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Post by Apotheon on Aug 18, 2018 16:00:08 GMT
Hey! I'm designing interplanetary infrastructure in between Earth (assumed still a superpower), the Moon, and Mars.
My first question: what is the ideal altitude for a space station orbiting around the Earth? - First, I assume the limits are dV to maintain altitude as the lower limit and the Van Allen belts radiation as the upper limit.
- The dV to maintain altitude increases by 1.75x-2x/50 km from 700 to 500 km altitude, by 2x-3x/50 km from 500 to 250 km altitude, and by 4x from 250 to 200 km.
- The Van Allen belts begin at 640 km (+/-20%) over the equator according to a 2003 NASA model, but varies with time and inclination. According to a South Atlantic Anomaly image, radiation starts at 1200 km 45 degrees above the equator, at 500 km by the equator, and at 150 km 45 degrees below the equator. However, I don’t know the variation with time.
- As such, 250 km appears to be a reasonable lower limit to avoid dV waste and 500 km appears to be a reasonable upper limit to avoid radiation, which requires hardening against.
- However, it's not readily apparent how to choose an altitude within the lower and upper limits. I assume this is a balance between ease of launching payloads up to the station and away from the station. Any other relevant factors?
My second question: what is the ideal way to move between Earth, the Moon, and Mars? - Earth-Moon costs a minimum of 4.14 km/s dV (5% more than the 3.94 km/s in dV maps) at 0.35 G of acceleration (more acceleration doesn't save dV, but less acceleration wastes it).
- Moon-Mars costs a minimum of 3.41 km/s (3.58 km/s with 5% extra) at 0.125 G acceleration.
- Earth-Mars costs a minimum of 5.71 km/s (6.00 km/s with 5% extra) at 0.35 G acceleration.
- I'm no super trajectorizer, so maybe someone else can improve on this with advanced special trajectories.
- Earth-Moon-Mars trajectories may be relevant, because it only costs you three days and you can fly a smaller ship between the Moon and Mars.
I've also spent about a hundred hours designing ships, including ships without payloads and ships with cargo, passengers, propellant, and a few stations also. I'll probably edit this post for inclusion later.
Edit 8-19: I've uploaded a diagram of the relationship between altitude and dV to maintain altitude HERE. The absolute numbers depend on daily, three-daily, monthly, annual, and decennial variation, but the relative relationship is always the same and apparently is mass-invariant.
Also, the dV and acceleration calculations in this post are based on a 250 km orbit around Earth, a 100 km orbit around the Moon, and a 200 km orbit around Mars.
Edit 08-20: I'm gonna put keep my current ships here.
The Apolunar as a chemical rocket and nuclear thermal rocket, 100% stock parts and can go Earth (250 km) to the Moon (100 km).
My current Apomartian nuclear thermal rocket. I rejected the chemical rocket, since going Earth-Martian-Lunar in the Apolunar costs 30% less propellant! Also, my Moon-Mars Chess-class cargo ship... scroll to understand the name.
My Chess-class cargo ships for Earth-Lunar and Earth-Martian transport of 1,000 t cargo. Stock power and propulsion, just like the two ships above them.
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Post by bigbombr on Aug 18, 2018 16:36:39 GMT
Hey! I'm designing interplanetary infrastructure in between Earth (assumed still a superpower), the Moon, and Mars.
My first question: what is the ideal altitude for a space station orbiting around the Earth? - First, I assume the limits are dV to maintain altitude as the lower limit and the Van Allen belts radiation as the upper limit.
- The dV to maintain altitude increases by 1.75x-2x/50 km from 700 to 500 km altitude, by 2x-3x/50 km from 500 to 250 km altitude, and by 4x from 250 to 200 km.
- The Van Allen belts begin at 640 km (+/-20%) over the equator according to a 2003 NASA model, but varies with time and inclination. According to a South Atlantic Anomaly image, radiation starts at 1200 km 45 degrees above the equator, at 500 km by the equator, and at 150 km 45 degrees below the equator. However, I don’t know the variation with time.
- As such, 250 km appears to be a reasonable lower limit to avoid dV waste and 500 km appears to be a reasonable upper limit to avoid radiation, which requires hardening against.
- However, it's not readily apparent how to choose an altitude within the lower and upper limits. I assume this is a balance between ease of launching payloads up to the station and away from the station. Any other relevant factors?
My second question: what is the ideal way to move between Earth, the Moon, and Mars? - Earth-Moon costs a minimum of 4.14 km/s dV (5% more than the 3.94 km/s in dV maps) at 0.35 G of acceleration (more acceleration doesn't save dV, but less acceleration wastes it).
- Moon-Mars costs a minimum of 3.41 km/s (3.58 km/s with 5% extra) at 0.125 G acceleration.
- Earth-Mars costs a minimum of 5.71 km/s (6.00 km/s with 5% extra) at 0.35 G acceleration.
- I'm no super trajectorizer, so maybe someone else can improve on this with advanced special trajectories.
- Earth-Moon-Mars trajectories may be relevant, because it only costs you three days and you can fly a smaller ship between the Moon and Mars.
I've also spent about a hundred hours designing ships, including ships without payloads and ships with cargo, passengers, propellant, and a few stations also. I'll probably edit this post for inclusion later.
NTR's, MPDT's, VASMR or laserthermal drives are probably best. NTR's have good thrust, but lowish exhaust velocity. MPDT's suffer from their low thrust but have excellent exhaust velocity. VASMR is inbetween. Laserthermal has equal or better exhaust velocity than NTR's and a similar or higher TWR. But you need a powerful laser nearby/pointed at them when they perform burns. The Earth-Moon Lagrange points might also be an interesting place for your station, though you'll have to bring radiation shielding as they're outside of the protection of the Earth's magnetic field.
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Post by apophys on Aug 18, 2018 20:52:51 GMT
My first question: what is the ideal altitude for a space station orbiting around the Earth? IMO, if you want to be below the rad belts, you want to be as high as possible without risking hitting them. This is because it's easier to move around farther away from a gravity well using ion drives. Rad shielding will be possible to do using magnetic fields at some point soon-ish, so location may not remain an important concern for too long. If you don't want to be limited by transfer windows to/from Mars, then you need ion drives. If you have plenty of dV available, like ion drives provide, the ideal is to use pseudo-brachistochrone transfers, because they take the least time. Acceleration is very low (milligee), but that's fine; it's still much faster than NTRs because it accelerates/decelerates continuously for the majority of the trip instead of just at transfer points. See the Homecoming speedrun thread. For Earth-Moon transfers, anything works (even chemical); they're close enough that ion drives aren't quite as dominating, though still good. In particular, if you have laserlaunch from Earth's surface (the best launch method, imo), you may as well use laser thermal for the transfer, using the same laser infrastructure.
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Post by Apotheon on Aug 19, 2018 0:32:10 GMT
Alright, regarding ship design, I've found the propellants worth considering are: (best-worst) fluorine-hydrogen, fluorine-methane, hydrogen-oxygen, and methane-oxygen for chemical rockets and (best-worst) methane, hydrogen deuteride, hydrogen, and deuteride for nuclear thermal rockets. Actually, fluorine-hydrogen appears to have a smaller dry and wet mass and dry and wet cost than nuclear thermal rockets based on my experience. However, it's apparently also quite an extreme hazard?
I use methane-oxygen and methane as my core propellants anyway. Creates a little cross-compatibility I guess.
My first designs are the Apotheon Apolunar and Apolunar X! 100%(?) optimization of 100% stock parts. Edit 8-20: images in original post. I also have a Apolunar Cynthian alternative, which uses fluorine-hydrogen. It's got user propellant tanks, but I only changed the amount of propellant and the aspect ratio and besides that it's stock. Edit 8-20: images in original post. I'm also happy with my Chess-class 1,000 t cargo ships. Again, stock except the propellant tanks and decorative radiation shields. Spacers are pretty fun! Edit 8-20: images in original post.
I've also worked on passenger ships, but I don't have anything I'm happy with yet, and I've worked a little on propellant ships, but they're annoying to design, in my own opinion. As you can see, I'm currently working mostly only with stock parts (which I hope are reasonably optimised, at least relative to one another) in order to not get too many variables.
I'm probably going to create a few more designs and start playing with weapons for a while before I invest heavily in user modules.
Edit 8-19: I uploaded a diagram of altitude vs dV to maintain altitude in the original post.
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Post by The Astronomer on Aug 19, 2018 3:10:37 GMT
God damn those spaceship nuclear lamps. I want to see some real stars. Why do you have to use high power ion drives on a ship that's just going to the Moon?
For ion ships to the Moon, you need to choose between giant ion drives with appropriately big, bright radiators/solar panels, and a few months of travel time.
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Post by apophys on Aug 19, 2018 4:01:48 GMT
God damn those spaceship nuclear lamps. I want to see some real stars. Why do you have to use high power ion drives on a ship that's just going to the Moon? Well, if you've already got it for transfers to/from Mars, you might as well use it for short distances also; simplifies construction and resupply. Nothing technically wrong with having a few thousand extra lights in the sky.
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Post by The Astronomer on Aug 19, 2018 4:04:57 GMT
God damn those spaceship nuclear lamps. I want to see some real stars. Why do you have to use high power ion drives on a ship that's just going to the Moon? Well, if you've already got it for transfers to/from Mars, you might as well use it for short distances also; simplifies construction and resupply. Nothing technically wrong with having a few thousand extra lights in the sky. How bright would those ships be, I wonder... ...nuclear fireflies, floating in the nightskies...
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Post by bigbombr on Aug 19, 2018 8:10:55 GMT
Alright, regarding ship design, I've found the propellants worth considering are: (best-worst) fluorine-hydrogen, fluorine-methane, hydrogen-oxygen, and methane-oxygen for chemical rockets and (best-worst) methane, hydrogen deuteride, hydrogen, and deuteride for nuclear thermal rockets. Actually, fluorine-hydrogen appears to have a smaller dry and wet mass and dry and wet cost than nuclear thermal rockets based on my experience. However, it's apparently also quite an extreme hazard? Fluorine will burn almost anything, and if you use fluorine to burn a molecule containing hydrogen (H 2, methane, ...) you'll get hydrofluoric acid, which can dissolve pretty much anything. This is why none uses fluorine IRL, despite it having a performance superior to oxygen as oxidiser. I personally also don't use hydrogen as it tends to leak through the walls of your propellant tanks (which is why hydrogen isn't the best choice for any spacecraft that will float for extended periods of time before making their final burn, IRL RP-1 was preferred). Dinitrogen tetroxide (DNTO) with unsymmetrical dimethyl hydrazine (UDMH) or monomethyl hydrazine (MMH) are viable choices for missiles that don't need too much delta-v. IRL they're hypergolic, which has it's advantages. I personally use water for MPDT's, pentane for NTR's and either methane/oxygen or MMH/DNTO for combustion rockets.
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Post by anotherfirefox on Aug 20, 2018 4:03:19 GMT
What does your infrastructure do? That's something we must agree before the discussion. The easiest thing to think of is orbital cryogenic fuel depot: Never put it in low orbit, no needs and they'll boil off much faster.
In any case, LEO is not a good idea. Note that US is gonna abandon ISS at LEO and going to build one at even farther, Earth-Moon L 1 point. Getting into orbit around Earth is not such a problem: You can use vast infrastructure we already have on the ground. Getting to another body is the problem. Note that we can easily build orbital booster with 9km/s with even 60's tech, yet never was able to build a deep space stage with more than 1kn/s with chemical rockets.
Unless you're using super powered VASIMR, never use an ion propulsion around deep gravity well: SUPER longer travel time, which leads to WAY more life support. With current/near future tech, the thing you have to care about is nothing with dV or TWR sorta propulsion things. They can be dealt with more funds. Life support is the crucial thing you have to deal with, which is not simulated by this game.
Above is a ship for just 6 crews for Earth-Mars transport, supposed by NASA. It's bigger than ISS, and more than two third of its volume is life support.
Lastly, if you're interested in building more civilian, interplanetary things, go and grab KSP Realism Overhaul. When it comes to such a thing, KSP Realism Overhaul is almost as detail as CDE module design. (Triple more, in my personal opinion.)
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Post by anotherfirefox on Aug 20, 2018 4:27:51 GMT
I personally also don't use hydrogen as it tends to leak through the walls of your propellant tanks (which is why hydrogen isn't the best choice for any spacecraft that will float for extended periods of time before making their final burn, IRL RP-1 was preferred). NASA going to develop Zero-Boil Off method because they just love cryogenic hydrolox. Really, they trying to take DCSS or ACES all the way to the moon surface or even mars to use the hydrolox. Whaat As far as I know, RP-1 was never an option for deep space maneuvering: Too dense for low isp, and still need cryogenic cooling for oxygen. Most of cases (with early tech, until now with big thrust needed) it was hypergolic, and now it's going to transit to ion thruster. (still NEVER for human mission soon)
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Post by anotherfirefox on Aug 20, 2018 4:37:30 GMT
Rad shielding will be possible to do using magnetic fields at some point soon-ish, so location may not remain an important concern for too long. The most viable method which is under research IRL is just put meters thick water around your wall.
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Post by The Astronomer on Aug 20, 2018 4:43:33 GMT
Rad shielding will be possible to do using magnetic fields at some point soon-ish, so location may not remain an important concern for too long. The most viable method which is under research IRL is just put meters thick water around your wall. Sounds pretty bulky, doesn't it?
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Post by anotherfirefox on Aug 20, 2018 4:47:10 GMT
The most viable method which is under research IRL is just put meters thick water around your wall. Sounds pretty bulky, doesn't it? Bulky and massy, but no need of constant power and can be used with variety of use. If you can rely on nukethermal reactor, like bimodal NTR which can produce power when not producing thrust, this will be depreciated.
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Post by anotherfirefox on Aug 20, 2018 6:06:34 GMT
My second question: what is the ideal way to move between Earth, the Moon, and Mars? - Earth-Moon costs a minimum of 4.14 km/s dV (5% more than the 3.94 km/s in dV maps) at 0.35 G of acceleration (more acceleration doesn't save dV, but less acceleration wastes it).
- Moon-Mars costs a minimum of 3.41 km/s (3.58 km/s with 5% extra) at 0.125 G acceleration.
- Earth-Mars costs a minimum of 5.71 km/s (6.00 km/s with 5% extra) at 0.35 G acceleration.
- I'm no super trajectorizer, so maybe someone else can improve on this with advanced special trajectories.
- Earth-Moon-Mars trajectories may be relevant, because it only costs you three days and you can fly a smaller ship between the Moon and Mars.
You may want to see this also: en.wikipedia.org/wiki/Interplanetary_Transport_Network
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Post by bigbombr on Aug 20, 2018 7:13:44 GMT
I personally also don't use hydrogen as it tends to leak through the walls of your propellant tanks (which is why hydrogen isn't the best choice for any spacecraft that will float for extended periods of time before making their final burn, IRL RP-1 was preferred). NASA going to develop Zero-Boil Off method because they just love cryogenic hydrolox. Really, they trying to take DCSS or ACES all the way to the moon surface or even mars to use the hydrolox. Whaat As far as I know, RP-1 was never an option for deep space maneuvering: Too dense for low isp, and still need cryogenic cooling for oxygen. Most of cases (with early tech, until now with big thrust needed) it was hypergolic, and now it's going to transit to ion thruster. (still NEVER for human mission soon) Zero-Boil Off method sounds bulky and heavy. Oxygen needs to be kept considerably less cool than hydrogen, and if you only have to cool your oxidizer instead of your oxidizer and your fuel you still save a lot of mass. And RP-1 has a decent I sp, it just isn't on the level of hydrogen. For hypergolics, the best choices are either MMH or UDMH burnt with DNTO. If you want excellent I sp with an easily storable, non-cryogenic propellant that won't melt your face off, just use water as propellant in a laser thermal drive.
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