One of the common mistakes I’ve noticed in the engineering is to overoptimize a subcomponent of a system at the expense of the system as a whole. I saw this a lot back in my manufacturing engineering days. You’d get someone working on a specific machine or process, who doesn’t have a good grasp of the overall problem, and he’ll tune his process to some metric (throughput for instance) that he thinks is important, and it will end up being completely out of sync with the rest of the production system. The end result is that even though his one process may be very “efficient” by some metric, the overall production system has a large increase in required inventory, produces more waste, has more issue with reacting to demand, etc, etc. Basically, one of the truisms in engineering is that overoptimization of a part often makes the system suboptimal.
A recent thread at NASASpaceflight.com brought home an excellent example of that. In this thread, Ross Tierney (of DIRECT fame) brought up the following graphic from the Exploration Systems Architecture Study:
Basically, this chart from Chapter-8 shows the main failures that could lead to killing a crew during a lunar mission, and their relative probabilities. The first thing that jumped out at me when looking at this chart was that fatal launch accidents are predicted to be only a tiny contributor to loss of crew in a lunar mission. By comparison, the Trans Earth Injection burn is predicted to be over 5x more likely to cost the loss of a crew. Another thing that stuck out is that if Ares-I really only has a 1/2106 (or whatever the 4-significant-figures odds were predicted to be) of killing a crew, and that is only 3% of the overall odds of losing a crew on a lunar mission, that means that the odds of losing a crew on a lunar mission are about 1-in-60 chance of losing a crew (this number is backed-up by the way in I think it was Figure 8-2 from the ESAS report). Over a decade of flights, that gives a 25% chance of losing a crew.
Think about that.
But that brings me to the two points I wanted to make with this thread.
First off, if the relative proportions here are even close to right, launcher reliability is really not super important to overall crew safety on a lunar mission. It’s kind of counterintuitive, but increasing the odds of a fatal launch accident by a factor of 2 only increases the odds of a fatal lunar mission from 1.6% to 1.7%, which is probably round-off error for a study like this. Most of the EELV options, DIRECT, and even a 2-launch Ares V architecture all supposedly fall within or near this category (and most of them probably do better if you actually work with the EELV manufacturers to figure out how to close the blackout zones). So, the vaunted Ares-I reliability, if it turns out to be anywhere near as good as claimed, only changes the odds of a lunar mission resulting in a dead crew from about 1-in-58 to about 1-in-63. With those numbers in mind, does spending an extra $10B+ on the launcher development, and an extra ~$1B a year from here on out really justify that modest of a safety increase? Once again, for its primary mission (launching crews for interplanetary missions), Ares-I only makes the overall system 6% safer, not 100% safer. Is that extra 6% really worth $10B? Or could that $10B be better spent elsewhere? Could it go to finding better ways of reducing the odds of a TEI failure or of a Lunar Ascent rendezvous failure?
The other major point I wanted to bring up has to do with the Zero Baseline Vehicle work that NASA and LM are doing for Orion. Basically due to Ares-I not really having any room for growth, they’re having to trim back backup systems and Orion capabilities. While some of those capabilities probably don’t affect the LOC numbers much at all, others of them do. Those LOC numbers shown above were based on two-fault-tolerant RCS and controls. But for the ZBV version of Orion, these are being pared back to single-fault-tolerant. Now, some of those systems may be added back over the next couple of weeks, but if any of them get left out, that’s going to affect the LOC numbers for the three largest LOC events: earth reentry, the TEI burn, and lunar ascent docking. Basically, if Orion loses during any of those phases, it increases the odds of losing the whole mission. Since combined those three failure modes are considered nearly 20x more likely than a launch accident, it only takes a tiny increase in the probability of any of those failures occurring to completely swamp any supposed gains from going with Ares-I as the launch vehicle.
This may end up being a classic case of premature overoptimization leading to a less safe system overall. Going with a slightly less reliable launcher like DIRECT, EELV-Heavy, or Atlas V Phase 2, would likely save money, save time, and make the overall architecture more capable and safer. So, why are we spending so much time and money giving our astronauts (and the American people as a whole) the “Shaft”?
Latest posts by Jonathan Goff (see all)
- SBIR Proposaling Advice - March 8, 2019
- FISO Telecon Lecture on LEO Propellant Depots for Interplanetary Smallsat Launch - November 28, 2018
- AAS Paper Review: RAAN Agnostic 3-Burn Departure Methodology for Deep Space Missions from LEO Depots (Part 2 of 2) - September 17, 2018