[Ed: I’m pretty sure I’ve used this argument before, but didn’t see it on the blog, so I figured I’d put it down in writing even if it ends up being repetitive.]
One of the most common criticisms I hear of propellant depots is that we can’t “put unproven technology on the critical path to our return to the Moon.” The idea being that doing something like that would be too risky, and should only be done after we have a basic approach that can do the job. The ironic thing is that had this mentality prevailed during Apollo, we probably wouldn’t be talking about “returning” to the moon, because there’s a good chance that we wouldn’t have made it there in the first place.
In the early days of Apollo, the mission mode had to be selected. The choice was between the massive “direct ascent” method, which while more technically conservative would’ve required a gargantuan (even compared to Saturn V) launcher, and the two approaches that traded riskier new maneuvers (orbital rendezvous) for much smaller launchers.
This debate came to a head in 1962, resulting in picking what was probably the “riskiest” of the three approaches, Lunar Orbit Rendezvous. It’s worth thinking about the context of when this decision was made. This was only five years after the first artificial satellite, Sputnik, had been put in orbit. It was only barely a year after the first US manned spaceflight, Mercury 6. The decision to bet the nation’s moon program on a risky architecture that required orbital rendezvous and docking–in lunar orbit no less–was made 4 years before anyone had actually demonstrated orbital rendezvous and docking. In fact, the spacecraft used for that demo, Gemini, had barely started development the year earlier, and wouldn’t have it’s first manned flight for another two years after the decision was made.
Had the Apollo team decided to take the “low technological risk” approach, and stuck with Direct Ascent and the enormous Nova vehicle, there’s a real chance that the program would’ve been canceled before they had ever even tried to make their first lunar attempt. Given enough time and money, Nova probably could’ve landed people on the Moon–it wasn’t technically impossible. But in the end, there probably wasn’t anywhere near enough time or money to make such a grandiose scheme work. Not only that, but even if it had been forced to work, it would’ve been even less sustainable than the chosen approach.
Now, it isn’t always a good choice to take risky bets by putting unproven technologies on your critical path. Doing so can sometimes be a mistake. But it’s important to realize that in a program like this, risk can’t entirely be avoided. At best risk can be managed, but if you delude yourself into thinking you can avoid technical risk, it often crops up in other forms instead.
I think that in many ways, the decision to make depots a larger part of the NASA lunar architecture going forward is actually less risky than the choice of Lunar Orbit Rendezvous back in 1962. The storage, handling, and transfer of cryogenic propellants on orbit is a lot more mature than the technology for orbital rendezvous and docking was at the point the mission mode decision had to be made in ’62. We have literally decades of launching and flying cryogenic stages in orbit. Hundreds of flights between Centaur, Saturn SIVB, and now the Delta-IV upper stage have demonstrated settled cryogenic handling, pressure control, mass gauging, and even fluid transfer. Projects such as Orbital Express, XSS-11, and others have increased our ability to do tugs, autonomous rendezvous and docking, and have even demonstrated transferring fluids from one spacecraft to another. Most of the pieces needed to make a small, first-generation propellant depot (one big enough to support manned lunar exploration missions) are either proven technologies, or at least well on their way to technical maturation.
I think that when looked at in the context of the decision to go with LOR, the decision whether or not to baseline propellant depots in NASA’s space transportation architecture is even more of a no-brainer. It may not be free of risk, but biting the bullet and aggressively pursuing the development of propellant depots as a vital part of NASA’s continuing human spaceflight activities is just the right thing to do.