Fuel is a major issue when it comes to long-duration spaceflights - its weight is a problem for launch and once a spacecraft runs out of fuel there's no place to get more. That's where in-space "gas" stations located in strategic spots along a route would be a boon to spaceflight.

Which is exactly what NASA is looking to do by beginning to solicit proposals for what it calls an In-Space Cryogenic Propellant Storage and Transfer Demonstration that will lay the ground work for humans to safely reach multiple destinations, including the Moon, asteroids, Lagrange points and Mars.

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NASA says the flight demonstration mission "will test and validate key capabilities and technologies required for future exploration elements such as large cryogenic propulsion stages and propellant depots. Today's Broad Agency Announcement solicits proposals for mission concept studies. NASA will use the results from these studies for project planning and implementation. Elements will include mission justification; technology identification and maturation; mission concept of operations; launch vehicle considerations; integrated flight system conceptual design; mission cost estimate; project schedule; concepts for government and industry partnerships; and project risk identification."

It won't be an easy task. Some of the requirements include the need to:

demonstrate long duration, in-space storage of liquid oxygen and liquid hydrogen cryogenic propellants.

demonstrate in-space transfer of liquid oxygen and liquid hydrogen cryogenic propellants.

demonstrate approach for zero boil-off storage of liquid oxygen in microgravity.

demonstrate approach for acquisition and bubble-free flow of liquid oxygen and liquid hydrogen in microgravity.

demonstrate approach for leak detection of liquid oxygen and liquid hydrogen in microgravity.

demonstrate approach for flow measurement of liquid oxygen and liquid hydrogen in microgravity.

The benefits of space fueling depots include minimizing the propellant weight for a given mission, allowing spacecraft to refill on long journeys to Mars or other locations. Space refueling could also involve developing commercial providers to deliver on-orbit propellants and could allow on-orbit assembly, satellite servicing missions, and resupply of empty or partially filled spacecraft.

NASA says the mission cost is approximately $200 million. Higher cost options may be considered if there is a substantial increase in benefit, but the mission cost shall not exceed $300 million. Also, lower cost options may be proposed if there is substantial decrease in mission cost with modest decrease in benefits, NASA stated.

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