A booms and mechanisms subsystem was designed, developed, and qualified for the geostationary scientific satellite GEOS. Part of this subsystem consist of four axial booms consisting of one pair of 1 m booms and one pair of 2.5 m booms. Each of these booms is carrying one bird cage electric field sensor. Alignment accuracy requirements led to a telescopic type solution. Deployment is performed by pressurized nitrogen. At deployment in orbit two of these booms showed some anomalies and one of these two deployed only about 80%. Following this malfunction a detailed failure investigation was performed resulting in a design modification of some critical components as release mechanism, guide sleeves of the telescopic elements, and pressure system

Schmidt, G. K.

English

NASA Technical Reports Server

GEOS Axial Booms 
Gunter K. Schmidt, Dornier Systems GmbH, Friedrichshafen, Germany 
ABSTRACT 
A Booms and Mechanisms Subsystem was designed, developed, and qualified for the geostationary 
scientific satellite GEOS. The project was sponsered by the EUROPEAN SPACE AGENCY as part 
of the GEOS development contract. Part of this subsystem are four Axial Booms consisting of one 
pair of 1 m booms and one pair of 2,5 m booms. 
Each of these booms is carrying one “bird cage” electric field sensor. Alignment accuracy require- 
ments led to a telescopic type solution. Deployment is performed by pressurized nitrogen. 
The main components of this system are: 
- Telescopic Section 
- Release Mechanism 
- Pressure System 
- Triax Cable Harness 
- Experiment Canister 
At deployment in orbit two of these booms showed some anomalies and one of these two deployed 
only about 80 %. Following this malfunction a detailed failure investigation was performed resulting 
in a design modification of some critical components as release mechanism, guide sleeves of the 
telescopic elements, and pressure system. 
INTRODUCTION 
Four special telescopic booms were developed for the scientific satellite GEOS as part of the Booms 
and Mechanisms Subsystem. 
GEOS is a spin stabilized satellite with a number of booms extending from the spacecraft body for 
positioning experiment probes. The experiments measure magnetic and electric fields in the low 
frequency spectrum. 
One of the experiments requires two pairs of wire sphere sensors of 100 mm diameter to be 
extended 1 m and 2,5 m above the spacecraft body in approximately ‘70 cm distance parallel to the 
spin axis. The design requirements led to the development of telescopic booms that are deployed by 
a nitrogen gas pressure system. 
GEOS was launched in April 77. After positioning the spacecraft in a final orbit, its booms were 
released and deployed. However, one of the “Long Axial Booms” deployed only about 80 to 90 %, 
resulting in a failure investigation. 
This presentation delineates the design details of several critical components and the failure investi- 
gation following the partial function failure of the failed “Axial Boom”. 
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FUNCTIONAL CHARACTERISTICS 
The four Axial Booms deploy and support GEOS’s four wire-sphere electric-field sensors. 
During launch the experiment spheres are stored at the base of the spacecraft in canister-like 
containers which are sealed by thin aluminium-foil membranes. The spheres are electrically connec- 
ted by helically-wound triaxial cables routed within the telescopic booms. (see Fig. 1) Sufficient 
stiffness was achieved by using aluminium-alloy tubes with a large outer diameter, and thin walls to 
withstand the bending loads due to centrifugal forces induced by the maximum deployment spin 
rate of 30 rpm. 
Torsional orientation is provided by a keyway. The axial booms are IJcked by a three-ball mecha- 
nism during launch (see Fig. 2) and are released and deployed by pressurised nitrogen, stored in 
pyrotechnically activated tank valve assemblies at 110 bar. Gas flow during deployment is control- 
led by orifices at the base of each boom, providing a deployment time of 1 to 4 sec. The gas system 
maintains a continuous flow for 15 to 20 set to provide an adequate safety margin. Numerous 
deployment tests in vacuum were necessary to ultimately define the optimum parameters for this 
pressure system. 
Because of the extreme magnetic and chemical cleanliness required for GEOS, a very careful 
selection of materials and processes was mandatory. 
DESCRIPTION OF THE OBSERVED ANOMALIES 
After positioning GEOS in its final orbit, its eight Booms and five Mechanisms were deployed. 
However, two Axial Booms showed some anomalies during deployment and one of these two, a 
Long Axial Boom, extended only about 80 to 90 % of its full length. 
The table below provides an interpretation of deployment data of the Axial Booms as observed in 
orbit during April 30. 1977. The observations are based on telemetry data from the accelerometer 
output and the boom status switches. Several of the interpretations are conclusive, while others 
remain inclusive (Ref. 1). 
Configuration Data: 
Deployment length 
overall length 
tip alignment 
release delay: 
t, nominal 
t, observed 
deploy time: 
td nominal 
td observed 
Deployment time: 
tD = tr + td 
tD nominal 
tD observed 
HK - Signal 
achieved 
deployment 
lenght 
Short Axial Booms Long Axial Booms 
+X/FS 1 -X/PM +Y/FM 1 -Y/FM 
1 2,5 m 
1,6 : 3,l m 
+0,02 m LO,02 m 
bed (set) (SIX) (se4 
0,5 LO,05 0,5 + 0,05 0,35 + 0,05 0,25 + 0,05 
0,5 0,5 or 10,9 6,6 0,8 LO,6 
1,5 +1 1,5 +1 2,7 t1,5 2,7 +1,5 
o,g 10,8 or 0,6 0,8 0,55 + 0,6 
2 +1 2 +1 3 +2 3 22 
I,4 11,3 1,45 + 0,6 
yes no Yes no 
full or 
full nearly 80 to 90 % full 
full 
212 
The information gained from spacecraft data is not totaly unambiguous. The -X PM/SAB data can 
be interpreted in two ways: 
a) Release occurred very late and was followed by a fast deployment. 
b) Release occurred for both Short Axial Booms (+X and -X axes) at the same time (ref. 1 Fig. la 
spike Al 1. This also occurs under normal conditions within a few miliiseconds time frame. 
For the +X SAB release was followed by a nominal deployment. However, deployment of the 
-X SAB may have momentarily halted after an initial start, due to stiction, and then continued 
some 11 set later. 
According to the HK-signal the release of the +Y Long Axial Boom occurred approximately 6 to 7 
seconds too late. Following the delayed release, a deployment action of this boom was observed at 
the accelerometer output. However, any conclusions are subject to doubt, as the accelerometer 
went into saturation when the -Y LAB deployed and as the connection of HK-data and accelero- 
meter readings were considered ambiguous for the Long Axial Boom. The actual deployed length 
achieved by the +Y Long Axial Boom was determined from attitude manoeuvre and shadow spike 
data from the LAB experiment sensors. 
POSSIBLE EXPLANATIONS FOR THE ANOMALIES 
1. Change in friction properties at seals, seal ring, and guide rings, due to: 
1 .l effects of hard vacuum 
1.2 effects of Van Allen radiation exposure 
1.3 temperature effects creating cold flow of the teflon guide rings 
2. Reduction of gas flow, due to: 
2.1 filter contamination 
2.2 orifice choking 
2.3 piping leakage 
3. Release Mechanism jamming, due to: 
3.1 ball-latch tolerances 
3.2 piston over travel 
3.3 increased friction between ball and ball cage 
4. An incident due to launch failure 
5. A zero-g Effect 
Most of the above explanations are essentially irrelevant since they would have caused different 
deployment data. Only the effects of 1.3 (stiction), 2.2 (orifice choking), and 3.3 (delayed release) 
can be considered as valid explanations for the anomalies observed. 
FAILURE INVESTIGATION AND SIMULATION TESTS 
To gain additional information about the failure behaviour, an investigative test programme was 
commenced. This programme consisted of three test series. 
- Single gas bottle tests 
- Release mechanism failure simulation tests 
- Orifice choke simulation tests 
213 
bar 
3 
release rod pressure 
telescopic section 
--Al 
Orifice diam.: 0.22 mm 
Deployment time: 2.9 set 
Feed Pressure: 110 bar 
Bottle size: 42 cm3 
I 
r” 
d 
I 
I 
I 
I I I 
boom deployed 
2 3 
+t 
4 set 
Fig. 3: Long Axial Boom - Pressure Profile 
GEOS-Spin Deployment Test of Short Radial and Axial Booms 
214 
Approximately one hundred vacuum deployments have been performed in the course of this investi- 
gative programme. * 
1. Tests with a Single Gas Bottle for each pair of the Axial Booms with increased charge pressure. 
For GEOS-2 it is proposed to keep a second gas bottle for each boom pair in reserve. For this 
reason, the single bottle pressure had to be determined duplicating deployment times of the 
G EOS-1 two bottle arrangement charged with 110 bar pressure. 
The tests demonstrated that the deployment times with a single bottle with pressures of 150 to 
160 bar are nearly identical to the required deployment time for two bottles with 110 to 
115 bar pressure. 
2. In a second test series, a jamming of the release balls in the release cage was simulated. 
The test setup allowed the release piston to move, but the release of the boom was delayed by a 
restraining wire which was melted following the required delay times. 
3. In the third test series, an orifice choke was simulated by a thin wire inlay in the orifice channel. 
During these tests the axial booms were equipped with a special release block which allowed 
recording of the pressure profiles in the release rod and the telescopic section. A typical pressure 
profile is shown in Fig. 3. 
RESULTS AND EVALUATION 
As a result of the above investigations, the release delay tr and the deploy-time td are determined to 
be the critical parameters. These parameters indicate a characteristic dependency for each of the 
possible failure explanations. The diagram Fig. 6 illustrates these characteristics. 
Curve I shows the relationship attributable to increased friction (1.3). The release delay is not 
influenced by increased friction in the seal and guide system. Therefore, this failure develops as a 
vertical line. (see Fig. 6). 
Curve I I shows the relationship of tr and td for the condition of a reduced gas flow. The reduced gas 
flow influences the deploy time significantly. Its influence on the release delay is less distinct. 
Curve I I I shows the tr, td dependency for the condition of release mechanism jamming. This failure 
mode causes the pressure to increase in the release rod and the stowed telescopic sections. When 
release occurres, there is an increase in deployment speed. This behaviour is more pronounced for 
the SAB than for the LAB, due to the lower deployed to stowed volume ratio of the SAB. 
The full deployment capability is limited by the amount of gas available. 
If we apply the orbit data to this diagram, the following can be derived: 
The SAB data substantiate a position on either curve I or curve I I I (point b and a), depending on 
individual interpretation of the orbit data. 
There is no substantive evidence from the orbit data that orifice contamination has to be consid- 
ered. The diagram demonstrates that any partial orifice choking would cause only a small release 
delay followed by a slow deployment of the SAB. The orbit data does not support this interpre- 
tation. 
If the delayed release of the Long Axial Boom has to be considered as definite, any friction has to 
be excluded, since the +Y LAB still deployed about 80 % following the delayed release. Orifice 
choking can also be excluded as it was impossible, during the tests, to generate release delays of 
greater than 4 seconds. The release pressure level could not be achieved for the delayed boom at 
higher choke rates. This condition is valid also during orbit. 
Therefore, a malfunction in the release mechanism can be assumed only for the Long Axial Boom. 
During the first few seconds of deployment, motion following this delayed release was relatively 
215 
Fig. 6 Axial Booms - Deployment Characteristics 
20 
15 
10 
5 
0 
tl 
sac 
25 
20 
15 
10 
5 
0 
Long Axirl Boom 
I 
/ 
Full deployment capability 
/ 
Y 
I Stiction characteristic 
II Orifice choke characteristic 
Ill Release delay characteristic 
0 +y LAB orbit data 
nominal deployment 
I I I , etrm _ 0 2 4 6 6 10 12 14 
A Short Axial Boom 
b 
J 
I Stiction characteristic 
II Orifice choke characteristic 
Ill Release delay characteristic 
0 -x SAB orbit data 
216 
rapid. However, as the accelerometer was in saturation it was not indicated in telemetry data. 
Following the initial rapid deployment, motion rate was reduced significantly for approximately 
five seconds and finally came to a stop at about 80 % deployment length. 
WHAT MAY HAVE CAUSED THE MALFUNCTION? 
Increased deployment times due to stiction (1.3) were observed some ew times during temperature 
tests. At higher temperatures the teflon guide rings show a cold flow tendency, which can result in a 
friction increase. However, this friction increase was found within acceptable limits at temperatures 
below 500C. 
Release Mechanism jamming: 
The friction conditions between ball and ball cage had been carefully calculated for GEOS-1. These 
calculations demonstrated a sufficient safety margin to jamming of I,5 to 2. However, an indenta- 
tion caused by the sharp edge of the titanium bushing (Fig. 7 part 1) could have resulted in 
increased friction, leading to a failure kinematic as indicated in Fig. 7a, b and c. 
Inspection of GEOS-1 spare unit release balls proved that the indentations exist. Repeated stowage 
and release increased the number of indentations and the likelihood of two indentation positions is 
highly probable; refer to Fig. 7c. 
The proper functioning of the +X FS/SAB, Flight Spare Short Axial Boom lends support to this 
theory, since this boom had not been exposed to repeated testing. 
Following actuation of the pyro valves for boom deployment, all four release pistons moved 
forward and 10 balls moved freely, except for one ball at the +Y LAB and most likely a second ball 
at the -X SAB may have stuck as shown in Fig. 7c. The gas expanded through the outlet hole into 
the telescopic sections and decayed as shown by the pressure profile. After some seconds the 
pressure in the telescopic section nearly equalized to the release rod pressure. At this time, the 
release spring, which was still compressed may have started to move the release piston in reversed 
direction due to the reduced pressure differential (see Fig. 2b). It is very likely that due to this 
piston motion the stuck ball in the jammed mechanism released and movement continued. 
DESIGN IMPROVEMENTS 
1. To reduce the possibility of friction due to cold flow of guide rings, the tightening torque for the 
fixation screws has been reduced to decrease the local stress concentrations at the,guide ring. In 
addition, the teflon guide sleeve for REl was slightly modified to prevent possible jamming 
forces caused by cold flow resulting in configuration instability. 
2. In GEOS-1 orifice and filter were screwed separately into the release block. During integration 
of the filter, loose particles may have been created by the screwing operation and trapped 
between filter and orifice. (Fig. 8a). 
During failure investigation this condition was observed at one time. 
For GEOS-2 a filter-orifice-assembly was developed which is assembled separately during in- 
creased cleanliness conditions with careful control under the microscope. This new design re- 
duces significantly the possibility of particle creation and trapping (Fig. 8b). 
3. The release mechanism modification mainly concerned the ball release piston and ball cage area. 
(Fig. 9). 
- The hard edge of the titanium sleeve was replaced by a soft aluminium chamfer to prevent 
indentation of the balls. 
- The ball cage holes which were cylindrical in GEOS-1 are now conical to improve the ball 
release. This modification significantly increases the friction safety margin for ball jamming 
from ~1 = 0,41 to ~2 = 0,77. 
217 
a 
I 
t ,’ 
d b 
C 
Fig. 7 Release Mechanism - Possible Failure Kinematic 
218 
Fig. 8 AXIAL BOOMS - Release Block 
a) GEOS-I design 
Filter Orifice 
1 Housing 
2 Screw 
3 Orifice 
4 Filter 
5 Clip ring 
7 O-ring seal 
\gesichert mit Solithane 
b) GEOS-2 Filter-Orifice-Assembly 
219 
a) GEOS-1 design 
Fig. 9 AXIAL BOOMS - Release Mechanism 
b) G EOS--2 design 
220 
CONCLUSION 
Considering the GEOS-1 housekeeping data and accelerometer data and the failure investigation 
results, the following can be derived: 
- there is no indication that an orifice contamination caused the observed anomalies 
- the data indicate that the reduced deployment length of the +Y LAB was caused by a malfunc- 
tion of the release mechanism 
- the anomaly in the SAB deployment behaviour was caused either by increased friction of one 
tube element or also by malfunction of the release mechanism. 
REFERENCE 
1) 
2) 
3) 
4) 
5) 
6) 
7) 
8) 
9) 
10) 
ESTEC-DOC. No. PG/O1.2/CCK/14.728/JR “GEOS Axial Boom Deployment Sequence - 
Observations in Orbit” by C.C. Kalweit 
Dornier - Study Note DO 5C SN 110 “Failure Investigation of FM Axial Boom Damages” by 
G. Schmidt 
PM/FM/FS Axial Booms Log Books 
BAC - “GEOS Axial Booms Improvement Ideas” by P.M. Grosse 
Dornier - ” G EOS Booms and Mechanisms Subsystem Manual ” Vol. I I Section 3.1.3 Axial 
Booms - Design and Analysis 
BAC - Dot. No. GBA 1611 “Axial Booms Deployment Simulation Results” by E. Crellin 
AER E Harwell-ESTEC Contract No. 2082/73 SK “The effects of radiation on the mechanical 
and electrical properties of the GEOS Satellite Long Boom Cable materials” by D. Verdin and 
P.R. Goggin 
ESTEC-Memorandum ref. PG/O1.2/BH/14542/HN “GEOS Axial Booms” try B. Henson 
ESTEC-Memorandum ref.: PG/146.3/CCK/15.31 l/JR “GEOS Axial Boom Review Documen- 
tation” by C. Kalweit 
ESTEC-Memorandum ref.: PG/510/BH/15.276/JR “GEOS-2 Axial Booms - Summary of 
Tests on Flight Model Booms” by B. Henson 
221 


GEOS axial booms

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