Under what conditions would fuel depots in earth orbit make sense?
As it costs $X/kg to launch something into space, wouldn't it cost the same to launch fuel by itself as it would to launch it as part of the vehicle that's going to end up using it?
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Sign up to join this communityUnder what conditions would fuel depots in earth orbit make sense?
As it costs $X/kg to launch something into space, wouldn't it cost the same to launch fuel by itself as it would to launch it as part of the vehicle that's going to end up using it?
The $X/kg is a simplification.
Rockets are only available in certain sizes; the launch vehicle with the greatest capacity that is presently operating is the Delta IV-H, delivering a little less than 28 metric tons to LEO. Even hypothetical rockets have limits due to the logistics of assembling and launching them.
If you want to launch a large and heavy mission - perhaps for the human exploration of Mars - you have to split up your mission into multiple launches. One way to do that is to establish a propellant depot in LEO, launch multiple tankers to it to top it off, and finally launch your main mission with empty or partially full tanks, fill up and depart for Mars.
There are alternative strategies - you could instead launch a fully fueled Earth Departure Stage, and separately launch the habitat and lander components, have them rendezvous in LEO and leave. That does put more time pressure on the launch and rendezvous since a full EDS can't hang around for too long before boiloff becomes a problem.
Humans like fast transit (less supplies, less radiation), so they want to use chemical propellant which is much less mass efficient than electric propulsion. One way to mitigate this and lower the overall mass required for human missions is to use electric propulsion to transport chemical propellant to high energy locations, such as high Earth orbit or at asteroid or Mars destinations. Then after you wait a few years to get the propellant to where it needs to be, the humans can zoot around the solar system using the stored propellant.
You would go ahead and store the engines with the chemical propellant, so there's no need to transfer fluids. Just grab a complete stage that has been stationed for you and go. The engines are a small fraction of the mass of a stage, so you might as well attach engines to your fuel tanks.
Normally LH2/LOX is the chemical propellant discussed for this sort of thing, but that will require technology development and demonstration of zero boil-off for the time spans required.
The other way "depots" makes sense is if the propellants didn't come from Earth. Asteroids and Phobos can be sources of water to make LH2 and LOX. High-Earth orbit is energetically much closer to those locations than Earth.
As it costs $X/kg to launch something into space, wouldn't it cost the same to launch fuel by itself as it would to launch it as part of the vehicle that's going to end up using it?
Why assume that the launch cost is the same?
Here are some hypothetical scenarios why storing fuel in space might be a feasible idea. (Assuming a need for fuel in space.)
Launching fuel from Earth without a specific purpose, waiting for a need to turn up, can't be a good idea. I've heard of fuel depots in the context of producing fuel on the Moon. So that is in a vision of a much more developed space flight than what we have today. The benefit would come from the lesser energy needed to launch from the Moon's lower gravity. And once produced on the Moon, the fuel might as well be put in a waiting depot in space. At a Lagrange point or some lunar orbits, the large distance to Earth makes any conventional Earth orbit easy to reach such as polar satellites, geostationaries, GPS. From Earth orbit, it would require a lot of fuel to navigate to different orbital inclinations.
The market for refueling satellites is limited by the economic lifetime of satellites. One might want to replace a 15 years old communication satellite with new hardware technology anyway. And it might be cheaper to fuel it for 15 years of station keeping already before launch, rather than hassling with docking and refueling systems.