Return to "SHUTTLE AVIONICS - Design Constraints & Considerations - A Guide Book"
GNC 1.
INDEPENDENT CONTROL OF ACTUATOR POWER
Constraint
When operational requirements call for simple, single surface movement, a lack of independent control of actuator power results in the mobilization of large, time consuming ground support (hydraulics, and other local & remote support) for control and monitor of all surfaces.
Impact
- Mobilization of large ground infrastructure (including large number of observers) for simple mechanical tasks
Design Objectives
- Default actuator power to a disabled state during ground operations. Allow power to be enabled to actuators independently.
GNC 2.
ENGINE COLLISION
Constraint
Engine collision should be mechanically impossible
Impact
- A great deal of software and procedures must be developed and maintained to prevent collision between engines
- Adds task-to-task constraints
- Adds time at console operator station to manage a safety issue that could be avoided during the design process
- Engine "Toe-in" clearance test requirement
Design Objectives
- Engine collision should be mechanically impossible in the propulsion system layout
GNC 3.
SSME HYDRAULIC CIRCUITS ISOLATION
Constraint
TVC hydraulics cannot be isolated from SSME cryogenic valve hydraulic circuits .
Impact
- When TVC engine positioning is required, opening the MPS TVC Hydraulics isolation valve applies hydraulics to both the TVC and the SSME cryogenic valve circuits. To hold the SSME cryogenic valves in their safely closed position, one of two conditions must be met :
- SSME Controller power must be applied
- MPS pneumatics must be applied to the SSME
- Thus the SSME system configuration is a constraint to the TVC engine positioning.
Design Objectives
- Provide isolated supplies to each subsystem.
GNC 4.
ENGINE MAINTENANCE DRIVES COMPLEX & LENGTHY MECHANICAL OPERATIONS
Constraint
Undependable engines for turnaround that require either intrusive, in-place maintenance or outright removal, drive a requirement for thrust vectored flight control systems to validate adequate clearance for gimballing. This is required to assure that newly reconfigured/rerouted/reconnected fluid and electrical lines do not exhibit cable stretch, interference, etc. In addition, complex heat shield mechanisms are required to be checked for binding, proper hot-gas sealing, etc. Our experience is that the propulsion system technicians do uncover anomalies after having restored the engine/propulsion system to its flight certified configuration.
Impact
- Requires extra thrust vector control (TVC) clearance tests (hours)
- Requires extra movement of nozzle for installation.
- Tremendous serial time delay associated with engine heatshield removal and re-installation (days and weeks).
- Large potential for flight hardware damage and personnel injury. Large amount of time and resources spent overcoming these hazards.
Design Objectives
- Design a propulsion system that leaves engine/flight control interface intact, i.e., flight certified configuration.
- (No de-pinning of engine thrust vector control actuators from engines, disconnection of engine to vehicle electrical cables, fluid lines, etc., as well as no removal of engine heat shields required for turnaround maintenance, servicing)
- Simpler engine heat shield design or open engine compartment with no need for complex heat shield mechanisms.
GNC 5.
SRB TVC FLEX BEARING RESTRICTION
Constraint
SRB TVC Flex bearing limit of 3 degrees requires control by procedure/software
Impact
- Added software/procedural management and overhead
Design Objectives
- Keep flight/ground operating limits common and design out special configurations in special conditions
GNC 6.
SRB NOZZLE FLEX BEARING TEMPERATURE
Constraint
Prior to vectoring the SRB nozzle, the average flex bearing temperature over the past 24 hours must be 50 ° F or greater. The average temperature is calculated using a sample every 3 hours for the 24 hours preceding nozzle vectoring
Impact
- The SRB must be powered up for at least 24 hours before nozzle vectoring
- Time and manpower is consumed performing the necessary data retrievals and calculating the averages.
Design Objectives
- Design a vectored nozzle with a flexible bearing that is less sensitive to temperatures above freezing.
GNC 7.
EXCESSIVE AEROSURFACE POSITIONING REQUIREMENTS
Constraint
Due to high maintenance activity on aerosurface structures, thermal protection and hinge-line thermal seal mechanisms, an excessive amount of support is required of the flight controls and hydraulics subsystems to reposition or stroke the surfaces.
Explanation: A large amount of maintenance activity (weeks) has been experienced on elevon and body flap hinge line areas. The maintenance has been due to thermal stress, erosion and general wear and tear on hinge line seals and mechanisms required to prevent hot plasma causing damage during entry flight between the upper and lower surfaces at the hinge line. Panel warp, slumping of thermal protection items and time spent re-aligning and re-rigging have been experienced.
In addition, low tolerance margins for corrosion due to low skin thickness design, atomic oxygen in the on-orbit environment and normal ambient humidity have been stated as contributing factors. This has been a problem most prevalent in the rudder/speedbrake area.
Impact
- Unnecessary mobilization of ground equipment and labor to reposition aerosurfaces for aerosurface wear and tear corrective action.
Design Objectives
- Design and demonstrate maintenance-free aerosurfaces including structural thermal protection as well as aerosurface hinge line thermal protection.
GNC 8.
NOSEWHEEL STEERING (NWS) TESTING
Constraint
Nosewheel steering (NWS) testing requires complex time consuming gear configurations and Ground Support Equipment.
Impact
- De-pinning torque arm
- Rate of caster ground support equipment (GSE) required
- Requires mobilization of ground hydraulic equipment and activation of flight hydraulic system
- Specific gear configuration required (gear extended). Control box access requires gear down (inaccessible in launch configuration)
- Steering Position Amplifier/Transducer (SPA/SPT) connectors are demated every flow due to landing gear pyro removal
- Requires pyro connector demates or interrupt boxes installed for testing some signals routed through Forward Reaction Control System pod
- Dedicated Display Unit power required for position transducer and amplifier (should be powered by control box to avoid inter-system task to task constraints)
Design Objectives
- Design, build and demonstrate NWS system that requires no special GSE for checkout, nor requires any de-configuration of vehicle
- Consider electromechanical actuation
GNC 9.
BRAKES/ANTI-SKID MAINTAINABILITY
Constraint
- Testing requires wheels and tires installed, ground support equipment which adds set-up time and serial delay and logistics support for the GSE
- Tires are replaced every flow. If flight tires not available for brake testing, roll-arounds must be used (another instance where the vehicle is required by design to be taken out of flight certified configuration)
- Brake balancing is not self-adjusting (requires manual setting of potentiometers by technician for proper puck/servo balancing)
- Brake stacks are designed to be removed every five flights unless damage found. Experience is that, even after the carbon brake and parachute modifications, we are still having to remove every two or three flights. This is an improvement over the old beryllium brakes and no parachutes, but wide of the mark of a dependable, maintenance-free landing gear/brake system.
Impacts
- Requires ground hydraulics systems mobilization/flight systems activation
- Requires landing gear extension for brake control and anti-skid system validation
- Weight-on-wheels (also known as "squat switches") and weight-on-nosewheel paddles requires man-in-the-loop for landing gear checkout
- Rudder/brake pedal ground support equipment required for brake balancing
- Special shop aid required to test new stacks
- Wheel speed sensor have been damaged during brake stack removals and replacements.
Design Objectives
- Need dependable, standalone, self-checking, self -balancing, autonomous brake/anti-skid test & checkout
- Self-adjusting (tuning) control box
- Drag chutes have alleviated brake problem somewhat, but still a high maintenance area. Heavy maintenance requirement on the current chute deploy scheme.
Note: Need for thoroughly understanding vehicle braking margins up-front. Don’t under-design the landing gear system. Do bring in control system design experts up-front for brake/anti-skid control box and actuation design.
GNC 10.
REACTION CONTROL SYSTEM (RCS) VERNIER DRIVERS
Constraint
RCS Vernier Jet driver circuit checkout requires area clears for personnel safety.
Explanation: A trickle current method of verifying driver circuit integrity is used for Built-in test. While this method has proven very successful and responsive for the larger Normal jets, the circuitry has a design flaw that allows the possibility of releasing hypergolic fuel for the smaller vernier jet trickle current testing. This constrains personnel from working near areas where there are vernier jets. This in turn causes a serial delay while waiting for areas to be cleared of personnel and equipment sensitive to hypergolics and prevents other testing a parallel maintenance work from continuing.
Impact
- Serial delay to test activities while areas are cleared of personnel and equipment sensitive to hypergolic fluids.
- Unable to perform parallel work activities not related to RCS testing because of area clears.
Design Objectives
- This operational constraint is due to a known design deficiency for which a simple known design fix has existed for many years. Due to up-front cost considerations for fixing the design, the fix has never been implemented.
- Future vehicle designs should provide ability to checkout these thrusters electrically without requiring any area clears.
GNC 11.
RCS DRIVERS - DRIVER POWER ACTIVATION
Constraint
Reaction Control System (RCS) driver power testing requires a complete pad surface clear or an OPF bay clear. This is due to the design of the driver circuitry
Explanation: The driver circuitry is very simple, consisting of a Darlington pair transistor module mounted on a heatsink. The driver circuit is controlled by much more complicated logic circuitry which requires two separate commands to activate the driver circuitry. Power for the logic circuitry and driver circuit is separate (i.e., logic power and driver power). With driver power activated, the driver circuitry is a single point failure that could lead to the firing of an RCS thruster without either command being on. Therefore, anytime the driver transistors are supplied with 28Vdc driver power, a single hardware failure would lead to a thruster firing.
Impact
- Stops all other work
- Personnel safety issues
- Single point failure with serious flight/mission concerns
Design Objectives
- Eliminate single point failures from reaction control system driver electronics which would cause an uncommanded thruster firing.
- This operational constraint is due to a known design deficiency for which a simple known design fix has existed for many years. Due to up-front cost considerations for fixing the design, the fix has never been implemented.
- Future vehicle designs should provide ability to checkout these thrusters electrically without requiring any area clears.
GNC 12.
ACCELEROMETER ASSEMBLIES (AA) INACCESSIBLE
Constraint
Accelerometer Assemblies located behind Avionics Bay
Impact
- Troubleshooting/repair/checkout requires removal of other components
- Possibility of collateral damage to other avionics/structures/active thermal systems during turnaround while accessing AA hardware
Design Objectives & Considerations
- Make all avionics components accessible such that no other functions are disturbed or taken "out-of-print" (reference General 8. Avionics LRU Access / Mounting)
- While mounting the box on a heatsink was good (prevented cold-plate mounting), accessibility was sacrificed.
- Reference General 8, Avionics LRU Access.
- Current test & checkout scheme is responsive (minutes) with little or no warm-up.
GNC 13.
INERTIAL MEASUREMENT UNIT (IMU) HEALTH ASSESSMENT
Consideration
IMU health determination more of an "art" than science
Impact
- Inordinate amount of time spent and a dedicated Inertial Systems Lab (ISL) support infrastructure required for tracking, calibrating and managing mechanical errors for turnaround
- Periodic calibration of spare units requires permanent design center lab support (ISL)
- Great deal of experience required to even speak the language
- Shuttle IMU’s require significant care with regard to warm-up time and time and cycle maintenance requirements
Design Objectives
- Need rapid, autonomous navigation system health determination
- Fully understand test margins required to "align and fly" the vehicle, and not just characterize hardware for maximum performance out of the device.
- Demonstrate self calibration and no need for on-line lab support nor software tailoring by LRU
- Investigate use of strap-down inertial systems using high accuracy Ring Laser Gyros and accelerometers, coupled with Global Positioning System (GPS).
- Incorporate new technology inertial sensors (solid state sensors, RLG, etc.) such that accuracy of system eliminates need for expert support
- Design navigation system such that IMU data can be supplemented by another navigation system such that IMU performance can have less strict tolerances
GNC 14.
STAR TRACKER LENS & LIGHT SHADE INSPECTION
Constraint
Star Tracker lens and light shade inspection requires clean room environment
Impact
- Clean room setup and access
- Cleaning
- Cover install and remove
- Light Shade sensitive to operational damage, causing unplanned work
- Thermal system damage due to door operation
Design Objectives
- Design out optical star tracker systems if simpler approaches are available
- If star trackers must be used, do not design-in dedicated mold-line penetrations (doors) and active doors mechanisms.
- There should be no special cleaning/access requirements
- Design robust hardware that is more immune to physical damage
Note: Electronics dependability of the Star Tracker avionics has been good overall (although, some false annunciation’s during self-test have been noted)
GNC 15.
STAR TRACKER CONTAMINATION
Constraint
Star Tracker optics is susceptible to contamination and damage caused by handling, outgassing, humidity, and tile debris.
Impact
- Requires periodic removal for cleaning and refurbishment of light shades and protective window assemblies
- Requires inspection to verify flight worthiness
- Storage of light shade not easily accommodated at KSC because of high humidity
Design Objectives
- Design out optics if simpler approaches are available
- Design robust hardware that is more immune to contamination and physical damage
- Design in ability to maintain hardware in place
- Eliminate or reduce the use of materials that outgas
GNC 16.
ENTRY AIR DATA SENSORS UNDEPENDABLE
Consideration
Current Air Data Transducer Assemblies (ADTA) pressure sensors are undependable due to transducer drift
Impact
- Requires complex ground support equipment (GSE) for calibration every turnaround
- Many recycles to vendor for sensor rework adds to spares level burden and overall logistics repair burden
- Air Data Test Set hook-ups and operation required every flight. Adds set-up time, more equipment that needs to be depended upon for a "green light GO" from the system and GSE maintenance and vendor support costs.
Design Objectives
- Provide pressure sensors with no drift characteristics allowing simple self test without pneumatic ground hookups during ground turnaround (only for depot level maintenance)
- Mean Time Between Maintenance (MTBM) of space flight air data technology must be improved by orders of magnitude and flight and ground operations certified.
- Build flight control system that computes air data parameters from navigation equipment, rather than utilizing traditional direct air-stream sensing.
GNC 17.
DEPLOYABLE ENTRY AIR DATA PROBE MAINTENANCE
Constraint
Deployable Air Data probes result in extra work
Impact
- Tile maintenance
- Tile repair
- Checkout, repair and alignment of deployment mechanisms
- Current technology makes contamination of ports critical
Design Objectives
- Design-out complex deployment mechanisms and mold-line penetrations.
- Demonstrate passive, simple, low maintenance air data collection system integrated with GNC functions. Re-explore flush-mounted air data sensors such as Langley’s OV-102 Shuttle Entry Air Data System (SEADS) for improved operations, maintenance and redundancy (not for improved performance).
Return to KSC Next Gen Site
Edgar Zapata, NASA Kennedy Space Center
Shuttle Process Engineering Directorate, Fluid Systems Division