Tetsuya Morimoto

Applicability of CFRP materials to the cryogenic propellant tank for reusable launch vehicle (RLV)

It is essential to utilize carbon fiber reinforced plastics (CFRP) for main structural materials of cryogenic propellant tanks in order to realize the drastic weight reduction needed for efficient reusable space transportation systems. Recently developed toughened CFRP materials, which are expected to show good cryogenic properties, are considered promising candidates for these kinds of applications. The present study investigates cryogenic properties of candidate materials and structural elements, including Y-joint structural models. 300 mm diameter filament wound tank and 600 mm diameter lay up tanks were fabricated and tested. Based on these experimental data, the feasibility of a CFRP cryogenic tank is discussed and future research tasks are proposed. This research is being conducted under the cooperation contract between NASDA and NAL.

Analytical prediction of resin impregnation behavior during processing of unidirectional fiber reinforced thermoplastic composites considering pressure fluctuation

In order to fabricate composite components more rapidly, a micro-braiding technique has been proposed as fabrication of intermediate material for continuous fiber reinforced thermoplastic composites. In the present study, a simple model considering pressure fluctuation during compression molding is proposed based on the Darcy’s law and the continuity equation to predict resin impregnation process using micro-braided yarns. In order to measure pressure fluctuation, a mold die including a channel from the cavity to a pressure gauge was fabricated. To confirm validity of the model, carbon fiber reinforced polypropylene composites were molded under various molding conditions. Once molding pressure was applied, pressure on the molding pieces rapidly increased and gradually decreased to equilibrium with time. Analytical results well-predict resin impregnation during molding and the effectiveness of the model proposed was confirmed.

Pressurization test on CFRP liner-less tanks at liquefied nitrogen temperature

Two liner-less CFRP concept tanks were prepared for internal pressurization tests at liquefied nitrogen (LN2) temperature. The tanks were designed in two patterns of eight-ply UD quasi-isotropic lay-up, in the shape as a cylinder of 600 mm in diameter and 1200 mm in length, covered with an aluminum flange at one end and with a CFRP hemisphere dome at the other. The maximum strain was applied as the damage onset condition so that the internal pressurization at LN2 temperature did not damage them up to 1.1 MPa. Damage onsets, such as transverse cracking and leak path formation, were monitored during the tests using helium flow detection, acoustic emission, and pressure-strain monitoring. The CFRP concept tanks showed no damage in the 1.1 mm thick cylindrical gauge section under pressurization up to 1.1 MPa at LN2 temperature. The design was thus shown to be successful in keeping the CFRP tanks intact.

Pressurizing Test of CFRP Model Tank in Cryogenic Temperature

The authors present in this study an approach demonstrating the performance of a toughened epoxy CFRP concept tank under internal pressure at a cryogenic temperature. Three tanks were prepared with eight-ply unidirectional (UD) quasi-isotropic lay-ups of two different patterns and cloth lay-up. Each tank was cylindrical with a diameter of 600mm and length of 1200mm, covered with an aluminum flange at one end and a 600mm hemispherical CFRP dome at the other. The gauge length used was the central 300mm of the cylindrical section with a wall thickness of I.imm and made from IM600/#133 toughened epoxy CFRP. Each of the tanks was installed into a metallic chamber with the outside being under vacuum in order to preserve the cryogenic condition. The inside of the tank was pressurized with liquid nitrogen (LN2) together with gaseous helium (GHe) with the aim of detecting the onset of damage from the GHe leakage. The experiments were performed as follows. Firstly, helium gas leakage was measured and indicated no damage resulting from a water pressure of 1 MPa at room temperature (R.T.). Secondly, LN2 storage was performed without pressurization in order to evaluate any damage onsets due to the cryogenic condition. Following LN2 storage, no damage was detected within the gauge section of the tank. Thirdly, the tank was pressurized with LN2 and GHe to 0.98 MPa at gauge pressure (MPaG) and indicated the gauge section to has kept intact. Therefore, it was concluded that the materials and concept CFRP tank structure were successfully demonstrated under pressurization in cryogenic conditions. Although technical steps remain regarding engineering structures, CFRP appears to be a promising candidate for the realization of lightweight pressure vessels such as launch vehicle cryogenic tanks.

Pressurization of CF/Epoxy Model Tank at LN2 Temperature and Identification of Leak Path Formation in Tank Wall

A review of six years research project for accumulation of basic knowledge about cryogenic composite tank of future RLV conducted in NAL/ NASDA will be given first. An emphasis is placed on the final part of pressurization of the model tank made of CF/Epoxy at liquid nitrogen temperature. Some typical results of preliminary steps in the research route will be also given. In the final cryogenic pressurization, an advantage of the difference in the matrix crack initiation strain levels at room temperature and at LN2 temperature played a key role to conduct the present test possible under safety requirements. The matrix crack free strain (stress) levels were obtained for the same composites as used for the tanks in the research programs conducted in NAL/University of Tokyo in parallel to this program. In the present cryogenic pressurization, leak path formation was detected at real time by nonlinear gas pressure – strain behavior and by sudden rise in gas pressure in the vacuum chamber. Later, the exact locations of the leak path were identified by a sniffer scan using a helium leak detector and by snoop tests using leak check solvent. Finally, the strain level of the leak path formation was compared with the matrix crack formation and evolution strain levels obtained by coupon tests. It is clarified that the leak path formation strain level coincides well with the strain level of matrix crack propagation in coupon detected by AE sensors, almost identical to the matrix crack initiation strain of tubular specimens. This conclusion seems to be appropriate if we notice that the present composite model tanks have no edges.

A new cost/weight trade-off method for airframe material decisions based on variable fuel price

This paper presents a simple method for analyzing the monetary trade-off between rising airframe material cost and reduced jet fuel cost for supporting the decision-making in designing lightweight but expensive new materials. The method considers the weight growth factor, Breguet range equation, specific strength, material cost, fuel price, and aircraft range. A model analysis reveals that rising fuel price can drastically change the optimum airframe materials from legacy aluminum alloys to carbon fiber-reinforced plastics.

Test of filament wound CFRP prototype for cryogenic propellant tank of space plane

National Space Development Agency of Japan (NASDA) is conducting a feasibility study of applying CFRP to the cryogenic propellant tank of a reusable vehicle system. As a part of the feasibility study, we are now conducting tests of small filament winding (FW) tanks with 300mm diameter to study the possibility of the filament winding method for manufacturing. The first phase test of a FW tank was conducted in 1999. We analyzed the weak points of the first FW tank and conducted the second phase tank test under the collaboration works with Italian Aerospace Research Centre (CIRA). The design and manufacturing processes of the second phase test were improved, and the first tank was completed with half surface of the cylinder good (healthy surface) and another half no good with large wrinkles produced in the curing process. Leakage occurred in the first pressurization test at 0.3MPa at room temperature along a large wrinkle. After the leak points were repaired, the tank was used for waterproof testing at 1.0MPa and 1.5MPa and the strain data was obtained. Finally, we conducted a pressurization test at liquid nitrogen (LN2) temperature at 1.5MPa, 1.9MPa, and 2.0MPa. The healthy surface of the tank cylinder remained healthy even at 1.9MPa and there were no leaks occurred. The first leak occurred at 2.0MPa in the healthy area at LN2 temperature. This is a promising result for the next series for realizing a perfect tank.