H. W. Babel, The Boeing Company, Huntington Beach, CA
Described is the approach and results used to define expected design allowables for cryotanks for 2099 Al-Li plate in the T6 and T8 temper. These design values were used to develop cryotank designs from which weight savings were established, 20% for upper-stage tanks, a weight savings that was very attractive to the program. As program judgments are made based on the projected weight saving, the expected allowables should not change when the allowables are established. As the designs were being developed, work continued to develop additional information leading to the establishment of allowables. Lower than targeted strengths were measured on one lot of plate; the approach used to resolve this question is discussed and the results obtained.
The properties that control the design were given the highest priority. Several of the ASTM standards did not provide the precision and reproducibility required, particularly elongation and modulus determination at cryogenic temperatures. The procedure selected to provide the required measurements are described.
Summary: The continued design maturation of 2099 Al-Li plate is discussed for T6 domes and T8 cylinders used for flight cryogenic tanks. Flight cryogenic tanks were designed to establish credible weight savings compared to 2219 tanks. Many detailed issues arose during the design process and resolved that will be discussed. To support this design effort, preliminary design values were established that were not overly conservative, but were not expected to change appreciably when allowables were established. The approach is described used to establish preliminary allowables with limited resources. Projected or estimated allowables that directly influenced the sizing of the components and measure these properties were measured with high precision as a function of temperature, an approach intended to ensure the credibility of the weight savings. The preliminary design for upper stage cryogenic tanks resulted in a weight savings of 20% compared to 2219 aluminum for upper-stage tanks. Such savings are significant and very attractive to the program.
As the designs were being developed, work continued to provide additional data leading to the establishment of allowables. Lower than targeted strengths were measured on one plate for the T6 temper; the approach used to resolve this issue is discussed and the results obtained. Several of the ASTM standards did not provide the precision and reproducibility required, particularly elongation and modulus at cryogenic temperatures. The procedure selected to provide the required measurements are described.
An assessment was made whether the alloy was sufficiently mature to make a program commitment for its implementation. For one program it was a “maybe yes, but more information is required first,” and another program “not ready.” The additional information required to achieve design and production maturity is discussed.
