A couple weeks back, I brought up some fundamental flaws I saw in the logic that the Exploration Systems Architecture Study group used to come up with their preferred lunar transportation architecture. In a discussion on NASASpaceFlight.com, I started realizing that even using their own methodology, a dry-launch propellant-transfer architecture might actually have a lower “Loss of Mission” probability than the preferred ESAS Architecture, in spite of requiring many more launches. And that is even ignoring almost all of the flaws I pointed out previously.
In the ESAS paper, they detailed three main sources of potential Loss of Mission:
- Rendezvous and Docking Failures
- Launch Vehicle Failures
- Launch Vehicle Availability Issues
On the surface, it would seem that an EELV based mission, that might require 6-12 flights per mission would obviously lose in this analysis. After all, more flights means more chances to lose a payload, more payloads that might fail to dock correctly, and more payloads whose schedules slip, right? Not necessarily.
One of the key things to remember about the ESAS architecture is that most of the Initial Mass in LEO (IMLEO) is propellants. A frequent commenter on this site said something elsewhere to the effect that “Most of what Ares V does is launch LOX”. Without the propellants, you could pretty much launch all of the ESAS hardware on two EELVs, one standard size, and one heavy, especially if the Heavy were an Atlas-V using the Phase I Wide Body Centaur upper stage mentioned in my last post. All of the rest of the flights are delivering propellants.
Upon a little thought, you realize that the only failures that can cause a loss of mission are failures on hardware flights. A lost propellant flight doesn’t cost you the mission, since you can easily fly a replacement. Only a failure involving the docking or launch of one of the hardware flights actually eliminates something unique that you needed for the mission.
So, comparing the ESAS architecture to a dry-launch architecture, we find that:
- Both architectures only have one mission-critical docking event that of the CEV to the LSAM/EDS stack. All other launches in the dry-launch architecture are not unique, and will not result in a loss of mission if the rendezvous and docking fail. At most they will result in a delay. So, the LOM numbers for rendezvous and docking failures are identical between the two architectures.
- Both architectures only have two mission-critical launches, the hardware launch (LSAM and EDS), and the crew launch. If a crew is lost or if the cargo gets put into a “fishing orbit” as I like to call it, the mission is over. ESAS claims that their all-new launchers will be safer and more reliable than EELVs, but only slightly so. And theoretical hardware-reliability numbers rarely reflect the true reliability of the systems. Witness Shuttle and Falcon I. But you could call this as slightly in favor of the current ESAS architecture. Their odds of losing a mission-critical launch are probably about 1% lower than for Dry-Launch.
- Where the current ESAS architecture loses the worst is ironically where they claimed a multi-launch architecture suffers the most–launcher availability. Basically, the current ESAS architecture has no way of topping-off the EDS/LSAM propellants before departure, so any delays above a certain amount end up costing the mission. Without a way of transfering propellant on-orbit, all of that expensive hardware ends up becoming next to useless if the CLV is late, or if the CLV suffers a non-fatal launch failure. The dry-launch architecture however can make up for delays. Any boiloff issues don’t cost the mission, they just delay it slightly and add a tiny amount to its marginal cost. The odds of losing a mission due to CLV delays and boiloff with the current architecture are non-zero, but the odds of losing a drylaunch architecture to that are much, much lower.
To be fair, one can definitely quibble about whether the slightly higher theoretical launcher reliability is more or less of a factor than the inability to tolerate launch delays. However, what one cannot quibble about is that we’re paying a very, very high cost for what is at best a very small, and entirely theoretical advantage.
An interesting sidenote from this though is that even if you insist on building Ares I and Ares V, developing on-orbit cryogenic propellant transfer still makes sense, as it can decrease your odds of launcher availability problems causing a loss of mission.
One can also claim that on-orbit cryogenic propellant transfer is so far off in the future that comparing it with the ESAS architecture is fantasy. However, Lockheed has a very valid point when they state that every time they relight a Centaur in orbit, they’re demonstrating on-orbit cryogenic propellant transfer. They’ve only done it 200 times more than Ares I or Ares V has ever flown. While there is some development and qualification yet to be done on the concept, it’s probably closer on both a monetary, and technical standpoint to reality than the J-2X or the 5-Segment SRB.
But the main takeaway I have for this is that the arguments for Ares I/Ares V are a whole lot less solid than ESAS tries to prove.