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In an answer to a related question, Jörg W Mittag wrote the following:

But more importantly, handling of the Superdracos is currently rather simple. They are safed after reaching orbit and then never turned on again. This means that there is no possible way in which they could accidentally fire in the vicinity of the ISS, for example, or during recovery operations. Otherwise, all of that would have to be certified, too. (The ISS guys are understandably very peculiar about what can be fired with how much thrust in which direction while in the vicinity of their station.)

Now I can't stop thinking about using the SuperDracos to reboost the ISS. Given that there's this chunk of delta-v aboard the Crew Dragon that costs good money to be carried all the way to orbit, it'd be great if one could put it to a good use once launch escape is no longer a concern.

Alternatively, could the small Draco thrusters be used instead? They do have a higher Isp (300 s) than the SuperDracos (235 s), but they would have to burn for minutes to make a difference for the entire 400-ton ISS.

I assume that there are currently no plans for that, with the likely reason being that certifying the Crew Dragon for ISS reboosts would be too expensive. However, it still feels nice to use this free delta-v somehow, thus freeing up some upmass capacity on the cargo resupply flights. Also, having a backup reboost mechanism feels useful as well.

Certification costs aside, would it be possible to use the Crew Dragon to re-boost the ISS, without requiring a massive redesign of the capsule, the docking mechanism, or the ISS? (Of course some flight software development for the Dragon is unavoidable.)

TooTea
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  • Delta-v wise it works out to 7.48m/s when the Crew Dragon were to be fully fueled. mass of crew Dragon is 8.9tons. Fuel capacity 1388kg when full. Isp is 235s. Mass of the ISS is 419,700kg, give or take. $\Delta v = 235s \cdot g_0 \cdot ln(\frac{419.7 + 8.9}{419.7 + 8.9 - 1.388}) = 7.48m/s$, so it definitely has enough punch, most re-boosts I could found were in the ballpark of 1.5m/s. – Polygnome Aug 04 '20 at 13:10
  • I'd be a bit surprised if the Dragon's main thrust vector is anywhere near the ISS's center of mass, meaning that it would be difficult to avoid imparting spin – Carl Witthoft Aug 05 '20 at 12:13
  • related: Can Dragon, or Dragon V2 be used to boost the ISS orbit? – uhoh Feb 09 '22 at 21:59

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Theoretically, it could, but why use the larger, more expensive rocket engines when you could just use a less expensive, smaller draco engines?

Super Dracos are off axis, meaning that any thrust from them will be less efficient than if they were directly in the desired direction. They burn real quick, which means more force that the station has to absorb, and less accuracy to the final burn.

The truth is, I doubt this would ever really be used, however. If it does happen, it will be with the cargo variant, and using the small Draco thrusters, not the larger Super Draco variant.

PearsonArtPhoto
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    Another good thing about the small Dracos is their higher Isp (300 s vs 235 for the SuperDracos). However, they're only 400 N each, so they'd have to burn for like 5 minutes to give the whole ISS the necessary ~1.5 m/s. Good point about off-axis cosine losses. The whole point about using the Crew variant is that is (presumably) carries extra fuel for aborts, doesn't it? – TooTea Aug 04 '20 at 13:27
  • Doesn't really matter the number of them, only the efficiency, and the amount of time they can maintain a burn. – PearsonArtPhoto Aug 04 '20 at 13:29
  • I have now edited the question to include the Draco thrusters as well. My original intention was about the Crew Dragon as a whole anyway, not a particular engine. – TooTea Aug 04 '20 at 13:40
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    +1. Progress spacecraft usually takes several minutes for ISS reboost burn, with acceleration about several mm/s^2. I would not be surprised if SuperDraco engines are too powerful and will violate some restrictions for equipment or experiments on ISS. – Heopps Aug 04 '20 at 14:01
  • @Heopps: I wouldn't be surprised if it just going to rip the entire ISS segment clean off. The SuperDracos are probably 150x more powerful than the Dracos. (They said the reentry burn, which currently takes 11 minutes, would take 5s with the SuperDracos.) It's a Launch Escape System, so it's designed to accelerate the fully-loaded vehicle to the limits of what humans can withstand. – Jörg W Mittag Aug 04 '20 at 15:19
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    @PearsonArtPhoto: The Dracos are also off-axis. The only 4 that are on-axis are the forward bulkhead thrusters under the nose cone, which would not be a good idea to fire while docked. – Jörg W Mittag Aug 04 '20 at 15:22
  • This is true. I don't know what the exact off angle is, but... – PearsonArtPhoto Aug 04 '20 at 15:34
  • So, what is the reason they don't do this (say, with Cargo Dragon and Draco thrusters)? Is it because it would use up too much of the delta-v margin? Are there some structural issues? Or is it just that SpaceX was not contracted to do that and doesn't have certification? – Frax Aug 06 '21 at 08:51