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Posts posted by wafflemoder

  1. 2 hours ago, vv3k70r said:

    Pod do no job, parachute does. You can issue a claim if parachute fail. Pod have no job in which it can fail and cause responsibiliti - it is chassi.

    If basically any part of the pod fails, it doesn't matter if the parachute works or not, you will die before you can come down, if not on the way up. And you can absolutely pin-point what part in the pod is responsible for the failure, that's part of what telemetry is for.

    Even for a dumb chasis (which a pod very much is not) it is still larger and hence requires more material and a larger workforce to construct, and so should cost more.

  2. 20 hours ago, vv3k70r said:

    Look at real dV - it is why launching from Kerbin looks cheap.

    Its not the actual cost of the vehicles, and this is arguably just to not hinder new career mode players. The prices of parts are weird in relation to each other. 

    21 hours ago, vv3k70r said:

    Only at the begining - later on when You "mass" manufacturing them (like soviets) they are dirt cheap.

    That is true, but that also applies to every other part in KSP, not just engines. Relative pricing should still hold if all parts are manufactured at equal rates. The engines in cars and aircraft are good examples of this in action, as both are very much mass produced and engines still make up a large fraction of the total cost.

    21 hours ago, vv3k70r said:

    This is reasonable. They are hard to manufacture in comparision with tanks. Try to roll a cone. Of course after You cut sheet in proper way that led You to trash lot of material. And in such aplication direction of rolling (on sheet) is not free, so You can cut only in certain area on the sheet and throw away the rest.

    The actual material costs should be minimal in comparison to the manufacturing process.  Rolling cone shapes also isn't that much harder than a cylinder. You'll need a new form, but that goes for tank diamters as well, so itsn't likely to increase the cost that much. 

    21 hours ago, vv3k70r said:

    Reasonable. Such parachutes are extremaly expensive due to certification.

    Yes, parachutes would be expensive to certify. But the same goes for a command pod, which is also larger and much more complicated.

    21 hours ago, vv3k70r said:

    Does pricing play any significant role in KSP?

    Not really, and that's kind of the point. You'd expect the price of your vehicle to be more than a passing thought in a gamemode about paying for your rockets, both for the players and the developers, but that isn't really the case right now.

  3. Part cost isn't something that is normally brought up with regards to balance or realism. Part cost is only important in Career, and even in career, it isn't really that big of a concern, so its understandable why it isn't brought up that much. Still, it is very much a part of the game that should be looked at, as there is a lot that can be improved upon.

    Before diving into KSP costs, it's good to look at the costs of components in real launch vehicles.  Now, pricing launch vehicles is surprisingly difficult, but fortunately just relative costs for parts is all we need for now. For the planned Vulcan launch  vehicle, it is estimated that the engines and related components are around 65% of the cost of the first stage. For the Falcon-9, propellants only make up around 0.3% of the costs of the launch vehicle. Now of course, we need to keep in mind that these values might not be accurate for all launch vehicles, especially things that are not conventional rockets, but hopefully its good enough to give a rough idea. From this, it seems engines  are the most expensive part of the launch vehicle, and particularly the first stage, with fuel being only a minor contribution.

    Looking at the first stages of various stock and dlc vehicles, engines only comprise around 15-55% of the first stage cost, lower than what one might expect. Fuel costs are much higher than expected, between 5-30% of the first stage. These two departures from realism could be resolved by simply reducing the cost of liquid fuel, oxidizer, and probably also monopropellant for good measure. In fact, liquid fuel is currently 4.4 times as expensive as oxidizer (0.8 funds/unit vs 0.182 funds/unit), so even just reducing liquid fuel and possibly also monopropellant would work (monopropellant has a cost of 1.2 funds/unit).

    Now you may have noticed that those price fractions varied a lot, and the reason for that is aerodynamic parts, with control surfaces in particular. As a point of comparison,  the smallest and cheapest control surface is the Elevon 4 (the small one), which has a steep cost of 400 funds. This is more than a LV-909 Terrier (390 funds), and two-thirds the price of a Mk1 Command Pod (600 funds). Now, Elevon 4 costs the exact same as an Elevon 1 (the normal one), so its really only tied for cheapest control surface, but that should show that something is wrong with these prices. The Elevon 5 (slanted one) is more expensive than the Elevon 3 (large triangle one), despite giving less lift and being unlocked in the same tech node. The AV-R8 winglet costs more than the Tail fin, despite the Tail fin giving more lift and deflection, and being unlocked in a tech node of the same price.

    Now, these are not the only oddly high prices with regards to aerodynamic parts. Wings are slightly less expensive than control surfaces, but not by much. The AV-T1 in particularly is especially crippling, costing a whopping 500 funds for a small wingless. This is 5 times the cost of similarly sized wings unlocked not soon afterwards, so they are clearly a rip off. Now, the probably-too-high costs of some of these wings can be excused by the fact that they are capable of withstanding reentry heat, not something most aviation wings need to handle, which can be seen with the cheaper but less capable FAT-455 series of parts. Nosecones, especially the smaller ones, are also probably a bit to costly, with costs approaching those of engines in the same size.

    Moving to structural parts, for the most part they are rather inexpensive, which would be expected, but there are some oddities. The Structural Fuselage actually costs more than the Mk1 Liquid Fuel Fuselage its based on (when empty), despite it presumably being far easier to manufacture without an integrated fuel tank. The size adapters are also pretty costly, being around as expensive as nosecones. The multicouplers have some odd prices too. The 1.25m tri and quad couplers are both more expensive than their 2.5m counterparts, with the TVR-2160C Quad-Coupler costing 2000 funds, compared to the TVR-400L Quad-Adapter costing just 800 funds.

    There are more examples of oddly high/low prices. Mk16 parachutes are nearly as expensive as the Mk1 command pod, radiators cost close to or more than the ISRU parts they are designed to cool, J-90 Goliaths are less than twice the price of the J-33 Wheesly while having a built in air intake and side mount, the Clamp-O-Tron Jr. being over twice as expensive as the regular Clamp-O-Tron, the list goes on.

    Hopefully this is enough to clearly show that prices need some revisions, if not a total overhaul. It would nice if this would be done in KSP, but knowing this community, I'm sure people would much rather the time be spent on something more exciting.

  4. Two things I noticed about the FX-2 and FX-3 fusion reactors that may not be the intended behavior:

    1) D-He3 mode consumes 10 times as much fuel as D-D mode, but only puts out twice as much power.

    2) Assuming I did the math correct, the specific energy of the D-D and D-He3 fuel cycles are less than that of enriched uranium in the NFE reactors. 

    For this, I used a two part formula. One for finding the energy per unit of fuel (generated power/ fuel use), and the other for converting in game units to kg (mass/units)

    - The NFE reactors give fuel specific energies between approximately 78 to 131 GJ/kg, varying between the reactors (which interestingly gives electrical efficiencies of around 3-5% for 3.5% enriched uranium)

    - In D-D fusion mode, the FX2 and FX3 have fuel specific energies of 23 and 28 GJ/kg respectively, between a third and fifth that of NFE reactors. Interestingly though there is still a use case for the D-D reactors despite their lower specific power (not counting radiators) and fuel specific energies than the NFE reactors, and that is that their fuel is comparatively inexpensive.

    - In D-He3 mode, the FX2 and FX3 have lower fuel specific energies of just 7.3 and 9.1 GJ/kg, although the high specific power makes them useful either way.

    Hope this helps

  5. 8 minutes ago, sevenperforce said:

    We already have a giant chunk of reaction mass orbiting once every month if we need to move the earth.

    We could use hundreds of mass drivers on small asteroids to gravitationally nudge a large asteroid into an earth-crossing orbit, timing it such that it would slowly nudge the moon into a higher orbit over time.  Repeating the process with additional larger asteroids would accelerate the process. Once the moon escaped, the network of Earth-crossing asteroids could be used to nudge it into a Venus-crossing orbit that would suck orbital velocity away from Venus on one pass and add it to Earth on the next. The moon would slowly transfer energy away from Venus and to the Earth, raising Earth's orbit.

    Of course you lose tides that way, but if the situation is so dire that you  need to fly the Earth out to Jupiter then tides are probably a minor problem.

    You can get the same effect by using the moon itself as a gravity tractor for earth. This simplifies the design, and saves both the tides and venus. A large solar laser could be used to push the moon, or even the earth itself, further simplifying things and allowing for much faster timescales.

  6. 1 hour ago, micha said:

    Am I missing something here?

    Well yes, but actually no. Like all things, it's complicated.

    So first off, my one and only defence what you were watching. We can do a pretty good job correcting for observation bias, as its something scientists know about going into things, and they have to work around it on a regular basis. When correcting for the planets we know we can't observe, and using the planets we can see to gauge their likelihood, we still find that most exoplanets are larger than Earth.

    That being said, there are still a lot of assumptions and misconceptions that can be unpacked here.

    Most of the exoplanets we've found were found by the kepler spacecraft, which was geared towards looking at K, G, and F type stars. These only make up 20-30% of all main sequence stars. The remainder is almost exclusively M type red dwarf stars. So we still lack a good census of what planets are like around these smaller, more abundant stars, which the TESS spacecraft and JWST will hopefully address as time goes on. From what we do know, it seems that planet mass may be loosely correlated with star mass. So worlds around these smaller stars will generally tend to smaller.

    In a further miscommunication, there is actually a frustrating amount of ambiguity in the compositions of exoplanets (even those with well defined densities). This is most apparent with mini-neptunes, which could either be rocky worlds with an extended hydrogen atmosphere, or water rich ocean worlds with little or no hydrogen. For smaller planets below about 1.5 Earth radii, where hydrogen is almost certainly not present, their compositions are slightly easier to pinpoint. As we don't know for certain how much iron an exoplanet has, or how much of that iron is oxidized, there is a lot of room for volatiles like water and carbon dioxide to sneak into a "rocky" planet's composition. In the small fraction of rocky planets with known and well constrained densities, most can allow for anywhere from 0 to 10% of their mass in water (for comparison, only 0.02% of Earth's mass is water). This doesn't seem bad, until you realize that an earth mass planet (or larger) only needs to be ~1% water for it to have a global ocean with a seafloor covered in high pressure ices, which would remove the rock-water interface which is thought to be a key requirement for abiogenesis. Also these ocean worlds have much smaller habitable zones, quickly going from frozen iceball to boiling steamhouse, much like the faucets in most buildings.

    What makes things worse, is that larger rocky worlds may also be unsuited for life. Planets larger than 2.5 earth masses will likely lack tectonic plates, and possibly magnetic fields as well. While neither are strictly required for life to develop (Earth's magnetic field and plate tectonics likely started after life developed), they are very very nice things to have for a developing biosphere. The thick atmospheres these worlds will have will likely make up for the lack of a magnetic field for surface life, and the larger masses of these planets will prevent atmospheric escape in the habitable zone.

    Hope this helps.

  7. On 4/18/2020 at 9:07 AM, Nuke said:

    i also have a hunch that fusion rocket engines would be significantly simpler than fusion power, so no point doing fusion-electric. something oddly ironic like having a fission power supply and a fusion engine might be a thing. 

    Fusion rockets are in fact easier to achieve than fusion power. This is because a fusion power plant needs to break even, making more energy than it uses. On the other hand, while a self sustaining fusion rocket would be very good to have, especially one that could be tapped for power, they aren't necessary. There are some designs for fusion rockets that use a fission reactor to provide power for their operation.

  8. 12 hours ago, Spacescifi said:

    So the question is: Can we ever increase the thrust of ion engines to 1g? Like if we put 50 kliograms of antimatter into it?

    Well yes, but actually no.

    You could, but it wouldn't be an ion thruster anymore. Ion thrusters specifically use electricity to energize their propellant, rather than through heating by use of fusion or antimatter.

    In principle, you could use an ion engine powered by a fusion reactor and antimatter fuel cell, but it would be better in almost every circumstance to just heat a propellant through fusion or antimatter instead. Thermal propulsion would be lighter, simpler, and would give higher performance. The only downside is that these thermal systems wouldn't be able to change throttle as quickly (needing to heat up an entire reactor versus using ultracapacitors to regulate power) This might make fusion/antimatter electric propulsion feasible for RCS, but not for a main drive.

  9. Realistically, any form of space elevator, space tower/space fountain, launch loop, or orbital ring will be more environmentally friendly in the long run as no propellants are needed, and all components are multi use. But, its nonsensical to say that any of these methods are "propellants". 

    The original question was about the most eco-friendly propellant, not the most eco-friendly surface to orbit transportation system, or even the most eco-friendly rocket design. These are all complex and intertwined questions for sure, and the question is framed in such a way that some pollution will be unavoidable from the production of the launch vehicle and the propellants. It is even fair to assume that a "nuclear space faring civilization" is one that uses nuclear propulsion (either in part or exclusively), which may make any chemical propellants considered irrelevant if that is the case.

    Based on all of this, and trying to pool everything together, here are 4 answers previously suggested that could be valid answers.

    • LH2 would be the cleanest nuclear propellant, if only the impact of the launch itself is considered.
    • H2O (water) would be the cleanest nuclear propellant if the impact of propellant production is considered, with minimal effort to produce. It remains a possibility for LH2 to be cleaner than water if there are significant advances in how its produced, but currently this is not the case.
    • Hydrolox would be the cleanest chemical propellant, ignoring the impact of propellant production.
    • Either methalox or hydrolox would be the cleanest chemical propellant if the impact of propellant production is included. As methane is more naturally abundant than hydrogen, and hydrogen typically being produced from methane or water, methalox may be cleaner overall. But this also depends on how these technologies would develop.
  10. 17 minutes ago, kerbiloid said:

    You anyway need to spend more energy by burning the biogas rather thatn you can store in the separated hydrogen.
    And the "biogas" is just methane and methanol, just biologically produced.
    It produces same amount of carbon dioxide, so it's just a way to save money on mining.

    Tthe Kværner process is just another way to apply external energy to the molecules of methane rather than electrolysis.

    First off, if a civilization's goal is to preserve the environment, monetary cost will be a non-issue. And biogas isn't even cheaper than minning, thats why we still mine.

    Secondly, Biogas is biologically produced methane and methanol, as are all fossil fuels. What makes biofuels "cleaner" than petroleum fuels isn't that they're a different chemical, its that biofuels are made from the carbon already in our atmosphere, rather than carbon which has been stored in the ground for hundreds of millions of years. Burning biofuels simply returns the CO2 that was used to make it back into the atmosphere, whereas burning natural gas adds more in.

    Third, the Kværner process is another way of turning methane into hydrogen. The difference is the waste product. Steam reforming creates ten times as much CO2 as it does hydrogen. The Kværner process produces no CO2 as a waste product, and so less greenhouse gasses. The kværner process instead creates carbon dust, which can be stored and removed from the carbon cycle.

    When taken as a whole: CO2 is taken from the air by plants to produce biomass. Some of this biomass is burnt, generating power and turning it back into CO2. The rest is put through the kværner process (using the power generated from before) and converted into hydrogen and carbon. The carbon is not returned to the air, so there is an overall net decrease in atmospheric CO2.

  11. 3 minutes ago, kerbiloid said:

    So, you need a powerplant for the plasma burner, and it requires more energy than CH4 bounds contain,

    So, this process just moves the pollution from the hydrogen plant to the power plant.

    Which is why I suggested running it off of a biogas powerplant rather than natural gas. Also its abilty to sequester carbon in elemental can reduce, or even give it a negative carbon footprint.

    Interestingly, the Kværner process is very similar (and possibly identical) to what happens in a nuclear rocket when you use methane as a propellant, only the carbon soot builds up in the engine, which can cause blockages or affect the neutron moderation in the reactor, and isn't desirable in that situation.

  12. 1 hour ago, kerbiloid said:

    Say, hydrogen is usually produced out of methane. This requires chemicals to purify it, and produces various by-products, especially carbon dioxide (because you have to remove carbon from the methane).
    Also it requires fuel to heat this kitchen, so usually the methane burning (and producing additional dioxide).
    So the chemically produced, such "environmentally friendly", hydrogen pollutes atmosphere much greater than if you just use that methane directly.

    If produce the hydrogen electrolytically, you need a lot of electric energy, so either need the same methane to run your powerplant, or nuclear fuel, or a lot of toxic chemicals to process the solar panels (and later utilize them, turning into same carbon dioxide).

    Introducing the Kværner process, which converts natural gas or biogas directly into nearly pure carbon and hydrogen. A carbon neutral biogas powered and biogas fueled system could be implemented relatively easily by any eco-conscious future civilization if so inclined. As a bonus, the carbon could be used in ceramics, composites, or high strength carbon allotropes like carbon nanotubes and graphene. So technically it would be a carbon sink, rather than simply carbon neutral.

    Much more broadly speaking, an eco-conscious space fairing future civilization could migrate all power, manufacturing, and living space off of Earth to preserve its environment. As I mentioned previously, though in a less serious way, you can't pollute an environment that isn't there. Because of this, any environmental impact caused by the manufacturing of any of the rocket's systems (fuel, structure, power, electronics) could be disregarded entirely.


  13. The best solution I could see is having space-based FTL capable superships with a fleet of surface-to-orbit shuttles and a few interplanetary shuttles.

    These supership would have to be constructed in orbit or on a small moon or asteroid, mass hundreds of kilotons to over a gigaton, and be up to several kilometers long. This lines up well with what you were originally looking for in terms of size (or many times larger) but be limited to space. They would be able to land on the smallest rounded objects like Ceres, but anything larger would be a one way trip down and cause significant damage to the ship and surface. The largest of these ships might even carry small space elevators (one megaton for a 100,000 km long carbon tube 5 cm wide) and 'lay anchor' to planets and moons from stationary orbit or their L1 point.

    As they can FTL between planets, their conventional propulsion should be optimized to facilitate quick travel within a planet's SOI. As both thrust and isp are important for high speed transit, a system that can vary between speed and efficiency would be desirable. Fortunately most AM and Fusion systems can facilitate multiple "gears", so this won't be an issue. This would also allow for the supership to travel interplanetary on conventional engines in a pinch. I'd go with an AM pion-beam core that gives 0.01 gee of acceleration and 3,000,000s ISP with a 1:10 AM:H2 fuel mix at high gear and 0.15 gee, 200,000s ISP, and a 1:2,250 AM:H2 fuel mix at low gear.

    This would on a continuous burn on low gear (high accel), go from LEO to the moon in ~10 hours using 54km/s dV, from ganymede to callisto (max sep) in a day with 130km/s dV, or Earth to Mars (min sep) in a week with 686 km/s dV (probably not enough fuel for this on low gear, but you get the picture)

    Depending on the limitations of their FTL drive, they could do a lot of work within a planets SOI on FTL alone.

  14. 1 hour ago, RoninFrog said:

    Most environmentally friendly?  Nuclear pulse, duh!

    Can't polute an environment that isn't there anymore Forehead :D

    On a more serious note, you need to consider not just the propellant itself, but what the exhaust products will be, how they can react to the air, and how the air responds to the intense localized heating of an engine.

    Regardless of the propellant, if it's hot enough, some of the surrounding air will be converted into the nitrogen oxides NO2, NO, and N2O, which are all powerful greenhouse gasses (N2O is ~15x worse than methane and 300 times worse than CO2) and are toxic. As these are also created by natural biological and geophysical processes, they aren't too big of an issue, and there really isn't anything that can be down about this while still using a rocket powered launch vehicle anyways.

    • Kerlox, Methalox, and Hydrolox all produce water as an exhaust product, with kerolox and methalox also producing some CO and CO2. Carbon monoxide is only ~0.6 times as potent of a GHG than carbon dioxide, but is toxic. Water is only ~0.4 times as potent of a GHG as CO2 (but contributes the majority of Earth's greenhouse heating because of its atmospheric abundance) and its clouds are reflective to create a small cooling effect, so is pretty mundane. Some unburnt RP1, or methane could be problematic though.
    • Alumilox (Aluminium + LOX) doesn't produce water or CO2, but does produce a fine Alumina dust that could potentially be harmful.
    • Solids, Hybrids, and Hypergolics are all less efficient than the big three cryo fuels, and will produce more toxic fumes to various extents.
    • Fluorine or Beryllium based fuels can get higher isps than hydrolox, and even rivaling some nuclear rockets, but both are pretty nasty substances to work with and would be quite bad for the environment.

    As for nuclear propellants: H2 is probably the most environmentally friendly; maximizing ISP while exhausting a gas with negligible GHG contributions (on par with O2). Some hydrogen may react with nitrogen and/or oxygen in the air to produce water and ammonia, which are greenhouse gasses, though in only small amounts. Water, as mentioned is one of the cleaner GHGs. Ammonia however, while very short lived (breaking down in a few hours or days), is nearly as potent a GHG as methane, and is pretty smelly. It is naturally produced by many organisms though, so can be tolerated by the environment to a degree.

    Helium could also be a good nuclear propellant, though there are issues with supply. Methane is a non-starter for a nuclear propellant, dissociating into H2 and soot in the reactor, which could be problematic for the reactor, or be exhuasted out and enter the atmosphere, neither of which are very good. Ammonia is probably a better option. Although is has a slightly lower ISP than methane does, it breaks down into inert N2 and H2, which will also be handled by the reactor better. Solid core reactors aren't hot enough to cause water to dissociate, but higher power liquid, colloid, droplet, vapor, gas, and plasma cores would be. The O2, H2, and H2O exhausted by such a rocket are non-problematic.

    Bipropellant mixtures like Hydrolox can also be used in nuclear rockets, but give lower ISPs in favor of higher TWRs. This may be useful in less developed NTRs, where TWR can be rather lacking. More advanced designs can get high enough TWRs and ISPs to be useful without bipropellants.

    As for what type of nuclear thermal rocket, it depends. A closed cycle is best, as no nuclear fuel can escape, but has reduced performance over open cycles, making them more difficult. Hotter engines also get better performance, but are also harder to contain. Solid cores don't get hot enough to take a performance hit from a closed cycle (other than TWR), so these types kind of blend together, especially since solid fissile materials can't be exhausted out unless something Really Bad happens. Liquid, Colloid, and Droplet cores get hot enough for open cycles to be problematic, but not hot enough for closed cycles to be much better than a simpler solid core. Vapor and Gas cores do get hot enough where closed cycles start to significantly out performing solid cores, but there are issues designing close cycles with current material science (especially the hotter gas cores). Plasma core are simply too hot to contain in any meaningful way, and despite magnetic confinement being a plausible option, it could still allow for some fissile losses during operation.

    All nuclear rockets have to worry about the potential environmental impact of a failure occurring, which would be significantly worse than the failure of any chemical rocket. When operating as intended would have a lower impact.

    For comparison of closed cycle options (assuming H2 as the propellant. Keep in mind that any other propellant would give worse performance):

    • Solid core NTR: ISP 800-1200 s, TWR 0.8-40
    • Vapor core NTR: ISP 1100-2000 s, TWR 0.5-10
    • Gas core NTR: ISP 1300-2800 s, TWR 2-15
  15. A ship like that would probably be able to refuel from water ice and hydrates. This would allow it refuel basically anywhere. The only locations in our solar system such a vessel wouldn't be able to refuel from would be Venus, Jupiter, Saturn, Uranus, Neptune, and possibly Io. Nearly all outer solar system bodies (with the exception of Io, and possibly some minor bodies) have thick crusts of icy material. Main belt objects like Ceres and Vesta have hydrate minerals at the surface, where water can be cooked out of them fairly easily. Mars has considerable amounts of ice under much of its surface, as well as hydrated minerals. Mercury and the Moon only have water ice in polar craters, and I don't believe they have any hydrated minerals.

    The ship would still be limited by its antimatter reserves, but it could dedicate a fraction of its payload volume to more antimatter.

    With the amount of antimatter present on this ship, (about a pound), the storage efficiency has a negligible impact on the overall craft specifications. Even so, the antimatter containment unit is 100 times more massive than the antimatter itself, while this is slightly optimistic for magnetic storage, it isn't something unattainable with the efficiencies of storing more than a few thousand atoms at a time. You could alternatively have antihelium stored inside fullerenes, for a tankage fraction of ~200:1, which is in the same ballpark as what is being used. Magnetic levitation of antihydrogen ices might be able to approach tankage fractions of 1:1 or lower, but I decided against using it from the potential danger of a single centralized antimatter container.

  16. Yeah, antimatter is kind of insane, and thats with using less efficient water as a propellant. This craft could probably only get 100t payloads off of worlds less than 2 Earth masses with thin earthlike atmospheres, which should cover most "Earthlike" planets. On smaller worlds however, its payload capacity skyrockets. Quick table of parameters for the same vehicle for different world sizes. (limiting vehicle parameter in bold)

    World Mars         Ganymede  Pluto        


    Required dV (km/s) 4 2 1 0.3
    Gravity (gee) 0.38 0.15 0.06 0.03
    Low Orbit Payload (t) 1460 4800 8760 27100
    TWR (local) 1.4 1.68 2.05 1.4
    Vessel dV (km/s) 4.9 2 1 0.35


  17. 2 hours ago, Spacescifi said:

    I am concerned that saltwater AM thermal won't be efficient enough to loft a 100 ton payload to orbit.... and if it did, the ship would be big.... probably bigger than Elon's Starship.

    No worries there. Here's a "basic" rundown of what a water propelled AM Gas Core SSTO could look like. 

    My assumptions:

    • 16000 m/s of dV.
    • Upright landing, change height to length if sideways landing is desired (doesn't make any real difference)
    • 8 meter diameter rocket, height determined by tankage volume. Additional 8 meters in height from engines, and 25 meters in height from payload bay.
    • 100t payload + 10t for misc structural and avionics.
    • Water: 0.8 kg/L in tankage. Tankage mass is 5% the propellant mass. (95.3% propellant)
    • Antiprotons: 0.01 kg/L in tankage. Tankage mass is 10000% the fuel mass. (1% fuel)
    • Water:Antiproton mass ratio = 1,000,000:1 (used at a rate of 10,000,000:1, allowing for 10 water refuelings before additional antimatter is required)
    • Vacuum ISP = 2400s, sea level ISP = 1600s. Engine vacuum TWR = 6. Engine sea level TWR = 4.
    • Minimum allowable surface TWR: 1.4

    Calculated values: (masses calculated after payload mass fraction is known)

    • Combined reaction mass (water+antiprotons): ~0.8kg/L in tankage. Tank mass is 5.1% the reaction mass. (95.1% reaction mass)
    • Gross mass/dry loaded mass: 1.9731        = 835.4 t
    • Tankage mass/dry loaded mass: 0.0496  = 21 t
    • Engine mass/dry loaded mass: 0.6906     = 292.4 t
    • Payload mass/dry loaded mass: 0.2362.  = 100 t
    • Misc structural mass: 0.0236                      = 10 t
    • Dry empty mass:                                            = 323.4 t
    • Water propellant mass                                  = 412 t
    • Antiproton fuel mass                                     = ~0.41 kg
      • Total vacuum (sea level) thrust: 17.21 (11.47) MN
      • Total height: 43.3 meters
    • dV (no payload): 19340 m/s
    • (no payload) Vacuum (Sea level) TWR: 2.39 (1.59)
      • Time to fill by US fire hydrant (500-1500 gal/min): 1.2 to 3.6 hours
      • Time to fill by average garden hose (17 gal/min): 4.44 days

    For comparison, Starship (just the upper stage) has a 9 meter diameter, is 50 meters tall, and weighs ~1420 tons fully fueled with payload.

  18. In terms of shuttle craft, there should be a lot of options available (I'm assuming shuttles are non-FTL craft primarily for ferrying small crews or payloads to and from planets and moons with various masses and atmospheres)

    With how large your ships are, you could probably get away with having a few types of shuttles for various purposes and/or of various sizes. If the technology is available, you could have smart ships that reconfigure themselves for a specific task. Shuttle craft probably wouldn't be needed (but nice to have) for more developed worlds with extensive orbital infrastructure in place, or smaller moons like those of saturn and uranus (excluding titan), where gravity and dV are minimal.

    Main categories I can think of are moon-mars sized (atmosphere is negligible), earth-superearth sized (low pressure), and earth-superearth sized (high pressure).

    ...and now I want to design a family of shuttle vehicles for an assortment of planetary environments of 5-10 crew/20t payload (space shuttle like) and 25-50 crew/100t payload (starship like)

    Any prefered propulsion system for shuttle craft.

  19. As none of them are chemical based, pretty much any fluid propellant can be used, with the exceptions of nuclear saltwater rockets and fusion augmented antimatter. Internal plumbing would have to be tailored to a certain propellant though (no jury rigging an H2O fueled craft to use O2, or at least not easily). The propellant used would affect the ISP and engine power with higher molar mass particles generally giving lower thrust and ISP.

    Some propellant processing will be required though, to remove any materials that could cause blockages in the plumbing where temperatures aren't 5000+ Kelvin. Salts, metals, and carbon soot would be the biggest offenders, remaining solid while the more volatile components are evaporated. Some propellants may also chemically react with the engine itself, oxidizing, corroding or otherwise jeopardizing the integrity of the engine. Halogens and Chalcogens like bromine, oxygen, sulfur, chlorine and especially fluorine should be avoided because of this. For this reason propellants would still need to be processed (often quite heavily), but could be sped up significantly by the shear power available from these spacecraft (The mainsail's description of being able to power a small nation isn't that far off the mark). Resource extraction would still take some time though, especially for larger craft. Distilled water would probably be your go to for exploration vessels in locations without the infrastructure in place to refuel with hydrogen. More civilized areas would probably stick with hydrogen though.

    Some of the more favorable and abundant propellants (ordered roughly by efficiency):

    H2, He, NH3, H2O, Ne, N2, O2?, Ar, CO2?

  20. It is quite probable that there are additional planets in the system. Here's my somewhat lengthy, but hopefully comprehensive summary on what types of worlds could be where in the system based on our current understanding of planetary science. For reference, these are the two types of planetary orbits in multiplanetary systems. S-type orbits go around a singular star, and P-type orbits go around multiple stars.


    There are four main regions where planets could reside in the Alpha Cen. + Proxima Cen. ternary system: S-type orbits around Proxima (C), S-type orbits around A, S-type orbits around B, and P-type orbits around both A and B, but not Proxima.

    Planets around Proxima (Alpha Cen C): There are currently two planets known to orbit Proxima: proxima b, and proxima c. Based on recent measurements of the inclination of c, the two planets likely mass 2.1 and 12 Earth masses respectively. As current detection limits are fairly low, and planets have already been found here, it is quite probable that there are additional bodies in the system.

    Planets around Alpha Cen A: It's certainly possible for there to be planets around A, but there are some complications. Alpha Cen A and B are separated by 17.6 AU on average (varying between 11.2 and 35.6 AU). While planets orbiting up to 2.8 AU away would be stable, it is unclear if planets could have formed in this region. Several planets have been found around single stars in multistar systems, but few as close together as Alpha Cen A and B. OGLE-2013-BLG-0341L B is currently the record holder for tightest binary with an S-type planet, and is separated from its companion by somewhere between 11 and 17 AU. This is very similar to Alpha Cen, but the stars in the OGLE-2013-BLG-0341L system are both red dwarfs, where planets would form much closer in, so its still unclear if planets could have formed around Alpha Cen A. There is some evidence for dust around A and/or B, so its certainly possible that planets could exist there.

    Planets around Alpha Cen B: A very similar case to A, certainly possible, but the situation is made complicated by the AB binary pair. Unlike A, there is an unconfirmed exoplanet around Alpha Cen B. Bc, if confirmed, would be a lava world about the same size as Earth. Alpha Cen Bb was found to be cause by data artefacts in 2015, and so is extremely unlikely to be present. With unconfirmed planets and possible dust, it is possible that one or more planets could be present.

    Planets in P-type orbits around both Alpha Cen A and B: This is possible, but less likely than planets around either A or B. There are numerous binary systems with planets in P-type orbits, but none have been found in systems with stars as far apart as Alpha Cen A and B. The current record holder is FW Tauri AB b, orbiting stars separated by 11 AU, which is only two-thirds the average separation in Alpha Cen. It is also unclear if FW Tauri AB b is a high mass planet, brown dwarf, or low mass star. Even if no planets were able to form in P-type orbits around A and B, it is possible that some worlds were flung out from A or B into orbit around the pair.

    Cool Info, but what do I actually think? I'd guess there are few more planets orbiting proxima. I'd also guess that there are a several planets in S-type orbits around A and B, but wouldn't be too surprised if this wasn't the case. I'd guess there aren't any planets larger than Earth orbiting A and B in a P-type orbit, but wouldn't be shocked if one was found eventually. I would however guess that there are several circumbinary dwarf planets around the pair. 

    Hope this helps give ideas and clears up more confusion than it causes.

  21. Now that have a slightly better handle on what your aiming for, here's a shortlist of what might work in your setting for planetary operations, and my best guess of performance and what they might look like.

    The Nuclear (Fission) Options:

    • Liquid Core Fission Rocket (Open Cycle): Very good TWR, but lower ISP. Liquid fissiles mix with and heat hydrogen propellant. Most fissiles are kept within the reactor, but some can escape, possibly bad for occupied worlds. ISP of 1000-2000s, engine TWR of 8~50. Bright translucent white exhaust. No residual smoke or steam trail.
    • Gas Core Fission Rocket (Open Cycle): Similar to Liquid cores, but fissiles heated to a gas for increased efficiency. Fissiles escape more easily than liquid cores, possibly bad for occupied worlds. ISP of 3000-7000s, engine TWR of 5~20. Bright translucent blue exhaust, with subtle hints of magenta from hydrogen plasma. No residual smoke or steam trail.
    • Gas Core Fission Rocket (Closed Cycle): Similar to above, but fissiles and hydrogen are kept separate, at the cost of added complexity and reduced performance. Not intrinsically bad for occupied worlds. ISP of 1500-3000s, engine TWR of 2~15. Bright translucent blue-white exhaust. No residual smoke or steam trail.
    • Salt Water Nuclear Rocket (Very Open Cycle): Liquid Fallout, very very bad for inhabited worlds. ISP of 4000-10000+, TWR of 10~40?. Very bright translucent blue exhaust, with hints of magenta from hydrogen plasma. Residual trail of white-light grey steamy clouds.

    External Fission and Fission/Fusion Pulse systems are a poor choice for planetary use for reasons already discussed, so wouldn't be a fit for the setting imo.

    Fusion isn't well suited for planetary use either. ICF and MIF systems suffer similar problems to other pulsed propulsion systems, but with lower TWRs in favor of much higher ISPs. MCF systems have very poor TWRs but fantastic ISP. You can thrust augment a Fusion drive with "afterburners" (injecting additional propellant to increase thrust at the expense of fuel economy), but this results in worse performances than fission thermal rockets.

    Antimatter Options: (probably the better choice for an FTL capable society)

    • Liquid Core AM Thermal: Cleaner than fission, antimatter annihilates before it can escape, but some x-rays can be emitted. Pretty safe. ISP of 1500-2500s, engine TWR of 10-40. Bright blue-white exhaust
    • Gas Core AM Thermal: Uses more antimatter than liquids. Emits some x-rays and gamma, but otherwise fairly safe. ISP 4000-10000s, TWR 3-10. Bright blue-indigo? exhaust. No trail
    • Fusion augmented AM Gas Core: Using deuterium in addition to hydrogen. Less antimatter is needed and x-ray emissions are reduced. Safe. ISP 3000-7000, TWR 3-10. Bright blue-indigo? exhaust. No trail

    Any higher power antimatter systems aren't well geared for planetary use, and put out significant amounts of x-rays and gamma.

    As for how you could handle your FTL PlotdriveProject Rho has a page for designing FTL in fiction which might be a useful resource.

  22. 2 hours ago, Dragon01 said:

    Also, MHD can work with any kind of hot gas (such as coal powerplant exhaust, which is what they're used for right now), so if you can use these gammas and pions to make hot gas, you can use them for MHD, as well.

    Oh yeah, that completely slipped my mind :/. Still beam cores might run into in efficiency issues with energy conversion with less reaction mass that can absorb the gamma (another reason why AM thermal is pretty good). AM thermal is a really good propulsion system, especially where high TWR is required and when antimatter is hard to come by. Just figured with the large amounts proposed in the OP that you might as well use it (that amount would also be ideal for a plasma core).

    Speaking of, just some math on the power requirements to produce 1 ton of antimatter every two months at various tech levels. Basically a swarm of specialized factories inside mercury's orbit should be able to crank it out no problem, just not today.

    At current efficiencies. (1E-13 %) Pretty ridiculous: 1.7E28 W, 9E14x Global power output, 44x Output of the Sun.

    Specialized facilities a billion times more efficient (0.0001%) Pretty reasonable for an advanced civ: 1.7E19 W, 900000x Global power output, 0.000004x Output of Sun.

    Pair production from black hole hawking radiation (~30%) Easily doable, assuming you can make and maintain the 5-10 Mt black holes: 5.7E13 W,  1100x Global power output.

    Max efficiency from Andreev Reflection in quark nuggets (~85%) Trivial, quark nuggets not included: 2E13 W, 380x Global power output.

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