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Conservatism and innovation in spacecraft design
katodomo
post Oct 20 2014, 05:00 AM
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QUOTE (Doug M. @ May 6 2014, 09:41 PM) *
IKAROS was certainly innovative, but it's not exactly an innovation that's been seized on with enthusiasm. There was Nanosail D2, and then Sunjammer got cancelled, and there's crowdfunded Lunarsail which IMS is supposed to launch in late 2016, and then... well, that's about it. The major space agencies' attitude towards solar propulsion seems to be profound disinterest.

ESA and DLR still want to fly the Gossamer solar sails, it just gets constantly postponed. Gossamer-1 is currently planned for launch in 2016, was originally 2013. And iirc we'll see some deorbiting sail demos next year too.
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spacejunkie
post Jul 24 2015, 12:58 PM
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I've got a few comments and questions about common failure modes that limit spacecraft lifetimes and also the path towards improving spacecraft communication. If anybody would care to take on the questions I'd be grateful.
REACTION WHEELS:
Kepler has been a fantastic mission. It could have been even more fantastic had the reaction wheels not failed that curtailed its ability to point accurately.

Off of the top of my head reaction wheels failed on DAWN, Hubble, Hayabusa. It has seemingly been a very common limiting factor in the life of spacecraft. Many if not most of the failures have probably met their design parameters, but we are used to "over design" and spacecraft elements often lasting well beyond their designed lifetime. I know with Kepler and DAWN innovative engineering work arounds have mitigated these failures and allowed continued spacecraft functioning (although not optimal functioning).

I have a few questions surrounding this seemingly common spacecraft limiting failure mode.

1) It appears NASA should recognize it is actually designing missions for longer duration than the very conservative "prime" mission. In other words "extended missions" are almost expected. In recognition of that fact, shouldn't the design expectations of the most common failure modes also be increased? So, it might well be worth the cost to enhance specifications for the reaction wheels on robotic spacecraft in particular.

2) I'm sure there are newer more innovative designs perform the functions for which reaction wheels are intended. These might solve the problem entirely by reducing the probability of failure very significantly for not much increase in cost. Does anyone in the forum have the expertise to comment about such new technologies? I would be interested.
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SPACECRAFT COMMUNICATION:
Another historically problematic area has been antennas and communication. It has been obvious to me though that NASA has put in a lot of effort to have redundancy so that at least there will be a lower bandwidth ability to communicate if the main fails. I know now that NASA is working on advanced communication modes to increase bandwidth (e.g. laser communication). I've been a long time advocate of orbital/remote teleoperation of robots to perform asteroid, moon, and/or planetary exploration and development duties. It just seems that Ballard has already proven teleoperation is cost effective method of exploration with our nearest unknown (the deep ocean) and similar exploration would be possible off earth. Can anyone point me to any NASA documents or studies or even private ones that have more details of what is required technically in terms of communication to support such teleoperation? Or even more simply just NASA's goals in terms of improving the communication "network" between various spacecraft elements and ground stations?
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stevesliva
post Jul 24 2015, 05:07 PM
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QUOTE (spacejunkie @ Jul 24 2015, 07:58 AM) *
1) It appears NASA should recognize it is actually designing missions for longer duration than the very conservative "prime" mission. In other words "extended missions" are almost expected. In recognition of that fact, shouldn't the design expectations of the most common failure modes also be increased? So, it might well be worth the cost to enhance specifications for the reaction wheels on robotic spacecraft in particular.


Depending on the statistical distribution of mean-time-to-failure, you get extended life with redundancy rather than having to overdesign. Dawn had 33% spares (needed 3, has 4). If you want to throw money and mass at the problem, why not 100%? My wild-ass-guess on the reaction wheel thing is that the number of failures early in life is far higher than expected. They expected <25% (1 in 4), obviously.
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JRehling
post Jul 24 2015, 06:01 PM
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I spent quite some time analyzing Kepler data and found an interesting story there about how the noise of the instrument was higher (in certain ways) than expected. This complicated the analysis enormously because the intended use of Kepler was to look for very fine variations in signal. Some of my discussion of this is here:

http://www.spacedaily.com/reports/Kepler_F...Earths_999.html

In brief, certain portions of the detector surface had more noise than other portions, and this varied with time because it varied with temperature, and the spacecraft performed a 90 roll about every three months. Therefore, some noise systematically resembled sub-Neptune-sized eclipsing planets with a period of about a year horrifically, exactly the kinds of real objects that Kepler was intended to find!

On a high level, Kepler was a very ambitious mission, and it pushed the limits of technical capabilities because it had such an ambitious goal. There's a payoff matrix whereby it's good to be a little ambitious, but devastating consequences if the goals are overly ambitious and the technology can't deliver. If they hadn't been at least a little ambitious, the mission might not have been approved at all. So it's a happy success story, but it was slightly perilous on technical grounds alone.
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