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Post by captinjoehenry on Oct 27, 2016 18:32:48 GMT
Yeah seriously I have tried using some ion engines and they don't produce any really usable amount of thrust for orbital maneuvers. They just take far too long to accelerate anything which means you need to do a whole crap load of orbits to get to the one you want. Really duel engine set up is pretty great though. Have main engines that are low efficency but high thrust and have other engines with the same fuel that are more efficient for orbital maneuvers outside of combat.
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Post by Durandal on Oct 27, 2016 19:39:45 GMT
Yeah seriously I have tried using some ion engines and they don't produce any really usable amount of thrust for orbital maneuvers. They just take far too long to accelerate anything which means you need to do a whole crap load of orbits to get to the one you want. Really duel engine set up is pretty great though. Have main engines that are low efficency but high thrust and have other engines with the same fuel that are more efficient for orbital maneuvers outside of combat. How do you arrange them? Combat thrusters on the rear, orbital thrusters on the side (assuming a needle/conventional layout)
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Post by captinjoehenry on Oct 27, 2016 20:55:10 GMT
I use broad side ships and place them on the side. Mind you my ships are fairly heavy and even with 8 GW I can't really get any acceleration of worth out of them. If I had a much lighter ship they might be more useful but I can't imagine they are capable of any sort of major orbital manuver in one burn.
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Post by lawson on Oct 27, 2016 21:53:28 GMT
"High thrust" The fuck are 60ish kN going to move? a 1 GW reactor is plenty enough to bring acceleration below 0.1g already Well for one, the DOOM laser drone shown below. A 1GW reactor, 1GW laser with 40MW beam, defensive missiles, and 10Km/s of delta-V for only 67 tons. Acceleration is a zippy 120mg0. A squad of 14 of these drones doing a flyby at 250Km is a bad day for almost any fleet. Civilian ships and high power light weight long range war-ships are where I plan to use MPDs. So that means laser ships right now and coil/rail gun ships once the physics are fixed. They're NOT for anything with armor. Better to use maxed out NTRs for heavy ships.
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Post by RA2lover on Oct 28, 2016 2:35:41 GMT
Seems like TWR isn't the best measure for MPD performance, but efficiency instead.
New challenge: Create a 99% efficient MPD with the lowest power consumption possible - preferentially with a viable chamber size.
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Post by captinjoehenry on Oct 28, 2016 3:01:11 GMT
Honestly what matters is thrust to power use ratio. MPDs weigh nearly nothing and cost about 0 what matters is how much power you need to pump into them to get a worthwhile amount of thrust.
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Post by RA2lover on Oct 28, 2016 3:30:22 GMT
That's kind of a mixed bag - you also have to take propellant cost into account. Mercury is the obvious choice for thrust-to-power ratios, but it's expensive. For example, take this thruster. 100MW power in, 15 kN thrust out at 12.6km/s. Lawson's laser drone uses 68.5kc worth of xenon to get 10.1km/s of delta-v. Getting 10.1km/s of delta-v with that MPD thruster requires 54 tons of mercury for a total cost of 918kc. That's nearly enough to double the drone's cost. The drone would weigh 98 tons. getting the same initial TMR would need 115.2kN as opposed to 78.7kN.
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Post by redmars on Oct 28, 2016 5:06:26 GMT
I was thinking about using MPDs, but I've had much better results from hydrogen resistojets. It's a similar concept -- electric engines with good ISP -- but the thrust is much more reasonable. You can actually get some ridiculous fuel efficiency out of hydrogen if that's what you want. Plus, hydrolox is great for combat thrusters.
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Post by dragonkid11 on Oct 28, 2016 8:01:43 GMT
Resistojet is kindaaaa the in between road of NTR and MPD.
Kinda, because I'm still tweaking it out.
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Post by draginea on Oct 29, 2016 5:53:32 GMT
I've been using some mercury MPDs and a 9.8t 500MW generator. I never found myself needing to move in combat, since my 500MW laser / coilgun setup kills everything before it gets within 50Km. The acceleration is very low, at 7mg0, but that is more than enough for strategic movement. Also, since mercury is so dense, I cut the surface area of my ships by about a factor of 4, with a corresponding decrease in armor cost and hit chance for the enemy. So while it may not always be true, for my play-style MPDs need very little thrust.
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Post by apophys on Nov 1, 2016 6:05:52 GMT
For something that follows the original premise of the thread, a 100GW MPD with TWR at 122 kg 0 (just because I can): Attachment DeletedIt's probably not very interesting for standard use, because mercury is dead weight in combat (mercury NTRs are bad). Also the TWR is utterly meaningless, because the power generated requires 1,510 tons of reactor alone with my mass-efficient 10GW design. Side note: 100GW through a 10cm disk; it's not a superconductor, so why is it not melting? Now, on the other hand, you can use decane with regular NTRs and/or resistojets, so this 100 MW thing here makes sense out of combat, considering its negligible weight & cost (and 90 o gimbal means it's even mountable radially on the sides to provide forward thrust): Attachment DeletedCode:
MagnetoplasmadynamicThrusterModule 17.6 km/s 100 MW Decane Gimballed Magnetoplasmadynamic Thruster CathodeRadius_m 0.0042 ChamberThickness_m 0.045 AnodeThickness_m 0.001 ThrusterLength_m 0.01 CathodeComposition Vanadium Chromium Steel AnodeComposition Vanadium Chromium Steel InsulatorComposition Polyethylene Propellant Decane Current_A 76000 Injector Composition Lithium PumpRadius_m 0.032 RotationalSpeed_RPM 310 Gimbal InnerRadius_m 0.047 ArmorComposition Silica Aerogel ArmorThickness_m 0.04 ReactionWheels Composition Polyethylene RotationalSpeed_RPM 23000 GimbalAngle_degrees 90
Note: From what I've seen, at the upper end of power, you can basically use any propellant at all to get to a desired thrust/exhaust velocity/power usage point that any other propellant can reach. So it makes sense to use propellants that are inherently interesting (mercury is superdense; decane is dense and very versatile).
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Post by redparadize on Nov 5, 2016 5:49:09 GMT
Seems like TWR isn't the best measure for MPD performance, but efficiency instead. New challenge: Create a 99% efficient MPD with the lowest power consumption possible - preferentially with a viable chamber size. On my part, I am working on a MPD NTR combo that use the same propellant. So far Methane look good enough. I don't plan on having the best MPD, but hopefully something that work within my mothership design requirement.
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Post by cuddlefish on Nov 5, 2016 9:05:36 GMT
Seems like TWR isn't the best measure for MPD performance, but efficiency instead. New challenge: Create a 99% efficient MPD with the lowest power consumption possible - preferentially with a viable chamber size. On my part, I am working on a MPD NTR combo that use the same propellant. So far Methane look good enough. I don't plan on having the best MPD, but hopefully something that work within my mothership design requirement. Curiosity - why an NTR rather than a Resistojet? If you're going to be having a massive reactor to power the MPD, having yet another reactor to power the high-thrust drive rather than just shunting power (given there's little reason to use both at once) seems odd.
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Post by redparadize on Nov 5, 2016 9:33:42 GMT
I won't have a massive reactor, if I did I will need massive flare, carried by even more massive flare missile. Thats what I meant when I said "requirement". I want something of 8MW or so. Just to get a extra leg when empty. But I could indeed have a extra reactor and deactivate it in battle.
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Post by cuddlefish on Nov 5, 2016 9:45:31 GMT
For clarity, by 'extra reactor' I meant the one built in to an NTR.
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