7 thoughts on “Earth-Departure Orbital Mechanics”

  1. I’ve heard similar arguments as why the ISS is a bad point of departure for interplanetary missions because of its high inclination orbit. If this will work for fuel depots why not for the ISS as a way point? FWIW I haven’t read the 2nd part of this paper yet. If it’s a question of fuel economy is it possible to leave a departure stage behind that acts like an orbital “tug” to get you in LEO alignment from the depot and then leave it for someone else to collect?

    1. Low Earth orbits are not optimal for immediate departure to destinations beyond earth orbit, but they are excellent as staging and refueling points on the way to L1/L2, which in turn are excellent staging and refueling points on the way to further destinations. Note that the most efficient transfers from L1/L2 use a powered Earth flyby and sometimes a Moon flyby. So you would return to LEO *altitude* for the final departure burn, but not to LEO *velocities*.

      IIRC you get daily transfer opportunities from LEO to L1/L2.

      As Heinlein said LEO is halfway to the rest of the solar system. We can add that L1/L2 are halfway from LEO to the rest of the solar system.

      The key points are that you want to make plane changes as high up as possible in Earth’s gravity well, and initial orbit insertion & refueling and the final departure burn low in the gravity well.

      1. Martin,

        One of the challenges of doing the kind of departures you’re mentioning (leave from L1/L2, swing by the Earth, and do your injection burn at perigee) is that it’s hard to target departure asymptotes that are far out of the plane of the Moon’s orbit. Some planetary missions are close enough to the ecliptic that that trick can work, but NEOs typically aren’t. But you can get some of those benefits with what I’ve sometimes called a “roving depot”. Basically, you figure out the HEO orbit you want to be in for the final set of launches from the depot before the final departure. Then, starting a while before that time, you launch a “roving depot” (basically a fancy tanker) to that HEO. And each time the depot intersects with that HEO orbit plane, you send tankers to gradually fill that depot up. And maybe send parts of your interplanetary mission stack. Then on the final orbit, you do any plane change you need, and then head out on your way. After the mission departs, the roving depot heads back to LEO for its next mission (same with the tankers filling it up along the way).

        That way you’re not limited to low declination targets, and get the benefit of being able to refuel just shy of escape velocity. The challenge is going to be making sure lunar/solar perturbation corrections don’t cost too much to deal with over that timeframe. But there may be some tricks to minimize that.


        1. Jon,

          Thanks for clarifying that complication. Couldn’t powered lunar flybys on the way from L1/L2 to the Earth flyby and/or from there to the departure asymptote help you target highly inclined departure asymptotes? I may be all wet on this. I need to study the sources that advocate L1/L2 as staging points more some and also further upgrade my orbital mechanics fu.

  2. David,

    Actually, the ISS works fine as an interplanetary departure destination. The high inclination does mean that you lose some performance in getting payloads there relative to a lower inclination site (at least when launching from a lower latitude launch site like Kennedy), but depending on the mission, the higher inclination more than makes up for it by providing the ability to reach a wider range of departure declinations. For the smallsat missions we focused on in the 2nd part of the review, ISS also has the benefit that you can probably scavenge cheap LOX/Kero from ISS delivery vehicles, which can help dilute down the high-cost of having a dedicated smallsat for the LEO launch part of the mission.


  3. Jon,

    In your co-authored & cited paper, Table 5 seems to accidentally omit Loop 1. I can’t believe that Delta-V plane change for a high inclination orbit is 0 for a lunar trip. It would appear that info should have been part of Loop 1. Also have you run any numbers for plane changes to the Lagrange Points? Would these in general follow similar results as for a lunar trajectory? Are these typically two loop trajectories? I’m specifically interested in L4/L5, because this could be a really neat way to get a cubesat to those points for very small expenditure, which would enable a survey of the space conditions in those volumes. I have always wandered if those areas are clean or contain orbital “detritus” swept up and trapped in those regions. If those “debris” items included large chunks of ice, that would be significant.

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