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Stationary fuel tanks seem problematic for rockets since your rendezvous would have to happen at such high speeds. You'd either have to decelerate the rocket b

Stationary fuel tanks seem problematic for rockets since your rendezvous would have to happen at such high speeds. You'd either have to decelerate the rocket back to zero or accelerate the fuel to the speed of the rocket.

philwelch 4y ago
Remember that all motion is relative. You can’t model space travel as simply going from point to point. Instead, you transfer from orbit to orbit and have to match velocities wherever you go. Relative to the propellant depot you have to reach zero velocity, but the propellant depot will be orbiting something at a very high speed just like you are. You will likely have to go out of your way somewhat to reach a propellant depot, so you don’t want your propellant depots at the bottom of heavy gravity wells or in hard-to-reach orbits. But it’s still a viable strategy.
Robotbeat 4y ago
There are some places that your relative velocity with a propellant depot may be near zero already. Natural staging points are not stationary, but they may be in fixed orbits, such as LEO (of a certain altitude, inclination), maybe a halo orbit of a Lagrange point (NASA’s Gateway is in NHRO, which is similar), or a highly Elliptical orbit near escape velocity. And you can do this for Mars as well. By refilling at these staging points, you can chop up the rocket equation so you don’t get hit hard by the exponential. You have to get fuel there in the first place, and to do that you can use slower, more efficient trajectories or solar-electric propulsion (much higher Isp but slower) or in situ propellant production. You can also have better cooling at these staging points, taking a mass penalty off your crewed vehicle and offloaded to the depot in the case of cryogenic propellants that may boil off over time without an active chiller. Propellant depots are a pretty powerful tool for expanding capability for space exploration. NASA picked SpaceX’s Starship as the lander for their Artemis lunar missions, and that will fuel up using a depot in Earth orbit.

philwelch 4y ago

Remember that all motion is relative. You can’t model space travel as simply going from point to point. Instead, you transfer from orbit to orbit and have to match velocities wherever you go. Relative to the propellant depot you have to reach zero velocity, but the propellant depot will be orbiting something at a very high speed just like you are. You will likely have to go out of your way somewhat to reach a propellant depot, so you don’t want your propellant depots at the bottom of heavy gravity wells or in hard-to-reach orbits. But it’s still a viable strategy.

Robotbeat 4y ago

There are some places that your relative velocity with a propellant depot may be near zero already. Natural staging points are not stationary, but they may be in fixed orbits, such as LEO (of a certain altitude, inclination), maybe a halo orbit of a Lagrange point (NASA’s Gateway is in NHRO, which is similar), or a highly Elliptical orbit near escape velocity. And you can do this for Mars as well. By refilling at these staging points, you can chop up the rocket equation so you don’t get hit hard by the exponential. You have to get fuel there in the first place, and to do that you can use slower, more efficient trajectories or solar-electric propulsion (much higher Isp but slower) or in situ propellant production. You can also have better cooling at these staging points, taking a mass penalty off your crewed vehicle and offloaded to the depot in the case of cryogenic propellants that may boil off over time without an active chiller. Propellant depots are a pretty powerful tool for expanding capability for space exploration. NASA picked SpaceX’s Starship as the lander for their Artemis lunar missions, and that will fuel up using a depot in Earth orbit.