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Old 12th January 2019, 01:58 PM   #41
RecoveringYuppy
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Elaborating on rjh01 it would make a lot of sense to completely divorce the payloads from the vehicles. Right now it would make sense to have "tug boats" carry satellites to circular GEOs rather than wasting fuel that could be used for station keeping. It would open up new possibilities such as doing final assembly of the satellite on the ISS or similar, avoiding deployment problems.
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Old 12th January 2019, 02:44 PM   #42
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Originally Posted by RecoveringYuppy View Post
We should take that seriously. It's you that should be ignored.

BTW I think they are actually understating the potential. There doesn't have to be water at every potential landing site. We got on and off the Moon without having to refuel. This technology won't need to either..
I agree.

It takes very little reaction mass to get off small planetary bodies. I seem to recall they had trouble getting the Philae lander to actually land on comet C67/P Churyumov-Gerasimenko. It bounced a kilometre off the surface and took almost 2 hours to come back down... then it bounced again - no reaction mass was involved in those bounces

Point of note - the Apollo Lunar Modules, and the Soviet Luna 16, 20 and Luna 24 landers are still the only bona-fide SSTO space craft that have ever flown, and the LM's were the only crewed ones.
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Old 12th January 2019, 02:56 PM   #43
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Originally Posted by smartcooky View Post
Point of note - the Apollo Lunar Modules, and the Soviet Luna 16, 20 and Luna 24 landers are still the only bona-fide SSTO space craft that have ever flown, and the LM's were the only crewed ones.

And to further the point they had already landed which also cost fuel.
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Old 12th January 2019, 04:00 PM   #44
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Originally Posted by smartcooky View Post
I agree.

It takes very little reaction mass to get off small planetary bodies.
True, but getting off the surface isn't the issue. Going from one asteroid to another is likely to require a few km/s of delta-V, and if your steam system has a poor specific impulse (Isp), you may be better off with ion thrusters (Isp ~3000) and a big never-to-be-refilled tank of some noble gas.

If the steam system is dissociating the steam to oxygen and hydrogen, then they can get a decent Isp (>400 sec) but that raises a host of other problems. If they're simply heating the steam using conventional heaters, the Isp is going to be badly limited (maybe 200 sec). And if they're using arc heating to get a decent Isp (400 to 600 sec), then the thrusters themselves will have limited life.
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Old 12th January 2019, 04:28 PM   #45
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Those numbers compare well against apogee kick motors in the Star family.
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Old 12th January 2019, 06:07 PM   #46
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Originally Posted by RecoveringYuppy View Post
Don't be ridiculous.
I do try not to be. So what, exactly, was I being ridiculous about? Do you think that reactors and/or solar cells last forever? Or did you think that I was seriously advocating reactor refueling/solar cell fabrication?


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True about water being the reaction mass but remember that sunshine you mentioned?
Sure I remember. Solar cells are only good out to a certain distance from the sun, and they get less useful the further out you go. After all, they have to get bigger for the same output power, so their power/weight ratio gets worse and worse. Likewise, their water-mining rate will drop in the same fashion, so the overall trip time (assuming multiple boosts) will increase.

So, could you be a bit (that is to say, a lot more) explicit about the failings you see in my argurments?

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Old 13th January 2019, 01:39 AM   #47
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Originally Posted by WhatRoughBeast View Post
I do try not to be. So what, exactly, was I being ridiculous about? Do you think that reactors and/or solar cells last forever? Or did you think that I was seriously advocating reactor refueling/solar cell fabrication?
I'm pretty sure it was taking literally the idea that the probe would last forever. That was hyperbole and no one (here anyway) is arguing that it could last forever.

What is being argued is that it could be a major improvement over current methods.
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Old 16th January 2019, 10:15 AM   #48
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Originally Posted by RecoveringYuppy View Post
In the first place we have Radioisotope batteries and even nuclear reactors that can power spacecraft.
Or we did. The russians haven't made good on their deals to sell us the Pu238.
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Old 16th January 2019, 10:19 AM   #49
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Originally Posted by RecoveringYuppy View Post
True about water being the reaction mass but remember that sunshine you mentioned?
It is something you don't get enough of to be all that useful in areas that you find ice in space?

And turning water into fuel is nothing new, they are just not going through the hassle of cracking it into hydrogen and oxygen, and using it as a reaction mass directly.
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Old 16th January 2019, 10:20 AM   #50
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Originally Posted by GlennB View Post
I was going to post about their still 'limited life', but WhatRoughBeast beat me to it. Then you retorted with 'sunshine', but that just gets us back to solar panels and batteries.

wtf? None of this points to such a thing buzzing around 'forever', and even a decade is huge stretch of the imagination.
A decade would barely get it to anywhere interesting though.
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Old 16th January 2019, 10:30 AM   #51
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Originally Posted by GlennB View Post
None of this points to such a thing buzzing around 'forever', and even a decade is huge stretch of the imagination.
Why? We have probes out there that have lasted far longer than that. New Horizons was launched over ten years ago and is still going strong.

Look at how long the Pioneer and Voyager probes have lasted. Look at the endurance of the various Mars rovers. Look at how long Galileo survived (13 years, including 8 in the searing radiation hell of Jovian space).

I don't think it takes any stretch of the imagination at all, to imagine a space probe that lasts even a decade. Hell, that's not even a stretch of the facts.

And here's the kicker: We haven't yet even really tried to make long-lived probes.
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Old 16th January 2019, 10:41 AM   #52
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Originally Posted by theprestige View Post
Why? We have probes out there that have lasted far longer than that. New Horizons was launched over ten years ago and is still going strong.

Look at how long the Pioneer and Voyager probes have lasted. Look at the endurance of the various Mars rovers. Look at how long Galileo survived (13 years, including 8 in the searing radiation hell of Jovian space).

I don't think it takes any stretch of the imagination at all, to imagine a space probe that lasts even a decade. Hell, that's not even a stretch of the facts.

And here's the kicker: We haven't yet even really tried to make long-lived probes.
But they're just cruising at their original speed (maybe with some slingshot thrown in), not landing and taking off repeatedly, as is proposed here. What's the launch velocity? 50mph might easily get you free of a small asteroid but when the next asteroid is 500,000 miles away and you're not even sure whether it even has water and that you can land in its watery regions, if it has any, then we're into the usual SF zone.
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Old 16th January 2019, 10:46 AM   #53
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Originally Posted by phunk View Post
Until the water filter clogs up.
So about two weeks, then.

Originally Posted by theprestige View Post
"How long could such a craft really operate?" is in my opinion a red herring.
It's central to the issue! It's also really interesting to me, so why shouldn't I ask and get a response? Sheesh.
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Old 16th January 2019, 10:59 AM   #54
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Originally Posted by theprestige View Post
Why? We have probes out there that have lasted far longer than that. New Horizons was launched over ten years ago and is still going strong.

Look at how long the Pioneer and Voyager probes have lasted. Look at the endurance of the various Mars rovers. Look at how long Galileo survived (13 years, including 8 in the searing radiation hell of Jovian space).

I don't think it takes any stretch of the imagination at all, to imagine a space probe that lasts even a decade. Hell, that's not even a stretch of the facts.

And here's the kicker: We haven't yet even really tried to make long-lived probes.
Those probes are solid state devices floating through the vacuum, with few moving parts, and none of them shoveling and processing dirt.
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Old 16th January 2019, 11:06 AM   #55
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Originally Posted by Hans View Post
It would depend on how long all the electronic and other parts would continue to function. What is the life expediency of electronics these days?
When does the warranty expire?

About 12 hours longer than that.
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Old 16th January 2019, 11:21 AM   #56
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Originally Posted by GlennB View Post
But they're just cruising at their original speed (maybe with some slingshot thrown in), not landing and taking off repeatedly, as is proposed here. What's the launch velocity? 50mph might easily get you free of a small asteroid but when the next asteroid is 500,000 miles away and you're not even sure whether it even has water and that you can land in its watery regions, if it has any, then we're into the usual SF zone.
These guys aren't cruising (coasting, actually) at their original speed:

https://en.wikipedia.org/wiki/Mars_Exploration_Rover

Opportunity drove around on Mars for 12 years. Curiosity has only been on Mars for five years so far, but its power source has a minimum lifetime of 14 years, and it's reasonable to expect Curiosity to be substantially operational for at least that long.

"Landing and taking off repeatedly" isn't a mysterious engineering challenge. There's no reason to expect it can't be solved if we have a reason to solve it.

So what if the next asteroid is 500,000 miles away? If you have the fuel to adjust your orbit and make rendezvous, most of the journey is just peaceful cruising across the intervening distance. And you at least accept that doing that for decades on end is no problem.

There's two real problems that I can see:

One, meeting the engineering challenges of making a robust refueling system. This is a new kind of thing, so while I'm pretty confident it can be done, I suspect it'll take some work. The first attempt might not even be successful (not all our space probes have survived as long as intended). But it doesn't really seem like a "we'll never get beyond a ten-year lifespan" kind of problem.

Two, the question of how to know there's enough water at the destination to refuel the probe. For that, it might make sense to start with a passive survey mission of some kind, that can assess asteroid water content at a distance. Maybe examining light reflecting off the surface or passing nearby the asteroid, similar to the kind of spectral analysis we do to try to learn more about exoplanets. Maybe combined with some kind of cheapo impactors to create plumes and debris fields that can aid easier to examine at a distance. Once we've identified a few likely candidates and done the orbital maths, we could build a probe that will last at least long enough to visit some or all of them. Again, not really a "ten years at best" kind of problem. More of a "let's do this right so we don't embarrass ourselves right out the gate" kind of problem. Nobody looks at the ill-fated Beagle and says "this just proves that you'd be lucky to even get to Mars intact".

Based on humanity's past performance with space probes, and with engineered solutions in general, I think "it could probably be made to last a very long time" is a much more reasonable starting point than "it probably can't last more than ten years at most".
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Old 16th January 2019, 11:29 AM   #57
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Originally Posted by Belz... View Post
It's central to the issue!
I disagree. We know we can build robust and long-lived devices when we want to.

Historically, it's never been central to the issue of a space probe. The central question there has been "can it last long enough to at least complete its initial mission?". The question of how long it will actually endure has always been incidental, and only answered by sending it out there and seeing what happens.

Same thing here. "Can it last long enough to visit three asteroids, refueling twice?" is kinda central. "How long can it keep going?" is entirely incidental. Based on past performance, though, the answer is probably going to be, "a lot longer than the original mission requires".

Quote:
It's also really interesting to me, so why shouldn't I ask and get a response? Sheesh.
I don't think there is a response, other than, "we won't know until we build one, send it out, and see what happens." Having the question is fine - I think we all wonder about this to some extent. But recognizing that the question has no good answer yet, and that it doesn't add much to the conversation at this point, is also good.
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Old 16th January 2019, 11:31 AM   #58
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Originally Posted by phunk View Post
Those probes are solid state devices floating through the vacuum, with few moving parts, and none of them shoveling and processing dirt.
You're absolutely right. Thank you for prompting me to go back and actually read what I'd written. Somehow I forgot that the Mars rovers were solid state devices floating through a vacuum. You've invalidated my entire claim, which sucks, but you've made me a more accurate and correct person, for which I thank you again.
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Old 16th January 2019, 12:27 PM   #59
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Originally Posted by ponderingturtle View Post
Or we did. The russians haven't made good on their deals to sell us the Pu238.
Oh, wow. I suppose Americans will never be able to figure it out again. Guess the human race is forever barred from space. Be incredibly stupid to think any country but Russia could ever figure out how to produce that again.

NASA contracting with Canadian firm for Pu238 production.

Originally Posted by ponderingturtle View Post
It is something you don't get enough of to be all that useful in areas that you find ice in space?
The answer to this question is "No".
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Old 16th January 2019, 12:34 PM   #60
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Originally Posted by ponderingturtle View Post
A decade would barely get it to anywhere interesting though.
That statement displays incredible ignorance.
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Old 16th January 2019, 01:09 PM   #61
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Originally Posted by RecoveringYuppy View Post
That statement displays incredible ignorance.
Indeed.

This might help
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Old 16th January 2019, 01:14 PM   #62
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Would I be right in saying that the only bit of the proposed device that hasn't actually been manufactured and flown is the engine/fuel scavenging thing? Are there navigation or interception challenges that haven't been overcome? The comet landing was a failure, I seem to recall?

Assuming that can be overcome, I see no reason why a device couldn't be built to last a long time, if the weight/reward equation allows redundant systems, a very long time.

The engine is the problem though, isn't it? I'm no engineer but isn't it heat that kills? And repeatedly heating and cooling really kills? Isn't that the challenge? Or am I missing something?
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Old 16th January 2019, 01:15 PM   #63
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Originally Posted by theprestige View Post
I disagree. We know we can build robust and long-lived devices when we want to.
Yes, but the thing about self-sustaining probes is that there's no such thing as a self-sustaining process. So, I want to know how long such a probe can last. If it's 450 years, great! Awesome! I still want to know.

Quote:
Historically, it's never been central to the issue of a space probe.
If the fact that it can operate for a long time the thing that makes this groundbreaking, how is it not central to the issue of THIS space probe?

Quote:
I don't think there is a response, other than, "we won't know until we build one, send it out, and see what happens."
Boo! I want my science answers NOW!
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Old 16th January 2019, 01:50 PM   #64
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A point seems to be missing in the discussion: I don't think anybody would seriously expect such a craft to be power-steaming from place to place. It would still coast most of the time, making minimal course corrections. But it would now have replenish-able source of thrust material. It would not need every object it visits to supply water, just one once in a while.

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Old 16th January 2019, 02:07 PM   #65
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Originally Posted by 3point14 View Post
Would I be right in saying that the only bit of the proposed device that hasn't actually been manufactured and flown is the engine/fuel scavenging thing? Are there navigation or interception challenges that haven't been overcome?
You'd be right. Some of the solutions are inelegant, but they've been done.

Quote:
Assuming that can be overcome, I see no reason why a device couldn't be built to last a long time, if the weight/reward equation allows redundant systems, a very long time.

The engine is the problem though, isn't it? I'm no engineer but isn't it heat that kills? And repeatedly heating and cooling really kills? Isn't that the challenge? Or am I missing something?
I just can't get excited about a hot steam rocket on a 50-year-old probe. In the inner system, the delta-Vs are just too big to make this usable; in the outer system, the available power is going to make for very low accelerations coupled with very long distances.

With careful mission planning, I think you could mitigate most of the wearout mechanisms. Keep the transmitters off most of the time, reorient the whole spacecraft to keep the solar panels on the sun when coasting, use the thrusters (not wheels) to maintain orientation, shield the crap out of everything for radiation and micrometeroids. Lots of redundant thrusters. The solar panels will gradually deteriorate, but even 50 years out you should be getting at least half of your original power.

Batteries really may be a problem, though. Even without much cycling, I'm not sure how they'd work after 50 years. Accelerated life tests sound nice, but they don't always predict real-world performance, and without accelerated life tests, it takes 50 years to find out if your battery will last for 50 years. But maybe you could design the beast to gracefully shut down and reboot as the sunlight comes & goes, as long as it didn't get too cold.

All of that redundancy and shielding makes your spacecraft heavier (and slower!), more complicated, and more expensive, without mitigating the obsolescence problem at all.

Just can't get that excited about it.

Rather than a portable ice-melter, maybe if we had a more permanent one on some particularly-icy asteroid. It would split the water to H2 & O2, and probes could stop there to refill. Yeah, there are a lot of issues with that one, too, but it's always fun to sketch this stuff out.
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Old 16th January 2019, 02:09 PM   #66
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Originally Posted by MRC_Hans View Post
A point seems to be missing in the discussion: I don't think anybody would seriously expect such a craft to be power-steaming from place to place. It would still coast most of the time, making minimal course corrections. But it would now have replenish-able source of thrust material. It would not need every object it visits to supply water, just one once in a while.
The way I see it, being able to replenish the fuel supply means it doesn't have to hoard fuel and stick to a narrow list of encounters. It can afford to make more radical orbital adjustments, and thus visit more objects. Still coasting along for the most part, but with bigger and longer transfer burns to encounter objects not within its original launch window.
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Old 16th January 2019, 03:12 PM   #67
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Yes, sure, and most importantly, not have to stick to a prepared mission plan.

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Old 16th January 2019, 03:17 PM   #68
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Originally Posted by theprestige View Post
The way I see it, being able to replenish the fuel supply ...
I have to ask - what do you mean by "fuel supply" here?
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Old 16th January 2019, 03:20 PM   #69
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Originally Posted by theprestige View Post
You're absolutely right. Thank you for prompting me to go back and actually read what I'd written. Somehow I forgot that the Mars rovers were solid state devices floating through a vacuum. You've invalidated my entire claim, which sucks, but you've made me a more accurate and correct person, for which I thank you again.
Excuse me for missing your mention of the rovers among that list of exactly what I described.

They still do nothing even remotely like processing large quantities of materials to extract water.
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Old 16th January 2019, 03:54 PM   #70
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Originally Posted by GlennB View Post
I have to ask - what do you mean by "fuel supply" here?
I assume the probe will burn water in its forge, and use the energy of combustion thus released to drive the pistons in its legs that allow it to hop from asteroid to comet and back again.
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Old 16th January 2019, 03:58 PM   #71
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Originally Posted by phunk View Post
Excuse me for missing your mention of the rovers among that list of exactly what I described.



They still do nothing even remotely like processing large quantities of materials to extract water.
You're excused.

And humans seem to have a pretty good history of engineering new mechanisms to accomplish new things.

If you think that this particular mechanism is going to be too difficult for anyone to figure out, just say so.
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Old 16th January 2019, 08:52 PM   #72
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Originally Posted by RecoveringYuppy View Post
Those numbers compare well against apogee kick motors in the Star family.
It just so happens that, early in my career, I spent a fair amount of time working on some of the issues associated with using a STAR-48 for an interplanetary mission.
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Old 16th January 2019, 09:28 PM   #73
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Originally Posted by theprestige View Post
The way I see it, being able to replenish the fuel supply means it doesn't have to hoard fuel and stick to a narrow list of encounters. It can afford to make more radical orbital adjustments, and thus visit more objects. Still coasting along for the most part, but with bigger and longer transfer burns to encounter objects not within its original launch window.
Even with some fairly generous assumptions (Isp=200s, propellant is 83% of wet mass, no arc losses), I think it would be hard to get more than 4 km/s total delta-V out of such a system before it had to be refueled, and 4 km/s is going to give it a fairly limited choice of targets even if going from one asteroid to another*, and that's about as easy as it gets. And with 4 km/s, it will need help getting to the asteroid belt in the first place.

*according to the back of my envelope, which I don't really trust for this sort of calculation, going from one random asteroid to another random asteroid should take about 19 km/s on average. Of course, some will be much lower, some will be much higher.
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Old 17th January 2019, 05:53 AM   #74
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Originally Posted by dasmiller View Post
Even with some fairly generous assumptions (Isp=200s, propellant is 83% of wet mass, no arc losses), I think it would be hard to get more than 4 km/s total delta-V out of such a system before it had to be refueled, and 4 km/s is going to give it a fairly limited choice of targets even if going from one asteroid to another*, and that's about as easy as it gets. And with 4 km/s, it will need help getting to the asteroid belt in the first place.

*according to the back of my envelope, which I don't really trust for this sort of calculation, going from one random asteroid to another random asteroid should take about 19 km/s on average. Of course, some will be much lower, some will be much higher.
Thanks, that seems to be quite relevant.

You did say:
Quote:
And if they're using arc heating to get a decent Isp (400 to 600 sec), then the thrusters themselves will have limited life.
Could you outline the limitations on the lifespan of the thrusters imposed by "arc heating to get a decent Isp"?

I have express my ignorance here: I don't know either what arc heating is, or specifically why it would cause a limited lifespan for the thrusters, though I can see the obvious fact that a more violent/high temperature thrust will induce more wear on the thrusters.
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Old 17th January 2019, 06:39 AM   #75
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Originally Posted by Roboramma View Post
I don't know either what arc heating is, or specifically why it would cause a limited lifespan for the thrusters, though I can see the obvious fact that a more violent/high temperature thrust will induce more wear on the thrusters.
So, resistojets vs. arcjets:

I am assuming that the steam thrusters are 'resistojets' (yes, it's a real word) which use simple resistance heaters to heat up the propellant as it flows down the tube to the nozzle. Resistojets can't heat the propellant above the melting temperature of the tube, and I suspect that they tend to waste a lot of heat, but they should be pretty long-lived since there's not much of a wear mechanism. One source said resistojet Isp could be near 300 if everything (including the heater itself) was made of tungsten, but that was for a different propellant. My spreadsheet gives a maximum resistojet Isp for steam at 269 s with an impossibly good nozzle. In the real world, I haven't (yet) found a pure resistojet that got above 150 s, so my 200 s was a compromise that assumed substantial improvement over the current (1970s) state-of-the-art.

Arcjets, in contrast, don't use heaters. Instead, they have a continuous electric arc zapping through the throat of the nozzle. This instantaneously heats the propellant (a lot!) and as I understand it, the propellant can be heated substantially above the melting point of the chamber. However, that arc is, well, an electric arc. Think of it as a tiny, continuous lightning bolt, constantly chewing up whatever it touches and you'll see why arcjets are likely to be life-limited.
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Old 17th January 2019, 06:54 AM   #76
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I was thinking of a different, albeit admittedly less efficient approach: Heating the water in a boiler till a sufficient pressure is reached, then release it through a nozzle as needed. For course correction bursts, and for landing and take-off "burns" in the feeble gravity of asteroids, I expect it would be efficient enough. The advantage is that you can use the relatively low energy from solar cells or isotope power generators, it just takes some time to build up pressure (but time is usually a plentiful ressource for unmanned space probes). Of course you would need a very effective insulation on your boiler.

Do you remember the crazy fellow with the steam rocket? He evidently used that method, and well, got thrust enough to fly ... a bit.

Hans
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Old 17th January 2019, 07:15 AM   #77
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Originally Posted by MRC_Hans View Post
I was thinking of a different, albeit admittedly less efficient approach: Heating the water in a boiler till a sufficient pressure is reached, then release it through a nozzle as needed. For course correction bursts, and for landing and take-off "burns" in the feeble gravity of asteroids, I expect it would be efficient enough. The advantage is that you can use the relatively low energy from solar cells or isotope power generators, it just takes some time to build up pressure (but time is usually a plentiful ressource for unmanned space probes). Of course you would need a very effective insulation on your boiler.
How quickly does a very hot thing radiate it's heat away in a vacuum?
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Old 17th January 2019, 07:22 AM   #78
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Originally Posted by 3point14 View Post
How quickly does a very hot thing radiate it's heat away in a vacuum?
Depends entirely on how well it is insulated. In fact, heat insulation in vacuum is not so hard because heat can only radiate away, you don't have to bother about conduction.

As you may know, the insulation in an ordinary thermos relies entirely on reflective surfaces and a fairly thin zone of vacuum.

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Old 17th January 2019, 07:40 AM   #79
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Originally Posted by MRC_Hans View Post
Depends entirely on how well it is insulated. In fact, heat insulation in vacuum is not so hard because heat can only radiate away, you don't have to bother about conduction.

As you may know, the insulation in an ordinary thermos relies entirely on reflective surfaces and a fairly thin zone of vacuum.

Hans

I ask because I figured you wouldn't actually need that much insulation, would you? Doesn't the vacuum do most of that for you?

A few sheets of quality tin foil should do it?

Then I realised that I have no idea how quickly very hot things radiate in a vacuum.
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Old 17th January 2019, 07:45 AM   #80
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Originally Posted by MRC_Hans View Post
I was thinking of a different, albeit admittedly less efficient approach: Heating the water in a boiler till a sufficient pressure is reached, then release it through a nozzle as needed. For course correction bursts, and for landing and take-off "burns" in the feeble gravity of asteroids, I expect it would be efficient enough.
In space, things aren't just floating far apart. They're moving very fast with respect to each other, so you have to change your speed a lot. Low thrust is okay, because you usually have a lot of time. But efficiency (Isp) is critical because the fuel required is enormous.

Quote:
The advantage is that you can use the relatively low energy from solar cells or isotope power generators, it just takes some time to build up pressure (but time is usually a plentiful ressource for unmanned space probes). Of course you would need a very effective insulation on your boiler.
Not easy to make a large, well-insulated, high-pressure light-weight tank. For the resistojet that I assumed, the steam was being heated to about 1600K, so the tank would be glowing yellow-white.

Quote:
Do you remember the crazy fellow with the steam rocket? He evidently used that method, and well, got thrust enough to fly ... a bit.

Hans
I believe he got about 150 m/s or 0.15 km/s out of it. I haven't been able to find much in the way of technical information about his rocket (quelle surprise), but his Isp may not be over 30 s.
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