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#280 by Hop_David on 10 Feb, 2015 20:48
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to posit fuel depots sitting at LaGrange points just waiting to support Mars missions in the 2030s.
I am positing no such thing. Try reading my posts.
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#281 by A_M_Swallow on 10 Feb, 2015 23:19
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{snip}
Again, this seems like energetic arm-waving to me. "Just toss on some sun shields" isn't exactly rigorous engineering.
Sun shields = curtains. I have those in my lounge. The technology is TRL 9 since Skylab had one. -
#282 by TakeOff on 11 Feb, 2015 07:48
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Concerning your cartoon, what about storing water ice in space and doing the electrolysis on demand in orbit/lagrange? No need to store leaking LH2. I think Doug Plata mentioned that idea. -
#283 by kkattula on 11 Feb, 2015 08:15
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Wow. You're posting like depot technology is some sort of techno-wizardry. It isn't. It's been researched and developed for decades, it just hasn't been flown. It's not fusion, or anti-matter.
It's just tanks with good insulation and a sun-shade. Some LH2 boils off, keeping everything else cold. Use it for station-keeping and power generation.
The trickiest part is zero-g fluid transfer. So you don't do it. Tiny amount of spin gives enough g for fluid settling. -
#284 by KelvinZero on 11 Feb, 2015 10:13
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yeah that keeps puzzling me. Nothing in space is easy, but People keep implying zero boiloff is hard, like violating thermodynamics hard.
Don't you just start with a reasonable passive cooling system then reliquify any boiloff with active cooling? If you can't do that, how can you expect to aquire water from regolith, split it into hot products, separate and THEN liquify those? (or liquify then separate, whatever)
Liquid gas is an extremely mature industry on earth. What am I missing?
(edit)
and thats ignoring the detail that passive systems look to be sufficient anyway, with any boiloff used for station keeping propellent
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#285 by Nilof on 11 Feb, 2015 11:46
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LH2 boiloff is hard, especially in LEO. LOX or Methane boiloff isn't.
However, since LH2 likely won't be the optimal propellant anyway, that is a complete non-issue. The limiting factors for a stage refueled at a depot is propellant volume(the depot capacity) and dry mass, not total fueled mass since the fuel is not shipped from Earth. You need very, very high mission delta-v's for the LH2 stage to outperform a methalox stage in this aspect since the methalox stage will have a better mass ratio. Furthermore, methane leads to a more optimal use of asteroid resources. -
#286 by jongoff on 11 Feb, 2015 14:28
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I m bemused at how the proponents of fuel depots keep popping depots up as the cure-all for all future interplanetary flight development, when the technology to create liquid hydrogen and liquid oxygen from lunar and asteroidal sources is still in the realm of energetic arm-waving, as is the technology for maintaining such fuels in liquid states for months or years.
Yes, fuel depots may one day be developed and used. But the technology just isn't developed, and won't be practical for decades to come. It makes no sense to posit activities in the 2020s or even 2030s being dependent on such depots when I can't see them being available for another 50 to 100 years...
I love it when people who have no idea about the state of technology or the remaining steps to depots being useful handwave about them not being available for another 50 to 100 years.
~Jon -
#287 by jongoff on 11 Feb, 2015 14:33
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yeah that keeps puzzling me. Nothing in space is easy, but People keep implying zero boiloff is hard, like violating thermodynamics hard.
Don't you just start with a reasonable passive cooling system then reliquify any boiloff with active cooling? If you can't do that, how can you expect to aquire water from regolith, split it into hot products, separate and THEN liquify those? (or liquify then separate, whatever)
Liquid gas is an extremely mature industry on earth. What am I missing?
(edit)
and thats ignoring the detail that passive systems look to be sufficient anyway, with any boiloff used for station keeping propellent
This. If you can take water, electrolyze it, and then chill it down to liquid temperatures, you've basically got a zero boiloff depot already. Sure, you probably want to settle the propellants via spin or gravity gradients or some other technique to make your life easier. But all the pieces past that are proven. We know how to do cryocoolers down to LHe temperatures. We know how to do solar panels and radiators. We know how to do insulation. Have all the pieces been integrated and scaled up to the size needed for a depot? Not yet. But this isn't anti-gravity or something we don't know will work if properly funded. If depots were given 1/10th the resources that SLS was, heck even 1/100th, we'd probably have them before SLS was flying useful missions.
But we're wandering pretty far afield from the topic of this thread. As much as depots supplied from asteroids or the moon are intriguing.
~Jon -
#288 by jongoff on 11 Feb, 2015 14:33
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yeah that keeps puzzling me. Nothing in space is easy, but People keep implying zero boiloff is hard, like violating thermodynamics hard.
Don't you just start with a reasonable passive cooling system then reliquify any boiloff with active cooling? If you can't do that, how can you expect to aquire water from regolith, split it into hot products, separate and THEN liquify those? (or liquify then separate, whatever)
Liquid gas is an extremely mature industry on earth. What am I missing?
(edit)
and thats ignoring the detail that passive systems look to be sufficient anyway, with any boiloff used for station keeping propellent
This. If you can take water, electrolyze it, and then chill it down to liquid temperatures, you've basically got a zero boiloff depot already. Sure, you probably want to settle the propellants via spin or gravity gradients or some other technique to make your life easier. But all the pieces past that are proven. We know how to do cryocoolers down to LHe temperatures. We know how to do solar panels and radiators. We know how to do insulation. Have all the pieces been integrated and scaled up to the size needed for a depot? Not yet. But this isn't anti-gravity or something we don't know will work if properly funded. If depots were given 1/10th the resources that SLS was, heck even 1/100th, we'd probably have them before SLS was flying useful missions.
But we're wandering pretty far afield from the topic of this thread. As much as depots supplied from asteroids or the moon are intriguing.
~Jon -
#289 by Hop_David on 11 Feb, 2015 15:29
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LH2 boiloff is hard, especially in LEO. LOX or Methane boiloff isn't.
Indeed. LOX is a much less difficult cryogen. And LOX comprises the lions share of bipropellent mass. I had wanted to use the word "propellent" but ran out of room and didn't have time to revise the cartoon.
The major straw man I was trying to draw attention to is two year wait times. Frequent use of a depot mitigates boil off loss. If Mars were the only worthwhile destination in the solar system, launch windows each 2.14 years would be a big problem for EML2 depots.
But there are multiple possible uses for EML2 propellent. Here are a few:
1) Reusable earth departure stages could eliminate an ion engine's slow spiral out of earth's gravity well, cutting years of asteroid retrieval missions and leaving more xenon reaction mass to move asteroids
2) Routine access to cislunar space. With 3 or 4 km/s delta V budgets, and no 8 km/s re-entry, re-usable intra-orbital vehicles are far more doable. It would be economical to maintain and upgrade our orbital sats as well as salvage dead sats. The launch, use and discard paradigm would be a thing of the past.
3) Access to the moon. EML2 is about 2.5 km/s from the lunar surface.
4) Supplying Mars transfer vehicles with propellent and life support consumables. When Trans Mars Insertion takes only 1 km/s, a reusable MTV becomes plausible. Or faster trips if using expendables.
Taken by themselves, I don't believe any of the above reasons justify the expensive infrastructure for extracting asteroidal or lunar propellent. But the sum of these might make a profitable market.
Depending on who is the loudest and most aggressive I alternately fight with Mars Firsters, Moon Firsters and NEA firsters. Each is a 1 legged stool. To break out of Cradle Earth, I believe we need to go for all three.
To get back on topic, a robust SEP vehicle capable of asteroid rendezvous and retrieval would be a very important step on the journey to move beyond cradle earth.
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#290 by AlanSE on 11 Feb, 2015 19:51
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by refueling at a LEO propellant depot (fueled from Earth surface)
Assuming lots of propellent at EML2 why would LEO depots be supplied by earth surface?
EML2 is 3.5 km/s from LEO. And 3.1 of that 3.5 is circularizing at LEO -- much or all of that 3.1 could be done by aerobraking. So delta V from EML2 to LEO can be as little as .4 km/s.
When the trip from earth's surface to LEO is cut out of the equation, trips between orbits in the earth-moon neighborhood would be around 3 to 4 km/s. This makes for much less difficult mass fractions and sturdier ships. Most inter-orbital vehicles could remain in space and thus never have to suffer the extreme conditions of an 8 km/s re-entry into earth's atmosphere. Re-use becomes far less difficult.
Routine access to our orbital assets in cislunar space is the goal. It's no coincidence that sounds like Spudis, Spudis has been talking about the same thing: water. Water which can be made into rocket fuel. Only an asteroidal propellent source in high lunar orbit doesn't have the moon's 2.4 km/s gravity well.
I'm not talking just reusable EDS but reusables vecicles for travel between LEO, GEO, EML1&2 and beyond.
Serious question: You've constructed a round trip from EML2 to LEO that requires at least 6 distinct burns over several days. Because of that, you would probably need to avoid cryo LH2. Or would you not? So I'm validly curious what your solution is.
Option 1: Have a spacecraft that can avoid boil off. If you can do it for a propellant depot, then I guess you can do it for this as well. But how much would that weigh? Would it very negatively affect other aspects of the design?
Option 2: Use some other propellant. Like what? Would that be some methane / LOX engine? How much worse would the specific impulse be? Would water even be a step in the processes to make these? -
#291 by jongoff on 11 Feb, 2015 21:07
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by refueling at a LEO propellant depot (fueled from Earth surface)
Assuming lots of propellent at EML2 why would LEO depots be supplied by earth surface?
EML2 is 3.5 km/s from LEO. And 3.1 of that 3.5 is circularizing at LEO -- much or all of that 3.1 could be done by aerobraking. So delta V from EML2 to LEO can be as little as .4 km/s.
When the trip from earth's surface to LEO is cut out of the equation, trips between orbits in the earth-moon neighborhood would be around 3 to 4 km/s. This makes for much less difficult mass fractions and sturdier ships. Most inter-orbital vehicles could remain in space and thus never have to suffer the extreme conditions of an 8 km/s re-entry into earth's atmosphere. Re-use becomes far less difficult.
Routine access to our orbital assets in cislunar space is the goal. It's no coincidence that sounds like Spudis, Spudis has been talking about the same thing: water. Water which can be made into rocket fuel. Only an asteroidal propellent source in high lunar orbit doesn't have the moon's 2.4 km/s gravity well.
I'm not talking just reusable EDS but reusables vecicles for travel between LEO, GEO, EML1&2 and beyond.
Serious question: You've constructed a round trip from EML2 to LEO that requires at least 6 distinct burns over several days. Because of that, you would probably need to avoid cryo LH2. Or would you not? So I'm validly curious what your solution is.
Option 1: Have a spacecraft that can avoid boil off. If you can do it for a propellant depot, then I guess you can do it for this as well. But how much would that weigh? Would it very negatively affect other aspects of the design?
Option 2: Use some other propellant. Like what? Would that be some methane / LOX engine? How much worse would the specific impulse be? Would water even be a step in the processes to make these?
Boiloff is a relative term. Depending on your heating and your level of insulation (and your amount of active cooling) you will have either zer boiloff or some finite rate of propellant loss. The heating rates once you get from from earth (which happens pretty quickly once you leave LEO) drop by almost an order of magnitude, so boiloff should also decrease substantially.
The question is how much propellant boils off, and can you live with it. With reasonably good insulation and/or sunshielding, it's probably not a big deal for missions shorter than a month, even with LOX/LH2. I know that's longer than has been demonstrated so far, but there's data on these things. The reason they haven't pushed this very hard so far is that for most Centaur missions you run out of either RCS propellants, battery juice, or pressurant long before then anyway. My guess is once IVF is working, you'll start seeing them doing longer missions.
~Jon -
#292 by TrevorMonty on 12 Feb, 2015 07:42
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3d printing regolith using a laser.
http://3dprint.com/30302/3d-printing-on-moon-mars/
Building radiation shelters from asteriod regolith was one possibility.
This rock may have more uses than just the oxygen that can be extracted from it. -
#293 by Nilof on 12 Feb, 2015 11:09
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Serious question: You've constructed a round trip from EML2 to LEO that requires at least 6 distinct burns over several days. Because of that, you would probably need to avoid cryo LH2. Or would you not? So I'm validly curious what your solution is.
Option 1: Have a spacecraft that can avoid boil off. If you can do it for a propellant depot, then I guess you can do it for this as well. But how much would that weigh? Would it very negatively affect other aspects of the design?
Option 2: Use some other propellant. Like what? Would that be some methane / LOX engine? How much worse would the specific impulse be? Would water even be a step in the processes to make these?
If you are using LH2, having it in LEO isn't a priority since it is only a very small part of the reaction mass and complicates the LEO depot design. For LEO depots a "bring your own hydrogen" approach should work best. So the only hydrogen fuel really needed would be the fuel needed for the tanker to perform the burns at LEO, which means you only need to store it for the first half of the trip which only lasts a week, which furthermore is mostly spent far away from the Earth's surface. With a sunshield the boiloff will be fairly small.
As far as methane vs Hydrogen goes, a good methalox engine should be able to give ~350-380 seconds of Isp depending on Fuel/oxygen ratio, while a similarily good hydrogen engine will give you ~430-470 seconds with a similar O/F variation. However, asteroids will tend to have more Carbon than Hydrogen.
One ton of Hydrogen will give you seven tonnes of Methane. If you burn it at a stoichiometric ratio, that one ton of hydrogen is nine tonnes of hydrolox propellant, while these seven tonnes of methane at stoichometric will give you ~20 tonnes of propellant, more than twice what you'd get from the same quantity of hydrogen with hydrolox. Furthermore, that larger mass of propellant will still easily fit inside a smaller volume than the nine tonnes of Hydrolox, which gives a smaller dry mass. In practice, you won't be burning at stoichometric, and hydrogen has a balance that is generally more shifted towards fuel-rich combustion than Methane, which further benefits methane.
At 360 seconds of Isp vs 440 seconds of Isp, the breakeven point where Hydrolox is better than twice it's mass in Methalox occurs at about ~15 km/s of delta-v. That is such a high delta-v scenario that it'd imply multiple stages, in which case you could have a Methalox first stage and a Hydrolox second stage which makes the comparison moot. For any scenario below that the Methane option will require less asteroid hydrogen for the same number of flights and will have more capable stages. -
#294 by AlanSE on 12 Feb, 2015 14:16
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This is a very fascinating discussion.
As a small complication, it seems to me that a "bring your own Hydrogen" approach would require some redesign of launch vehicles, because it would decrease the average density of the payloads to LEO. It's something like 7% the density of water. In order to systematically lift extra hydrogen with most missions, you'd be looking at a more egg-like upper stage to accommodate more Hydrogen tank volume. -
#295 by Hop_David on 12 Feb, 2015 14:40
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by refueling at a LEO propellant depot (fueled from Earth surface)
Assuming lots of propellent at EML2 why would LEO depots be supplied by earth surface?
EML2 is 3.5 km/s from LEO. And 3.1 of that 3.5 is circularizing at LEO -- much or all of that 3.1 could be done by aerobraking. So delta V from EML2 to LEO can be as little as .4 km/s.
When the trip from earth's surface to LEO is cut out of the equation, trips between orbits in the earth-moon neighborhood would be around 3 to 4 km/s. This makes for much less difficult mass fractions and sturdier ships. Most inter-orbital vehicles could remain in space and thus never have to suffer the extreme conditions of an 8 km/s re-entry into earth's atmosphere. Re-use becomes far less difficult.
Routine access to our orbital assets in cislunar space is the goal. It's no coincidence that sounds like Spudis, Spudis has been talking about the same thing: water. Water which can be made into rocket fuel. Only an asteroidal propellent source in high lunar orbit doesn't have the moon's 2.4 km/s gravity well.
I'm not talking just reusable EDS but reusables vecicles for travel between LEO, GEO, EML1&2 and beyond.
Serious question: You've constructed a round trip from EML2 to LEO that requires at least 6 distinct burns over several days. Because of that, you would probably need to avoid cryo LH2. Or would you not? So I'm validly curious what your solution is.
Option 1: Have a spacecraft that can avoid boil off. If you can do it for a propellant depot, then I guess you can do it for this as well. But how much would that weigh? Would it very negatively affect other aspects of the design?
The Farquhar route from LEO to EML2 is about 9 days. So a tanker's round trip from EML2 to LEO and back would take about 20 days.
I was going to reply with links to the ULA pdfs on cryogenic depots. I believe tolerable boiloff over 20 days is doable.
But reviewing the ULA pdfs I'm looking at conical umbrellas of Multi-Layer Insulation (MLI).
3.1 km/s aerobraking to circularize at LEO would inflict some punishment on the tanker, probably enough to destroy these delicate sun shields.
However a conical umbrella might be helpful for aerobraking -- it would increase the cross sectional area and thus make for a better ballistic coefficient. Perhaps the outer layer of the cone could be reinforced with some structure and TPS.
It would not have to be TPS on the same order as a spaceship enduring an 8 km/s re-entry. (3.1/8)^2 is about 1/6. It would be shedding about 1/6 of the energy of a full blown re-entry.Option 2: Use some other propellant. Like what? Would that be some methane / LOX engine? How much worse would the specific impulse be? Would water even be a step in the processes to make these?
This is also an option. As Nilof mentioned, carbonaceous asteroids have carbon (as do the lunar cold traps). So it's possible to use methane or other hydrocarbons as the fuel portion of the bipropellent.
If memory serves, exhaust velocity of LOX/methane is 3.5 km/s vs the 4.4 km/s of LOX/LH2. Still a pretty respectable ISP!
I believe water would be the best source of hydrogen, whether the fuel is hydrogen or a hydrocarbon. -
#296 by arachnitect on 16 Feb, 2015 18:45
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http://spacenews.com/op-ed-10-reasons-why-an-asteroid-redirect-mission-is-worth-doing/
By Jonathan Goff of Altius Space Machines -
#297 by jongoff on 16 Feb, 2015 19:49
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http://spacenews.com/op-ed-10-reasons-why-an-asteroid-redirect-mission-is-worth-doing/
By Jonathan Goff of Altius Space Machines
It's mostly just a toned down and professionalized version of the blog post I did a few weeks back on Selenian Boondocks. The SpaceNews guys liked it and thought it would make a good op-ed, so we decided to give it a bigger audience.
It'll be fun to see the comments and/or hatemail I get from this one. :-)
~Jon -
#298 by Robotbeat on 16 Feb, 2015 23:56
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Good article! I think you've won me over to Option B, by the way (although Option A is still neat).
Also, I think Lunar DRO is essentially interchangeable with EML1/2, since getting between them takes delta-vs measured in tens or hundreds of m/s, not multiple km/s. At least in my mind. If you put a habitat in EML2, you can move it to EML1 or lunar DRO or even Earth-Sun Lagrange point just with on-board station-keeping propellant (if you're very careful with it). -
#299 by Hop_David on 17 Feb, 2015 01:39
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Good article! I think you've won me over to Option B, by the way (although Option A is still neat).
Also, I think Lunar DRO is essentially interchangeable with EML1/2, since getting between them takes delta-vs measured in tens or hundreds of m/s, not multiple km/s. At least in my mind. If you put a habitat in EML2, you can move it to EML1 or lunar DRO or even Earth-Sun Lagrange point just with on-board station-keeping propellant (if you're very careful with it).
It's my understanding a DRO can endure for quite a few decades without the need for station keeping.
EML1 and EML2 needs minor station keeping but when the object you're keeping in place is hundreds of tonnes, even a small delta V station keeping budget can be hard.