Orbital Transfer Vehicle, Ion engine powered or Nuclear powered?

[8749236]

Currently I have two design, both has a payload of 10 Tons with dV 4500m/s

But the first one has dry mass of 6~ tons, wet weighs 21~, nuclear powered.

The second one, the ion engine powered, has dry mass of 16~ tons! and wet mass of 23~ Tons.

Half of the dry mass are contributed by the huge solar panel and nuclear reactor along with the big capacity battery system.

That is a lot of dead mass and it have a terrible TWR of 0.05 when carrying 10T of payload.

Does that really worth it? In terms of economy?

Mods Related: NearFuture Propulsion

Edited April 1, 2014 by 8749236

[michaelphoenix22]

wait until the next patch ION engines are getting a buff. My crystal ball says it should drop soonTM.

[vger]

I do have to wonder what the benefits/drawbacks of each will be. It seems like those two engines are more or less suited to the same purpose. Both feel like 'end game' engines, both capable of taking you pretty much anywhere. Does it all just boil down to personal preference?

[Red Iron Crown]

Currently, the only economy that matters is player time, so unless you love doing those long ion burns I would suggest you go with the nukes.

[Random Tank]

If you want efficient, go for a few Dual Stage 4-Grid Thrusters (19300 Isp!!), but if you want decent TWR, use the 12 HI-SNAP part, its got something like 8200 Isp and 28kN of thrust? Awesome, they make NERVA's pointless!

[Jouni]

I'm currently using nuclear engines in transfer stages, but I'm getting fed up with the long burn times. When the next update comes, I'll probably start using liquid boosters as transfer stages when possible. With 10 tonnes of payload, you get almost 3900 m/s of delta-v, with initial (Kerbin) TWR 4.25.

[Champ]

If you want efficient, go for a few Dual Stage 4-Grid Thrusters (19300 Isp!!), but if you want decent TWR, use the 12 HI-SNAP part, its got something like 8200 Isp and 28kN of thrust? Awesome, they make NERVA's pointless!

Don't know which mod you are talking about, but that is exactly why i don't use mods that give new engines. Seams like cheaty.

[cantab]

How many ion engines are you running? I'm pretty sure you shouldn't need a nuclear reactor to power them, unless you plan on going really far from the Sun or you've got loads of engines. Presumably there does come a point where adding more ion engines in order to increase TWR stops being worth it.

[Random Tank]

Don't know which mod you are talking about, but that is exactly why i don't use mods that give new engines. Seams like cheaty.

Nearfuture Propulsion, the one the OP said in the first post...

I wouldn't say it's cheaty, the 12 HI-SNAP engine part requires 780e/sec, which is.... a lot. All it does is to add more variety to Ion engines... The highest thrust engine in the pack is 210kN, and it uses 6500e/sec with and ISP of 3000.... which is..... some...

[biohazard15]

Don't know which mod you are talking about, but that is exactly why i don't use mods that give new engines. Seams like cheaty.

Actually, NearFuture is far from cheaty. Sure, it got all kinds if huge Isp engines, but these require an exorbitant amount of electricity (which comes from nuclear reactors - and they are heavy).

Speaking of OP, all engines from NFPP (except for big VASIMR, maybe) doesn't suit for heavy cargo hauling - much of your weight will be comprised of things required to run the engine. Use them for satellites, interplanetary probes, small orbital crew transfer vessels with external seating - in short, things that doesn't weigh much.

[Starstrider42]

Currently I have two design, both has a payload of 10 Tons with dV 4500m/s

But the first one has dry mass of 6~ tons, wet weighs 21~, nuclear powered.

The second one, the ion engine powered, has dry mass of 16~ tons! and wet mass of 23~ Tons.

Half of the dry mass are contributed by the huge solar panel and nuclear reactor along with the big capacity battery system.

That is a lot of dead mass and it have a terrible TWR of 0.05 when carrying 10T of payload.

Does that really worth it? In terms of economy?

Mods Related: NearFuture Propulsion

I actually used a similar design for an ion-powered Dres probe -- an ion engine pushing 10 tons, about half of it xenon. I didn't realize until I got into orbit that, with a TWR of 0.1, it would take more than a full orbit to get to escape speed. By the time I got into a high enough orbit that I could do a reasonable escape burn, I'd already used too much fuel and had to scrap the original mission.

So my advice is to go with the nuclear engine, and save the ion engine for light (up to 2-3 tons) spacecraft. It's not just a question of player patience, but whether you actually have time to do the maneuvers you need your ship to do.

Edited March 31, 2014 by Starstrider42

[vger]

So my advice is to go with the nuclear engine, and save the ion engine for light (up to 2-3 tons) spacecraft. It's not just a question of player patience, but whether you actually have time to do the maneuvers you need your ship to do.

How about a hybrid? Use the nuke while orbiting smaller bodies and switch to ion when the sun takes over?

[biohazard15]

How about a hybrid? Use the nuke while orbiting smaller bodies and switch to ion when the sun takes over?

Too much deadweight. You'll lose nuke dV, and ions will struggle to push nukes, fuel and payload.

[Starstrider42]

How about a hybrid? Use the nuke while orbiting smaller bodies and switch to ion when the sun takes over?

I think it depends on the mission plan. It probably can be made to work, especially if you can discard the nuclear engine once you're done with it.

(To clarify: my example was specific to large burns around Kerbin. For smaller planets, with smaller delta-V requirements, an ion engine might still cut it. It's just important to remember that acceleration actually can matter for things other than launch and landing).

[Dispatcher]

Its preferences. If you want the best iSP and mass savings, use ion and expect burns that take ages to complete. If you want burns taking more reasonable timeframes, use LV-N and be prepared to launch more mass; however, the LV-N has the best iSP of the current stock chemical rocket engines.

[8749236]

to ALL of you complaining LONG BURN TIME (excuse me for caps..)

Go here --> http://forum.kerbalspaceprogram.com/threads/70881-0-23-Orbit-Manipulator-Series-(WIP)-(Updated-March-12-2014)

NBody simulation still sucks..

But warppable engine works... It lets you fire your ion engine while time-warp (non-physic)...

Thats how I ended up designing Ion Propelled Vehicle...

Don't know which mod you are talking about, but that is exactly why i don't use mods that give new engines. Seams like cheaty.

I've already stated the Mod =\

I'm currently using nuclear engines in transfer stages, but I'm getting fed up with the long burn times. When the next update comes, I'll probably start using liquid boosters as transfer stages when possible. With 10 tonnes of payload, you get almost 3900 m/s of delta-v, with initial (Kerbin) TWR 4.25.

The link

If you want efficient, go for a few Dual Stage 4-Grid Thrusters (19300 Isp!!), but if you want decent TWR, use the 12 HI-SNAP part, its got something like 8200 Isp and 28kN of thrust? Awesome, they make NERVA's pointless!

Haven't unlocked them yet... And yeah, I'd look forward to it...

wait until the next patch ION engines are getting a buff. My crystal ball says it should drop soonTM.

Today I buffed it myself =P

How about a hybrid? Use the nuke while orbiting smaller bodies and switch to ion when the sun takes over?

Tried it, seems complicated for me and non-stock-alike.. so I scrapped it...

I think it depends on the mission plan. It probably can be made to work, especially if you can discard the nuclear engine once you're done with it.

(To clarify: my example was specific to large burns around Kerbin. For smaller planets, with smaller delta-V requirements, an ion engine might still cut it. It's just important to remember that acceleration actually can matter for things other than launch and landing).

The mission is an cargo-tug...

Used to move huge pieces within Kerbin's SOI or some other planet's SOI....

Or do a two-way-trip or even one-way-trip to other body...

It is supposed to be General Purpose and left in orbit for years...

And for acceleration (TIME!), see the link I gave in this reply...

Its preferences. If you want the best iSP and mass savings, use ion and expect burns that take ages to complete. If you want burns taking more reasonable timeframes, use LV-N and be prepared to launch more mass; however, the LV-N has the best iSP of the current stock chemical rocket engines.

Time frame does not matter, but thanks for the advice...

Today I just tried a refueling mission to my previously expended lander in orbit for another landing on Mun, I noticed that even my biggest rocket had to launch at full payload in order to resupply it...

I think i'll go Ion? Because the LV-N powered version requires 15~ tons of fuel for 4500m/s where the ion powered version only needs half...

And I don't have a launching system that fires 15~ tons of trash into orbit yet...

At the end, see the HoneyFox's Orbital Manipulator Series for WARPPABLE Engine!...

You can actually Warp while Ion Engine still Burning!...

Edited April 1, 2014 by 8749236
highlight...

[Starstrider42]

And for acceleration (TIME!), see the link I gave in this reply...

I already knew about Orbit Manipulator; that's not the point. If you have only two hours (of in-game time) to make a burn that takes four hours (again, of in-game time) to complete, you won't get enough delta-V even if you warp through it in a couple seconds of player time.

[8749236]

I already knew about Orbit Manipulator; that's not the point. If you have only two hours (of in-game time) to make a burn that takes four hours (again, of in-game time) to complete, you won't get enough delta-V even if you warp through it in a couple seconds of player time.

Multiple burns body...

I can set up a burn that takes 2 or more times to complete...

I don't need to finish them all at once...

That is how I do it even with LV-N...

[Jouni]

Multiple burns don't necessarily help that much. If you start from LKO, the preliminary burns can't be more than 950 m/s, or you'll end up escaping Kerbin too soon. When going to Jool, for example, you still have to burn around 1000 m/s near the last periapsis, if you don't want to waste delta-v.

[RockoDyne]

Unless you have a well defined mission planned out, you'll probably be better off with nukes. I'm just thinking that if you wanted to conserve/refuel/make use of whatever fuel you had around, then nukes will be the thing that didn't waste anything, but that's just my preference these days. The only serious question in all this though is can you tolerate the burn time and if you can, then it doesn't really matter.

[Streetwind]

Currently I have two design, both has a payload of 10 Tons with dV 4500m/s

As a cargo tug, stick with the LV-Ns, at least until you unlock the high TWR magnetoplasma thrusters; then you can check again if electric is worth it. With only the stock ion and Near Future's starter engines, it will definitely not be worth it from a TWR standpoint.

However, if your mission profile consists of "only" 4km/s dV max (the remaining 500 being hopefully reserved as buffer), then electric propulsion isn't really being played to its strengths. The point of those engines is to give you 20-30 km/s dV (yes, you read that right) by substituting abundant electric power for some of the reaction mass. You then use that massive amount of dV to dramatically shorten interplanetary trips, or to build vessels capable of long-term missions and grand tours without refueling. Short-range cargo tugs with access to regular refueling don't gain anything from that massive dV potential but you still get stuck with the long burn times. Better use something with more thrust.

Now, the MPDTs have TWR ratings close to some of the lower-end chemical rockets, more than twice as high as the LV-N. Those might be a useful upgrade, even with the added mass of the reactor, but you'll have to wait and see until you research them.

[Skorpychan]

Nuclear engines. I use them on cruise stages to tug craft around the solar system, and it works wonders. Just create a space tug from a fuel tank with nuclear engines mounted on pylons, and dock the payload behind it.

[8749236]

As a cargo tug, stick with the LV-Ns, at least until you unlock the high TWR magnetoplasma thrusters; then you can check again if electric is worth it. With only the stock ion and Near Future's starter engines, it will definitely not be worth it from a TWR standpoint.

However, if your mission profile consists of "only" 4km/s dV max (the remaining 500 being hopefully reserved as buffer), then electric propulsion isn't really being played to its strengths. The point of those engines is to give you 20-30 km/s dV (yes, you read that right) by substituting abundant electric power for some of the reaction mass. You then use that massive amount of dV to dramatically shorten interplanetary trips, or to build vessels capable of long-term missions and grand tours without refueling. Short-range cargo tugs with access to regular refueling don't gain anything from that massive dV potential but you still get stuck with the long burn times. Better use something with more thrust.

Now, the MPDTs have TWR ratings close to some of the lower-end chemical rockets, more than twice as high as the LV-N. Those might be a useful upgrade, even with the added mass of the reactor, but you'll have to wait and see until you research them.

DS01 is already available, but its isp is way too low while consuming huge amount of electricity which I cannot provide it with...

I guess I'll stick with LV-N but since I am playing with RT2, 20-30km dV seems point less since I do not have a well constructed deep space network to support such a mission yet while only 4km dV are required to reach the farthest distance where my signal can reach. (Duna or some where between Duna and Dres..) For manned mission it is possible, but only I'm now way on unmanned side so manned tech sucks =(..

Maybe I should try a alternative design such as a huge battery bank?

Nuclear engines. I use them on cruise stages to tug craft around the solar system, and it works wonders. Just create a space tug from a fuel tank with nuclear engines mounted on pylons, and dock the payload behind it.

I used that design before, having a 100 ton payload rated space tug that can bring me to anywhere...

But refueling is a pain-in-the-****..

So everytime I crash it into kerbin or something instead of refueling it..

Edited April 1, 2014 by 8749236
Merging reply..

[Streetwind]

3600 Isp is not "way too low". It's 4.5 times that of a LV-N! That is a big deal, especially when it has a TWR of 9.84. For comparison: the LV-N has 2.72, the Poodle has 8.97, the LV-909 has 10.19, and the PB-ION thruster has... wait for it... a whole 0.2. In other words: the DS01 has nearly ten times the Isp of other engines with comparable TWR. That is really, really, really good.

You may however be getting deceivingly low dV ratings from it anyway. The reason for that is that it runs on hydrogen, which has an abysmal density. Even the biggest tank you can mount has hardly any significant mass in it. And because the rocket equation looks at mass, and not at the number of arbitrary units of fuel that the fuel tank uses to count its contents, that means you don't get much dV from just one tank. If you want to go places with hydrogen, you need to build big. Really big. Bundle up those 2.5 meter tanks. The fun fact is that this won't actually make your vessel heavier than if you used a single small liquid fuel tank. It will just be much larger. In fact, because the Isp is so much higher than that of a LV-N, and the thruster itself is so much lighter too, the ship will weigh a whole lot less despite being much larger.

But then, yeah, the power generation. The plasma thrusters buy their impressive TWR by paying in electric charge. Let's calculate an example:

Mounting three DS01 MPDTs and one MX-1 reactor weighs 6.87 tons, plus weight for the radiators, which I don't know off the top of my head. Let's guess blindly and say 7.5 tons for the whole assembly. That gives you 84 kN of thrust, for a "propulsion section only" TWR of 1.14. Compare that to the LV-N: 2.72 for the propulsion section (in this case only itself). For the weight of the plasma thruster/reactor assembly, you could mount three and have a little weight left over.

However, now comes the Isp difference. You say your LV-N fueled craft weighs 21 tons wet and 6 tons dry, and is meant to carry 10 tons of payload. Let's quickly check the rocket equation: 800 * 9.82 * ln(31/16) = ~ 5196. That doesn't really match up, to get 4500 dV you'd need about 17.5t dry mass + payload, not 16. Or the wet mass is lower, but that's ultimately arbitrary, so let's just go with this. Assuming one LV-N in those 7.5t dry mass gives a total vessel TWR of 0.197; if it's two LV-Ns, then that's 0.394.

Replacing the LV-N's with the MPDT/reactor assembly, assuming one LV-N we remove 2.25t and add 7.5t. Together with the 10t payload that makes 22.75t dry mass. How much fuel mass does this need to get 4500 dV? 4500 = 3600 * 9.82 * ln ((22.75+x)/22.75), solve for x. The result is roughly about 3.1t, for a wet mass of 15.85t, or 25.85t with payload. Note how this is less than the 31 that the nuclear rocket tug features. In fact, since the MPDT assembly gives 84 kN thrust instead of the 60 of one LV-N, the total vessel TWR with payload is now 0.331 - which is almost 70% higher than the LV-N! The reason the MPDT craft weighs less despite the huge reactor is all down to the fact that the Isp is so high that it requires barely any fuel mass.

What if the original tug had two LV-N's? Then the new dry mass is 10.5t plus payload, and total vessel mass with payload and enough fuel for 4500 dV would be 23.3t, which gives a total vessel TWR of 0.367. This is slightly less than the twin LV-N config, showing that higher-thrust, lower Isp engines still have a good reason to be used in certain cases. But the MPDTs are certainly competitive still, and if there was a higher dV requirement (such as longer distance or higher payload) they would once again come out on top. You could double the dV of this ship and the vessel TWR with 10t would still be over 0.3 - not something you can easily match with LV-Ns.

Now, if you paid attention, you'll have noticed that the MX-1 reactor is actually slightly short of being able to supply three DS01 MPDTs. They would drink 2250 Ec/s, and the reactor outputs 2000 Ec/s. The remaining 250 could be supplemented by a MX-4 reactor, or solar power and/or by utilizing buffer storage. Near Future actually adds parts predestined for this, namely the capacitors. They store 8 times as much Ec per weight as normal batteries do, but cannot be drained directly; they must dump their contents into regular batteries for it to be used. So what you do is have enough regular battery space to receive one capacitor worth of power, and then just stack a large number of capacitors and dump them one by one when necessary during burns. Then afterwards the capacitors can be recharged. This will of course add weight, but that should be under 2 tons so the craft will still easily remain over 0.3 TWR.

All in all, this shows that electric propulsion using the DS01 MPDT can be competitive to LV-Ns even in a medium dV light cargo tug application, but technically plays its real strength only when more dV (or bigger payloads) are required.

[Skorpychan]

I used that design before, having a 100 ton payload rated space tug that can bring me to anywhere...

But refueling is a pain-in-the-****..

So everytime I crash it into kerbin or something instead of refueling it..

Dock it to a station or something, or fly disposable tankers up to it to restock it with fuel. Mechjeb can handle rendezvous nicely, but docking is best left to manual controls.