Propulsive Fluid Accumulator Satellites

[Northstar1989]

Back in 1959 the idea was first seriously proposed to set up a Propulsive Fluid Accumulator satellite at the edge of Earth's atmosphere, to gather Oxygen and use it for propulsion on missions deeper out in space (useful for LH2/LOX of Kerosene/LOX systems). The LOX would be periodically carried by the satellite (once full) to a propellant depot in a higher orbit (where it would experience far less drag), and then the satellite would return to further LOX-collection:

http://en.wikipedia.org/wiki/Propulsive_fluid_accumulator

It was an excellent idea, but a little before its time, and more importantly, the original design required an onboard nuclear reactor to meet the vessel's extensive power demands (cooling/compressing Oxygen to liquid form, keeping capture Oxygen cool, and running a Nitrogen plasma thruster being the main ones...)

The other technology that was missing at the time, besides space-capable nuclear reactors (which the Russians first deployed not all that many years later with their TOPAZ satellites), was nitrogen-electric propulsion. However now, in the modern day, this is finally also becoming a reality:

http://en.wikipedia.org/wiki/Helicon_Double_Layer_Thruster

So, all the necessary technologies finally exist! (There are also plenty of designs out there for orbital propellant depots- the main reason they haven't been built is because there hasn't been seen as being a demand for them- as all the fuel to fill them would still haven to be launched aboard tankers, and the total launch mass would actually *increase*.)

And, for those too scared to put a nuclear reactor in orbit, as unreasonable as I think those fears might be, there's always Microwave Beamed Power, which is an equally viable (and safer+cheaper) alternative to power the satellite:

http://en.wikipedia.org/wiki/Microwave_transmission#Microwave_power_transmission

http://en.wikipedia.org/wiki/Wireless_power#Electromagnetic_radiation

http://en.wikipedia.org/wiki/Space-based_solar_power

http://nextbigfuture.com/2014/02/escape-dynamics-and-microwave-power.html

The power-transmission equipment could equally well be placed on the ground or in space (beaming from solar-power satellites in higher orbits if in space). Although ground-based infrastructure would obviously be cheaper, space-based transmitters could transmit in much shorter wavelengths that penetrate poorly through Earth's atmosphere, but carry better over distances and require smaller receivers on the Propulsive Fluid Accumulator satellite... One could even beam power from Earth via long-wavelength microwaves, then convert them to short-wavelength beams at relays above Earth's atmosphere, to transmit the power back down to the Propulsive Fluid Accumulator at the edge of the atmosphere- although such a system involves multiple transmission losses...

So, there are a variety of options to power the satellite (onboard nuclear, ground-based microwaves, space-based microwaves, or a hybrid system of some sort), and even is a system to propel to the satellite using Nitrogen skimmed from the atmosphere!

The question is, then, not so much why this hasn't been done in real life (remember, the necessary nitrogen plasma-thrusters are still in the final stages of testing, Beamed Power to spacecraft is still considered a novel idea despite its relative simplicity and long history going back to Tsiolkovsky, and political willpower to put a nuclear reactor in orbit for any reason is lacking), but more how do we go about changing that?

And also, why hasn't anyone doe it in KSP yet!?

All the necessary components now exist: KSP-Interstellar has high-powered plasma thrusters with realistic ISP and thrust-energy relationships- and there is now an "extension" config that allows plasma thrusters to use Nitrogen (and Atmospheric Scoops to operate *just* outside the edge of atmosphere so you can use time warp- an idea which was proposed by me and graciously accepted...), RealFuels already has a Nitrogen resource which could be used, Interstellar already has Atmospheric Scoops capable of collecting said Nitrogen (and LOX/Oxidizer for a fuel depot- which is the whole point of the Propulsive Fluid Accumulator), and Karbonite even has code to allow its native parts to scoop atmospheric resources from *just above* the atmosphere (so players don't have to set up a 65 x 65 km "orbit", and only be able to operate the satellite at 4x time-warp: they can set up a 72 x 72 km orbit instead and still scoop resources using the Karbonite code) which might be able to be adapted to KSP-Interstellar...

Join me in installing the KSP-Interstellar 0.90 port "extension" config, and giving this a try. And ask/pressure Fractal_UK to integrate the extension config into his main version, whenever he comes back from his current long leave...

And, of course, feel free to discuss the science/politics/etc. behind all this- that's why this thread is here!

Regards,

Northstar

Edited January 26, 2015 by Northstar1989

[K^2]

You can always just dip your orbit just bellow atmosphere threshold in KSP. Atmo is thin enough there to maintain altitude using ion/plasma propulsion. Is there a mod that would actually let you store ox from scoops, though? I haven't seen anything like that.

[shynung]

Is there a mod that would actually let you store ox from scoops, though? I haven't seen anything like that.

Here's one.

[Northstar1989]

You can always just dip your orbit just bellow atmosphere threshold in KSP. Atmo is thin enough there to maintain altitude using ion/plasma propulsion. Is there a mod that would actually let you store ox from scoops, though? I haven't seen anything like that.

KSP-Interstellar lets you store Oxygen from scooping Kerbin's atmo, as I pointed out in the OP.

The problems with using ion propulsion this way are two-fold, though. One, the stock ion engines have a MUCH lower ISP (though higher thrust) than many of the more advanced real-world ion engines: such as a Dual Stage 4-grid ion thruster (which the NearFuture mod accurately replicates the ISP of, but with higher TWR than the original). Two, Xenon-based ion thrusters are non-renewable in such a situation- you'll have to constantly launch more Xenon propellant for the system to work...

Using Nitrogen scooped from the atmo to maintain altitude/speed lets you run the system with no external inputs except power (beamed from the ground or solar power satellites if you don't use an onboard nuclear reactor and occasionally ship up fresh Uranium rods...), and essentially get "free" Liquid Oxygen (LqdOxygen in RealFuels or Oxidizer in the stock game) without any cost other than the initial setup cost of the system. Such a Propulsive Fluid Accumulator system QUICKLY pays for itself many times over- which was the whole point of the originally-proposed design...

Regards,

Northstar

[Aghanim]

So, thermal jet powered with nuclear reactor/microwave receiver and atmospheric scoop?

[magnemoe]

So, thermal jet powered with nuclear reactor/microwave receiver and atmospheric scoop?

No some sort of vasmir who can handle nitrogen. http://en.wikipedia.org/wiki/Helicon_Double_Layer_Thruster

Should work but have some problems, microwaves to an satellite in low orbit requires an network of senders.

An nuclear reactor might have heat issues, problem with cooling fins here, also an safety issue, this will deorbit fast if the engine or reactor fails.

[Dodgey]

No some sort of vasmir who can handle nitrogen. http://en.wikipedia.org/wiki/Helicon_Double_Layer_Thruster

Should work but have some problems, microwaves to an satellite in low orbit requires an network of senders.

An nuclear reactor might have heat issues, problem with cooling fins here, also an safety issue, this will deorbit fast if the engine or reactor fails.

A small supply of chemical propellant and a thruster or two could serve as a contingency plan, something happens to the main propulsion then it engages, pushing the sat into a higher orbit to await rescue.

[SargeRho]

Couldn't this also work for gas giants, to harvest Hydrogen and perhaps Helium 3?

[andrewas]

I think you'd have to go deeper for helium, it is twice as dense as hydrogen.

[Northstar1989]

So, thermal jet powered with nuclear reactor/microwave receiver and atmospheric scoop?

Turbojets of any sort (thermal or otherwise) don't work at the relevant altitude and speed ranges (plus their ISP is much too low at the edge of their performance envelope, so going deeper wouldn't work...) We're talking about *satellites* inside the edge of the atmosphere (think between a 64x64 km and a 72x72 km orbit in KSP), either just above or below the Karman line here- not planes. Some type of nitrogen-electric propulsion is necessary to efficiently counteract drag using locally-available resources.

Couldn't this also work for gas giants, to harvest Hydrogen and perhaps Helium 3?

Yes, the exact same technology would work on gas giants, That's correct. You'd just be scooping Hydrogen/Helium-3 (enough atmospheric mixing occurs that you could still find *some* He-3 at the edge of the atmosphere- although you'd probably still want to go deeper for higher concentrations...) instead of Oxygen and Nitrogen. And using Hydrogen for electric propulsion instead of Nitrogen...

Regards,

Northstar

P.S. If you didn't infer it from what I just wrote- yes, you can already create Propulsive Fluid Accumulator satellites just inside Jool's atmosphere, using KSP-Interstellar's atmospheric scoops and hydrogen fuel-mode for its plasma thrusters. It works great with FAR installed. But this can also theoretically be done on Earth in real-life, and Kerbin in-game, and I want to see KSP-Interstellar create nitrogen-electric propulsion to enable it in KSP...

Edited October 27, 2014 by Northstar1989

[Northstar1989]

Hey,

Just an update for y'all (sorry to necro an old thread, but it's better to necro a relevant thread than to create a redundant thread on the same topic and lose all existing conversation/thought...)

FreeThinker has created an Extension Config for the KSP-Interstellar 0.90 port, which now allows any player to utilize RealFuels-compatible "Nitrogen" (or a new KSP-Interstellar specific "Liquid Nitrogen", which represents the cryogenic rather than gaseous form of Nitrogen, and is much denser but more expensive to purchase and store...) as an electric propellant for the KSP-Interstellar plasma thrusters, and allows Atmospheric Scoops to work *just* outside the edge of the atmosphere (in circular orbits up to +10% of atmospheric height, which represents the fact that most Propulsive Fluid Accumulators are designed to operate above the 100 km Karman Line in real life- which 70 km acts like in KSP...) thus allowing players to create Propulsive Fluid Accumulators schemes of their own in KSP-Interstellar without any additional mods or modding required (although installing RealFuels will also allow you to use "Nitrogen" for RCS, like in real life).

Please take a look at the Extension Config, and let's get some Propulsive fluid Accumulators of our own up in KSP! Maybe it'll draw some attention (if it becomes a "thing" in KSP- remember many NASA engineers and administrators play KSP in their free time), and make NASA realize that for the past 10 years (since Microwave Beamed Power became feasible with recent advances in gyrotrons- no longer requiring a nuclear reactor to be launched to orbit for a PFA when you can just keep the reactor on the ground and beam the power to operate a Propulsive Fluid Accumulator to Low Earth ORbit instead...) we've had all the technology we need for *virtually unlimited* FREE (no marginal cost for additional fuel- decently expensive set-up cost for the whole system) propellant mass in the form of unlimited quantities of Nitrogen in space...

(Nitrogen can be utilized for either electric propulsion, or in thermal rocketry where higher thrust-levels are required for manned missions... A heat source is still required for thermal rocketry, however- either a nuclear reactor, or Microwave Beamed Power from somewhere else, including coal/wind/natural gas/hydroelectric/nuclear power plants on Earth if desired...)

Regards,

Northstar

Edited January 26, 2015 by Northstar1989

[FreeThinker]

Alright, I made a first crude but working implementation of the Propulsive Fluid Accumulator in KSPI modifying the the Admospheric Scoop.

In short, what I currently do is measure the time past since the last login and use it as the delta time for calculation of collecting OSR resource.

The amount of resources collected depends on [the amount of air] * [percentage of power] * [delta time]. Note that I fudge the amount of air by a small amount (which is configurable) to speed up resource KSP collecting.

Although this works, I need a better theoretical base for a exact amount you collect and lose for propulsion

For example, I do now yet spend any fuel to counter the drag.

The amount of spend fuel could depend of the [vessels average mass] * [iSP electric engines] * [delta time].

Only if the amount of gained mass is higher than the amount of spend mass, it would be profitable, other your just wasting time and resources.

Another issue is the tool I use for resource colecting, the Admospheric collector is ment to be used in the admosphere, at high speed, not at the edge of space. How would a real accumulator work, what would it look like? would it benefit from some kind of large contracption, or would it only increase drag ...

Another issue is that I want to addres is that the Atmosheric Scoop pretent the composition of gasses, does not differ near the edge of space. This is probably false as I can imagine that resource like Hydrogen are much higher quantities at this altitude than at lower altitudes. Is there any hard data we can use?

Edited January 26, 2015 by FreeThinker

[Northstar1989]

In short, what I currently do is measure the time past since the last login and use it as the delta time for calculation of collecting OSR resource.

That seems like a valid method if the vessel is close enough to the edge of the atmosphere to collect gasses (remember, the edge of the atmosphere in KSP is at an altitude/pressure more analogous to the Karman Line in real life- vessels still encounter very dilute gasses they can scoop above it, and still experience *greatly* reduced levels of drag...)

The amount of resources collected depends on [the amount of air] * [percentage of power] * [delta time]. Note that I fudge the amount of air by a small amount (which is configurable) to speed up resource KSP collecting.

What do you do for scooping *beyond* the official edge of the atmosphere? I thought you already said over on the KSP-I 0.90 port thread that you implemented code to allow Atmospheric Scoops to function beyond the Karman Line/ "edge" of the atmosphere (where KSP thinks no air is present).

Although this works, I need a better theoretical base for a exact amount you collect and lose for propulsion

For example, I do now yet spend any fuel to counter the drag.

Didn't you say that in time-warp the vessel just collects 95% of what it would w/o time-warp, and you assume the remainder to be going to fighting drag?

The amount of spend fuel could depend of the [vessels average mass] * [iSP electric engines] * [delta time].

I think you mean 1/ISP (the inverse of ISP). Otherwise vessels with better ISP will expend MORE fuel fighting drag, which makes no sense.

Using mass to approximate drag is also a TERRIBLE idea. In real life, drag is completely unrelated to mass- only to shape- and even the TWR affects of increased mass don't matter here- more massive vessels may experience less acceleration from the same thrust due to their lower TWR, but they also experience less acceleration due to drag as a result of their greater momentum. Once you're in orbit, you could keep a 1 MILLION ton vessel at speed with a 1 kN thruster if the vessel only experienced 1 kN of drag...

In stock KSP, mass and drag are currently directly related (which makes no sense- it means a feather and a lead ball dropped at the same time inside the atmosphere would hit the ground at *exactly* the same time and speed...) but the stock aerodynamics model is soon getting a much-needed overhaul. I suggest just sticking with your 95% approximation (that is, 5% of collected fuel being used to fight drag- which is actually EXTREMELY conservative... A streamlined vessel will expend considerably less fuel on fighting drag...) for the time being, until stock aerodynamics gets its overhaul at the very least...

Once stock aero is overhauled, hopefully it will internally record the number you need to make an accurate approximation of drag- which is the Ballistic Coefficient of the vessel when facing prograde (this is an inherent property of a vessel of a certain mass and shape) although it changes as a vessel gains or loses mass, what you want to do is multiply that in as an additional factor. So, fuel-consumption is proportional to:

[Delta Time] * [1/ISP] * [ballistic Coefficient] * [Vessel Mass]

You can simplify this further by choosing an arbitrary vessel mass and the corresponding Ballistic Coefficient (say when the Propulsive Fluid Accumulator's Nitrogen tanks are completely full) and multiplying them together, and then using that number in *ALL* calculations for that vessel unless it gains or loses parts (parts, *NOT* mass- mass is irrelevant, but parts can change the shape and thus drag of the vessel).

However, drag varies greatly by altitude, planetary atmospheric composition (Duna's Thermosphere would behave very differently from Kerbin's, for instance), magnetic field strength (having little or no magnetic field, as with Duna, will influence Thermosphere conditions GREATLY), and even distance from the sun (again, Duna differs from Kerbin). The requisite math involved, which must take into account all of these factors is far too difficult for a single feature of KSP-Interstellar in my opinion- and I suggest you just stick with the 95% approximation permanently. However, if you really want to figure it out, I suggest you approach Ferram4 (the creator of FAR, he will know a lot more about the Thermosphere above the Karman Line than me) and the creator of the Atmospheric Trajectories mod (he already figured out a way to get his mod to access FAR's data on Ballistic Coefficient, and I think also temperature/pressure variations with altitude, in order to make re-entry predictions fro spacecraft...)

Only if the amount of gained mass is higher than the amount of spend mass, it would be profitable, other your just wasting time and resources.

That's going to happen at pretty much any altitude much above the Karman Line, so long as the vessel doesn't have huge flopping solar panels to generate excess drag (and then again, even *with* those panels at a slightly higher altitude). I can easily design craft that can operate profitably *BELOW* the Karman Line with FAR (and a streamlined designed) and Nitrogen-utilizing plasma thrusters, even in Real Solar System 64K: I just don't WANT to operate there, as then I can't use nonphysical time-warp speeds...

Another issue is the tool I use for resource colecting, the Admospheric collector is ment to be used in the admosphere, at high speed, not at the edge of space. How would a real accumulator work, what would it look like? would it benefit from some kind of large contracption, or would it only increase drag ...

Actually, the Atmospheric Scoop part looks a lot like what the parts on a real Propulsive Fluid Accumulator would. A real PFA would probably have an aerodynamic nosecone (to minimize drag, especially while ascending to orbit in the first place, and if it ever dipped below the Karman Line while collecting...) with a couple inline/radial intakes for scooping desired gasses (the Atmospheric Scoop part looks a lot like those intakes probably would).

A PFA with REALLY high power levels available would probably have something that looks like a Ram intake on the front (which would generate a lot of drag, but also collect proportional amounts of gasses from that atmosphere), and all sorts of cooling equipment arranged directly behind that to chill the gasses into liquid form. Basically, a Propulsive Fluid Accumulator is a lot like a spaceplane without wings (it operates too high up for there to be any lift- which becomes impossible above the Karman Line)- it should be designed to make as much of the surface, and especially cross-sectional, area it exposes to the "airflow" (though hardly a flow at that density- proper airflows only occur below the Karman Line) intake area, and the rest of it as streamlined as possible. So, it could look like anything from a fat cylinder with a big scoop over the entire front end, to a long/narrow cylinder with radial scoops scattered along its length.

The current Atmospheric Scoop parts suffice for a first-generation design (which would likely take a more streamlined approach to minimize drag in case of power or engine-failure, to give rescue craft more time to reach it, rather than a design based on filling the tanks as quickly as possibly by maximizing scoop area...) I wouldn't worry too much about it...

Another issue is that I want to addres is that the Atmosheric Scoop pretent the composition of gasses, does not differ near the edge of space. This is probably false as I can imagine that resource like Hydrogen are much higher quantities at this altitude than at lower altitudes. Is there any hard data we can use?

I assume that, once again, you're talking about gas composition *ABOVE* the Karman Line (the edge of space in KSP, and also the approximate start of the Thermosphere in real life...) Below the Karman Line, gasses are comparatively well-mixed. You know what, I think a couple lists are in order...

Atmospheric Traits BELOW the Karman Line (100 km in real life, 70 km in KSP)

- Gasses are comparatively well-mixed, gas composition does not vary greatly with altitude

- Gas density varies GREATLY with altitude: pressure falls off in accordance with the planet's Scale Height

- Gasses are comparatively dense compared to above the Karman Line (this is a RELATIVE term- at the Karman Line, atmospheric pressure is incredibly tiny at this altitude)

- Aerodynamic Lift is possible

- Drag is comparatively high relative to atmospheric density

Atmospheric Traits ABOVE the Karman Line (100 km in real life, 70 km in KSP)

- The Karman Line is roughly where the Thermosphere begins. The temperature of the atmosphere climbs to very high levels, increasing gradually with height.

- Gasses start to become stratified by molecular weight above this point...

- Atmospheric pressure falls off VERY gradually compared to at lower altitudes.

- Aerodynamic lift is no longer possible

- Drag is comparatively low relative to atmospheric density (which is even lower) as atmospheric particles become too far apart to be properly compressed above this point... (they are simply moved aside by a spacecraft)

Regards,

Northstar

[FreeThinker]

What do you do for scooping *beyond* the official edge of the atmosphere? I thought you already said over on the KSP-I 0.90 port thread that you implemented code to allow Atmospheric Scoops to function beyond the Karman Line/ "edge" of the atmosphere (where KSP thinks no air is present).

Well technically KSP density property still returns a number > 0 when above the 70.000 m but it's incredicably low (< 0.00001 g/L). I simply add a constant (bonus) value (called MinAtmosphericAirDensity) which adds 0.001 g/L to the measured athmospheric density. This is enough to collect significant amount of gas. If you think this is wrong, we can easily tweak the value to a lower/higher value.

- - - Updated - - -

Gasses start to become stratified by molecular weight above this point...

Alright, this is intresting, I could make light gases (like Hydrogen) more abundant the higher you go, this could also add an intresting gameplay meachanic "find the optimal altitude for resource collecting". To implement it, I could first order the gasses by gas density, and spike their abundance at diffent altitudes. The distance between the spikes might depend on the differnce in density between the gases. Now I need some good math formula.

- Atmospheric pressure falls off VERY gradually compared to at lower altitudes.

Great, instead of limiting resource collection to 10% above the karman, we might extend it to 1000% (70.000 m - 700.000 m for Kerbin)

That also requires I replace the constant density bonus value (density bonus) by function that zerros out at 1000% above the edge of space. The following simple function might work well enough.

density =  [KSP density property] + max(((vessel altitude / karman altitude ) * -(MinAtmosphericAirDensity * 10) )  + MinAtmosphericAirDensity), 0)

- - - Updated - - -

A PFA with REALLY high power levels available would probably have something that looks like a Ram intake on the front (which would generate a lot of drag, but also collect proportional amounts of gasses from that atmosphere), and all sorts of cooling equipment arranged directly behind that to chill the gasses into liquid form. Basically, a Propulsive Fluid Accumulator is a lot like a spaceplane without wings (it operates too high up for there to be any lift- which becomes impossible above the Karman Line)- it should be designed to make as much of the surface, and especially cross-sectional, area it exposes to the "airflow" (though hardly a flow at that density- proper airflows only occur below the Karman Line) intake area, and the rest of it as streamlined as possible. So, it could look like anything from a fat cylinder with a big scoop over the entire front end, to a long/narrow cylinder with radial scoops scattered along its length.

Intresting, before I started I though it was odd the the Atmospheric Air intake wasn't required for the Atmospheric Scoop to function. We could make the efficiency of the PFA depend on the availability of Atmospheric Air intakes. We should rename it a different name like "Propulsive Fluid Accumulator air intake", to make it clear it's suitable for this Job. We make it require a lot of power. One advantage of this part is that it would make the Propulsive Fluid Accumulator Satelite something dedicated, it would require some serious dedication and design.

Edited January 27, 2015 by FreeThinker

[FreeThinker]

In stock KSP, mass and drag are currently directly related (which makes no sense- it means a feather and a lead ball dropped at the same time inside the atmosphere would hit the ground at *exactly* the same time and speed...) but the stock aerodynamics model is soon getting a much-needed overhaul. I suggest just sticking with your 95% approximation (that is, 5% of collected fuel being used to fight drag- which is actually EXTREMELY conservative... A streamlined vessel will expend considerably less fuel on fighting drag...) for the time being, until stock aerodynamics gets its overhaul at the very least...

Once stock aero is overhauled, hopefully it will internally record the number you need to make an accurate approximation of drag- which is the Ballistic Coefficient of the vessel when facing prograde (this is an inherent property of a vessel of a certain mass and shape) although it changes as a vessel gains or loses mass, what you want to do is multiply that in as an additional factor. So, fuel-consumption is proportional to:

[Delta Time] * [1/ISP] * [ballistic Coefficient] * [Vessel Mass]

You can simplify this further by choosing an arbitrary vessel mass and the corresponding Ballistic Coefficient (say when the Propulsive Fluid Accumulator's Nitrogen tanks are completely full) and multiplying them together, and then using that number in *ALL* calculations for that vessel unless it gains or loses parts (parts, *NOT* mass- mass is irrelevant, but parts can change the shape and thus drag of the vessel).

I understand but I might found a better solution. I asked allista and he advised my to make A 2D convex of the in vessel in prograde direction.

var vT  = vessel.transform;
var dir = vessel.transform.InverseTransformDirection(prograde_vector_in_world_space);
var rot = Quaternion.LookRotation(dir);

var all_verts = new List<Vector2>();

foreach(Part p in parts)
{
    foreach(MeshFilter m in p.FindModelComponents<MeshFilter>())
    {
        //skip meshes without renderer
        if(m.renderer == null || !m.renderer.enabled) continue;
        var verts = m.sharedMesh.vertices;
        for(int i = 0; i < verts.Length; i++)
        {
            //transform mesh vertices into vessel's space;
            verts[i] = vT.InverseTransformPoint(m.transform.TransformPoint(verts[i]));
            //then exclude prograde part from each vertex; this gives the projection
            verts[i] = Vector3.Exclude(dir, verts[i]);
            //finally rotate vertices so that one of their components became 0 and they may be used as Vector2
            verts[i] = rot * verts[i];
            //store a 2D vector; double check if it is thy z component that is zeroed by the rotation
            all_verts.Add(new Vector2(verts[i].x, verts[i].y));
        } //the above operations may be combined into one
    }
}

//I won't try to write the 2d-hull implementation here.
//You can find one on the net.
var hull = ConvexHull(all_verts);
var area = hull.Area();

This should prevent space stations from acting as efficient accumulators, as their cross sections would create a lot of drag.

[Northstar1989]

Well technically KSP density property still returns a number > 0 when above the 70.000 m but it's incredicably low (< 0.00001 g/L). I simply add a constant (bonus) value (called MinAtmosphericAirDensity) which adds 0.001 g/L to the measured athmospheric density. This is enough to collect significant amount of gas. If you think this is wrong, we can easily tweak the value to a lower/higher value.

- - - Updated - - -

Alright, this is intresting, I could make light gases (like Hydrogen) more abundant the higher you go, this could also add an intresting gameplay meachanic "find the optimal altitude for resource collecting". To implement it, I could first order the gasses by gas density, and spike their abundance at diffent altitudes. The distance between the spikes might depend on the differnce in density between the gases. Now I need some good math formula.

Great, instead of limiting resource collection to 10% above the karman, we might extend it to 1000% (70.000 m - 700.000 m for Kerbin)

That also requires I replace the constant density bonus value (density bonus) by function that zerros out at 1000% above the edge of space. The following simple function might work well enough.

density =  [KSP density property] + max(((vessel altitude / karman altitude ) * -(MinAtmosphericAirDensity * 10) )  + MinAtmosphericAirDensity), 0)

All of these sound great to me. In real life, the Thermosphere starts at 80 km and reaches to 500-1000 km (depending on relative solar activity), so making it run to 700 km sounds good to me...

Intresting, before I started I though it was odd the the Atmospheric Air intake wasn't required for the Atmospheric Scoop to function. We could make the efficiency of the PFA depend on the availability of Atmospheric Air intakes. We should rename it a different name like "Propulsive Fluid Accumulator air intake", to make it clear it's suitable for this Job. We make it require a lot of power. One advantage of this part is that it would make the Propulsive Fluid Accumulator Satelite something dedicated, it would require some serious dedication and design.

Not a bad idea- considering most Propulsive Fluid Accumulators rely on energy-hungry high-vacuum pumps in order to concentrate the gasses they use. However, keep in mind that the Atmospheric Scoop itself already has some intake area built into it, and should be capable of operating on its own at a slower rate. Giving the Atmospheric Intake part the *additional* ability to increase the gathering rate (but also the energy consumption) of a Propulsive Fluid Accumulator would be a good idea. I wouldn't create a separate part, as there's no reason to think a simple intake couldn't funnel the gasses to the same vacuum pump as the intake built into the Atmospheric Scoop part itself- rather the Atmospheric Intake should be toggleable to act as additional intake-area for the Atmospheric Scoop through the right-click menu. This should also be made to somehow work in-atmosphere, so if a player decides to scoop BELOW the Karman Line, the Atmospheric Scoop can still be used to increase their gathering rate (perhaps by allowing the Atmospheric Scoop to convert the IntakeAtm resource into the relevant resource being scooped, at an efficiency depending on the abundance of that gas?)

The important thing about Atmospheric Scooping Ship design (whether a Propulsive Fluid Accumulator that operates near the Karman Line, or a winged vessel that operates lower in the atmosphere) is the ratio of drag to intake area. Intakes necessarily improve this ratio, with a theoretical (but impossible) maximum for a vessel being when 100% of drag comes from the intakes themselves. This same principle actually also applies to spaceplanes- you want the best ratio of intake flow to drag possible (hence the phenomenon of Air Hogging in KSP, which takes this to physically-impossible extremes)- but lift and the necessary to pre-cool air for the engines at high compression-factors also becomes essential in that case...

- - - Updated - - -

I understand but I might found a better solution. I asked allista and he advised my to make A 2D convex of the in vessel in prograde direction.

This should prevent space stations from acting as efficient accumulators, as their cross sections would create a lot of drag.

That's a good way to simulate drag due to cross-sectional area, but there is also such thing as skin friction (although it is a less important factor). A 80 meter long cylinder with a 2 meter diameter encounters more drag than a 8 meter long cylinder with a 2 meter diameter... Ballistic Coefficient * Mass already gives you the drag experienced by the vessel in terms of Newtons...

Keep in mind that a space station could still make an efficient Propulsive Fluid Accumulator if enough of its cross-sectional area were coated with intakes for the scooping unit... (the unit that cools/compresses the harvested gasses to usable densities) Once again, its a matter of total drag vs. intake flow...

Regards,

Northstar

Edited January 28, 2015 by Northstar1989

[FreeThinker]

I wouldn't create a separate part, as there's no reason to think a simple intake couldn't funnel the gasses to the same vacuum pump as the intake built into the Atmospheric Scoop part itself- rather the Atmospheric Intake should be toggleable to act as additional intake-area for the Atmospheric Scoop through the right-click menu. This should also be made to somehow work in-atmosphere, so if a player decides to scoop BELOW the Karman Line, the Atmospheric Scoop can still be used to increase their gathering rate (perhaps by allowing the Atmospheric Scoop to convert the IntakeAtm resource into the relevant resource being scooped, at an efficiency depending on the abundance of that gas?)

Alright, I think I can indeed implement this with exsting part. Basicly the Admospheric Scoop will look at all directly connected (stock) air scoops , and if activated, add its scoops surface area to its internal surface area. This in turn will increase overal power requirement but allow a single Atmospheric scoop to collect resources at an increased rate. But perhaps a specialised part like the one seen on the picture below might be intresting as well

- - - Updated - - -

That's a good way to simulate drag due to cross-sectional area, but there is also such thing as skin friction (although it is a less important factor). A 80 meter long cylinder with a 2 meter diameter encounters more drag than a 8 meter long cylinder with a 2 meter diameter... Ballistic Coefficient * Mass already gives you the drag experienced by the vessel in terms of Newtons...

Indeed, the problem is very complicated, if only I could simply retreive the Ballistic Coefficient of a vessel from a simple API call, perhaps FAR has a function like that ...

Edited January 28, 2015 by FreeThinker

[Northstar1989]

Alright, I think I can indeed implement this with exsting part. Basicly the Admospheric Scoop will look at all directly connected (stock) air scoops , and if activated, add its scoops surface area to its internal surface area. This in turn will increase overal power requirement but allow a single Atmospheric scoop to collect resources at an increased rate. But perhaps a specialised part like the one seen on the picture below might be intresting as well

http://upload.wikimedia.org/wikipedia/commons/2/21/LEO_Propellant_Depot.jpg

Great! Does it work with the KSP-Interstellar "Atmosphere Intakes" as well as the stock air intakes?

Indeed, the problem is very complicated, if only I could simply retreive the Ballistic Coefficient of a vessel from a simple API call, perhaps FAR has a function like that ...

I'd suggest looking at how Atmospheric Trajectories works. I suspect the mod's creator has already found a way to call on the Ballistic Coefficient of the vessel in FAR (the mod actually is designed to work best in FAR), since that would be the main relevant number for determining re-entry profiles...

Also, a slight error on my part- I forgot to mention before that you want the INVERSE of the Ballistic Coefficient when determining fuel consumption. With Ballistic Coeffcient, like ISP, a high number (indicating a streamlined design) is a good thing for a Propulsive Fluid Accumulator. You still need to multiply by mass though- you're interested in the total drag force on the craft (in Netwons/kN), not the rate at which it slows down... So the PROPER formula would be proportional to:

(1/ISP) * (1/Ballistic Coefficient) * Mass * Thermosphere/Atmosphere Density

Alternatively, it might be possible to do something with the Terminal Velocity of the vessel- which already combines Ballistic Coefficient and Atmospheric Density into a single number (keep in mind both BC and Terminal Velocity *are for a particular Angle of Attack*, but you can assume a Propulsive Fluid Accumulator is always heading directly along the surface prograde vector to minimize drag and maximize intake...) The problem is, FAR doesn't calculate Terminal Velocity for any vessel above the Karman Line... (as, when KSP assumes no atmosphere, this number is effectively infinite)

Regards,

Northstar

[Northstar1989]

Hey Freethinker,

I must apologize- it turns out I was wrong about the efficiency and ISP of Nitrogen in a plasma thruster.

Both should be *significantly* higher than I initially guessed. I went back and applied hard mathematics to figure out the correct values... Unlike with a Thermal Rocket, where they're relative, (and ISP should be 75.675% and thrust 132.14% that of the next-heaviest thermal propellant Methane for any given reactor temperature...), it's possible to come up with a single concrete value for the plasma thruster, since a plasma thruster operates effectively the same at all reactor temperatures...

CORRECT Nitrogen efficiency/ISP: 78% efficient, ISP = 3090.34 s

Please see this post for more details:

http://forum.kerbalspaceprogram.com/threads/104943-0-90-KSP-Interstellar-port-maintance-thread?p=1702243&viewfull=1#post1702243

Also, I noticed something else that is off before- you assumed that the new 2.5 meter Nitrogen cryostat you added to KSP-Interstellar should have 4 times the surface area of the 1.25 meter Deuterium/Tritium cryostat, and thus require 4 times the power top operate.

At the time, that comment effectively slipped through the cracks because I had so much else I was thinking about. But unfortunately, that assumption is also wrong for two very important reasons...

First, due to the Square-Cube Law a larger fuel tank (or cryostat) actually has LESS surface area relative to its volume. So, while a 2.5 meter cryostat may hold 4 times the volume per meter of length (if you simply up-scaled the 1.25 meter cryostat, the length also doubled as well though- and it should hold EIGHT times the volume...) it actually has only twice the surface area per unit of length- meaning it has half the lateral surface area per unit of volume. This is looking only at the lateral surface area- the part that will be exposed to the outside environment. The cross-sectional area is indeed 4 times as great (and in many ways this actually dominates the power-usage of the cryostat, as conduction from the rest of the rocket will be the main way heat flows into the cryostat), but the cryostat is also twice as long- meaning it has half the end-plate surface per unit of volume.

So, in summary, the 2.5 meter cryostats should hold exactly EIGHT times the volume of the 1.25 meter cryostats, but require 4 times the power usage IF HOLDING THE SAME PROPELLANT (more about that is a second). You were right about how the power-usage increases with size, but wrong about its relation to volume- which increases even more (by contrast, a cryostat with a 3.75 meter cross-section and triple the length of the 1.25 meter cryostat would hold 27 times the volume, but only require 9 times the power consumption... Square-Cube Law again for ya'...)

The second error was assuming that a Deuterium-Cryostat would require the same amount of power to operate as a Nitrogen Cryostat (of the same size). That absolutely couldn't be further from the truth...

A Deuterium-Tritium Cryostat has to maintain its contents at an EXTREMELY cold temperature: at -249.84 degrees Celcius or colder (as this is its boiling-point of Deuterium, the lighter and therefore more easily boiled of the two Hydrogen isotopes). This requires a LOT of energy. (a Helium Cryostat should require even more power- it has to maintain its contents at -268.9 degrees Celcius or colder, *just* above absolute-zero...)

A Nitrogen Cryostat, on the other hand, only has to maintain its contents at a "modest" -195.79 degrees Celcius or colder. This requires A LOT LESS power.

If one makes the simplifying assumption that one only has to expend an amount of power proportional to the difference between these propellant's boiling points and 0 degrees Celcius, then a Nitrogen Cryostat should only require 78.37% of the power of a Deuterium-Tritium cryostat of the same dimensions.

Add all this up, and a 2.5 meter Nitrogen Cryostat only requires 3.13 times the power of a 1.25 meter Deuterium-Tritium Cyrostat, despite holding 8 times the volume! However we're not done yet! There is still one more minor factor that must be taken into account...

Any fuel tank in a vacuum is a pressure-vessel. Meaning that the mass of the container increases linearly with the volume, despite the Square-Cube Law. This is as larger pressure-vessels require thicker tank walls (the change in ratio in volume: surface area increases the stresses on each square meter of tank wall proportionally). However, thicker tanks walls are better insulators- meaning that even if no additional insulation is added on, the larger Cryostat will have to contend with slightly less heat leaking inside the fuel container!

If we assume this relatively minor effect only decreases heat-leakage by 5% (this is probably under-estimating the effect of nearly DOUBLING the tank wall thickness), and then round-up to stay on the conservative side, then we get the following nice, clean number which I suggest you use:

A 2.5 meter Nitrogen Cryostat should contain 8 times the volume of a 1.25 meter Deuterium-Tritium Cryostat, but only require 3 times the power (15 kW vs. 5 kW) to operate.

Regards,

Northstar

Edited January 30, 2015 by Northstar1989