Author Topic: Does electric propulsion still have a future? (if BFR is successful)  (Read 7286 times)

Offline Pipcard

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Because brute-force "abundant chemical" architectures with fully reusable super-heavy-lift launchers carrying "cheap propellant" would apparently make SEP look complex, expensive, and obsolete by comparison.

MikeAtkinson:
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MCT seems to be about reducing cost by taking advantage of low launch costs to use a simple chemical based architecture. SEP adds complexity, vehicle cost and operational difficulties to reduce IMLEO - which is exactly the one thing you don't need to optimise for if you have cheap launch.

Offline AncientU

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Because brute-force "abundant chemical" architectures with fully reusable super-heavy-lift launchers carrying "cheap propellant" would apparently make SEP look complex, expensive, and obsolete by comparison.

MikeAtkinson:
Quote
MCT seems to be about reducing cost by taking advantage of low launch costs to use a simple chemical based architecture. SEP adds complexity, vehicle cost and operational difficulties to reduce IMLEO - which is exactly the one thing you don't need to optimise for if you have cheap launch.

SEP and the PPE only make some sense if SLS/Orion are the rides.  None of the three make an ounce of sense if/when BFR becomes operational.  Alternative architectures, such as propellant depots and broad use of existing and near-future launchers, also negate the 'performance' advantage of SEP.  (Basically, who cares if ISP is 3000 if the architecture is expensive, slow, and complicated.)
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Offline Nilof

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High power SEP is more or less required for large scale colonization of the asteroid belt and/or mercury.  For Mars, it allows transport to Mars outside of the very limited launch windows that chemical is subject to, and with good enough solar arrays it can reduce travel times relative to chemical.

The BFR does not reduce the viability of electric propulsion at all. It just increases our presence in space, which is strictly beneficial for the development of pure in-space propulsion. The complexity argument is mostly unwarranted because SEP is a much simpler technology than your typical staged combustion chemical engine, with space storable propulsion and practically no moving parts.

O'Neill made a convincing argument in the high frontier that for an in-space civilization, individual SEP craft could potentially be produced at a price affordable to the middle class, while high performance chemical would have to be owned by corporations because of the much higher complexity needed to make it safe. This makes sense. SEP thruster development budgets are typically in the low millions, while chemical rocket engines have development budgets in the billions. The cheaper access to space is, the more it makes sense to build cheap spacecraft with no chemical propulsion (including pressure fed hypergolic), using SEP and cold gas thrusters instead.
« Last Edit: 01/10/2018 12:22 AM 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 Zed_Noir

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High power SEP is more or less required for large scale colonization of the asteroid belt and/or mercury.  For Mars, it allows transport to Mars outside of the very limited launch windows that chemical is subject to, and with good enough solar arrays it can reduce travel times relative to chemical.

The BFR does not reduce the viability of electric propulsion at all. It just increases our presence in space, which is strictly beneficial for the development of pure in-space propulsion. The complexity argument is mostly unwarranted because SEP is a much simpler technology than your typical staged combustion chemical engine, with space storable propulsion and practically no moving parts.

SEP requires very large steerable solar arrays with complex electrical conduits from arrays to some sort of storage system then to the engines. Somehow don't see a direct feed of electricity from the arrays to the engines. Also you need a cooling system for the waste heat from the solar arrays. So high power SEP is not simple or non-complex, therefore not cheap.

Don't see high power SEP entering service in the near to mid time frame. However maybe nuclear electric propulsion will be economical beyond Jupiter's orbit and away from the propellant depot network.

Offline Darkseraph

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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.


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Offline Nilof

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High power SEP is more or less required for large scale colonization of the asteroid belt and/or mercury.  For Mars, it allows transport to Mars outside of the very limited launch windows that chemical is subject to, and with good enough solar arrays it can reduce travel times relative to chemical.

The BFR does not reduce the viability of electric propulsion at all. It just increases our presence in space, which is strictly beneficial for the development of pure in-space propulsion. The complexity argument is mostly unwarranted because SEP is a much simpler technology than your typical staged combustion chemical engine, with space storable propulsion and practically no moving parts.

SEP requires very large steerable solar arrays with complex electrical conduits from arrays to some sort of storage system then to the engines. Somehow don't see a direct feed of electricity from the arrays to the engines. Also you need a cooling system for the waste heat from the solar arrays. So high power SEP is not simple or non-complex, therefore not cheap.

Don't see high power SEP entering service in the near to mid time frame. However maybe nuclear electric propulsion will be economical beyond Jupiter's orbit and away from the propellant depot network.

You don't need to actively cool solar arrays, that is the weirdest thing that I have ever heard of. They radiate heat passively. Large chemical engines on the other hand may need hundreds of megawatts of active cooling to prevent their combustion chamber from melting.

Gimbaling solar arrays is a complete non-issue as long as your spacecraft has attitude control. You only need the gimbals to support a tiny weight and you only need it to gimbal on a timescale of weeks. On a manned craft you could literally have a hand-cranked system like the ISS cupola shutters.

SEP does require some engineering for power systems that work in space, but not nearly on the same level as a chemical rocket engine. The Earth equivalent of those solar power systems exist and are considered boring technology that any private individual could buy and own.  By comparison, there is no boring non-aerospace equivalent of chemical rocket engines with power outputs comparable to the power consumption of large cities.
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 Coastal Ron

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The SpaceX BFS is a vehicle that will always be a high-value and fast transit form of transportation. What's the latest estimate for sending a human to Mars? $200,000? Hard to build a colony at that price.

But if you want to move less valuable cargo, and it does not degrade quickly in space and zero G, then slower forms of transportation like SEP will probably make more sense - and be less expensive.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline Robotbeat

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Because brute-force "abundant chemical" architectures with fully reusable super-heavy-lift launchers carrying "cheap propellant" would apparently make SEP look complex, expensive, and obsolete by comparison.

MikeAtkinson:
Quote
MCT seems to be about reducing cost by taking advantage of low launch costs to use a simple chemical based architecture. SEP adds complexity, vehicle cost and operational difficulties to reduce IMLEO - which is exactly the one thing you don't need to optimise for if you have cheap launch.
Yes, but it'll have to be made a lot cheaper. Currently, it's over-priced. Space solar cells are like $200/watt while terrestrial ones are like $0.20/Watt, so there IS a huge room for improvement. (Similar improvements are possible for the power electronics and the thruster itself.)

SpaceX is still using SEP for their constellation. It'll be especially critical for the lower VLEO constellation that will need to counter the drag propulsively.

There are also handling advantages to SEP over conventional chemical thrusters for spacecraft.

The implication of all of this is that SEP for cargo missions to deep space can definitely still make sense, but for bulk cargo it'll have to use much cheaper SEP.
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Offline TrevorMonty

For LEO-moon trips SEP take hammering from Val Allen belt, degarding solar arrays significantly on every pass, not to mention effects on cargo. For this run chemical has an advantage especially if lunar fuel available for return leg.

SEP are probably better for Mars cargo. I can see a case for hybrid systems, use chemical stage for earth departure then return chemical stage, leaving SEP for rest of trip.


Offline Nilof

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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.

The way to eventually replace chemical for the Earth-Moon leg would be with infrastructure like tethers, in-orbit mass drivers, or orbital rings, i.e. reusable reaction mass of some sort. SEP works best in heliocentric orbits.
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 TrevorMonty

Another one is solar sails driven by lasers from larger solar satellite. Forgot name of project buts one that wants to send nano size probes interstellar. Same concept can work for larger payloads at slower speeds, if driven by 100MW-GW significant payloads can be sent between earth and moon.

This could be one way to build larger GEO space solar power satellites for beaming power to earth. The same satellites can use some of there power for delivering cargo to GEO to build more. The vehicles can be very simple, large sail, cargo container, avoinics and some cold gas thrusters. As vehicle spirals out, GEO SSPS spaced around earth take turns boosting it.

« Last Edit: 01/10/2018 09:10 AM by TrevorMonty »

Online Eerie

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Electric propulsion has a bright future in the form on powering launch loops and orbital rings. The economy just needs to get to the point where BFR is not enough.

Offline AncientU

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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).
« Last Edit: 01/10/2018 12:33 PM by AncientU »
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Online SWGlassPit

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SEP requires very large steerable solar arrays with complex electrical conduits from arrays to some sort of storage system then to the engines. Somehow don't see a direct feed of electricity from the arrays to the engines. Also you need a cooling system for the waste heat from the solar arrays. So high power SEP is not simple or non-complex, therefore not cheap.

Don't see high power SEP entering service in the near to mid time frame. However maybe nuclear electric propulsion will be economical beyond Jupiter's orbit and away from the propellant depot network.

You don't need to actively cool solar arrays, that is the weirdest thing that I have ever heard of. They radiate heat passively. Large chemical engines on the other hand may need hundreds of megawatts of active cooling to prevent their combustion chamber from melting.


You don't need to cool the arrays, but unless your SEP hardware is 100% efficient in terms of converting electrical energy into kinetic energy (hint, it's not), you will have some waste heat in the system that has to be eliminated somehow.  It's possible that passive radiant cooling may be enough, but if it's not, there will have to be a cooling system of some kind.

Offline Pipcard

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Electric propulsion has a bright future in the form on powering launch loops and orbital rings. The economy just needs to get to the point where BFR is not enough.
By "electric propulsion," I am referring to low-thrust, high-Isp technologies that would be only used in space.

SEP only will make sense if commodity transport to LEO/EML-1/2 is never realized, or if 'N'uclear becomes the power supply for the 'EP'.  BUT the OP isn't about NEP...
Sorry for mentioning only SEP in the first post, I meant all types of electric propulsion.
« Last Edit: 01/10/2018 03:59 PM by Pipcard »

Offline DrRobin

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It's an interesting point, that higher performance technologies (by whatever metric you are using for performance) may still have lower overall value when all costs are factored in. This has been argued for decades , for example, by proponents of hydrocarbon fuels versus hydrogen, and by enthusiasts for Big Dumb Boosters. It can be tricky, though, to really know which costs are truly mostly fixed and which may fall with technological progress and economies of scale. In the case of SEP, the cost of terrestrial photovoltaic cells has fallen by more than two orders of magnitude in my lifetime, so there may be a lot of room for the costs of space-based systems to fall, particularly if one settles for cheaper, lower performance cells.

As others have pointed out, some destinations like Mercury are just very difficult to get to without high-ISP propulsion. Relevant to this discussion, Ceres is sort of a special case, as an attractive source of volatiles with not too large a delta-V to get them _from_ Ceres to Earth orbit (with aerobraking) but a punishingly high delta-V to get  _to_ Ceres given the need to match orbits at the destination (without aerobraking). Maybe this is a case where SEP can actually augment the chemical propulsion-propellant depot overall architecture: Slowly hauling equipment up from Earth orbit to Ceres to allow hauling propellant back down to near-Earth depots.

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Because brute-force "abundant chemical" architectures with fully reusable super-heavy-lift launchers carrying "cheap propellant" would apparently make SEP look complex, expensive, and obsolete by comparison.

I think the answer is "it depends".

For the Earth surface / Mars surface roundtrip chemical propulsion has a really large advantage. First of all, a large part of the total delta-v, Earth surface to LEO and Mars surface to LMO, can only possibly be done chemically. Then there is the fact that for both departures (TMI and TEI) you get a significant oberth effect because both earth and mars have a significant gravity well. And last but not least you can use aerocapture/aerobraking basically for free if you have a spacecraft that is capable of surviving reentry, which you need anyway to land on earth or mars.

For destinations with significant mass but without atmosphere such as the moon, electric propulsion is more competitive. But where it really shines is when your source and/or destination has neither atmosphere nor significant mass, so you can neither use the oberth effect nor aerobraking/aerocapture, and you do not need significant chemical propulsion for launch from the surface. E.g. asteroids or minor planets. For example the trip that the dawn spacecraft has done would have been almost impossible without electrical propulsion.

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. And it will work with lower Isp to reduce energy usage. Something as simple as an solar electrothermal rocket using water might be viable if you can easily refuel at your destination.

If you can use in-situ resources such as water from asteroids to refuel, there is no point in going for a very high Isp like current xeon ion engines. Something in the range of 1000s or even lower is perfectly fine. Extremely high efficiency is not even all that important, since modern thin film solar arrays have such a high power to weight ratio that you can afford mediocre efficiency, and low Isp means lower energy expense.

Future SEP spacecraft will maybe look like the IKAROS solar sail. Very lightweight structure with thin film solar sails. Solar cell efficiency does not matter at all. The only thing that matters is power to weight, and in this area thin film cells can be even better than very efficient triple junction cells.

Major breakthroughs for widespread use of solar electric propulsion for e.g. earth to mars transfer would be
- ability to use water with reasonable Isp and efficiency
- ability to do aerocapture with an electrically powered spacecraft (e.g. magnetoshell aerocapture)
- ability to use the oberth effect by having high thrust to weight for a short time, even with low Isp similar to chemical propulsion
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Offline Bob Shaw

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Another one is solar sails driven by lasers from larger solar satellite. Forgot name of project buts one that wants to send nano size probes interstellar. Same concept can work for larger payloads at slower speeds, if driven by 100MW-GW significant payloads can be sent between earth and moon.

This could be one way to build larger GEO space solar power satellites for beaming power to earth. The same satellites can use some of there power for delivering cargo to GEO to build more. The vehicles can be very simple, large sail, cargo container, avoinics and some cold gas thrusters. As vehicle spirals out, GEO SSPS spaced around earth take turns boosting it.



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Offline pathfinder_01

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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.

There are more costs than just launching propellant propellant. A trip to Mars would tie up an BFR craft for years.  If another type of craft did the trip say SEP then the BFR could focus on generating more revenue via Earth launches.  The BFR would deliver an bulk delivery of propellant to orbit it just would be propellant for SEP.  In addition on Mars the ISRU could just produce enough propellant to move things to Mars Orbit instead of to escape and Earth landing.

For Space X however attempting to develop SEP for MARS would be a bad idea and abundant chemical is probably the best idea for an initial landing. However given enough development and demand SEP could find a use as the colony grows.

It would depend on the economics of how much the SEP costs, how many  trips could it do and how much does maintenance cost.

Offline AncientU

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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.

There are more costs than just launching propellant propellant. A trip to Mars would tie up an BFR craft for years.  If another type of craft did the trip say SEP then the BFR could focus on generating more revenue via Earth launches.  The BFR would deliver an bulk delivery of propellant to orbit it just would be propellant for SEP.  In addition on Mars the ISRU could just produce enough propellant to move things to Mars Orbit instead of to escape and Earth landing.

For Space X however attempting to develop SEP for MARS would be a bad idea and abundant chemical is probably the best idea for an initial landing. However given enough development and demand SEP could find a use as the colony grows.

It would depend on the economics of how much the SEP costs, how many  trips could it do and how much does maintenance cost.

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.
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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.

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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!

Offline 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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Offline 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.

Offline 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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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).

Offline AncientU

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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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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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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 »

Offline AncientU

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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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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.

Offline 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.

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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.

Quote
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.

Quote
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.

Quote
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 »
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To put things on scale I worked out some rough performance numbers for an SEP system using Electrodeless Lorentz Force (ELF) thrusters as described in Pancotti et al. 2015 and a MegaROSA type array.  I assumed a 1 MWe system, which for the most conservative specific power (5 gm/Watt) gives a 5 tonne array.  The thrusters have a specific power of 1.5 gm/Watt (total mass 1.5 tonnes, including the PPU).  I assumed H20 as a propellant with a thrust efficiency of 0.55 and Isp of 5000 (~50 km/sec).  Add a couple of tonnes for spacecraft structure and tankage and the dry mass comes to around 8.5 tonnes.  A single BFR launch could orbit a dozen of these things, depending on packing efficiency.

Suppose this vehicle starts with 100 tonnes of H2O and does a low-thrust (naturally) transfer from LEO to EML-1, which requires a delta-v of around 7 km/sec.  Its acceleration is around 2e-5 gee, and it can produce that delta-v in about a year.  By the time it arrives at its destination it's consumed about 15% of its initial propellant.  The return trip without cargo would be much faster. Of course for such a short trip the Isp I assumed is much higher than optimal if you're in any kind of rush to get your cargo to its destination.  As others have noted, lower Isp won't hurt your mass ratio nearly as much as it would for a chemical system.

These SEP vehicles can act as a force multiplier for BFR.  In cislunar space, a fleet of them enables one BFR launch to put > 100 tonnes in EML-1.  I'd guess that's an improvement by a factor of around 4 compared to BFR refuelling in LEO and carrying the payload itself.

For Mars one could imagine pushing chemical prop to EML-1 or HEO or into Mars orbit, thereby expanding the launch windows, reducing the transit time, and/or increasing the payload that can be carried. 

Offline TrevorMonty

One of asteriod mining companies plan use solar concentrator to heat asteriod in bag and capture water and volatiles. Same solar concentrator will heat water and volatiles giving 300-330m/s thrust. Big plus is water doesn't need purifying.

They plan use same system for cargo and tanker vehicles. These vehicles may refuel from asteriod on route to destination, makes for low cost fuel depots, that only require occasional replacement capture bags before moving onto next asteriod.

Low ISP doesn't matter if fuel depots are ever 1-3km/s.

System should work out to asteriod belt, thrust will drop off but ISP should stay same.



Offline AncientU

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Water is ubiquitous in the solar System (and across the Universe).  EP SEP/NEP will proliferate if/when they use such an available propellant, and after a vigorous, ongoing inter-planetary program is running. 

The PPE model and scale, which will apparently be built soon, has all the disadvantages of low power SEP and none of the advantages of NEP or water as propellant.  As such, it will play near zero role in initial settlements on Mars, IMO, or anything beyond.  On the other hand, high power NEP with water as propellant could be the ticket to the outer Solar System.
« Last Edit: 01/13/2018 06:45 PM by AncientU »
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Offline Exastro

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It's important to note that omnivorous electric thrusters are not hypothetical: they've been up and running in the lab environment with good performance for years now.  One imagines that only a modest development effort (as noted above, likely a tiny effort compared to a large chemical engine) would be needed to make them ready for flight.

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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.

Is this actually right? That is 40 watts per square meter or 3% efficiency. From what I understand, they were 14% efficient BOL. For most BEO stuff, you get near continuous solar with very short infrequent eclipses if the trajectories are designed right. That is one of the reasons for the NRHO, the moon doesn't block the sun except rarely. And historically, solar power available on spacecraft has doubled about every 4 years...which puts megawatt class systems in the 2020s extrapolating from the ISS today(or extrapolating from end of construction at the beginning of the decade).

And it isn't like the BFS architecture avoids megawatt class power systems deployed in space. They are integral to the architecture.
« Last Edit: 01/13/2018 04:25 PM by ncb1397 »

Offline Nomadd

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 ISS arrays are more like 240kw. The system, which is in shadow half the time, can supply about 100kw average. The arrays supply half to systems and the other half to charge batteries while in sunlight. (rough numbers. Probably more like 140kw is for battery charging since there are inefficiencies)
 Even NASA keeps getting that one wrong in it's releases. They should specify that the system, including batteries, puts out about 100kw average.
 That's still only about half the claimed 14% efficiency. All I can think of is the actual electrical generating part of the cells only covering half the area of the array. It's even more confusing when you figure that there's 1350 watts available per square meter in space as opposed to the 1000 watts on Earth.
« Last Edit: 01/15/2018 04:32 AM by Nomadd »

Offline the_other_Doug

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And while I know it's been said, it bears repeating that any low-thrust, high-isp system is out of the question when departing from LEO or any other body with a radiation-trapping magnetic field with a crew on board.  You will surely kill your crew slowly spiralling out through the van Allen belts for weeks or months.

You'll kill all but the most rad-hardened electronics that way, too.  We basically need Treklike shields and screens if we think we can run EP cargo through high-radiation environments for years.  And if we have those we likely have better propulsion systems than shoving dirt through a linear accelerator.

EP likely has a role to play between planets, but not so much near several of them...
-Doug  (With my shield, not yet upon it)

Offline alexterrell

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I know it's claimed above that chemical has an advantage in the Earth - Moon system, but....

From EM-L1 you only need about 0.8km/s to get to Mars, with aero-capture.

One option would be to stage in HEO - perhaps at EM-L1. If we can use solar cells that are not effect by Van Allen belts, then a SEP tug could be used to carry full BFR tankers and cargo modules from LEO to HEO, adding about 3.75km/s.

This could happen over a 2 year period before the launch window.

On the other hand, I think chemical has an advantage for Mars transfer, as:
1. It can make use of the Oberth effect at Earth.
2. It can aeorcapture at Mars.

So we could colonise Mars with a fuel depot and space station at EM-L1, with a mix of SEP and chemical propulsion between LEO and EM-L1, and then chemical to Mars. Crew would still go taht route with Chemical.





Online speedevil

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And now, look at the streak of landing success where they have actually tried to land F9 with non-extreme or unusual landings.

Offline Bob Shaw

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These SEP vehicles can act as a force multiplier for BFR.  In cislunar space, a fleet of them enables one BFR launch to put > 100 tonnes in EML-1.  I'd guess that's an improvement by a factor of around 4 compared to BFR refuelling in LEO and carrying the payload itself.

For Mars one could imagine pushing chemical prop to EML-1 or HEO or into Mars orbit, thereby expanding the launch windows, reducing the transit time, and/or increasing the payload that can be carried. 

This is exactly right; CEP is an extra which will be most suitable for delivery of large masses which don't mind radiation and the use of which will relieve limited transport resources. SEP may also open up Mars orbital activities (smaller gravity well and no radiation belts).
« Last Edit: 02/24/2018 09:09 PM by Bob Shaw »

Offline ArbitraryConstant

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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.
If we're talking about "settlement scale" then it seems like electric propulsion can consider beamed power to remove some of the sun's constraints. A laser can provide more than the sun's 1.4 kw/m2 at 1 AU, and it can provide it at a specific wavelength rather than black body, which is potentially much more efficient for photovoltaics provided the band gap and wavelength are well optimized for each other. We've seen this demonstrated with eg lasermotive.

If this approach could, say, temporarily turn a 5 MW solar array into a 500 MW solar array, not only is that sufficient for high power ion propulsion, but it might even be easier to switch to thermal. The thrust would potentially be high enough for an oberth burn.

Earth departure could be handled by infrastructure near or perhaps even on Earth, and we would be able to use that to send solar laser arrays anywhere else we wanted to do lots of round trips like Mars or Ceres.

I don't think this is mutually exclusive with BFR/BFS, it probably enabled by BFS because it's the kind of infrastructure nobody would invest in without demand for space transportation that's created by BFS and others like New Glenn. Getting to orbit is still hard, there's still limits, and increasing ISP still increases the possible missions. Even if BFS offers quick transits to Mars inside transfer windows it's not like better ISP wouldn't offer improvements like quick transits without waiting for a window.

Offline AncientU

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These SEP vehicles can act as a force multiplier for BFR.  In cislunar space, a fleet of them enables one BFR launch to put > 100 tonnes in EML-1.  I'd guess that's an improvement by a factor of around 4 compared to BFR refuelling in LEO and carrying the payload itself.

For Mars one could imagine pushing chemical prop to EML-1 or HEO or into Mars orbit, thereby expanding the launch windows, reducing the transit time, and/or increasing the payload that can be carried. 

This is exactly right; CEP is an extra which will be most suitable for delivery of large masses which don't mind radiation and the use of which will relieve limited transport resources. SEP may also open up Mars orbital activities (smaller gravity well and no radiation belts).

How large of masses do you imagine?   ...what you are probably doing is thinking in terms of tiny, 1-3t landed payloads that we've been struggling to get delivered as status quo, and electric tugs could deliver cargoes that are factors of several tens larger (10-30 t?).  But then what do you do with them once they've spent two years in transit and another in aero-braking?  Who carries them to the ground?

BFR is supposed to be capable of landing 150t... three months or so after it departs Earth.  If you were on Mars, which supply line would you prefer?
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Online speedevil

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How large of masses do you imagine?   ...what you are probably doing is thinking in terms of tiny, 1-3t landed payloads that we've been struggling to get delivered as status quo, and electric tugs could deliver cargoes that are factors of several tens larger (10-30 t?).  But then what do you do with them once they've spent two years in transit and another in aero-braking?  Who carries them to the ground?

BFR is supposed to be capable of landing 150t... three months or so after it departs Earth.  If you were on Mars, which supply line would you prefer?

For 150 ton class masses - the scale of the SEP also gets quite daunting indeed too.
Also - as a truly ridiculous point.

Impulse delivered by SEP to Mars by SEP may in fact be comparably as expensive as just sending a BFS.

I have not done in detailed calculations, but as a sketch if you were imagining going from GTO to Mars and braking into low mars orbit with SEP takes some 4km/s, compared to the HEO BFR sets out in.
This is - neglecting the SEP mass, some 50 tons of xenon at ISP=1500.
Various sources I find say this is around $50M.

That is before the cost of a 2MW or so solar panel, ion engines, ...

It seems at least plausible that this can end up costing you half of what a BFS in total costs to send to Mars.
If ISRU works, it may actually be cheaper, even only counting the propellant.

(Propellants other than Xenon are of course possible for SEP)




Offline alexterrell

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http://ieeexplore.ieee.org/document/7355859/

It would appear that Perovskite based solar cells don't suffer from radiation effects. They are also suitable for thin film deployment, so could eventually (i.e. quite soon) be suitable for space based electric propulsion through the Van Allen belts.

This would allow a couple of solar tugs to provide heave lift from LEO to HEO. From there, chemical propulsion could be used to get to Mars, benefiting from the Oberth effect (near Earth flyby) and Aerocapture on Mars.

This solution could combine the best of chemical and electric propulsion. Electric propulsion provides most of the delta V, chemical propulsion provides the manouverability, as well as crew transport.

Offline MATTBLAK

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Yes; an advanced Solar Electric and chemical combo would be a powerful exploration enabler. The chemical could be hypergolics or LOX/CH4 - ideally paired with propellant transfer (depot) for that system. The SEP could of course be refuelable by changing out or refilling the Xenon (or Argon) tanks. And for beyond Martian or Ceres orbit - there could be a chemical/Nuclear-electric combo. The nuclear could be based on the coming 'Kilopower' technology.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170002010.pdf

https://www.nextbigfuture.com/2017/09/higher-power-and-high-density-nuclear-electric-for-space-missions.html
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