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.
Are there any launch systems that operate as reliably and as often as a truck? I.E. a rocket that can be prepped and launched within hours of a launch failure?
Are there any launch systems that… can be prepped and launched within hours?
Do you think all those ICBM silos are empty? Just a big bluff?
So the plan is to use ICBMx?
No, Mark, the “plan” is to use launch systems that can launch more than three (at best) times a year.
How often would these hypothetical launch systems launch?
As often as need be. And there’s nothing “hypothetical” about them. In the JSC study, even Delta IV was cheaper than SLS, by tens of billions.
So you are not willing to assign a hard number. Thanks.
Zero. Is that hard enough for you? Though, actually, if it was twice or three times what it was actually worth, you’d have to pay me. 🙂
If you are using a surface supply chain analogy, you might want to research how it works. When is an 18 Wheeler used as opposed to a panel truck?
When is an 18 Wheeler used as opposed to a panel truck?
I don’t know, Mark. When?
More to the point, what is the point to his question?
My point that if Rand is going to use the trucking industry as a model for his scheme it might be of some benefit to understand how it works.
OK, Mark, then please explain to us how it works. Do they figure out the biggest possible thing they might want to transport, and then build the biggest possible truck, and the biggest possible roads to carry it (or, rather, bigger than practicable)? Because that seems to be how the SLS boosters think.
So, Mark, are you saying I can quote you as subscribing to the idiotic argument I laid out? I’m always eager to find someone willing to go on the record.
If not, make some other argument on the record that I can mercilessly shred in my paper. Though it won’t really have that much value, since most people would say “Who the hell is Mark Whittington? This seems like a straw man.”
I think it was this year, Mike Griffin and two coauthors made the case for SLS on an online article. I forget the publication, the date, and the coauthors. I wish my memory was better.
Do you know which op-ed I’m talking about? I think you posted a fisking of it. The reason I bring it up, is I seem to remember Griffin making something like your SLS supporter comment.
How much are you willing to pay for my time as a consultant? If nothing, then I suggest that you do the work yourself.
Twice what it’s worth. Maybe three times.
Really, I doubt that you would ever sign a contract for a job on that basis. In any case, it seems that this latest scheme seems destined to fail, just like the property rights proposal and the space flight safety proposal, Sad.
Oh, and if you do quote me, please be accurate and in context.
The truck decision is made to reduce the number of drivers (who have to be paid) when possible, along with limits on the number of available dock doors. If we ever switch to driverless trucks most of the big rigs might disappear. Is there any other place we use driverless vehicles? Oh, yes there is! Space.
Mark, I seriously don’t know when you use an 18 wheeler vs. a panel truck. I’ve moved twice in the past 7 years. Roughly the same amount of stuff, roughly the same distance. One time Mayflower used one tractor trailer. One time they used two of their larger panel trucks. I don’t know why.
One time I had to ship a helicopter rotor blade for work. It wouldn’t fit in a panel truck, so it had to ride on the back of a tractor trailer flatbed by itself.
Based upon the estimated load, big movers like mayflower will put several people’s stuff on one truck. So if yuo have 2 panel trucks worth that won’t fill a big rig. But if the leftover space is enough to put someone else’s junk in there, and if they are in roughly the same direction as your new place, they will use the big rig.
One truck
One driver
2 moves.
I was referring to literally moving houses, not moving the contents.
If there has been anything useful gained from the ISS, it’s the knowledge that on orbit assembly of reasonable sized modules is possible. The greatest mass for a Mars mission will be propellant and there’s no requirement that it be lofted in a single launch. Loss of any single propellant supply launch (or even several losses) will have little impact on the mission. As for the vehicle itself, loss of a major module would be serious but that can be mitigated with redundancy.
The question of how large a launcher must be depends on the mass and size of the largest atomic (non-severable) component. Analyze and design the components and use that to set the launcher requirements, not the other way around. If at all possible, using existing or soon to exist (e.g. Falcon Heavy) rockets instead of spending tens of billions to develop the SLS (plus billions more per launch) makes far more sense and saves enough money to build those redundant modules. Also, if the Shuttle is any guide, there’s no guarantee that the SLS will be 100% reliable, either.
Currently, the most economical method for building houses is partial factory assembly and transport of the modules to the job site for final assembly. You gain better quality control by having an environment that allows for maximum efficiency and safety from the elements (no rainstorms to slow that part down). Bring the completed modules to the work site and assembly. You are all done. Basic site prep is all that is required on location.
If we did more of this with the space program and just accepted assembly as well mass production, costs begin to drop and spare parts are readily available. Always build in threes.
The argument of SLS is largely argument of history- it’s already had +20 billion dollars spent on it. Or the $100,000 prefab house 1/2 paid for and the truck that tows it already has an engine and frame build- though no where near going to Mars or the house is useless as in the truck.
Or we have already wasted a decade and tens of billions for no reason or purpose.
Or basically one should count on using SLS in same way one counts on using ISS [another “mostly” waste of billions and decades of time].
But one should think of having a limited use of SLS. Or there is no sanity connected to the idea of spending more billions and years trying to make SLS have a large enough launch rate [say 2 or more launches per year] so it actually is the major launcher for a Mars exploration program or even major launch vehicle for a Lunar exploration program.
Also SLS doesn’t have to be a big deal, one could say it amounts to 1 billion dollar loss per year to NASA, and give NASA a potential of a bit of flexibility- having something that can lift 50 to 70 ton to LEO. Or one more way to lift about 50 tons to LEO- and if 50 ton payload grows to 59 tons instead, then it’s only thing on the books which might lift 60+ tons.
Or NASA is always making things which become bigger than they planned, so SLS is back up option for NASA’s incompetence.
But plan on most tonnage being shipped into space using the best launcher- and part of best is cheaper.
Another part of this is that NASA could be restricted by the US government to only using US launch, rather use Russian, European, Indian, Chinese, or whoever.
In theory it would be better that NASA could choose whatever launch rocket which was available. But this is rarely the case. So SLS, gives SLS and US launches as an option, as it’s unlikely US government will prevent US launch companies [unless they are buying and using Russian rocket engines].
Or maybe we need BOTH big and small lifters. Analogous to global transport of hardware for major long term military ops. Much as some would like to do it all by air transport, most of the heavy gear still has to go by sea.
The issue isn’t about needing “BOTH big and small lifters.”
Another flaw I see in the pro SLS argument: the assumption that launcher reliability is independent on number of launches made. When in reality actual launches are an effective way of working out bugs. But I recall Rand making that argument that we need flight rate for reliability.
If you fly only once every year or two, it’s difficult for the ground and mission crews to gain and maintain proficiency. They simply don’t do their jobs often enough. On the other hand, a very high flight rate can lead to mistakes induced by fatique unless you can rotate your crews and automate as much of the work as possible.
The limited understanding of risk reminded me of this video, which I compared to NASA management during STS-107. Some of those people really need to stop with their heads down in number crunching and actually see exactly what is happening and why.
And to head off any moronic comments; no, standing on a rock isn’t like launching a rocket or managing trucking logistics, but if your only argument is about the analogy, then the concept is beyond your grasp.
That’s a cool video. I hadn’t seen it before.
Were you looking for this article by Mike Griffin and Scott Pace?
http://spacenews.com/propellant-depots-instead-heavy-lift/
Thanks, Ed. This might be it, though I don’t recall propellant depots being the focus of the article I’m thinking of. I think it was more recent than 2011 and I don’t think it was on SpaceNews.
Not sure why my memory is suffering so terribly today. :-/
Here’s one from this year by Mike Griffin and Dan Dumbacher:
“Bush’s former NASA Administrator Michael Griffin says Obama’s Space Launch System ‘next essential step’: guest opinion”
http://www.al.com/opinion/index.ssf/2015/06/former_top_nasa_administrators.html
That one’s just a regurgitation of the same talking points in the Cooke/Cook/King editorial. It doesn’t explicitly make the probability-of-mission-success argument. But I know I’ve heard it. In fact, refuting it is why Cheauvront et all did the JSC study four years ago, that fed into the Next Gen LLC report this past summer.
Thanks, Rand. It was the Cook/Cooke/King editorial that I was thinking of.
It doesn’t explicitly make the probability-of-mission-success argument.
The C/C/K editorial doesn’t make the argument as thoroughly as your hypothetical SLS proponent, but it does say:
“More launches necessarily mean more cost and risk.”
and
“Packaging [landers and habitats] into a space six times smaller would be extraordinarily challenging and would dramatically increase cost and risk while limiting overall mission capabilities.”
Yes, but they don’t say why it adds more risk. Dumbacher, in his interview with Jason Rhian, says it’s because orbital assembly is complicated, and puts crew more at risk. They don’t talk about it in terms of failure to deliver payloads.
Thanks, Ed. I think my tired old brain blended together the Griffin/Dumbacher and Cook/Cooke/King editorials.
Easy to do, because they used the same talking points, often word for word.
The truck/house analogy doesn’t quite work because trucks do haul houses all the time. Not just manufactured homes but also conventional houses that must be moved for one reason or another.
The limiting factor isn’t the risk of an accident per se but width, height, and weight restrictions on the route from point a to point b. These can lead to accidents but they are mostly avoidable and are not related to the reliability of the truck/trailer.
In a conventional setting the risk of truck/trailer failure could be rather low considering all the things done to mitigate risk. However, what about using trucks hauling mining or other industrial equipment to remote areas? In that setting, trucks break and payloads can be lost or damaged. There might even be some actuarial data you could obtain or you could interview someone in the industry about what their best practices are for hauling equipment, supplies, and people to remote areas.
Or maybe that is a too much work to support a couple paragraphs but I would flesh out or change the analogy.
I do expand on it. This is just an excerpt.
Also consider that SpaceX may soon have a significant supply of slightly-used first stages. We’ve all been assuming (mostly?) that they’ll just drop prices a bit, but it could be more complicated.
Suppose instead a brand-new rocket from SpaceX will cost about what it does now, but you can get a refurb launch for (at most?) half-price. Using those cheaper launchers for (e.g.) propellent or any relatively cheap cargo would seem to be a no-brainer at that point, even if you have big launchers…
Start the discussion off talking about shipping a -large- house, perhaps. The house has to be large “because that’s what’s needed to store the supplies and fuel to get to Mars.” There’s no real dispute on this point, although NASA’s seven-year-Salisbury Steak plan still seems daft.
“Previously, we built ISS with pre-built modules of a couple sizes. These modules were ‘entire-truckloads’ for the Shuttle and the Russian lifters, and assembled in orbit – with the bulk of the fuel, food, and people coming on separate trucks.
But SLS is basically this plan:
https://www.youtube.com/watch?v=svpfCKGsmJE&ab_channel=bigmrsteve
Or talk about shipping an ice rink instead of a house. Even if you do take the silly route of using not-just-a-semi but the so-large-we-took-down-the-nearby-powerlines house movers, it would be a gigantic hassle. And, at the end of the day, you don’t have something useful (a semi) you have something niche – house-moving-treads.
For an ice rink, the pieces of the puzzle line up with the pieces of a mission to Mars. There’s the shell, the living/locker areas, a large mechanical space, and a huge volume of material that can be shipped in whatever shape is most convenient – the ice.
Start the discussion off talking about shipping a -large- house, perhaps.
I did. I just didn’t include that in the excerpt.