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Posted by: stevesliva Sep 2 2020, 08:41 PM

From fall 2020 OPAG https://www.lpi.usra.edu/opag/meetings/opag2020fall/

Persephone:
https://www.lpi.usra.edu/opag/meetings/opag2020fall/presentations/Howett_6005.pdf

There is talk of extended missions out to 2079. Building cathedrals for our grandkids here...

Posted by: Explorer1 Sep 3 2020, 03:25 AM

The Voyagers have lasted that long, we know.
Slide 40 (Data volume) is particularly poignant and rams home the scale, in time and space, of these endeavors: 584 megabytes of data over a generation; that's not even an hour episode of a TV show today. How much smaller will that seem by the 2070s? I cannot imagine.

Posted by: volcanopele Sep 3 2020, 05:45 AM

I think that's supposed to be 584 Gb over the course of the Pluto tour which seems pretty good given the distance.

Posted by: Explorer1 Sep 3 2020, 06:42 AM

Yes, I missed the digits. Still pretty modest by the next half century's standards, I would guess.

Posted by: vjkane Sep 3 2020, 02:53 PM

Biggest problem that I see is finding the plutonium for 5 RTGs. I don't believe that the production rate would allow that in the next decade. And we probably are in game of which mission gets the Pu: Ice giants orbiter, Pluto mission, Enceladus lander, Europa lander, something else?

Posted by: Steve G Sep 8 2020, 10:04 PM

About that 27 years trajectory . . . That puts a lot of lifetimes to the wayside when it arrives. With a launch ten years from now, new and better rockets may be available, such as StaceX's Starship should it deliver to expectations, or even Blue Origin's New Armstrong (should it be ever come to pass). If launch costs are reduced by the new (reusable) space rockets along with the massive payloads they are advertising, you could conceivably launch it really fast with loaded with tons of fuel and jam on the breaks hard. That would cut trajectory significantly. Based on the cost per flight, SLS may not even be around by then.

Posted by: Steve G Sep 8 2020, 10:07 PM

Posted in error

Posted by: Explorer1 Sep 8 2020, 10:28 PM

The trouble is, what sort of fuel will last long enough on the journey to still be available for a Pluto orbit insertion? Cryogenic fuels like liquid hydrogen tend to evaporate, don't they? Would solids be better (like for the Mars Ascent Vehicle being planned?)

Posted by: Steve G Sep 9 2020, 01:47 PM

Cassini used a Bipropellant system- Nitrogen Tetroxide (NTO)/Monomethylhydrazine (MMH). Solids would be more reliable, but less flexible.

Posted by: HSchirmer Sep 9 2020, 04:16 PM

IIRC, ESA has been looking at hydrogen peroxide monopropellant for attitude control thrusters; and New Shepherd used hydrogen peroxide and kerosene for their oxidizer and fuel. So some propellant combinations could do double duty.

With RTG supplying electric , more likely an ion drive for long term course correction, and reaction wheels for fine attitude control. Would need a battery to store power for fast slewing at encounter, so you might still need thrusters as a backup if the reaction wheels failed.

IIRC some papers found that eddy currents from solar storm could cause arcing across the bearings that chews them up and causes failure; but until somebody perfects and proves ceramic bearings work on reaction wheels, you'll have redundant systems.

Posted by: stevesliva Sep 9 2020, 04:39 PM

I wanted to see if you recalled correctly, because I myself could not recollect this:
https://hackaday.com/2018/09/11/do-space-probes-fail-because-of-space-weather/

Posted by: mcaplinger Sep 9 2020, 04:45 PM

Persephone already uses electric propulsion, though the viewgraphs are not terribly clear about exactly how the orbit insertion works -- it takes almost 10 years between the KBO flyby and arrival at Pluto. If you want really fast transit times, electric probably won't help. But you are talking a lot more launch vehicle delta-V than anything available now.

Solids have no particular advantages over storable biprops. A solid motor was used for Magellan; it worked but would not be anyone's first choice.

Posted by: HSchirmer Sep 9 2020, 04:54 PM

In 2013, they figured launch shaking or radiation.


By September 2017 they had research and published a paper


About a year later that led to a Youtube episode on reaction wheels that has about 700,000 views.



Time to put up some cubesats for small scale testing of different types of bearings.

Posted by: mcaplinger Sep 9 2020, 05:08 PM

There's no consensus that this has anything to do with the failures as far as I know. But Ithaco has started using ceramic bearings so maybe that will help if this was the root cause.

Note that Persephone would have 5 reaction wheels total, 2 for sparing.

Posted by: HSchirmer Sep 9 2020, 05:27 PM

Technically radiation and magnetic/electric fields.

Corrected above, added the reaction wheel company's September 2017 "tech service bulletin."

Posted by: stevesliva Sep 9 2020, 07:40 PM

Would be a worthwhile use of the space station to bolt two (or any multiple of 2) of these new improved reaction wheels to the outside, and have them apply opposite forces and stress them for a few years, starting as soon as they're ready. If they fail, could actually do PFA. Probably can't isolate them enough. Oh well.

We're headed for Chit-Chat on this one.

Posted by: HSchirmer Sep 9 2020, 08:06 PM

Eh, whether it's smart to use reaction wheels on a Persephone multi-year missions is on topic.

I like the "hard drives as cubesat reaction wheels" as a possible 2-for-1 (data storage AND attitude control)
Take a couple old hard drives, put in new test bearings, (metal, diamond coated metal, ceramic, magnetic) put them in a RAID array and run to failure while writing the test results to the disk.

Posted by: JRehling Sep 11 2020, 06:42 PM

The long timelines for this mission suggest several obstacles beyond those of engineering. When a mission returns science only decades after it launches, one has to consider what the payoff is for the individuals and organizations involved. The true economics of space exploration has always been a little fuzzy – What makes a mission worth a certain amount of cash compared to another mission for another amount of cash? I think a mission like this pushes it beyond the breaking point of philosophy; every person involved with it would be making an investment that would not be repaid. The tenure-track professors would either have their tenure or not. The agency administrators would have moved on. Many people present at the beginning would literally be dead before the main mission commences. And if the agency depends on public goodwill for its sustained support, this is an up-front sink that might do real damage in terms of the opportunity cost. And all of that is to say nothing of the lack of continuity in the operations team and the possibility that future technology could "pass" the probe that's on the long way there and make it possible to get the same science sooner or cheaper or possibly sooner and cheaper.

I think a proposal like this is DOA out of pragmatic considerations. It's not that the science couldn't be lovely and the thought of altruistically giving the gift of that science to future generations is nice and hopeful, but for the price tag, it can't possibly beat out missions that would get some science back in the one-decade timeframe or less.

Posted by: Steve G Sep 11 2020, 07:19 PM

I agree! My whole point at the beginning of this discussion, is that new low-cost SpaceX and Blue Origin launch services are game changers. For years we have minimised fuel and weight penalties and took years just to get into orbit around Mercury. Just look at BepiColombo's journey. Having the luxury of massive propellant modules will shorten trajectories, which has a cost reduction right there. Also, the emergence of cube sats offers other opportunities. Look at Rocket Lab's planned Venus mission. We'll know in the next few years how the new generation of super rockets pan out.

Posted by: HSchirmer Sep 12 2020, 02:43 AM

There is no paradox. Launch now based on a classic Hohmann transfer orbit.
When (not if) we do eventually develop exponentially better rockets that have torchship levels of specific impulse, we send a "helper" rocket out to dock with the probe. Then blasts along on a brachistochrone trajectory-full throttle acceleration to the half way point, flip 180', full throttle deceleration until the destination.

The next step for space exploration should be to systematically refurbish the defunct US & Russian "Keyhole" class optical spy satellites that are optimized for imaging at 250km, and put them into 250km orbits around Mercury, Venus, the Moon, Mars, large asteroids, Jupiter, Saturn, Uranus Neptune and Pluto. Every planet large circular body orbiting the sun, whether or not it has cleared it's orbit should have a re-purposed spy satellite mapping it at 6 centimeter resolution.

Posted by: stevesliva Sep 12 2020, 02:55 AM

Well... the Pluto orbit mission concludes 30 years after launch in 2030, after 3 years in orbit. The additional 20 would probably be the point of substantial debate. Cassini lasted 20 years after launch.

Launch after 2032 means Jupiter's no longer in the right place... for Pluto. Makes me wonder where else direct-to-Jupiter (<1yr) + 26 yrs of electric propulsion can put you.