During the Space Exploration Initiative, NASA's Planetary Missions and Materials Office at the Johnson Space Center developed a concept for a spacecraft that would land cargo on the moon in a horizontal landing configuration. Our initial analysis of this configuration leads to some significant questions about the placement of the main propulsion system's rocket engines. This image was produced for NASA by John Frassanito and Associates here in Houston. It shows a pressurized rover being deployed from the of common cargo lander. (Note the resemblance of the rover to the little moon bus we use in the Artemis Data Book tables of contents as a "You are here" pointer. I based that little pointer image on the design by the JSC Planetary Exploration Office -- another terrific spinoff from the U.S. space program!) We aren't concerned about the rover design in this essay, but rather the design of the lander that carries it. The second image, below, shows the lander with its engines firing as it descends toward the moon. Note that the two main engines are located at either end of the lander, quite a distance from the vehicle's center of gravity.
The second image shows the automated cargo lander again, but this time it is delivering an oxygen production plant to the moon rather than the manned roving vehicle. (You might correctly guess that the NASA studies from the Space Exploration Initiative lead to much of the thinking today about using lunar resources as a means to leverage private investment in space; but that is yet a different story.) We're worried about the placement of the engines. In this design, the lander has its descent engines placed at the far ends of a horizontal lander. It appears that, with this design, if a single engine should fail, then the entire mission is lost. With this design, having two engines doesn't buy us any additional redundancy, because a single engine failure still results in loss of the spacecraft. Splitting the thrust between two engines actually makes the vehicle's reliability worse because doubling the number of engines at least doubles the probability of engine failure. In practice, the chance of an engine failure increases more than double because now we have to deal with additional sets of redundant valves and control systems, and we have the added complexity of precisely balancing the thrust between the two engines. Even if the remaining engine has sufficient thrust to continue the landing, it wouldn't be able to support the rocket because of the long moment arm between the engine and the vehicle's center of gravity. The moment imparted by the remaining engines would cause the spacecraft to spin up so fast that there would be no way of recovering from the failure -- no ability to do a fast separation and climb back to lunar orbit, and no way to continue the mission with the remaining engine.
That's why I showed the engines clustered at the lander's center of gravity in the Horizontal Landing Option shown at the left. This design concept has a cluster of engines right in the middle, beneath the pressurized habiat. The pink things beneath the habitat are hydrogen tanks, and the blue things are oxygen tanks. The Ascent Stage is mounted side-saddle at one end of the platform. As they do in the current version of the baseline reference mission stack, the crew will ride down to the moon in the Ascent Stage, ready to abort the mission at any time. In an earlier version of the Horizontal Landing Option, I showed engines at the four corners of the lander stucture. We rejected this design early on because of the disastrous single-engine failure mode. In that version, I showed four engines; which of course makes the probability of engine failure even worse. The Guidance, Navigation, and Control Technical Committee didn't think there would be much chance of recovering from a single engine failure, even gimbaling the three remaining engines and a robust set of maneuvering rockets to augment the thrust. Nevertheless, if we can rely on those engines to keep running at the nominal performance, the corners of the spacecraft would be a very convenient placement for the descent stage engines. It allows a lot of flexibility in the cargo compartment, accommodating many different cargo configurations and providing a mechanism for convenient deployment of the cargo. For example, note how easy it would be to deploy the rover, as shown in the NASA's image at the top of this page. Given that introduction, here are the tough questions:
A complete answer to these questions, considering all the technical and financial factors, would be worthy of a PhD thesis, so, well ... enjoy the adventure! Images courtesy of NASA. See the JSC Exploration Server for the full details on this program. |
