Dragon tug
On the first glance, Dragon 2 could never be used as an efficient space tug. It is big and heavy, has lots of redundancies, a heat shield, Super Dracos which are really a dead weight on cargo missions. And it can carry too small amount of fuel for any significant delta-V maneuver.
But it will exist in both crew and cargo variants. We can expect at least 12 flights, and probably much more. It will have IDSS for docking, standard on current ISS and future deep space gateway station. It has full GNC, solar panels, communication package that will be working in LEO. Since SpaceX has announced circumlunar mission already, we can assume that high velocity re-entry, thermal management, radiation and long range communications in cis-lunar space are also being addressed. So Dragon 2 is basically missing more powerful engine and a lot more delta-V (meaning propellant tanks). SuperDraco is not very efficient for in-space propulsion, while Draco is low power (400N). However, cluster of ten or more Dracos would be sufficient of in-space propulsion needs, giving ready made and tested solution, with high redundancy and acceleration between 0.1m/s and 1m/s.
But a Cargo Dragon 2 has 10m3 of internal space and 14m3 of external space that is mostly unused. There was (or still is) a plan for extended trunk with 34m3 of external space. By using this space for NTO/MMH tanks it would increase its mass for fuel capacity for 42t using 80% load factor. With additional structure, tanks and engines, the whole stack would approach 50t launch mass. With Isp of 300 of Draco engines that gives 5.5km/s of deltaV. Put more complex engine with larger exhaust bell, and Isp could rise above 330. Which gives Isp of 6km/s. This is enough performance for LEO to GEO transfer and back to GTO and reentry. It could dock a small payload in LEO, push it to Deep Space Gateway, dock there, come back and land. All that with one Falcon Heavy (for Dragon tug) and one F9 launch (for payload). It could be also used for moving light modules from ISS to DSG. Or deorbiting heavy LEO/MEO artifacts like spent stages and defunct satellites. Or moving failed GEO artifacts to graveyard orbits.
Major advantage of this approach is that SpaceX will have operational and partially reusable launchers (F9 and FH) and partially reusable spacecraft (Dragon 2). So on each mission SpaceX would spend (maybe) central stage, 2nd stage, Dragon trunk with tanks and solar panels. Plus most major components of the system would be already flight proven and tested. Thus it would reduce development cost to a minimum, using ready made components. Major risk would be associated with redesigning trunk to contain large tanks and transfer of fuel between internal and external (trunk) tanks. Unlike any other space tug (which would require some docking, fuel transfer and communication capabilities), major/expensive components of Dragon could perform EDL, and be refurnished and reused for subsequent missions. Unlike any satellite derived tug, it would not require in-space refueling. Instead Dragon tug can reentry and land.
If we estimate fully expendable Falcon Heavy price to 135 million USD and extended trunk to 20 million USD, this tug could go to DSG and back. Although NTO/MMH is on the expensive side (around 10USD/kg), total fuel cost would be less than a million USD. Therefore SpaceX could price reusable Dragon tug missions for less than 200 million USD. At that price point, it would probably be far cheaper than any other Vulcan/ACES or DSG resupply mission AND reusable. It would also enable "distributed launch" concept promoted by ULA at far lower price point. Unlike ACES, it could provide loiter time measured in months and possibly years. It could refuel most satellites directly from its tanks (since most satellites use NTO/MMH combination for the propulsion). And the best part is that most of the development needed to make this happen is already underway.
But it will exist in both crew and cargo variants. We can expect at least 12 flights, and probably much more. It will have IDSS for docking, standard on current ISS and future deep space gateway station. It has full GNC, solar panels, communication package that will be working in LEO. Since SpaceX has announced circumlunar mission already, we can assume that high velocity re-entry, thermal management, radiation and long range communications in cis-lunar space are also being addressed. So Dragon 2 is basically missing more powerful engine and a lot more delta-V (meaning propellant tanks). SuperDraco is not very efficient for in-space propulsion, while Draco is low power (400N). However, cluster of ten or more Dracos would be sufficient of in-space propulsion needs, giving ready made and tested solution, with high redundancy and acceleration between 0.1m/s and 1m/s.
But a Cargo Dragon 2 has 10m3 of internal space and 14m3 of external space that is mostly unused. There was (or still is) a plan for extended trunk with 34m3 of external space. By using this space for NTO/MMH tanks it would increase its mass for fuel capacity for 42t using 80% load factor. With additional structure, tanks and engines, the whole stack would approach 50t launch mass. With Isp of 300 of Draco engines that gives 5.5km/s of deltaV. Put more complex engine with larger exhaust bell, and Isp could rise above 330. Which gives Isp of 6km/s. This is enough performance for LEO to GEO transfer and back to GTO and reentry. It could dock a small payload in LEO, push it to Deep Space Gateway, dock there, come back and land. All that with one Falcon Heavy (for Dragon tug) and one F9 launch (for payload). It could be also used for moving light modules from ISS to DSG. Or deorbiting heavy LEO/MEO artifacts like spent stages and defunct satellites. Or moving failed GEO artifacts to graveyard orbits.
Major advantage of this approach is that SpaceX will have operational and partially reusable launchers (F9 and FH) and partially reusable spacecraft (Dragon 2). So on each mission SpaceX would spend (maybe) central stage, 2nd stage, Dragon trunk with tanks and solar panels. Plus most major components of the system would be already flight proven and tested. Thus it would reduce development cost to a minimum, using ready made components. Major risk would be associated with redesigning trunk to contain large tanks and transfer of fuel between internal and external (trunk) tanks. Unlike any other space tug (which would require some docking, fuel transfer and communication capabilities), major/expensive components of Dragon could perform EDL, and be refurnished and reused for subsequent missions. Unlike any satellite derived tug, it would not require in-space refueling. Instead Dragon tug can reentry and land.
If we estimate fully expendable Falcon Heavy price to 135 million USD and extended trunk to 20 million USD, this tug could go to DSG and back. Although NTO/MMH is on the expensive side (around 10USD/kg), total fuel cost would be less than a million USD. Therefore SpaceX could price reusable Dragon tug missions for less than 200 million USD. At that price point, it would probably be far cheaper than any other Vulcan/ACES or DSG resupply mission AND reusable. It would also enable "distributed launch" concept promoted by ULA at far lower price point. Unlike ACES, it could provide loiter time measured in months and possibly years. It could refuel most satellites directly from its tanks (since most satellites use NTO/MMH combination for the propulsion). And the best part is that most of the development needed to make this happen is already underway.
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