There are too many problems with Michael Griffin and Scott Pace’s op-ed on propellant depots [“Propellant Depots Instead of Heavy Lift?” Oct. 31, page 17] to address them all in a single letter to the editor.

So ignoring the issue that economies of scale in any transportation system are more strongly driven by utilization rate than vehicle size, I will focus on just one — their use of marginal cost in justifying a heavy-lift system. Marginal cost (the cost of the next flight, given that a system is already operating) makes sense in deciding which existing vehicle to fly on, but it’s useless by itself in deciding whether or not to develop a new vehicle. For that decision, one must take into account the total life-cycle cost, flight rate and average cost per flight, including amortization of development costs and annual fixed costs.

Per congressional mandate, the Space Launch System (SLS) is going to cost a minimum of $18 billion just to be developed to its first, smaller version. Total cost estimates into the 2020s to get to full capability range from twice to three times that amount. NASA officials say it will fly once every year or two. If they get an annual flight out of it for 13 years, that would imply a cost of at least $3 billion per flight, and a cost per metric ton (for 130 ton) of $23 million, or $23,000 per kilogram. Even if they somehow fly four times a year, and I generously grant a marginal cost of zero (unlikely with a large expendable vehicle), it’s still $5,750 per kilogram when all costs are included.

In contrast, the Space Exploration Technologies Falcon Heavy is priced at $120 million for 53 tons, or about $2,300 per kilogram, with no development costs from the taxpayer. If you don’t believe that this vehicle will ever fly (we’ll know in a year or two), then use the now-existing twice-flown Falcon 9, whose quoted price is $60 million for just over 10,000 kilograms or $5,750 per kilogram — about the same as the most optimistic case for SLS, and available today, not a decade from now.

As Mr. Griffin himself pointed out in congressional testimony recently, the Chinese could send humans to the Moon in this decade with the Long March 5. We could do the same thing, with the Falcon or Delta 4 Heavy and Atlas 5.

But if we want to send humans beyond low Earth orbit, we have to stop spending money on developing new grossly oversized rockets that we won’t get for many years and that we do not need, and start spending money on the things that we actually do need — e.g., propellant storage and transfer technologies, radiation protection, closed-loop life support, Earth departure vehicles and landers. When we see Congress demanding that NASA develop those kinds of things, and provide funding for it, we will know that U.S. lawmakers are serious about human spaceflight. Until they do, it will remain clear that they are serious about nothing except jobs in the desired ZIP codes, and prestige rockets.


Rand Simberg

, Wyo.