Tubular composite core stiffens launch vehicle structure
ChamberCore, an alternative to traditional sandwich laminates, also improves vibro-acoustic damping.
By Barry Berenberg, Web Editor/Technical Writer | September 2004
The payload fairing on a launch vehicle protects the payload during ascent, maintaining it in a controlled environment. Although a simple monocoque laminate could otherwise provide sufficient environmental protection, the fairing is located at the front of the vehicle, so the construction must have a high stiffness to react the aerodynamic loads and prevent excessive vehicle bending. As a result, most modern payload fairings are constructed of a composite sandwich laminate, usually carbon/epoxy facesheets over a honeycomb or foam core. Conventional sandwich constructions provide acceptable stiffness but offer insufficient protection from acoustic vibration during take-off. "At launch, acoustic waves reflect from the ground and excite the vehicle, which, in turn, excites the payload," explains Joe Padavano, president of aerospace engineering firm Delta Velocity Corp. (Purcellville, Va., U.S.A.). The potential for damage to satellites and sensitive instru-mentation is high. According to the U.S. Air Force Research Laboratory (AFRL), researchers estimate that vibro-acoustic stress may account for nearly half of all payload failures that occur within the first 24 hours after launch. Reducing such stresses, therefore, could go a long way toward improving payload reliability and reducing launch risks. Furthermore, a payload fairing that better damped stresses would enable payload engineers to create lighter satellite structures, leaving more weight for sensors, propulsion units and other non-structural elements.
Source: USPTO
The original Douglas Aircraft Company patent illustrates the layup process for a ChamberCore-type structure. Core mandrels (13) are overwrapped with prepreg (19), laid parallel, sandwiched between facesheets (18 and 20), and finally cured in a matched tool (14 and 23).
In pursuit of that goal, AFRL launched a small sounding rocket on Aug. 20, 2003 from Wallops Island Flight Facility, off the coast of Virginia, U.S.A. The Vibro-Acoustic Launch Protection Experiment (VALPE-2), as it was called, carried a number of new vibration isolation and acoustic mitigation technologies. Active system experiments designed by CSA Engineering (Mountain View, Calif., U.S.A.) and The Boeing Co. (Chicago, Ill., U.S.A.) monitored vibro-acoustic loads during launch and drove actuators to partially cancel out harmful loads. The VALPE-2 payload fairing, designed and built by Delta Velocity, was itself part of the experiment, designed to demonstrate a lightweight composite fairing based on ChamberCore, a unique manufacturing process that promises to significantly reduce vibro-acoustic stresses.
Updating an old idea
A ChamberCore laminate is basically formed by placing a number of square (or nearly square) composite tubes in parallel and bonding them together with composite facesheets. This construction achieves the stiffness of a sandwich laminate but also possesses unique geometric features that can be used to further reduce acoustic transmission.
The concept was originally developed by Douglas Aircraft Co. and patented in 1956. These "multi-ducted shells," as Douglas called them, were used for aircraft radomes. In the late 1990s, AFRL and Dr. Stephen Tsai (Stanford University, Stanford, Calif., U.S.A.) began developing a simplified ChamberCore manufacturing process. Later, AFRL determined that, by drilling holes through the inner skin into the hollow cores, a ChamberCore laminate could act as an acoustic damper (based on the same principles as a Helmoltz resonator).
In 2001, AFRL awarded a Small Business Innovation Research (SBIR) contract to Delta Velocity. Phase I required Delta Velocity to further streamline the design and manufacturing processes; Phase II would provide initial flight demonstrations of ChamberCore, first in the VALPE-2 fairing, and later in the larger Scorpius (Microcosm, El Segundo, Calif., U.S.A.) fairing.
Source: Delta Velocity
The fairing, assembled with a nosecone and aft vehicle adapter, is set up on a shaker table for vibration testing. Instrumentation cables are run out through the access door.
Delta Velocity engineers came up with an alternative to drilling holes: by filling the cores with water, they realized they could reduce the acoustic loads even further. "Most payload fairings add mass, in the form of heavy acoustic blankets, to attenuate these loads," says Padavano. "The extra mass must be carried until fairing jettison -- about two to five minutes into flight -- but it is really only needed for the first 8 to 20 seconds." The Delta Velocity design would drain the ChamberCores after those first few seconds, reducing the effective flight mass and allowing for heavier payloads. The water-filled and Helmholtz modifications will be tested in the future. For now, the VALPE-2 and Scorpius fairings use standard ChamberCore.
Adapted software optimizes design
In Phase I of the SBIR contract, Delta began by sizing the ChamberCore tubes and facesheets for expected flight loads. Analysis methods are well defined for standard sandwich structures, but the unique ChamberCore geometry required new analytical techniques. The tube geometry introduces new failure modes, such as buckling of the walls between the tubes. The tubes and facesheets each can be made of a different layup, and possibly even different materials. Finally, the size of the tubes is also important -- if they are made too large or too small, the structure weighs more than is necessary.
Delta analysts used HyperSizer software from Collier Research Corp. (Hampton, Va., U.S.A.) to size the cores. HyperSizer is able to analyze the individual components of a stiffened panel, sizing each component to minimize weight while maintaining positive margins against dozens of failure modes. Although HyperSizer models six different panel families (honeycomb, corrugated, etc.), with a number of subtypes within each family, it does not include a ChamberCore panel. Instead, the ChamberCore panels were treated as I-beams (a supported model type) sandwiched between two facesheets. By making the gaps between the I-beam flanges very small, the resulting panel was a good approximation of the ChamberCore geometry. HyperSizer was able to analyze literally thousands of combinations of core height and width, core layups, and facesheet layups in a matter of minutes.
A T700/E-765 prepreg from FiberCote Industries Inc. (Waterbury, Conn., U.S.A.) was chosen for the tubes and facesheets because it had been qualified under the AGATE program and material property data was readily available (see "AGATE Methodology Proves its Worth," HPC May 2003, p. 38). Combinations of unidirectional and fabric plies were examined, but the HyperSizer analysis ultimately settled on an unusual +45°/-45° (antisymmetric) layup of unidirectional tape (0.127 mm / 0.005 inch ply thickness) for the cores and a three-ply, 0°/90°/0° tape layup for the facesheets. The core cross-section was square, with 23 mm (0.9 inch) sides.
Maximizing mandrel performance
AFRL fabricated the tubes for early ChamberCore tests by overwrapping a solid rubber mandrel with prepreg. The mandrel was made from Aircast 3700 (Airtech International, Huntington Beach, Calif., U.S.A.). This material is easy to cast into any shape, and its high thermal expansion coefficient provides good compaction pressure during cure. The wrapped cores were laid up between the facesheets and the entire structure was cocured under autoclave pressure. AFRL also tried curing the cores first, then placing them between uncured facesheets and curing the resulting structure. The latter method had the potential to reduce the number of mandrels needed -- the cores could be made in bulk, then placed between the facesheets without mandrels -- but the cores tended to deform under cure pressure without the internal mandrel support.
The rubber mandrels worked well for straight cores with small aspect ratios (relatively fat cross-sections). For typical aerospace structures, however, the cores would be smaller in cross-section and might bend or curve, as well. Preliminary tests indicated that pulling rubber mandrels out of such cores would be difficult, if not impossible. As an alternative, Delta decided to use water-soluble mandrels that could be washed out after cure. These mandrels would not expand like the rubber mandrels, but the autoclave pressure would provide sufficient compaction.
A flat test panel was made using core mandrels cast from Aquacore (Advanced Ceramics Research, Tucson, Ariz., U.S.A.), a water-soluble ceramic material that is environmentally friendly and requires no special disposal procedures. ACR built the mandrels for Delta Velocity by packing the material into a metal tool and then drying it in an oven. When the Aquacore material proved to be very brittle, the mandrels were reinforced with a thin metal rod through the center and overwrapped with a release tape to prevent flaking.
For the VALPE-2 fairing, ACR used a newer Aquacore formulation that was tougher and did not require an internal reinforcement. The bare cores were shipped to Composite Tooling Corp. (Albuquerque, N.M., U.S.A.), where Delta Velocity sprayed them with Aquaseal. This gave a smooth, less porous surface, eliminating the need for a release tape overwrap. The mandrels, however, absorbed moisture from the sealer, causing them to become flexible and warp. "We ended up spraying one surface at a time," explains Delta integration and test engineer Warren McCrary. "After that surface dried, we rotated the cores and sprayed the next surface, until all four sides were covered twice."
