This third post in a series on Constellation and the National Research Council's document Launching Science: Science Opportunities Provided by NASA's Constellation System takes us back where we started at the beginning of Part 1 of the series: Satellite Servicing. A recent NASA RFI titled Feasibility of using Constellation Architecture for Servicing Existing and Future Observatory-Class Scientific Spacecraft (PDF) uses the NRC document as one of the main sources for examples of the types of satellites that would be the objects of servicing. In Part 1, we already looked at the types of science missions under consideration by the NRC document. Now, let's look at the types of satellite servicing that's described there.
Without going into a great deal of detail repeating the NRC report, here's a summary of some of its points:
- Although some level of satellite servicing is possible without special design considerations on the satellite to be serviced, designing for servicing helps a great deal. This includes grapple points, foot and hand holds, easy access to components to be replaced, safe fuels, and sharp points.
- Missions like Orbital Express and ESA's Automated Transfer Vehicle (ATV) demonstrate that cost-effective robotic satellite servicing is practical for many tasks.
- Designing for servicing may increase costs, but there are benefits to counter these costs, such as relaxed requirements for long lifespans of components that can be replaced.
- NASA has gained a lot of experience with satellite servicing.
As far as Constellation is concerned, the document concentrates on the satellite servicing potential of Orion:
"The Orion spacecraft, although capable of traveling beyond low Earth orbit, has a number of limitations with respect to acting as a servicing mission. Orion has limited capability for change in velocity (delta-v) and would require additional propulsion to leave low Earth orbit. The spacecraft is not equipped with an airlock, which would therefore require that the entire cabin be depressurized or that an airlock be provided for the crew. In addition, Orion has limited extra mass and volume capacity for carrying equipment for a servicing mission, such as a robotic manipulator system, toolkits, and any equipment to be installed. Any plan to use the Orion for a servicing mission will have to address these limitations. Options include launching a second spacecraft to carry servicing equipment, designing spacecraft to include many of the necessary tools and even an airlock, and launching a dedicated servicing spacecraft to which Orion would rendezvous and dock before journeying to the spacecraft to be serviced."
The document proposes using Orion (or robotics) and a servicing node at the Earth-Moon L1 or L2 point to allow servicing of various satellites planned for the Sun-Earth L2 point:
"Several of the mission concepts evaluated in this report would operate at the libration points a significant distance from Earth ... Although these sites are attractive for operating heliophysics and astronomy observatories, they are less than ideal for servicing purposes owing to their distance and communications lag times and for other reasons. One possibility is to move the spacecraft closer to Earth for servicing purposes and then move it back to its operating location. Several proposals exist for establishing a servicing node or way station at a closer location that could be visited either by humans in an Orion spacecraft or by a robotic servicing spacecraft. Transferring the observatory from one Lagrangian point to another requires very little change in velocity and subsequently only small amounts of fuel."
The document goes into more details about the potential servicing station at Earth-Moon L1 or L2:
"Such a servicing station would have a servicing node, which would remain in orbit and would require some additional avionics and propulsion capabilities beyond those found in a simple airlock. In addition to servicing spacecraft out at Earth-Moon L1, it could also possibly be used to aid lunar surface exploration. This servicing node could enable two types of robotic servicing: a robot operated by astronauts and a robot operated autonomously from Earth."
This type of servicing node could have other purposes, too:
"Ultimately, a servicing node at the Earth-Moon L1 or L2 point would make it possible to construct large astronomical observatories that surpass even Ares V single-launch capabilities ... It could also be used to facilitate lunar exploration goals, and in the far term, an Earth-Moon L2 point servicing mission could provide a stepping stone between lunar missions and Mars missions. It could be used as a test site for issues such as duration in space, distance from Earth, communication delays, and supply issues."
The document makes it fairly clear that by itself, Constellation is not very useful for satellite servicing. Perhaps, with a limited budget, robotic satellite servicing with modest capabilities focused on the most likely servicing needs, and no use of Constellation hardware, is the best route.
However, with budgets permitting, Constellation combined with other capabilities (such as a Centaur and a servicing node with an airlock) could provide significant science-enabling capabilities, and thus allow Constellation to deliver a lot towards one of its 3 main goals, science benefits, well before the lunar return. The question then becomes "will the science benefits be worth the cost"?
Satellite servicing clearly shows promise if it can be done in a cost-effective way. However, there are a number of potential pitfalls in using Constellation for satellite servicing beyond the obvious budget consideration. One pitfall is that a Constellation-based satellite servicing mission could lead NASA to simply engage in more in-house and cost-plus contract work with little benefit to the commercial space industry or U.S. economic health in general.
For example, the NRC document depicts a NASA concept for linking Orion with a servicing node that looks much like an ISS Node. The big question then immediately becomes "will this Node be a commercial system that helps to enable non-NASA business (for example, a Bigelow module, or some commercial LEO or GEO satellite servicing venture), or will it simply be another NASA cost-plus contract?" In other words, will it be a force-multiplier for the space economy, or will it absorb even more funds that could otherwise have been used as force-multipliers?
Another diagram in the NRC document shows a NASA concept where Orion is launched by Ares I and then docks with the Centaur and Servicing Node. Well, that's to be expected, since after all NASA is looking into the "feasibility of using Constellation architecture for servicing existing and future observatory-class scientific spacecraft". But there's more ... how are the Centaur and Servicing Node launched? Of course in the diagram they're launched by an unmanned Ares 1 rocket!
One of the central goals of the Vision for Space Exploration is economic benefits to the nation, in the context of expanding the commercial space industry, rather than just the cost-plus contractors. This was later emphasized by the Aldridge Commission. Even now, one of the 4 main objectives of the Human Spaceflight Commission is "stimulating commercial space flight capability". Encouraging commercial space has been a consistent focus in all of these policy directions that NASA has been given.
If we're to consider developing the expensive Constellation system for ISS and lunar crew transportation, and then operating that system for decades, all with very little benefit to commercial space, and if the Constellation components are also to be used in a satellite servicing system that will entail additional costs, if we're to gain any economic benefits from the whole effort, it's vital that the additional components used to enhance Orion's capabilities (eg: unmanned launch, servicing node) be commercial. Adding more cost-plus components to the satellite servicing architecture will only get us more deeply in the trouble that's already caused the need for a Human Spaceflight Review Commission.
On the other hand, if we do use commercial components that are also used for non-NASA business to enhance Constellation's capabilities into the satellite servicing arena, then Constellation will be that much closer to achieving at least 2 of the goals of the VSE: science and economic benefits. Constellation will be fulfilling at least part of its purpose.
Of course we could flip the partnership in the other direction, too. A COTS satellite servicing effort, perhaps building on COTS A-D efforts, for commercial satellite servicing capabilities could be offered. This would use commercial launch and a commercial human transport spacecraft to link with an Ares 1 launched cost-plus servicing node. Again, both NASA in-house/cost-plus contractor and commercial space would benefit.
This brings us back to the recent NASA RFI for satellite servicing ideas. Although the RFI does mention robotic and commercial servicing capabilities, it's clearly oriented towards the Constellation system. It would be preferable to look broadly at all satellite servicing options.
As a note, the original RFI mentioned a pre-RFI workshop around June 27, 2009. However, the workship has been postponed until sometime after June 27th (date TBD).
I have one final observation. The focus of the satellite servicing strategy outlined in the NRC document, and thus stressed by the NASA RFI, is on large heliophysics and astrophysics missions at L2. Are those the best candidates for satellite servicing? It seems to me that constellations of similar satellites are also good candidates for servicing. With such satellites, you can design the satellite servicing capabilities into multiple satellites with a single design, unlike the big single-case science missions. Then a single satellite servicing capability can operate multiple times on numerous satellites. There are many such satellite constellations, including some where the value of individual satellites is quite high. Many of these are not NASA satellites, though. Can NASA enable commercial space capabilities to be able to service these non-NASA satellites, and at the same time encourage the designers and operators of these satellite constellations to move towards serviceability?