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Ask OCE — February 8, 2006 — Vol. 1, Issue 3


“It’s the wrong thing to be doing,” I told the Director of Engineering, trying to head off a last-minute change in our X-ray Timing Explorer (XTE) project.

The spacecraft was nearly integrated and had passed some of its early mechanical and electrical testing. One of its instruments, the Proportional Counter Array (PCA), had a gas leak in one of the five modules that made up the array. The science division wanted a gas replenishment system added to assure the PCA would last for the entire mission.

Adding a gas replenishment system would mean: interrupting integration and testing; developing and integrating a new subsystem; modifying all the PCA modules; and implementing a more complex performance and environmental test process. It made a simple design more complex and added little value to the mission at a major cost in time and dollars. We couldn’t afford the additional budget and schedule risks.

I summarized my arguments to the head of the Directorate responsible for the development. His response was music to my ears. “Jim, I won’t stand in your way, but you’ll have to convince the scientists and engineers.”


I had entered the fray because the XTE Project Manager had asked me to get involved. I was the Explorers Program Manager, but I worked with my project managers in a way that encouraged them to consider me their deputy. The project manager and the spacecraft manager wanted to stay focused on the mission development while I fought the political and technical battles necessary to prevent disrupting progress.

As part of the plan to produce XTE for a fixed price, we worked closely with the science team to document the mission requirements. We struggled together to make the requirements specific, clear, and to the greatest extent possible, quantitative. As a result, the XTE project plan identified performance requirements on all the major mission elements. Two of the requirements bore directly on this situation: 1) the mission had to last two years, and 2) four of the five PCA modules had to be operative. Of course, the scientists wanted XTE to last much longer than two years and to have all five PCA modules operating as long as possible.

We didn’t negotiate the requirements in isolation at the beginning of the project; they were well thought out and realistic because they were established after we had taken implementation approaches into consideration during initial formulation. First, the scientists set out broad goals. Eventually, we gained an understanding of the architecture, implementation, and programmatic issues, and we were able to sign realistic requirements.

Now comes the real work

I knew that the scientists and instrument engineers thought they needed the system in order to assure a longer life for the PCA. There was real disagreement on the benefits of the change and the impacts. Four of the PCA modules were sealed well and leak tests had confirmed they would last beyond two years. Only one module had a problem. This was within the parameters of the project’s basic requirements. I knew that I was going to rely on those requirements to win this argument.

Sealing the deal

I met with the instrument team and explained the technical and programmatic situation. All the key scientists and managers had signed the XTE project plan. “These are your requirements, guys,” I said. “You signed up for them, and you agreed to them.”

My unspoken implication was clear: If you didn’t mean this, why did you sign up for it?

They weren’t happy, but they agreed not to pursue the gas replenishment system. We were meeting our mission requirements. The engineers then refocused on the question of the leak. The instrument development team redoubled their efforts to seal the fifth PCA module. They identified a tolerance build-up and fixed it in time to get the module delivered on schedule.

The XTE mission was ready on time and well within budget. It is now on orbit as the Rossi X-ray Timing Explorer (RXTE) mission, and it is still scientifically productive after eleven years with no degradation of the PCA performance. The time spent working to define the right requirements has reaped a far greater return on investment than we dared to imagine.


  • Clear, documented requirements help control the scope of a project. Have all key parties sign off on the requirements to assure mutual understanding and commitment.
  • Get project requirements right as soon as possible, but not prematurely. To assure they are realistic, requirements should not be finalized until after implementation approaches have been considered.

James Barrowman worked for 22 years as a program and project manager at Goddard Space Flight Center.


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