The thrust vector control systems of the solid rocket boosters are turbine-powered, electrically controlled hydraulic systems which function through hydraulic actuators to gimbal the nozzles of the solid rocket boosters and provide vehicle steering for the Space Shuttle. Turbine power for the thrust vector control systems is provided through hydrazine fueled auxiliary power units which drive the hydraulic pumps. The solid rocket booster auxiliary power unit resulted from trade studies which indicated significant advantages would result if an existing engine could be found to meet the program goal of 20 missions reusability and adapted to meet the seawater environments associated with ocean landings. During its maturation, the auxiliary power unit underwent many design iterations and provided its flight worthiness through full qualification programs both as a component and as part of the thrust vector control system. More significant, the auxiliary power unit has successfully completed six Shuttle missions

Hughes, R. W.

English

NASA Technical Reports Server

THE SOLID ROCKET BOOSTER AUXILIARY POWER UNIT - MEETING THE CHALLENGE 
Robert W. Hughes 
Structures and Propulsion Laboratory 
Marshall Space Flight Center, Alabama 
ABSTRACT 
The thrust vector control systems of the solid rocket boosters are turbine-powered, electrically 
controlled hydraulic systems which function through hydraulic actuators to gimbal the nozzles of the 
solid rocket boosters and provide vehicle steering for the Space Shuttle. Turbine power for the thrust 
vector control systems is provided through hydrazine fueled auxiliary power units which drive the 
hydraulic pumps. 
The solid rocket booster auxiliary power unit resulted from trade studies which indicated sig-
nificant advantages would result if an existing engine could be found to meet the program goal of 20 
missions reusability and adapted to meet the seawater environments associated with ocean landings. 
During its maturation, the auxiliary power unit underwent many design iterations and provided its flight 
worthiness through full qualification programs both as a component and as part of the thrust vector 
control system. More significant, the auxiliary power unit has successfully completed six Shuttle 
missions. 
THE SOLID ROCKET BOOSTER CHALLENGE 
The challenge associated with the development of the Solid Rocket Booster (SRB) Auxiliary Power 
Unit (APU) was to develop a low cost reusable APU, compatible with an "operational" SRB. This challenge, 
as conceived, was to be one of adaptation more than innovation. As it turned out, the SRB APU 
development had elements of both. 
During the technical t~ade studies to select a SRB thrust vector control (TVC) system, several 
alternatives for providing hydraulic power were evaluated. A key factor in the choice of the final 
TVC system was the Orbiter APU development program, then in progress at Sundstrand Aviation. This 
program was implemented under contract with R0ckwell International Corporation and Johnson Space 
Center. The Orbiter APU design requirements very closely approximated or exceeded the ascent phase 
performance needed by the SRB and was physically compatible with the SRB concept! (Table I). Because 
of this comparability in requirements and physical compatibility, the Orbiter APU was selected as the 
basic power element for the SRB. With this selection came the expectation of reduced development costs 
and early hardware availability. 
This paper deals with the challenge in adaptation of the Orbiter APU to meet the SRB need. 
TABLE I. APU OPERATIONAL REQUIREMENTS 
PARAMETER ORBITER SRB 
Horsepower Shaft (HP) 135/151 135/148 
Operation/Mission (Min) 82 2.4 
Missions (Min) 40 20 
Total Operating Time (Hr) 50 2 
Useful Life (Hr) 250 10 
Cold Gas Checkout No Yes 
All Attitude Operation Yes No 
Zero G Opera t ion Yes Ho 
Redundant Control Yes Yes 
690 
https://ntrs.nasa.gov/search.jsp?R=19850008637 2020-03-20T20:38:02+00:00Z
.,.,. .... 
SRB AND ORBITER 
Although the Orbiter APU is used as the basic power element for the SRB APU and both fly on the 
same Shuttle vehicle, the APUs are, in fact, quite different . These differences range from the obvious 
to the barely detectable and are the factors which make the SRB APU of interest. 
In order to understand the development challenge of the SRB APU it helps to have a basic under-
standing of the differences between the Orbiter and the SRB missions and the environments these missions 
induce (Fig. 1). 
The Orbiter mission requires the APUs to be functional during all phases of operation (Table II) . 
Orbiter APUs start about 5 min prior to lift-off and operate continuously throughout ascent into Orbital 
insertion before they are shutdown. In preparation for de-orbit and reentry the APUs are restarted and 
operate throughout atmospheric maneuvering to a runway landing. Total APU operating time for each 
Orbiter APU is about 82 min. 
In comparison, the SRB APUs are started approximately 25 sec before lift-off and function only 
during the ascent phase of the mission terminating at SRB separation 161 sec later (Table II). From 
separation at approximately 200,000 ft altitude, the remaining 6 min of the SRB flight mission consist-
ing of atmospheric reentry, parachute slowed descent, and ocean splashdown are performed with nonfunc-
tional APUs. Once in the water, the SRB mission becomes one of survival. From SRB splashdown at 
91 ft/sec to SRB removal from the ocean at dockside, the APUs are subjected to various combinations of 
seawater immersions and water pressures for 7 days during SRB recovery and retrieval. 
Complicating the SRB APU situation is its installation. Where the Orbiter APU is installed in an 
aircraft type compartment offering protection from aerodynamic, vibration, and thermal extremes, during 
all phases of operation, the SRB APU is located in the aft skirt section of the SRB next to the engine 
where protection is available only during ascent (Fig. 2). 
Figure 1. SRB Mission Sequence. 
691 
J 
TABLE II. APU MISSION COMPARISON - SRB VERSUS ORBITER 
APU OPERATION ORBITER SRB 
PRELAUNCH YES YES 
BOOST YES YES 
ORBITAL YES NO 
REENTRY YES NO 
LANDING YES NO 
POSTLANDING YES NO 
TOTAL TIME 4200 Seconds 161 Seconds 
+z 
FLT~+Y 
-Y~+X 
-z 
SRM PROPEL 
SRB AFT BOOSTER ASSY 
......... SUPl'ORT POST 
(TYr.4) 
Figure 2. SRB TVC Subsystem and Actuators. 
Table III shows the major environmental differences between the SRB and the Orbiter APUs. The main 
differences were in the vibration and landing loads, and in the seawater pressures and immersions. It 
was obvious the Orbiter APU was not designed for the SRB severe conditions, and modifications would be 
necessary. The majority of the SRB APU developmental effort was expended in these areas. 
In this developmental phase of the SRB program, the decision to use the Orbiter APU proved most 
valuable since test hardware for modification became available without long lead times. 
Modifications made to create the SRB APU may be categorized in four basic ways: 
a. 
b. 
c. 
d. 
Elimination of unneeded features 
Hardening of existing components 
Reduction of production/replacement costs 
Implementation of servicing and checkout aids. 
692 
TABLE III. ENVIRONMENTAL COMPARISON 
ENVI RONMENT 
VIBRATION 
LIFT-OFF 
BOOST 
REENTRY 
LAN 01 HG LOADS 
ACCELERATIOII (ASCEIfT) 
WATER IMPACT PRESSURE 
WATER IMMERSION PRESSURE 
SALT WATER IMMERSION 
ZERO G 
VACUUM 
ORBITER 
1 B.l gnns 
1 B.l gnns 
5.7 grms 
1 .5 9 (9311/S) 
3.3 9 
None 
None 
None 
Yes 
Yes 
SRB 
21. 7 grms 
32.0 grms 
47.0 grms 
40 9 (l40M/S) 
3.3 9 
120 psi 
57 psi 
4 to 7 days 
No 
No 
Axial 1 atera 1 
An example of a modification for each of the categories is presented. To facilitate understanding 
these examples, a bas ic description of the APU is necessary. 
DESCRIPTION OF THE APU 
The SRB APU is a monopropellant hydrazine-fueled gas turbine engine used to drive a variable dis-
placement hydraulic pump (Fig. 3). The APU features a gear type fuel pump, driven through the APU 
speed reduction gearbox, in a bootstrapping mode, to provide high pressure hydrazine fuel to a catalytic 
type hydrazine gas generator. Flow of fuel to the gas generator is controlled by the gas generator 
valve module (GGVM), which contains two control valves in a series configuration. In the gas generator, 
the hydrazine is decomposed to create hot gas for driving the reentry type turbine, turning the gearbox, 
and driving the hydrauliC pump. Speed control of the turbine is accomplished through a pulse counting, 
logic circuit called the controller. The controller issues open or close signals to the GGVM to control 
fuel flow in response to electrical pulses generated by the turbine. The controller can control APU 
speed at 100 percent speed (72,000 rpm) for normal control; at 110 percent speed (79,200 rpm) for 
redundant TVC operation; or at 112 percent speed (80,640 rpm) for redundant internal APU control. The 
100 percent and l10 percent control modes operate the primary control valve of the GGVM and the 112 
percent control mode operates the secondary control valve of the GGVM. 
ELIMINATION OF UNNEEDED FEATURES 
The best example of how "elimination of unneeded equipment" was used in the development of the 
SRB APU is found in the gearbox. 
The Orbiter gearbox, as it was developed, had an intricate externally attached lubrication oil 
cooling loop; externally attached oil accumulator; and an externally attached gearbox pressurization 
system to support long duration and on-orbit operations in zero G and vacuum environments (Fig. 4). 
If necessary for the SRB, these features would have meant additional complication of the TVC system 
and additional weight and volume in the aft skirt. In addition, the projected effort to design the 
externally mounted components to meet the vibration and water entry loads; to select materials which 
would survive in the corrosive seawater environment; and to flight qualify these components would also 
have been expensive. 
The solution to the problem was obvious. Eliminate as many components as possible. Through 
analysis of APU operational times and the heating loads induced, it appeared to be feasible to tailor 
the SRB APU operational profile to stay within the 300°F thermal limit of the gearbox without the external 
components. The results of this analysis were confirmed by a series of tests duplicating expected worst 
case APU operations. With this verification, the coolant loop, the accumulator, and the pressurization 
693 
A£nM 
/ 
\ 
Figure 3. SRB APU. 
system were eliminated. An advantage in this elimination was the fact that the components in question 
were all bolted on externally so that their removal did not significantly alter the design of the basic 
gearbox. 
HARDENING OF COMPONENTS 
Hardening, as used in this paper, is meant to describe efforts to improve the resistance of the 
Orbiter APU components to damage and make them suitable for SRB. 
The largest example of hardening is the SRB APU vibration isolation system. 
In all cases, the vibration loads expected for SRB APU exceeded those expected for Orbiter. This 
exceedence caused concern for the operability and life of several APU components such as the gas 
generator and the fuel pump. To preclude having to redesign these components, a vibration isolation 
system was developed for the APU (Figs. 5, 6, and 7). The system consisted of three individually tuned 
vibration damping mounts attached between the APU at its mounting lugs and the primary mounting struc-
ture. The result was attenuation of the vibration loads input to the SRB APU to levels well below those 
for a hard mounted APU and, in some cases, below those experienced by the Orbiter (Table IV). 
694 
O'l 
\0 
<.TI 
COHTROLLEI!.-....--" 
8}= 
DOLT ON 
ACCUMUlATOR 
INSULATION 
~i 
WIRING HARNESS HYOR~IC 
~ ~' ~~~ 
0 * MOOULE o 0 ;1> LUBE OIL COOLER IROCKWELL SUPPUED) 
6M2 'iYSTm 
• Chanaes 
COMMON 
ITEMS 
BOOSTER 
<) rtJ ~ _0Ul PlUG, 
tdl 
I:P CONTROlLER 
(PRINTED 
CIRCUIT BOAROS) 
II(N1FlCA11Oll 
run: 
NtmoGEN SPIN NOZZLE 
~, 
-U- TUBE MAROTTA 
CLuBE OIL) VAlYE 
MODULE 
~~ 
""'"01[ ftAl"" 
~ 
.... TOIOIIocF 
.-r~_ 
,... - - - - - - - - --, 
I t=-(G~~ I • I 
I .' I 
I 
I BOOSTER STRUCTURAL " 
I ISOlATlOH M<XMT 0' 
I J 
-------- --, 
Figure 4. Pictorial Hardware Comparison - Booster Versus Orbiter. 
,. 
., BEARING CAP 
'"I"" /IIIII""/IIIII,'.I1//I"IW'/IIII IIIIIIIIIII'III/II"/ IIII/U "/"1/'" 11 1111'1/11'111111111'11111 111 111111"1'1'111,'1" 1'1111111 1111" IIII'IUII I" 
K ~ U U ~ ~ $ ~ U • M " a ~ " ~ • • 
01 • • I.. Ii» • .. [: 
",',/II, I,/""IJ,/"" """", 1""""""",I,',1 ,1."1,/, /,/11"""""'/"1' ,1". 1,111,',1,1, 1,1,',,,I,',I,I,I,I''''''t' .\, 
Figure 5. Isolation Mour.t M-3. 
1111'1""'11"1'11"""1"'1' "1',,/1/1/'1"'1111"1"""'/'1'1"""'1'1111/11'1'" 1/111'\'11111'1'\'11111'1'\'1'1'1'\1 \'1' '\1\1\1\1\1\.\1,,\, \, 
o t 2 3 " 15 e 7 tI 
o 10 ~ D ~ • • n _ _ ~ 
'1111 11 /111111111111111111111111111111111111111 111111,'1111111111\ ,\".,1". 
Figure 6. Isolation Mount M-2. 
696 
.I;> ); 
, - -
x (AXIS) 
LI FT -OFF 
BOOST 
REENTRY 
Y (AXIS) 
LI FT -OFF 
BOOST 
REENTRY 
Z (AXIS) 
LIFT-OFF 
BOOST 
REENTRY 
Figure 7 . Isolation Mount M-l. 
TABLE IV . APU VIBRATION ISOLATION SYSTEM 
IN PUTS TO SRB APU2 
HARD MOUNTED 
(G RMS) 
21. 7 
32 . 0 
41. 5 
21.0 
31.1 
47.0 
16. 9 
24.0 
28.1 
697 
ISOLATED 
(G RMS) 
5.3 
7.B 
10.0 
11.0 
16 . 0 
lB.5 
5.8 
5.7 
8.8 
ORBITER 
HARD MOUNTED 
(G RMS) 
lB .1 
5. 7 
5.7 
lU:DUCED COMPLEXITY 
This category of modification is mentioned separately, because it covers circumstances where SRB 
made changes to the Orbiter APU primarily to achieve significant cost advantages. The gas generator 
valve module is presented as a good example. 
The Orbiter gas generator valve module (GGVM) is a complex component utilizing many sophisticated 
production and operational techniques (Fig. 8). The design incorporated features like welded actuator 
torque tubes, torque motors, and metal to metal seats. These features were necessary in the Orbiter to 
meet the demands of reduced fuel consumption; prolonged cycle life; and elevated operating temperatures. 
Associated with these features were high development and unit costs. 
In evaluating GGVM requirements, the SRB realized that its needs were far exceeded by the Orbiter 
GGVM. Coupled with high Orbiter development and unit costs, this realization lead the SRB to investigate 
the availability of a less sophisticated GGVM to meet the SRB needs. The resulting SRB GGVM was a 
direct actuating, poppet type, solenoid valve module with elastomeric type seats (Fig. 8). This unit 
was produced at approximately one quarter of the unit cost of the Orbiter GGVM. This savings was con-
siderable when multiplied by the projected 400 SRB APU unit buys, and an attrition rate as high as 
12 percent. 
ORBITER 
HYDRAULIC RESEARCH 
WEIGHT 
ORBITER 
BOOSTER 
CONSTRUCTION 
LIFE 
REFURBISHABILITY 
HYDRAULIC RESEARCH 
3.2 POUNDS MAXIMUM 
FLEXURE TUBE, HARD SEATS 
1.5 x 106 CYCLES 
LOW 
BOOSTER 
MARO'ITA 
2. 5 POUNDS MAXIMUM WITH SWITCh"E5 
SLIDING SPOOL, SOFT SEATS 
ESTI~ 130,000 CYCLES 
HIGH 
Fig~e 8. Comparison of Gas Generatox. Valve Modules. 
698 
1 
,'--
PROCESS STREAMLINING 
The Shuttle program had a very ambitious operational schedule projected to be as high as one 
per month. This high rate of activity made it mandatory for the SRB to incorporate all practical 
veniences which would decrease the n~ers of in-flow operations and reduce the total operational 
around times. Several streamlining features were developed for the SRB APUs to accomplish this. 
of these will be discussed. 
flight 
con-
turn-
One 
The most successful streamlining modification undertaken for the SRB APU was the cold gas turbine 
spin (Fig. 9). This feature was added in the reentry nozzle block of the SRB turbine to provide an easy 
method to rotate the APU turbine without a full hot-fire operation or disassembly of the TVC system. 
Some of the advantages of this are: easy checkout of the APU at speeds up to 76 percent of full opera-
tional speed; easy checkout of the SRB TVC system at demands up to 26 horsepower; easy servicing of the 
hydraulic system; and easy fuel system servicing. The total TVC system time savings realized through 
the incorporation of cold gas spin is measured in days (approximately 6 days/mission). This is sig-
nificant when compared to an operational turnaround time measured in weeks. 
The examples presented were intended to explain the challenge presented in developing the SRB APU 
from the Orbiter APU and to demonstrate how the SRB APU became a unique entity in the answering of this 
challenge. Other examples of how the challenge has been met are found in the following: 
a. SRB controller 
b. SRB fuel pump 
c. SRB fuel system 
d. SRB gearbox 
e. SRB turbine 
f. SRB gas generator 
g. SRB electrical system. 
Figure 9. SRB GN2 Spin Nozzle. 
699 
THE SCORE CARD/RESULTS 
To evaluate how well a challenge has been met, a score in some form must be kept. Table V gives 
an indication of the score. 
TABLE V. SRll APU OPERATIONAL SCORE CARD 
STS SUCCESSFUL APU ASCENT/OPERATI ON RECOVERED APU'S APU DAMAGE 
1 4 4 20% 
2 4 4 15% 
3 4 4 10% 
4 4 SRB'S LOST 100% 
5 4 4 3% 
6 4 4 1% 
To date, all APUs recovered have been slated for return to service. It should be noted that all 
APUs from STS-5 and STS-6 could have been reused without off-line repair. 
A second challenge was to reduce development cost. In this, the program can again be declared 
successful. The cost for development of the SRll APU resulted in a program savings of approximately 
50 percent over the costs anticipated for a totally new development . This savings can be attributed 
almost totally to the decision to use the Orbiter APU as a basis for development . 
WHAT NOW 
The challenge of the SRll continues and will until the damage column on the score card reads 0 and 
hardware turnaround becomes routine. 
Efforts to improve the APU continue with the objectives of: 
a. Reducing preparation, servicing, and turnaround times. 
b. Reducing costs. 
c. Improving reliability. 
Toward this end, the SRll is presently pursuing major product improvement programs with a primary 
focus on the development of a low cost SRll gas generator, the most expensive component of the APU. A 
40 percent reduction in APU unit costs is a goal for this effort. 
Also in work is the development of an unpumped fuel system for the SRll APU. This effort will have 
significant impacts on APU reliability, servicing, preparation, turnaround, and cost. 
CONCLUSIONS 
An overall assessment of the SRll APU program leads to several important conclusions. 
a. The challenge to use the basic Orbiter APU as the design basis for the SRll APU has been met 
and has produced positive dividends in cost and schedule . 
b. The present SRll APU has been highly successful and has met the challenge of SRll APU reusability. 
c. The APU problems encountered during Shuttle operations pose a new challenge wanting real 
solutions which are cost effective and timely. 
d. The challenge to bring an SRll APU into operational status has been met and future challenges 
are in the category of product improvements. 
700 
REFERENCES 
1. McCool, A. A.; Verble, A. J.; and Potter, J. H.: Space Transportation System Solid Rocket Booster 
Thrust Vector Control System; Journal of Spacecraft and Rockets, Volume 17, Number 5, September -
October 1980, pp. 407-412. 
2. Koenings, D. A. and Johnson, B.: Qualification Report for Solid Rocket Booster Thrust Vector Control 
System Auxiliary Power Unit Model APU-02; Sundstrand Aviation AER-1539-6, Rev. A., 1979. 
3. SRB Systems Data Book, Marshall Space Flight Center, SE-019-083-2H, Rev. A, Vol. I, June 1977. 
701 


The Solid Rocket Booster Auxiliary Power Unit: Meeting the Challenge

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server