Author Topic: Does electric propulsion still have a future? (if BFR is successful)  (Read 7235 times)

Offline A_M_Swallow

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SEP could be used for Earth space station to Mars space station. Payloads in the ~100 tonne range include habitats, propellant and heavy landers. Big mixtures include ISRU processing equipment, rovers, mining equipment and food for a couple of years. In a continuing mission these can be sent out in advance of the people.

Offline DrRobin

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* No one in their right mind would suggest using SEP for people.

Well, not exactly SEP, but I actually remember as a little kid in the mid-1960's being shown this Disney movie in school (when we were cooped up in the Cafeteria during a storm and they were trying to keep us occupied) where Werner Von Braun and Co. told how they were going to use NEP to send people ("twenty men" per ship) to Mars on a 14 month voyage, most of which was spent spiraling out from Earth and in to Mars. The animated movie was originally made less than a year after Sputnik, but almost fifty years before NASA actually got around to using EP as primary propulsion for a mission.



So, while the question of whether EP has a future is probably above the pay grade of this humble microbiologist, as old space fan who's been paying close attention for a very long time, I can attest that it certainly has an illustrious past, as least in the world of imagination!

Online AncientU

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SEP could be used for Earth space station to Mars space station. Payloads in the ~100 tonne range include habitats, propellant and heavy landers. Big mixtures include ISRU processing equipment, rovers, mining equipment and food for a couple of years. In a continuing mission these can be sent out in advance of the people.

What Earth space station?  What Mars space station?  You've hit the nail squarely, though... without all this other infrastructure and an large 'continuing mission', SEP isn't going to cut it.  And if you have the spacecraft/launch systems to get these 100tonne chunks to that non-existent space station, why not just use them to get the delivery finished at Mars?

And if you are going to send people (of course you are), what ride will they use?  SEP?  And will they go from space station to space station, or surface to surface?
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Online AncientU

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* No one in their right mind would suggest using SEP for people.

Well, not exactly SEP, but I actually remember as a little kid in the mid-1960's being shown this Disney movie in school (when we were cooped up in the Cafeteria during a storm and they were trying to keep us occupied) where Werner Von Braun and Co. told how they were going to use NEP to send people ("twenty men" per ship) to Mars on a 14 month voyage, most of which was spent spiraling out from Earth and in to Mars. The animated movie was originally made less than a year after Sputnik, but almost fifty years before NASA actually got around to using EP as primary propulsion for a mission.



So, while the question of whether EP has a future is probably above the pay grade of this humble microbiologist, as old space fan who's been paying close attention for a very long time, I can attest that it certainly has an illustrious past, as least in the world of imagination!

Mars is actually too close to use SEP or NEP*.  First, you are going to want to get out of Earth's gravity well quickly and efficiently using Oberth effect.  Second, you are going to want to decelerate efficiently at Mars, using aero-braking of direct entry.

If you are heading to Jupiter and/or Saturn, departing from Earth or Mars, then use chemical to get going and SEP/NEP to shorten the trip.

* The only people who think Mars is a year or more away are those stuck in the minimalist architecture mindset. 
They should move on...
« Last Edit: 01/10/2018 05:40 PM by AncientU »
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Offline DrRobin

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I think solar electric propulsion has a bright future for space colonisation, but it will look completely different than current electric propulsion. It will use cheaper and easier to obtain propellants such as argon, water, ammonia, CO2 or even regolith.

Also, there are other high-ISP possibilities for SEP propellants aside from noble gases like Xenon. Magnesium, for example, is relatively abundant for ISRU, and not too hard to work with, so one could imagine using asteroidal sources for both chemical and EP propellants.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160005357.pdf

Offline Nilof

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Does shipping still have a future? (If airplanes are successful?)

Oceanliners are pretty much a thing of the past since the invention of winged machines that go over 10 times faster. However, a gargantuan amount of the worlds trade is still conducted with far slower vessels over the world's waters. I fully expect a similar dynamic to develop for human expansion into the solar system. Bulk cargo being hauled by slow but efficient umanned craft, people being sent by faster and pricier means.

Although propellant is a small part of the cost of launch today, in any large settlement effort it would rapidly become a large portion of the costs of the endeavour. Fuel costs put enormous strain on "rapidly reusable" airliners today even though jet fuel is relatively cheap and the oxidizer is free, courtesy of the atmosphere. More efficient ways to use the same amount of resources will always have a future.

This is not really true.  Since something like 75% of what is delivered to orbit is fuel, 75% of the cost of launch is 'fuel' cost (plus the few $100k to buy the fuel itself).  This is the argument for SEP... don't have to bring as much fuel to orbit when launching a fully fueled spacecraft.  The counter argument is that bulk commodity deliveries of fuel to orbit could become very inexpensive... in that case, launch the spacecraft dry, and fuel it on orbit.  This gets you a factor of a few more spacecraft for the launch costs.

SEP only will make sense if commodity transport to LEO/EML-1/2 is never realized.  If/when 'N'uclear becomes the power supply for the 'EP', and EP thrust grows by a couple orders of magnitude, then we have a freighter power supply or even an augmentation for distant human travel (Jovian system, for instance).

No, the increased Isp is not just about reducing mass to IMLEO. It's about making your spacecraft physically capable of performing it's mission. Chemical propulsion has a maximum delta-v per stage, and if you go past that you are forced to introduce staging.

For example, an Earth to Ceres or Earth to Mercury transfer is relatively straightforward with electric propulsion, but requires multiple expendable stages if done with chemical propulsion. Mars missions can be done without staging for near-minimum energy transfers only because of hard aerobraking. It is physically incapable of doing an emergency Earth to Mars or Mars to Earth transfer outside of transfer windows.

Secondly, in the outer solar system, Beaming power from the inner solar system is very viable for ships with solar arrays in the square kilometer range and up. SEP with beamed power from the inner solar system will generally have significantly higher performance than any plausible NEP design.


Mars is actually too close to use SEP or NEP*.  First, you are going to want to get out of Earth's gravity well quickly and efficiently using Oberth effect.  Second, you are going to want to decelerate efficiently at Mars, using aero-braking of direct entry.

If you are heading to Jupiter and/or Saturn, departing from Earth or Mars, then use chemical to get going and SEP/NEP to shorten the trip.

* The only people who think Mars is a year or more away are those stuck in the minimalist architecture mindset. 
They should move on...

Mars is not too close for SEP, if your SEP has good enough performance. Stop comparing colonization scale chemical efforts to minimalist SEP. Non-minimalist SEP can easily allow 3-4 month transfers with better mass ratios than minimum energy transfer chemical. Or it can do 6 month transfers at time where chemical can't do the trip at all, and which would enable two Earth-Mars trips per synod.

There is no need for a space station. You capture into a high elliptical elliptical mars orbit, have the lander drop into the atmosphere (which can be a full BFS), and the lander comes back up to the SEP spacecraft in elliptical orbit with ISRU fuel. You can have a BFS acting as the lander doing multiple trips back and forth between the surface and the ship, since not having to do aerocapture means that the BFS heatshield can be reused. You can have it permanently based on Mars so you don't have to carry it around, and use more efficient Bigelow-type habitats that give much more living space and are much more mass efficient.
« Last Edit: 01/10/2018 10:11 PM by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Online AncientU

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No, the increased Isp is not just about reducing mass to IMLEO. It's about making your spacecraft physically capable of performing it's mission. Chemical propulsion has a maximum delta-v per stage, and if you go past that you are forced to introduce staging.

For example, an Earth to Ceres or Earth to Mercury transfer is relatively straightforward with electric propulsion, but requires multiple expendable stages if done with chemical propulsion. Mars missions can be done without staging for near-minimum energy transfers only because of hard aerobraking. It is physically incapable of doing an emergency Earth to Mars or Mars to Earth transfer outside of transfer windows.

Secondly, in the outer solar system, Beaming power from the inner solar system is very viable for ships with solar arrays in the square kilometer range and up. SEP with beamed power from the inner solar system will generally have significantly higher performance than any plausible NEP design.

...

Square kilometer* arrays and up?(!!!)  PPE at 40kW is the biggest SEP currently envisioned...  by the time you are driving around on square kilometer solar arrays, chemical rocketry could be in the full colonization mode.  SEP will have near zero role because it is so trivial in capacity.  It is like parachutes on Mars... nice for the one tonne payload, but doesn't scale.


* ISS has 2,500 sq meters generating about 100kw -- 0.25% of a single square kilometer.  So, take ISS arrays times 400... 40 MegaWatts -- easy peasy.  PPE is 40kW... so just strap 1,000 PPEs together and off you go.
« Last Edit: 01/10/2018 10:52 PM by AncientU »
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Offline Nilof

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Square kilometer* arrays and up?(!!!)  PPE at 40kW is the biggest SEP currently envisioned...  by the time you are driving around on square kilometer solar arrays, chemical rocketry could be in the full colonization mode.  SEP will have near zero role because it is so trivial in capacity.  It is like parachutes on Mars... nice for the one tonne payload, but doesn't scale.


* ISS has 2,500 sq meters generating about 100kw -- 0.25% of a single square kilometer.  So, take ISS arrays times 400... 40 MegaWatts -- easy peasy.  PPE is 40kW... so just strap 1,000 PPEs together and off you go.

1 GW is long term for large colonization ships of the kind that you would need to settle the outer solar system, so working on a large scale should not be surprising for these applications. 1 GW PPU's is only unusually large for in-space applications, not terrestrial ones. By the time you're seriously considering large scale colonization of Jupiter or Saturn, you would have a very significant in-space industrial park and you would be working at that scale. Just surviving in the outer solar system in the long term would require something on the order of a megawatt per person for agriculture.
« Last Edit: 01/10/2018 11:52 PM by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline Robotbeat

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No, the increased Isp is not just about reducing mass to IMLEO. It's about making your spacecraft physically capable of performing it's mission. Chemical propulsion has a maximum delta-v per stage, and if you go past that you are forced to introduce staging.

For example, an Earth to Ceres or Earth to Mercury transfer is relatively straightforward with electric propulsion, but requires multiple expendable stages if done with chemical propulsion. Mars missions can be done without staging for near-minimum energy transfers only because of hard aerobraking. It is physically incapable of doing an emergency Earth to Mars or Mars to Earth transfer outside of transfer windows.

Secondly, in the outer solar system, Beaming power from the inner solar system is very viable for ships with solar arrays in the square kilometer range and up. SEP with beamed power from the inner solar system will generally have significantly higher performance than any plausible NEP design.

...

Square kilometer* arrays and up?(!!!)  PPE at 40kW is the biggest SEP currently envisioned...  by the time you are driving around on square kilometer solar arrays, chemical rocketry could be in the full colonization mode.  SEP will have near zero role because it is so trivial in capacity.  It is like parachutes on Mars... nice for the one tonne payload, but doesn't scale.


* ISS has 2,500 sq meters generating about 100kw -- 0.25% of a single square kilometer.  So, take ISS arrays times 400... 40 MegaWatts -- easy peasy.  PPE is 40kW... so just strap 1,000 PPEs together and off you go.
Oh, please, bro. It's space. Square kilometer is no prob. Really, it ain't.

And if you took ISS's 2500 m^2 arrays and actually used modern cells in them, that'd produce a Megawatt, not 100kW (and 100kW is really more of an average figure... i.e. averaged over night and day). Do the math:

2500m^2*1350W/m^2*0.30 = ~1MW.

1350W/m^2 solar flux, 30% efficiency (we can do slightly better, but I assume coverage isn't perfect).
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Offline pathfinder_01

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This is where the complexity arises... why transfer 'things' -- that might weigh a hundred tonnes* -- to Mars orbit to await the slow boat SEP (which also needs to be built, fueled, operated, etc.).  Your heavy payloads would spend months spiraling out of the Mars gravity well, then a year or more getting to Earth.  Once there, they'd have to be transferred again to a vehicle to bring them to the surface.  All of this hardware costs money... and time is money, too, so going slow means your SEP craft is busy for a long time each trip... so make a lot of them, which costs more money.  Much easier for your ship that brings this hundred tonnes to Mars orbit to just load more fuel on the surface or in orbit, and finish the trip to Earth's surface.

* No one in their right mind would suggest using SEP for people.

Here is the thing there are reasons why NASA would prefer an SEP system and reasons why Elon should avoid it. SEP for transportation as on earth it is going to depend on what, where, how much and when the cargo or people need to be there.  i.e. Rail and Boat are slow but much cheaper than flight for cargo(but not people).

SEP can be faster than chemical. Can carry more mass for a given amount of propellant and has much longer launch windows(in theory you could leave at any time with SEP). It really depends a lot on how the system is implemented.

For instance in you example:

1. BFR cannot carry 100 tons to Mars orbit(with reuse). It would have to land on mars refuel and relaunch to orbit(unless a depot is present in orbit). SEP could making trips to Phobos and Demois much easier. In fact BFR can only send 20-50MT back to Earth from the surface of Mars. I don't have any figures for how much it can land. I am guessing 150(max to LEO).

2. Depending on the type of Cargo a long trip might not be a problem at all. .  Who cares how long an hab unit takes to get to Mars just so long as it arrives before the crew. People need a fast trip, but some items donít.

3. You don't need to transfer just dock. Imagine a BFR like cargo pod that is sized to carry 50MT from Mars surface to an Mars Orbit, dock with SEP and land it on Earth.  It would require far less propellant (which is likely to be limited and expensive on Mars vs. Earth).

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For some people and companies that have to generate their own revenue, time is money.  Not caring how long something takes is a luxury only certain programs can tolerate.  (And they aren't going anywhere with that mindset.)

And space is big, mind-numbingly so.  But we aren't.
Let's put on the big boy pants and see if we can get to Mars... and build a settlement there. 
Then we'll start building megastructures.
« Last Edit: 01/11/2018 10:29 AM by AncientU »
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Offline rklaehn

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I think solar electric propulsion has a bright future for space colonisation, but it will look completely different than current electric propulsion. It will use cheaper and easier to obtain propellants such as argon, water, ammonia, CO2 or even regolith.

Also, there are other high-ISP possibilities for SEP propellants aside from noble gases like Xenon.

Yes, but I think once your ISP is high enough that the propellant mass is less than the empty mass of the spacecraft, there is not much point in having a higher Isp. E.g. going from 0.5 of the empty mass to 0.25 of the empty mass almost doubles the energy you need for a certain thrust to weight. Once you are below the point where the exponential nature of the rocket equation really hurts, low Isp is good. Something like v_e = 2 * total_mission_delta_v is probably the sweet spot.

Quote
Magnesium, for example, is relatively abundant for ISRU, and not too hard to work with, so one could imagine using asteroidal sources for both chemical and EP propellants.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160005357.pdf

Certainly better than Xenon, but you still have to have significant infrastructure to refine the ore and to create elemental magnesium. Water would be much easier to obtain. Could be as simple as putting some regolith or other asteroid material into a slightly heated envelope.

Ideally you would be able to just (electrostatically?) accelerate raw regolith. That would really open up the solar system.

2. Depending on the type of Cargo a long trip might not be a problem at all. .  Who cares how long an hab unit takes to get to Mars just so long as it arrives before the crew. People need a fast trip, but some items donít.

The issue is not with the cargo but with the spacecraft. If a roundtrip takes 10 years, you only get 2 to 3 reuses or so over a reasonable time frame, so you might as well make the spaceship expendable. The reason BFS does one roundtrip per synod is not to shorten the transfer time but because you need the spacecraft back for another reuse ASAP for economic reasons...
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Offline Oli

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Agreed. For Earth-moon, chemical has a massive advantage. Not just because of radiation, but simply because the shorter trip times let you reuse the ship much more often, so you get economies of scale. You don't have to worry about synodic periods and coast times are short.

You can only reuse a chemical tug more often if it has a longer lifetime in terms of number of transfers. Today it's likely easier to reuse an SEP tug 10x than a chemical tug, despite the SEP tug requiring a lot more time for the transfers.
« Last Edit: 01/11/2018 05:03 PM by Oli »

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Agreed. For Earth-moon, chemical has a massive advantage. Not just because of radiation, but simply because the shorter trip times let you reuse the ship much more often, so you get economies of scale. You don't have to worry about synodic periods and coast times are short.

You can only reuse a chemical tug more often if it has a longer lifetime in terms of number of transfers. Today it's likely easier to reuse an SEP tug 10x than a chemical tug, despite the SEP tug requiring a lot more time for the transfers.

I don't understand your rationale...  why would a chemical tug be limited to ten transfers?  If it does several (or several tens) per year to the Moon, how is this more limiting than an SEP that takes months to get to the Moon?  For Mars, one round trip every synod for a chemical ship like BFS vs. one round trip every 3-4 synods for a SEP tug? 

And you still need a fleet of service vessels to supply fuel to the SEP tug and lift/lower its payloads from/to the surface.
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Offline Chasm

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With real cheap access to space larger SEP systems become feasible.

Then it is more a question of power to weight ratio and how fast their solar cells degrade. If you can treat the arrays more like long term infrastructure and do the moving parts in a modular and redundant configuration things get interesting.

At the end it is still about using the right tool for the job.

Offline pathfinder_01

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Agreed. For Earth-moon, chemical has a massive advantage. Not just because of radiation, but simply because the shorter trip times let you reuse the ship much more often, so you get economies of scale. You don't have to worry about synodic periods and coast times are short.

You can only reuse a chemical tug more often if it has a longer lifetime in terms of number of transfers. Today it's likely easier to reuse an SEP tug 10x than a chemical tug, despite the SEP tug requiring a lot more time for the transfers.

I don't understand your rationale...  why would a chemical tug be limited to ten transfers?  If it does several (or several tens) per year to the Moon, how is this more limiting than an SEP that takes months to get to the Moon?  For Mars, one round trip every synod for a chemical ship like BFS vs. one round trip every 3-4 synods for a SEP tug? 

And you still need a fleet of service vessels to supply fuel to the SEP tug and lift/lower its payloads from/to the surface.

Ok there is a big difference between SEP to the Moon and SEP to MARS. The way electric propulsion works is that it  accelerates constantly for months even years if need be and in such a fashion it can travel faster than a chemical rocket. It is sorta turtle(electric propulsion) vs. hare(chemical). It's engines are built to last for a very long time.

The problem is that the moon is too close for electric propulsion to gain an advantage of speed. Mars isn't.

The other problem is that you are comparing an mission to Mars orbit(NASA) to a mission direct to the surface (BFR). For instance if you did electric propulsion direct to the surface you would easily beat BFR to Mars but it would be hard to design a reusable spacecraft that could do it (due to the big arrays) and it would be an extremely fast reentry! Likewise BFR can't stop in Mars orbit and have enough proplant to go back home without refueling(SEP can).

A SEP powered system might be able to make as many trips per synod as a chemical system. It may have more travel time total(i.e takes a year to get back to Earth vs. 3- 6 months) but it takes 2 years for Mars the launch window to open for Mars and most chemical systems can't arrive at Mars or Earth before the window closes on the other end.

The low thrust of SEP does have a weakness in that it bites when you need to slow down to get into orbit around the planet or get out of orbit around a planet hence the interest in Hybrid systems(Chemical/SEP) at the moment.

You don't need a fleet to supply or lift  it's payloads. BFR takes five refueling flights for every Mars landing and requires ISRU on Mars to return.  SEP could move the same payload with fewer flights and not need or require less ISRU production on MARS.  One BFR(or other rocket)could lift(or swap) the payload and another supply it with proplant. That is where the advantage could come from.

« Last Edit: 01/12/2018 04:47 AM by pathfinder_01 »

Offline DrRobin

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I think solar electric propulsion has a bright future for space colonisation, but it will look completely different than current electric propulsion. It will use cheaper and easier to obtain propellants such as argon, water, ammonia, CO2 or even regolith.
Also, there are other high-ISP possibilities for SEP propellants aside from noble gases like Xenon.
Yes, but I think once your ISP is high enough that the propellant mass is less than the empty mass of the spacecraft, there is not much point in having a higher Isp. E.g. going from 0.5 of the empty mass to 0.25 of the empty mass almost doubles the energy you need for a certain thrust to weight. Once you are below the point where the exponential nature of the rocket equation really hurts, low Isp is good. Something like v_e = 2 * total_mission_delta_v is probably the sweet spot.
Quote
Magnesium, for example, is relatively abundant for ISRU, and not too hard to work with, so one could imagine using asteroidal sources for both chemical and EP propellants.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160005357.pdf
Certainly better than Xenon, but you still have to have significant infrastructure to refine the ore and to create elemental magnesium. Water would be much easier to obtain. Could be as simple as putting some regolith or other asteroid material into a slightly heated envelope.
Ideally you would be able to just (electrostatically?) accelerate raw regolith. That would really open up the solar system.
*putting on my Caltech Chem major hat here*
There are enormous differences in the difficulty and efficiency of using different materials for Electric Propulsion! You can't just throw regolith in the hopper and (electrostatically?) accelerate it in a useful fashion for practical missions. Xenon (and way back when Cesium) were picked for high atomic mass and low ionization energy. Neither, however, is available for near-term ISRU. Magnesium (and Aluminum, though this is somewhat more difficult to extract from regolith) are attractive since they are abundant and can be used both for propellant for high-ISP Electric Propulsion (given low ionization energy and manageable melting/boiling points) as well as as fuels for low-ISP, high-thrust chemical propulsion. http://php.scripts.psu.edu/users/p/n/pnu/AIAA-2004-4037%20Miller.pdf
My original point, though, was that EP can sometimes complement rather than compete with chemical propulsion, especially in the special case of Ceres, where the high outbound delta-V might be done with EP and then then volatile chemical propellants mined at Ceres transported back to near-Earth orbit (with much lower delta-V needed using aerobraking and potentially Mars flyby) for use in the Earth-Mars BFR/BFS-architecture.

Online AncientU

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With real cheap access to space larger SEP systems become feasible.

Then it is more a question of power to weight ratio and how fast their solar cells degrade. If you can treat the arrays more like long term infrastructure and do the moving parts in a modular and redundant configuration things get interesting.

At the end it is still about using the right tool for the job.

With real cheap access to space, a propellant rich architecture becomes feasible --actually mandatory if we are going to get serious about exploration.  SEP needs to compete against that architecture, not this SLS/Orion/DSG minimalist approach.

This is why SEP may have a role down the road when there are niches that it can exploit, but first we have to get a vigorous program going to create those niches.  We are not going to establish the first beach head on Mars using SEP.
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Offline DrRobin

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With real cheap access to space, a propellant rich architecture becomes feasible --actually mandatory if we are going to get serious about exploration.  SEP needs to compete against that architecture, not this SLS/Orion/DSG minimalist approach.

This is why SEP may have a role down the road when there are niches that it can exploit, but first we have to get a vigorous program going to create those niches.  We are not going to establish the first beach head on Mars using SEP.

Right, but the OP was "does EP have a future?" Sounds like you are saying yes, just not in the immediate future. I agree that it's an important distinction, since with limited resources, things that would be great eventually (EP, nuclear, cis-lunar depots, etc) can actually get in the way of establishing the first Martian settlement. In contrast, Musk is putting ISRU -which in standard NASA proposals has been something they'd get around to eventually- right in the initial critical path.

Offline rklaehn

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Certainly better than Xenon, but you still have to have significant infrastructure to refine the ore and to create elemental magnesium. Water would be much easier to obtain. Could be as simple as putting some regolith or other asteroid material into a slightly heated envelope.
Ideally you would be able to just (electrostatically?) accelerate raw regolith. That would really open up the solar system.
*putting on my Caltech Chem major hat here*
There are enormous differences in the difficulty and efficiency of using different materials for Electric Propulsion!
I know the physics. Looking at just the engine in isolation, xenon is an almost perfect propellant. But if you take ISRU into consideration I think the tradeoffs change completely. E.g. which of the following electric engines would be most useful for large scale settlement:

1. xenon ion engine, 80% efficiency, 30000 m/s Isp
2. arcjet using water, 40% efficiency, 8000 m/s Isp
3. regolith mass driver, 3000 m/s Isp (less than chemical!), 50% efficiency

I would argue that probably 2 and 3 are the most useful despite having low efficiency and Isp.

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You can't just throw regolith in the hopper and (electrostatically?) accelerate it in a useful fashion for practical missions.
It should be possible to accelerate fine regolith to a few 1000m/s somehow. E.g. a bucket full of regolith driven by a linear electric motor. Why wouldn't it work? It would probably benefit from being very long, but remember that we are talking about "settlement scale" here, not small space probes like dawn.

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Xenon (and way back when Cesium) were picked for high atomic mass and low ionization energy. Neither, however, is available for near-term ISRU. Magnesium (and Aluminum, though this is somewhat more difficult to extract from regolith) are attractive since they are abundant and can be used both for propellant for high-ISP Electric Propulsion (given low ionization energy and manageable melting/boiling points) as well as as fuels for low-ISP, high-thrust chemical propulsion. http://php.scripts.psu.edu/users/p/n/pnu/AIAA-2004-4037%20Miller.pdf
That could definitely be useful for the occasional chemical kick to use the Oberth effect, and e.g. for launching from ceres.

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My original point, though, was that EP can sometimes complement rather than compete with chemical propulsion, especially in the special case of Ceres, where the high outbound delta-V might be done with EP and then then volatile chemical propellants mined at Ceres transported back to near-Earth orbit (with much lower delta-V needed using aerobraking and potentially Mars flyby) for use in the Earth-Mars BFR/BFS-architecture.
Yes, good point. But probably using a water arcjet for the ceres departure would also be an option. In any case I think that Ceres is a valuable destination of its own and not just an enabler for mars...
« Last Edit: 01/12/2018 09:41 PM by rklaehn »
Try the ISS 3D visualization at http://www.heavens-above.com/ISS_3D.aspx

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