
for station-keeping and other orbit manoeuvres. The latter would tend to increase the useful life of the spacecraft.

Analysis of data in the Atlas V Launch Services User's Guide, Revision 11, Table 2.6.2-1: Atlas V 401-431 Geo-transfer
Orbit Performance, revealed the following relevant performance data.

     Inc   mass
     Deg    kg
401  20.7  4324
411  13.4  4205
421  13.4  4914

I caution that the 411 and 421 values are the result of extrapolating well beyond the published data, which covers the
range between 18-30 deg inclination. I guess the extrapolated values to be accurate to about 100 kg.

I included the Atlas V-411 to provide perspective. It could place slightly less mass into a 13.4 deg GTO than the 401
into a 20.7 deg GTO, so the only possible advantage would have been the aforementioned 122 m/s reduction in delta-V to
GEO. I have not made any analysis of this.

The 421 could deliver about 590 kg more to a 13.4 GTO than the 401 to a 20.7 deg GTO, which tends to support the
hypothesis that the spacecraft is a new model. Another clue is the use of a longer payload fairing than on NROL-33 and
38, as reported by Spaceflight Now.

I cannot rule out the possibility that the payload is a new GEO SIGINT spacecraft; however, a new model of GEO SIGINT
(USA 250) was launched on NROL-67 in March 2014 on an Atlas V-541. It seems unlikely for two new models to be introduced
at about the same time.

Ted Molczan

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Received on Tue Jul 26 2016 - 11:08:42 UTC

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