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Thrust to weight ratio of a magnetoplasmadynamic thruster?


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Trying to balance the ones in Near Future Propulsion for the Real Fuels plugin. Can't find any solid info online. The wiki says one was tested on the "Japanese Space Flyer Unit" as part of EPEX in 1995 but I can't find any straightforward stats on that engine either. Anyone know? If there isn't any data on that particular unit what about theoretical stats for one that could be used on a manned interplanetary transfer vehicle?

Edit: Also I'm guessing the TWR probably depends on the amount of power available. Let's assume the MPD in question is designed to utilize a 1MW power source.

Edited by The Pink Ranger
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There's very little solid info out there as there's comparatively little practical work done on MPDTs. Ion thrusters and hall thrusters are much preferred test subjects since you can actually power them. A MPDT on the other hand requires a lot of juice to get going, which mandates either solar panels exceeding the size of those on the ISS (all of them together!), or an onboard nuclear reactor. Neither of which is readily available for test vehicles, and even laboratories struggle to pull that much current. So they mostly just test subscale models and do a whole lot of theorycrafting.

What How2FoldSoup quoted is a fairly rough general guideline, as you can build electric propulsion to a whole lot of specs, down to the uselessly small. And there's only an upper bound in the form of your engine glowing itself to slag from the current throughput. But it's probably the best you're going to get, and somewhat useful in the sense that you need to balance them in relation to other engines anyway. Probably start with a power usage that seems sensible for a MPDT of that size, find where on Mr Choueiri's scale that is, and estimate what thrust you get at that point in the scale.

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Typical ion drives are a little better than 1mN/kg. MPD can be several times better. An order of magnitude isn't a stretch. The Atomic Rockets lists a design giving you a bit over 10mN/kg. I find that to be quite plausible. So if you need a conservative estimate, I'd go with 10 to 15mN/kg.

That said, vanilla ion drive in the game is 8N/kg, which is absurdly high TWR for an ion drive. So you should either scale TWR of that way down, or scale relative to that. The problem with using realistic ion drive TWR, and the reason it's so high in the game, is that it would take forever to accelerate anything. And unlike real world, you can't run an ion drive for a year to get a probe to go where you need it to. If you are dealing with a mod that takes care of that, allowing high physics time warp, then you can go with realistic value for both. Otherwise, I would advise increasing TWR of the MPD accordingly.

If you want MPD to compete with vanilla ion, go with 30 to 100N/kg, increasing power drain accordingly. That would be proportionally correct, and make use of MPD drive practical, yet balanced.

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That said, vanilla ion drive in the game is 8N/kg, which is absurdly high TWR for an ion drive.

Err nope, that's 8N/ metric ton, or 8 mN/kg, which kinda puts it in the MPD range.

[Edit]

Wait. You're right, it really is 8N/kg in the game, since it's using kN as thrust unit. Never mind.

Edited by Jesrad
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the problem with mpd is its not the mass of the thruster, its the mass of the power supply and the thrusters (clusters are preferable for fault tolerance). i think they would be most viable for use with a small scale fusion powerplant, preferably one with direct conversion. solar with a lot of panel area or fission/fusion with a lot of radiator area are also options, but not as good. solar/radiator panels and their support structures can get heavy and needlessly complicated.

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That said, vanilla ion drive in the game is 8N/kg, which is absurdly high TWR for an ion drive.

The TWR isn't even the worst of it... the really absurd part is the power draw (or lack thereof) :P

This is why Near Future Propulsion ships a MM config to nerf it. You simply cannot have a mod that adds a sensible electric propulsion progression if you leave the PB-ION at stock levels.

the problem with mpd is its not the mass of the thruster, its the mass of the power supply and the thrusters

And this is why I had to invent all sorts of new performance indicators while balancing NFP :P One of them is "operational TWR", which basically adds the mass of the power supply system required to the mass of the engine and calls the whole thing "drive section". All engines are balanced according to OpTWR, and regular TWR is completely ignored.

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  • 1 year later...
On 29/08/2014 at 11:50 PM, K^2 said:

Typical ion drives are a little better than 1mN/kg. MPD can be several times better. An order of magnitude isn't a stretch. The Atomic Rockets lists a design giving you a bit over 10mN/kg. I find that to be quite plausible. So if you need a conservative estimate, I'd go with 10 to 15mN/kg.

That said, vanilla ion drive in the game is 8N/kg, which is absurdly high TWR for an ion drive. So you should either scale TWR of that way down, or scale relative to that. The problem with using realistic ion drive TWR, and the reason it's so high in the game, is that it would take forever to accelerate anything. And unlike real world, you can't run an ion drive for a year to get a probe to go where you need it to. If you are dealing with a mod that takes care of that, allowing high physics time warp, then you can go with realistic value for both. Otherwise, I would advise increasing TWR of the MPD accordingly.

If you want MPD to compete with vanilla ion, go with 30 to 100N/kg, increasing power drain accordingly. That would be proportionally correct, and make use of MPD drive practical, yet balanced.

Here I am, on a Friday night, having a scotch and feeling annoyed that the wiki has the thrust, but not the weight, of an MPD drive. Google around, and obviously the answer is in here (with an obligatory reference to @nyrath, of course). I love this forum :)

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16 hours ago, monstah said:

Here I am, on a Friday night, having a scotch and feeling annoyed that the wiki has the thrust, but not the weight, of an MPD drive. Google around, and obviously the answer is in here (with an obligatory reference to @nyrath, of course). I love this forum :)

Just keep in mind that MPDTs are not miracle engines; they do the exact same thing all other electric engines do as well. They ionize a propellant into the plasma state and then shunt it out via a magnetic nozzle. And as such, they are bound by the same rules as all other electric engines, which result in an inverse relationship between Isp and thrust/power ratio.

Engine                           Source     Year    Power      Thrust      Isp      Thrust/Power
SPT-100B hall thruster           Fakel      1994    1.35 kW    0.082 N     1,600s    0.061 N/kW
BHT-8000 hall thruster           Busek      ????    8.0 kW     0.449 N     2,210s    0.056 N/kW
NSTAR ion thruster               NASA       1990    2.3 kW     0.092 N     3,120s    0.040 N/kW
NEXT ion thruster                NASA       2004    6.9 kW     0.236 N     4,190s    0.034 N/kW
VX-200 VASIMR                    Ad Astra   2010    200 kW     5.000 N     5,000s    0.025 N/kW
HiPEP ion thruster               NASA       2003    39.3 kW    0.670 N     9,620s    0.017 N/kW
Dual-Stage 4-Grid ion thruster   ESA        2005    250 kW     2.500 N    19,300s    0.010 N/kW

Basically: you get somewhere between 0.01 and 0.06 N per kW of power input. How much precisely? Depends on where you put your Isp, take your pick. Doesn't matter who you are, or how old or new your technology is. You're going to end up on the same curve - the laws of physics say so.

As such, talking about TWR is problematic. Of course a MPDT could get 10 times the TWR of a gridded ion thruster, if you compare a MPDT with low Isp against an ion thruster with high Isp. The latter will naturally have low thrust - but not because ion thrusters have poor TWR. Rather, it has low thrust because it has a high Isp.

And keep in mind that the power generation equipment is usually dominating the engine weight anyway. That DS4G prototype? Less than 10kg in mass. Now consider how much mass you need to invest to get 250 kW of solar power; then double that figure to account for the power distribution unit (PDU). Does it really still make sense to talk about the TWR of the engine alone...? :wink:

 

Edited by Streetwind
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On 8/29/2014 at 10:25 AM, Streetwind said:

The TWR isn't even the worst of it... the really absurd part is the power draw (or lack thereof) :P

This is why Near Future Propulsion ships a MM config to nerf it. You simply cannot have a mod that adds a sensible electric propulsion progression if you leave the PB-ION at stock levels.

And this is why I had to invent all sorts of new performance indicators while balancing NFP :P One of them is "operational TWR", which basically adds the mass of the power supply system required to the mass of the engine and calls the whole thing "drive section". All engines are balanced according to OpTWR, and regular TWR is completely ignored.

I was playing around with real ion numbers (namely for the NEXT) and it appeared that whatever the rated mN/kg was, divide it by a factor of 10 for the solar panels used (assuming in Earth orbit, you might do ok going closer to the Sun.  NTRs are likely to be even worse).  Note these simply used the ISS solar panel numbers, so involved a bit more structural parts (but still should be close to your needs).  But still, each kg of NEXT needed about 1kg of solar panels, and other means could easily have worse thrust/mass(solar panels).

In the end, you probably want more mass in cargo than in solar panels, so your effective TWR decreases and dry mass increases.  Expect to bring a lot more Xe than you ever imagined (although vastly less than hydrazine or any other appropriate chemical), I think 1-2kg of Xe gets you roughly 1kg (cargo) from LEO to LTO.  Note that for sufficiently long-term planing, it would certainly be possible to build three solar-generated beamed-power units in GTO (actually I think four equally spaced in LEO would provide one bird which is not eclipsed by the sun and still in line of sight.  They might want to be a *little* further out, but not by much) and use them for an ion-powered space tug.  Don't forget your tug still needs *radiators* to remove the heat even if it doesn't have solar panels.

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