13 thoughts on “Propellant Depots”

  1. My question:

    Why isn’t NASA doing any of this? Yes, from the article, there is more to this than it looks, but why isn’t the ISS or some uncrewed platform developed to move this technology, even in a proof-of-concept-of-a-key-element instead of just floating around doing whatever “they” do at the ISS?

      1. The Russians have been using orbital refueling since the 1970s. Progress tankers regularly refuel the Russian Orbital Segment of ISS. What’s not widely appreciated is the ROS of ISS can act as a depot and refuel Soyuz and Progress spacecraft as needed. For example, the fuel systems and tanks of all the Russian modules are interconnected. They can move fuel from a Progress docked at Rassvet to a Soyuz docked at Poisk if they need to. The US segment has no fuel tanks.

  2. The biggst objection to low earth orbit depots is orbital mechanics, including inclination. Secondarily, orbital depots suffer from what I call “Tom Swift and His Ocean Airport” Syndrome. The book, published in 1934, tackles the problem of airplanes having too short range for intercontinental transport by building a floating airport in the middle of the ocean. It doesn’t take a genius to see the real solution is to build better airplanes.

    Looking at the mechanized ocean travel paradigm (which the best [but still poor] fit for interplantary travel), rather than depots, fleets were supported by things like oilers and sub-tenders (i.e., tankers). Which also helps one recognize that depots have to be refueled by tankers. So is it the right solution (reload depots with tiny tankers so big spaceships can refuel with few docking events)? No. The solution is to build bigger tankers.

    One day, maybe 50 years down the road, things like Starship will be lighters for large, nuclear powered interplantary liners that will resemble enormous versions of NautilusX. At that point we’re likely to have high-orbit (more likely L-point) shipyards, with associated fuel storage systems. And the shuttles bringing up passengers will be the third stages of much larger 2-stage super heavies, delivering them fully fueled to LEO. (George, I remember your trimaran SuperHeavyHeavy calculations. Probably the right idea if it turns out this happens a lot sooner. Which it might!)

    1. Thanks for remembering! Elon mentioned that he’s thinking about going to an 18 meter diameter for the Starship follow on. If the stack remains the same height, that means a four-fold increase in mass and payload. If he can shift more of the delta V to the booster (by giving it additional thermal protection and landing it further out to sea), then a relatively smaller upper stage could get to orbit with a higher fuel fraction. Or he could stick with getting a bigger upper stage to orbit with the same fuel fraction as the current Starship. If the fueling ship is 18 meters in diameter and just as long, it would more than fill up a regular 9 meter Starship. Considering his low anticipated launch costs, I can’t see any reason anyone would launch fuel to a depot on something else.

      As an aside, I think the 18 meter Starship would be tougher and more reliable, because the current skin should scale up to 1/4″ thick, which is going to weld better and, if they kept the same internal reinforcement spacings, deflect an eighth as much from point loads. It would also provide a much more robust attachment for thermal tiles and anything else.

    2. Yep I agree with this reasoning. A depot makes sense only if it’s in a critical location where fuel is hard to come by and getting a tanker to rendezvous on time is difficult. LEO doesn’t strike me as a case for that.

      I was strict by the artist’s depiction of the proposed depot with large solar arrays. Very large. Is that because water is being electrolyzed into hydrogen? Or are they running the Sabatier Process in Space?

      I also agree that for long term exploration of the solar system a migration of space stations from LEO to what I call near-Earth orbits or NEO to prove out long term sustainability and then on to the other planets big and small. Nautilus-X like architectures. These are exploration oriented not settlement oriented. The latter would benefit more from a Mars Direct type approach. Which Zubrin must be happy to see what Musk is doing.

    3. Also-a tanker is required to have depots-the opposite isn’t true. Both you need something to fill the depot and because most technical issues you have to solve for a tanker are also issues you have to solve for a depot. But the depot adds the need to keep cryogenic propellant from boiling off. Or at least keep losses low enough that the depot makes economic sense.

  3. It seems if we have settlements on Mars, one will need depot at Venus and if NASA ever does Mars exploration program it could also find depot at Venus as useful.
    As people probably know the hohmann travel time to Mars is faster from Venus than hohmann transfer from Earth to Mars. A simple hohmann from Earth to Mars is 8.4 months and Venus to Mars is 7.2 months. I don’t know what hohmann + patched conic is from Venus to Mars. but it seems it should about 4 months. And from Venus one has shorter launch windows as compared to from Earth to Mars.
    Currently it’s not “better” to go to Venus in order to get to Mars, though some mission plans have had leaving from Mars, and going to Venus, in order to return to Earth. Which is largely about having more return Earth launch windows from Mars to Earth.
    But if you had depots in space, can one consider going to Venus to get Mars from Earth as a better way to go to Mars?

    The reason going to Mars from Venus is faster, is mainly because you travelling a shorter distance as compared to Earth to Mars.
    So, going to Venus from Earth to get to Mars would easily “better” in terms getting “another launch window” to Mars from Earth, what mean by better is traveling a shorter total distance AND not using “too much” total delta-v.
    So, how fast can get to Venus from Earth.
    An extreme example is Parker solar probe, launched from Earth
    August 12, 2018, flyby of Venus October 3, 2018
    or less than 2 months. Maybe one do something like 2 months, but I was roughly thinking 3 months to Venus and 3 month to Mars.
    So get another launch window, and get to Mars fairly fast and you don’t need too much delta-v. And it’s actually traveling less distance compared to 7 month Hohmann with patched conic from Earth to Mars.
    Now, for years I having thinking 3 month to Mars from Earth by doing non-hohmann {use Earth’s gravity, and rocket power to change vector- but requires fair amount rocket power. But requires basically getting into Venus {or Mercury} hohman transfer to Mars from Earth. Or you leave from Earth in trajectory heading to Mars and it’s return leg, doesn’t return to Earth distance- it returns closer to the Sun than Earth distance [it could return to Venus {or Mercury or anywhere between planets] distance.
    So, going to Venus to get to Mars from Earth to be “better” would/should also have use less delta v, than changing vector at Earth distance and going directly to Mars. Though couldn’t be a shorter distance in that comparison.

  4. To use a fuel depot around Venus would require getting a fuel depot to Venus and keeping it fueled, which doesn’t sound easy. It would also require an orbital insertion burn at Venus, and then a Hohmann transfer burn, which the Venus flyby trajectories won’t have.

    1. Well I’m in a fanciful mood tonight so George if you’ll indulge me, what about a fuel depot with methane powered robotic “dippers” that descend into the Venusian atmosphere just far enough to scoop up lots and lots of CO2 and the zoom back into orbit to dock back with the depot that uses its ginormous solar arrays to run the Sabatier Process to make more Methane? Of course I don’t know where you get the H2O from. I leave that as an exercise for the student.

    2. You wouldn’t put depot at Venus in low Venus orbit for number of reasons. The global clouds reflect sunlight and seems in comparison it would make LEO seem quite cool, And another reason is why going are down in Venus gravity well {though if not sunlight, Venus emits less Longwave IR than Earth does}.
      Though you can take rocket fuel from a high Venus orbit and deliver to low orbit- if one needs it there.
      So a Venus depot could be in a highly elliptical orbit. Another thing is it could be at Venus L-2- and thereby allow you to have a cryogenic environment by being the shadow of Venus. And you also do same with using the L-2 of Mars.

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