Helios is the German-American solar probe which comes within 0.31 Astronomical Units of the sun. A special thermal design and a dry lubrication system have resulted in successful operation up to now, with Helios having finished its first orbit around the sun

Mueller, E. J. W.

English

NASA Technical Reports Server

TIIE IIELIOS MECIIANICAL DESPIN DRIVE ASSEMBLY FOR THE
tIIGII-GAIN ANTENNA REf LECTOI/
by E. J. W. Miiller, Dipl.-lng.
DKVLR-BI°T, Germany
41
I
ABSTRACT t
The tlELIOS mechanical despin drive assembly for the high-gain antenna
reflector is described in this paper. HELLOS is the German-Amrrican solar
probe which comes within 0.31 Astronomical Units (about 50. 106 kin) of the
sun. A qpecial thermal design and a dry lubrication system have resulted
in successful operation up to now, with HELLOS having finished its first orbit
around the sun.
IN TRODU C TION
tlELIOS is a German-American solar probe with an elliptical flight path
around the sun. HELIOS A was launched on Dec. 10, 1974, and HELIOS B
follow d in Jan. 1976. Its aphelion is one Astronomical Unit (I ,kU) and its
perihelion 0.31 AU from the sun. The solar impact to the spacecraft varies
between one and eleven solar constants. Figure 1 shows the orbits of earth .,
and HELIOS around the sun. The HELIOS spacecraft is spin*stabilized at
60 rpm and has three antenna systems to achieve ground connection: a low-
gain antenna for the near-earth part of the mission before the spacecraft
spin axis is oriented orthogonal to the orbit plane; a medium*gain antenna,
and a high-gain antenna for the main mission. The high-gain antenna is on
the upper side of the spacecraft. It consists of an axial antenna feeder
rotating with the spacecraft and a despun reflector, which is also pointed
towards the earth. Figure 2 shows the spacecraft's configurat,'on with its
high-gain antenna system and the despin drive assembly (DD_' .'_efright
time for one orbit is about six months; three monthe after le ,._,i_,IELIOS
reaches it,sclosest diFtance to the sun, which is 46.106 km ,r tiF_,IOSA
and 43. 10t)km for HELIOS B. The launch vehicle -a Titan _, • en*a
TE 364-4 combination, the varying thermal environment during tl._m; : on,
and the ground testing time influenced the design of the reflector _,. .:
drive assembly.
The HELIOS Despin Drive Assembly
Figure 3 shows a photograph of the DDA. The design concept of the despin
drive, assembly had to take into consideration
a) the general HELIOS system aspects
b) the launch conditions
c) the orbit environments
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The general HELIOS system aspects--including storage, shipment, and S/C
system tests--called for materials which are corrosion-free and for a lubri-
cant which operates in air as well as in vacuum. The launch conditions--
acceleration, vibration loads, and acoustic noise-- det_ 'mined the mechani-
cal design and the size of the bearings. The orbit environments--vacuum
and an extremely changing thermal impact between one and eleven solar
c,_nstants -- dictated the kind of lubrication system used: dry film lubrica-
tion with self-replacement of the film by transfer.
The Mechanical Design of the Despin Drive Assembly
To accommodate the expected load impact during lift-3ff (14 g at 60 Hz), a
three-inch angular contact bearing with 4Z balls on each side was chosen.
The contact angle varies between 18" and 25", depending on the axial load
impact. The lower bearing, supporting the axial load during the launch
._ phase, is a "fixed" bearing with respect to shaft and housing. The bearing
on the other end is preloaded via a preload spring to five pounds. The pre-
load force was optimized during the tests to slightly more than the minimum
force necessary to have the balls running in the same track. It was found
out during the preselection tests at DFVLR, Munich, how the preload force
influences the lifetime of the bearings. Figure 4 shows the DDA configura-
tion, the arrange:nent of the beax'ings, and the materials used. The DDA is
driven by a brushless DC motor which can overcome a drag torque of
30 oz-in. The nominal drag _orque range is from 3.5 oz-in, to 6.4 oz-in.
A magnetic oick-up devi:e makes possible the pointing of the reflector
towards the earth throughout the mission. The pointing accuracy is speci-
•_ fled to be better than 0.7 °.
The materials used for _he DDA are:
! • 440 C stainless steel for the high-precision bearings
• Aluminum for shaft and housing
• Titanium for the bearing seats (similar thermal expansion factor
to 440 C)
These materials were chosen because of the wide range of expected tem-
peratures and thermal gradients across shaft and housing.
The goal wa_ not to jam the DDA under all possible flight conditions, which
means not closing the radial or axial gap between top bearing .nd housing.
Figure 5 shows the temperatures and thermal gradients recessary to close
the radial gap between upper bearing and housing, and Fig. 6 shows the
thermal gradient {housing versus shaft) required to close the axial gap. At
a shaft temperature of -40"C, a thermal gradient of 20"C could be tolerated
{assuming an isothermal housing and an isothermal shaft).
The Lubrication System
The lubrication system chosen is the dry film l,_bricant "D.-'y Vac Kote" of
Ball Brothers Research Corporation {USA), which is based on molybdenum-
disulfide (MoS Z) combined with a phenolic (RULON-A) retainer for
$PL Technical Memorandum 33-777 81
1976021184-088
relubrication b_, transfer. The reasons for not using "common" oil or
greas, • lubricants and _oing to a dry lubrication system are:
a) Expected bearing temperatures from -40°( to _60°C J
b) Evaporation of oil at high temperatures i_ vacuum
c} Danger of contamination ,_f spacecraft surfaces.
The Ground TeJting of the HELIOS Despin Drive Assembly -.
J
ttELIOS is the first civilian spacecraft wh'-h uses a dry film lubricant for
such a mechanical device. Up to now, dry lubricants were used only for
instrument size bearings. Because it was not possible _o get lifetime data /
for these lubricants or even design data (e. g., temperature resistance
across dr,/ lubricated surfaces or lubricant break-away forces}, var'ous
tests had to be carried out during the design and development phase cf the
ttELIOS program. In the be_inni ,_, a functional support contract was estab-
lished between the HELIOS project office and the DFVLR Institute for
"Flugtreib- und Schmierstc_re '' in Munich. The work do,_e at DFVLR can be
summarized as follows:
• Literature research
$ Preselection tests of bearings, lubricants and separators
s Long-life tests of dry film lubricated bearings in UHV
• Long-life tests of dry film lubricated bearings in thermal vacuum
• HELIOS DDA therma! gradient tests in UHV
• Stand-by for trouble-shooting during the HELIOS mission using a
HELIOS DDA in a thermal vacuum chamber which allows indepen-
dent temperature settings at the shaft and the housing .-_
Details about these tests and the results can be found in IRef. I. In the fol-
lowing, the HELIOS DDA tests carried out at the subsystem and system
levels at the supplier (UialtBrothers} and the HELIOS main contractor
(Iviesserschmitt-B61kow-Blohm) .rillbe discussed.
HELIOS DDA Lifetime Test i,IThermal Vacuum
In the beginning of the program, itwas recommended by the HELLOS project
office that a therm_*.lvacuum lifetest b_.performed in real time on a corn°
plete DDA in addition to real-time tests on the DDA bearings. This would
more nearly simulate the real environment for the bearings and provide a
lifetest of all other DDA components. To simulate the real environment, a
random vibratiov test (acc. level} was performed before the DDA was put
into the thermal vacuum chamber. The lifetime test was done successfully
between Apr. '7Z and Nov. '73. Figure 7 shows the test history, ir,cluding
the measured torque values at the various temperatures. In order tc com-
plete the test in the allotted time, itwas decided that the time lost through
interruptions could be made up by' operating It double speed.
HELIOS DDA Tbc_-rnal Vacuum Tests
i
The next DDA system tests were the thermal vacuum tests of the prototype
flightunits one and two. During the prototype tests, it could be seen that
the friction torque went up to 7 t_z-in, at shaft temperatures between =40°C ,
8Z JPL Tec_dcal Memoral,dum 33-777
h
1976021184-089
!and -50°C. When the flight units were tested, the friction torque reached
12 oz-in, at the same cold temperatures. Many tests followed, with differ-
ent bearing separators ar q different gaps between upper bearing and housing.
Finally, it turned out that the drag torque was a function of the torque ripple
which occurred due to an end plate (at the lower end of the DDA) misalign-
ment at cold temperatures. During manufacture, the final machining was
done with th,_ end plate fixed to the housing. The rotational position of the
end plate with respect to the housing was marked, and during the assembly
of the DDA, the end plate was sctewed on in the same position. During the
therr_.3.1 vacuum tests -- especially during the cold phase -- it turned out that
the a ,_ve-men t aed fabrication method was not the optimal one. A better
way _._as to fabricate and machine each part by itself with close tolerances
and tc optimize by test the final position of the end plate on the housing. The
resaltmg drag torque during the cold phase tests was about 5 oz-in, for both
flight units.
HELIOS Spacecraft System Solar Simulation Test
The most important test to improve the thermal design of the spacecraft
and its components was the HELIOS solar simulation test performed at the
Jet Propulsion Laboratory (3PL, California}. During this test, the DDA
was operated by its motor to simulate flight conditions. The measured
temperatures were nearly coincident with the calculated ones for the hot
phase at II solar constants, but they deviated for the cold phase at 0.9 solar
constant. Thermal gradients on the order of -40"C (shaft warmer than hous-
_ ing) were read out.
This was later explained by a heat leak in the area of the upper flan_e. As :
already mentioned, the maximum allowable thermal gradient is -Z0_C for
the cold phase. Figure 8 shows a summary of the solar simulation test with
., respect to the behaviour of the I)DA. One can see the influence of the tem-
peratures on the friction torque, which influences the motor current and :
also the pointing accuracy ....
f
The DDA Thermal Gradient Test
This test wa_ necessary to simulate the JPL test and to check the design
_ changes made to obtain warmer temperatures and lower thermal gradients
during the cold mission phase. In the first step, the DDA temperatures
were set to the JPL measured values. Then the following design changes
we re made:
a) The heat leak between reflector hub and DDA flange was closed.
b) A heater was added inclde the reflector hub to warm up the upper
part of the housing and thus to reduce the thermal gradient during
the cold phase.
After implementation of these changes, a shaft temperature of -30 *C and a
thermal gradient of 8 *C were measured for the cold flight condition.
JPL Technical Memorandum 33-777 83
1976021184-090
Flight Data of the HELIOS DDA
Up to now, the performance of the HELIOS DDA ha_ been excellent during
the mission. Figures 9 and 10 show the performance Gf the DDA during the
HELIOS A and B missions through March 1976. As can be seen, the mea-
sured friction torque {via motor current} is always within the specified
values. As a result of the last design changes, the measured temperatures
are warmer during aphelion and colder during perihelion.
Conclusion
The flight performance of the dry lubricated :IELIOS despin drive assembly
for the high-gain antenna reflector shows that the choice of the unusual
lubricant was a good one. It confirms a concept which could be used not
only for spacecraft but also in extreme earth environments.
References
1. E. 3antzen: A Dry Lubrication System for Spaceproofed Beatings.
ESRO Proceedings, 1975.
2. BBRC, Calculation of DDA Endplay, March 1974.
3. MBB, DDA Design Specification.
4. MBB, Test Procedures for DDA.
L
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84 JPL Technical Memorandum 33-777 :
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1976021184-091
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Fig. 1. Orbits of earth and HELIOS around the sun
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OS
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Fig. 2 HELIOS spacecraft dimensions
IPL Technical Memorandum 33-777 85
1976021184-092

500-
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u 400-
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.+ ' Th - TR= -14°C (-25°F)
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' UPPERBEARINGOUTERRACETEMP.
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Th - HOUSING TEMP.+,
i Fig. 5 Thermal gradient required to close upper
bearing radial gap
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SHAFTTEMP- °C +
Fig. 6 Thermal gradient (housing vs. shaft) to close an axiat gap of
0.0055 inch, assumin E an isothermat housing and an isother-
mal shaft
/PL Technical Memorandum 33-777 87
1976021184-094
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Fig. 7 Summary ofDDA T/V lifetest
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Fig. 8 Summary of HELIOS solar simulation test at JPL (for DDA)
'_ _ 88 JPL Technical Memorandum 33-777 i
1976021184-095
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Fig. 10 HELIOS B DDA performance i
$PL Technical Memorandum 33-777 89 *_
1976021184-096


The Helios mechanical despin drive assembly for the high-gain antenna reflector

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