Watch This, Space: Manufacturing MLAS
By George N. Bullen, FSME
President and CEO
Smart Blades Inc.
On July 8, 2009, the earth shook as four Thiokol solid rocket motors ignited, propelling a bullet-shaped vehicle weighing better than 45,000 lb (20,000 kg) up and away from its launch pad. Approximately seven seconds later, the engines stopped and the vehicle silently coasted upward as the boost skirt drawn by gravity fell away toward Earth, 7000' (2.1 km) below. In a sequence of events separated by seconds as the vehicle continued skyward, the coast skirt separated and deployed parachutes; the forward fairings fell away; and the crew capsule deployed its parachutes and began its decent toward Earth. The flight test performed at NASA’s Wallops Island (VA) Flight Facility was meant to test the Max Launch Abort System (MLAS), and it resulted in a flawless performance.
The test was not just highly successful because of MLAS’s performance, but also because of how the system was made, what it was made of, and where it was made. The manufacturing methods used included liquid-resin infusion, filament winding, and out-of-autoclave cure of composite materials.
The boost skirt, coast skirt, and forward fairings were each made in quarter sections at Northrop Grumman Corp.’s shipbuilding facility in Gulfport, MS, using a liquid resin infusion process. The quarter sections were infused over wooden tools that created the shape of the Outer Mold Line (OML). Once cured, foam core and stiffeners were bonded to the OML skin with Plexus paste adhesive and the inner skin liquid-resin infused.
Each quarter section was removed from the mold and the process repeated until all of the sections for the boost skirt, coast skirt, and forward fairings had been completed. As each section was in the mold being fabricated, the section just removed was trimmed and then drilled for the attachment of fittings, brackets and other hardware necessary for the final assembly of the vehicle. All of the work was performed using standard shipbuilding technology, personnel, and facilities from designs and process specifications provided from the NASA/NGC design team.
When completed, the sections were packed and loaded on trucks for transport to the NASA Wallops Island MLAS assembly building. When the boost skirt, coast skirt, and forward fairings were assembled and stacked together they were about 33' high (10 m) and approximately 10' (3 m) in diameter.
Eight struts were attached inside the boost skirt angling inward to join it to the motor mount and crew module bracket.
The manufacturing process used to fabricate the struts was filament winding over a Paraplast mold. The Paraplast material was heated until liquefied and then poured into a mold representing the inner mold line (IML) of the strut. A steel shaft ran through the center and was supported at each end by the mold. When solidified, the mandrel was removed from the mold and aluminum attach-brackets slid over each end of the steel shaft and onto the Paraplast mold. The mandrel was then placed in a slow turning hand lathe.
Once the struts were wound with composite material they were bagged, vacuum was applied for compaction, and cured. When cured, the bag was removed and hot steam used to remove Paraplast from inside the solidified strut. The struts were designed to take the stress and vibration of the 45,000-lb vehicle as it accelerated from the launch pad to 7000' in seven seconds.
Affixed to the outside of the MLAS vehicle were eight fins for guidance and stabilization. Four larger fins are attached to the boost skirt and four smaller fins attached to the coast skirt. The fins are made of autoclave-cured-type composite material bonded to a foam core. The autoclave composite material is cured out of the autoclave using a process called in-a-bag cure.
The out-of-autoclave cure (OAC) process involves bringing the temperature of the composite material above water’s boiling point but below the cure temperature of the composite material while it is under controlled vacuum and sealed in a bag. The temperature is held until the water contained in the composite material is boiled off. The temperature is then raised to the cure temperature of the composite and held until compacted, consolidated, and cured.
The critical nature of the skins as the primary guidance mechanism for the MLAS vehicle made the dimensional integrity of the fins a critical factor in their manufacture. The protrusions from the skins fit into precisely trimmed slots in the boost and coast skirts. They align the centerline of the MLAS vehicle with its flight trajectory and therefore any misalignment can cause error in the desired flight path.
The out-of-autoclave cure of the fins performed in a shop in Huntsville, AL, was used to validate the process for manufacture of flight hardware. Besides the elimination of the autoclave, the OAC process used to produce the skins also incorporated low-cost manufacturing solutions such as collapsible/transportable low-cost clean room and oven. The elimination of the autoclave when combined with the transportability of the clean room and oven reduces facility cost for the cure of autoclave composite materials.
Once the fins were inspected and tested, they were packaged and shipped to the MLAS assembly facility on Wallops Island.
The struts, fins, coast and boost skirt quarter sections, and the struts arrived on Wallops Island to be married with the other components of the vehicle. The other components included the Thiokol solid rocket motors, parachutes, pyros, fittings, brackets, fasteners, and electrical components necessary for the system to be completed.
All of the components needed to be coordinated to arrive simultaneously with a team of experts from geographically diverse locations across the US as "best athletes" for the vehicle’s assembly.
The assembly of MLAS was critical because the vehicle did not have a guidance system to correct errors in flight. It was a launch-and-leave system that relied on the precision of its assembly aligned with the thrust of the motors to maintain proper trajectory. Therefore the mass properties of all the combined components had to be weighed, measured, and constantly aligned during the assembly process using precision measuring instruments. Without proper alignment of all the pieces and parts of the assembly to the centerline of thrust, the vehicle would tumble or veer off course. Critical to the success of the precision assembly process was the use of laser alignment systems. In addition the fins had to be aligned precisely with the vehicle as it was stacked and assembled. The fins of the coast skirt had to be clocked 45° from the fins located in the boost skirt in order to be positioned between them at their precise midpoint.
Other hard point details were also used to facilitate proper alignment during the assembly and launch of the MLAS vehicle. They included alignment stacking pins and frangible joints. Four drag plates were used to facilitate the separation of the boost skirt from the rest of the vehicle after the rocket thrust had stopped.
The assembly process included the stacking and unstacking of the MLAS vehicle to ensure the separation and fitting would realign and not bind. The assembly of the MLAS vehicle also included the 27.6' (8.4-m) diameter conical ribbon parachutes for the coast skirt, 27.6' diameter conical ribbon parachutes for reorientation of the capsule simulator and fairing, and two more 27.6' parachutes to slow descent until splashdown.
The NASA Engineering and Safety Center (NESC) had several partners during the coordinated effort to develop and conduct the demonstration and perform the assembly work. Besides Northrop Grumman Corp., NASA personnel based at Wallops Island conducted structures and mechanism assembly as well as flight test support.
The Meaning of Success
The successful launch of MLAS from Wallops Island was characterized by more than the operation of the vehicle and its systems. Embedded in the system were manufacturing technologies and materials that had never been used to fabricate a launch vehicle prior to MLAS. They included liquid-resin infusion using ship construction technology, personnel, and facilities; out-of-autoclave cure of autoclave composite materials in portable facilities; and hand filament winding over Paraplast washout mandrels. The use of these innovative manufacturing processes and materials came with risk. The demonstration of their ability to be included as an option for manufacture of future space launch vehicles is supported by their successful application on MLAS. They offer lower-cost solutions to the fabrication and assembly of space launch vehicles by meeting the precise specifications and strength requirements that these vehicles demand. ✈
George N. Bullen was deputy director of manufacturing
for Space Technology during the MLAS, ARES I, CCM,
Composite Cryo-Tank, In-situ, and ALTAR Lunar Lander projects.