As I report in the second part of my Adrift series on America’s human spaceflight program, work is well underway on NASA’s first heavy lift rocket in four decades, the Space Launch System.

Whatever you think about the $9 billion rocket, its development is running several months ahead of schedule and it now appears likely to at least make an initial test flight in 2017.

There are many good questions that can be asked about whether NASA really needs a heavy lift rocket, and perhaps more importantly whether it has the funding to adequately use one. I’ll write more on this is the next installment of the series.

For today I wanted to flesh out the issue of sustainability. The National Research Council, in its report published last week, found that future flight rates of the SLS would be far below that of NASA’s previous rockets, the Saturn V, and the space shuttle. And for this reason it found the SLS would likely be unsustainable. This is consistent with the reporting I have done this year.

But what does that really mean?

In the spaceflight business weight is gold, and the key to having a sustainable program is one that gets a lot of stuff into orbit safely, regularly and at the lowest possible cost. This is perhaps where the SLS faces its greatest difficulty. Let me explain.

NASA’s budget appears to allow for one flight of the SLS per year, at most. And that assumes NASA can find some money — which it doesn’t have — to build a payload or two for the rocket to launch.

This issue of cadence is critical because with any launch system there are fixed and variable costs. Fixed costs are those associated with the “standing army” costs of keeping supply lines open and flight crews ready. Variable costs are those for an individual flight.

“What you need to understand is that, from a cost standpoint, there’s an optimum number of flights,” Doug Stanley, president of the National Institute of Aerospace, a non-profit research and education institute, told me.

Although actual SLS flight costs have yet to be disclosed; assume a heavy lift system that has $2 billion in annual fixed costs, and $500 million per flight cost.

If the rocket launches five times a year it would cost $900 million per flight, and lift payload to orbit at a price that is more expensive than private, smaller rockets, but not inordinately so. But if the rocket flies once a year, the projected rate of the SLS at best, the per-flight cost is $2.5 billion.

“At a low flight rate the costs can become ridiculous,” Stanley said.

And of course those costs don’t factor in the $9 billion spent to develop the SLS and billions more on ground systems to support it.

For more on these and related issues, please do check out Adrift.