aiyel
Junior Member
Posts: 83 
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Post by aiyel on Oct 25, 2016 13:14:57 GMT
Assuming power consumption is a non-issue, (and one need only peek into the UNLIMITED POWER thread to see that this is, in fact, the case) what's the most raw thrust one can get from a MPD thruster?
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Post by ross128 on Oct 25, 2016 13:56:08 GMT
You mean if your only concern is not physically breaking the thruster? Well, I can technically design one with 299GN of thrust, but you'll need about 12EW (Exawatts) to power it.  |
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Post by redmars on Oct 25, 2016 15:41:25 GMT
Firstly, I should note that power is still in issue, since you need to carry radiators and these add mass, which cuts into how effective your thrust is... If you're looking to build a super-powerful MPD in place of a nuke, you might be out of luck.
That said, my (admittedly brief) experimentation suggests that thrust for even a truly monstrous ion drive would be measured in kilowatts at best. You might be better off with a resistojet.
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Post by wafflestoo on Oct 25, 2016 15:41:26 GMT
The problem with MPD's seems to stem from the radiator mass needed to deal with the reactor's waste heat.
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Post by ross128 on Oct 25, 2016 16:45:39 GMT
Well, it's quite possible to build 100% efficient MPDs, though it's probably a bug.
Thing is though, your typical NTR is putting out several GW of thrust power already, probably at around 5-8 km/s. Your thrust power is your thrust in newtons, times the exhaust velocity in m/s (a N*m is a joule, so a N*m/s is a watt). A high-power MPD is likely to have around 3-5 times the exhaust velocity of an NTR, so for the same power it will have 1/3 to 1/5 of the thrust.
So besides having maybe 1/5 of the thrust for the same power (amazing exhaust velocity though), you're now looking at how you get that power. An NTR gets it by running the propellant over a nuclear pile, extracting the heat, and throwing it right out the back of the rocket. Very efficient, probably 90% or something like that, and conveniently self-cooling.
An MPD is going to be getting its power from a reactor, which will run coolant over a nuclear pile to extract the heat (much like the first part of an NTR), run it over a thermocouple using a turbopump to extract electricity and dump the heat into a secondary coolant, then run the secondary coolant through the radiators with another turbopump to radiate the heat away. Efficiency? Maybe 15% if you're lucky.
So an MPD is going to be burning nuclear fuel at a tiny fraction of the NTR's efficiency, its reactor is going to be bigger and heavier than an NTR of the same power, and then you've got to deal with radiators on top of that. The thruster itself may look amazing, but generating the power to run the damn thing and getting that power to the thruster is what kills it. Resistojets have the same problem, they don't produce their own power (though they don't share the very lopsided thrust-velocity balance).
So building a power-plant to drive an externally powered thruster is simply a losing proposition. Externally-powered thrusters are only potentially viable if you're going to have a huge powerplant for other reasons anyway (like you're building a doom laser), and you'd like to get some more use out of it.
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Post by thorneel on Oct 26, 2016 0:27:22 GMT
Remember that once in tactical combat, where MPDs are useless (and you have to switch to secondary manoeuvring thrusters), you can use your monstrous generators to power weapons.
Corollary: if your weapons require monstrous generators, you can easily add MPDs for strategic movement not requiring point acceleration.
MPDs themselves are essentially free, the actual cost comes from the tanks (and placement, and strategic acceleration is bugged for side-facing multiple thrusters)
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aiyel
Junior Member
Posts: 83
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Post by aiyel on Oct 26, 2016 2:46:27 GMT
Totally not the point. I know they're not effective, I just want to know how good one can be made. I DO have a purpose for them, though, rest assured.
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Post by ross128 on Oct 26, 2016 3:08:52 GMT
Well, after some additional fiddling with the turbopump (to squeeze a bit more performance out of it without shattering the rotor) I managed to make one that had 306GN of thrust at about 77Mm/s. You'll still need about 12EW to power it though (to be more precise the previous one required 11.4EW, this one requires 11.8EW). It has a thrust/weight ratio of about 5k, and it's made almost entirely out of van-chrome steel.
Obviously you want to use Mercury, you're going to get a completely excessive exhaust velocity anyway so using Mercury to convert some of that into thrust is a good play.
Of course, if we're going to go for a more reasonable goal of "something that can be reasonably put on a Doom Laser" rather than straight up "power is no object", the exawatt thrusters can of course be scaled down to gigawatt thrusters. But even using Mercury, a 35GW MPD is only going to put out 570kN (at 119km/s).
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Post by lawson on Oct 26, 2016 5:56:14 GMT
Been playing with MPDs a lot my self. Attached are three of the best designs I've found so far. For high thrust I've found that it's best to use the highest molar mass propellant, and cluster a lot of low power MPDs. For high exhaust velocity you want a low molar mass propellant, 1-2 high power MPDs. |
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Post by lawson on Oct 26, 2016 6:00:02 GMT
Just made a usable MPD with a TWR of 10.8Kg0 While power consumption is reasonable, I don't know if I'd use it on a real ship because it wouldn't survive while saving 750g is just noise compared to generator and radiator weight. Attachments:
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Post by dragonkid11 on Oct 26, 2016 6:17:29 GMT
I still have no idea what to actually used MPD for though.
Besides civilian ship I meant.
MPD just seems well....you have all the delta-V you want but your ship will be really, REALLY slow.
By the way, is it possible to use resistojet as main thruster?
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Post by someusername6 on Oct 26, 2016 6:51:36 GMT
Hmm. It occurs to me you could build a dual-propulsion system on a ship with doom lasers -- keep some Xenon tanks for the long journey, and whatever you were using in combat to manuever. I was playing around with the 10 GW reactor and the 1 GW lasers (a violet variant), and got a huge ship with 10 lasers and a turnaround time in seconds. Once you add some high-TWR MPD and some tanks for that, you also get a large amount of delta V:   |
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Post by cuddlefish on Oct 26, 2016 6:52:48 GMT
I think so, but I'm not sure why you'd want to? The ability to get chamber temperature higher than an NTR exists, but not by much, and it's introducing the inefficiency of needing to cycle the energy through a thermocouple on the way. As far as I can tell the upsides to them are propellant flexibility (because it doesn't need to be a good reactor coolant) and that the engines themselves are very small and light, so you can use many in different locations with minimal weight change (good for 6DOF, not overly useful for single cluster).
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aiyel
Junior Member
Posts: 83
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Post by aiyel on Oct 27, 2016 1:01:41 GMT
Ultimately my intent was to use them on drones that already had serious energy requirements and didn't need to maneuver tactically. It's one of the many solutions in looking into for Giants rather than my Matroyshka system. While otherwise fine, Matroyshka was right up against the mission budget in both mass and cost.
And yes, Matroyshka was exactly what it sounded like. A carrier launched a drone which launched another drone which launched another... Which had a 60mm conventional gun.
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Post by leerooooooy on Oct 27, 2016 18:28:42 GMT
Been playing with MPDs a lot my self. Attached are three of the best designs I've found so far. For high thrust I've found that it's best to use the highest molar mass propellant, and cluster a lot of low power MPDs. For high exhaust velocity you want a low molar mass propellant, 1-2 high power MPDs. "High thrust" The fuck are 60ish kN going to move? a 1 GW reactor is plenty enough to bring acceleration below 0.1g already |
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