Full 127 page DAC-2 presentation on L2.









The study, called the Design Analysis Cycle Review (DAC-2), reveals that the current CEV is overweight- as per DAC-1 study - by about 5,200 lbs. The changes being made by NASA address this problem by including a significant shrink of the SM, decreasing its mass by almost 3,000 lbs. The SM's length is to be reduced by about half, such that it only accommodates the volume of the spacecraft propellant tanks.

The study also confirms that NASA plans to use pressure-fed NTO/MMH hypergolics for the SM engine, with a helium tank for pressurization. The SM RCS (Reaction Control System) will also be pressure-fed NTO/MMH thrusters. The SM radiators will also be significantly shrunk from the original version.

The documents also reveal that NASA believes the SM RCS would be able to act as a backup for a TEI (Trans-earth injection) burn in case the main SM engine fails.

A number of materials were examined for the SM propellant tanks. However, composites were determined to have no weight advantage, which led to spherical titanium tanks were chosen. The spacecraft will get power from advanced triple-junction solar cells, with single axis gimbal tracking of the Sun. Like Apollo, the CEV will run on 28 volts of DC.

Changes in the CM (Crew Module), including a switch from an aluminium structure to an Al-Li alloy - and reduction in some equipment and potable water margins. This will reduce the CEV mass by another 2,000 lbs.

A redesign of the escape tower accounts for another 1,200 lbs of saved weight, added to the spacecraft adapter section that connects the CEV and CLV being redesigned to save almost another 500 lbs. In total, about 6,400 lbs in reduced weight in the redesigned CEV has been achieved in DAC-2.

New details about the design of the CEV have also been discussed for the first time. The CEV CM will support a crew of 4 astronauts for slightly longer than two weeks and can remained docked to the ISS (International Space Station) for about a month longer than the original six-month goal.

The CM remains at a 5 meter diameter - and will use GOX/ethanol for its RCS system. The CEV will use an ablative, non-reusable heat shield on the blunt end, which will be attached to the CM at eight points, to be jettisoned before landing. The remainder of the capsule is covered in thermal insulators.

There is some question at this time as to whether the CM can transport six astronauts to the ISS; cargo transport values also need to be reassessed in light of the 5-m resizing of the CEV.

These changes reflect a significant shift in CEV sizing and capabilities, but are necessary due to the baseline weight requirements with the vehicle. NASA is conducting ongoing studies to test the effects of these changes and to ensure that the CEV that results is light enough to fly.

Evaluations to the baseline is an on-going process, with the aim to create a level of margin ahead of the final baseline, before being used as the template by the winning contractor.

Further articles will follow, looking at the CLV (Crew Launch Vehicle) baselines. New CLV image with re-designed CEV courtesy of Richard Par



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