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#1 2015-01-08 17:35:19

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

LOX - Ideal Solar Electric Propellent?

I've been wondering, a Solar Electric Propulsion systems have traditionally using Propellents entirely different from conventional rocketry, generally noble-gas mono-propellents.  Some speculation on use of easily store-able compounds like water or hydrocarbons.  Pure oxygen never seems to be considered.

Oxygen would have comparable ISP to Argon due to similar atomic weight, lower electrical efficiency might result from the higher energy needed to ionize it but the broad performance characteristics look comparable to many of the normally considered propellents.

The obvious downside is LOX is cryogenic, and if you can avoid that on a small satellite sized vehicle on a multi-year trajectory it's an obvious choice to go with non-cryo propellents.

BUT, as soon as we start to look at a reusable architecture which involves a rocket lander shuttling between the SEP vehicle and a planetary surface then a lot of things change.  The rocket vehicle MUST have LOX tanks and MUST be able to refuel on the planets surface which correspondingly MUST have a robust LOX production capability.

If we wish to also re-propellent (can't really call it refueling when theirs no fuel) of the SEP vehicle in orbit then we would need to ADD a 3rd tank to the Rocket vehicle AND a 3rd propellent production capability to the planets surface.  Both would be added cost and complexity, where as making excess LOX is a predicted output from most ISPP processes on Mars like Sabiter and is likely to be the lowest energy per unit mass propellent to produce other then CO2. 

Likewise LOX is ALWAYS going to be present on every vehicle launched from Earth and if the upper stages of rockets had their LOX tanks stretched slightly then could devote a fraction of payload to extra LOX that would be offloaded to a SEP vehicle.  If done at the right ratio this would archive both the loading of cargo and propellent onto a vehicle with the use of a single launcher rather then needing a separate 'tanker' launch.

Now if we also had some Rocketry on that SEP vehicle (making it hybrid) for the purpose of orbit circularization, Oberth-effect optimized escapes, emergency thrust etc.  Then the potentially exists for carrying small amounts of fuel along with us to Mars from Earth and in fact we would likely just attach one of thouse stretched-LOX upper-stages to the vehicle.  Now we can depart for Mars on LOX-SEP, brake into orbit with some fuel and remaining LOX, then refill the LOX tanks from Mars ISPP, then depart Mars on LOX-SEP and finally brake at Earth with the last of the fuel.

The whole architecture from Earth-surface to Mars-surface and back is just 2 fluids and it only requires that LOX be transferred in orbit at Mars, simpler and safer then transfer of two fluids.  And no real pressure is put on the Mars propellent production infrastructure as no precious hydrogen is needed, just a slightly larger LOX tank on each lander and the ability to transfer it on orbit.

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#2 2015-01-08 17:52:20

RobertDyck
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From: Winnipeg, Canada
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Re: LOX - Ideal Solar Electric Propellent?

Ion propulsion requires a propellant that can easily be ionised. More energy for ionization, requires more electric power. More electric power means heavier power system, slowing the vehicle. Furthermore, ionized oxygen is highly reactive, causing oxidation of engine parts. One reason for using a noble gas is to avoid reaction. Remember, ion engines use a screen (mesh) of metal. Corrosion due to impact of exhaust with engine parts is already a known issue. That was the primary issue of the research opportunity for the next generation ion engine. The winner of that was was expected to be the engine for the New Horizons probe. It didn't work out, but that was the intent.

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#3 2015-01-08 19:04:18

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

Re: LOX - Ideal Solar Electric Propellent?

Aren't the erosion issues caused by the kinetic energy of collision rather then the chemical reactivity of the propellent?  The general research trend has been to get lower and lower amounts of impacting on grids by keeping the propellent under tighter magnetic control so it just flows around the elements of the grid while still providing thrust, so it would seem that this problem is being reduced and may allow the head-room for a more reactive element like Oxygen.

Also consider that non-grid based SEP systems like a HAL or a VASIMIR should have less issue with corrosion.  What tech ends up being used could be a make or break for use of Oxygen.

As we try to go to higher and higher ISP in development of any Electric propulsion system, the ratio of total energy input used to ionize the propellent drops as a percentage of the total, I don't think the Ionization energy is all that different for O and Ar. 

Simple search here but this actually seems to indicate it is lower then Argon, this doesn't include energy needed to break the O2 bond though which might be significant.

http://oxygen.atomistry.com/energy.html  (314.33 Kcal/mol)
http://argon.atomistry.com/energy.html  (363.73 Kcal/mol)

Both are higher then Xenon at 280.00 Kcal/mol which explains one of the advantages of thouse heavy elements, easy to Ionize and high thrust, but lighter elements give better ISP.

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#4 2015-01-08 20:20:42

RobertDyck
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From: Winnipeg, Canada
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Re: LOX - Ideal Solar Electric Propellent?

And when I bid on that contract, my idea was to treat the accelerator as a proton particle accelerator, with rings instead of a mesh. And stacked rings that act as a particle accelerator, so a multi-stage ion engine. Yes, I'm the one who proposed a non-grid ion engine. I did get on NASA's short list. The contract was open to foreign organizations, both for profit and non-profit, but there was small print that said anyone outside the US wouldn't get paid. I don't know how they think it's "open to foreign organizations" when they won't get paid.

Anyway, corrosive propellant is a major issue. They resolved that by using a noble gas. It's no longer an issue because the propellant is not reactive. But you propose using a propellant that is reactive.

And you aren't talking about ionization energy for "O", you're talking about O2. That's diatomic oxygen. And more than two electron charge does require breaking the O2 bond, which is very significant. And corrosion of mono-atomic oxygen is dramatic.

Ion engines use the heaviest noble gas available. Chemical engines attain higher Isp with lighter propellant, but ion engines get higher performance with heavier propellant.

If you want a paradigm shift, consider nuclear thermal propulsion. It has both high thrust and high Isp.

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#5 2015-01-08 22:38:22

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

Re: LOX - Ideal Solar Electric Propellent?

I explicitly said the number did not include breaking O2 bonds, and I know that needs extra energy, this is where a helpful person would go find the numbers, looks like I have to do it myself.

http://en.wikipedia.org/wiki/Bond-dissociation_energy

119  Kcal/mol, so O2 takes a total of 433.33 Kcal/mol to disassociate and ionize, a whooping 19% more then Argon!  If we Ionize to the 2+ level then Oxygen looks worse as it's Ionization energy grows rapidly after the first electron, I don't know if Ion engines typically go beyond a 1+ level, it would seem a big waste to do so.

Ion engines are also higher ISP for lower atomic masses as a smaller atomic mass translates to higher exhaust velocity given the same acceleration field.  The use of heavier propellents is to get higher thrust levels.  But higher per unit-propellent power inputs (which are growing steadily with each generation of thrusters) can also generate higher thrust, so I'm assuming the evolution will be towards lighter propellents to trade thrust levels for even higher ISP.  Once we reach the thrust levels necessary to complete a decent round-trip in one synod their will be no need for any more thrust (unless we find that human health is being significantly improved by shortening trip times even more) and we would rather trade that for lower mass fraction.

If oxygen reactivity is a fatal flaw for gridded Ion engines then that only rules out that one combination, VASIMIR concepts and a number of others are still viable, these all plan to use a wide range of gases including water which would be mostly Oxygen so what matters is the different damage/erosion rates for different ionized gasses and plasmas, and they are ALL going to be quite nasty in this regard requiring a 'don't touch' magnetic bottle to use, so the chemical reactivity of the gases really seems to be moot at that point.

NTR is just 900 ISP and we don't need the thrust level, we have been over that a million times, stop bringing it up, it is a thread derailment.

Last edited by Impaler (2015-01-08 22:40:35)

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#6 2015-01-08 23:06:53

RobertDyck
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Re: LOX - Ideal Solar Electric Propellent?

The guy who invented VASIMR claimed he could achieve 9000 second Isp using liquid hydrogen. However, Princeton University did the first work on Magneto-Plasma-Dynamic thrusters (MPD). Glenn Research Centre did further work. They have demonstrated 8400 second Isp in the lab, using liquid hydrogen. VASIMR only achieved 5400 seconds using xenon. The VASIMR guys haven't gotten LH2 to work. And the magnetic containment bottle of VASIMR consumes a hell of a lot of power. MPD does not need magnetic containment. VASIMR claims they can vary Isp, producing high thrust and low Isp, or low thrust with high Isp. MPD only works one way, one thrust and one Isp. MPD is either on or off, that's all. But MPD is already demonstrated technology. And because of the magnetic containment bottle, MPD consumes a hell of a lot less power.

But all of these produce low thrust. If you want high thrust and high Isp, then go nuclear. Gas Core Nuclear Thermal has been designed on paper so it should be able to achieve 5000 second Isp with as much thrust as chemical. Solid core NTR has been shown to achieve 925 second Isp.

Don't like nuclear? We can go to Mars now with chemical rockets. With SLS.

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#7 2015-01-08 23:21:52

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

Re: LOX - Ideal Solar Electric Propellent?

How many times do I have to tell you to stop thread-jacking with this Nuclear obsession of yours.  Stick to the propulsion system in the title or don't post.  If you want a comparative debate between SEP and Nuclear go to the thread I made for that debate with GW http://newmars.com/forums/viewtopic.php?id=6177 or start a new one.

MPD may prove to be the better propulsion system over VASIMIR, I'm not married to any implementation of Electric Propulsion.  Even old simple HALL thrusters are still being improved and could reach the ISP levels we currently get form Ion engines now.  Energy efficiency is a major concern but all current techs are reasonably efficient and none seem to be truly eliminated from competition due to an inefficiency problem, rather choices seem to hinge on factors of ISP, thrust and system scalability and mass.

Last edited by Impaler (2015-01-08 23:36:54)

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#8 2015-01-08 23:35:02

RobertDyck
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From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 6,501
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Re: LOX - Ideal Solar Electric Propellent?

You can use water as propellant for various propulsion technologies: microwave, nuclear thermal. Oxygen is not good propellant. You can use oxygen as oxidizer for chemical propulsion. But I've already mentioned problems with oxygen for ion engines.

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#9 2015-01-09 08:55:16

Terraformer
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From: Logres
Registered: 2007-08-27
Posts: 3,356
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Re: LOX - Ideal Solar Electric Propellent?

Why not use Argon? Any significant atmospheric processing on Mars is going to produce a large amount of Argon as a byproduct, since it makes up 1.9% of the atmosphere. I can see why you'd want Oxygen for Lunar or Mercurian vessels, but Martian ones can use Argon fine.


"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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#10 2015-01-09 10:05:07

Antius
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From: Cumbria, UK
Registered: 2007-05-22
Posts: 1,003

Re: LOX - Ideal Solar Electric Propellent?

An arc-jet could conceivably use any propellant.  It is just a matter of adjusting voltage to overcome resistance. 

If O2/N2/CO2 or Ar are the propellants, then you have the option of gathering them from residual atmospheric gases in low orbit, or indeed, from processed lunar materials.  Nitrogen is non-corrosive.  Corrosiveness might not matter so much if the energy source is microwaves or a rotating magnetic field.

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#11 2015-01-09 11:36:25

GW Johnson
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Re: LOX - Ideal Solar Electric Propellent?

OK,  look.  The dynamics of fast interplanetary travel demand propulsion that has at least a low-1000's of sec of Isp,  and lots of vehicle acceleration capability: 0.1 to 5 gees.  We have some technologies that have shortfalls,  and we have those benchmarks.  Not both!  And we have some concepts we haven’t tried yet. 

There are a bunch of electric propulsion schemes,  some more mature than others.  The best ones have the low-1000's of sec Isp,  but all fall way short on vehicle acceleration:  0.001 gee or less.  The most critical need is for a lighter-weight,  more powerful electric power supply.  That's what needs to be worked on.  If the SLS budget were going toward that,  we might have a solution in sight.  But we do not.

Chemical and solid core nuclear thermal meet the thrust benchmark,  but fall short on Isp.  Many folks have objections to nuclear for any of a number of reasons,  but we may just "have to get over it",  and be careful how and where we employ it.  Both technologies are essentially ready-to-use,  especially the chemical.  Here in the US,  nobody is making LOX-kerosene engines but the newcomers like Spacex and XCOR.  That needs to change.  We also need to work on in-space cryogen transfers and long-term storage.  The practicality for real interplanetary travel is not quite there yet,  but could be. 

Gas core nuclear thermal is absolutely immature,  but holds the promise of meeting both the thrust and Isp benchmarks simultaneously.  So,  we ought to be working on it,  too.  That technology could enable travel if the search for a practical electric propulsion power supply fails.  It's stupid to put all your eggs in one basket,  as we all already know.  If that word "nuclear" offends,  well tough.  Life ain't fair.  Neither is physics.  The electric power supply we seek for electric might well be nuclear.  So,  get over it. 

BTW,  my candidate power supply concept would be a nuclear thermal rocket device feeding an MHD generator.  None of that heavy steam loop nonsense.  Use it as a rocket for getaways and captures,  use it for MHD electric on the transits between to raise midpoint speeds.  Get thrust from the electrics and the released plume.  Same rocket hardware does both,  which saves vehicle weight.  That might really start looking good if gas core nuclear was the rocket device.  Then we could start looking at excess electricity to use for processing propellants while electric-thrusting on the transits.  It pays to dream big.  Tells you what you really need to be working on,  too. 

The only other technology holding the promise of meeting both benchmarks simultaneously (that I know of) is the nuclear explosion drive.  We have known since 1959 that this scheme would work.  The side effect to fear is not so much fallout from launches to LEO,  but EMP effects,  and by far!  The devices make lousy blast weapons,  and blast weapons make lousy explosion-propulsion devices.  Trouble is,  it has been ignored and forgotten since 1965.  Plus,  it is only efficient in very large (10,000+ ton) vessels.  I suggest we ought to be working on this,  too,  but slower and longer term,  with an eye toward giant colonization transports  by next century. Could always be accelerated if we find we need it sooner. 

OK,  that being said,  now return to electric propulsion.  The problem is power supply,  as I said just above.  The thrusters themselves are also a bit short on basic device thrust/weight,  even when you don't count the power supply,  so that's another area we ought to be working on,  as well. 

But,  a good power supply could make the simple arc jet feasible,  which could use just about anything as propellant,  at least in principle.  To use oxygen-containing materials as electric propellants,  will require solving the reactive-oxygen problem,  so we should just belly up to the bar and get on with solving that issue.  That solution makes in-situ electric propellants a lot more feasible-looking. 

Some (few?) parts of Mars have big buried glaciers,  we think,  so there is massive ice we could mine,  just not at every site where we'd like to base.  Maybe not very many at all.  Who yet knows?  The atmosphere there is mostly CO2,  which at those temperatures is not all that hard to freeze for easy storage.  How about CO2 as an electric propellant?  Could we make that work?  Venus also has CO2,  but it has hellish conditions and big gravity well,  which make it not so attractive at this time in history. 

Titan has a largely-nitrogen atmosphere and a very weak gravity well,  although Saturn's is considerable.  Could we use Titanian nitrogen as an electric propellant to support activities in the outer solar system?  The surface "rocks" seem to be water ice,  so you have that as well.  Plus,  methane and ethane in liquid form in the lakes.  Could any of these work as electric propellants?

There's the ices on the surfaces of 3 of Jupiter's big moons.  Water,  CO2, perhaps ammonia,  and some other things.  Could these be used?  The moons have weak gravity wells,  but Jupiter's is enormous,  as is its radiation hazard.  Not sure how soon we might be capable of safely recovering those resources,  but somebody ought to be looking at it. 

Guys,  the power supply issue and the what-can-we-use-as-propellant issue are the two biggest issues facing electric propulsion.  Those are what we need to be working on,  very big time.  I just don't see that going on in anything being funded through NASA or the other space agencies,  nor through DARPA or anybody else.  Which is why I really don't believe any of those agencies are serious yet about sending men beyond LEO.  Not even to the moon,  or the SLS/Orion test schedule would look more serious about actually accomplishing something. 

Solve those two issues with electric propulsion,  and we can ride the ions where we want to go.  Right now,  the acceleration is way too low to use it for manned flight.  The travel times are just too long.  Vehicle acceleration at departure and capture needs to be 0.1 gee or better. Can tolerate 0.01 to 0.001 gee during transits,  but higher would be better.  That’s what we need. 

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 2015-01-09 16:02:25

Terraformer
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From: Logres
Registered: 2007-08-27
Posts: 3,356
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Re: LOX - Ideal Solar Electric Propellent?

If you're going to exploit the Oberth effect (and why not, you're probably going to be launching from L1...), you need a fairly high thrust. That can cut your delta-V requirements to a half or a third. So you might be better going with chemical propulsion, rather than something with a higher Isp but too low a thrust to use the Oberth effect. Once you're launched, unfurl your thin-film solar array and start your electric propulsion. Remember, 0.01g will accelerate you by over 0.8 km/s each day - over a week, that builds up. Even if you're not using it during the journey to accelerate, perhaps it will be useful for allowing an efficient capture into orbit? Maybe allowing for transit times of a few months?

Solar thermal is another option that needs to be looked at. You could make the mirrors inflatable.

As far as nuclear goes, forget it. Governments won't touch it, and they won't let anyone else touch it either. It doesn't matter how  much you say we need it, you won't get it, so you have to figure out how to do without.


"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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#13 2015-01-09 17:30:31

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

Re: LOX - Ideal Solar Electric Propellent?

The next person to so much as breath a word about any other propulsion system other then SEP is getting reported to the moderators, this is a thread about SEP propellent choices, nothing else, I am sick of every thread turning into a mess as everyone throws out their pet propulsion type and starts trying to compare them.

In the context of Mars I believe water, while it's storage on-board of a Reusable SSTO capsule while it would not be very difficult it's rarity on the surface of Mars and high cost of production and high demand for use in habitats/farming etc etc makes it seem unlikely that it would be used as propellent in large quantities.  If the destination were some water rich body in the outer solar-system then this would change dramatically and I can easily see water as the preferred propellent.

CO2 on the other hand would be available in basically endless supply on Mars (indeed liquid CO2 may kind of take the role of universal solvent in industrial process that water takes here on Earth) and it can be compressed and stored without too much trouble.  I think it comes down to the complexity of a third tank system for the vehicle vs the reduced energy cost of production on the surface.

The actual quantity of propellent that ends up being needed to re-propellent the SEP vehicle will determine how much of an issue that 3rd tank is, if the amount of propellent being offloaded to the SEP vehicle on each round-trip is very small compared to the total vehicle mass and tankage then a 3rd tank is not much trouble.  If on the other-hand the landers need to offload significant amounts of propellent then the tank is large and starts to compromise the vehicles design, it might be better to simply have external tanks on the SEP, take these down to the surface, fill them and then relaunch them and re-attach that to the exterior of the SEP vehicle.  That would basically eliminate any propellent transfer and the propellent for the SEP system is completely independent of what ever fuel/oxidizer mix the landers use.

At Earth orbit I expect propellent delivery to a SEP will generally be done by taking excess propellent from the upper-stage of a rocket, and this is where the biggest advantage of having commonality with that stages 2 existing tanks and fluids would be found.  Upper stages are very light, very tightly engineered things, being able to modify them as little as possible and just connect to the LOX ports and draw the stuff off would be much simpler then having a 3rd tank stuck on top of the stage as cargo.

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#14 2015-01-09 21:35:14

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 21,910

Re: LOX - Ideal Solar Electric Propellent?

I do agree Impaler about the threads not staying organized and on topic but when they put the forum back together the society decided that they would control the forum and the moderators would have very limited power to correct the issues to just a few choices for action.



Solar panels are around the 35% efficiency with the convertor around 95% to store the energy in a battery which we know is not 100% but to then convert the stored power back to the ionizer system means another drop in what we will be able to make use of to make the space craft move and that is just until the power to the panels drops as you move farther from the sun in terms of battery health as they are drained....

Last edited by SpaceNut (2015-01-09 21:43:02)

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#15 2015-01-09 21:53:52

SpaceNut
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From: New Hampshire
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Posts: 21,910

Re: LOX - Ideal Solar Electric Propellent?

http://en.wikipedia.org/wiki/Ion_thruster

According to Edgar Choueiri ion thrusters have an input power spanning 1–7 kilowatts, exhaust velocity 20–50 kilometers per second, thrust 20–250 millinewtons and efficiency 60–80%

Due to their relatively high power needs, given the specific power of power supplies, and the requirement of an environment void of other ionized particles, ion thrust propulsion is currently only practical on spacecraft that have already reached space, and are unable to take vehicles from Earth to space, relying on conventional chemical rockets to initially reach orbit.

Power supplies for ion thrusters are usually solar panels but, at sufficiently large distances from the Sun, nuclear power is used. In each case the power supply mass is essentially proportional to the peak power that can be supplied, and they both essentially give, for this application, no limit to the energy.

Electric thrusters tend to produce low thrust, which results in low acceleration. Using 1 g is 9.81 m/s2; F = m a ⇒ a = F/m. An NSTAR thruster producing a thrust (force) of 92 mN[9] will accelerate a satellite with a mass of 1,000 kg by 0.092 N / 1,000 kg = 0.000092 m/s2 (or 9.38×10−6 g).

OVERVIEW OF ION PROPULSION

An ion is simply an atom or molecule that is electrically charged. Ionization is the process of electrically charging an atom or molecule by adding or removing electrons. Ions can be positive (when they lose one or more electrons) or negative (when they gain one or more electrons). A gas is considered ionized when some or all the atoms or molecules contained in it are converted into ions.

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#16 2015-01-09 22:14:15

RobertDyck
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From: Winnipeg, Canada
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Re: LOX - Ideal Solar Electric Propellent?

SpaceNut wrote:

Solar panels are around the 35% efficiency with the convertor around 95% to store the energy in a battery

I would like to argue for gallium-indium-nitride. Current photovoltaic cells for space use gallium-indium-phosphate for the top layer. So they're already dealing with those metals. And blue LEDs, which are part of every white LED, are gallium-nitride. That means electronic engineers know how to impregnate nitrogen into a semi-conductor. There's a patent to implant nitrogen into gallium-indium-nitride photovoltaic cells. Ok, it's known. So do it. This semi-conductor was discovered by Los Alamos National Laboratory, they went to the materials lab at University of Californian in Berkeley to make one to test it. They found two junctions convert 56% sunlight to electricity, three junctions 64%, and 36 junctions 72%. Later work found 8 junctions convert 70.2%. Ok so 8 junctions is optimal for this chemistry. Corporate executives want to milk the industry for all the money they can get, they don't want to jump directly to 70% efficiency. But for Mars, we can't afford to pander to them. Just do it.

::Edit:: I corrected blue LEDs. Current blue LEDs are made of gallium nitride. That means manufacturers are already impregnating nitrogen into gallium.

Last edited by RobertDyck (2015-01-10 05:42:22)

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#17 2015-01-09 22:52:18

SpaceNut
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From: New Hampshire
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Re: LOX - Ideal Solar Electric Propellent?

I sure would like to be able to make them as the benefits would be great.

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#18 2015-01-10 17:26:33

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

Re: LOX - Ideal Solar Electric Propellent?

I realize now that I need to account for molar mass to get a fair comparison of total Ionization energy costs when comparing O2 to other gases.

O2  27.7 kCal/g, includes disassociation and ionization

Ar   9.11 kCal/g

And for comparison the SoA Xenon is 280.00 Kcal/g to ionize and thus 2.13  kCal/g to ionize.

This explains a lot of the motivation for Xenon, going from a Xe to Ar is a rather big jump on it's own, going to O2 is three times as much again and 13 fold more energy then the Xe.  This may just not be worth the energy cost unless Solar power density just gets to really really high levels like 1000 W/kg.  If vehicles remain power limited then use of Oxygen just on energy basis my be prohibitive.  Argon sourced from Mars may be preferable.

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#19 2015-01-10 23:18:59

SpaceNut
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From: New Hampshire
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Re: LOX - Ideal Solar Electric Propellent?

http://www.buildtheenterprise.org/propellant

Most ion propulsion engines today use Xenon as the propellant. But Argon can be used as an alternative propellant, and it’s much less expensive. Argon may not work as efficiently as Xenon in some engine designs, but it provides a very high cost savings. Argon is $18 per pound while Xenon is $2273 per pound. If 220 million pounds of propellant need to be sent into space to fill the propellant tanks on the Enterprise (55 million pounds) as well as the tanks in three propellant depots (55 million pounds each), then the cost savings when using Argon is enormous. Here is the cost comparison:

■Xenon: 220 million pounds x $2273 = $500 billion
■Argon: 220 million pounds x $18 = $4 billion


http://en.wikipedia.org/wiki/Electrical … propulsion

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#20 2015-01-11 08:14:17

Terraformer
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From: Logres
Registered: 2007-08-27
Posts: 3,356
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Re: LOX - Ideal Solar Electric Propellent?

What about a Microwave Electrothermal Thruster, possibly using water as propellent, and possibly using beamed power when close to a planet?


"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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#21 2015-01-11 10:59:28

SpaceNut
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Re: LOX - Ideal Solar Electric Propellent?

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#22 2015-01-11 13:12:34

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

Re: LOX - Ideal Solar Electric Propellent?

The cots of Xenon is definitely one of the reasons I think it's going to be abandoned as a propellent, particularly if SpaceX can get the cost to orbit down to their goal of $1000 per lbs, the propellent cost would actually be significant compared to the cost of launch.  The next fall back would seem to be Krypton though, I don't know the cost but as a lighter more common element I assume it's significantly less, I have seen NASA proposals for the ARM mission talking Krypton instead of Xenon.

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#23 2015-01-11 19:37:43

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
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Re: LOX - Ideal Solar Electric Propellent?

http://en.wikipedia.org/wiki/Krypton_gas

The Earth has retained all of the noble gases that were present at its formation except for helium. Krypton's concentration in the atmosphere is about 1 ppm. It can be extracted from liquid air by fractional distillation.
Krypton costs about 100 times as much as argon

http://www.nasa.gov/centers/glenn/techn … sion1.html

Deep Space 1 used less than 159 pounds of fuel in over 16,000 hours of thrusting. 200 days (4800 hours). Since much less fuel must be carried into space, smaller, lower-cost launch vehicles can be used. For the Deep Space 1 probe, ions were shot out at 146,000 kilometers per hour (more than 88,000 mph).

http://www.nasa.gov/centers/glenn/about … y/ds1.html


The NASA Glenn Research Center, which pioneered solar electric propulsion, played a key role in the development of DS1's Ion Propulsion System (IPS) and Solar Concentrator Arrays (SCA) that were demonstrated on this trailblazing mission

The technology is so efficient that it only consumes about 3.5 ounces (100 grams) of Xenon per day, taking about four days to expend just one pound (0.4 kg).

The spacecraft is equipped with two solar wings, each of which is composed of four panels measuring about 44 by 63 inches (113 by 160 centimeters). At launch, the wings were folded up so that the spacecraft would fit into the launch vehicle's fairing; now fully extended, the wings measure 38.6 feet (11.8 meters) from tip to tip. A total of 720 cylindrical Fresnel lenses made of silicone concentrate sunlight onto 3,600 solar cells made of a combination of gallium indium phosphide, gallium arsenide and germanium.

The arrays produce 15 to 20 percent more power than most modern solar arrays of the same size - about 2.4 kilowatts (kW) at the beginning of the mission (declining over the life of the mission as the array ages and the spacecraft recedes from the Sun) with a voltage of 100 volts. An operational assessment was done on November 30, 1998 to determine that the peak power point for the solar concentrator arrays is approximately 2.15 kW.

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#24 2015-01-12 16:43:34

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

Re: LOX - Ideal Solar Electric Propellent?

So Krypton looks to be only slightly cheaper the Xenon at ~$1800 based on the x100 cost factor.  I guess the improved ISP is the main driver for it's adoption.

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#25 2015-03-04 22:15:46

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 21,910

Re: LOX - Ideal Solar Electric Propellent?

Why not use Methane for a fuel for the ion drive?

NASA eyes ion engines for Mars orbiter launching in 2022

Engineers also want to add ion engines to the orbiter and fly the efficient electrically-powered thruster system to Mars for the first time, testing out a solar-electric propulsion package that officials say will be needed when astronauts visit the red planet.

Ion engines produce just a whisper of thrust, using electric power to ionize atoms of a neutral gas and spit out the particles at high speed. While the drive given by the thrusters is barely noticeable in one instant, they can operate for months or years, burning scant fuel compared to traditional chemical rockets.

A Mars orbiter launching in 2022 is a prime candidate to test out new technologies — like ion drive engines, better solar arrays, and lightning-fast broadband communications between Earth and Mars — to help scientists return samples from the Martian surface, and eventually send humans there, according to Charles Whetsel, who oversees formulation of future Mars missions at NASA’s Jet Propulsion Laboratory in Pasadena, California. Adding solar-electric propulsion to the 2022 Mars mission would allow the probe to swoop through different orbits around the red planet, passing by the Martian moons Deimos and Phobos and through a so-called “areostationary” orbit about 11,000 miles above the Mars equator.

The areostationary orbit is similar to the type of orbit around Earth used by communications satellites, which circle the planet at the same speed as its rotation, appearing to hover over a fixed location.

mars_sep.png

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