The Rudder/Speedbrake (R/SB) Actuation Subsystem for use on the NASA Space Shuttle Orbiter is an electro-hydro-mechanical system which provides the control and positionary capability of the orbiter aero-dynamic primary flight control surface. The system is located in the vehicle's vertical stabilizer. The geared rotary actuators provide a power hinge feature of the split panel rudder. Actuation of both panels in the same direction provides conventional rudder control; actuating the panels differentially provides a speedbrake function intended to control both speed and pitch. The commands may be superimposed on one another. The system consists of one power drive unit which responds to quadredundant avionic signals to generate a rotary output, four geared rotary actuators, which develop rotary position and torque as outputs, and ten torque transmitting drive-shifts

Naber, R. A.

English

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SPACE SHUTTLE RUDDER/SPEEDBRAKE ACTUATION SUBSYSTEM
Ruth A. Naber
Sundstrand Advanced Technology Group
Rockford, Illinois
ABSTRACT
An electro-hydro-mechanical actuation system for control of the rudder and speedbrake aerosurfaces of
the Space Shuttle orbiter was developed to meet the strict operational requirements imposed on this
mission cd{ical function. (Reference 1) The re-entry and landing portions of the Space Shuttle mission
included a set of environmental and operational requirements which presented a significant engineering
challenge• The design solutions derived during this effort served to provide a reliable, safe-life actuation
system and to provide technology that will be useful in future engineering challengeS.
INTRODUC_ON
The Rudder/Speedbrake (R/SB) Actuation Subsystem supplied by the Sundstrand Corporation for use
on the NASA Space Shuttle Orbiter is an electro-hydro-mechanical system which provides the control
and posibonary capability of the orbiter aero-dynamic pdmary flight control surface. The system is located
in the vehicle's vertical stabilizer• The Geared Rotary Actuators provide a power hinge feature of the split
panel rudder• Actuation of both panels in the same direction provides conventional rudder control;
actuating the panels differentially provides a speedbrake function intended to control both speed and
pitch• The commands may be superimposed on one another.
The system consists of one Power Drive Unit (PDU) which responds to quadredundant avionic signals to
generate a rotary output, four Geared Rotary Actuators, which develop rotary position and torque as
outputs, and ten torque transmitting Ddve-shafts. (See Figure 1)
@
GEARED ROTARY
ACTUATORS
SHAFTS ._
ORIGINAL PAGE f._
OF POOR QUALITY,
DRIVE UNIT (PDU)
Rgure 1
Rudder/Speed Brake Actuation System
905
https://ntrs.nasa.gov/search.jsp?R=19850008656 2020-03-22T18:09:18+00:00Z
ThePower Drive Unit (PDU) responds to electrical command s,gnals that operate electrohydrauli¢
servovalves. The servovalve operation ports hydraulic pressure to hydraulic motors which in turn provide
rotary power to PDU gearbox. The PDU is presented schematically in Figure 2.
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Figure 2
Power Drive Unit Schematic
Within the PDU design, the areas of engineering challenge to be discussed in this paper include the
following:
1. Electrical/Hydraulic Fault Detection and Isolation,
2. Gearbox design to mechanically mix the rudder commanded movement with speedbrake
commanded movement.
The Geared Rotary Actuator shown in Figure 3 presented a significant challenge on its own. As in all flight
control systems, weight and volume are critical factors in determining the actuating mechanism package.
This results in the use of highly stressed gears to accomplish the task. This gear design required
refinement of the analytical technique and extensive testing.
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Figure 3
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FAILURE DETECTION AND ISOLATION
The operational capability of the R/SB system is critical to the successful completion of the orbiter
mission. Therefore considerable design emphasis was placed on electrical and hydraulic system
redundancy and further fault detection and isolation. Referring to Figures 2 and 4, the electronic input
commands are quadredundant, coming tethe PDU from four independent avionic sources. These four
commands act on four separate electro-hydraulic servovaives within the PDU. Pressures from the
servovalve second stages act on areas of a single structural power spool valve, causing it to move from its
null position. This movement opens ports that supply hydraulic pressure to the three hydraulic motors
providing rotary power to the PDU gearbox.
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Channel Fault Detection
Should a servovalve output to the power valve be significantly different from the other three (due to failure
of the valve, or its associated avionics, for instance), the second-stage differential pressure of that
servovalve will rise as it force-fights the three opposing good servovalves. This rise in pressure (Figure 4)
is monitored by second-stage pressure transducers, and, when a trip level is attained indicative of a failed
channel, a solenoid valve is commanded to open, bypassing the failed channel. The PDU, meanwhile,
continues operating normally in response to the commands of the three good channels. Should a second
servovalve failure occur, a similar two-versus-one vote would result in its bypass. In this way, the PDU is
tolerant of two avionic failures while maintaining full performance.
During early development testing of the PDU, it was found that channel failure and isolation, while the
PDU was operating at a high output rate, could result in very high ihternal pressure pulses as a result of
transfer of kinetic energy to potential energy by compressing the oil as a spring between the motor and the
valve. This problem was resolved by making several system modifications, including avionic command
filtering, elimination of a power valve positioning linkage, and strengthening of pressure passages within
the PDU.
A hydraulic system failure is monitored by and managed by the switching valve. This pressure actuated
valve is arranged.to accept the three on-board independent hydraulic systems. The valve automatically
switches from an out-of-tolerance hydraulic source to a standby source. Isolation of motor pressures is
maintained.
MECHANICAL MIXER
The Space Shuttle Orbiter combines in the rudder flight control system two functions: conventional rudder
operation and a speedbrake function. In order to produce the desired output of the functions the PDU
gearbox design was required to accept the power inputs of either or both the rudder and speedbrake
controls and integrate and deliver those inputs to the geared rotary actuators.
907
A combination of gears and differentials was selected to perform this PDU function (See Figure 5). in the
first series of gears/differentials the three motor outputs of each flight control system (rudder or
speedbrake) are speed summed into a single shaft entering the mixer, in the final series of
gears/differentials, the mechanical mixer algebraically sums the rudder and speedbrake motor outputs
and delivers the resultant to the aerosurface via the system drive shafts and actuators. Successful design
and development of this mechanical mixer concept has greatly reduced the system complexity and
weight while permitting simultaneous rudder and speedbreke motion.
SUMMING DIFFERENTIALS
i "_ _'_/ TO LEFT PANEL OF RUDDER/
I i \ / I,_\ SPEEDBRAKE ACTUATORS
To PANE"OF.UDDE 
\,\ /=u'-=" S-J SPEEOeRAK  rUATORS
SPEEDBRAKE INPUT TO DIFFERENTIALS
Figure 5
Mechanlca! Mixer
PLANET GEAR DESIGN
The Geared Rotary Actuator is a planetary-type gearbox with a 474-to-1 gear reduction system. The unit
acts as a power hinge for the aerosurfaces. Within the system there are two basic sizes, with barrel
diameters of 8.8 in. and 11.4 in. The two sizes are designed for maximum load capacity of 338,000 in lb.
and 732,000 in lb. ,,.-.
The planet gears in the actuator assembly a_e the critical, life limiting component. These gears are
designed to withstand high stresses in order to minimize weight and envelope. During eddy program
testing a problem was encountered with root cracking in these highly stressed gears. This test result lead
to extensive evaluation of material and manufacturing processes as well as load distribution analysis and
cumulative damage analysis.
The material and manufacturing processes' evaluation lead to requirements for additional shot peening of
the gear roots to increase precompression and for a new process: gdt-blasting of gear roots to remove the
presence of intergranular oxidation. The absence of these two items was not considered the cause of
initial failure but provided added reliability to final design configuration.
Considerable test work and analysis was conducted to study the effect of load distribution on the planet
gears. Testing revealed that under maximum load conditions the relatively long planet gears deflected
several thousandths of an inch, resulting in a considerable load concentration on the inside comers of the
gear teeth. Using the test information the analytical model was reviewed and after several iteration a
solution was recommended. That solution required optimizing the gear tooth thickness and providing a
crown on the mating ring gear so that under loaded conditions the planet would deflect into a more uniform
gear contact. These changes were thoroughly tested and verified before implementation into production.
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The ability to predict the fatigue life of a gear critical actuation system became the subiect of concern. It
was determined that a more detailed model of the mission load spectrum was required, in studying this
design, Dr. Philip S. Oyoung developed a method to synthesize the load spectrum for the
Rudder/Speedbrake mission profile so that actuator life capability could be calculated. The method,
applicable to general fatigue analysis, provides a much improved accuracy to gear life prediction.
Conclusion
All of the design challenges of the Rudder/Speedbrake Actuation Subsystem, and specifically those
addressed in this paper, have been met and solutions dedved that yield a highly reliable actuation system.
The channel self test features of the PDU and its ability to isolate bad control channels and to switch
hydraulic power supplies combine to give a depth of redundancy that is unique to a flight control system.
The mechanical mixer provided a light weight, simple gear system that permits simultaneous rudder and
speedbrake operation. Safe-life design concepts for primary load carrying components were utilized to
maximize output potential for given weight and envelope.
Successful test programs and, now, successful operational status of the orbiter vehicle are the evidence
of meeting the challenge this aircraft has presented the technical community.
References
1. Technical Report Woolhouse, Dwight: Space Shuttle Orbiter Rudder/Speedbrake Actuation
System. STS 81-0231
2. Technical Report Oyoung, Shio-ping Philip: Fatigue Life Optimization for Gear Critical Actuation
Systems by Load Spectrum Synthesis and an Algorithm for S-N-R Curve
Generation
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Space shuttle rudder/speedbrake actuation subsystem

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