The Cost and Equivalent System Mass of Space Crew Time

In “Theory and Application of the Equivalent System Mass Metric,” Levri, Vaccari, and Drysdale computed the Equivalent System Mass (ESM) of crew time. ESM is a cost-type metric based on allocated mass that is often used in life support systems. The previous paper suggested that the cost per hour of crew time should be equal to the ESM of the life support system, divided by the number of available crew work hours. We suggest here that the mass cost for additional crew time may be as large as the total mission mass or as small as the added mass of consumables, depending on how much more crew time is needed. If the increased mission work load requires flying additional crewmembers, the total mass and cost of the mission increases roughly proportionally to crew size. But if the needed work can be done merely by extending the mission duration, the required additional mass is only that of the food and supplies to be consumed during the time extension. The resulting upper and lower bounds on cost per hour of crew time are within an order of magnitude and can help resolve design decisions even when the total demand for crew time is unknown. However, the cost of crew time used in mission planning should not always be the actual cost to provide that time. The cost should be set at a level that ensures that the crew is neither under or overloaded. If little work is needed, we should set the price of crew time low or at zero to encourage more tasks. If the crew time demand is excessive, the cost should be set high to reduce the task requests. Imposing a low cost for low total demand and high for high will help guide the sum of crew time requests to converge to the desired workload.