An excerpt from my project that I just wrote:
I’ll conclude with a discussion on mission risk and reliability. SLS supporters make an argument like this:
The greater the number of flights it takes, the more the probability of successfully delivering the elements needed for a Mars mission is reduced, because that probability is a multiplication of the individual probabilities of success of each rocket flight. For instance, if it takes thirty flights of a vehicle with 98% reliability, the probability of mission success (POM) will be 0.98 to the thirtieth power, or only 55% or so. A rocket with the same reliability for which only six flights are required will have a much better POM: 0.98 to the sixth power, or 89%.
Moreover, because of NASA’s superior experience and processes developed over decades, SLS will probably be greater than 98% reliability, compared to those amateur commercial rockets. SpaceX just blew one up last summer, so their demonstrated reliability is only 20/21 (as of the end of 2015), or 95.2 percent. That means that a mission that required thirty successful Falcon flights would only have 0.952 to the thirtieth POM, or about 23%, less than a one in four chance. Why are you trying to sabotage our Mars plans by insisting on using these dinky, unreliable rockets?
I know that because I don’t have a specific quote, this sounds sort of strawmannish, but there have been arguments like this made by SLS proponents for years. And the logic and math seems indisputable, right?
Well, it’s a lot more complicated than that. In fact, a greater number of flights to accomplish a given job actually increases the chance of mission success. And also in fact, this would only be counterintuitive to someone in the warped space industry, stuck in the Apollo and general “mission” mindset. Let’s unpack the above “analysis,” to see why.
First, it assumes without basis that the loss of a single flight causes the loss of an entire Mars mission, ignoring the fact that the lost payload could be delivered on another flight. The only flight failure that could cause mission failure would be one carrying an irreplaceable mission element. But if we have irreplaceable mission elements, we’re clearly doing it wrong.
We could do a reductio ad absurdum, and assume that we are going to deliver everything for a human mission beyond earth orbit and back in a single launch.
Oh, wait! That’s what we did in Apollo!
For Apollo, the Saturn V carried all of the hardware elements described in the DRM, plus propellant. If the launch system failed, the mission failed. The reliability of the launcher put a ceiling on the POM; if the Saturn was 95% reliable, the POM could be no greater than that, and of course it would be less, because of the potential for failure of any of the other mission elements (as happened with Apollo 13, when the liquid-oxygen tank in the service module exploded). For the launch system, it was all or nothing.
Let’s go back to the truck analogy. Suppose we build the house in the factory, ready to live in, and then deliver it to its final destination on a giant truck. It’s a very expensive payload, because of all the value added in the factory where it was built.
Now the success of getting your house to your building site is totally dependent on the truck not crashing somewhere along the way. Would you really want to make that bet? Because trucks do crash with some regularity. And if it happens, you’ve lost a hundred-thousand-dollar (or more) house. Who would insure that?
That’s why we build houses on site from much smaller, less expensive parts, and we add value by assembling them there. That way, if you lose a shipment, it’s not that big a deal. You just send out another load of cheap cement or plywood or studs or drywall, or whatever.
This is the way we do things on earth. There is nothing magical about space that means we should do it any differently there, except that the one time we successfully did what we’d like to do again—send humans beyond earth orbit—we did it the crazy way, because we were in a hurry, and got away with it half a dozen times.
There will be more to come. BTW, I’ve been struggling to find a quote like that, but we all know that people have made that argument. Anything folks can come up with via crowdsourcing would be appreciated, particularly from NASA officials.