Structural Mechanics

COPV, Carbon Overwrap Pressure Vessel

This form of pressure vessel consists of a liner, usually aluminium, and a carbon fiber composite tape or fiber wrap. The choice of tape or fiber depends on the preference of speed with less control or smoothness and better control respectively.

The reason for going to fiber composite is of course ease of manufacture and stiffness as well as strength advantages. A useful rule of thumb is that we can get the same performance for 1/5 the weight. One could consider the fuselage of an airplane as a special form of COPV. There is not much research on COPV. A good summary is provided in [1]. The work barely scratched the surface of the stress conditions that can exist in a COPV. However it does recommend some design rules for its actual usage in launch rockets. Very early research at NASA recommended an ellipsoidal closure to achieve similar membrane stresses in the cylinder and the closure. A three D analysis by the author showed significant bending stresses at the intersection. It was also established that the traditional axisymmetric analysis produced bad comparison with experiment.

In this article, we consider a recent application of COPV. That is the use of COPV as helium filled containers within a liquified Oxygen tank. Two highly publicized failures have emphasized the incomplete knowledge of the COPV. The reasons for failure are sketchy. In the first failure, a COPV built by an outside maker, the failure was blamed on a defective strut support. The second Failure was on a vessel built by SpaceX who decided to take the design in house. The failure Investigation was inconclusive but attributed to interaction of leaking Helium and oxygen which formed solids and exploded. This however does not explain why the pressure liner integrity was violated. An impact test with a bullet projectile repeated the failure with the same symptons. My first reaction is that there was a brittle fracture. The other liners in the other tank showed significant buckling that may be explained by the explosion.

Its instructive to consider the sequence of events that applies load on the liner.

  1. When the frozen liquid helium is introduced, the liner is first shrunk but since it has little strength, the liner simultaneously yields elastic-plastically and balances out its load with the fiber composite.
  2. Then the cold diffuses into the fiber composite and it contracts ( by a much smaller amount) this places the liner in compression.
  3. Soon after, the Oxygen is introduced in the surrounding tank and the COPV is subjected to further external pressure. The liner suffers further compression. The actual conditions in the liner can only be determined by a full FEA analysis that includes all the failure parameters. Similar analysis have been succesfully carried out on simpler structures [2]. The one factor that worries the writer is that it is known that flat spots in the closures can cause failure [3]. What level of imperfection is generated by the composite tapes?

Discussion

In considering the design and use of COPV for liquid fueled rockets, we have identified a number of quantities that must be calculated in order to minimize the risks attending its use. These quantities can be obtained by a careful nonlinear finite element analysis

References

[1] John C. Thesken Ohio Aerospace Institute, Brook Park, Ohio

Pappu L.N. Murthy Glenn Research Center, Cleveland, Ohio

S.L. Phoenix Cornell University, Ithaca, New York

N. Greene National Aeronautics and Space Administration, White Sands Test Facility, Las Cruces, New Mexico

Joseph L. Palko Connecticut Reserve Technologies, Inc., Cleveland, Ohio

Jeffrey Eldridge and James Sutter Glenn Research Center, Cleveland, Ohio

R. Saulsberry and H. Beeson National Aeronautics and Space Administration, White Sands Test Facility, Las Cruces, New Mexico

‘A Theoretical Investigation of Composite Overwrapped Pressure Vessel (COPV) Mechanics Applied to NASA Full Scale Tests’, NASA/TM—2009-215684

[2] P.V.Marcal,J.T.Fong, and N.Yamagata, A Macro Model for 3D Fiber-Reinforced Polymer Composites, Proc. WCCM, Barcelona, 2014. Also Design and Analysis of Reinforced Fiber Composites, editors P.V.Marcal,N.Yamagata, Springer, 2016.

[3] Marcal, PV, “Large Deflection Analysis of Elastic-Plastic Shells of Revolution”; AIAA Journal, 8 (9),   1627  (1970).  Also Brown University Report, N00014-0004/1,  (December  1968).

Note on Initiation of turbulence

Introduction
Turbulence is a much studied subject in Fluid Flow. Though we are able to predict some of its behavior numerically, other
features remain somewhat obscure. In particular,
at what point does a laminar flow
transition into turbulence. In this note we will make some assumptions as to the nature of the
phenomenon and then using a Least Squares Finite Element Method (LSFEM, Jiang [1]) implemented in the MPACT code [2], we carry out a simulation to gain some further insight.