Chair Force Engineer

Tuesday, May 23, 2006

George Washington's Hatchet

Historians have a joke about a museum claiming to display the hatchet that young George Washington used to chop down his father's cherry tree. The only problem is that the handle rotted and had to be replaced, then the head rusted and had to be replaced.

The joke cuts to the heart of the question, how much can you replace on something and still call it original? NASA is having a hard time with this stumper. After promising "shuttle derived" rockets for returning to the moon, the CLV and CaLV are looking more and more like the old Saturn series. The only shuttle-derived component is the 5-segment SRB (which still uses a different fuel grain and nozzle than the original SRB.)

With aerospace vehicles, designs are often optimized for a certain performance. When off-the-shelf parts and designs are reused, they often require significant re-engineering. Some programs will try to reuse aircraft avionics inside spacecraft, but this often requires significant re-engineering to make the parts radiation-tolerant. Similarly, the A-7 Corsair II light bomber started as a redesign of the F-8 Crusader, a supersonic fighter. By the time the subsonic A-7 took to the skies, it had almost no commonality with the F-8.

The CaLV is looking more and more like a resurrected Saturn V with SRB's, and CLV now uses an Apollo-derived engine on stage 2. We can't call it "shuttle derived" anymore, but I never had much hope that reusing shuttle hardware would lead to appreciable cost-savings. Inevitably, it would require re-engineering to meet the needs of Project Constellation. If it wasn't the CaLV being enlarged to 10 meters, it would have been a significant re-engineering of the SSME into the "expendible SSME," or another significant change to the SSME to make the "air start" SSME for the CLV.

On the plus side, NASA's recent decisions actually reduce the amount of money that would have to be spent on engine development. RS-68 would be minimally modified, while J-2X requires a new thrust chamber and nozzle (but reuses the old J-2S turbopumps.) By contrast, the air-start and expendible SSME's would have both been significant new developments, on top of the J-2X work that is necessary for the CaLV's second stage.

Similarly, the tankage issue is not a big change over what was in last fall's ESAS study. A stretched Shuttle ET with thrust structure on the aft end would be no trivial modification from the old ET. The 10m tankage isn't much worse off. In fact, it will probably reuse the external plumbing found on the Shuttle ET. The tank will probably be built with modified jigs from the Shuttle ET, unless there are Saturn V jigs and other pieces of tooling still stored at Michoud.

For people who were sorely disappointed by the lack of commonality with the shuttle, I think that they were sailing on a sea of false hopes. The new rockets are significantly different from the shuttle system and will inevitably evolve away from their shuttle roots.

The new CaLV is like South Park's "Manbearpig." It's part Delta IV (engines,) part Saturn V (tankage and upper stage turbopumps,) and part shuttle (SRB's.) Space cadets shouldn't be disappointed; it will be more capable and cheaper than the original CaLV design.