NASA Doesn't Want To Talk About A Messy Dragon
A SpaceX Delivery Capsule May Be Contaminating The ISS, Wired
“Part of the problem here, though, is NASA’s reluctance to talk about both the problem and the plans to fix it. The presentation, shared during the Payload Operations Integration Working Group meeting back in April, was approved for unclassified and unlimited public release and placed on the NASA Technical Reports Server in early September. I asked for an interview about it on September 25. The next day, the presentation was gone. “The record details page you tried to access cannot be found on this server,” the page now says. I inquired about the dead link, and more than three weeks later, I received a response: “The document is under review,” wrote Meagan Storey, of the NASA Scientific and Technical Information Program, “and we advise that you make a FOIA request for the item.” Statistically, that’s probably a losing prospect.”
It would be interesting to know how other cargo carriers and Soyuz compare on this and what requirements NASA levies on contamination.
That would be interesting, and why I agree with the author about how this sort of thing should be more transparent. But the transport process, from the source to the sensor, could easily make much more of a difference than the vehicle. With only one sensor, at one location, Dragon could be the cleanest spacecraft to visit ISS and just in the wrong place relative to the sensor. Without more sensors, I don’t see how anyone could know.
Let’s the review this,
The experiment was placed close enough to visiting vehicles that contamination was possible. VV”s can produce nasty things such as thruster combustion products even during normal operation.
Why?
Presumably, NASA had a veto over the paint or coating SpaceX used on Dragon 1, but outgassing wasn’t discovered until a more sensitive instrument was present at ISS. Now they’re working on a replacemant coating.
The main question I have is why wasn’t this detected during a thermal vacuum test? Was outgassing even checked for? If not, why not? Was it an issue of instrument sensitivity, such as the one on ISS?
Also, Dragon 1 only has a few more launches before Dragon 2 takes over under the CRS2 program. Does Dragon 2/Crew Dragon use a different exterior coating? Is it the same as the new coating for Dragon 1?
We don’t know paint was the problem. That’s a speculation by an outside expert on outgassing. He didn’t have access to all the data, so he could be wrong. In fact, he thought it was odd that the team seemed to be worrying about things he wouldn’t worry about, and not worrying about things he would worry about. To me, that just says how complicated a problem this is; the experts can’t even agree on what the important issues are.
As far as thermal vacuum is concerned, those chambers don’t provide a very good or a very clean vacuum. That isn’t what they’re designed for. I’ve never heard of anyone trying to measure outgassing in one, and I’m not sure it would be possible to notice it over the rest gas.
And, in terms of thruster plumes, the instrument has a cover (transparent, so they can still observe through it, with some loss of sensitivity) which they close when a spacecraft is around. The engineering sensor measuring the contamination is there, as I understand it, to let the team know how long they need to keep the cover closed, and tell them whether or not they needed that cover in the first place. (A useful thing to know when and if they build a SAGE IV.)
The article was a bit strange. It seemed to me like it was taking swipes at SpaceX because it’s a profit making company.
I think it’s important to realize that the SAGE III team does not have a problem, and they specifically said that to the author of this article. In fact, the article says NASA polled everyone with instruments on the outside of ISS and no one reported a problem.
The sensor which measured the contamination is _not_ the SAGE III science instrument. It’s an extra sensor, added to SAGE III as an engineering diagnostic. Since they were concerned about contamination, they did something smart but relatively uncommon: They added a sensor to measure contamination. From the sound of it, this is the only sensor on ISS capable of making such measurements.
It’s also worth mentioning that this is a extremely difficult problem to model. Those gas molecules aren’t colliding with each other they way they do at sea level. They are basically moving in a straight line until they hit something, then they either bounce or stick to the surface. That’s an oversimplification, but it means all sorts of weird things can happen, and getting the answer right involves knowing all about the gas (how volatile is it?) and the composition and temperature of every surface between the source and the location of interest. It’s easy to get an answer wrong by a factor of ten, just messing up one subtle detail.
One usual way to deal with that, and the one they seem to have used in this case, is to impose a requirement which is _extremely_ conservative. So much so that the contamination could be orders of magnitude worse than estimated, and it still wouldn’t be a significant problem. Then you can be sure you don’t have to worry about it. That drives up cost, but so would the person-years of analysis required to make a reasonably accurate estimate.
The funny part is that, quite often, once they decide to do this, they do stop worrying about it. And don’t actually do anything to measure how bad it is, or whether it’s a complete non-issue. When they do, the results are often a surprise (sometimes good, sometimes bad.) Rosetta was one example for outgassing and contamination. The first webcam inside a payload faring on a rocket was another (there is a terrifying amount of particulate material and, well, junk, flying around during launch…)
From the text of this article, I think they’ve discovered that outgassing and contamination can be as much as a hundred times worse than the requirement and _still_ not be a noticeable problem.
Someone at NSF mentioned the possibility of water vapor being absorbed into Dragon’s SPAM thermal coating, then slowly outgassing on orbit.
Hmmmm. ..
Hmmmm? Someone mentioned a possibility and you think that means anything? There are tons of possibilities and someone mentioning one doesn’t say much. I’m not sure what your point was.