Here’s one other interesting paper, which was presented at the SPACE 2008 conference a few months ago. As I mentioned in a previous post, I was given a copy of a few of the ULA papers before the conference, but decided to wait until after the conference to write about it. The past few months have been busy enough that it took me being home with a stomach flu today to have the time to finally write up a brief summary.
For those of you who were there for the propellant depot panel that I chaired at Space Access this year, the paper covers in more detail many of the things that Frank Zegler presented.
After an introduction where the benefits of propellant depots for the planned Constellation architecture (such as allowing the architecture to actually, you know, work…), a concept for a first-generation propellant depot was given. This concept was designed around some of the work they’ve done on their ACES stage (aka the Wide Body Centaur that I’ve written about previously), combined with some recent work on deployable sunshields.
The paper hit on several of the key concepts that I’ve mentioned on this blog:
- The benefit of “settled” cryogenic fluid management (CFM) instead of “zero-G” CFM. To reiterate, if you can force the propellant to assume a preselected orientation, almost all CFM tasks go from being science projects to being straightforward adaptations of terrestrial CFM techniques. Basically you want to keep liquid from going out the vent, and gas from being ingested into the transfer lines. They propose a combination of the propulsive settling that they’ve demonstrated over almost 200 Centaur flights, combined with a rotational settling technique similar to what we’ve discussed on this blog in the past. This rotational approach, and the transition to and from the axial settling to rotational settling is set to be demonstrated after the DMSP launch this next year. They’ll have almost 11klb of unused propellants to play with after delivering the primary payload, and they plan to squeeze as many experiments as possible out of that excess propellant. There’s another approach that Frank Z. and I were working on for an SBIR proposal a year ago that could potentially also work if this one doesn’t turn out.
- Proper thermal design can allow for passive systems that minimize or eliminate boiloff. While you can add an active cooling system to compensate for a poor passive thermal design, it’s much better to do what you can first with a good passive design.
- Almost all of the technologies for propellant depots are already developed, many of them to high TRLs. Especially if you go with settled cryo handling instead of insisting on zero-G.
- If NASA opened up its lunar architecture to allow for the use of propellant depots, it would greatly expand the current demand for orbital launches. As the authors point out, even just topping-off the Earth Departure Stage’s LOX tanks would provide something like 10x the mass demand as COTS will.
- They also discussed the importance of having experimental facilities for flight testing and maturation of these technologies before they’re implemented on real systems. They mention their Centaur Test Bed concept for cryogenic experiments as secondary payloads on Atlas V, but they also link to an interesting paper by Dr. Chato of Glenn Research Center about the history of suborbital and orbital flight testing of CFM technologies. I think this is one of those areas of research for which a low-cost, unmanned suborbital vehicle like we’re developing at Masten could greatly aid the development and maturation of critical spacefairing technologies.
There were a few issues I had with their presented concept that are probably worth mentioning. First, they focus on only providing LOX. While this may still be useful for NASA missions, it’s not as useful for commercial missions. Since hydrogen boils off a lot faster than LOX, not having a way to top off your LH2 tank on orbit eliminates one of the big benefits of propellant depots. Even if you don’t go with LH2 for your fuel, having both oxidizer and fuel at the depot gives you far more flexibility than just the one fluid. Of course, this disagreement is mostly just a matter of taste on my part.
My other concern is more substantial. They only briefly mention this in the paper, but the sunshields they’ve been working with use aluminized plastics. Unfortunately, the LEO environment is somewhat nasty on plastics due to atomic oxygen. In order to minimize degradation of their sunshield, as well as minimizing damage from space debris, they selected a 1300km altitude for their analysis. While this makes the sunshield work better, that altitude is not a great place for a depot operationally. First off, it’s inside the edges of the inner Van Allen belt. Once you get much higher than about 500-600km, the radiation dosage goes way up. This makes it a lot trickier on the electronics, and I don’t know if you could keep the rendezvous, docking, and transfer periods short enough in such an environment to avoid radiation damage to the crew (the point of this depot after all was for providing propellant for crewed lunar missions). Ideally, a propellant depot should be a place where you can loiter for a while in case something comes up that delays the mission. Lastly, 1300km is high enough up that there’s a significant penalty for delivery to that altitude. Especially for future potential RLVs. Now of course, a tug could relax that constraint a bit (and as I mentioned in my previous post would make operations a lot better in general). But I think the reality is that in order to close this case operationally, they really need to find a way to make the sunshield survivable at lower altitudes. For non-LEO propellant depots (L1/L2, LLO, Mars Orbit, etc) this shouldn’t be a problem, and the idea can probably be used as-is. But the concept probably needs some rethinking if they can’t get it to work at a reasonable orbital altitude.
[Update: I was able to dig up a bit of additional information about the concept. Apparently the 1300km number was somewhat arbitrary. The concept can work at more reasonable altitudes (ie 400km would probably be fine), it's just a question of how long of a lifetime you want for the sunshield. With the petals concept as Frank explains it in the comments section, it sounds like down the road you might be able to replace the sunshield if it wears out, but depending on the lifetime, it may make more sense to just retire the module at that point and launch a new one. Basically, it sounds like a tradeoff between lower altitudes for easier access vs. more maintenance/replacement costs due to more wear. But this information more or less ends that key concern of mine with the concept.]
One last thought is that ULA is still pulling its punches on this technology. They talk about how it could help aid the existing Constellation architecture, but the reality is that once you have this technology, you could completely transform the Constellation architecture, or get rid of large chunks of it entirely. Once you have propellant depots you no longer nead super heavy lifters like Ares V. Depots allow you to store the propellants you need for long durations, so that the ESAS concerns about losing a mission if a given launch is delayed or failed are greatly reduced. Depots allow you to split propellant launches up over as many redundant launchers as you want. If you look carefully, you’ll notice that the ACES stages they mention at the end of the paper could carry quite a bit more propellant if you have a Depot to top them off than an un-topped EDS stage. And if you launch that stage dry, you can have a system that has better cryo thermal properties, much better performance overall, and it would be part of a system that was commercially useful for other markets. Once you go to a propellant depot architecture, you could launch all of the actual dry hardware from the ESAS architecture on two existing or near-term EELV Heavies, and then the rest of your launches you really don’t care about launcher reliability. Basically with a propellant depot architecture, you can keep the number of mission-critical rendezvous and docking opportunities to the same number as ESAS, while greatly increasing performance, reducing cost, and stimulating the private launch industry.
Like Space Tugs, propellant depots are an idea whose time has come.