In several posts now, I have criticized the use of nuclear thermal rocket (NTR) engines. In the case of Earth departure stages, I have shown through mathematical analysis that they either do not have a performance improvement over chemical engines (for the overall system) or that the performance improvement is insufficient to merit the titanic expenditure that would be required to develop them. In the case of a hypothetical Earth-to-orbit application, I have shown that there is simply no hope whatsoever for their use.
My writings have elicited strong responses, both here and over on the NSF forum. People have asserted that I am simply wrong, or that I have gone into the analysis with a bias that has somehow compromised my results, or that I have ignored some obscure advanced version of an NTR that they assert can solve the problems that I have identified. Of these criticisms, it is the criticism that my bias compromises my results that troubles me the most. As an engineer, we want to believe that we are immune to opinions and biases but my experience has been that that is not the case at all. They creep into our judgement, and sometimes they sit flat on our face. I have seen for years now at NASA how even the most clever engineers can be seduced or bullied into accepting terrible vehicle designs, and in a perversion of the Stockholm Syndrome, eventually come to “love” the fatally-flawed design that they would have initially rejected.
I cannot be certain that my biases do not affect my work, but I am always striving to reduce them as much as possible. The most effective way I have found to do this is to get to “the numbers” as quickly as possible. We can talk all day long about how much better this technology is than that technology, but when we get to the numbers we can begin to improve the signal-to-noise ratio of our discussions more quickly than anything else I know.
Let me tell a story about a time when my opinion was changed significantly through solid engineering analysis. When I first got to NASA in 2000, I was part of a study called by a number of different names. Some called it “Decadal Planning”. I thought of it as “go to Mars and back in a year”. Within a few months of getting there, I was running a part of the study looking at propulsion technologies, and we were comparing a number of them. I met a lot of people in the field and began to develop a distaste for nuclear electric propulsion (NEP). The study concluded quietly and in my opinion, was a failure. Some of that was my own fault. But that’s some other post.
Near the end of 2002, my boss asked me to be a part of an MSFC response to a JSC study on a new and different NEP vehicle. As I recall they were interested in launch vehicle options. So in January 2003 several of us went to JSC to talk to them. I was very impressed by what they had done.
Essentially, they had asked “what are the big barriers towards sending people to Mars?” and then “how do we deal with them?” The biggest barrier they had identified was uncertainty about what happens to people after many years in microgravity. So they decided from the outset to design a vehicle that incorporates artificial gravity, thus cutting the “gordian knot” that had driven previous mission planners to “fast” trip times. My own experience on the one-year-round-trip Mars study had convinced me that it was utterly foolhardy to try to go to Mars and back too quickly. But I know the idea still rattles around in the Internet and is carried by the astrodynamically misinformed.
To accomplish the artificial gravity approach, the JSC study anticipated using the natural countermass of the nuclear reactor that would power the NEP vehicle to counterbalance the mass of the inflatable crew habitat. The boom that would be present anyway to keep the reactor away from the crew would now double as the separator needed for artificial gravity. Any time you can get a “two-for-one” value like that in space vehicle design, you want to take it.
The persistent problem in the design was the need to point the engines along an inertial direction while the vehicle was rotating. The JSC planners had rejected the idea of rotating slip or roll rings, for good reason and based on their experience with the ISS. To keep the engines body-mounted and yet pointing along an inertial direction required rotating the vehicle in inertial space, nearly 180 degrees during the transit to Mars. Once they got to Mars, the “spiral-in” proved very difficult, since now they would need to move the rotation axis of the vehicle through 360 degrees on every orbit. They came up with a compromise, called “minor-axis rotation” that mitigated some of the issues associated with this maneuver, but I don’t think they were terribly satisfied with it.
The design wasn’t complete or perfect, but it was a real step forward. And I was very impressed by their willingness to challenge their pre-conceived notions about Mars travel and examine a design completely different from what they had looked at before. And it changed my mind about the value of nuclear electric propulsion technology. In a future post, I’ll describe how a design innovation I came up with could solve the remaining architectural concerns with the AG-NEP vehicle and make it far more feasible.
My contrast, the response of some of the MSFC personnel I was travelling with was not so open minded, at least with regards to launch options. Before we went over to the meeting, we met at a Denny’s for breakfast. There were two senior personnel, a mid-level manager, and me. One of the senior folks (who’s no longer with MSFC) began to lay down the “MSFC position” for the meeting, which he said would be Shuttle-C. I gently began to demur, saying how the use of Shuttle-C would commit us to using the expensive Shuttle infrastructure for decades to come. He quietly but firmly cut me off and said:
“The answer…is Shuttle-C.”
I understood from the tone of his voice that this decision wasn’t technical, it was strategic. Shuttle-C was based on propulsion hardware developed and controlled at MSFC. If the Mars program went forward, if this vehicle was developed, if Shuttle-C was baselined for its launch, then MSFC would be supporting it for many years to come. I looked over at the other senior person and he was nodding his head in agreement. I looked at the mid-level manager and he wasn’t saying anything. He and I were like enlisted men getting our orders from the officers. And so Shuttle-C it was…the only option we were permitted to present to the JSC study team.
As it was, several weeks later the Columbia was destroyed on reentry. Everything changed at the agency and everyone forgot all about Mars studies. But I still remember what I learned and think that there are lessons to draw from it.
Latest posts by Kirk Sorensen (see all)
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- Sorensen TEDxYYC Thorium Talk - April 23, 2011