Kerbin to Minmus space tug?

[M_Rat13]

Hello,

I am relatively new to KSP (on Xbox one). I've been going through the tutorials and messing round in creative and feel I'm improving little by little.

Now I have set a challenge for myself. I want make my stamp on my first step to space, the Kerbin system, by having space stations around Kerbin and Minmus, and having a mining colony on Minmus to help me on my way to reaching further into the Kerbol system.

My main problem is: I can get into orbit with a spaceplane, and have a small orbital monoprop tug tug design to start building the Kerbin station, but I need a more efficient tug to get large space parts to Minmus. The max weight for my loads is likely 50 tons (fuel tank system). 

So, what type of tug would you recommended for taking these type of loads to Minmus? Please bear in mind the tug has to fit into a mk 3 cargo bay.

Added pictures of your designs would be appreciated. Please also understand this is mostly a discussion of ideas, so my final design may be a joining of different ideas.

So with that out of the way, what are your ideas?

[Snark]

Moving to Gameplay Questions.

[Jarin]

Fitting in a Mk3 cargo bay is a toughie, considering I prefer my tugs to use the Mk3 LF fuselage as a core. I'll take a look and see what I can hash out, but are you willing to look at rocket-lifting a larger design?

 

Edit: Tossed together a quick design. Since KER for 1.2 isn't out yet, I'm not sure what the dV is (the math makes my head hurt). This could be terribly inefficient. 

Just barely fits into 2 mk3 cargobays, with a docking port in the bay to hold it in place. Total weight 33 tons

 

Edit2: 5192 dV, with 240kn (0.73 to 1.4 unloaded TWR) of thrust. Could be worse.

Edited October 3, 2016 by Jarin

[M_Rat13]

39 minutes ago, Jarin said:

Fitting in a Mk3 cargo bay is a toughie, considering I prefer my tugs to use the Mk3 LF fuselage as a core. I'll take a look and see what I can hash out, but are you willing to look at rocket-lifting a larger design?

I've never been good with rockets weirdly. I can use them in the tutorial as they tell you what you need to do, but when flying a rocket or in fact building, I don't really know what to do, but I try to learn.

Space planes being single stage (and the fact I understand regular plane mechanics pretty well), make them easier for me personally to fly.

I'm pretty weird compared to most players in that regard. 

I suppose eventually I'll learn, but I prefer working with things I know for certain before I start with harder things.

Edit: I don't know if it's just affecting me but my posts are being hidden till looked at by a moderator.

But looking at the edited post, do I really need that much thrust/delta-v to get to Minmus with my load requirement?

I may be new to this but from what I've read/watched on YouTube, that sounds enough to get to Duna, maybe I'm missing something. And I don't mind a slow burn time if it means I can carry less fuel.

Edited October 3, 2016 by M_Rat13
New information posted

[Jarin]

Honestly, my favorite strategy is to use Extraplanetary Launchpads and just build large ungainly things in orbit rather than trying to lift them off of Kerbin. Then you only need to launch the relatively simple rocket part containers to your spacedock.

[M_Rat13]

1 hour ago, Jarin said:

Honestly, my favorite strategy is to use Extraplanetary Launchpads and just build large ungainly things in orbit rather than trying to lift them off of Kerbin. Then you only need to launch the relatively simple rocket part containers to your spacedock.

I might use that idea later down the line, when I have more experience under my belt/want to explore further. However, that seems much more long term than what I am currently aiming at.

The designs I have are small enough to where a normal laugh works just fine. My space station design would be small enough for two launches, one for the main body (which would include power), and one for the fuel tanks, so for now a tug works. 

I could just have a nuke powered space plane (my only other option other than getting better at rockets), but the added weight of nukes which are useless till you get high enough around Kerbin, just seems to give me diminishing returns as of right now.

So thanks for this idea, but currently I'd like to stick to the question I've asked, if that's ok with you.

[Palaceviking]

2 hours ago, Jarin said:

Fitting in a Mk3 cargo bay is a toughie, considering I prefer my tugs to use the Mk3 LF fuselage as a core. I'll take a look and see what I can hash out, but are you willing to look at rocket-lifting a larger design?

 

Edit: Tossed together a quick design. Since KER for 1.2 isn't out yet, I'm not sure what the dV is (the math makes my head hurt). This could be terribly inefficient. 

Just barely fits into 2 mk3 cargobays, with a docking port in the bay to hold it in place. Total weight 33 tons

 

Edit2: 5192 dV, with 240kn (0.73 to 1.4 unloaded TWR) of thrust. Could be worse.

Use an adapter to reduce the number of lvn's (cos they're bloody heavy) and that might make 6k-7k dv.

[Aegolius13]

Other than nukes, your other major option would probably be a Poodle based rig.  But with loads as heavy as 50 tons, nukes will likely come out ahead. Looks like you'd need more than half an orange tank to move 50 tons to Minmus.

If and when you move into heavy rockets, I'd consider skipping the tug for Minmus runs and just building an upper stage with enough delta v to get there. But I'm lazy and try to avoid docking.  

Duna actually takes very little more delta v to reach than the moons, if you hit a good launch window.  Especially if you aerobrake to avoid the capture burn.  It's more expensive to return from, though.

[M_Rat13]

11 minutes ago, Palaceviking said:

Use an adapter to reduce the number of lvn's (cos they're bloody heavy) and that might make 6k-7k dv.

What if you halved the fuel load also? Not only would that bring TWR back up a little, wouldn't it also not lose too much delta-v due to the reduced weight?

Also, do you have to use nukes just for Minmus? You can get to the Mun without them, and I didn't think Minmus was that much further away, but I might be wrong.

Edit: Seems someone just answered my question, but isn't that assuming using the same thrust? How do say, terriers, perform, especially if I decided to cut down to two of whichever engine I chose.

Granted it'd be a long burn time but that doesn't bother me, because in the end I'm more efficient so use less fuel, especially since I'd be refuelling at space stations, distance between fill ups would be nice if it were higher, but a round trip is the minimum.

Edited October 3, 2016 by M_Rat13
New information posted

[Aegolius13]

Two Terriers might slightly beat out a Poodle in delta v, but your TWR would be quite low.  Not even sure about that since you lose a little ISP. I would not do three  Terriers, and certainly not four (Poodle beats four Terriers in thrust, mass and ISP).  You could also look at the Dart as an in-between option vs the Poodle and Terrier.  

However, the heavier your payload, the more ISP matters and the less engine mass matters.  Which is why nukes may make sense in this role, even though you're not going very far.  If you're looking at a total mass of 70 tons or whatnot, another ton of engine for Poodle vs Terrier is just not that big of a deal.  That's a long way of saying this will take a lot of fuel if you use any LF engine.

 

Edited October 3, 2016 by Aegolius13

[Jarin]

1 hour ago, M_Rat13 said:

Granted it'd be a long burn time but that doesn't bother me, because in the end I'm more efficient so use less fuel, especially since I'd be refuelling at space stations, distance between fill ups would be nice if it were higher, but a round trip is the minimum.

The problem is the large amount of weight you're wanting to move. 50 ton payload is well into the range where nukes give much better efficiency than other engines. Throwing TWR out of the equation, the LV-N is better from 500 dV on up. 

See other charts here. Those numbers should be solid until 1.3 or whenever the new engine overhaul is fully released. 

Edit: If you want to push to .4 or .5 TWR, then you might be better off with the 909 or the poodle, but only if you run with less than 1500 dV, which cuts your margins pretty close.

1 hour ago, Palaceviking said:

Use an adapter to reduce the number of lvn's (cos they're bloody heavy) and that might make 6k-7k dv.

The problem with the tree structure of KSP craft assembly is that you can go from fewer lines to more, but not more to fewer. I can't just use another quadcoupler to bring that back down to mount fewer engines (only one of the four points would attach, which would lead to breakage or even kraken). The best bet would be to drop two engines and leave two on opposite sides, then use fuel lines to make sure the flow works from the tanks that don't have engines on them now. 

Edit: Really, the primary problem is the lack of decent LF tanks in stock.

Edited October 3, 2016 by Jarin

[Jarin]

Okay, it looks like in 1.2 at least, the LV-N draws evenly from all tanks by default. That's... probably not the case in 1.1.3, but I can't look to find out right now. Regardless, dropping two engines gives a buttload of dV

Fuel lines may be necessary to make this work for you.

With a 50 ton payload strapped to the front, it's got about 1800 dV, so 50 tons to mimus without trouble. Four more LF tanks (requiring 2.5 mk3 cargo bays to transport) would push that up to 2600 dV (easily a round trip if it's coming back empty). I don't know how big your primary lifter plane is.

Edited October 3, 2016 by Jarin

[M_Rat13]

51 minutes ago, Jarin said:

Okay, it looks like in 1.2 at least, the LV-N draws evenly from all tanks by default. That's... probably not the case in 1.1.3, but I can't look to find out right now. Regardless, dropping two engines gives a buttload of dV

Fuel lines may be necessary to make this work for you.

With a 50 ton payload strapped to the front, it's got about 1800 dV, so 50 tons to mimus without trouble. Four more LF tanks (requiring 2.5 mk3 cargo bays to transport) would push that up to 2600 dV (easily a round trip if it's coming back empty). I don't know how big your primary lifter plane is.

It's designed to haul a big orange and large monoprop tank to orbit. It should do fine with this. Might need to make multiple trips, but I can get everything to orbit.

Plus what you've told me is I need 2 nukes with 12 mk 1 tanks and I should be able to do a round trip. Shame nukes are the only viable engine, especially since I only want to go Minmus, at least for now. 

I would go the mun if it meant I used less delta-v, but then I'd need a larger lander for a mining rig and that would probably be very heavy considering muns gravity.

Only other option is capture an asteroid into say a 100 km orbit around kerbin and build a mining base on that. Don't know how effective that would be however.

[Jarin]

I'd say get your fuel base on minmus and get more comfortable with rocketry before trying asteroid capture. 

20 minutes ago, M_Rat13 said:

Shame nukes are the only viable engine

Nukes are the best engine, but not the only viable one. You can get about  70-80% of the dV with the same payload just by slapping a poodle onto an orange tank where that quadcoupler splits on the design above. It cuts the margins a bit closer, but it's a lot cheaper and easier to use. Your call.

[M_Rat13]

41 minutes ago, Jarin said:

I'd say get your fuel base on minmus and get more comfortable with rocketry before trying asteroid capture. 

Nukes are the best engine, but not the only viable one. You can get about  70-80% of the dV with the same payload just by slapping a poodle onto an orange tank where that quadcoupler splits on the design above. It cuts the margins a bit closer, but it's a lot cheaper and easier to use. Your call.

At what weight does it become better to use other engines? I'm asking to see if I could build a smaller station for the time being (it might not have as much fuel, but would do for the time being, and since I'd be mining, I could always wait till I've filled it up using what I mine). I'll have to think about it.

Edit: ok so if I use the the biggest white tank (x200-32), I will have half the fuel but also half the weight, at about 22 tons.

What are my options for tugs at that point?

Edited October 3, 2016 by M_Rat13
Learned new information

[Aegolius13]

Minmus may well be cheaper than the Mun.  The ejection burn is a little more expensive, but the injection is cheaper.  The bet difference is not significant.

So if you have a system that works for the Mun, it should work here too.  But lifting a 50 ton payload to the Mun should take a similar amount of engines / fuel.

[Aegolius13]

35 minutes ago, Aegolius13 said:

Minmus may well be cheaper than the Mun.  The ejection burn is a little more expensive, but the injection is cheaper.  The bet difference is not significant.

So if you have a system that works for the Mun, it should work here too.  But lifting a 50 ton payload to the Mun should take a similar amount of engines / fuel.

For a 22 ton payload, looks like another x32 tank and a Poodle could get you over 1500 delta-v, which is a little aggressive but ought to be enough with careful flying.  Adding one more x8 tank might buy a little more insurance, though.    Using one Terrier instead only buys you about 30 more delta-v, but TWR is pitiful, so I would definitely go Poodle.  

Or with nukes, you could do 4 Mk 1 LF fuselages and 2 nukes, for a delta-v of around 1800.   TWR is around .3 which is workable.  I tried a cross shape, with 2 fuselages inline and, the nukes and remaining 2 fuselages radially mounted.  This would keep the rear node free for earlier stages.

As far as round trips, keep in mind that you will use MUCH less fuel when the tug is not moving a payload, plus you can aerobrake at Kerbin.  So return fuel usage can be quite minimal (pun intended).  

[M_Rat13]

4 minutes ago, Aegolius13 said:

For a 22 ton payload, looks like another x32 tank and a Poodle could get you over 1500 delta-v, which is a little aggressive but ought to be enough with careful flying.  Adding one more x8 tank might buy a little more insurance, though.    Using one Terrier instead only buys you about 30 more delta-v, but TWR is pitiful, so I would definitely go Poodle.  

Or with nukes, you could do 4 Mk 1 LF fuselages and 2 nukes, for a delta-v of around 1800.   TWR is around .3 which is workable.  I tried a cross shape, with 2 fuselages inline and, the nukes and remaining 2 fuselages radially mounted.  This would keep the rear node free for earlier stages.

As far as round trips, keep in mind that you will use MUCH less fuel when the tug is not moving a payload, plus you can aerobrake at Kerbin.  So return fuel usage can be quite minimal (pun intended).  

From what someone else mentioned, a round trip with 50 ton payload was 2 nukes and 12 mk1 tanks. 

So with half the payload I assume I'd need half the tanks, correct me of I'm wrong. But, from what you've said I need more nukes with if I included more tanks to get a decent TWR. Starts to sound like diminishing returns even with only half the load.

However on the bright side you mentioned I could use other engines. It's got me thinking, would an aerospike work for 22 tons? Sure it has less ISP than the poodle but uses less fuel anyway, and would probably still give me a good TWR. Would me idea work?

[Aegolius13]

46 minutes ago, M_Rat13 said:

So with half the payload I assume I'd need half the tanks, correct me of I'm wrong. But, from what you've said I need more nukes with if I included more tanks to get a decent TWR. Starts to sound like diminishing returns even with only half the load.

Not totally sure I follow.  This stuff should scale in that way - with half the payload, you can theoretically use half the fuel, and half the number of nukes, and end up with the same TWR and delta-v.  In practice it does not work exactly like that, because you have to add probe cores, solar panels, etc., but it's relatively close if you don't build extravagantly.  My suggestion (4 tanks plus 2 nukes) would obviously offer higher TWR than the 50 ton / 2 nuke option.  It's partly a matter of how little thrust you can put up with.  I personally refuse to go under about .25 for most non-ion applications, but that's not a hard limit by any means.  If you go too much lower, though, you have to either burn well off the node (which hurts efficiency and accuracy) or split up your burn (which is putzy, and can have accuracy problems as well).

46 minutes ago, M_Rat13 said:

From what someone else mentioned, a round trip with 50 ton payload was 2 nukes and 12 mk1 tanks. 

So with half the payload I assume I'd need half the tanks, correct me of I'm wrong. But, from what you've said I need more nukes with if I included more tanks to get a decent TWR. Starts to sound like diminishing returns even with only half the load.

However on the bright side you mentioned I could use other engines. It's got me thinking, would an aerospike work for 22 tons? Sure it has less ISP than the poodle but uses less fuel anyway, and would probably still give me a good TWR. Would me idea work?

Yep, you can.  It's not going to very different than the Poodle option - slightly less thrust, comparable delta v to the Poodle (inferior ISP offsets lower weight).  If in career, you're paying a lot of extra money for atmospheric efficiency you're not using, but if that's no big deal, the Dart is works fine for stuff like this. No gimbal on the Dart, but I'd imagine you have adequate reaction wheels anyway so this probably won't matter.

The only significant advantage I see over the Poodle, though, is that the Dart is smaller physically.  Though that can be a big difference if you're constrained by cargo bay size.  In my Jool 5 I used a Dart in a Tylo descent stage, rather than a Poodle, for the sole reason that only the Dart would fit into my cargo bay.  

However, when form factor is not an issue, the Poodle is likely going to be better than the Dart for this kind of tug work.  The Poodle's niche in this game seems to be hauling medium-ish loads to and from the moons.  Plus the Poodle is cheaper and has gimbal. 

 

EDIT: one thing to add - you mention that the Dart "uses less fuel."  Fuel consumption per second is not really meaningful in most contexts, because when the Poodle consumes more fuel, it's producing proportionately more thrust (and then some), which means you have to burn for less time.  And unlike, say a car, KSP does not penalize you for flooring the throttle (at least in space). Stripped of math, specific impulse is basically the ratio of thrust to fuel consumption, so it captures what really matters about an engine's fuel efficiency.  Of course, to figure out the right engine for a job, you also have to take into account the engine's mass, and thrust (to get the TWR ou need).  

Edited October 4, 2016 by Aegolius13

[Jarin]

4 hours ago, M_Rat13 said:

At what weight does it become better to use other engines?

Look above for the chart I posted. Right below it is a link to an album with engine efficiency charts for all desired TWR. Assuming you're aiming for a TWR of .3 to .5 (minimal, but enough to not go mad) with a 50 ton total craft, the LV-909 and poodle are both acceptable choices. The poodle even has a nice spike around 1500 dV and .4 TWR to be the best engine overall. 

Basically, the poodle gets better the more TWR you want, while the Nuke gets better the more dV or mass you want. When designing something to fit inside a Mk3 bay, you can use any 2.5m parts you want, so there's really no reason to go for the 909 or the aerospike.

Edited October 4, 2016 by Jarin

[Palaceviking]

9 hours ago, Jarin said:

The problem with the tree structure of KSP craft assembly is that you can go from fewer lines to more, but not more to fewer. I can't just use another quadcoupler to bring that back down to mount fewer engines (only one of the four points would attach, which would lead to breakage or even kraken). 

I use upside down quad couplers all the time with nukes, attach stack separators to nukes then take quad coupler, flip upside down on 1x symmetry, hold alt and attach to one of the separators.

Apply struts liberally.

[Pthigrivi]

My last stock implimentation used a lander with a converter on minmus and skipped using a base/station. It just cuts down on the number of rendezvous you have to do. The lander transfers to a tanker that aerobrakes down to LKO and can fuel up a station or dock with modules directly. It works best if you time your transfers for Minmus's ascending and descending nodes to reduce inclination burns. 

http://imgur.com/a/F9xRo

 

(have I lost my mind? how do you embed albums now?)

Edited October 4, 2016 by Pthigrivi

[M_Rat13]

14 hours ago, Aegolius13 said:

Not totally sure I follow.  This stuff should scale in that way - with half the payload, you can theoretically use half the fuel, and half the number of nukes, and end up with the same TWR and delta-v.  In practice it does not work exactly like that, because you have to add probe cores, solar panels, etc., but it's relatively close if you don't build extravagantly.  My suggestion (4 tanks plus 2 nukes) would obviously offer higher TWR than the 50 ton / 2 nuke option.  It's partly a matter of how little thrust you can put up with.  I personally refuse to go under about .25 for most non-ion applications, but that's not a hard limit by any means.  If you go too much lower, though, you have to either burn well off the node (which hurts efficiency and accuracy) or split up your burn (which is putzy, and can have accuracy problems as well).

Yep, you can.  It's not going to very different than the Poodle option - slightly less thrust, comparable delta v to the Poodle (inferior ISP offsets lower weight).  If in career, you're paying a lot of extra money for atmospheric efficiency you're not using, but if that's no big deal, the Dart is works fine for stuff like this. No gimbal on the Dart, but I'd imagine you have adequate reaction wheels anyway so this probably won't matter.

The only significant advantage I see over the Poodle, though, is that the Dart is smaller physically.  Though that can be a big difference if you're constrained by cargo bay size.  In my Jool 5 I used a Dart in a Tylo descent stage, rather than a Poodle, for the sole reason that only the Dart would fit into my cargo bay.  

However, when form factor is not an issue, the Poodle is likely going to be better than the Dart for this kind of tug work.  The Poodle's niche in this game seems to be hauling medium-ish loads to and from the moons.  Plus the Poodle is cheaper and has gimbal. 

 

EDIT: one thing to add - you mention that the Dart "uses less fuel."  Fuel consumption per second is not really meaningful in most contexts, because when the Poodle consumes more fuel, it's producing proportionately more thrust (and then some), which means you have to burn for less time.  And unlike, say a car, KSP does not penalize you for flooring the throttle (at least in space). Stripped of math, specific impulse is basically the ratio of thrust to fuel consumption, so it captures what really matters about an engine's fuel efficiency.  Of course, to figure out the right engine for a job, you also have to take into account the engine's mass, and thrust (to get the TWR ou need).  

So really, I'm looking at either Poodle or nukes for tug work.

But with the Poodle I'd have to refill at least every round trip, but the nukes can go further. The design shown at the top of this thread can carry the original 50 ton load and has a delta-v of 5192, correct me if that's wrong, but a poodle carrying 22 tons has a delta-v of 1500. Seems like if I want fuel efficiency I have only once choice.

On the bright side using nukes allows me to go back to my original payload.

[Aegolius13]

9 minutes ago, M_Rat13 said:

So really, I'm looking at either Poodle or nukes for tug work.

But with the Poodle I'd have to refill at least every round trip, but the nukes can go further. The design shown at the top of this thread can carry the original 50 ton load and has a delta-v of 5192, correct me if that's wrong, but a poodle carrying 22 tons has a delta-v of 1500. Seems like if I want fuel efficiency I have only once choice.

On the bright side using nukes allows me to go back to my original payload.

I doubt you could make two trips without refueling, even with the nukes and the 22 ton-payload.  That is, unless you add so much fuel it would likely not fit in a cargo bay.  The nuke tug at the top has a delta v of 5192 empty, but that number is going to plummet once you attach the tug to its payload.  See @Jarin's two nuke option later in the thread - it has only 1800 delta-v loaded.  So the 4-nuke option will pack even less.

But anyway, nukes are pretty much the superior option for heavy vaccum cargo hauling.  I too don't really like them - they're expensive, underpowered, and don't work with half the fuel tanks (including anything in Mk 2).  But that 800 ISP is such a huge advantage, sometimes you have to put up with them.  

[Kerbart]

Does the 2.5m docking port work these days? I've gotten into the habit of avoiding it when frequent docking/undocking is required as it's the one that never undocks in my book. Or is that a problem that was fixed many moons ago?