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In the question: "Can Dragon, or Dragon V2 be used to boost the ISS orbit?" I asked if the Dragon V1/V2 can provide station reboost functionality that is currently provided by the ATV or Progress cargo vehicles docked to the back end of the Zvezda module.

I am pretty sure Dream Chaser would not be able to, since its two engines flank its airlock/docking port, and it back up to the station and docks tail first. Thus its engines are blocked.

But how about the Boeing CST-100? Can it provide sufficient thrust, when docked to the front of Harmony on the PMA for station reboost?

Perhaps a necessary precursor question is, can the LIDS/NDS docking system attachment, connected to the PMA, connected to the CBM port on the Harmony module take the thrust level needed for station reboost? One assumes it should, since the Shuttle provided some level of station reboost while docked to that port. Though because of the geometry of the connection, I wonder if the Shuttle did not apply a very large amount of thrust. Perhaps there is a limit.

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    $\begingroup$ While this is not an answer to the question that you asked, I think that Ad Astra (company) is planning to test a VASIMR (electrically-powered thruster) engine on the ISS in another year or so. If that test is successful, it might allow NASA to avoid needing the periodic re-boost from visiting vehicles. $\endgroup$ – Kirkaiya Oct 6 '14 at 23:22
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    $\begingroup$ The VASIMIR they are proposing is high power (about as much as the station can provide) and low thrust, so ya, low thrust forever is nice, but destroys Zero-g research still always under thrust, but also, you run out of Zenon or whatever working fluid they use. $\endgroup$ – geoffc Oct 6 '14 at 23:23
  • $\begingroup$ I think the idea for the VASIMIR, if it was going to be used for actual station-keeping (ie, not for just the test), is that a bank of capacitors would be trickle-charged, and it would then be used at max thrust for short periods of time. How much it would reduce reliance on periodic re-boosts is something we could figure out (from published specs on the VASIMR, mass of the station, etc), but I'm too lazy to do it now ;-) $\endgroup$ – Kirkaiya Oct 6 '14 at 23:44
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    $\begingroup$ Additionally, as someone pointed out on nasaspaceflight.com forums, the reason the ISS needs re-boosting is that it's not in zero-g, due to the drag with the upper atmosphere. So a low-powered thruster (and it would be very, very low thrust) continually-operating would, ideally, restore zero-G by negating the acceleration due to drag. $\endgroup$ – Kirkaiya Oct 6 '14 at 23:49
  • $\begingroup$ This answer says that "Anything over a couple of thousand Newtons should have a measureable effect." I don't know the thrust of the CST-100, which seems about the only missing parameter for this question to be a duplicate of that one. $\endgroup$ – a CVn Oct 10 '14 at 13:04
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The answer is yes, it can used to re-boost the ISS.

According to the information in spaceflightinsider.com and in rocket.com Aerojet Rockedyne site in the RCS engines paragraph, in a declaration of Terry Lorier, Aerojet Rocketdyne’s CST-100 service module propulsion system program manager, are confirmed the capabilities of Boeing CST-100 Starliner spacecraft.

Propulsion system of CST-100 includes different types of engine thrusters with different roles. In spaceflightnow.com are explained what includes the propulsion system.

CST-100 has four 39700-pound thrust (176.6 kN) launch abort engine (LAE) for the CST-100’s ; 24 orbital maneuvering and attitude control thrusters (OMAC) 1500 pounds of thrust (6.67kN) each, for launch abort attitude control, stage separation functions, large orbital maneuvers.

Also are included 28 reaction control system engines (RCS), each of them with 100 pound thrust (0.44 kN) using similar technology to other engines in CST-100. RCS role is to re-boost international space station (ISS). Let say that even the above engines the bigger ones, could be used sometimes to help for re-boosting faster the ISS if there is enough fuel in their respective tanks. But with or without them, the RCS can accomplish the re-boosting procedure for ISS.

This hasn’t to do with your question but I see that you have written Dream Chaser would not be able to re-boost. Actually in sncspace.com Sierra Nevada Corporation (SNC) site and in this article of spaceflightinsider.com, is confirmed this capability. In the official site of SNC is included also the info for cargo and crew Dream Chaser similarities and differences that you have asked in another question. Anyway for the part “engines are blocked”, it could be if we are speaking for the crew version. But at least for cargo version, now I am just supposing that since info said that can re-boost, probably this could happen because in spacecraft is attached the cargo module, and when will be attached to ISS distance between Dream Chaser and ISS module will be considerably bigger and its engines could be lower as level. Probably Dream Chaser cargo and crew roles could be the same as Progress and Soyuz spacecraft situation for re-boosting procedure part.

Anyway current NASA’s request for the role of this spacecrafts CST-100 and Dream Chaser, is to accomplish crew and cargo missions respectively, but if sometimes it will be necessary to re-boost ISS each company of this two spacecrafts has confirmed this capability.

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