Technologies cooperate to control critical mixing operation
Is sophisticated batch control rocket science? Mixing ablative coating for space shuttle rocket boosters requires very high levels of precision and repeatability.
In April 1981, Columbia, the first Space Shuttle, launched and orbited the earth 37 times before landing on a runway at Edwards Air Force Base in California. It was the first U.S. manned space vehicle launched without an unmanned powered test flight. NASA described the mission as: "The boldest test flight in history.”
NASA’s space program has been the source of many such feats that have sparked the imagination of children and writers for the past half century. But each of these missions is preceded by years of stringent adherence to safety and production requirements.
Safety, precision, and accuracy
After each countdown to ignition, a shuttle is propelled into space by trademark twin flames streaming from solid rocket boosters. Those twin boosters provide 80% of the shuttle launch thrust before they burn out, separate, and fall by parachute into the Atlantic Ocean. NASA recovery ships retrieve the shells and tow them to Hangar A/F, Cape Canaveral Air Force Station (CCAFS) in Florida, where they are disassembled.
Refurbishing the boosters for reuse takes place in two locations. The solid rocket booster subassemblies—the frustum, forward skirt, and aft skirt—are initially refurbished at Hangar A/F then transported to the United Space Alliance (USA) Assembly and Refurbishment Facility (ARF) at NASA's Kennedy Space Center in Florida for final assembly and testing. Parachutes are refurbished and packed at the Parachute Refurbishment Facility and then shipped to the ARF. The reusable solid rocket motor segments and the nozzle, which steers the rockets during flight, are transported via railcar to Alliant Techsystems (ATK) in Utah, where they are reassembled, tested, and returned to Kennedy for remating. The entire process from retrieval to completion takes approximately a year.
Refurbishment and reuse
The engineering teams at USA’s ARF are bound by incredibly rigid production specifications, because anything more than a hairline deviation can severely affect the safety of a mission. Of the booster’s total weight of 1.25 million pounds, propellant accounts for 1.1 million pounds, which burns hot enough to damage the enclosure’s structural integrity. One of the materials used to protect the rocket boosters during ascent, descent, and splashdown is a USA-developed thermal protection system called booster trowelable ablative (BTA). Its consistency resembles that of automotive body filler but holds much better thermal properties. This is important because it protects the booster components from damage, enabling them to be reused time and time again.
The batch mixing of the insulation for use on the flight components is an automated process, and handled by two functionally identical machines. Micro Motion flowmeters release precise measures of resin and a catalyst into a mixing vessel, where a Charles Ross mixer blends them together to form the BTA insulation. The Kennedy facility prepares an average of five 3,000 g batches per day, but with specifications allowing for only ±2% deviation on any given batch, USA has a challenging job.
One of the machines is controlled by a Rockwell Automation Allen-Bradley ControlLogix PAC and the other by an SLC-500. USA’s engineers tried using 4-20 mA feedback between the controllers and the flowmeters, but found they were unable to obtain the needed level of accuracy and precision because the standard analog input module could not reach the data transfer rates required.
“In normal industry you can produce a similar product and get away with being 10-15% off and it wouldn’t make any difference,” explains Dan Dermody, control systems engineer at USA and the machine builder for this application. “But because of the environment that these solid rocket boosters operate in, there is absolutely no room for error.”
“We contacted Micro Motion and they pointed us to ProSoft Technology’s Modbus communication modules, which integrate directly into the ControlLogix and SLC-500 platforms,” Dermody explains. “We tested them out and quickly discovered that they provided the accuracy and precision we needed. The module collects flow data and feeds it directly into the ControlLogix data tables. This type of flow-control system maintains all of the process parameters, ensuring that nothing goes out of specification during mixing. The ProSoft module made the architecture we wanted to use possible, and we’ve stuck with that type of philosophy on our flowmeters ever since.”
While the new approach was a major improvement, it still required some fine-tuning to deliver the full benefit. “Once we brought the information over digitally it was a night-and-day difference,” Dermody adds. “Still, we were only barely achieving our goal and I knew something wasn’t right with the update rate. So, we worked with ProSoft to identify a controller programming problem which essentially caused the controller to write over data within a millisecond of when I was trying to read it. We now have the performance we need. We’re getting millisecond update times and we can control down to the gram level in a 2,000 gram batch.”
Because of the level of repeatable precision USA is able to accomplish with this solution, they are not required to test the adhesive delivery system continually to prove their accuracy.
NASA and the space program are currently undergoing a major directional shift with the end of the space shuttle program. Presently, USA is building up the parts for a second test flight for the Ares Program. While there has been no official decision on the exact architecture of the post-shuttle human spaceflight program, one fact will remain: the demanding environment in which rockets must perform will require materials with the highest quality standards made possible by innovative solutions.
Adrienne Lutovsky and Danetta Bramhall are staff writers for ProSoft Technology.