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Solar-electric might well work pretty good in the inner solar system, maybe even to Mars or the Main Belt. But there is a very good reason most of the probes to the outer planets have been nuclear-powered. The lower the sunlight intensity, the bigger and heavier your solar hardware has to be. It's either that, or accept lower power and lower thrust (which is already pretty low for the weight to begin with).
As for Zubrin, he's pretty smart, and he can be very vocal. But, like all of us, he can sometimes come up very short on being right. I know he has no patience for technology development. Sometimes that's the right thing to promote, and sometimes it's wrong. When you are putting together an expedition, it only makes sense to use proven technology, otherwise it's just a gravy train, and you never really go. That's where Zubrin is coming from. I've been there, too.
But, 10+ years down the road, you'd really like to have some better technologies available to choose from. That's mostly where I come from. And I don't mind resurrecting an old one for a new use (not many come from there!). The trade-off comes from expedition schedule: just when do you really expect to go? Does that give you time to add some new proven technologies to the mix of proposals, when it actually comes time to choose? And, just when do you have to choose? All expeditions are different in that respect.
The pitfall is not "having some technology programs", it is "incorporating new technology development into your baseline expedition program". Those need to be separate functions until such time as the technology program(s) actually bear fruit. Ideally, the technology development effort is a whole separate plethora of parallel and serial projects, funded continuously over time, with the stated purpose of providing "new stuff later" when it is needed.
Funding these vs specific expeditions is not necessary a zero-sum game, either. But usually, we can only afford one or two expeditions at a time (those usually have huge price tags). Technology efforts are usually very modestly-priced in comparison to expeditions, and are also much more amenable to trading off funding vs schedule time.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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It should be VERY clear by now that a mission to Maris is NOT going to be funded at the kind of price tags that would result from using conventional rockets and our present off-the-shelf tech. So opposition to improving and refining our tech is opposition to going to Mars EVER, and that's ware Zubrin is now. I think it's high time for him pass the torch and leave Mars advocacy to people like Musk.
Musk embraces doing real engineering work and taking the necessary step by step approach that produces results, Musk's results show the fallacy of Zubrins "Now Now Now, damn the costs" philosophy, ware Zubrin condemns everyone who disagrees with him as "having no stomach for risk" anyone with a brain can see that Musk has taken more REAL risks with his own money then Zubrin has ever had. Musk's generated more interest in space in a decade then Zubrin has in his whole carrier, partly it's cause he's young and 'dashing' and a more effective public speakers but mostly it's cause he DOSE what he says rather then just SKOLDS omnidirectional.
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Actually, we could actually go to Mars with the current stable of rocket and space capsule technologies. It could be done right now, but not the way everybody has in mind. Most, even Zubrin, still see it almost like a one mission-one launch thing, and most folks (even most of those at NASA) still think astronauts can ride all the way to Mars in nothing but a space capsule.
Both those ideas are utter BS for an expedition to Mars. 6-8 months one way to Mars is absolutely nothing at all like 3 days one-way to the moon.
You're going to need a big spacious habitat to live in, in order to keep them sane, you're going to need artificial spin gravity to keep them healthy, and to simplify the design of all the life support functions, you're going to need a serious radiation shelter in the event of a solar storm (a near certainty on such a voyage), you'd be very smart to make that radiation shelter the vehicle's flight control deck, and you're going to need multiple landers and all the propellants to fly them.
Yep, I really did say multiple landers. What's the point of going all that way under circumstances so difficult with the technologies we have, if you don't visit several different sites all over Mars while you're there? I'm serious as the grave about that question. What would be the point of one landing-one mission?
With chemical propulsion, the fleet of vehicles to do this (a manned transport and each of several landers pushing its own propellant supply) is quite large, and each individual vehicle is also quite large (several hundred tons). This requires assembly in LEO of numerous docked modules, thrown up there by whatever rockets we already have, that can fling things in the same 15-25 ton class from which we built the ISS.
Guys, it's just expensive. It's not a technological leap. It'd be somewhere around $200B, not the $trillions "everybody knows". None of these assemblies are "Battlestar Galactica", either. They're big, but not that big.
You can cut the required total launched mass, and thus total expedition expense, by a factor between 2 and 4 by resurrecting the old solid core nuclear NERVA technology for use pushing the manned transport and the landers. That technology was flight ready 4 decades ago, and could be again in about 5 years, if done by the right team. (It'll take at least 2 decades if done "business-as-usual" by all the usual suspects.) That puts you in the ballpark of $50-100B.
There's nothing else on the horizon that could actually be flight-ready within 2-3 decades, nothing else that could do a better job as "hotter propulsion". If it ain't "hotter propulsion", launched masses and expedition costs stay high, simple as that. There's nothing on the horizon, except maybe nuclear explosion propulsion, but that works best at ignition masses 20,000 tons and up, not the several hundred ton vessels we are considering here. No cost advantage, and a lot of side-effects to deal with. Wrong mission for that technology.
It ain't nothing but rocket equation work, tempered with the knowledge of what is actually required to keep people alive and fully healthy for long durations in space. That second phrase is what most mission plans I have seen always leave out.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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What if, say, we already that the propellent in orbit, off the shelf habitats and landers, and a launcher capable of putting 50 tonnes into orbit? I can see the price tag coming down to a few billion dollars in such a scenario...
Use what is abundant and build to last
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As Terraformer said: What if, say, we already that the propellent in orbit, off the shelf habitats and landers, and a launcher capable of putting 50 tonnes into orbit? I can see the price tag coming down to a few billion dollars in such a scenario...
So can I.
I just want to see how propellants really can be made in quantity off-Earth.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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GW: Whats with you Obsession with NERVA, I've debated you into the ground on why that Technology IS DEADER THEN DISCO. But you keep coming back to it over and over and over again holding it up as the holy-grail of space flight (and like literally in every thread you post in, nearly to the point of thread-jacking)
http://www.newmars.com/forums/viewtopic.php?id=6177
But it seems I was wrong in my earlier assessment, I was TOO GENEROUS TO NTR!!
http://selenianboondocks.com/2010/02/pf … s-example/
Here a fellow named Kirk Sorenson http://energyfromthorium.com/kirk-sorensens-corner/
Utterly Destroys any argument for NTR with a well done number crunching that show that for a TMI scenario they have trouble even MATCHING a plain Chemical booster in payload fraction. The main problem is Hydrogen tanks are big and bulks and very inefficient and the T/W ratio of the engine itself are terrible which both cut directly into payload.
A long debate revolving around an online Presidential petition (you know those ones Obama will officially 'respond too' if they get enough votes) to increase funding for NTR then started over at nasaspaceflight.com and people debated the merits of NTR, both vs conventional chemical and vs SEP and draw heavily on Sorenson's analysis. The NTR proponents in this thread can't rub two numbers together to save their lives and basically repeat a mantra of "Twice the ISP of chemical" and cover their ears. This post basically sums it all up.
http://forum.nasaspaceflight.com/index. … msg1006138
GW, I think you need to recognize that you have an illogical attachment to this tech and are investing it with far more potential then it ever had as a way to explain away the general failure of reality to live up to science-fiction, rather then having to admit that science-fiction was just absurdly unrealistic and naive you can point to this miracle technology which WOULD have made it all possible if not for the foolish/evil cancellation that doomed what SHOULD have happened. It wasn't your aspirations which were too big, it was the minds of politicians that were too small!!
SEP dose not promise us a fabulous Buck-Rogers future either, it's going to be slow and is thus going to be more conducive to robotic then manned exploration. But it's what the technological pipeline is giving us because its a viable commercial technology used for station-keeping commercial satellites.
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Impaler:
I think your anti-NTR rhetoric may be a bit overblown. Most of this stuff you read about it are exaggerations feeding off the real safety concerns. And they are real, but not actually as bad as is often trumpeted.
NTR as NERVA suffered from high engine weights for the thrust, although not all future NTR need be the NERVA design. They may not all suffer from that inert weight problem.
It would be rather silly to use NERVA for launch to LEO, although that was the first slated application back about 1973: a replacement upper stage for Saturn-5. It was ready-to-fly, and was about a year from actually flying, when the whole thing got cancelled along with Apollo itself.
As an orbit-to-orbit transport, that engine weight objection is far less severe, as long as the engine thrust/engine weight ratio remains near 1, and the nuclear improvement in Isp can "shine", as an indirect cost reduction.
As an old NERVA-type NTR, such vehicles usually size out 2 to 4 times smaller, compared to the various chemical options, including those who have the same hydrogen packaging and storage problems. I don't know what numbers you might be looking at, but these are the typical ranges I see when I work it out for myself.
Having been flight-ready 4 decades ago, the "right team" really could resurrect this NERVA technology quickly, if we decide it is the prudent thing to do. SEP will likely always suffer from a low thrust/weight, far worse than NERVA, as you indicated. The other separate problem is that SEP is much further away from anything like "flight-ready" status. That's simply not a near-term option for anything.
VASIMR is very little different, in that its power supply problem, identical to that of all the other electric thruster types, leads to an extreme low thrust/weight problem. Any of the electric schemes could "shine" if there were a flightweight electric power plant in the MW range. There will not be such a thing anytime soon, as near as I can tell. Some of the fundamental science solutions are still missing, much less the engineering technologies.
So for a Mars expedition, or anywhere else nearby (visiting an asteroid has about the same orbit-to-orbit delta-vee requirements as going to Mars), we are stuck with the various chemical means or the old NERVA. Chemical Isp could be 460 s with LOX-LH2, but only if you can successfully store very-demanding cryo-propellants, in vacuum and in tankage exposed to all the other risks of the space environment, for about a 3 year voyage. I'm not at all sure that is a flight-ready technology. Not for missions of that duration.
All the other practical liquid combinations have performance closer to Isp 280-310 s, although many are easily long-term storable. You compare them to NERVA at 700-1000 s, and that's where the size and cost reductions I mentioned start looking "real".
If you make landing and Earth-return propellants in-situ at Mars (a still-unproven but very promising idea), you have water-as-ice and CO2 to work with. You still have to launch the propellants off the surface to refuel the orbiting vehicle for its return. That's not going to be a trivial operation: you will need a reusable lander to act as the propellant ferry, and you will have to refuel it for each trip up, too.
There will necessarily have to be a lot of trips. (I don't think I would choose an ablative heat shield for this, more weight to carry for replacement, and all that.) And, the landings will require direct rocket braking in those sizes, aero-decelerators simply won't work at ballistic coefficients that high, in "air" that thin. That's a lot of delta-vee required out of your lander on each trip.
Water-as-ice on Phobos or Deimos might help relieve that refueling difficulty somewhat, although that rules out carbon: you're pretty much stuck with LOX-LH2 at that point. The difficulty with that refueling-on-the-low-gravity-moons scenario is that we don't know if there really is any water-as-ice in either of those two places, and we're not really going to know, until somebody or something visits them with a big drill rig. Ain't really gonna happen before we send the big expedition, if we ever do at all.
Water-as-ice requires energy to melt, and more yet to electrolyze into LOX and LH2. Both chemical and NERVA suffer from this need to produce hydrogen. Neither would be a very effective refueling solution, due to the infrastructure required to produce these cryo-propellants, especially the LH2. The leak problem alone is tremendously-difficult to solve handling hydrogen down here on Earth, much less out in space encumbered by these idiotic space suit designs we are still using.
Water-as-water is really easy to handle. You only need to melt the ice. It's on Mars, some asteroids, and probably most comets, plus most of the larger outer-planet moons. Chemical cannot use this directly as propellant, but a water-variant of the old NERVA could. So could some forms of solar propulsion, at least in principle. We don't have a water NERVA, never did. But we could, probably about 5-10 years after resurrecting the LH2 NERVA. It's not a big technology leap. A solar water rocket is decades away from being flight-ready.
Refueling-as-you-go makes a lot of sense in terms of vehicle size and launch costs. The most abundant potential propellant resource that seems to be very widely available is water-as-ice. Does it not make sense to choose a propulsion scheme that can use the water directly as propellant with little or no processing infrastructure? That's how you reduce launch costs and vehicle sizes without getting someone killed.
Can you think of any such schemes besides a water-NERVA? If so, the crucial question is how soon could we make it flight-ready? Most of us want to see the Mars expedition mounted in about 10-15 years at most. Certainly no more than 20. I'd rather go sooner with what we have, even if it's chemical.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Your response leads me to believe you did not read anything I linked too, but were instead responding to Sorenson's other article ware he shows NTR could not reach orbit with any payload at all. I didn't link to that article and instead linked to one that addresses a purely in-space use of NTR. Their the central criticism is that NTR can not even out perform chemical in the Trans-Mars-Injection role. Safety is not even mentioned, and the T/W ratio is relevant because of vehicle dry-mass that it gobbles up, not because it will be relevant in a ground lift-off scenario. The Sorenson analysis aimed for an initial total vehicle T/W ratio at the start of TMI of only 0.5, he indicates this is a normal T/W for performing such a maneuver, but if you have a lower value that would probably go a long way to reducing dry-mass. Given the nature of fissile materials it should be feasible to run a NTR core for a long period of time at low thrust but then you would give up Oberth effects and most of the ISP advantage over chemicals. Please show me your numbers which directly counter the Sorenson analysis
The latter half of your response dealing with ISRU is entirely irrelevant.
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Impaler:
Actually I did look at the links. Sorenson is using the lowest numbers he could find for NERVA to make his case, and that's cheating.
I would trust data from the source a lot more than blog sites with axes to grind, and I recommend that approach to everyone. There are still 3 engineers alive who actually worked on the Rover project, all in their 80's now. I met them at the 2011 convention in Dallas.
One of them, David Buden, has written books about Rover/NERVA, starting in 1985. His recent 2011 book has all of the declassified data in it, and it is quite complete. I recommend that you acquire and read it, and find out for yourself the truth. You'll find out you have been wrong about NERVA and what it might do for us.
As for the "ISRU" thing that you dismissed out-of-hand: deliberately selecting a propulsion scheme that could take direct advantage of the most plentiful volatile in the universe as its propellant, with little or no processing at all, is very far from "irrelevant", very far indeed.
GW
Last edited by GW Johnson (2013-04-03 09:49:29)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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The T/W ratios he tests are 4:1 and (and a GENEROUS, even dare I say speculative) 5:1 as well as the 850 ISP. These are the numbers from the actual TESTED NTR systems and are not in the least low balled. He excludes paper rockets and speculative NTR concepts, and as you have always said this technology is literally on the shelf ready to be utilized with NO REFINEMENT that is absolutely appropriate.
As you clearly have the book your referring too, why don't you enlighten us with the numbers in it that show the TRUE test results for NTR if you disagree with the above numbers. The only area I see ware the analysis is tough on NTR is having a 0.5 initial vehicle T/W ratio, I honestly don't know how low you can get that T/W and still make a good TMI burn.
As for ISRU, your being quite intellectually dishonest here saying Hydrogen is "most plentiful volatile in the universe" which is both true and irreverent. Because its in fact very hard to get in the Inner solar system ware we live. Their might be some at the moons south pole but that's both speculative and an enormous undertaking to get. Mars itself is even worse, its gravity well is sufficiently deep that it's never going to be economical to use fuel produced on the surface for anything but Mars assent. And we know that NTR is terrible when performing that high Thrust lift-off, not to mention in difficulty of landing the huge Hydrogen tank, electrolysis 8 tons of water for every 1 ton of hydrogen produced then cool it to cryogenic temperatures and store it for long durations on the Martian surface, and that's assuming you even find available water on Mars. This is why everyone who talks about ISRU talks about sucking in and processing the Martian atmosphere which is unfortunately devoid of Hydrogen.
A Water variant of a NTR losses all the ISP bonus (due to higher molecular weight of exhaust, its now got ISP identical to a H2/LOX rocket) which justify the technology in the first place, you should know this. The whole 'wiz-bang' of the Nuclear in NTR is really a sham, its the Hydrogen low molecular weight which is really giving the high ISP, any rocket that could heat Hydrogen to that temperature would have that ISP. They should be called HTR (Hydrogen Thermal Rockets) because that's what's really going on under the hood.
Last edited by Impaler (2013-04-04 06:15:59)
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I said water, not hydrogen, was the most abundant volatile around.
One would accept the lower Isp of a water NTR just so one could refuel anywhere there is ice, without any propellant processing other than melt and filter out the dirt.
I will post some data from the book, but not now. Most of devices tested were developmental/research. There was one baseline engine (the XE device) tested as an engine. Lots of improvements to that engine were tested in the other devices. Several of those paper designs showed large improvement in T/W. Many of those were candidates for the first or subsequent flight test engines, which were never built.
When I said flight-ready, I was referring to experimental flight test. Not operational (routine) use. If you knew anything about the engineering development process or the actual events of Project Rover, you would have known that. But OK, from now on I will say ready-for-flight-test, not flight-ready.
At the start of Sorensen's first article he quotes 15,000 lb thrust at 5000 lb weight. Sounds like T/W 3 to me. Some of the Enabler designs were thought capable of 10:1. Never got the chance to be tested though.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Robert Zubrin wrote a critique of VASIMR propulsion here:
The VASIMR Hoax
By Robert Zubrin | Jul. 13, 2011
http://www.spacenews.com/article/vasimr-hoax
The primary criticism is that it would require unrealistically lightweight nuclear propulsion. However, Zubrin doesn't even like the idea of fast propulsion to allow short travel times to Mars. He argues in favor of using 6 month or more one-way travel times to allow free return trajectories at Mars. But the health disadvantages of long travel times such as radiation exposure, bone and muscle loss, and the recently found eye damage and vision loss suggest we should investigate such short travel times.
Now we find there is also another reason: mechanical breakdowns on such missions of 2 or more years round trip length, such as found with the coolant system on the ISS.
Note that the argument about free return trajectories does not hold with respect to planets with atmospheres. The Apollo missions did do a free return around the Moon, but there was no non-propulsive method to slow down at the Moon. On return to the Earth though, even Apollo had a trajectory that would send it off into space if the angle was too shallow or plunging too steeply into the Earth's atmosphere to burn up if the angle was too steep. The same could be used in addition to the propulsive method whose high efficiency would also allow it to be used for slow down at Mars.
So it is important to note we may have a short term power source instead of nuclear power, for plasma propulsion such as Vasimr at the needed lightweight.
The key point is that the power source does not need to be nuclear. According to Zubrin's article on the Vasimr it requires a power source of 1,000 watts per kg power density. This is 100 times better than what has been done with nuclear space power at 10 watts per kg. However, it is only 10 times better than standard solar space cells at 100 watts per kg. Actually more recent space solar cells get 200 watts per kg, so it is only needs to be 5 times better than those.
Now the key fact is that solar cells can put out more power if they have more concentrated light shone on them. Estimates of how much power solar cellls put out are based on the solar insolation at the Earth's distance from the Sun. But if that light is concentrated they can put out more power. In fact some Earth solar power systems get more power by using inexpensive mirrors or lenses to concentrate light over a larger area rather than using expensive solar cells over that larger area.
A disadvantage is this increases the loss due to heat and also if the light is too intense it can overload the solar cells so they don't work at all. However a recent report claims they can use concentrated light at thousands of times higher than solar insolation:
SEPTEMBER 07, 2013
Stacked Solar Cells Can Handle Energy of 70,000 Suns.
This work is important because photovoltaic energy companies are interested in using lenses to concentrate solar energy, from one sun (no lens) to 4,000 suns or more. But if the solar energy is significantly intensified – to 700 suns or more – the connecting junctions used in existing stacked cells begin losing voltage. And the more intense the solar energy, the more voltage those junctions lose – thereby reducing the conversion efficiency.
http://nextbigfuture.com/2013/09/stacke … nergy.html
Several reports in fact claim solar concentration at hundreds to thousands of Suns:
FEBRUARY 20, 2009
Breakthrough Solar Concentrator:low cost with high efficiency.
http://nextbigfuture.com/2009/02/breakt … -cost.html
FEBRUARY 17, 2011
Concentrated solar power at half the cost of thin film solar.
http://nextbigfuture.com/2011/02/concen … -cost.html
DECEMBER 16, 2011
Tiny Solar Cell Could Make a Big Difference
http://nextbigfuture.com/2011/12/tiny-s … e-big.html
This will be dependent on having lightweight mirrors or lenses. However another key fact is that the parabolic mirrors do not have to be telescope grade accuracy. Indeed you can find on the net videos of amateurs making their own homemade solar furnaces that also require light to be concentrated to high intensity. These homemade mirrors can be as simple as aluminum foil spread onto a cardboard frame and still concentrate light to generate thousands of degrees. Not requiring high accuracy for the mirrors suggest they can be made lightweight.
DARPA is also funding lightweight space lenses:
DECEMBER 08, 2013
DARPA shoots for 20 meter folding space telescope.
http://nextbigfuture.com/2013/12/darpa- … space.html
These methods would concentrate sunlight onto solar cells. However, solar cells are typically low efficiency, in the range of 30%. Another method would eliminate the need for solar cells. That is to use a solar furnace. These can get temperatures as hot as the surface of the Sun by concentrating sunlight. By thermodynamics very high temperatures correspond to high efficiency conversion of heat to other forms of energy, 90% and above.
Bob Clark
Last edited by RGClark (2014-01-10 10:34:29)
Old Space rule of acquisition (with a nod to Star Trek - the Next Generation):
“Anything worth doing is worth doing for a billion dollars.”
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VASIMR has a number of problems. It uses magnetic confinement to hold hydrogen while it is heated to plasma, and pressurized. The pressurized plasma is ejected for propulsion. But holding any significant quantity of material under pressure with a magnetic vacuum bottle? That requires a LOT of power! So much so that fusion researchers found they just can't make more power than it consumes. I've argued before that a fusion reactor requires a different approach, something that doesn't include magnetic confinement. It is possible, but not with magnetic confinement. VASIMR uses it, but not for power production. It can work, but VASIMR will suck a hell of a lot of power.
The developer for VASIMR claimed it could produce 9,000 seconds Isp with liquid hydrogen. This was after the Glenn Research Centre already achieved 8,400 seconds with a MagnetoPlasmaDynamic thruster, and Russians achieved the same with their largest Thruster Anode Layer Hall Thrusters. So the claim for VASIMR is 9,000 seconds; that sounds like he pulled a number out of thin air just to claim he can do better than his competition. Is this credible? No VASIMR thruster has so far worked with liquid hydrogen, only with xenon, and that one achieved 5,400 seconds. Impressive, but not as good as Glenn's MPD or Russia's TAL Hall. And these others don't use magnetic confinement, so they consume a lot less electrical power. One design feature of VASIMR is adjustable configuration: low thrust / high Isp, or high thrust / low Isp. Call it high gear or low gear. It's actually achieved by adjusting the magnetic nozzle at the exit of the magnetic vacuum bottle. Nice trick, MPD and TAL Hall cannot do that, they have fixed thrust and Isp, they are either fully on or fully off. "Throttle" is achieved by pulsing the thruster. But VASIMR claims it is fully throttleable, and has this "gear" capability. Nice claim, but at what cost? The power requirement is very significant.
Then there's the propellant problem. As I said, VASIMR has only been demonstrated in the lab using xenon. But there's only so much world wide production of xenon. Whether there's enough xenon to fuel a human mission is highly questionable. Hydrogen is plentiful and cheap. MPD has already been demonstrated with liquid hydrogen.
I'm not arguing against the idea of electric propulsion; rather arguing to use MPD instead. It has already been proven, and uses less power.
By the way, the Glenn Research Centre are the same guys who developed the NSTAR ion engine for Deep Space One. And they developed MPD after Princeton University did, taking Princeton's work and improving on it. They achieved such high performance by significantly increasing electric power, but again not as much as VASIMR. And they were able to achieve this dramatic result after receiving data and education from the Russian lab that produced their TAL Hall thrusters. So this work with MPD is already the synthesis of everyone else's work.
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IMO the VASIMR guys are not being totally honest about the performance of their system. They've been quite big on plans (They keep repeating that 39 days to Mars lie over and over again, after all) and very nonspecific on achievements.
Anyway, given actually available power sources I don't think Isp really matters that much once it gets higher than 3000 s or so.
-Josh
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IMO the VASIMR guys are not being totally honest about the performance of their system. They've been quite big on plans (They keep repeating that 39 days to Mars lie over and over again, after all) and very nonspecific on achievements.
Anyway, given actually available power sources I don't think Isp really matters that much once it gets higher than 3000 s or so.
A 39 days to Mars may be achived only coupling VASIMR with a Bussard's Polywell fusion generator, if it will work. But Bussard has just designed the probably more efficient high thrust-high Isp electric propulsion QED engine, to couple with his Polywell.
However VSIMR stody may be of some interest, for undertanding magnetic nozzle.
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If there were such a thing as a lightweight electric power supply in the megawatt range (and there is not), then electric propulsion (and there are many types) could be used for faster transit to Mars. I don't know if 39 days would be feasible, but even 90 days would be good. This problem needs solution, but we cannot wait on its eventual development. You add that later when you have it in hand.
At the few-kilowatt level, we already have fairly lightweight electric power, and we'll need it anyway for any manned mission to Mars. (Shuttle's panels were 25 KW, I believe.) Why not combine electric and conventional propulsion in the same vehicle? Do the departure and arrival burns at Hohmann-transfer levels with conventional rockets, but also add continuous acceleration and deceleration during the long transit with electric propulsion. That would cut down total transit time.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Hi Josh:
To answer your question about cutting transit time: I dunno. I haven't run any numbers, and I know little about the electric propulsion devices. I suspect that the key driver for tradeoffs will be solar panel weight. Do you have any figures for weight per unit power? For thrust per unit power? What about for the gravity-losses of using non-impulsive micro-thrusts?
But, the idea is an intriguing one.
GW
Last edited by GW Johnson (2013-12-29 11:57:26)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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I could give you figures for current space rated solar panels. The last time I checked, the best today were the Improved Triple Junction panels sold by SpectroLab, purchased by Boeing a number of years ago so now a division of Boeing Satellite Systems. They had a multi-decade plan to very slowly improve sunlight conversion efficiency. Today their website lists Ultra Triple Junction, and NeXt Triple Junction (capitalization is from their website), so I guess they are proceeded on schedule. Their XTJ panels are listed to produce 366 W/m^2 @ 28°C beginning of life, for panel area > 2.5 square metres. For smaller panels they're listed at 345 W/m^2 BOL. Mass with 3 mil ceria doped coverslide is 1.76 kg/m^2. Cell thickness is 5.5 mil, not including coverslide. They say mass is add-on to substrate, so I guess you would have to add something for the backing that holds the panel together. Their link for arrays just points to a page with some sales verbiage about panels; no numbers.
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MPD
http://www.nasa.gov/centers/glenn/pdf/1 … 11-022.pdf
Glenn is currently developing high-specific-impulse, megawatt-class, hydrogen-fueled MPD thruster technology. ...
Testing for these thrusters has demonstrated exhaust velocities of 100,000 meters per second (over 200,000 mph) and thrust levels of 100 Newtons (22.5 pounds) at power levels of 1 megawatt.
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RobertDyck-
Better Panels are available. These are space rated and have a mass of .84 kg/m^2. I don't know exactly what this includes, though.
-Josh
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Interesting. And impressive. To be fair though, their mass does not include cover glass (known as coverslide). Their ZTJ cells are rated for 135.3 mW/cm^2 = 1353 W/m^2. That's 3.7 times as much power! Conversion efficiency is not that different, so what's up? Emcore lists "Electrical Parameters @ AMO". What is "AMO"?
That same table has "Vmp = 2.41V" and "Jmp = 16.5 mA/cm^2". The letter capital "I" is usually used for impedance, I don't know why they use "J". And they have "Jsc" as well as "Jmp". Since volts are lists as "Vmp" and "Jmp" is lower, let's use that. Calculating: 2.41 * 16.5 = 39.765 mW/cm^2, or 397.65 W/m^2. You could round off to 3 significant figures. That is a lot closer to what Spectrolab lists, comparing apples to apples.
Of course all these figures are Beginning Of Life, because space radiation will degrade them over time. But they're designed for satellites that last multiple years, so a 6 month journey to Mars won't have significant degradation. The other qualifier is this power output is in Earth orbit, sunlight intensity will diminish as you travel from the Sun.
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Spectrolab has an option for 3 mil or 6 mil coverslide. The mass I listed is with 3 mil.
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