Fairing Model
Dateline: 03/10/99Introduction
About this time last year, I started a series of articles on the development of a fairing for the OSP launch vehicle (later named Minotaur). As often happens with government projects, this one has moved into a state of limbo. It's not dead, and it will hopefully pick up again in a few months, but not much has been happening for the last six months or so.
Part of the OSP fairing project was to develop an acoustic damping system for launch vehicles. CSA Engineering was going to take one of the non-flight articles, develop an acoustic suppression system, and test it on the ground.
In order to verify their models, they needed to characterize the acoustic behavior of the fairing. The easiest and fastest way to do this was to build a full-scale model of the fairing. The model would then be shipped to a test lab (which turned out to be Duke University), assembled over a microphone tree, and blasted from speakers.
This new series of articles covers the manufacture of that fairing model. The fairing is complete and has been shipped to Duke, so there won't be any delays like with the OSP series.
Problem Statement
To run the acoustic characterization tests, all that was needed was a model with the same shape and size as the flight fairing.
The main portion of the fairing is a cylinder, with about a 61 inch diameter. The forward end is bi-conic (two conical sections with different angles) ending in a hemispherical nosecap. The aft end is a conical boattail, which transitions from the cylinder to the vehicle's third stage motor (about a 50 inch diameter). The overall length is about 20 feet.
The purpose of the tests is to measure the acoustic behavior of the fairing cavity. Thus, the inner mold line (IML) dimensions are what's important. Furthermore, the acoustic wavelengths are larger than the ribs, so they don't need to be included in the model. This also means that imperfections in the surface such as wrinkles are not important.
(The flight fairing is designed as a grid-stiffened structure. For more information on this type of structure, see the OSP series of articles.)
Aside from size and shape, not much else affects the acoustics. Materials aren't important, because this is for a cavity test, not a transmission test. The model doesn't have to carry any loads, but it does have to maintain its shape under its own weight.
Manufacturing Approach
The fairing is a big part, and any sort of traditional tooling would be much too expensive. This project didn't have an official budget, but it was funded from "slush" money. Therefore, everything had to be cheap.
Fortunately, the design requirements all fit in with the cheap approach.
The tool would be the most expensive part of the project. Only one part was needed, so the first decision was to build an expendable tool. The initial concept was a foam tool which would be torn out when the part was completed. This is similar to some of the one-off approaches listed on the How To's page, only on a much larger scale.
Because materials were irrelevant to the test results, E-glass with a wet layup resin was chosen. The resin had to be a room temperature cure system, because the part was too big to fit in any of our ovens. The layup would consist of a layer or two of glass fabric directly over the foam (for stability), followed by a to-be-determined thickness of filament-wound glass. (We have a large stock of glass roving, so that material would be free.)
We started by ordering blue Styrofoam billets and glass fabric from Aircraft Spruce. We already had a good supply of E-Z Poxy, so resin wasn't an issue. Other materials, such as framing for the tool, would be purchased from Home Depot as necessary.
Some of the design details changed as we got into the project. For example, we used a glass fabric and core layup rather than filament winding. Even with the changes, though, the finished part was surprisingly similar to the initial concept.
Next week, I'll cover the details of the initial tool design and start showing how we built it.