Section A.5.2.12 Composite Tankspg.1
A.5.2.12Composite Tanks
We first examined carbon fiber composite tanks in preliminary design phase. For preliminary design, we assumed a simplified material model consisting of an isotropic pre-preg carbon fiber layup, which gave us conservative material strengths for a carbon fiber tank. Even with this handicap, the performance of the carbon fiber tank was very attractive from a structural perspective. The carbon fiber tank designed with these conservative assumptions would still be half the weight of an aluminum tank of the same specifications, and we knew that we could achieve even better performance with further optimization.
However, a decision was made to abandon carbon fiber early on in the design process for several reasons. First and foremost was the difficulty in accurately assessing carbon fiber material and manufacturing costs. These figures are closely guarded trade secrets for aerospace companies. What quotes we did obtain suggested that carbon fiber tanks were up to an order of magnitude more expensive than metallic tanks. Since the main goal of the project iss to minimize cost and not maximize performance, we made an executive decision to discard carbon fiber from our final design unless the required performance absolutely could not be reached with metallic parts. In this section, we will discuss the design of aluminum tanks in a little more detail than was covered in preliminary design.
Carbon fiber by itself is not suitable as a primary tank material for rocket tanks. It has poor corrosion and temperature resistance, and in resin-impregnated form, is too porous to contain hydrogen gas. Fabrication of a purely carbon fiber tank is also problematic, as it requires a mold for layup and is difficult to join in parts once cured – if formed in a single piece, it would require that the mold be melted out of the exit port of the tank. For these reasons, a pure carbon fiber tank is not practical or desirable. Instead,a carbon fiberwinding over a metallic tank is more useful in space applications.
Design of a carbon fiber wound tank follows slightly different principles than a metallic tank. The first item of consideration is the wind angle. As the hoop stress in a cylindrical tank is twice the axial stress (see Section A.5.2.1.4), the optimal winding angle is one which provides twice the circumferential strength to the axial strength. Working out this value1
hoop stress, / (A.5.2.12.1a)axial stress, / (A.5.2.12.1b)
/ (A.5.2.12.1c)
/ (A.5.2.12.1d)
where T is the stress in the longitudinal fiber direction (Pa), D is the tank diameter (m), trank is thetank wall thickness (m), wind is the filament winding angle measured from the long axis of the tank. Refer to Figure A.5.2.12.1 below.
Fig. A.5.2.12.1 Filament winding on tank
(Chii Jyh Hiu)
This wind angle wind is both optimal for structural strength and necessary to avoid out of plane deformation from pressure loading.
A fiber wound tank consists of unidirectional carbon fiber filaments wound around a metallic tank. The stress analysis1 of such a compound tank is more complicated than for a single material tank. Stresses in the material are now a function of the material properties.
deformation, / (A.5.2.12.2a)after solving for contact pressure,
metallic hoop stress, / (A.5.2.12.2b)composite hoop stress, / (A.5.2.12.2c)
where m and c are the deformation in the metal and composite respectively (m), P is the internal tank pressure, Pcontact is the contact pressure between the metal lining and the composite sleeve, rm and rc are the radii of the metal and composite tank respectively (m), Emis the Young’s modulus of the metal lining (Pa), Ec,1 is the Young’s modulus of the carbon fiber layup in the principal fiber direction, wind is the winding angle of the carbonf fiber measured from the long axis of the tank, tm and tc are the thicknesses of the metal and composite respectively (m), m and c are the hoop stresses seen in the metal and composite respectively (Pa). Refer to Figure A.5.2.12.2 below.
Fig.A.5.2.12.2 Carbon fiber sleeve over metallic lining
(Chii Jyh Hiu)
A carbon fiber sleeve allows us to withstand higher loads than a metallic tank itself could withstand. Due to the high elastic modulus of carbon fiber in the longitudinal direction (~145 GPa2), a substantial improvement in metallic hoop strength can be obtained and significant weight savings realized. We predict a weight savings of 2.5-3 times over a comparable metallic tank, as opposed to a 2x savings from the preliminary design runs using isotropic layup assumptions. Due to the appealing structural characteristics of carbon fiber, it may be prudent to keep composite tanks in consideration for future designs, as the cost effectiveness of aerospace carbon fiber continues to improve with industry adoption, so costs will only continue to fall.
References
1 Roylance, D, “Pressure Vessels”, Massachussets Institute of Technology, August 23, 2001
2. Callister, W.D. Jr, Fundamentals of Materials Science and Engineering, 2nd Ed., Wiley & Sons, 2005
Author: Chii Jyh Hiu