Author Topic: Does electric propulsion still have a future? (if BFR is successful)  (Read 7236 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.

Online 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.

Online 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.

Online 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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Offline 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.

Offline rklaehn

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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.

Online 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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