Hlynkacg
Aspiring rocket scientist
I guess the question now becomes, where do we put it? and how massive does it need to be?
Discussion Powell & Pelligrino's Skytrain (aka ISV Venture Star)
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I guess the question now becomes, where do we put it? and how massive does it need to be?
Slightly larger than the diameter of the centrifuge, or the size of your largest payload (whichever is larger).
The shield can be pretty thin- maybe even, 1-2 millimeters, though it will need a support structure to stop it from buckling... but this can be made pretty lightweight as well.
The shield can be pretty thin- maybe even, 1-2 millimeters, though it will need a support structure to stop it from buckling... but this can be made pretty lightweight as well.
Eli13
Fish Dreamer
Hlynkacg
Aspiring rocket scientist
Been messing around with math and proportions.
With the planned DV of 100 km/s our vessel wil be capable of transfer to Mars in 45 days or to Saturn in 2.5 years. Obviously a higher DV results in shorter transit times.
For the moment we will assume that our colonists will be awake and active for the trip. NASA reccomends a minimum 17 cubic meters per crew-person to avoid issues with over-crowding. This means that our 450 colonists will need a minimum of 7,650 cubic meters of living space. This fits comfortably in a 20m by 30m TransHab module but does not include storage mass and volume for consumables (Food, Water, and O2).
Atomic Rockets puts total consumables for 455 people over 45 days at 3970 tonnes of water, 52 tonnes of food, and 1.3 tonnes of O2. To determine internal storage requirements for a 45-day Mars transit let us assume that our packaged foods have a densisty of approximatly 0.5 tonnes/meter (i'm using canned veggies as my baseline) and that water and O2 will be stored externally. This leaves us with a requirement for 26 cubic meters of storage space. This means that 45 days is sufficiently short that food/water can be packaged and shipped along with the passengers.
2.5 years on the other hand will be problematic as the required mass of consumables will quickly out-pace the mass of whole ship. This raises the questions of whether to reduce the number of passengers on outer planet voyages? or to create a closed biological cycle (grow food on-route)? and how will this effect our mass/volume requirements?
Note: For a closed cycle system, I favor algea tanks for the volume savings and ability to cultivate fish and shrimp as a suplimental food source.
With the planned DV of 100 km/s our vessel wil be capable of transfer to Mars in 45 days or to Saturn in 2.5 years. Obviously a higher DV results in shorter transit times.
For the moment we will assume that our colonists will be awake and active for the trip. NASA reccomends a minimum 17 cubic meters per crew-person to avoid issues with over-crowding. This means that our 450 colonists will need a minimum of 7,650 cubic meters of living space. This fits comfortably in a 20m by 30m TransHab module but does not include storage mass and volume for consumables (Food, Water, and O2).
Atomic Rockets puts total consumables for 455 people over 45 days at 3970 tonnes of water, 52 tonnes of food, and 1.3 tonnes of O2. To determine internal storage requirements for a 45-day Mars transit let us assume that our packaged foods have a densisty of approximatly 0.5 tonnes/meter (i'm using canned veggies as my baseline) and that water and O2 will be stored externally. This leaves us with a requirement for 26 cubic meters of storage space. This means that 45 days is sufficiently short that food/water can be packaged and shipped along with the passengers.
2.5 years on the other hand will be problematic as the required mass of consumables will quickly out-pace the mass of whole ship. This raises the questions of whether to reduce the number of passengers on outer planet voyages? or to create a closed biological cycle (grow food on-route)? and how will this effect our mass/volume requirements?
Note: For a closed cycle system, I favor algea tanks for the volume savings and ability to cultivate fish and shrimp as a suplimental food source.
Eli13
Fish Dreamer
Depends on what you are growing. Are you talking about your basic fruits and veggies or are livestock included with that as well? But i see how your algae tanks would work well. But I'm pretty sure that protein would be fairly important on a nice little trip like like that. Then again, there are supplements for that :hmm: it could go in a lot of different directions depending on what you grow.
100 km/s? Is that the dV? Or the transit velocity?
Assuming it is overall dV, we can set aside at least 10 km/s for auxilliary manuvering, you have 45 km/s transit velocity.
At the 2011 window for Mars (in December), you get 76 days (roughly 2.5 months) to Mars. At the 2011 window for Jupiter (in July) you get 293 days to Jupiter (9.6 months). I am not sure about Saturn, presumably a flight there would take even longer.
A closed gas-core NTR will have a dV of 100 km/s if it had a mass ratio of over 134! Atomic Rockets gives an all-out maximum exhaust velocity value of 98 000 m/s for an open gas-core NTR which can achieve such a dV with a mass ratio of under 3, but this is implied to come with bad thermal issues as well.
Moreover, an increase in exhaust velocity will increase both the exhaust temperature and thrust power, and a physical exhaust shield might be called into question at those levels.
I would suggest growing some food onboard, and supplementing it with stored food from back home. Eating only green slime for years can't be good for you. I have a feeling stored food will not only be an easier option, but it would be more palatable as well (though fresh fish would be pretty good, plain algae or yeast-based food would be pretty boring, maybe even disgusting... you can spice it up a bit though, it's all in the preparation).
Another problem is space. There's a certain amount of time after which a bunch of people just can't be in that confined space anymore. The space that is acceptable for a certain number of people for a 3 month mission, would be too cramped for a 2 year mission, and so on. I would suggest, instead of grossly overdesigning the crew modules for a Mars mission, decreasing the number of people for an outer planets mission. 110 people will already consume
Assuming it is overall dV, we can set aside at least 10 km/s for auxilliary manuvering, you have 45 km/s transit velocity.
At the 2011 window for Mars (in December), you get 76 days (roughly 2.5 months) to Mars. At the 2011 window for Jupiter (in July) you get 293 days to Jupiter (9.6 months). I am not sure about Saturn, presumably a flight there would take even longer.
A closed gas-core NTR will have a dV of 100 km/s if it had a mass ratio of over 134! Atomic Rockets gives an all-out maximum exhaust velocity value of 98 000 m/s for an open gas-core NTR which can achieve such a dV with a mass ratio of under 3, but this is implied to come with bad thermal issues as well.
Moreover, an increase in exhaust velocity will increase both the exhaust temperature and thrust power, and a physical exhaust shield might be called into question at those levels.
I would suggest growing some food onboard, and supplementing it with stored food from back home. Eating only green slime for years can't be good for you. I have a feeling stored food will not only be an easier option, but it would be more palatable as well (though fresh fish would be pretty good, plain algae or yeast-based food would be pretty boring, maybe even disgusting... you can spice it up a bit though, it's all in the preparation).
Another problem is space. There's a certain amount of time after which a bunch of people just can't be in that confined space anymore. The space that is acceptable for a certain number of people for a 3 month mission, would be too cramped for a 2 year mission, and so on. I would suggest, instead of grossly overdesigning the crew modules for a Mars mission, decreasing the number of people for an outer planets mission. 110 people will already consume
Hlynkacg
Aspiring rocket scientist
Yes that is 100 km/s Dv overall.
I got the cited numbers by printing out Atomic Rocket's mission-planner Nomograph and assuming a transit speed of approx 46-47 km/s. Unfortunatly it seems I forgot to consult the performance tables for a NH3 based NTR before doing so. :facepalm:
Did you see the size of the radiators on the orgiginal ISV? :lol:
That said, I wanted to figure out the total wheight of the structure / Payload before I started worrying too much about the engines. Afterall, the propulsion section's specs will be largely dependant on what it has to tow. Using blender's volume calculator and some back of the envelope comparisons we're looking at 440 tonnes for the Hab Section and 38 tonnes for the (2000m carbon fiber) tether. Its seems your initial guess of a 500 tonne empty wheight was pretty close.
So given an 1,800 tonne Payload and 478 tonne structure how ludicrous of a Propulsion Section are we talking about? From there we can also extrapolate how much shielding is required, how much actual Handwavium we're going to need, and whether or not this whole project is a pipe dream.
I completely agree. For the moment we will treat a 450 person "Mayflower Run" to Mars as the baseline and then look at how the numbers scale for longer voyages. I.E Half as many people getting you twice as far.
Based on NASA's studies the break even point for shipping food vs. growing it en route it is around 6 months for mass and 9 months for volume. The break even point for using Chlorella Algae to Process CO2 and Biological waste is about 5 months. So let's just say that the Algae tanks are primarily for atmosphere/waste processing and that the occasional fresh seafood dinner is a bonus.
For a mission to the outer planets the bays no longer occupied by colonists can be devoted to a hydroponics garden.Those radiators? Those are small. Trust me. I came up with some outcomes, that were plain painful to look at. Anyway, those thermal issues deal primarily with worries of melting the engine, etc.
The thruster calculator can do the work for you, if you are too lazy with math, just like I am.
We have an exhaust velocity of... let's say, 18500 m/s, to be on the low side. A dV of 3.86 km/s requires a mass ratio of around 3.862. Now the key is thrust. Your required acceleration determines your thrust. I would say, go low. 3 MN can push over 600 tons of ship at 0.1G, so it should be more than enough. That is roughly 7 hours of continuous burning, if you were to finish off your entire dV capability. But your transit and deceleration burns will take less time than that. While a three hour burn for example, might sound like a lot, it is actually quite short and one must remember that the lower the thrust (and thus the lower the acceleration, and thus the longer time it takes to accelerate to a set velocity) the lower the thrust power is. The key here is energy over time.
3 meganewton should not be that much, especially if spread over several engines. I am not sure what the mass of these engines would be, although Atomic Rockets gives a mass of over 56 tons for a 445 kN engine, which doesn't sound too promising. But that might, for example, include shielding, and other things that can be ommitted, refined, etc. If you want, you can reduce acceleration even further. A 1 MN thruster can accelerate roughly 520 tons of spacecraft at 0.04G. A transit burn would then maybe be around 8, 9 hours. That is not that bad, in general. Maybe slightly higher thrust would be desirable. In any case, it is possible; it is just a matter of either slimming your engine mass (somehow) or dealing with it (somehow).
I am unsure of what the mass of the propellant tanks would be. The best would be to calculate their volume based on the mass and density of the propellant, and then compare with other rocket propellant tank technology. Not only are the densities of NH3, CH4 and LH2 different, but they also have different thermal requirements, for example. Liquid hydrogen has appallingly low density, CH4 is much better, and NH3 is even better still. I think, but I'm not sure, that NH3 has the highest boiling point as well. Either way, CH4 might be the ideal propellant considering that Mars is the primary destination. There, nitrogen is not very common. If you are planning on mining ice from the Martian moons, than liquid hydrogen would probably be the best propellant. I don't know if the extra mass of using water would warrant the decrease in exhaust velocity.
And always... always add space to grow. Your engines and propellant tanks are going to add quite a bit of mass to the vehicle, so be prepared.
Well, it isn't as much distance, as it is time. For example if you are carrying an exceptionally heavy payload, you might also decrease your travel time as well. But generally the faster passenger trips are, the better.
That sounds like a good idea. Just because you're switching to a closed system doesn't mean you need to ditch stored food, and just because you have a primarily expendable system doesn't mean you should go without fresh fruit or fish, or not have a regenerable oxygen supply.
Hlynkacg
Aspiring rocket scientist
I initially chose NH3 for it's density, ease of storage, and the fact that it burns cleanly.
I'm currently looking at other options for the engine as like you I find Atomic Rocket's numbers for the NTR Gas/Closed system to be less than promising. Looking at the Pratt & Whitney Triton Proposal and scaling up, a 5 MN thruster would mass 330 tonnes including radiator. The cited thrust/ISP is based on LH2 not NH3 so I'll need to crunch some numbers but this sounds promising.
The problem is that the Triton is a totally and utterly differing design from the NTR and the exhaust velocity it offers is far lower. It doesn't make much sense to compare the two.
Even with the very low values seen in the Atomic Rockets drive table, I have seen numerous references to gas core rockets having a thrust/weight over 1, which would mean a far more promising mass.
I'm not sure if there are any figures for a more... capable closed-core NTR out there, though.
If you want a bimodal reactor, I'm also not sure a lightbulb engine can do that... due to certain facets of the engine design.
Carbon deposits from CH4 could become a problem in a NERVA engine, but it should be noted that the openings in a gas core rocket would be larger (and not the finely packed core of a NERVA-like design), which might mitigate the problem of soot deposits.
Nitrogen is pretty rare in the inner system; I would not regard it as a worthwhile option, even when considering soot deposits. If soot deposits are that problematic, then I would suggest hydrogen, or even water if things were bad enough. NH3 just isn't very abundant. And it probably has a poorer exhaust velocity when compared to CH4, too.
At least you can make methane out of CO2 and hydrogen. But launching stuff from the surface of Mars might outweigh the advantage. Ice from one of the martian moons, if available and easy enough to mine and process, electrolysed for the hydrogen, would probably be the best propellant option logistics wise.
But the density of LH2 really is annoying... :dry:
Even with the very low values seen in the Atomic Rockets drive table, I have seen numerous references to gas core rockets having a thrust/weight over 1, which would mean a far more promising mass.
I'm not sure if there are any figures for a more... capable closed-core NTR out there, though.
If you want a bimodal reactor, I'm also not sure a lightbulb engine can do that... due to certain facets of the engine design.
Carbon deposits from CH4 could become a problem in a NERVA engine, but it should be noted that the openings in a gas core rocket would be larger (and not the finely packed core of a NERVA-like design), which might mitigate the problem of soot deposits.
Nitrogen is pretty rare in the inner system; I would not regard it as a worthwhile option, even when considering soot deposits. If soot deposits are that problematic, then I would suggest hydrogen, or even water if things were bad enough. NH3 just isn't very abundant. And it probably has a poorer exhaust velocity when compared to CH4, too.
At least you can make methane out of CO2 and hydrogen. But launching stuff from the surface of Mars might outweigh the advantage. Ice from one of the martian moons, if available and easy enough to mine and process, electrolysed for the hydrogen, would probably be the best propellant option logistics wise.
But the density of LH2 really is annoying... :dry:
Hlynkacg
Aspiring rocket scientist
While hunting around for some more optimistic numbers I found this.
Air Force Advanced Propulsion Survey
It gives a T/W ratio of 12, but with a lousy exhaust velocity of only 9km/s.
So let's consider some alternatives, since we do not intend to land (or even enter the atmosphere) perhapse we should be looking at an open or liquid core design.
Even a 1st Gen open core off Atomic Rocket's Drive Table would make this a lot easier.
It is also a pain-in-the-butt to store safely which is why I keep hunting around for alternatives, CH4 would work for mars missions, and I seem to remember reading about a Russian NTR proposal that used alcohol as reaction mass. As I recall the T/W and ISP were quite promising.
Air Force Advanced Propulsion Survey
It gives a T/W ratio of 12, but with a lousy exhaust velocity of only 9km/s.
So let's consider some alternatives, since we do not intend to land (or even enter the atmosphere) perhapse we should be looking at an open or liquid core design.
Even a 1st Gen open core off Atomic Rocket's Drive Table would make this a lot easier.
It is also a pain-in-the-butt to store safely which is why I keep hunting around for alternatives, CH4 would work for mars missions, and I seem to remember reading about a Russian NTR proposal that used alcohol as reaction mass. As I recall the T/W and ISP were quite promising.
It shouldn't be a problem to store safely in space; there is no abundant oxygen for combustion in the event of a leak. The problem is boiloff, LH2 has to be kept very cold and that can be a problem in space. You can find some data on LH2 boiloff here.
I don't know about alchohol. It would probably be pretty expensive to get out at Mars...
I don't think it is impossible to build a enclosed core with high thrust/weight, likely the study cited there was a conservative design. Since you are in space, an open-core design might be a better option (and it provides an ability to have higher dV!), because you do not have to worry as much about radionuclides escaping in the exhaust. Still, you will lose nuclear fuel in the exhaust, so this might affect engine effectivity. An open-core might also be easier to build than a closed core, Atomic Rockets explains some of the finities of enclosed gas core NTRs here, some of the details will make you cringe (for example, the lightbulbs have to be filled with coolant tubes, the uranium somehow has to be seperated from the transparent walls so that it doesn't vaporise them, and injecting nuclear fuel into the engine would be problematic as well).
But I don't imagine that would make an open-core engine super-easy to construct, either.
I don't know about alchohol. It would probably be pretty expensive to get out at Mars...
I don't think it is impossible to build a enclosed core with high thrust/weight, likely the study cited there was a conservative design. Since you are in space, an open-core design might be a better option (and it provides an ability to have higher dV!), because you do not have to worry as much about radionuclides escaping in the exhaust. Still, you will lose nuclear fuel in the exhaust, so this might affect engine effectivity. An open-core might also be easier to build than a closed core, Atomic Rockets explains some of the finities of enclosed gas core NTRs here, some of the details will make you cringe (for example, the lightbulbs have to be filled with coolant tubes, the uranium somehow has to be seperated from the transparent walls so that it doesn't vaporise them, and injecting nuclear fuel into the engine would be problematic as well).
But I don't imagine that would make an open-core engine super-easy to construct, either.
Hlynkacg
Aspiring rocket scientist
Been kicking some numbers around, and it seems that a 1st Gen Open/Gas NTR using CH4 as Rection mass will have an unimpressive exhaust velocity of 12,420 m/s.
ISP and T/W ratio on the other hand are both very promising.
With a mass ratio of 7 we are looking at an approximate Delta-V of 24 Km/s. While this easily gets us to Mars and back it limits our voyages to the outer planets. I am prepared to accept this though as the Martian Mayflower Run is supposed to be our baseline mission.
Edit:
The more I read the more I'm liking Methane as a reactant. It's a lot easier to store than hydrogen, and Methane/Oxygen chemical rockets could be used for RCS thrusters or landing craft. Most importantly it is readily available throughout the solar system.
ISP and T/W ratio on the other hand are both very promising.
With a mass ratio of 7 we are looking at an approximate Delta-V of 24 Km/s. While this easily gets us to Mars and back it limits our voyages to the outer planets. I am prepared to accept this though as the Martian Mayflower Run is supposed to be our baseline mission.
Edit:
The more I read the more I'm liking Methane as a reactant. It's a lot easier to store than hydrogen, and Methane/Oxygen chemical rockets could be used for RCS thrusters or landing craft. Most importantly it is readily available throughout the solar system.
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How did you arrive at the exhaust velocity figure of ~12km/s?
According to my very crude calculations, and by messing around with IMFD, I think I came up with a figure that indicated a transit time reduction from the parameters the MFD automatically gave me of 3 days with an 11km/s burn (propellant for one-way, 2km/s reserve). This is with a travel time of around 9 months... :dry:
But my dates could have been a bit off, you might be able to cut it down further, with better timing.
According to my very crude calculations, and by messing around with IMFD, I think I came up with a figure that indicated a transit time reduction from the parameters the MFD automatically gave me of 3 days with an 11km/s burn (propellant for one-way, 2km/s reserve). This is with a travel time of around 9 months... :dry:
But my dates could have been a bit off, you might be able to cut it down further, with better timing.
Hlynkacg
Aspiring rocket scientist
Assuming equal thrust (35,000 Kn) and Reactor Temp (26,000 kelvin), I took the molar mass of liquid methane and plugged it into the equation given my Atomic Rockets for calculating NTRs and solved for "Ve".
Qe = (Ve / (Z * 129))2 * Pw
I then fed that value into Tsiolkovsky's rocket equation.
Δv = Ve * ln[R]
Now this value is intentionally pessimemistic as I assume a low operating pressure and did not factor in the possibility of the liquid methane breaking into it's component atoms.
Both would serve to increase "Ve" by at least 50%.
