Chair Force Engineer

Friday, December 15, 2006

Centaur Stage

Lockheed Martin has publicly talked about a wider Centaur upper stage for its Atlas V rockets. The new Centaur would be scaled up to match the 5.4 meter diameter of the Contraves payload fairing on some Atlas V variants. The bigger Centaur could carry six times the propellant mass of the original Centaur on the Atlas V, and have as many as six RL-10 engines (much like the Saturn I first stage.)

Jon Goff has been somewhat bullish about the idea. The benefit is that payloads that might have needed one or two SRB's can use the bigger upper stage instead. This is significant, in my opinion, because SRB's are a leading cause of failure for rockets in this class. The obvious drawback is that development of the new stage will cost a significant amount of money. With the Atlas V currently not viable in a commercial sense, there appears to be little motivation for pursuing the idea further.

As an interim step, I propose that Lockheed Martin adapt the Centaur G, used on some Titan IV missions, for use on the Atlas V. While the 4.33 meter-wide Centaur G would not have the benefits offered by the 5.4-meter Centaur, it wouldn't cost a lot to develop, either. It wouldn't face any aerodynamic loads because it's probably small enough to fit inside the 5.4 meter fairing instead of supporting the fairing's weight (it would assume the same position that the current Centaur fills inside the fairing.)

Jon also suggests cutting costs by using the Centaur on both the Atlas V and Delta IV. The Centaur's structure is lighter than the Delta IV upper stage. This is because the Centaur uses a partially pressure-stabilized structure (the "balloon tank" concept) like the old Atlas did. In fact, Centaur was originally proposed by Convair as a natural extension of what they had developed on the Atlas program, except that hydrogen was used instead of kerosene (this was a radical idea for the time, as hydrogen was not in widespread use.)

I would assume that the Delta IV would require a wide-bodied Centaur to perform its standard missions. However, the current Single-Engine Centaur on top of a Delta IV first stage could be a suitable replacement for the Delta II (which will soon be phased out, even though there's really nothing in the US fleet that meets the same requirements.)

The Centaur series is one of the oldest families of upper stages that's still being produced. Even so, there's plenty of life left in the design. The balloon-tank concept and RL-10 series engines continue to serve as good solutions for a unique design challenge. Future versions of Centaur may even fill the role that NASA has carved out for its proprietary "Earth Departure Stage" design. That will be the subject of an upcoming post.