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#1 2012-05-22 02:03:18

Impaler
Member
From: South Hill, Virginia
Registered: 2012-05-14
Posts: 286

Forget NTR, SEP is the Future

Many people think Nuclear Thermal Rockets such as NERVA are 'THE' propulsion answer for Mars.  I beg to differ, their are several reasons we will never see NTR and instead the most likely propulsion solution for replacing chemical will be Solar Electric.  I am NOT saying that Chemical can't be used, it's clear that given enough Initial Mass LEO a chemical only mission is possible.  But IF you believe some form of propulsion more IMLEO efficient is needed to make a mission politically/economically viable then SEP is the only near term technology worth considering.

First the reasons why NTR will never happen.

First they are getting absolutely zero R&D investment from anyone anywhere, sure NASA might come up with reference mission architecture that use NTR but their money is not ware their mouth is, a tech that's got no active R&D budget will never be delivered no matter how 'simple' and 'done' it is from past research, it must be a living area of engineering to be used.

Second it is a political non-starter, this is of course the reason no ones wasting R&D on it because it would be impossible to use for public opposition reasons even if it was at technology readiness level 10.  Even if the Radioactive exhaust is utilized only in space the launch of such a large amount of nuclear material is beyond the public tolerance threshold which currently allows for only small Radioactive Thermal Generators on scientifically ambitious deep space probes.

Third the performance of ISP in the ~900 range on paper only double that of chemical is pulled down by the large non-propellent engine mass.  Thus the whole systems performance in payload delivery is not sufficiently large compared to Chemical propulsion (~50% more payload) to be sufficiently advantageous to justify the development of the technology even if the first two barriers did not exist.  More chemical rockets would probably be cheaper then development costs of such a technology.


Now to the advantages of SEP and why it is the future.

1:  Solar Electric Propulsion IS being actively researched by government AND private industry.  The commercial satellite industry is increasingly moving to SEP as THE propulsion method for both station-keeping and orbit raising of satellites this is driving electrical efficiency, power-2-mass ratio, ISP and reliability up and price down.  Government is interested in SEP for deep space missions and continues to fund development.  Likewise the power-source solar is being developed with even MORE money from the home PV market but their is significant spillover into space usage and additional research is specifically done to improve power-2-mass ratios the critical value for use in space.  This makes SEP a LIVING technology which has huge potential for improvement in the near and mid-term that is effectively guaranteed.

2:  Obviously SEP faces no political hurdles or public back-lash as would anything propulsion system that carries the N-word.

3:  The power requirements for Electric Propulsion to transport CARGO on long slow (2 year) flights is very reasonable and achievable with Solar technology power-2-mass ratios only slightly above current levels (but larger in area).  Criticism of SEP and particular VASIMIR made by Zubrin are valid only if attempt to use the technology for what it was not naturally inclined for aka fast crew-transport, in which case we would need Nuclear power of extraordinary energy density.  If not for the fact some VASIMIR advocates had actually proposed such a preposterous vehicle Zubrin would have been guilty of an egregious straw-man.  Once fast crew-transport is eliminated and slow cargo transport 'tugs' become the focus SEP looks attractive.  Any manned mission will simply use Chemical propulsion for movement of crew, but because all mission architectures send multiple unmanned cargo modules for every 1 manned module a system that moves cargo can have the greatest impact on IMLEO.

4:  Because of the high efficiency of propellent use and the relative small fraction of outbound mass that the engine, solar array and fuel tank represent it is very cheap in terms of sacrificed payload mass for a SEP tug to be reused.  The tug simply needs to retain a small residual fuel after separating from the payload to allow it to reverse course and achieve Delta-V equivalent to the outbound mission that brings it back to it's starting point.  The tug could then be refueled and reused, albeit with a multi-year duty cycle that would likely necessitate a large fleet in transit at any time.

5:  Payload fraction is at-least comparable and potentially superior to NTR ranging from +50% to +100% more then Chemical (between half and 2/3 of IMLEO or even 3/4 if the Payload separates and airobrakes for orbit or landing.

My figures come from this Ad-astra (VASIMIR developer) paper, but other forms of SEP that can achieve the 5000 ISP (easy) and the thust-2-mass ratio (harder) would be applicable.  Plan Ion drives have a large head-start and the NEXIS currently in development which has an ISP target of 7500, it could easily scale up to a tug though it's Xenon propellent would be expensive then VASIMIR which can use anything as propellent.  My own architecture would split the mission into a LEO <-> EML1 tug and an EML1 <-> Mars Capture tug, each tug being reused multiple times and probably being a bit smaller in mass and moving payloads in the 50mt range.  The first leg tug to EML1 would be developed first for Moon exploration so the later Mars infrastructure builds on-top of an existing asset rather then needing to be created entirely from scratch.  An Argon depot at EML1 is used to refuel and avoids the difficulty of cryo LH2 storage, liquid Argon being similar to Oxygen in it's boiling point but denser.

http://spirit.as.utexas.edu/~fiso/telec … -19-11.pdf

Last edited by Impaler (2012-05-22 03:47:32)

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#2 2012-05-22 07:59:23

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 3,754
Website

Re: Forget NTR, SEP is the Future

NERVA is a 40-year old technology that was a first step toward the upgrade waiting in the wings:  gas core NTR.  Gas core was at the academic lab project stage when NERVA was ready to fly,  and NASA very unwisely cancelled the whole thing.

The best-informed design calculations of the time indicated gas core open cycle had Isp's in the 1000-2500 sec range at engine T/W in the 10-30 range without need for a waste heat radiator.  It appeared regenerative cooling was adequate.  They had bench tested a flow scheme as good as perfect containment,  and they had demonstrated controlled gas-phase fission.  The rest was just technology development in a company lab,  because this was way beyond what academic institutions could handle. 

You look at 2000 sec and you find out that a Buck Rogers-type single stage ship could fly to the moon and back,  at both high payload and high structural fractions.  That was LH2.  Nobody had yet really done a water NERVA,  much less a gas core of any kind.  Water is the really attractive propellant,  because it's "everywhere" and you can refuel to come home.  That ups payload massively. 

Operated above 2500 sec Isp,  a very large waste heat radiator was required.  There was an ill-defined "transparency limit" to reactor power levels somewhere around 10,000 sec.  The design target was a spherical injection scheme operated at 6000 sec,  with an engine system (including radiator) T/W was thought to be near 0.01 to maybe 0.1.  This would have made a very good orbit-to-orbit engine,  with enough vehicle acceleration to be "impulsive" and avoid the long-burn delta-vee losses associated with electric propulsion.  (I don't know,  but I suspect SEP will suffer the same low-acceleration long burn losses. It happens when vehicle acceleration falls below crudely about 0.02 gee.)

That 6000 sec gas core engine was one of three candidates being considered in 1969 for the manned Mars mission actually on-the-books at NASA for 1983.  By the the time the whole thing was cancelled in 1973,  that Mars shot had been pushed back to 1987.  The other two were NERVA and chemical. 

You look at 6000 sec,  and you can go to Mars single stage,  two-way,  at very,  very reasonable mass ratios.  You can even fly very fast:  how about 75 days one-way?  There is enough mass ratio for nice payload and structural fractions.  The structural fractions can cover both the equipment required for cry-storage of propellants long-term,  and for far more structural durability than we have ever used before in a rocket vehicle (except the X-15).  The prospect of a fully reusable ship that could serve for decades,  even centuries,  in space,  thus becomes real. 

These are the "small" applications:  vehicles between a few dozen to a few thousand tons.  Mars,  Venus,  Mercury,  the NEO's,  and maybe,  just maybe the Main Belt could be reached with men using tinkertoys like these. 

For the biggies (which would be needed for planting real colonies anywhere later on) there is nuclear pulse propulsion,  which we already knew in 1959 would actually work.  That set of physics is quite peculiar:  it works better and easier at launch weights above 10,000 tons. 

That kind of ship is not built like anything we have ever seen before.  You build it like a warship,  of heavy steel.  In something resembling a real marine shipyard.  Isp potential is in the 10,000 to 20,000 sec range (at least),  and it is difficult to hold vehicle accelerations under 2 gees.  There are side effects:  fallout and EMP effects.  But it would be worth it to launch maybe half a dozen of these things over about 50 years,  sometime in the not very distant future. 

Propulsion advancement is the real key to just about anything we would ever want to do out there.  The best place to test/develop and maybe base the launch of stuff like that is the moon.  No air and water to pollute,  no neighbors to annoy.  Close enough to reach with what we have right now. 

More wild ideas offered to spark out-of-the-box thinking.  But,  these ideas really are/were supported by the numbers.  These could actually be turned to reality,  if we as a people decided to do it. 

Enjoy!!

GW

Last edited by GW Johnson (2012-05-22 08:07:19)


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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#3 2012-05-22 13:39:59

Impaler
Member
From: South Hill, Virginia
Registered: 2012-05-14
Posts: 286

Re: Forget NTR, SEP is the Future

Yes, the on-paper advantages of the NTR are all VERY impressive but none of that refutes my points as to why it will never happen.  NTR ISP >900 would require considerable development and even the basic NERVA was only developed to TRL 6, even ignoring skill 'rust' that puts it well behind current SEP at 9 and on-par with VASIMIR.  The more advanced super high ISP variants you refer to are down at TRL 1-2 according to this pro-NTR NASA study.

http://ntrs.nasa.gov/archive/nasa/casi. … 015551.pdf

The best 'off-the-shelf' NTR provides such a modest improvement over Chemical that it's not worth the effort to take it off the shelf if we were going to Mars tomorrow.  If were talking about a mission decades down the line (which is the only realistic time-frame) then improved NTR should be compared to equally improved SEP, but even that comparison is unfairly in favor of NTR because advanced SEP is guaranteed by the existing research efforts, ware as NTR is still on the shelf.  So the comparison in 20 years time will be between advanced SEP and still 1970's level NTR tech at 900 ISP.

The current NTR tech doesn't provide any transit-time advantage over Chemical either, only a small payload boost, because you still use standard transfer orbits, only the super high continuous thrust NTR concepts give short duration transit.  Zubrin argues that even if we had some fantastic super-rocket (NTR or SEP) the appropriate use is to ALWAYS increase payload rather then shorten transit time and I agree with him on that.  All the dangers in a mission in space be it in Radiation, micro-gravity, EDL on mars, return fuel, mental health etc can be improved by adding more payload mass, likewise scientific value increases with payload.  Also keep in mind all NTR mission concepts send the Cargo by normal chemical means and save time/mass only with the crew transit, as I said earlier because cargo is dominant a tech that moves cargo efficiently has much more impact of IMLEO, SEP can even be leveraged to move the yet-2-B-manned crew transit vehicle to EML1 reducing the amount of chemical propellant needed.

Now of course IF someone actually starts aggressive development on these advanced NTR concepts then it might fly someday.  But I don't see that happening under NASA or the west in general, maybe in a country with major space ambitions and ware a bit of radioactive exhaust is tolerated (aka police-states) like Russia or China would invest in and use NTR but I don't even think they would 'win' a hypothetical new space race against SEP based nations/programs.  Also given the Chinese cultural value on patience and diligence I don't see NTR being appealing to them the same way giant phallic high thrust rockets are to Americans and Russians.

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#4 2012-05-22 16:52:27

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 3,754
Website

Re: Forget NTR, SEP is the Future

So,  let's go further than Mars.  Say,  to the asteroid belt.  Somewhere near 2 years one way,  something more like 6+ years round trip.  Don't you want to fly faster and cut that down?  Especially if you could refuel out there from frozen volatiles?  That engineering target ought to be a water-propellant gas core NTR.  Further,  depending on the solar cycle,  something like 2 to 4 years in space will incur a career limit radiation dose from galactic cosmic rays,  for which all known practical shielding techniques are rather ineffective.  Flying faster gets really important beyond Mars. 

When I looked at either gas core NTR or solid core NTR for Mars,  my vehicle accelerations were around .05 gee as I sized them.  That's a very few days burn for the very largest delta-vees,  orbit-to-orbit.  Electrics (and VASIMR is one),  have vehicle accelerations well under .001 gee,  which means you "burn" for much of half the trip there, just to accelerate,  and then you "burn" for much of the other half of the trip to decelerate.  Acceleration times over a very few days start incurring very significant solar gravity losses.  Fact of life. 

BTW,  the final NERVA testing had a rather radiation-free exhaust,  though few today believe (or want to believe) it.  They solved the core erosion problems,  and the daughter products stayed in the core.  900 Isp,  engine T/W about 3,6,  47 minute burn,  and multiple restarts is what they finally demonstrated.  And a large throttle ratio,  too.  In comparison,  there were major core mass losses in the early Phoebus and Kiwi series.  Very radioactive exhausts.  Some of that stuff is still out there on the nuclear test site in Nevada,  and will be dangerous for decades,  even centuries,  to come.  (Yes,  I do understand the dangers.)

Open-cycle gas core does have a radioactive exhaust,  but it's daughter products only,  not uranium or plutonium.  That's a different beast by orders of magnitude in terms of collateral radiation exposures.  (Myself,  I'd use thorium-bred U-233,  and avoid plutonium entirely,  but that's just me.)  The real advantage of open-cycle gas core is that it's an empty steel can when you shut it down.  Secondary-induced radioactivity in the engine structures decay in hours (at most) to safe levels.  No core on board to worry about when flying home,  or in a crash scenario.  The exhaust stream velocity generally exceeds solar system escape,  so if you don't point it at the ground,  you'll never pollute the atmosphere in the orbit-to-orbit application.  A truly abortable,  rather safe,  nuclear rocket.  Wow!  Is that ever at variance with widely-held public perceptions!

A lot of the fears being used even today to justify not doing nuclear propulsion,  are actually just completely irrational,  and at quite a variance with actual facts.  Not all,  but most of these fears.  There's real problems to design around,  of course,  but the advantages are simultaneous high thrust/high Isp,  especially when compared to every electric scheme I ever heard of. 

The only verifiably-doable thing I ever heard of in the last 50 years,  offering even higher thrust and Isp simultaneously than NTR,  is nuclear pulse propulsion. 

GW

Last edited by GW Johnson (2012-05-22 16:58:12)


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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#5 2012-05-22 20:23:20

Rune
Member
From: Madrid, Spain
Registered: 2008-05-22
Posts: 191

Re: Forget NTR, SEP is the Future

Impaler wrote:

Third the performance of ISP in the ~900 range on paper only double that of chemical is pulled down by the large non-propellent engine mass.  Thus the whole systems performance in payload delivery is not sufficiently large compared to Chemical propulsion (~50% more payload) to be sufficiently advantageous to justify the development of the technology even if the first two barriers did not exist.  More chemical rockets would probably be cheaper then development costs of such a technology.

Wait, what? I stopped reading here. First, T/W in a NTR goes from ~10 if you use a straight NERVA design to over ~30 if you have fancier cores and turbopumps. So much, much, MUCH lighter engine group mass and unfueled "tug" mass (I prefer propulsion stage or module, 'cause that's what they are). Also, Oberth savings (missions to in cislunar space can cost x4 in delta-v if you don't use it) and you don't fry your electronics, solar panels, and battery system by staying for a year in the Van Allen belts, slowly spiraling your way out of LEO.

Second the "small payload advantage" can make a MTV single stage, and hence fully and rapidly reusable, using a 875s core like NERVA's and a storable soft cryogenic propellant like methane. Is that really such a "small advantage"? Because for me that opens Mars to mass deliveries of cheap payloads with a modest fleet of reusable propulsion stages. Doing that with chemical propulsion looks very, very on the verge of what is possible at difficult to believe mass ratios and using every delta-v saving trick in the book (aerocapture into a very high elliptic orbit at Mars, departure from lagrange points, you name it).

You actually point out yourself the need for nuclear reactors when considering either manned or big cargo electric ships, but you don't state a very important point: a >>1000s isp propulsion system is really good to move to the belt and beyond. Which is where the sun intensity drops like a rock. Good luck not using nuclear reactors there, and that is why I think that SEP is a dead end for anything other that unmanned small ships. Also, the "frying itself on the radiation belts" thing because it takes it years to leave a planet doesn't help either.

And about a NTR's readiness level... do you know what a solid core rocket is, basically? Pick a chemical rocket, switch the combustion chamber for a nuclear core with a cooling loop instead of a preburner to run the turbopump, and you are done. Oh, yeah, and eliminate the oxidizer loop. That simple. The only item that has been upgraded in the following designs after NERVA has been the core. The rest is the same LH2 turbomachinery taken from the J-2, and slightly upgraded as the J-2 itself got upgraded. And if you don't want to certify a new core to use the fancier materials available nowadays, as I said, a NERVA or DUMBO core can do very nicely for anything this side of the belt. The only expensive thing in all of this is the facility where you test-fire the thing, and the industry capable of manufacturing the core fuel elements (that is, the nuclear one, though US' is a bit on the down slope nowadays, maybe you'd better talk to the germans, they are experts in fuel). I would also bet that with today's material science and a modest deign goal, the exhaust ends up looking so harmless that open tests in some nuclear site are perfectly acceptable form a rational environmental point of view (never mind you can convince some hardcore greenie of that) and an earth launch doesn't look that dirty. Though, to be clear, I wouldn't never ever launch an activated reactor, that is just asking for trouble. So NASA can say it's at TRL 6 all they want. I say someone with "cojones", and access to a decent nuclear fuel industry, could have one flying in a very short time. Most of the technical stuff, other than fuel element production, is either open source by now, or depends on your particular design and you have to engineer yourself. Of course, NTR core grade nuclear material is also weapons-grade nuclear material. Hence the bad language in the previous phrase and the need of a government, which always complicates this things.

And as a last note, NASA has never been pro-NTR. They have always seen it as a crude thing that will get killed by politics, IMO, and are only interested in the super-complicated state-of-the-art things like NEP for the outer planets. They were the ones that killed the most promising solid core concepts like DUMBO (which, with much simpler turbomachinery and lower design goal, looked like it had better isp than NERVA at better T/W).


Rune. Ok, I'll admit. I went back and re-read very politely most of your post to reply adequately. wink

Last edited by Rune (2012-05-22 20:37:28)


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#6 2012-05-23 16:46:34

Impaler
Member
From: South Hill, Virginia
Registered: 2012-05-14
Posts: 286

Re: Forget NTR, SEP is the Future

GW:  Your looking for a mission to fit a vehicle rather then selecting a vehicle for a mission.  I'll grant that SEP starts to look weak when your going beyond Mars, but the name of the forums suggests that Mars is the priority and as any credible Mars mission is still decades away the destinations beyond Mars are even further.  Also note that the only vehicles going to the Asteroids right now are SEP that just suffer the low-power conditions by going slower, the desire to reach these destinations with robotic probes has not resulted in NTR tech development.  Also I suspect that SEP could be used for descent speed one-way trips to the outer solar system by 'sun-diving' a tight parabolic around the Sun to generate around x10 the power output of our solar array relative to Earth, burn through most of the fuel and be on your way.

Oh and 'daughter products' might sound benign to some people but I know that stuffs the MOST radioactive, shortest half-life and most dangerous of all biologically compatible stuff like your Strontium-90 and Iodine 131.  Not losing your un-fissioned core material is of course an obvious engineering goal because losing it a a pure waste of potential energy, but in order to be non-radioactive the exhaust would have to be pure uncontaminated propellent (even erosion of irradiated core-cladding would be an issue after a long enough operation) which is a very hard engineering challenge because your fundamental goal of high flow rate, temperature and heat conductivity all work against it.


Both:  The Oberth effect dose indeed provide a Delta-V advantage to a high thrust propulsion system, a slow thruster like SEP needs more Delta-V to achieve the same mission but this is FULLY accounted for in the numbers and papers I presented earlier, a SEP still comes out ahead of NTR in payload for a given mission after the higher Delta-V requirement.


Rune:  When I said the performance of the NTR was reduced I was referring to the Payload Mass, because percent (6% in the paper I site) of a NTR vehicle will be the Engine/Core that's mass which is unavailable for Propellent or payload.  I was not implying anything about a low or lowered Thrust/Mass ratio.  The Radiation belts of Earth are clearly not a concern for electronics of spacecraft that transit the belt slow or fast as many SEP craft have already done this as has every chemical rocket too.

The power needs for a cargo hauling SEP system are only 1 MW and that's for a 100mt vessel with a payload fraction above 50% doing a slow multi-year trips to Mars, that's an amount of power only about 3 times what the ISS generates so Nuclear is NOT required for slow cargo, I said that SEP is not appropriate for fast crew transport.  I also didn't say NASA was pro-NTR, I said the paper was pro-NTR in that it advocates for the use and development of NTR/NERVA and it rates the Tech at TRL 6 which means demonstrated but not yet at a flight ready versions.

Last edited by Impaler (2012-05-24 01:23:22)

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#7 2012-05-23 23:54:14

Terraformer
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From: Lancashire
Registered: 2007-08-27
Posts: 3,160
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Re: Forget NTR, SEP is the Future

What about Nuclear Electric Propulsion? If you're going to have NTR's, you'll need the lightweight reactor anyway, and surely NEP would allow you to convert the nuclear energy into kinetic energy much more efficiently? What sort of power-mass ratio can we get from a nuclear reactor that we could build today?

Do remember that only a small minority complain about placing nuclear reactors on board ships.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#8 2012-05-24 05:04:25

Rune
Member
From: Madrid, Spain
Registered: 2008-05-22
Posts: 191

Re: Forget NTR, SEP is the Future

Impaler wrote:

When I said the performance of the NTR was reduced I was referring to the Payload Mass, because percent (6% in the paper I site) of a NTR vehicle will be the Engine/Core that's mass which is unavailable for Propellent or payload.  I was not implying anything about a low or lowered Thrust/Mass ratio.

I get your point, but you will admit, for a reusable vehicle that refuels through depots (whose propellant is supposed to be cheap), empty weight is important. I mean, it is what you have to launch from earth. Also, in general, the price of aerospace-grade equipment is a function of weight. 50mT of high-power electrical equipment is going to cost at the very least as much as a single 500kg engine and a big empty ~10mT fuel tank.

Impaler wrote:

The Radiation belts of Earth are clearly not a concern for electronics of spacecraft that transit the belt slow or fast as many SEP craft have already done this as has every chemical rocket too.

Yup, they just size the solar panel 40% bigger and rad-protect all the electronics to account for the radiation damage. Good luck reusing a solar tug after crossing the Van Allen belts a few times.

Impaler wrote:

The power needs for a cargo hauling SEP system are only 1 MW and that's for a 100mt vessel with a payload fraction above 50% doing a slow multi-year trips to Mars, that's an amount of power only about 3 times what the ISS generates so Nuclear is NOT required for slow cargo, I said that SEP is not appropriate for fast crew transport.  I also didn't say NASA was pro-NTR, I said the paper was pro-NTR in that it advocates for the use and development of NTR/NERVA and it rates the Tech at TRL 6 which means demonstrated but not yet at a flight ready versions.

That is what bugs me. You yourself say SEP is probably never going to get a man on the way to mars. Yet you want to develop it as a way to, ultimately, get people to mars. I say if you can't develop the rational propulsion choice to get people there cheaply, then at least go with the one (developed already) that can, plain chemical expendable rockets. Don't overcomplicate things and get lost in the fancy engineering like NASA has been doing for the last 40 years. Just pick something that can get the job done and actually use it to get the job done for a change.

Tough I do admit, let me be clear on that, that if you restrict SEP to unmanned payloads and high orbits/interplanetary flights in the inner solar system, it not such a bad propulsion system. My point of contention is that nukes are a better, more universal one, that can handle manned payloads.


Rune. So don't forget them!


In the beginning the universe was created. This has made a lot of people very angry and been widely regarded as a "bad move"

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#9 2012-05-24 05:16:25

Rune
Member
From: Madrid, Spain
Registered: 2008-05-22
Posts: 191

Re: Forget NTR, SEP is the Future

Terraformer wrote:

What about Nuclear Electric Propulsion? If you're going to have NTR's, you'll need the lightweight reactor anyway, and surely NEP would allow you to convert the nuclear energy into kinetic energy much more efficiently? What sort of power-mass ratio can we get from a nuclear reactor that we could build today?

Do remember that only a small minority complain about placing nuclear reactors on board ships.

Well, NEP's have the worst problems of both worlds. First, the reactor is quite different, it is not cooled by the propellant and therefore requires huge radiators, so no weight advantage vs a solar system, at least in the inner planets. Same T/W, too, so huge flight times also. It is inherently more radiation-robust, of course, but that is a small consolation for me. And don't get me started on efficiency... electric drives are about 60% efficient, and nukes never get above 50%. So about 70% of your energy gets wasted as heat. Tough that is better than solar panels, of course. A thermal rocket is in the very high nineties, in practical terms 100%, courtesy of the thermodynamic trick of open cycles and regenerative cooling.

And about complains about nukes... well, honest-to-god nuclear reactors have been flown and nobody on the general public seems to have noticed in the first place. My point of view? They will buy whatever you sell them. Hate it, too, but that is always a fraction of any response.


Rune. I think the people that ought to know better are the most scared of trying to use it.


In the beginning the universe was created. This has made a lot of people very angry and been widely regarded as a "bad move"

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#10 2012-05-24 05:39:07

Terraformer
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From: Lancashire
Registered: 2007-08-27
Posts: 3,160
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Re: Forget NTR, SEP is the Future

Ah. What about the greater Isp that can be attained by electric propulsion, though?

I think Solar Thermal ought to be looked at more. It's far simpler than SEP, and gets higher efficiencies - you're just using sunlight to heat a propellent, after all, rather than turning it into electricty and then using that to accelerate a propellent.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#11 2012-05-24 18:01:37

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 3,754
Website

Re: Forget NTR, SEP is the Future

Last I heard,  traceable numbers supported by actual test data for electric propulsion said Isp was in the neighborhood of 6000 sec,  about the same as the projections for gas core NTR.  That actually includes VASIMR,  too. 

All of these electric methods need a nuclear power source to operate for more than just a very few minutes.  That power plant weight pushes vehicle accelerations well below 10^-3 gee.  As for gas core NTR,  the best estimates from 40+ years ago said the heavy radiator pushed vehicle accelerations down in the neighborhood of 0.05 gee,  if you operated above 2500 sec.

Below 2500 sec Isp,  no radiator was required,  based on the best estimates from 40+ years ago.  Without that radiator,  you are looking at around 2000-2500 sec at engine T/W 30+,  for orbit-to-orbit vehicle accelerations somewhere near 0.1 gee,  unless you over-size vehicle T/W.  You don't need to do that.  It does skew results very seriously. 

I don't know about solar electric,  but I don't at this time see how the solar electric power plant can weigh much less than the nuclear electric power plant,  for the same MW-level outputs.  Both are then pretty heavy for the power,  at least we already know the nuke is. 

That being the case,  I don't see much advantage to SEP over the others we are discussing,  unless the Isp is way beyond the 6000 sec class.  This is given that we are probably looking at practical vehicle accelerations under 10^-3 gee,  just like all the other electric schemes.  As far as I know,  Isp estimates for SEP are supported by no test data as of yet.  (But I don't know much about it.)  It's certainly worth a look,  but I suspect it's a long way from flying,  even as a laboratory demo. 

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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#12 2012-05-24 18:18:59

Rune
Member
From: Madrid, Spain
Registered: 2008-05-22
Posts: 191

Re: Forget NTR, SEP is the Future

Oh, no, GW, turns out ion tugs are on the rise, and have been for some time. Just a few months ago Boeing announced it was rolling out the new 702 GEO comm birds as all-electric, to launch in pairs in Falcon 9's. Xenon ion thrusters with 3,000-5,000s isp are quite commonplace now for stationkeeping, especially on the russian side of things because they developed them first way back when. I think the failure in the last USAF's AEHF satellite, where they lost the apogee engine and had to salvage the mission by using the station-keeping ion thrusters to get to orbit, gave electric propulsion a big boost confidence-wise... and the sat a radiation dose it was definitely not designed for, we'll see how long it lasts. For unmanned satellites everything, of course, and mainly because of the station-keeping advantages (though they do manage to fit two GEO commsats on a single Falcon, using this and other weight savings). Solar panels are also getting better all the time, giving nuclear power-to-weight ratios a run for their money, though I am most definitely not up to date on that.


Rune. As I said, they have their uses.


In the beginning the universe was created. This has made a lot of people very angry and been widely regarded as a "bad move"

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#13 2012-05-25 03:00:03

Impaler
Member
From: South Hill, Virginia
Registered: 2012-05-14
Posts: 286

Re: Forget NTR, SEP is the Future

GW:  You don't need Nuclear power to run a SEP system of large size for the inner solar system, the mass efficiency needs to improve yes but by only a modest amount and the potential gains for Solar panels are far better then the potential gains in nuclear-electric reactors because of the radiator limitations and energy conversion equipment.  If your panels can produce 100W per kg then 10mt of panels produce 1 MW.  Of course to account for the power-management and Thruster structure you would want a bit better then that so that the total SEP engine system was at-least 100 W/kg but the thrusters themselves are generally small so this isn't a very big challenge, the Solar collector mass is the dominant factor.  According this this paper current state of the art panel is/was 60 W/kg and development efforts towards 300 and eventually 1,000 W/kg is underway and considered achievable.  Again this is research already IN the pipeline that has near-term application and which will see use in missions as soon as it's available.  The use of thin film Fresnel concentrators is the key.

slasr.com/Papers/STAIF04.pdf

Likewise the ISP of IN DEVELOPMENT Ion thrusters is going even higher, NEXIS is the next evolutionary improvement of the DeepSpace1 spacecrafts engine and it's development target is an ISP of 7500 and an energy consumption of 20 kW.  And this system is little more then enlarging, increasing the ruggedness and upping the Voltage of the last generation of engines so it is again basically in-pipeline tech we will see in the next 10 years.

http://hdl.handle.net/2014/38479

SEP has basically already WON the race to be the dominant post-chemical propulsion system in that it has secured the lions share or both public and private research dollars and is on a sustained development trajectory that will not be abandoned because it's bearing fruit already and shows no signs of reaching physical constraints (unlike Chemical which is basically maxed out as a tech).  I'm not really 'championing' anything here in the sense that I'm saying "We SHOULD go research tech X, because X is great" which is the position of someone who advocates for anything not currently being worked on like NTR, I'm just saying SEP is a what the development pipe has decided to give us for the next 10-20 year time-frame, it is a Fait accompli.  Wishing for and planning around technologies that aren't in development while ignoring useful tech what is coming down the pipe is foolish and wasteful.

I can see a very viable Mars mission conducted with SEP and an EML1 depot.  Just move all your payloads and chemical fuel to EML1 with SEP tugs, pass the unmanned payloads over to interplanetary tugs which go to Mars and assemble the manned payloads to modest sized chemical booster for fast transits.  This should easily deliver something like half the Initial Mass in LEO to Mars capture while setting up a reusable fleet of tugs that can sustain multiple repeat missions and eventually more permanent bases as well as large robotic missions to the entire solar system.

Last edited by Impaler (2012-05-25 03:04:00)

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#14 2012-05-25 09:49:11

GW Johnson
Member
From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 3,754
Website

Re: Forget NTR, SEP is the Future

Impaler:

Well,  may be.  Being an old-school guy,  it worries me about delta-vee losses when vehicle acceleration is so low,  but then that's a mission-specific concern.  My old brain finds it hard not to get hung up on that,  but that's just me.  It applies to all the electric schemes:  tons of engine for one or two digits of newtons (or pounds) of force. 

Actually,  the smart thing to do would be to work seriously on all these things,  and bring them into developmental flight demonstration status where they can be flown and evaluated on real experimental missions.  If any of them look really good at that point,  then it is prudent to spend the effort to make them fully ready for reliable and routine use. 

I haven't seen NASA effectively do a thing like that since the 1960's.  It all got shut down by 1973,  after Apollo got cancelled in the middle of the planned landings,  and all human flight out of LEO was forbidden by then-President Nixon.  That's part of why we have floundered in orbit for 4 decades,  the rest of the cause being political crap. 

I really would like to see it all flown and evaluated:  SEP,  ion,  VASIMR (another magneto-plasmic thing),  solid NTR,  gas NTR,  nuclear pulse propulsion,  solar sail,  arcjet,  laser propulsion,  and maybe even whatever the latest chemical ideas might be (such as light gas gun launch).  Each has a use,  if it can be made ready.  Each different kind of mission has a "best" propulsion mix,  which these various technologies can fill. 

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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#15 2012-06-02 18:07:16

Impaler
Member
From: South Hill, Virginia
Registered: 2012-05-14
Posts: 286

Re: Forget NTR, SEP is the Future

An 'all of the above' strategy isn't going to happen, their are only so many research dollars to go around and Nuclear propulsion methods on their technical merits do not deserve any of those dollars in my opinion.  With Nuclear currently being allocated zero dollars to say we "should" be developing them all is just to say money should be removed from SOMETHING else to be spent on Nuclear.  Money doesn't grow on trees so unless you can simultaneously convince government to massively increase funding any money spent on Nuclear propulsion is a loss to development that would otherwise be more fruitful.  All of the above development strategies are only justified if you have no clue what the correct solution is but even then you start try to down select towards to more promising options and Nuclear would be the first thing to cut, their are SO many techs beyond even SEP that are critical to Mars exploration that are either mandatory or offer greater cost reduction then Nuclear propulsion.

With respect to the Radiation degradation of Solar panels, I've done some research and it dose indeed look to be a factor for normal panels.  But the recently developed Concentrator panel which was first demonstrated in DeepSpace-1 (along with the first use of Ion-drive) will allow a small strip of high efficiency PV that can be shielded heavily enough to give a reasonable life-span of 5-10 back and forth transits through the VA Belts, while retaining high power density.  This 2007 paper discusses the predicted performance under known VA radiation environment, it looks like a version of this panel was launched on a highly eccentric spy satellite which will expose it to considerable radiation.

http://www.slasr.com/Papers/IEPC07_SLA_Paper.pdf

This type of Panel is expected to have an initial power density of ~300 W/kg which will degrade to ~250 W/kg by the end of lifespan (better at end of its life then standard ones are at the start).  A hypothetical SEP tug craft will likely have suffered enough erosion of its Ion grids to be in need of refurbishment/replacement by that point anyway.  A tug of 10 mt (3 mt panels, 6 mt thruster hardware, 1 mt Propellent tank) running on 1 MW of power and utilized 5-10 times amortizes to just 1-2 mt per payload, factoring in fuel at already archived ISP and the IMLEO needed to get a payload to EML1 or Mars is HALF what would be needed with Ideal chemical propulsion.   A decade ago before DeepSpace-1 skepticism of SEP was quite justified, it had never been used before outside a laboratory and was 'Sci-fi', but after a decade of rapid advancement and validation (and flawless performance btw) it should be obvious which way we are going.   A MW class SEP tug 10 years from now is very very conservative given what has been achieved in the last 10 years, no real breakthroughs, the rendezvous and fuel transfers are the the biggest outstanding issues to making a tug and that's in the research plan already.

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#16 2012-06-03 11:22:23

GW Johnson
Member
From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 3,754
Website

Re: Forget NTR, SEP is the Future

"All of the above" is not limited to just NASA's budget.  Funding can come from a variety of agencies,  as it did decades ago.  There are many paths here to explore.  And they all need exploring.  Not as a precursor for going to Mars,  but for later on. 

I for one would like to see a means be found to uprate the engine thrust/weight ratios for electric devices,  out of the 10^-4 class.  They have the Isp,  it's just that vehicle accelerations that are ridiculously low.  Until that changes,  those are not a viable path for fast trips to Mars (it's too close for the buildup of speed to have much effect). 

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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#17 2015-04-25 20:19:17

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 17,333

Re: Forget NTR, SEP is the Future

http://www.nasa.gov/sites/default/files … or_SEP.pdf

One problem to note is the table with power and fuels for the ISP to time for arrival one should note that as power goes up so does the mass for the system that creates it and that is not accounted for in the table....



http://www.newmars.com/forums/viewtopic.php?id=5346

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#18 2015-04-25 23:42:50

kbd512
Moderator
Registered: 2015-01-02
Posts: 3,098

Re: Forget NTR, SEP is the Future

NTR doesn't receive any funding because stupid people are deathly afraid of anything with nuclear in the title and that's all there is to it.  The fact that no one has been killed in the US by nuclear power generation, for the entire history of our use of nuclear power, does not dissuade the stupid people from associating nuclear power with nuclear weapons.

There is no comparison, none whatsoever, between the energy density of nuclear fuels and the theoretical maximum energy density of solar panels.  There is a real problem with converting all the energy released in fission into electricity, though.  That's a different problem.

Uranium, even HEU, is not an optimal fuel for space power and propulsion applications.  Naturally, NASA has decided to spend what little money it devotes to NTR research on producing sub-optimal nuclear fuel rod assemblies for sub-optimal methods for using nuclear reactors for propulsion.  Space nuclear reactors wouldn't be nearly as big and heavy as they are if Americium was used instead of Uranium.

SEP is a technology worthy of continued development because it's ISP is better than chemical and the nuclear thermal alternative, which provides both high thrust and high ISP, is unpopular with stupid people.  That does not mean that nuclear power and propulsion are not worthy of future development.

I'm not holding my breath, but if Lockheed manages to produce a working fusion reactor of the size and power they're currently working on within the next five years or so, this debate will be entirely academic.  Fusion research receives plenty of funding from a variety of universities, corporations, and governments.

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#19 2015-04-26 09:08:34

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 17,333

Re: Forget NTR, SEP is the Future

The trouble with the technology is a false equating that it is a miricale cure as it can not with all its isp get a rocket off the planet so its isp is not as good as chemical. The isp is that of time dependant which means it needs time to build up forward velocity, now the solution to forward velocity is to use more engines to obtain the same reactions as to that of a chemical engine to get the same time with acceleration.

Pg 14's slide shows the progression of ion drive isp with mass, wattage and delta v derived but it does not place into the equation time to accomplish.

Pg 16 shows the roadmap of development with the DRA 5 being the manned mission on pg 19 using the combination of nuclear and solar to get cargo but the slide only assumes manned flight....

The slide that got my attention was pg 21 with  table

Delta V, SEP Power, and Specific Impulse
• Sample Comparison -- Earth-to-Mars transit with 2000 kg
spacecraft dry mass with varied power and specific impulse

saying that doubling the isp causes a slow down in time to arrival... huh....

On the same slide was the delta needed:

Mercury, 17.1 km/s
Venus, 5.2 km/s/s
GEO, 3.9 km/s
Moon, 4.1 km/s
Mars, 5.6 km/s
Jupiter, 17.1 km/s
Saturn, 15.7 km/s
Neptune, 15.7 km/s
Uranus, 15.9 km/s
Pluto, 15.5 km/s

Delta V’s based on a Hohmann Transfer to a circular orbit with no gravity assist

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#20 2015-04-26 13:10:27

kbd512
Moderator
Registered: 2015-01-02
Posts: 3,098

Re: Forget NTR, SEP is the Future

There are intelligent ways to use SEP in conjunction with chemical rockets to minimize the mass of the propulsion systems required for a manned Mars mission.

In my DRM, chemical rockets (F9H only) are used to lift the individual pieces to ISS for assembly and testing as well as transfer crews to MTV's at L1.

There are some reasons why this works well and it requires an explanation of how mission hardware is assembled and utilized:

Individual mission hardware components are launched using F9H for assembly at ISS.  The SEP system is integrated with the MTV rather than being a separate module mated to the MTV.  The total mass of the MTV is low enough for this to be practical since no massive multi-person lander or heavy capsule system is attached to it.

A chemical kick stage with storable propellants is mated to the MTV at ISS as part of final assembly.  Once assembly and checkout have been completed, the unmanned MTV is transferred to L1 using a SEP tug.  This eliminates crew time for the transfer from LEO to L1, eliminates crew radiation exposure problems associated with spiraling out through the Van Allen belts, and permits the MTV to protect its solar arrays during its transit to L1.

The crew boards the MTV at L1 using Dragon/F9H.  The Dragon capsule would then be returned to ISS using the SEP tug that brought the MTV to L1.

The chemical kick stage sends the MTV to Mars from L1, eliminating the time required to spiral out.  The MTV's onboard SEP propulsion system is only responsible for LMO insertion, transfer to L1 from LMO, and L1 insertion.  If a SEP tug mated a chemical kick stage to the MTV in LMO for the MTV to use to depart LMO, then all we require our SEP system to accomplish is LMO insertion from L1 and L1 insertion from LMO.

After the MTV returns to L1 another Dragon capsule takes the crew from the MTV to ISS to return to Earth with a departing ISS crew or the MTV crew can simply return to Earth.  A SEP tug would take the MTV back to ISS for repairs, refurbishment, and refueling.

The synergistic use of chemical and SEP can keep mission hardware component masses within the lift capability of single F9H flights.  Refraining from adding the mass of multi-person landers and capsules to the MTV permits use of integrated SEP hardware.

The point to all of that is that there are ways to make SEP work for manned missions, but only in conjunction with chemical rockets and only for missions that don't start in LEO (neither of which is a big deal, IMO).  If you don't use chemical rockets with SEP, your transfer times go up dramatically or your ISP takes a big hit.  From NASA's own DRA 5.0 presentation, using NEP resulted in the lowest IMLEO and using NTR resulted in the greatest number of days on Mars.  Combined chemical and SEP resulted in missions with the longest duration, with the fewest days on Mars.  However, IMLEO was on par with NEP and we're all aware of how launch costs affect mission costs.

Using an Americium fueled reactor, gas core for NTR or thin film for NEP, would make the comparison more favorable for the nuclear options.  The insistence on using Uranium fueled reactors that require large core diameters and heavy moderators to sustain fission is what makes nuclear power unattractive in terms of power density.  There's such an extreme difference in volume and mass between a reactor fueled with Uranium vs Americium that there's no case to be made for continuing research into using Uranium fuels for space nuclear power.

Last edited by kbd512 (2015-04-26 13:19:34)

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#21 2015-07-09 20:00:27

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 17,333

Re: Forget NTR, SEP is the Future

Aerojet Rocketdyne Successfully Tests PowerTrain(TM) Solar Electric Propulsion Concept

Aerojet Rocketdyne (NYSE:AJRD) has successfully built and tested a prototype system designed to improve the way power is delivered from the solar arrays to the high-power electric thrusters on Solar Electric Propulsion (SEP) spacecraft. The prototype was tested in a simulated mission environment to show that it could achieve improved system efficiency over the current Power Management and Distribution (PMAD) approaches used on satellites.

"For high-power SEP modules, such as those that NASA envisions for transporting cargo to Mars, it will be critical to efficiently process and deliver the power from the Advanced Solar Arrays to the High Power Electric Propulsion thrusters on spacecraft," said Julie Van Kleeck, vice president of Advanced Space and Launch Programs at Aerojet Rocketdyne. "This is a critical step toward achieving that goal as we prepare to journey further into space."

The PowerTrain™ SEP system, which uses a peak power tracking capability, is a way to increase overall vehicle electrical system efficiency, and it is compatible with current and future advanced Hall propulsion systems. The system was developed using breadboard power conditioning modules provided by Aerojet Rocketdyne. The concept was tested at Aerojet Rocketdyne's Los Angeles site, where the majority of International Space Station power-system hardware was developed.

As the article meantion good for cargo to far off places....

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#22 2015-07-11 16:46:18

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 17,333

Re: Forget NTR, SEP is the Future

AeroJet Rocketdyne conducts successful test of PowerTrain solar electric system

Aerojet-Rocketdyne-SEP-647x467.jpg

Aerojet Rocketdyne’s SEP thrusters were first used on the RCA Satcom 1R satellite in 1983. Aerojet Rocketdyne has flown over 500 electric thrusters since then, and over 200 satellites currently in orbit use their SEP systems. A mission concept study conducted by Aerojet Rocketdyne in 2012 indicated that mission costs for NASA Human Exploration cargo missions could be reduced by over 50 percent through the use of existing flight-proven SEP systems.

“Our team has constructed an approach that leverages the benefits of SEP to improve affordability and reduce the risk for NASA Human Exploration,” said Van Kleeck. “Using an SEP tug for cargo delivery, combined with NASA’s Space Launch System and the Orion crew module, provides an affordable path for deep space exploration.”

http://www.rocket.com/article/aerojet-s … xploration

Self-funded under a collaborative Space Act Agreement, Aerojet’s SEP mission concept study yielded a solution that uses the same flight-proven electric propulsion approach that supports the Air Force Advanced-Extremely High Frequency (AEHF) SV-2. By providing the aerospace community with SEP vehicle designs that operate at power levels ranging from 9 to 30kW and that use flight-proven technologies, Aerojet provides both a low-risk, affordable approach to NASA Exploration cargo missions and a stepping stone to next-generation higher power SEP transportation vehicles.

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#23 2015-12-25 23:24:26

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 17,333

Re: Forget NTR, SEP is the Future

Sometimes when I am researching other topics I come across documents that apply to others and this is just another of them.

Adequate Electric Propulsion System Parameters for Piloted Mars Missions
http://erps.spacegrant.org/uploads/imag … ex/219.pdf

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#24 2016-01-24 22:24:15

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 17,333

Re: Forget NTR, SEP is the Future

I got looking at the JPL images and the ion drive engines require 200kwe to make them work. That said here is the level of solar power and storage that we can put together to create that power level for the ion drive engine.

The current space mission solar array designs seem to be the firmular circular fans of Nasa seen on Orion and others in the UltraFlex solar array.

solar_array_photo1.jpg

http://nmp.jpl.nasa.gov/st8/tech/solar_array4.html
or start with the beginning page for the array
http://nmp.jpl.nasa.gov/st8/tech/solar_array1.html

Solar energy striking a square meters worth of panels is 1367 watts at earth orbit. The 5.5-m diameter UltraFlex 175 array will be capable of providing 7 kW of power with a deployed specific power of 175 W/kg. The solar cells used in the UltraFlex 175 solar array can attain 28% efficiency, meaning 28% of the energy that strikes them is converted to electricity. The on orbit Mars will be 715w to 492w for Perihelion to Aphelion with the surface being much lower.

We do know that there are more efficient cells but by time we get to go to mars hopefully there will be even higher to chose from that will be of less mass per wattage of output.

Just a sudo array number for earth orbit would be 200kW / 7 kW = 28.5 approximate panels to start with the power level required but as we approach Mars the power level is steadily dropping.


Lithium ion battery Charging basics

The chemistry is basically the same for the two types of batteries, so charging methods for lithium polymer batteries can be used for lithium-ion batteries.
Charging lithium iron phosphate 3.2 volt cells is identical, but the constant voltage phase is limited to 3.65 volts.

The lithium ion battery is easy to charge. Charging safely is a more difficult. The basic algorithm is to charge at constant current (0.2 C to 0.7 C depending on manufacturer) until the battery reaches 4.2 Vpc (volts per cell), and hold the voltage at 4.2 volts until the charge current has dropped to 10% of the initial charge rate. The termination condition is the drop in charge current to 10%. The capacity reached at 4.2 Volts per cell is only 40 to 70% of full capacity unless charged very slowly.

•Charge temperature--must not be charged when temperature is lower than 0° C or above 45° C.
•Charge current must not be too high, typically below 0.7 C.
•Overdischarge protection--stops discharge when battery voltage falls below 2.3 volts per cell (varies with manufacturer).
•A fuse opens if the battery is ever exposed to temperatures above 100° C.

Here is the value for C in battery charging
http://www.microchip.com/stellent/group … 028061.pdf

LI-ION CHARGING
The rate of charge or discharge is often expressed in relation to the capacity of the battery. This rate is known as the C-Rate. The C-Rate equates to a charge or discharge current and is defined as:
I = M x Cn
where:
I = charge or discharge current, A
M = multiple or fraction of C
C = numerical value of rated capacity, Ah
n = time in hours at which C is declared.

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#25 2016-01-31 22:11:44

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 17,333

Re: Forget NTR, SEP is the Future

That makes for Earth orbit a panel of 40 meters and 4 of them to get the power to start the mission but we need to keep power constant so we need more panels for mars as its only getting 500 W - 700 W so with that we need to double the count of panel wings or make the 4 panels twice as wide in order to keep the 40 meter lengths....

further hints are in the jpl mission topic http://www.newmars.com/forums/viewtopic.php?id=7260

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