Mothership to duna

[StupidAndy]

so I have a mission I'm building called Kartemis (Artemis is an actual Greek god, I don't know of what...) and I'm building a mothership for a mission to duna, it has 5 landers, 4 unreusable Duna Landers for a total of 20 kerbals landing on duna, they have an Mk1-2 cockpit with a Mk2 lander can on the bottom, and I have 12 kerbals going on the mission, so 1-10 will land on duna twice, and 11 and 12 will land on ike once, so that makes everyone have a fair landing, and that's the plan at duna, when we get back, standard return, enter orbit, send up return vessels, repair, off to jool or eve, who knows, but this is about DUNA!!

I'm having trouble with the Delta V issue, currently, I'm having maybe 5 modules of 4 of the biggest Mk1 fuel tanks, so it goes like this Mk2 to Mk1 adapter x4, Longest Mk1 fuel tank, Mk2 to Mk1 upside down, relatively simple, so I don't know...

the trouble is that the Delta V always goes down when I add fuel, and I can never get above 4000 Dv, so its hard to get up, and I think its adding the fuel from the landers, so I'm not sure, do I have an asparagus staging like a rocket but with docking ports for the fuel, do I have it with nuclear? Ion? elephant? just random questions on building a mothership for a mission to duna

if this is in the wrong thread, just move it mods!

[Aegolius13]

14 hours ago, StupidAndy said:

so I have a mission I'm building called Kartemis (Artemis is an actual Greek god, I don't know of what...) and I'm building a mothership for a mission to duna, it has 5 landers, 4 unreusable Duna Landers for a total of 20 kerbals landing on duna, they have an Mk1-2 cockpit with a Mk2 lander can on the bottom, and I have 12 kerbals going on the mission, so 1-10 will land on duna twice, and 11 and 12 will land on ike once, so that makes everyone have a fair landing, and that's the plan at duna, when we get back, standard return, enter orbit, send up return vessels, repair, off to jool or eve, who knows, but this is about DUNA!!

I'm having trouble with the Delta V issue, currently, I'm having maybe 5 modules of 4 of the biggest Mk1 fuel tanks, so it goes like this Mk2 to Mk1 adapter x4, Longest Mk1 fuel tank, Mk2 to Mk1 upside down, relatively simple, so I don't know...

the trouble is that the Delta V always goes down when I add fuel, and I can never get above 4000 Dv, so its hard to get up, and I think its adding the fuel from the landers, so I'm not sure, do I have an asparagus staging like a rocket but with docking ports for the fuel, do I have it with nuclear? Ion? elephant? just random questions on building a mothership for a mission to duna

if this is in the wrong thread, just move it mods!

Artemis is the goddess of the moon and hunting, I believe.  

Sounds like your design is inherently pretty heavy - if you consider the "payload" everything you're bringing to Duna, it's a lot.  It's not unfeasible to move a lot to Duna, since fortunately, it's the lowest delta-v of interplanetary destinations. But if you can lighten things up, it certainly makes it easier.

One possibility would be going to a resuable Duna lander - that will save a lot of duplicated weight on those Mk 1-2s.  It is not too hard to build an SSTO Duna lander, and since you can use chutes on the way down, it does not need all that much fuel in total. If you do go this route, just make sure you have an engineer to repack your parachutes.

 You could also save some mass by using lighter parts - both Mk 1-2s are disproportionately heavy for what they offer.  A few alternatives are:

-A Hitchhiker crew cabin with an Mk 1 command pod or landing can.  This is very light but might be a little light on torque, though. 

-An Mk 1-2 landing can and Hitchhiker.  This would also add an extra Kerbal seat.

-An Mk1-2 command pod and Mk 1 passenger cabin sticking out the top.  I like this combo myself, since the Mk 1-2 pod has more than enough torque to go around.  Also keeps center of mass low for entry stability and landing.  

As far as engines, your best options are probably going to be nukes, Poodles, or Rhinos.  Since it only costs around 1000-1200 delta-v to get from Kerbin to Duna, chemical rockets are more practical than they would be for most interplanetary destinations.  Asparagus staging works well in space, just like on launch.  For example, with nukes, you can do stacks of a few Mk 1 LF containers with a nuke on the bottom.  

I have never used the Elephant but understand it has stats similar to a Vector or Mammoth.  If so, it's not a vacuum optimized engine, and you'd probably do better with a Rhino or a bank of Poodles. Note that even on Duna surface, the atmosphere is thin enough that vacuum engines work better than Kerbin atmo-oriented engines.   

Depending on how your ship is designed, you might be able to use your lander engines for the transfer burns as well, to avoid the mass of another set of engines.  For example, you could have a central column with fuel and the other stuff that stays in space, and a small engine for the return burn, and have your landers sitting on the sides, kinda like radial boosters, using their engines for the Kerbin-Duna transfer but but drawing from the central fuel tank.

If you're not now, you might think about aerobraking to capture into Duna orbit to save a few hundred delta-v.   It's got quite a forgiving atmosphere.  

 

Edited January 19, 2017 by Aegolius13

[DerekL1963]

Take a gander at the Jool-5 mission in my sig for some design ideas...  

[Streetwind]

5 hours ago, StupidAndy said:

I'm having trouble with the Delta V issue, currently, I'm having maybe 5 modules of 4 of the biggest Mk1 fuel tanks, so it goes like this Mk2 to Mk1 adapter x4, Longest Mk1 fuel tank, Mk2 to Mk1 upside down, relatively simple, so I don't know...

the trouble is that the Delta V always goes down when I add fuel, and I can never get above 4000 Dv, so its hard to get up, and I think its adding the fuel from the landers, so I'm not sure, do I have an asparagus staging like a rocket but with docking ports for the fuel, do I have it with nuclear? Ion? elephant? just random questions on building a mothership for a mission to duna

First thing I'll say: when asking for design help with specific spacecraft, then showing a screenshot (not at night, please!) will help us help you tremendously.

Second thing I'll say: it looks like you're a bit ahead of yourself. You're building a fairly complex design, but they way you describe your issues, it sounds like there are a few fundamentals of rocketry that you missed picking up on in your time so far. The problems can probably be narrowed down to three things: one, fuel flow rules. Two, engine choice. Three, you don't know what mass fractions are. I suspect it's mainly the third, but it can't hurt to quickly touch on all of them...

Regarding fuel flow rules: not all fuel you add to a spacecraft actually gets used by your main engines. Decouplers, for example, block fuel crossfeed. Else your first stage would drink the fuel of your second stage during launch, and nobody wants that! There are other parts that do this too. So if you're adding fuel somewhere and your dV goes down, that (most likely) means that you are adding it in a place that is crossfeed blocked from your main engine. Investigate the parts between where you added fuel and the engine, and check if everything is set up the way you need it to. Remember: you don't want the transfer stage to consume the fuel of your landers. But you do want the transfer stage to be able to access all of its own tanks.

Regarding engine choice: you didn't tell us which one you're currently using, but make sure you have a vacuum rated engine. Ideally a nuclear one, but for Duna that's almost overkill. A Duna transfer costs like 500 m/s more than a Minmus transfer, and that includes capturing at Duna. It's the cheapest interplanetary destination of them all. Just... try not to fly around in space with atmosphere rated engines like the Mainsail

Regarding mass fractions: this may be the most important thing you missed out on learning up until now. Do go and read this post I once made. Read it twice and ask me questions if necessary, but don't skip on it just because it's a little longer and contains grade school math! It explains another possible scenario how dV can go down while you add fuel, which is rooted in poor spacecraft design.

 

Edited January 19, 2017 by Streetwind

[Kryxal]

Just a minor correction, decouplers now block fuel flow by default, but it's a tweakable.  Docking ports default to not blocking it, but that's also a tweakable.

[StupidAndy]

if anything the thing blocking would be the docking ports, so...no

[Carl]

Without a picture we really can't help anymore than the second poster, but honestly for dragging that many landers along, your going to flat out need bigger tha mk1 parts even with nukes. Mk1's just aren't up to carrying the huge gobs of fuel you'll need. Don't know what else you have on the Duna landers besides the capsules or what your using for crew carriage on the mothership. But assuming you have mk2 cans as mothership crew space your hauling 41.6 tons without the fuel or engines on the landers, (and get that cross feed disabled if it isn't, you never get a functional lander design till you do), I'm not sure what duna return and landing needs there, but i'd guess you've probably got another 15 tons on those each minimum. So your trying to drag a 116 tons or so to duna. Thats a lot Like a bloody hell of a lot. Perfectly doable mind, but your going to need to go a lot bigger in terms of fuel tank sizes. A 2.5m Orange holds as much as 8 1.25m max size tanks and your probably going to need several of those. Without seeing your payload though i can't advise anymore than that.

 

EDIT: To try and put things into perspective. The Nuke setup for my DER5 design which had 14kps of d/v with a 22 ton payload massed 600 tons, despite dropping most of it's tankage as it went. I'm not sure off the top of my head what it's dv with a payload like yours would be, (i need to figure out how to take base log with my calculator), but it would probably be barely adequate at best. 

 

EDI2: after setting up an excel sheet, it would be quite good actually, a 154 ton payload would achieve 8.5kps of dv. Not bad.

Edited January 20, 2017 by Carl

[WanderingKid]

To add to the discussion (in a lengthy form), a technique I used back when I did my Let's Play for the Duna mission was to attach one of my landers (it was my emergency lander, in fact) to the back of an orange tank with the Docking Port Seniors.  It's mighty poodle engine was engaged during transfer and added some TWR for the orbital transfer, and when it was detached it had plenty of TWR for landing/takeoff from Duna.

Part of the idea here is just to help you think laterally and generate some 'fun' ideas.

If you do this with all your one-shot landers, those engines can be reattached for the trip home, and you won't need to drag along the empty tanks they used either.  Just use another set of docking port seniors from the engine itself and (if you're bringing a claw tow along) re-attach the engines right to your mothership again, and ditch the lander completely.  So, from top to bottom, think this way for your lander:

• DockingPort Sr.
• Gas tank(s)
• Crew compartments
• DockingPort Sr. (x2)
• Poodle Engine

The docking ports won't add much weight, comparatively, but doing this will save you a tremendous amount of Delta-V.  You won't have wasted tonnage on 'dead' engines, you'll ditch unnecessary mass for the return transfer, and the tow itself can be very light, only used for local maneuvers.  Think Okto-2, a few RCS ports and a tank, a docking jr, a claw, and an SAS component to help with rotations.  Slap on a few massless batteries and solar panels and you've got a short range tug that comes in at ~1 ton total.  Each of those poodles outweighs that tug by itself, nevermind all the lander components.  You can even leave the mini-tug behind when leaving orbit to save a little extra dV.

Also, while I like having a backup 'rescue' lander (my missions rarely go as planned, I've noticed, particularly at Duna) a reusable lander with discarded fuel tanks on the mother ship will save you mass overall.  Here's the general principal:

A lander going to Duna takes up mLand mass (cockpit/hitchikers/etc) + mLFO fuel for landing and takeoff.  Unless the lander is staying there, you need enough fuel on the lander to de-orbit, land, takeoff, re-orbit, and rendezvous.

Four trips to Duna with four landers = 4 mLand + 4 mLFO, along with mLFO(extra) being wasted, which is your safety margin of fuel per lander.  If you re-use the same lander four times, you're only bringing along mLand + 4*mLFO mass.  Additionally, your mLFO(extra) can now be used for either a 5th landing (if you've got enough) or can be kept as an additional safety margin for other explorations, or your return trip home.  If the fuel tanks for the 3 mLFO extra fuel landings are detachable from the mothership, you've saved 3 mLand in weight on your insertion to Duna, and you can still discard the additional dead weight tanks when you're done, leaving you in the same state on the way home.

Just some food for thought.

Edited January 20, 2017 by WanderingKid