Author Topic: Does electric propulsion still have a future? (if BFR is successful)  (Read 3909 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.

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.

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 »

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)