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Timeline Problems with Project Constellation
Initially, Project Constellation was supposed to be a straightforward, realistic approach to getting back into space after the Shuttle. Relying on lessons learned from existing technology such as the Space Shuttle’s Main Engines (SSMEs) and Solid Rocket Boosters (SRBs) was a way to reduce the risk, cost, and development time.
However, much like a number of NASA programs before them, the Ares boosters and Orion capsule have acquired a number of technologies that are less than mature and made changes to existing hardware that are far from cosmetic. The SSME's were dropped and the SRB's extended, both of which added complexity, cost, and time to the design. Complicating this fact is the tight timeline, one that requires concurrent development of systems that rely on each other. For instance, a GAO report on Constellation points out that the design review for the first stage of the Ares I booster occurs only 5 months before the review for the entire vehicle, allowing insufficient time for full testing of the development rocket motors. Changes to those motors later could have a very adverse effect on the schedule.
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Taking the DIRECT Approach
Enter DIRECT as an answer to some of these criticisms. Based on studies that go back to the beginnings of the Shuttle program, worked out by NASA employees on their own time, and based on real-world numbers, the DIRECT architecture is so called for it’s much stronger linkage to the Shuttle program heritage. At its core are the four-segment SRBs and the External Tank from the Shuttle and the RS-68 rocket motors from the Delta IV. Because development would be more of an alteration effort than a design effort, the timetable could be moved up or the stress reduced on critical components of Constellation (like the J-2X rocket motor). The “as-is" approach also increases confidence on both safety and cost issues, as NASA has extensive documentation on existing hardware.
The report for the current version, 2.0.2, discusses lower life-cycle costs as well. The program would produce two boosters, the Jupiter-120 and Jupiter-232; these boosters are identical but for the inclusion of an upper stage on the -232. This upper stage would enable the insertion of crews and/or payloads into lunar orbit. The large degree of commonality between boosters stands in contrast to the Ares I and V, which are essentially different rockets, and is one of the main advantages of DIRECT. (For more detailed information on DIRECT, go here. To see more images of the Jupiter boosters, go here.)
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Possible Issues with the DIRECT 2.0 Proposal
So where’s the catch? NASA argues the numbers, saying that the performance figures used are unrealistic or inaccurate. On the project website, the DIRECT team addresses the critique of the first version of DIRECT; issues were raised concerning the development of an uprated version of the RS-68 rocket motor. These issues were addressed by redesigning to assume a standard RS-68. A NASA white paper addressing DIRECT 2.0 disputes their figures, claiming that the weight of the system is higher and the payload capacity smaller than the stated values. One cited difference is in propulsion system mass- from the tables in the NASA document, it appears that the mass for the upper stage is significantly less in the DIRECT design (version 2.0). The DIRECT website states that the team "believes NASA's analysis to be deeply flawed, and is working on a rebuttal".
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Project Constellation vs. DIRECT 2.0
At this point, the numbers for both DIRECT and Project Constellation are probably optimistic- it’s hard to know the actual performance values for an aerospace program until after the dust settles and you’ve actually built a vehicle. However, just looking at the approaches, DIRECT 2.0 certainly appears to be the less risky and more straightforward of the two. It’s also hard to see where it hurts to have our development eggs in more than one basket, especially as we’ll likely be launching the new rockets well into mid-century… Further, given the uncertain economic environment, DIRECT's budgetary requirements are likely to be far more feasible than Constellation's.
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Side-by-side booster comparison courtesy of Antonio Maia