California--The design process is never complete. Or so is the case for the Reusable Space Systems Div. of the Boeing Information, Space and Defense Systems Group. These engineers are responsible for improving the complex operational mechanisms that make up the NASA Space Shuttle Orbiter.
When the Orbiter's External Tank (ET) door drive system started to show signs of wear, this team was put to work. They determined that an overall fine-tuning of the entire system's rigging procedure would reduce the wear on the attaching fasteners and optimize the efficiency of the system's load on the drive motor.
"Everything has a wearing limit," says Andy Messina, project leader and senior engineering specialist. "Criteria have to be met. You cannot afford the risk of 'what are we going to do if the system doesn't function?' because you cannot return the Orbiter from space with the door in the open position." The system closes off a valve opening on the shuttle's belly to protect it during the extreme heat of re-entry
Two main factors were crucial to the redesign process. First, to achieve the re-entry position, 17 different adjustments must work together in a chain reaction to reach over-center in the open or closed position. The second complication, making modifications onboard the shuttle are nearly impossible since designers need to be in the belly of the Orbiter where structural components such as wiring and feed lines are in the way.
"You can't rely on trial and error with a piece of production hardware because every move or adjustment has to be documented," says Messina. "It has to be a very close copy to the actual device, otherwise it won't behave in the same way." That's why carbon copies of the Orbiter's parts were required for the redesign process.
To complete this task, Boeing created prototype parts using the SLA-250 stereolithography (SL) machine from 3D Systems (Valencia, CA). They used the machine to produce solid 3-D objects that mimicked the ET door-drive system. From this they changed the rigging procedures and reproduced the apparent wear patterns. "You reverse-engineer everything to produce a working model," notes Messina. "Once you have the working model, you can start breaking new ground."
A module in CATIA changes the original part models' format and converts the information to an .stl file in less than 15 minutes. The bigger the part, the longer the process.
The file is then sent to Maestro processing software on the SLA-250. As many components as possible are oriented on the machine, which has a build envelope of 10 x 10 x 10 inches. Then the user selects a build style, in this case the ACES method, which determines how the selected parts are scanned. These instructions are then sent to the build station where the parts are built one layer at a time.
A horizontal plane sliced through all of the parts tells the SLA-250 build station how to scan each particular layer and where to turn the laser on and off. For example, if there is a hole in the component the laser beam skips across it. The machine scans a layer and dips itself in the resin which changes from liquid to solid because it is photoreactive to the frequency of the sensor. The machine continues to build in this process--the hardened layer remaining at the bottom. At the end the platform raises and reveals the part(s).
Production time for each part is geometry-dependent. New machines like the SLA 5000 have a 40 mW laser and can produce a prototype of a telephone receiver in 2 to 4 hours. The SLA-250 has a 400 mW laser and can produce the receiver overnight. "We've squeezed 4,500 hours out of our machine's laser, which cost $13,000," says Messina.
The Boeing team only charges for the time it takes to finish the parts, not for machine time or for the cost of the resin, which is $770 per lb. "We try to use the SLA-250 as an entry tool for customers who may want to start building models in our lab," says Messina. All of the real metal parts and actual product tests are conducted in a machine shop.
The SLA-250 runs at about 40A at 110V per hour and measures about the size of a desk--30 x 60 inches, and 5 ft tall. That includes the control system, the laser, and the optics. "We installed the system in December of 1996," says Messina "It's been running 14 months and only been down for a total of a week and a half."
Boeing built the eleven parts in two weeks using the ACES build style and CIBA 5170 epoxy resin. By combining these parts with commercial components the team created a manually operated, full-scale, table-top version of the Orbiter system. From this they were able to perform instrumentation tests and make adjustments in CATIA. The results were seen immediately.
Boeing fine-tuned all 17 different adjustments and sent new rigging specifications to the people at Kennedy Space Center. After every flight they will check certain mechanisms and readjust according to the conditions the Boeing team stipulates.
"We use stereolithography in place of mold-making any time we make parts," comments Messina. "This has cut time and cost for us. If you do it right, you can recover the cost of the machine in about 10 months with the money you save." Messina also mentions that a new machine is on order, the SLA 5000. Installation is top to bottom.
"There are usually no surprises in stereolithography," adds Messina. "If you build the part right, the machine will produce the part right. If you feed the machine garbage, it will give you garbage."