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Post by Apotheon on Oct 21, 2018 15:50:08 GMT
I've never built an MPD, a MPD spacecraft, or flown the stock ships more than once. Can someone give me an explainer of MPDs? The stock ones suck, with almost no acceleration, which means you can't get close to any planets and they also have this really strange property where adding more MPDs doesn't increase acceleration... how does more propulsion not increase your acceleration? Does it have anything to do with power? With rockets, I can create one, place it on ships, then fly around and do stuff, but with MPDs I don't know what's a good design, how the ships should be designed, or what I should be designing them to do... how close can you get to Earth? LEO is too low... GEO is too low...
If I spawn a stock Scout, unless it's outside the orbit of the Moon, it can only go in circles!
Homecoming is the only Campaign mission with MPDs.
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Post by Pttg on Oct 21, 2018 17:57:29 GMT
MPDs use electric power, so make sure you have enough to power all of them simultaneously (the game doesn't warn you if it can only power one at a time).
As for designing them, it's finicky. I still haven't mastered them.
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Post by AtomHeartDragon on Oct 21, 2018 20:56:51 GMT
MPDs are basically coaxial railguns firing plasma they create by ionising injected propellant.
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Post by doctorsquared on Oct 21, 2018 22:24:52 GMT
MPDs work by ionizing the propellant of your choosing (denser/higher molar mass propellants like xenon, carbon dioxide, hydrogen deuteride, or neon are more effective) which accelerates out the back of the thruster as a jet of particles that push you along. You only see circles around the moon because it takes way longer for the MPD to accelerate the ship out of its orbit than the game is capable of simulating in a single turn. MPDs Pros - - Highly fuel-efficient (consumption for even the highest-powered ones is in the grams to kilograms per second range)
- Light, much smaller and cheaper than other thrusters.
- Cheap.
Cons -
- MPD output is limited by how much power you can pump into them, adding more increases the load on the power supply.
- MPDs accelerate very slowly since they output a continuous stream of particles that accelerate the ship instead of an instantaneous pulse of acceleration like combustion/NTR/resistojet thrusters. When you set a burn with an MPD it takes much longer to accelerate so you can't change speed/heading as quickly as you can with other designs.
- Adding more really doesn't increase the acceleration of your ship by much since that jet of particles is all pushing at the same rate.
A straight MPD build is really only going to be optimal for operating outside of a large gravity well, so if you're hopping between rocks in the Asteroid Belt or traveling through interplanetary space it's a much more efficient option since it consumes less dV to make a maneuver. You'll find that a lot of 'Laserstar' designs use massive gigawatt-level MPDs for propulsion since they're already packing massive reactors to power the lasers and engage at distances long enough to make maneuvering irrelevant.
An alternative option is to build a 'hybrid' system using either a resistojet that consumes the same amount of power as your MPD thruster, an NTR, or a combustion rocket that uses the same propellant. You can select the ship and shut off the other thruster and get the fuel economy of the MPD when you need to travel long distances and then switch over to the more traditional 'rocket' thruster when you need to maneuver within a large gravity well or make quick, fast maneuvers.
Here's a couple of designs, one's an MPD and a resistojet that use RP-1 as a propellant as a part of a hybrid system (RP-1 is quite dense and a large amount of it takes up less space than methane or decane) and one that's designed to be used by itself that uses hydrogen deuteride.
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Post by AtomHeartDragon on Oct 21, 2018 23:22:20 GMT
Resistojets seem to be broken, though, as their thrust power can greatly exceed input power.
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Post by airc777 on Oct 22, 2018 1:01:26 GMT
An MPD will let you burn the wrong way around the sun and let you do planetary transfers in half the time of any other propulsion option.
Pics and craft file.
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Post by treptoplax on Oct 22, 2018 2:39:26 GMT
I've never built an MPD, a MPD spacecraft, or flown the stock ships more than once. Can someone give me an explainer of MPDs? The stock ones suck, with almost no acceleration, which means you can't get close to any planets and they also have this really strange property where adding more MPDs doesn't increase acceleration... how does more propulsion not increase your acceleration? Does it have anything to do with power? With rockets, I can create one, place it on ships, then fly around and do stuff, but with MPDs I don't know what's a good design, how the ships should be designed, or what I should be designing them to do... how close can you get to Earth? LEO is too low... GEO is too low...
If I spawn a stock Scout, unless it's outside the orbit of the Moon, it can only go in circles!
Homecoming is the only Campaign mission with MPDs. The key thing to understand here is that rocket engines are always a tradeoff between mass efficiency and energy efficiency. High-mass-flow, low-power engines have a great thrust-to-weight ratio but burn fuel like crazy. (Because they put little energy into each mass unit they eject). Low-mass-flow, high-power engines have a lousy thrust-to-weight ration but are efficient with fuel. (Because each unit of mass ejected is going at very high speed). High-mass-flow, high-power engines are wonderful in theory but are likely to melt themselves and the ship they're attached to (because there's a huge output of incredibly hot exhaust). This is fundamentally a mathematical/physics limitation, not an engineering one. In CoaDE, chemical engines are high-mass low-thrust; resistojets and nuclear-thermal are low-to-intermediate, and MPD are low-mass, high power. You can probably find some blackbox fusion drive modules if you want high-mass/high-energy (a 'torchship'). Also, low-thrust engines are inconvenient in gravity wells because burns take so long they have to be broken up across multiple orbits or otherwise finagled around.
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