Mike Griffin and Scott Pace wrote in their commentary “Propellant Depots Instead of Heavy Lift?” [Oct. 31, page 17] that the huge Space Launch System (SLS), formerly called Ares 5, would have a lower marginal cost than smaller rockets with propellant depots. I believe this was very misleading, simply because NASA must pay not only marginal cost for each flight but total costs, including large development costs and large fixed costs.

The SLS will certainly be costlier to build and own than smaller rockets for NASA’s human exploration program. Big rockets like SLS require dedicated big facilities, and many more people to operate them, even with only one or two launches per year. Following an $18 billion-plus SLS development program, maintaining and operating manufacturing, assembly, test and launch facilities would result in fixed costs amounting to 80 percent to over 90 percent of total operating cost when the launch rate is very low. Large fixed costs of the SLS would be with us for decades — unless it was canceled after several years for lack of funding, as was Constellation. Smaller rockets with higher launch rates definitely have proved to operate at lower overall cost.

SLS was characterized as being seriously underfunded in an Aug. 19 Independent Cost Assessment report by Booz Allen Hamilton. Accordingly, the SLS is likely to suffer the same fate as the canceled Ares 5 of the Constellation program, and for the same reason.

The SLS comes before its time. It is not an unreasonable design, but under the current plan would not be needed until sometime in the 2030s. Even then, NASA internal studies, referenced by Rep. Dana Rohrabacher in his commentary “Fueling Stations vs. Monster Rocket” [Oct. 24, page 19], have shown that a propellant depot would be a lower-cost and much more efficient way to assemble elements in low Earth orbit for transport outward.

In the near term, NASA should utilize existing systems as soon and as much as possible to restore U.S. leadership in human spaceflight and minimize transfer of scarce dollars to Russia. A slimmed-down version of Orion, together with existing and proven vehicles, such as Atlas 5 or Delta 4 and Centaur, could fly U.S. astronauts to early destinations beyond low Earth orbit, and do it within existing budgets.

Further, an international effort using elements derived from the international space station could create a human cislunar transportation system, starting with robotic elements on the Moon to begin to harvest water found there and make it into propellant to fuel this system. Capability development will be incremental, cumulative, pay-as-you-go and sustainable.

Building such a system creates long-term spacefaring capability, is useful for future missions regardless of destination, and can be achieved quicker and more easily for less money. It is a logical approach to the current lack of direction we see in our national civil space program.

O. Glenn Smith