When all is said and done, NASA’s Constellation project to return to the Moon and go on to Mars will achieve remarkably little. Now the agency plans to undo one of the few decisions where Constellation got it right — the use of RS-68 engines on the Ares 5 heavy-lift vehicle.

As originally conceived, Constellation was a very good idea. Rather than go out on another expensive, new-technology limb that had no economic or practical reason to exist — another National Aero-Space Plane or X-33 — we would use the launch vehicles and technology we already have. We would create a “constellation” of relatively simple and low-cost vehicles to support a small outpost on the Moon. As an added benefit, the logistics requirements of a lunar base would provide real, practical reasons for the development of commercial transportation — a “lunar Commercial Orbital Transportation Services (COTS)” model.

Building the international space station has provided the inspiration, motivation and market for the current COTS contracts. These subsidize private development of cargo delivery to low Earth orbit. There is no reason a lunar base could not provide a similar market.

Unfortunately, NASA got sidetracked developing hardware applicable to Mars, and the Constellation project, as it was eventually executed, proved almost entirely a waste. No rockets or capsules were deployed to return to the Moon, and no landers or habitats were developed to live there. The hardware needed to go on to Mars remains firmly in science fiction stories and computer-aided design files.

Rather than develop any of these deep-space skills, Constellation spent billions of dollars and five years reinventing the medium-lift wheel, something many nations already knew how to do. The United States cannot reach a consensus on how to go forward and Congress won’t let NASA kill Constellation — which survives in the latest continuing resolution. Long after the project is effectively dead it continues to consume money.

In one key aspect, Constellation-as-executed got it right. If we are not going to use existing medium-lift rockets, at least we should use their engines. NASA planned to adapt the Delta 4’s RS-68 to an RS-68B for Constellation’s Ares 5 heavy-lift vehicle.

The agency’s latest plan to fulfill the congressional requirement to develop a heavy-lift launch vehicle appears to reverse that decision. It relies instead on space shuttle main engines (SSMEs) in a rocket otherwise very similar to the canceled Ares 5.

It is true that SSMEs are the most efficient rocket engines ever built. All other things being equal, any rocket that uses them is likely to have extraordinary performance. They are also hugely complex and expensive — they were not designed to be flown once and discarded — thus ensuring that any expendable rocket using them will be unaffordable.

The RS-68 is the largest high-energy liquid hydrogen/liquid oxygen engine in the world. It is not as efficient as the SSME. The basic RS-68 has a vacuum specific impulse of 409 seconds, compared with the SSME’s 452 seconds, and it operates at a lower chamber pressure. However, it is less expensive, with a far lower part count than the SSME. It is good enough for most likely applications.

The RS-68 has flown on numerous Delta 4s without a single failure. An improved RS-68A is nearing operational status.

Ares 5 would have used up to six RS-68Bs clustered on each first stage. Three RS-68s fly on every Delta 4 Heavy used by the American intelligence agencies, and one on each basic Delta 4 flown by the Air Force and civil agencies. If NASA had adapted the Delta 4 Heavy for the Orion capsule, still more RS-68s would have been needed.

This huge market for one engine for multiple customers would have resulted in economies of scale, production efficiencies, and learning curves that could have greatly reduced costs. It would have further increased the reliability and safety of what already has proved to be a very reliable engine. 

Lower launch costs could have made the Delta 4 more competitive in the commercial market, potentially increasing launch rates, and thus RS-68 production runs, still further — making both engine and rocket more competitive still.

Instead, NASA is pushing space shuttle main engines. This is hardly a surprise. NASA likes high technology and hardware it builds itself. Almost four decades after their development, SSMEs remain the most advanced rockets in the world, and they were developed by NASA, not the Air Force.

After the shuttle, if NASA continues using SSMEs, they will have exactly one customer on one rocket: two or three annual launches on the heavy-lift vehicle Congress has directed NASA to build. The Delta 4 and Delta 4 Heavy will not help pay for the SSME engineering sustainment team and the production line. After the current inventory of used shuttle engines runs out, they will be expensively produced in very small lots, with little opportunity to increase production efficiencies and to lower costs. Reliability of these overly complex engines then will depend on relentlessly sustained vigilance — like the space shuttle — without the benefit of high launch rates.

There is no conceivable commercial application for the SSME. Its continued use will lead United States space exploration down another long and expensive road to nowhere.

If we are going to build a heavy-lift rocket with no market, let us at least use engines that are used on other vehicles, engines that do have a market and a future — RS-68s.


Donald F. Robertson is a freelance space industry journalist based in San Francisco. For further examples of his work, see www.DonaldFRobertson.com.

Donald F. Robertson is a retired space industry journalist and technical writer based in San Francisco.