Author Topic: Does electric propulsion still have a future? (if BFR is successful)  (Read 7237 times)

Offline Exastro

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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.



Online 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.

Offline ncb1397

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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 »

Online 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)

Online 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.





Offline 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.

Online 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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Offline 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)




Online 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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