Just reading some of the comments from the Constellation used-car sale pitch going on in Huntsville today. One of the topics discussed was how Ares-V enables manned missions to Mars.  The Marshall guys put up a chart showing that depending on whether we go with NTRs or chemical propulsion, Ares-V could place the needed mass into orbit in only 7-12 launches.
To quote the former senior NASA official who was poo-pooing depots in the article I linked to earlier
Rocket malfunctions are not uncommon, and the more launches are needed for each moon mission, the more likely it is that something will go wrong, a former senior NASA official told New Scientist.
ESAS had a whole section slamming lunar architectures that used more than two launches as having too high of a probability of losing the mission. The ESAS study pointed out that the odds of not losing any of the launches is much lower than the odds of not losing any one given launch. The probability of successfully doing N launches without any failures is the probability of success for a single launch raised to the Nth power (ie 6 launches with a 98% reliable vehicle only has 88.5% chance of not losing one vehicle). They also went into rendezvous reliability, launcher availability, boiloff losses etc…
…but for some reason that doesn’t matter for Mars missions. Why are large numbers of launches, rendezvous, and long duration cryo storage considered perfectly acceptable for Mars, but completely unnacceptable for the Moon?
Jonathan Goff
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With depots the N-th power thing doesn’t matter. That only matters if losing one launch means losing everything, in other words only if you have a brittle architecture without depots.
Because Mars is w a a a y kewler!
And because, in this postmodern era, facts don’t exist, only narratives. Narratives compete for power, and their value is contingent solely on the power they wield.
Hence, lotsa launches is BAD for a lunar program because that works AGAINST Ares I/V. Lotsa launches is GOOD for a Mars program, because that works FOR Ares I/V.
Less snidely, the NASA yokel would likely say fewer launches is always better because the mission is more reliable that way. To which we all would render the proper 3 finger salute with multiple Bronx cheer flourishes.
“Why are large numbers of launches, rendezvous, and long duration cryo storage considered perfectly acceptable for Mars, but completely unnacceptable for the Moon?”
Because in our political system you may only get one shot at buying a big rocket for Mars, and if you have to use ‘Moon Direct’ to justify it, so be it.
Loss of the manned launch still matters. It is now just one of N launches.
The Mars argument is just someone running away from the implications of his own argument. Where x is the probability of a launch success, with N launches
p(f) = 1 – (x ** N)
Where say 6 times as many launches are involved
p(f) = 1 – (x ** 6N)
For x = 0.98
p(f, 7) = 1 – (0.98 **7) = 0.868
Where there are 6 * 7 = 42 launches
p(f, 42) = 1 – (0.98 **(6*7)) = 0.572
Seven to twelve Ares V launches for a Mars mission? Does that strike anyone else as being a bit high?
It’s been a while since I looked at manned missions to Mars but, if I recall correctly, Zubrin’s Mars Direct concept involved only two or three HLV’s that were shuttle derived and thus somewhat smaller than Ares V.
Given the cost of Ares V, NASA’s mission profile seems like it would be impossibly expensive.
Andrew,
It doesn’t really matter as long as a loss of a single launch doesn’t lose you the entire mission. And with depots it wouldn’t. Not all 7 launches need to go perfectly, just the crew launch. For the other launches all that matters is the reliability of a single launch, since that determines cost. If the launch vehicle is 95% reliable, that means an “insurance premium” of 5% on top of launch costs, which is perfectly acceptable. Your numbers merely show that the probability that you may need to redo a launch or two is significant.
Compare this to an Ares moon mission where you lose the entire mission (1 very expensive launch and 1 very expensive lander) through boil-off if you can’t launch your CLV on time.
Even the crew launch doesn’t matter, if you have a backup crew. Which NASA always does. In fact, while it’s politically incorrect to say it, the crew are the most affordably replaceable mission element — we have no astronaut shortage. A little rant on that subject was one part of my essay for The New Atlantis (probably for page count, rather than political incorrectness).
There is a false assumption that I see appearing over and over again in the arguments against multiple launch architectures. There is an unstated presumption that the reliability of a given launcher is a fixed quantity that is based on its past performance (or in the case of paper rockets – their imagined performance).
For example, in the past year, I have read several articles where the author cited the three failures of the Falcon I as reason for not trusting our astronauts on the new and unproven rockets from SpaceX. For the fourth flight, the Falcon I had 0% proven reliability, based on it’s track record. For flight five, it was only 25%. Of course, such numbers are nearly useless for the purposes of predicting future reliability. More importantly, if there ever _is_ a failure of a launcher, it is very likely that the cause will be determined and corrected – thus further improving the _actual_ reliability of the vehicle.
Perhaps I’m over-simplifying things a bit. I’ve been a little out of sorts since I watched the presentation on the supposed safety of the Ares I/V architecture over all other options considered. I think it was Jeff Greason who made the comment that “one should be careful when comparing the reliability of actual working launch vehicles against vehicles which have yet to fly.” I think he also said something about the dangers of making decisions about the launch architecture based on factors of two or three when the error bars are so large. I did notice that the chart he showed had large error bars on all of the alternative launch vehicles, but none on the Ares I/V baseline that he was comparing them to.
One other related thought on this multi-launch paranoia is that you wouldn’t need 7-12 launches (or however many) each time you wanted to launch a mission if we had vehicles that were actually reusable. Sure it may take more launches to build such a thing, and more fuel to send it on its way. But that is a one time price to pay. Thereafter, you would only need to refuel and re-provision the vehicle for each new mission. If NASA is really looking for something interesting to do with all of its bright engineers, how about looking into building real spaceships (ships that actually stay in space for multiple uses, rather than being discarded or crashed into the ocean after each mission).
An anecdote from the original t/Space proposal for COTS: we proposed a two-launch solution to meet the cargo transport (and human crew) requirements as we interpreted them. This approach reduced the development cost and improved flight rate, thus decreasing cost per launch, plus it allowed us to use a smaller launch aircraft. Cargo would go first, followed a day later by a crew capsule and service module that would then dock with the cargo module, and tug it towards ISS, where docking would be done manually. This technique eliminated the need for automated docking development.
We lost (narrowly) because one of the major arguments against us was we needed two launches to conduct one mission. We were never given a chance to defend the approach in oral followups so this was a “drive-by” attack that came out of the blue. A rather typical NASA technique, of course.
What is “funny” is that the NASA flagship mission that Constellation is working towards is going back to the moon: the architecture chosen is a two-launch architecture.
I, too, have noticed the discrepancy between the “official” comments about Lunar and Martian missions. I have characterized these discrepancies, these cognitive dissonances as hypocrisy also. It is good to see the word “hypocrisy” used in the above posts. Something is not right in Denmark, or whatever that Shakespearean quote is. For example,I have seen LOC ratios bandied about as predictors of future safety, and this is wrong. I have also seen an argument used to promote a strategy, such as in the OP above, and the same argument used to deride an alternate strategy.
The “narrative” argument posted above bears something in common, I think, with the slippery ideas of George Lackoff, where “truth” is specifically not seen as a necessary component of human culture.
It’s nice to see some familiar names above.
Meanwhile, it would be helpful to have more official honesty in discussing the pros and cons of HSF.