2009 was a busy year for me. Around the same time that we were slogging away at getting Xombie and Xoie prepped for the Lunar Lander Challenge at Masten, and around the time that the Augustine Committee was trying to prepare yet another study on NASA’s future that Congress would immediately ignore, I was also working on a propellant depot paper with some friends from ULA and Boeing to present at the AIAA SPACE 2009 conference. This was back in the relatively good old days when a ULA engineer could actually coauthor a paper with the word “depot” in the title without getting in trouble with Boeing or Lockheed execs.*
The paper was focused on near-term options for propellant depots that could be fielded for relatively low cost, and one of the important ideas we discussed was how much LOX/LH2 depot capacity could you launch on a single EELV. We wanted to know if there were any ideas that could enable a “dual-fluid” (ie LOX and LH2) depot with sufficient capacity to be interesting for manned missions beyond LEO.
One of the creative ideas that one of the ULA folks suggested was simultaneously bleedingly obvious, yet profound enough that I had never heard anyone suggest it before: build a tank whose walls are integral with the launcher’s payload fairing. Typically, when you launch a payload inside a fairing, you end up wanting to give it some healthy clearance between the payload and the walls of the fairing–because launch vibrations can cause the cantilevered payload to vibrate back and forth and if you don’t leave enough clearance, you could hit the inside of the fairing with your typically fragile payload. So in many cases, payloads inside a 5m diameter EELV fairing might only be 4.5m in diameter (about the same diameter as the existing ISS modules). But if you make the wall of your pressure vessel double as part of the barrel section of the payload fairing, that problem goes away, and you can now fit a much bigger volume onto the same vehicle–just going from 4.5m to 5m diameter increases cross sectional area by 25%! By going with an integral payload fairing design for the Atlas-launched dual-fluid depot, we could fit a LH2 tank of a bit over 200m^3, enabling (with a few tweaks) a dual fluid depot with 75mT propellant capacity.
In case you’re having a hard time visualizing it, here’s Figure 9b from that paper:
The cool thing is that the same idea can be interpolated for any pressure vessel, probably including habitat modules.
The conventional wisdom says that the only way to put up spacious pressurized volumes is to use inflatable habitats like what Bigelow is doing, or to launch on an HLV that has an oversized fairing. But in an existing EELV fairing, you can fit a pressure vessel with somewhere around 200-220m^3 of pressurized volume, and since the fairing diameter is similar to the tank diameter on the Delta-IVH upper stage LH2 tank, there’s existing fabrication tooling for making lightweight cylindrical tanks in those dimensions. Some additional ideas/benefits include:
- If you used some of the Launch-Vehicle MLI and MMOD-IMLI concepts that Quest Thermal has been developing with Ball Aerospace (previously discussed on Selenian Boondocks here), you can provide lightweight thermal insulation and excellent MMOD protection that can be launched on the external skin of the pressure vessel without requiring any on-orbit deployment, and without changing the outer-mold-line of the payload fairing.
- Since it’s not an inflatable structure, you can have all of your structural attachments, cable harnesses, and cooling systems pre-installed on launch, and you also don’t have to have a central core structure or inflation systems.
- The outer structure can be made as solid as you want it, making it possible to have interfaces allowing you to plug-in external structures such as solar arrays, radiators, robotic manipulators, and communications systems anywhere on the outside surface–unlike inflatable modules that have to fit most of their hardware at the two endcaps.
- It might even be possible to integrate flexible solar cells into the LV-MLI if the external sheet was both robust and transparent.
- Also, if you ever lose pressure, the structure retains its shape.
Admittedly it’s not quite as big as you can do with a Bigelow module (something like 2/3 the volume), but it should be cheap, low technical risk, and easily doable, especially if having a large amount of pressurized volume was a key concern. It’s also not entirely a new idea, as it’s more or less how Skylab was done, just in this case using existing, commercially available launch vehicles. Now I’m not saying that this is the only way things ought to be done–there are probably many situations where inflatable structures would be as or more preferable. But if the goal is to create large open volumes in space, affordably, and soon, it seems like the kind of idea makes a lot of sense. And with how bleedingly obvious it is, I’m somewhat baffled it isn’t discussed more. Am I missing some hidden flaw in this concept?
* I wish I was kidding