Frank

Why rethink the polar orbit? I don't think you've read the paper I linked. The tether material is Spectra 2000 (and is readily available) and a Hoyt tether (sp?) has multiple strands that can withstand a break. That part is known. Everything in space seems "extremely difficult", the question is always "can it be done with reasonable resources?". This is the paper: http://www.tethers.com/papers/InterplanetaryTetherTrnsprt.pdf I've been looking for (papers on the web, written by smart people) that explain why it can't work and haven't found any. I have found other versions of it like the MXER. Here's one from NASA: https://www.nasa.gov/centers/marshall/pdf/115871main_MXER_TS.pdf Also, the Carbothermal process uses the Sabatier reaction which is similar to what would be used on Mars.

I agree that the tank would likely be deformed now that I've read how thin the material is: 2.5 to 5 mm of aluminium alloy. I've thought of a better way to build a habitat anyway. Assuming constant deceleration, ((2380 (m / s))^2) / (2 * 45 000 (m / (s^2))) = 62.9377778 meters, we need a crater less than 100 m long for hardened payload (4500g) to survive. Krafft Arnold Ehricke (March 24, 1917 – December 11, 1984) was a German rocket-propulsion engineer and advocate for space colonization. https://en.wikipedia.org/wiki/Krafft_Arnold_Ehricke Krafft Ehricke invented the Lunar Slide Lander as an alternative to powered descent to the lunar surface, which would use 90% less propellant by taking advantage of the Moon’s sandy and glassy soil to slow the vehicle. http://schillerinstitute.org/educ/sci_space/2017/0324-lifting_mankind/lmo.html I wasn't proposing we land humans, just hardened payloads.

What was the effect of viscosity being too low, did it leak? I had a project in mind and was going to use bladder actuators... Wonder if it would work for that.

Maybe https://en.wikipedia.org/wiki/Propylene_glycol? Might not be "slippery" enough. Non-toxic. I assume you already checked all the hydraulic fluids (brake, transmission, power steering etc...) https://en.wikipedia.org/wiki/Hydraulic_fluid

Given a 40t lunar momentum exchange tether with solar power (powervator?) we get 2.5 t of payload that can be either powered or transferred to EML-1. A hydrolysis module can be powered directly instead of charging batteries. So we have ~60 kW to play with. I found a 1991 paper for a concept "Conceptual Study of on Orbit Production of Cryogenic Propellants by Water Electrolysis" https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910010004.pdf Scaling their findings to 20%, I get a module mass of 2122 kg and power use of 58.2 kW. But I removed the cryogenic hydrogen generation because it is to be reused in the production of LOX only. So we get ~70 t of LOX per year of operation. If a lunar base is thought of as a Deep Space Gateway, there may be an economic argument for producing LOX and placing it at EML-1. Basically, each 20 t of LOX is equivalent to a launch's worth of payload on its way to Mars for example. Of course the cost of maintaining the base, collecting the water, putting the LOX into orbit, etc... may ruin the economics. If nothing else, it may end up being cheaper than maintaining the ISS. The mining part can produce water (directly or via reaction lunar oxygen with earth hydrogen) which can also be exported to EML-1 and potentially other materials (silicon for solar panels, aluminium and iron). https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110006938.pdf I may look into the Solar Carbothermal Regolith Processing and Gathering costs later. The solar thermal part is linked to solar thermal propulsion.

According to a Stanford study, it's possible to use WWS (wind water and solar) to power everything by 2050: US: https://news.stanford.edu/pr/2015/pr-50states-renewable-energy-060815.html World: http://www.thesolutionsproject.org/ http://thesolutionsproject.org/why-clean-energy/

Those are all good points. You may also be correct that the Musk/SpaceX plan is wrong and ill-conceived. But the biggest trick is money, no money, no space travel. I don't know how to compare the cost of a cryogenic hydrogen tank VS the cost of a pressurized methane tank. I'm not the one trying to prove SpaceX is wrong. My risk is thinking they can do what they claim and I may be dismissing a better concept plan than their concept plan. Anhydrous Ammonia is conveyed by train all over the place in tankers that are many tens of tons each and they do crash and rupture (sadly). Fuels are dangerous, some are poison, some explosive. Risk assessment must be done. I wasn't suggesting it be used blindly, just not rejected out of hand because of toxicity. The SpaceX plan either has a rocket fuelled and ready to launch remotely, or people are there refuelling a rocket from ISRU or maybe it's all meant to be done robotically. In any case, the plan is that there is a rocket (or many) sitting there from a previous supply/cargo mission. There would need to be a way to transfer fuel from a cargo rocket to a crew rocket. The hop with another rocket was MY idea addressing your concern about accuracy, SpaceX has only said they would land on target. What I found was that the time it takes to accelerate a ship using STP can be 30 or more days and that the distance travelled in that time is greatly inferior compared to a rocket burn, so that higher speed must be attained to compensate for the penalty. When greater speeds are attained, the extra arrival speed must be burned off to reach a capture orbit which also takes more time compared to a rocket burn or direct aerobraking. There MAY be a way to do it, I haven't found one that is significantly faster than 80 days, so I'm putting it aside. I did make a spreadsheet of my own, put haven't posted it. "probably useful" means that STP is a way to do it, but it might be cheaper to do a one-way trip with a rocket than a two way trip with STP or there are other, better ways to do each of the missions. I haven't done the math yet, or looked at all the possibilities (who can?) someone who's already done it can post the results or I may do it in the future. Yes, I'm just starting to learn this stuff. It's interesting and I've learned a lot already.

I found it encouraging that money was spent on STP and that 1200s Isp is thought to be achievable based on prototype results (even with their design). I'm still unsure how the ruminator works, but it sure would be neat to get a (small?) grant to build a one and test it at that facility. Most of the infrastructure is already in place! Most high efficiency CPV works on multiple wavelengths, but light generally isn't selectively filtered and heat IS an issue. It's surprising that there is a way around that but it isn't being used (that I know of). I must admit that I don't understand how the selective wavelength filter works either.

Yup. http://planetforlife.com/h2/h2vehicle.html But it will still produce con trails and probably NOX, both contribute to climate change Personal opinion is Ammonia (NH3), less bulky, liquefies under pressure, no cryogenic freeze, but it's toxic, so handle with caution.

How embarrassing, given a ~20t system mass, a payload of 1/16.7 is possible, so up to 1.25 t (1250 kg) instead of 250 kg. Of course, 250 kg of batteries can still be used and recharge would still be 2h. This version seems a bit clearer: http://www.tethers.com/papers/InterplanetaryTetherTrnsprt.pdf

Yes, and the economic argument on Earth is getting weaker as well as solar PV cost drops and efficiency rises. https://en.wikipedia.org/wiki/Cost_of_electricity_by_source#NREL_OpenEI_.282015.29 Plus, the lunar tether power is stable and predictable. (I like V2G to fix that on Earth... someday)

Well, Germany and other countries a phasing nuclear out. https://en.wikipedia.org/wiki/Nuclear_power_phase-out My personal opinion is that no new nuclear sites should be developed and that reprocessed uranium should be used up in the existing sites. Regardless, no harm in exploring solar as a main power source. The thing is that putting 2x (more or less) the solar panels on the moon than would be needed in space means more launch mass per delivered kW. Not only are more solar panels needed, but they need to be soft-landed on the moon which isn't free. I suppose if a working lunavator/tether were working, that last penalty would be reduced. With some slight corrections of the lunavator over the course of 29 days, a single spot close to the pole could be supplied with charged batteries ready to be inserted into rovers and other lunar robots and plants.

It sounds really stupid at first - a survivable crash landing... The idea is similar to aerobraking where regolith is used to decelerate a space ship. Not one with humans aboard of course, it's a crash landing and bad things can happen. So bring the elliptic orbit of a ship to a grazing height and touch down, then skid along the lunar surface. But what can this get us? A bunch of otherwise useless upper stages of rockets could have their material recycled once dropped onto the lunar surface. If we do this enough times, we might become able to clear out a runway or learn how to do it without much damage, then more sensitive stuff could be "landed" this way. But what if we send a rocket into a mountain or crater wall? Maybe it creates an automatic habitat - a space covered by the 1 to 10 m of regolith needed to protect humans from cosmic rays and solar proton events. So forget trying to build lunar excavators and complex 3D printing for habitats, just(tm) drop a rocket into the regolith, cut out the unneeded parts and replace that space with some sort of inflatable Hab. Done. A less random/gross way might be to use a spent rocket as a form and drop the edge of a mountain or crater down on it instead of lifting and gathering regolith from all around. Use the potential energy already in the mountain to quickly bury a habitat. Maybe use the buried rocket tanks as storage and the regolith would help regulate the temperature, acting as insulation. If the rocket goes in engine first, access to the tanks or even tank fittings might be available.

A working version of the tether animation. Can't seem to edit the original post...

Certainly, the upsides of NP are significant, but so are the downsides. If we stop using nuclear on Earth because of the downsides, it's hard to understand why we would want to do it where there are limited resources to clean up a mess. In any case, offering an alternative isn't going to hurt anyone. The tether end, from the lunar perspective, just drops down on a particular spot and pops back up. There is an animation on wikipedia: https://en.wikipedia.org/wiki/Momentum_exchange_tether#Rotovator I started a new thread for ideas on how to make the swap here (I don't know exactly what would work): http://www.scienceforums.net/topic/110975-fast-battery-swap-for-a-lunavator/

A rotovator/lunavator with a 200 km long tether rotating in space above the lunar surface drops momentarily to swap a large battery pack (mass of 250 kg) for a depleted one to be charged. Is there a simple and effective mechanism that would do this? If the operation is too fast for a release then a catch, maybe two "bays" could be used. Instead of exchanging position on the tether on route, the base (where the exchange takes place) could coordinate which side or position is open when the exchange happens so that the spent battery can be placed at the correct location. Some king of catch and release mechanism - hook and eye? Ideally, an object in flight could also be grabbed. Here is a clever rotovator catch and release mechanism, it is at 3:00 into the video: Longer description of the lunavator power here: http://www.scienceforums.net/topic/110962-how-to-cheaply-and-efficiently-power-a-moon-base/

According to wikipedia, "No peaks of eternal light have been positively identified on the moon, but many peaks have been detected that, via simulations based on imaging and laser and radar topography, appear to be illuminated for greater than 80% of a lunar year." And "Further data from the SELENE spaceprobe determined that one peak at Peary Crater receives sunlight for 89% of a lunar year, the highest level of illumination predicted for any peak of eternal light on the moon.[3]" https://en.wikipedia.org/wiki/Peak_of_eternal_light A battery swap from a tether seems easier, faster, cheaper to me. Add to that the ability to ferry stuff to and from the moon efficiently and it seem worth looking into.

Yes, this is possible, in fact likely the best place for it given a polar orbit, otherwise it might be ~29 days until the tether returns to the same location. And ice should be easier to extract than oxygen from regolith, but some object to using the limited resource as fuel. I don't know enough to make an informed argument. Another nice thing about a system like this is it's easier to move than a solar field if we decided we wanted a base in a different location. I guess it's not all roses if a base is anywhere but the poles, the equator orbit would have less sunlight and other places are only visited once per rotation... I say we go with the polar moon base for this to work. Hi Ken, oh, I see I should quote when responding... I missed the part about constant sunlight in Moontanman's post. Axial tilt should be OK for a tether system but wobbling might mean a larger landing target. The mechanism to do a battery swap isn't obvious either. When it comes to money, I'm also mystified. Bigelow thinks he can set up space tourism to a space station https://en.wikipedia.org/wiki/Bigelow_Commercial_Space_Station. Blue Origin wants a polar lunar colony https://en.wikipedia.org/wiki/Blue_Origin_Blue_Moon.

How to cheaply and efficiently power a Moon Base. Powering a Lunar Moon Base isn't simple because the moon is in shadow for long periods of time. Solar power must have weeks of backup which would require a lot of batteries or some sort of backup generator that runs on maybe hydrogen and oxygen. Solar panels must be oriented to the sun to maximize efficiency, or more panels must be employed. One idea is to place the solar panels in space, perhaps at EML-1 and beam the power back to the moon. This still requires a very large array to capture the energy on the moon. This idea is similar, it uses a satellite in a polar orbit such that it is always exposed to the sun (except for short eclipses with the Earth). Power collected by the solar panels is transmitted down a rotating tether which has a battery charger and swapable battery at the end. This arrangement (without the charger and power cable) is known as a Momentum Exchange Tether or rotovator/lunavator or skyhook. There it is. Use the mass of a solar panel array as an anchor for a rotovator, charge a battery, drop it and pick up another to be charged. Batteries are convenient to move around and place where they are needed as opposed to fixed power lines and they are their own backup. A single launch of a falcon heavy could put ~20t of solar panels, chemical rockets and tethers with power cables into lunar orbit providing around 30 kW of power. A single ~2h orbit would likely be enough to charge a 50 kWh battery, basically delivering continuous 25 kW power with battery reserves in units of 50 kWh. The battery swap doesn't change the momentum energy in the system. A 50 kWh battery using 200 Wh/kg is 250 kg (but would weigh 42 kg on the moon) which is the maximum mass of this first stage tether. Not only does this arrangement not require landing 20t of solar panels on the moon, it also allows transfer of payloads up to 250 kg to or from lunar elliptic orbit or EML-1. This saves rocket fuel launched from earth or mined from the moon to do the lifting. Also anything that can be packaged into 250 kg chunks can be sent to the moon or from it. And if the transfers are bidirectional, no net energy is used! The idea of a momentum exchange tether isn't new, but using battery swap to power a moon base is, as far as I know. Here is a paper on a momentum exchange cislunar system which is meant to send cargo to and from LEO. http://www.tethers.com/papers/CislunarAIAAPaper.pdf It requires two rotovators. I think a first step of just the lunar portion would allow efficient transfers between 400 km elliptic lunar orbit (or EML-1) and the lunar surface. This could mean water at EML-1 by combining sent hydrogen with lunar oxygen or LOX from electrolysis of the water in a reusable cycle scenario. Once power and transportation is solved, building a lunar moon base seems attainable.

So, it seems using Solar Thermal Propulsion for fast transit to Mars isn't helpful. Aerobraking a chemical rocket (the SpaceX way) is better. STP is probably useful to bring payloads to higher orbits or to EML-1 or to low lunar orbit maybe. STP is probably useful to haul (non-human) payloads to Mars and back, as well as hauling from Mars lower orbit to a higher orbit. I used this spreadsheet to get there https://space.stackexchange.com/questions/18396/how-is-80-to-150-days-to-mars-possible-at-6-km-s-tmi-delta-v and here http://clowder.net/hop/railroad/Hohmann.xls This work on STP is interesting: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160003173.pdf

Here are a couple of link to STP that might be of interest: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160003173.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050203997.pdf "Reduce tank volume with increased STP specific impulse approaching 1200 seconds - Higher temperatures above 3000K increase Isp - At higher temperatures above 3000K and low pressures, hydrogen starts dissociating and lowers the average molecular weight, which also increases Isp" I guess dissociation of hydrogen will allow more Isp for a given temperature (which is limited by material properties), at the expense of efficiency, requiring more energy input.

I haven't looked closely at the numbers, but 7x more efficient seems optimistic to me. Here on Earth, it's VERY difficult to make a case for solar thermal based on cost, even where thermal should obviously be better. When it comes to mass as the currency, I don't know yet. The working example of a concentrating solar collector I found is SCARLET https://pdssbn.astro.umd.edu/holdings/ds1-c-micas-3-rdr-visccd-borrelly-v1.0/document/doc_Apr04/int_reports/Scarlet_Integrated_Report.pdf It's a fresnel concentrator and thermal management is on page 7 of the report. In 1998, 20 years ago, they managed 45W/kg for the array and DS1 with its ion engine worked. Using a 4 m diameter dish and concentrating on a PV module will generate some heat. Quite a bit of it, because up to 60% of the energy will dissipate in heat if 40% efficient.

Hydrogen has lots of handling issues, maybe they are all solved, but I suspect that the SpaceX team did the cost comparison and decided to go with methane. True, the Lunar ISRU is different, but experience from a Lunar settlement would still be helpful. I don't feel it's an either-or case anymore with private money going into space exploration. Both Lunar outposts AND Mars outposts/colonies are possible concurrently. SpaceX mentioned the Sabatier reaction https://en.wikipedia.org/wiki/Sabatier_reaction to convert CO2 + 4H2 → CH4 + 2H2O   ∆H = −165.0 kJ/mol, so Mars CO2 with Earth hydrogen to get methane and water. I'm guessing the boil-off rate is manageable to feed the reactor? Carbon-fibre tank vs hydrogen tank plus insulation.. Energy density of hydrogen 8.491 MJ/L, liquid methane (LNG) 22.2 MJ/L compressed methane (CNG 250 bar) 9 MJ/L. So both tanks are the same size. Mass difference? https://en.wikipedia.org/wiki/Energy_density Ammonia can be handled safely, it's used on crops everywhere, regardless, it has lower energy content than methane, so not ideal as rocket fuel, though it might combine well with thermal thrusters. A space suit is probably enough protection and must be worn anyway. Well, that lander crashed, so there was no recovering. A rover is one solution. A Mars short hop is another if enough fuel is left in the rocket - this would assume a navigation failure. Risky business for sure. A short hop using one of the rockets previously landed and partly refuelled is another.

If we get 40% conversion efficiency from a concentrating solar array, is pumping this energy into an ion engine more efficient when its high Isp is considered? All this technology is also known and working so the risk of development isn't there (or as high). Also, electricity is a generally useful form of energy (compared to heat) and may have use along the coast phase or even when parked waiting for the next mission. I believe there is some cooling needed for those concentrating collectors, but no mirrors needed, which may end up adding to the mass of the array in the balance - hard to tell.

One thing about moon based Oxygen production is that it would be nice to know that ISRU works before we send stuff to a far-away planet. In other words, if someone starts putting moon hotels in place (Bigelow) or decides that it's about time we have a moon base, then the Oxygen creation infrastructure must work. Then sending fuel to EML-1 or Earth orbit or even Mars is possible and may even be economical. I didn't begin to calculate the cost difference. I'm just leaving the door open for that possibility, same with the mass drivers. If these things ever get built and work as advertised, that's probably an order of magnitude cheaper at least. " last person putting time in it? " - I don't follow. In any case, I was under the impression the Solar Thermal Propulsion threads were about long stay and short trips, so I wondered if short stays and long trips were possible and how short can the trips can be given Solar Thermal Propulsion. Well, methane can be stored in pressure tanks, but would need to be liquefied before use, as can oxygen. Freezing is probably avoided by burying the tanks. Hydrogen is tricky to get any kind of density as is evident from our lack of hydrogen cars, efficient storage is a problem. Active cooling is a possibility - sure. I mostly went with methane based on the Raptor engine and the idea that fuel can be synthesized on Mars (someday). I'm not a chemist, so I assume smart people are working for Musk. Aerobraking I took from the Musk plan. Assuming it would work for a smaller craft. Now that I think about it, it isn't clear how the dragon capsule and the glider would work together. I guess they don't which is too bad, because I liked the escape feature. Maybe a fairing style of glider skin with a spare one on the main ship for the Earth reentry. Needs more work... As for re-use, returning upper stages to Earth seems too expensive still, so my thought there was to re-use the upper stages in space. Even though the upper stage used for the Mars mission would return to Earth it seems that the 5 refills and two re-entries would be enough re-use . Maybe burn up on Earth re-entry with a capsule landing instead of a glider landing? Accurate landings on Earth have been going very well for SpaceX hitting the bull's eye 18 times in a row now. I had though both orbit and land drops, no ISRU at first, though setting up the equipment for future missions might be a part of the short stay mission. The main idea for me is to re-fuel at Mars and make the manned trips as short as possible. The important thing about the Raptor vs the Merlin was the fuel type as RP-1 isn't going to help anyone. 3 MN would sustain gravity acceleration for a 300 ton ship with a single engine! Artificial gravity anyone? It is also more efficient. Another thought is that having at least two engines would be nice redundancy should one fail on landing for example. They did develop a scaled version of the Raptor that might work better for this application. Hydrogen vs Methane - still not convinced either way. Ammonia or Ammonia hydrogen-capable might be good too. Don't know. If you're saying that chemical rockets are more efficient than solar thermal for LEO to staging area (EML-1 or high orbit), then why bother with staging at LEO at all? Just go straight there, robotically assemble/refuel at EML-1.