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Neptune Orbiter, Another proposed mission
nprev
post Apr 6 2007, 04:20 AM
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I might be off-base here entirely, but based on the discussion & Helvick's usual incredible technical insight it seems as if it might be possible to design microthrusters with low propellant consumption that have very high specific impulses which could propel microsat payloads (enabled by quantum leaps in electronics technology) to outer system targets within reasonable timeframes with consequent very significant cost savings.

Such devices might also find a considerable commercial market for boost to GEO from LEO as well as station-keeping applications. With respect to the latter, propellant consumption is now the chief limiting factor for satellite longevity due to improvements in payload/bus electronics reliability, so obviously increasing this margin would have a tremendous positive effect.


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djellison
post Apr 6 2007, 08:30 AM
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Where are you going to get the power from...AND....where do you put the instruments - not seen a single mention of those yet in terms of mass, volume or power. I've not seen anyone, realistically, add up the figures here yet - just immediately jumping to "well - if the tank can be only 10cm then the whole thing will be 50 pence and return amazing science". You're jumping about 25 steps too far forward. You're talking about MRO's X-Band. That's 100 Watts. Where are you going to get 100 Watts from at Neptunian distances? That's just about 1/2 of NH's power budget gone right there. The mass of that power supply alone is going to be an order of magnitude above what you're talking about. So your delta V is going to be an order of magnitude less.

If you're going to do the outer solar system, as small, light and cheap as possible - the only logical place to start is New Horizons, the most advances, lightest, cheapest outer planetery spacecraft to date, and work from there.

Doug
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edstrick
post Apr 6 2007, 09:06 AM
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(evil grin)
I got it all figured out...
A dilithium crystal ore body is what's powering all the thermal activity and plumes on Enceladus.
We go there and mine it so we can explore the rest of the solar system.
(you can shoot me now)
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helvick
post Apr 6 2007, 11:23 AM
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Doug,

Just to be clear - I don't think any of this is possible today. What I'm trying to work out for myself is whether this is closer to reality or fiction and whether R&D efforts aimed at maturing these types of technologies would be worthwhile or not. Certainly any deep space probe that is going to be planned and built today or in the near term (5 years) is going to be based on proven technologies that are mature on the ground today and will result in something in the 100-2500kg mass range.

This works but it results in having rare (once per decade) and very expensive missions and more importantly (to me) it is very inefficient at developing\maintaining general purpose solar system exploration capabilities. The reliance on mega budgets and the rarity of missions means that there is always a cancellation risk and a single cancellation can result in an entire generation of scientists missing out on any significant deep space exploration experience. That is why Algorimancer's suggestion appeals to me - if it could be done then it could change the economics of this endeavour dramatically.

I think you are not being totally fair about my arm waving numbers. smile.gif You are right though that there should be less arm waving and more presenting things with precision so they can be properly judged and torn apart if they are wrong.

However they aren't as weak as you describe - let me re-state my telecoms example since you specifically call that out.
I used MRO's X-Band system as a baseline and then worked out from that what the practical bit rate for a highly power constrained transmitter around Neptune would be. I used their published data rates because that allows me to work out the comms performance of the DSN without knowing anything about how the DSN itself performs.
I then confirmed that my method made sense by applying the same transformations to the New Horizons comms design and came up with a number (~300bps) that agrees with the number published by the NH team for their bit rate at Pluto.
So I am certain that a 1 watt Ka-Band transmitter with a fairly small antenna (1m parabolic) at Neptune could communicate with the DSN at 50bps today if we could put it there. That is poor but it is as good if not better than the crippled rate Gallileo had to work with.
Edited to add: Here's a quick spreadsheet with the two sets of comms rate estimates. Also available in XLS format for those who do want to tear it apart.
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algorimancer
post Apr 6 2007, 02:22 PM
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QUOTE (Greg Hullender @ Apr 5 2007, 11:07 PM) *
Algorimancer and nprev: What am I missing here? Why is it desirable to scale the ion engine down below 10W? ...

We've been discussing developing a moderately competent spacecraft capable of exploring the outer solar system which masses on the order of a few kilograms. This implies low power availability, as well as a few other challenges w.r.t. propulsion and communication. Current ion engines have good performance, but require too much power and are too massive for a vehicle of this scale. Otherwise they're great smile.gif

Doug, I'm in agreement with helvic in that the motivation for this discussion is a transformational change in the way we develop spacecraft, which would make deep space exploration trivially inexpensive and accessible. I've been attending a lot of biomechanics/bioengineering conferences these past few years where I've seen a lot of the current work in the MEMS/Microfluidics realm, and I am genuinely convinced that these capabilities are within reach. I think that if we could motivate the right group of people working with this technology (or if we could come up with the money to fund them) we could be manufacturing these sorts of vehicles in the next five years, and while we might not send a CubeSat to the outer solar system, we could certainly use it as a container vehicle during the launch. There are still details to be sorted-out (obviously) before we're in a position to firm-up specific masses for the various components, but considering the capabilities of my Windows Mobile driven cell phone, I don't see a problem with including such instruments as a several megapixel (telephoto even) camera and particles/fields instruments within the mass constraints. It appears thus far that hardware scales linearly, while performance (delta-v capability) remains constant.

I'm looking forward to working on the spacecraft software smile.gif
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centsworth_II
post Apr 6 2007, 02:47 PM
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Sorry I'm late, but I've got to know:
While waiting on new technologies
(and a budget) for a Neptune orbiter,
why not send NH2 on a flyby?

I can't think of another mission that the
craft would be better suited for. They aren't
going to let it just rot on the shelf, are they?
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djellison
post Apr 6 2007, 03:02 PM
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QUOTE (algorimancer @ Apr 6 2007, 03:22 PM) *
don't see a problem with including such instruments as a several megapixel (telephoto even) camera and particles/fields instruments within the mass constraints


QUOTE
So I am certain that a 1 watt Ka-Band transmitter with a fairly small antenna (1m parabolic) at Neptune could communicate with the DSN at 50bps today if we could put it there


4 megapixels, R, G, B, 12 bits, 20:1 compression. 40 hours of downlink. I see a problem.

Lets talk local - no one's put multiple megapixels on any cubesat in LEO, when you've got (comparatively speak) orders of magnitude more power and downlink that what you speak of.

Helvick - your 650kbps - is that for the 100w Xband or the 35 Ka Band on MRO? Are you scalling apples with apples here? And have you visited http://descanso.jpl.nasa.gov/DPSummary/sum...orce_external=0 - it says that at Max range, the Ka band can actually manage only 331kbps, and the Xband, just over 500kbps ( so 600 seems reasonable ) - That document also tells us that the 35W Ka band on MRO requires 81W of actual spacecraft power. The 100W Xband, 172W of actual power - your 1W Ka band is now looking at more like 1.5 watts of consumed power, with a downlink of 25bps. That one colour frame now takes 80 hours.

I grant you - something smaller than NH is possible with near-future technology....but in the Microsat ( 50-100kg 50-100W ) bracket, not the multiples of cubesat ( 1-3kg and a few watts ). Pick a target of 100kg and then decide what power you can get, by what means, from that. Even then you're talking about technology found in PDF's - not in ATLO. The Stirling RTG is something like 34kg, for 110W - that would be my starting point.

Doug
(PS - NH2 was proposed by Alan et.al. http://www.boulder.swri.edu/pkb/NH2_community_info.ppt - but sadly turned down sad.gif A real pity as the Voyager/Pioneer/Viking/MER/Mariner syndrome of getting two for less than the price of two would have kicked in here )
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Greg Hullender
post Apr 6 2007, 08:23 PM
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When I first read a proposal for "Grand Tour," which later became Voyager, one of the options was to use a laser for the comm link instead of radio. The biggest objection was that you had extra cost because you'd need a satellite on this end to receive the laser and then beam it to the ground with regular radio waves.

Forty years later, I never seem to see this discussed. Is there some other reason that makes it nonviable? I'd think we could put a receiving satellite in Geosynchronous orbit and design it to cope with up to a half-dozen missions at once. Would that really be so expensive amortized over a few missions?

Note: I'm NOT proposing running a really long fiber-optic cable out there! :-)

On a separate note, I just discovered this report on NASA's advanced propulsion efforts.

http://books.nap.edu/execsumm_pdf/11432.pdf

I thought it was very apropos, and not too out of date (copyrights between 2004 and 2007).

--Greg
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nprev
post Apr 6 2007, 10:28 PM
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The cancelled Mars Telecommunications Orbiter would have tried to demonstrate lasercomm DTE...think that's looking more like a real loss in terms of technology development every day.

Every so often I drag up that idea I had (which was actually first described in the 1930s) for putting DSN-augmenting data relay sats at the Earth/Sun L4 & L5 points. Maybe we need another set at Jupiter's Lagrange points for outer system support. Although that doesn't shorten the range all that much, at least we can avoid some significant atmospheric attenuation...plus, if they happen to carry a couple of cameras to check out any Trojan asteroids they may pass near from time to time, that'd be cool, too... wink.gif


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JRehling
post Apr 6 2007, 10:42 PM
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QUOTE (nprev @ Apr 6 2007, 03:28 PM) *
Maybe we need another set at Jupiter's Lagrange points for outer system support. Although that doesn't shorten the range all that much, at least we can avoid some significant atmospheric attenuation...


That doesn't shorten the distance at all when eg, Neptune is on the other side of the Sun from Jupiter. They're not even that close to Jupiter.
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helvick
post Apr 6 2007, 10:48 PM
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Algorimancer sugggested the multi-megapixel camera not me, Boss.! smile.gif

That's a really interesting link - it does force me to correct errors in my baseline assumptions and that will push the data rate down to ~25bps. I really will have to dig out more info on those phased array antennas.

The Ka band numbers are strange - especially the min data rate of 331kbps. Oviously they must be correct but I would have thought that the S/N ratio would be much better even after reducing the Tx power to ~35%. Lots of reading.

Ah well we just have to ask the VLA to lend a hand like they did for Voyager 2 at Neptune That apparently gave V2 the same downlink rate at Neptune as it had at Saturn - which would be a 10x or more gain.
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nprev
post Apr 6 2007, 10:55 PM
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QUOTE (JRehling @ Apr 6 2007, 03:42 PM) *
That doesn't shorten the distance at all when eg, Neptune is on the other side of the Sun from Jupiter. They're not even that close to Jupiter.

True, of course; the maximum possible advantage would only occur when Earth, one of Jupiter's Trojan points & Neptune were all at opposition to each other, and even then we'd only get a repeater boost to compensate for the last half-billion miles...lots of constraints there, might not even happen more than once every few thousand years or so. (Hidden agenda revealed...wanted to get closeup looks at the Trojan asteroids, looking for 'ride-on' extra mission justifications! tongue.gif )


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Greg Hullender
post Apr 7 2007, 03:49 AM
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Frankly, I proposed a geosynchronous Earth orbit simply because it'd only need one ground station to support it. Might even be a step towards replacing the Deep-Space Network, assuming it were feasible to switch to all-laser missions after some point.

Anyway, one good question would be "ignoring the other issues, could a laser communications link be substantially lighter than a radio one for the same bandwidth?" Does anyone either know or know a good reference?

--Greg
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