Dragon Tug

With the Dragon capsules fully capable of ISS interaction now, how hard would it be to use the dragons for ISS tugs as is?

One of the quite useful pieces of on orbit hardware is a space tug that off loads the critical maneuvering capabilities from the launch vehicles. A launch vehicle that does not need those capabilities will be simpler and cheaper, not to mention safer, than one that does require all the bells and whistles. I am wondering if the capability to accept cargoes from multiple  suppliers for the ISS has stealthed its’ way into operational use without any of us noticing.

It would be interesting to know if Atlas and Delta, or even Sea Launch and foreign suppliers, could deliver payloads to just outside the exclusion zone for a Dragon to tug to ISS. Financially it wouldn’t seem to make sense. Politically though it might get several more players pulling in a positive direction. If it suddenly became blindingly obvious that several suppliers could safely and reliably surge major tonnage to an orbital berth, the argument for the Big Rocket would take another hit.


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I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.

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15 Responses to Dragon Tug

  1. ken anthony says:

    I would think the biggest issue is refueling?

  2. The ISS has a machine making methane on-board out of water and Co2
    place said machine onboard a Dragon modified with a methane engine or make ethylene out of it.
    make and store and fire engine periodically,
    when cargo and crew approach undock and fly out to keep out zone and loiter.instead of orbit reboost for ISS a Dragon tug could demonstrate this mission,


    reminds me of Gemini Agena mission, perhaps this is a Cygnus lab free flyer in need of a boost to higher orbit or perhaps this demonstrates the use of Cygnus and Dragon as a space telescope repair mission
    I am sure the Methane Dragon could not make the ISS to Hubble trip.
    I am sure Zubrin has suggested reverse gas shift using astronaut waste metabolites in one of his early papers so our hats off to him!
    I am sure the paper suggested that anywhere in the solar system you have humans a engine could maneuver them.
    the tug makes more sense since it could perform multiple missions and such a tug could be a work horse between L2 and L1 both earth moon and earth sun!

    So this idea means that a small spaceX Methane or Ethylene Merlin does make sense?


  3. I am thinking that most people smarter then I am are saying any Methane based MCT system would not have any bearing on the existing commercial spaceX RP1/LO2 and NTO/MMH
    The Tug idea using astronaut metabolites might suggest otherwise if it had a commercial purpose
    who is to say that small Dragon Methane engines would not have a role to play in mars orbit?

  4. ken anthony says:

    Steven did you just say they could refuel by farting?

  5. Andrew Swallow says:

    Without a service module the Falcon 9 or Atlas V can put more cargo in orbit. This is now a two launch situation so the payload would have to be heavier than the total payload of two ordinary launches.

    The payload could be to LEO, possibly a new module for the ISS. Alternatively the payload could be for GEO.

    The cargo needs to have a docking port, homing radio, attitude control and power. The Dragon or Cygnus would fly to the cargo, dock and then push to GEO/ISS.

  6. Ken
    why no!
    Zubrin uses a process to turn CO2 and water from human pee, sweat and breath into Methane
    Currently on the ISS this Methane is dumped overboard
    Google Zubrin reverse gas shift + ISRU



  7. Andrew,
    this scenario still involves Dragon and or Cygnus having pressurized volumes?
    “The Dragon or Cygnus would fly to the cargo, dock and then push to GEO/ISS”
    end Quote

    if so this opens up the possibility of a propositioned GEO hab

  8. john hare says:


    The idea would be for the Dragon to deliver a full cargo to the station first. Then a week or two later, it could depart to a rendezvous with another cargo ship that it would then bring back to the station. Possibly triple the cargo of a single launch in two launches with one of them nominally incapable of the mission without assistance.

    If refueling could be incorporated, then a Dragon could stay up for months fetching multiple cargoes from outside the exclusion zone. If the Dragon systems could handle an indefinite orbital stay, the heat shield and other reentry gear could be left on the ground in favor of larger fuel tanks, power systems and docking ports as required. This could lower the bar for future SpaceX and other companies to deliver not only to ISS, but other destinations in the cislunar system as you suggest.

  9. so you leave the heat shield on the ground as John suggests you would want to insure the Dragon/Cygnus other vehicles avionics can be repaired on orbit just like Hubble.
    The proposed Dragon lab could be one of those vehicle outside the exclusion zone that could be repaired
    transfer Dragon lab material back to ISS? ***
    *** my brother does NASA grant research on mouse reproductive tissues in simulated zero gee( the news is bad so far) mice and rats on ISS causes allot of problems so I proposed this idea to spaceX three years ago;
    rats and mice on Dragon lab
    centrifuge on Dragon lab( to mimic 1/6 and 1/3 gravity)
    biological s on Dragon lab
    from time to time Dragon lab docks with ISS and their is a suit port between the ISS/Dragon hatch……………..
    no exchange of atmosphere between dragon lab and ISS
    astronaut in suit port performs vivisection and other chores
    how to return tissues?

  10. The Cargo version of Dragon uses berthing, which would require a robotic arm for capture and connection. It also uses the passive-side of a CBM, which means the other system would need the active side (that includes nut drivers to secure the connection). The crew version of Dragon (as well as CST-100, Dreamchaser, etc) do include actual docking connections, that would allow tug operations with something that has at least a dumbed-down version of that interface, without requiring an arm.

    Once those are flying, I guess you could use them potentially as tugs, provided you had a way of providing the cargo 3-axis stabilization prior to the Dragon (or CST-100 or Dreamchaser arriving to pick them up), since I don’t think the NASA Docking System can grapple something that’s tumbling.

    You can definitely make any of these work as a tug with enough extra work, and it isn’t a silly idea, but it’s not trivial either, IMO.


  11. So Jonathan
    A crew version of Dragon even if uncrewed (the tug)could dock with a passive Cygnus? or should I say a Cygnus with a passive CBM?
    I would hope that dream chaser or CST or Orion could dock with one another.

    I remember when I was young that a treaty was signed that stated all space craft should be able to rescue another crew
    we should honor this

  12. Andrew Swallow says:

    @steven rappolee I was think of lifting satellites to GEO. A hub needs people who may wish to return to the Earth. That makes everything harder since you need a bespoke manrated tug.

    According to Wiki LEO to GEO has a delta-v of 4.33 km/s. This is similar to the return trip propellant on a lunar lander without refuelling. However they were not designed for 10-20 tonne payloads.

    In about 5 years time docking ports will be off the shelf products. Inspace navigation equipment already is.

  13. Andrew Swallow says:

    Once those are flying, I guess you could use them potentially as tugs, provided you had a way of providing the cargo 3-axis stabilization prior to the Dragon (or CST-100 or Dreamchaser arriving to pick them up), since I don’t think the NASA Docking System can grapple something that’s tumbling.

    An expendable cargo docking “interstage” can be designed. Use off the shelf docking ports, add some minor avionics and station keeping thrusters. I assume that we do not have to maintain orbit so small thrusters can be used to control roll and attitude. The hydrogen peroxide RCS thrusters from the Mighty Eagle may work.

  14. Andrew Swallow says:

    To use a cargo Dragon as a tug it would have to be equipped with a docking port since berthing is far too complex to be performed by remote control.

  15. Kevin Crady says:

    If your Dragon tug could be launched with deployable solar panels instead of the heat shielding, perhaps they could also use electrodynamic tethers for orbital transfer propulsion, so that the astronaut-generated methane biogas could be reserved for RCS maneuvering jets. I don’t get the impression that astronaut manure would generate enough methane to provide all the delta-v you’d want your tugs to have, especially if you want to do things like maneuver from LEO to GEO.

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