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One of the roles that Progress (and ATV) fulfill at the ISS is station reboost.

This works because they can dock at the back of the Zvezda module, and boost inline with the station. I cannot find a value for the thrust level of the main engine, but it suffices to boost the station.

A Dragon currently berths to the CBM ports on the bottom (or top as backup) of the Harmony (node 2) module on the US side, and is thus perpendicular to the main line of the station.

But a Dragon V2 will be docking to the PMA on the front end of Harmony (or the PMA which is due to be moved to the top of the Harmony node, and the backup CBM port will be the bottom of Unity, as the MPLM berthed there is due to be moved. See the answer to "Will the ISS need more docking ports" question for more info and images of how the re-arrangement will look).

The front end of Harmony is basically the opposite end from where ATV and Soyuz currently do their reboost tasks.

So will Dragon or Dragon V2 be able provide station reboost if needed?

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I shall answer myself, since in wondering and writing about it, I think I know the answer.

Probably not. The issue is that a Dragon capsule (Cargo version) only has Draco thrusters for maneuvering. These have a thrust of 90 lbs. That is just too little to make much of a difference. Now if they could thrust for hours, sure that would help, but of course there is a fairly limited supply of fuel for each Draco on the Dragon so it is very unlikely.

Now Dragon V2 is a slightly different vehicle and will have 8 Super Draco engines, with a thrust closer to 16,400 lbs. That starts to be enough thrust to be useful, but again it is fuel constrained, since they are intended to be used for powered landing, and depleting that fuel supply before landing seems like a generically bad plan.

But even more so, the Dragon and Dragon V2 will have a trunk section attached that has the solar panels. (V1 has extendable panels, V2 looks to have them flat on the surface of the trunk itself, an interesting concept, which begs the question of why the change!)

The Draco engines can fire around the trunk, after all that is how it maneuvers in orbit and for docking/berthing. But the Super Dracos are meant to thrust after the trunk is discarded. So the trunk is likely 'in the way'.

It is interesting that while Dragon can fulfill upmass and downmass requirements, there is still a hole in the US capacity to reboost the station. (Which begs the question, could Dream Chaser or CST-100 reboost the station. Dream Chaser, no, since it docks tail end first to the station, which is where its engines are. CST-100? A good question).

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  • $\begingroup$ But if the service module is in the way for the Super Draco engines, how could it maneuver in space? The sidewall angle where the engines are built in, is 15 degrees. Wouldn't that be enough to stay clear of the service module? And maybe it explains why they skip the extendable solar panels. It would seem to be a strange mistake for NASA to not require the ability to boost the station. $\endgroup$
    – LocalFluff
    Oct 5, 2014 at 16:34
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    $\begingroup$ Super Dracos are NOT used for in space manuevering. They are used for launch abort, and if not used to abort, for powered landing. Regular Draco's are meant for use in space. $\endgroup$
    – geoffc
    Oct 5, 2014 at 18:25
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    $\begingroup$ Personally, I don't think NASA made a mistake not requiring station-boost ability. The geometry of the station means only the Zvezda module is suitable for boosting. Boost any other place, and you risk adding angular momentum and/or shear stress on module couplings. Besides, now that the Russians are no longer receiving the 'taxi fare' for cargo (and soon with Dragon 2, passenger) deliveries, we don't want them to also lose income from boost missions, right? :) $\endgroup$
    – pepoluan
    Oct 7, 2014 at 9:55
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In addition to Geoff's answer:

There was no requirement for the CRS spacecraft to provide ISS reboost. The two reboost methods available now (Progress, and the ISS' own engines on Zvezda) were seen as sufficient. NASA was working on a third method: they wanted to install a VASIMR ion engine to test it for reboost purposes. This plan was canceled in 2015.

The SuperDraco is too powerful. Reboost is done at lower thrust levels to limit the forces acting on the station.

Progress is connected to the station's guidance system during the reboost. Dragon doesn't have this capability.

A Dragon reboost from the front end of Harmony would require that the station be turned through 180º (twice), which is doable but increases the station's exposure to micrometeoroids.

A reboost from Cargo Dragon's current berth position would place bending forces on the docking port. This might be an issue, but something similar happened when the Shuttle was used for reboost.

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If NASA needed to reboost ISS a Dragon could be fitted with a propulsive trunk. 4/8 Super Dracos mounted on its sidewalls & fuel tanks in the unpressurised cargo space.

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    $\begingroup$ Where would you berth/dock such a souped up Dragon on the ISS, that could handle the structural load a 16 Klbs thrust (asuming you fired on Super Draco at a time)? Seems more likely you would tear the station apart trying. CBM port or PMA are the options, and the currently available CBM ports are perpendicular to the axis of the station. $\endgroup$
    – geoffc
    Dec 19, 2014 at 12:22

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