tasks must be accomplished for the metallic and/or composite tanks to meet the Phase II go-ahead criteria:

1. At least one metallic (Al-Li) tank will be constructed and integrated with the required TPS, health monitoring, and attachment subsystems and will be under test. Current plans call for two such tanks to be manufactured and integrated for test. Appropriate coupon and other element testing (e.g. LOX compatibility, reusability) required to achieve this goal will be completed and documented. All applicable subscale testing will have been conducted to scaled (to full-scale RLV) pressures and loads.

2. At least one graphite composite tank will be constructed and integrated with the required TPS, health monitoring, and attachment subsystems and will be under test. Current plans call for two such tanks to be manufactured and integrated for test. Appropriate coupon and other subscale testing (e.g., LOX compatibility) to achieve this goal will be completed and documented.

3. The material selection for both fuel and oxidizer tank subsystems will be completed and documented. The selection must consider performance (e.g., weight, strength) producibility, inspectability, and operability characteristics.

4. A documented analysis will have been completed demonstrating that the selected materials and tank subsystems are scaleable to a full-scale RLV and will adequately be demonstrated by an X-33 vehicle. This analysis will contain the correlations between analytical predictions and experimental test results. These correlations will be at a level of confidence sufficient to ensure that analytical tools are valid for purposes of full-scale vehicle design. Estimated requirements for the RLV, which will be supported by this analysis are a minimum of 100 lifetime missions including depot maintenance not more than every 20 missions, volumetric weight targets (which will be updated for selected X-33 configuration) of 0.7 lb/ft³ or less for an oxidizer tank and 0.5 lb/ft³ or less for a liquid hydrogen tank, with leakage rates within the limits set for the space shuttle.

The NASA/industry programs and the findings and recommendations for Al-Li alloys and composites are discussed separately in the following sections.