[Editor’s note: A good friend of mine from Santa Clara, Henry Cate, is starting up a Carnival of Space. I’m usually not a huge fan of blog carnivals, but I think this is a creative idea, and wanted to support him on this, so this is my first “Carnival” post.]
Of the top ten technologies that I discussed previously as being critical for a spacefaring society, one of the technologies that I’ve repeatedly stressed has been orbital propellant transfer and storage. And for good reason. Other than lower cost launch (which I think has been discussed to death already), these two technologies are probably the ones that can have the largest impact on space exploration and development. I’d just like to summarize some of the key benefits I see of using orbital propellant transfer and storage in a space transportation architecture.
Adaptability: Propellant transfer and storage technologies (especially in the form of propellant depots) allow a space transportation system to take advantage of improvements in launch vehicles over time. By separating the launcher from the interorbital transfer stages, landers, and other in-space hardware, it makes it a lot easier to take advantage of upgrades over time.
In a way it’s kind of like the computer I’m writing this post on. This computer started out as a machine I bought on eBay back before my mission (in ’99 I think). Over time as new chips and better hard-drives came out, I was able to incrementally upgrade things without having to fork out all the money for a brand-new machine. By now, the only hardware I have on this computer that was on the original machine is the smaller of the two hard disks. The modularity of a PC architecture has allowed me to inexpensively upgrade things as I had the time and money available, instead of forcing me to buy a whole new system. Now, not everyone does it that way, but the option is there if you want to.
Reusability:Propellant transfer and storage makes it much easier to move towards a more reusable transportation infrastructure. In fact, without the ability to transfer propellants on orbit, there are some segments of a lunar or Martian transportation infrastructure that really can’t be reused. With propellant transfer capabilities (eventually augmented by ISRU capabilities), there really aren’t any parts of the transportation architecture that need to be expendable.
The economics of reusing in-space hardware may actually be even more compelling than reusing orbital launch vehicles (and the case for reusing orbital launch vehicles is pretty darned compelling). Unlike orbital launch vehicles, reuse of on-orbit vehicles doesn’t involve adding much if any hardware that wouldn’t be needed already for just performing the basic mission. Design for reusability does tend to drive you in different directions from design for expendable vehicles (such as pushing you to multi-engine landers with engine-out capability instead of rolling the dice every time you land with a single-engine lander), but in many cases those changes can actually make things less expensive in the end.
But all of that is moot if you can’t refuel the vehicles except on planetary surfaces.
Capability: Orbital propellant transfer and storage can allow for much more capable missions than you could perform without them. Dallas Bienhoff (of Boeing) recently presented a paper at the recent STAIF 2007 conference discussing how much you could increase the lunar surface mass of the planned ESAS architecture if you used orbital propellant transfer and “dry-launch” techniques for the EDS and LSAM (Dry Launch is where you launch the transfer stage and lander empty, and top them up on orbit from a depot or from fuelers). I don’t have the exact numbers handy, but the increase was substantial. It may have been over double the cargo to the lunar surface.
More interestingly to me, these technologies can allow you to get much more capability even if you don’t develop new launch vehicles. Every component of the planner lunar stack is light enough to be launched dry on existing EELV equivalent vehicles. And if you then top them off in orbit, you can send a lot more in a given mission than could be done with a non-dry-launch architecture. You could probably send 6-8 person missions, or land entire Sundancer modules along with the 4-6 person crew. All without needing heavy lift launch vehicles.
Dependability: In a world of expendable launchers, where launcher reliability is still depressingly low, a propellant depot serves as a buffer or capacitor between a lunar or martian mission, and the launch vehicles that put the components up. A commercial propellant depot can buy from whoever can launch to it, and with the likely propellant demands for even modest lunar transportation architectures, it will be buying from lots of suppliers. If one launcher starts having problems, the show still goes on. Much like how many companies will put UPS systems between their computers and the main power grid, especially in areas where the power can be flakey or unreliable.
Incremental Developability: [Yes, I think I may have just created that word on the spot.] One of the main issues raised with propellant depots, is that they sound like big, very complex projects. When people hear propellant depot they often think of some ISS sized monstrosity and then extrapolate that only NASA could run something like that, and therefore it would cost as much, take as much time, and be as poorly run as ISS. The reality is that the first “propellant depot” probably isn’t going to be some sprawling 100% custom designed facility that has all of the features, bells and whistles. More likely you’ll see a gradual buildup of capability.
At first, you might see missions that don’t even use a depot–but transfer propellants directly from tanker to tankee without any special infrastructure. Some of that may be in the form of hitchiker satellites that tap the surplus propellants from their launcher so they don’t have to store propellants onboard during the flight (thus reducing the risk to the main, paying customer). Then, you might see someone moving into a first generation propellant depot. This will probably be nothing more than an upper stage possibly docked to a Sundancer module. It won’t have zero-boiloff capabilities, probably won’t have fancy sunshields or meteorite protection, it probably will only handle two propellants, and much of the propellant handling may involve manual connections and valves. Only once there starts to be serious money being made by depots will you start seeing them branching out, growing in size, adding bells and whistles, etc.
You’ll also likely see a lot of the technologies needed for these depots being developed not by big expensive NASA or DoD demonstration satellites like DART or Orbital Express, but by companies like Lockheed piggybacking experiments on the postflight portion of Atlas V launches, and other such, low-budget partially IR&D funded experiments.
Feasibility: One of the best things about propellant depots is that there really is a lot of prior art and experience that demonstrates that we should be able to make this a reality. Every time a Centaur upper stage performs an in-flight relight, it is settling propellants, transferring them through a series of valves and pumps, and then sending them into another system (in this case an engine). Starting with Gemini and Russian programs at the same time, we’ve demonstrated the ability to do orbital rendezvous and docking. The Russians have been doing autonomous rendezvous and docking for decades, and now that we finally got around to it with Orbital Express, we’re doing it too. The Russians for decades, the Shuttle, many other programs, and now Orbital Express have demonstrated the ability to make fluid couplings between spacecraft, both with and without manned intervention.
There are subtleties and tricky parts to tying everything together in the case of cryogenic propellants, but almost all of the toughest techniques and technologies needed for transferring propellants on orbit have been demonstrated already. Based on the plethora of past experience, one can have high confidence that this orbital capability can be refined and brought into practice in the near term. There is a lot of detail work to be done, and it’ll probably take a lot of hands-on experience and several iterations before we start converging on the best ways of doing things, but the initial capability is relatively low-risk, and near-term.
Anyhow, that’s a basic introduction to some of the benefits I see from orbital propellant transfer and storage. I’ve got lots of other articles on this blog detailing some of the technical challenges and some ideas for how to handle them. I’d strongly suggest doing some searches if you have the time and are interested.