X-37B OTV 4 search elements

From: Ted Molczan via Seesat-l <seesat-l_at_satobs.org>
Date: Mon, 18 May 2015 15:37:31 -0400
The AFSPC-5 mission will orbit X-37B OTV 4 and ten cubesats. It is scheduled for launch from CCAFS on an Atlas V-501
rocket, on 2015 May 20, during a launch period that begins at 14:45 UTC. Based on the NOTAMs, I believe that the launch
is scheduled to occur ~20 min later, at 15:05 UTC.

A timely query from Craig Covault got me working on the orbit estimate sooner than I had planned, the results of
which are reflected in his article below, and in my following analysis.


1. OTV orbit

The NOTAM coordinates reveal a launch azimuth of 61 deg, which indicates a 39 deg inclination orbit:


Earlier OTV launches were to azimuth ~108 deg, which normally would have resulted in a 33 deg inclination; however, yaw
steering enabled OTV 1, 2 and 3 to reach respective inclinations of 40.0 deg, 42.8 deg and 43.5 deg.

Assuming launch at 15:05 UTC, and no yaw-steering, I estimate the following OTV 4 elements for 350 km and 390 km
altitude. The epoch is 20 min. after lift-off.

OTV 4                                                    352 X 354 km
1 74996U 74996A   15140.64236111  .00000000  00000-0  00000-0 0    08
2 74996  39.0000 342.1967 0001000   0.0870 108.4874 15.73000000    05
OTV 4                                                    390 X 391 km
1 75996U 75996A   15140.64236111  .00000000  00000-0  00000-0 0    00
2 75996  39.0000 342.1980 0001000   0.0853 107.9999 15.60000000    00

The first northern hemisphere visibility window will be in the morning. It will begin to open about May 29.

The first southern hemisphere visibility window will be in the evening. It will begin to open about May 25.

2. Secondary payload orbit

The NOTAM coordinates for the de-orbit of the Centaur in the Southern Indian Ocean indicate an orbital inclination of
~56 deg:


That is consistent with the following document for the AeroCube-8 secondary payload:


"Identification of the anticipated launch vehicle and launch site: AeroCube-8 has been
manifested on ULTRASat, which will fly as a secondary payload on AFSPC-5, an Atlas V
launch scheduled for May 2015. The launch vehicle will deploy AeroCube-8 to a roughly 350 x
700 km altitude orbit inclined at 57 degrees."

The following paper on Ultrasat, is in general agreement, but reports a higher perigee and inclination:


- May 1 2015 on ELaNa XI - possible if there's a "vacancy"
- AFSPC "ULTRASat" mission
- Planned orbit is 389 km x 700 km _at_ 58 degrees inclination

For this discussion and the search elements, I assume that after deploying OTV 4 into a 39 deg, 350 km or 390 km orbit,
the Centaur will manoeuvre to a 350 or 390 x 700 km, 57 deg orbit. If this occurs at a node, then for the 350 km OTV 4
orbit, the delta-V will be 2,380 m/s. A de-orbit burn at apogee would require a further ~180 m/s. The nearly 2,600 m/s
total delta-V is well within the demonstrated capability of the Centaur after separation of the X-37B.

On OTV 1, the Centaur manoeuvred to a heliocentric disposal orbit. Based on the reported duration of the burn (148 s),
and the thrust and Isp of the Centaur, a total of 3,323 kg fuel and oxidizer were consumed. The Centaur's inert mass is
2,243 kg; however, based on data from an earlier heliocentric disposal of the same model stage, I estimated a final mass
of 2,673 kg. Applying that to OTV 1 Centaur's disposal manoeuvre, yields a delta-V of 3,569 m/s - more than sufficient
to escape Earth orbit from the orbit of OTV 1.

I have insufficient information to constrain my estimate of the time of the manoeuvre to increase the inclination and
raise the apogee. Assuming manoeuvre at a node, then the first opportunity is the descending node near T+00:38, about
21 min. after MECO1. That may be too soon for the Centaur to have separated itself sufficiently from the X-37B. OTV 1
Centaur's disposal burn occurred 27 min. after MECO1. A burn at the first descending node would result in the following
approximate orbit for the two perigee heights under consideration:

Centaur                                                  348 X 699 km
1 74997U 74997A   15140.65493058  .00000000  00000-0  00000-0 0    06
2 74997  57.0000 342.1159 0254000 180.0000   0.0000 15.14000000    06
Centaur                                                  387 X 700 km
1 75997U 75997A   15140.65512731  .00000000  00000-0  00000-0 0    08
2 75997  57.0000 342.1175 0226000 180.0000   0.0000 15.07600000    02

If the burn occurs at the first ascending node, near T+01:24, then this would be the approximate orbit for the two
perigee heights under consideration:

Centaur                                                  348 X 700 km
1 74998U 74998A   15140.68671298  .00000000  00000-0  00000-0 0    05
2 74998  57.0000 341.9115 0255000   0.0000   0.0000 15.14000000    08
Centaur                                                  385 X 700 km
1 75998U 75998A   15140.68717593  .00000000  00000-0  00000-0 0    06
2 75998  57.0000 341.9153 0228000   0.0000   0.0000 15.07800000    01

The Centaur's manoeuvre need not occur at a node. I experimented with a couple of alternative manoeuvre locations, none
of which appeared to better fit the available facts. If past practice is followed, USSTRATCOM will provide TLEs to the
owners of the secondary payloads, some of which may make them public.

The above orbits will be immediately visible pre-dawn from the southern hemisphere, but the secondary payloads are tiny,
which would make searching for them difficult, especially given the considerable uncertainty in the argument of perigee.
However, one of them will deploy a solar sail sufficiently large to observe.

2.1 LightSail

The most interesting secondary payload for observers is likely to be the Planetary Society's experimental solar sail,
named LightSail.


"LightSailT is a citizen-funded project by The Planetary Society, the world's largest non-profit space advocacy group.
We're sending two small spacecraft into Earth orbit carrying large, reflective sails measuring 32 square meters (344
square feet). Our first mission is a May 2015 test flight that will pave the way for a second, full-fledged solar
sailing demonstration in 2016."

"In May 2015, our first LightSail spacecraft will hitch a ride to orbit aboard an Atlas V rocket for a shakedown cruise.
We won't fly high enough above the Earth's atmosphere for solar sailing, but we'll test our sail deployment sequence and
snap some pretty pictures."

According to the following timeline, the deployment of the sail is planned for 28 days after launch:


By then, the orbit will be visible from the northern hemisphere during the middle of the night. The southern hemisphere
may have marginal evening passes. Note that the high area to mass ratio with sail deployed, combined with the low
perigee height, is expected to result in decay as soon as a couple of days after deployment.

3. OTV 4 unlikely to target 57 deg orbit

Could the manoeuvre to ~350 x 700 km, 57 deg occur prior to separation of the OTV? I doubt this because if 57 deg was
required for OTV 4, then the Atlas V-401 could easily perform a direct ascent to that inclination. Also, the mass of the
X-37B, ~5400 kg, would require in excess of 6,000 kg fuel and oxidizer for the total 2,600 m/s delta-V - far more than
the demonstrated 3,323 kg of fuel available after OTV 1 reached orbit.

Ted Molczan

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Received on Mon May 18 2015 - 14:38:34 UTC

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