Looking for ideas for a trans-Moho tug

[Barefoot Friar]

I've gone around and around in the VAB and I just can't seem to design what I'm wanting. I was hoping someone could perhaps point me in the right direction.

I want to place this into low Moho orbit:

As you can see from the stats panel in the picture, it's 35 tons. It's 3.75m diameter, not counting the four landing boosters. Getting it to the surface is no problem at all; I already hyperedited it there and tested it. Launching it from KSC to low Kerbin orbit is also no sweat. It's getting it from Kerbin orbit to Moho orbit that I can't quite figure out. I keep designing craft that have enough dv but ridiculously low Kerbin twr, or else the other way around. I'm also trying to keep the tug small enough that orbital maneuvers aren't a major pain -- although that I can get around if need be.

I'm using KW Rocketry, Stockalike Station Parts Expansion, and Near Future Technologies parts mods, along with several others whose names I'm not remembering at the moment. Those two should be most relevant, however, since the others are mainly for probes and small landers. Also, I have a part that combines two stock nuke engines into one 2.5m part. It's exactly the same as having two stock nukes, but reduces part count and looks better on larger ships.

I'm not necessarily asking for you to make my craft for me. I'm just asking for design tips. Although, if you have a tug already that could do the job I'm certainly interested -- I try out stuff from the spacecraft exchange all the time.

Thanks for the help.

Edited March 6, 2015 by Barefoot Friar
Question answered

[LethalDose]

You're probably not gonna like my advice, but I think the two points below are among the best you'll get in this situation.

#1 Don't use a tug. Tugs are intended to be reusable, and you're sending this to the surface. Why does the stage that got you there need to be reusable? You'll be better off with a disposable transfer stage. I like a KR-2L for those.

#2 Make the thing smaller. 35 tons is a LOT of payload to move to Moho. If you're having trouble with this part of the design (and the x'fer to Moho is the hard part), start with a smaller mission, get some practice, and then revisit this.

Hope that helps.

[Barefoot Friar]

Good points. I really kind of misspoke when I called it a tug. I'm not really interested in getting it back. This is definitely a one-way trip.

I looked at using the same kind of base I have on Minmus and the Mun, but it would require too many launches and the overall cost would be too high. I'll revisit my design, however, and see if I can downsize it.

[Foxster]

A few thoughts on your lander...

Do you really need all those Kerbals? One (or two if have a science lab) is usually really all you need.

Do you need RCS? If its going to re-dock then probably yes (but you could use (massless) Vernor engines instead and save carrying mono-prop), otherwise no.

If you aren't re-docking then you don't need the docking port(s). Use just one on the mothership, doesn't need a matching one on the lander. Or, even better, a small decoupler.

Do you need landing struts? Some angled stacks of (massless) Cubic Octagonal Struts would do as good a job.

Using one LV-N is heavier than your engines but you'll need half the fuel and so the craft might weigh less overall.

I doubt you need four inline stabilizers. Try it with one.

Consider (massless) OX-STAT single solar panels rather than PB-NUKs (assuming that's what you are using).

I doubt you need that huge battery pack. Use (massless) Z400s instead.

Do you need those radial decouplers? If you don't actually need to detach the engines then use some (massless) Cubic Octagonal Struts instead.

The secret to getting this to Moho easily will be to reduce your payload. Simplify and skinify.

Edited March 4, 2015 by Foxster

[magnemoe]

First, I agree with others, rcs, you don't need so much if needed at all, two small round tanks.

Don't use the 48-7S, its an excellent engine however not for heavy stuff. use the 909, or to be an power gamer, the LV-N, benefit of the LV-N is that you can also use it for the trip to Moho.

put 45 liter tanks radialy on the outside of the long ones, LV-N below, pipe from long tank to the 45 liter, pipe from core to the four long tanks.

This gives me 4.2 km/s dV with 32 ton before adding the LV-N and an TWR of 0.56

Now add a drop tanks below the docking port. a large grey tank adds 2.3 km/s, another below that again adds 1.7 km/s, an total of 8.3 km/s and an starting TWR of 0.3

Now if you want an tug, you could dock it below this and burn until you pass Minmus orbit, fill up the ship from tug if needed, you have now done 1 km/s burn with higher TWR and the tug returns to LKO after aerobrake.

Optionaly replace the lower tank with an orange one with 4 other LV-N on it as an first stage.

[Jouni]

Don't listen to the people with tiny rockets.

A reasonable initial TWR for the transfer stage might be 0.5. Roughly speaking, this means that there must be one nuclear engine for 10 tonnes of vehicle mass (not including the engine itself). With 35 tonnes of payload, you need 3.5 engines for just the payload.

Let's assume that our standard fuel tank is the FL-T800 / X200-8, with 4 tonnes of fuel and 0.5 tonnes of dry mass. Because the transfer stage may need other parts than just fuel tanks and engines, 0.5 engines/fuel tank should be a nice round figure. Then it's just a matter of adding more fuel to the ship, until it has enough delta-v. You'll probably end with something with 10 or 12 engines (120-140 tonnes at LKO).

[Aethon]

I agree with the others that your ship could use some optimization, but I'm guessing you're not leaving for Moho in the most efficient window. Due to Moho's eccentricity, inclination and orbital speed it's really a different animal. The best method to Moho I've used is maccollo's method.

http://forum.kerbalspaceprogram.com/threads/61478-Oh-bugger-Injection-burn-at-Moho?p=835667#post835667

Read Kashua's post 2 down from maccollo's as well.

Edited March 4, 2015 by Aethon

[Barefoot Friar]

Okay, I've been tinkering around in the VAB some more, and I've decided that I can shave a wee bit of weight off my design, but I think that the basic configuration is the best option. The biggest issue I'm running into is that the contract stipulates:

• Space for 10 Kerbals
  
• A viewing cupola
  
• Power
  
• Radio
  
• A docking port (unspecified size or placement)
  

My usual habitation module only has room for 5. I decided launching two hab modules, a base crawler, and a connecting truss module along with their respective landers would be heavier and more expensive than this craft.

So next I played around with stock components, and then a few other stockalike mod parts I have at hand, and I still cannot hit the mission parameters for space for less than 8 tons. The original, just the cupola and living quarters, are 8.8 - just .4 more than any other combination I could put together, and was the simplest.

After that, it was just a case of implementing suggestions. I removed the 4k battery and replaced with two massless 400 batteries. It's hard to see in the shot I provided, but power is from four stock OX-4L solar panels, weighing in at 0.0175 each (0.07 total). I removed the bottom docking port, and replaced the top standard port with a junior. The junior only saved me 0.03, while removing the senior saved me 0.2.

Switching from LT-2 to LT-1 legs saved me 0.3.

So now, not counting whatever method I use to land it, I'm down to 21 parts and 9.2 tons. Playing around with other configurations and living arrangements didn't net me very much difference in weight, but increased the part count.

For the landing engines, I played around with several different configurations. I did remove all RCS and SAS modules, which immediately saved 5.4 tons -- a significant savings. Ideally, I want to be able to detatch the lander engines after I reach the surface, sending them away to crash (or deleting them from mission control). I believe (and will test this tonight) that I only need in the neighborhood of 1000dv to safely reach the surface. This gives me enough room to tweak my landing area if need be and leaves enough fuel that the separated stages can fly safely away to crash and burn. I switched the four FL-800 tanks for FL-200s, saving me another 3.375 tons and increasing the Moho TWR to 3.12. So now as it stands my total payload to LMO is 14.4 tons -- roughly half of what it was when I posted this thread this morning.

I may can shave a bit more off by completely redesigning it, and I might play around with it some more tomorrow. However, I'm fairly confident that this is the most useful and least complicated design that will meet my mission parameters.

I just saw the suggestions to try different engines, so I will do so. I can almost guarantee, however, that the 48-7S is probably going to be the one I go with; the LV-Ns were too heavy and way too long. I'll try the LV-909s just to see what changes. Maybe I can go with two 909s and larger fuel tanks instead of four 48-7Ss and smaller tanks.

We're supposed to get an ice storm tonight so I may not respond for a day or two if we lose power. But I'll be back as soon as possible.

- - - Updated - - -

I agree with the others that your ship could use some optimization, but I'm guessing you're not leaving for Moho in the most efficient window. Due to Moho's eccentricity, inclination and orbital speed it's really a different animal. The best method to Moho I've used is maccollo's method.

http://forum.kerbalspaceprogram.com/threads/61478-Oh-bugger-Injection-burn-at-Moho?p=835667#post835667

Read Kashua's post 2 down from maccollo's as well.

Thanks for the link!

I'l using Alex Moon's transfer calculator. Is that what you meant, or are you referring to bouncing off of Eve a couple times before finally making Moho orbit? If it's the latter, no, I'd rather burn straight for Moho because I don't want to be floating around any longer than need be. However, one does what one must.

[Kryxal]

It might be worth running up two designs, one with the small engines and the other with LV-Ns serving double duty (landing and transfer burn). It wouldn't be too hard to do, use a small fuel tank as a mounting point and feed the small tanks from a larger, central one. You'd probably need to shut off gimballing and put one of the SAS modules back (maybe under the docking port) but landing without a wide base is easier since the new options for SAS were added.

[Aethon]

http://forum.kerbalspaceprogram.com/threads/26656-Delta-V-to-reach-Moho-orbit/page2?highlight=moho

Quote from myself to avoid typing.

Basically you leave LKO on Mohos' AN/DN.

The first time this occurs ( IIRC the AN ) is ( Earth ) Year 1 Day 21, on or about Hour 17. Set Moho as your target and drag a dummy Kerbin escape node out to see where the AN/DN is. I combined my Kerbin escape burn with my inclination burn at a total cost of 2300 dv. At my solar periapse ( which was also Mohos' solar periapse ) I burned 1800ms retro ( as Kashua suggests. See link ) to make sure I encountered Moho on my next orbit, which lowered my solar apoapse and slowed the eventual Moho encounter down dramatically. My capture burn at Moho was around 1000ms.

[Streetwind]

Your lander is honestly fine. You've already tested this design and it works, so stick with it - shamelessly exploiting massless parts may earn you a ton or so less weight, but it'll also look dumb, leave a bad aftertaste and the small savings won't actually absolve you of the need to design a giant transfer vessel.

Regarding the transfer vessel itself, you have basically three options:

1.) Gigantic, staged chemical rocket. Pro: high TWR. Con: annoying to build, launch and fly because of tedium, part count lag and sheer mass/size. Likely needs orbital assembly.

2.) Classic nuclear transfer vessel. Pro: doesn't need to be as big. Might be able to do it in one launch. Con: low TWR.

3.) Nuclear-electric high tech boat. Pro: will give you the smallest vessel. Might even have the fuel to return itself to Kerbin for reuse. Con: Low TWR, very expensive (in career mode).

Going from Aethon's numbers, you want at least 5200 dV when starting your transfer burn in LKO. Accordingly, the rocket equation can tell us your required payload fraction...

Payload mass: 35t

Fuel tank mass: 8t fuel for 1t dry mass

Engine mass: 2.25t per LV-N

Isp: 800

One Rockomax X200-32, two LV-N's and a bit of spare change dry mass to mount the LV-N's radially and fit a decoupler/Docking port for your payload: this stage with your payload on top does about 2500 dV with a TWR of 0.21. Low, yes, but you can use this stage for the two non-critical burns - the Sun periapsis maneuver and the orbit insertion. TWR will improve as you burn and be highest when you need it most. For Kerbin departure, let's stack a beefier stage below it...

Payload mass: 58t

Fuel tank mass: 7.2t fuel for 1t dry mass

Engine mass: 6.5t per KR-2L

Isp: 380

One Kerbodyne S3-14400 fuel tank, one Kerbodyne KR-2L and a bit of spare change dry mass for adapters/decouplers/reaction wheels: this stage with all the previous stuff on top does another 2400 dV, while having a TWR of over 1.7 fully fueled - that ought to be enough for anyone!

Now this thing has a total of 4900 dV in its first two stages, give or take a few depending on how you build. You'll notice that this is a few hundred short of Aethon's example, meaning you'll run out of juice probably after capture but before low orbit circularization. Thing is, though: your landing stage has 2100 dV, not counting RCS, which is more than enough - you need about 1400 to land from a 50km circular orbit. So you can use the lander to supplement the missing dV.

Total transfer vessel weight: ~150 tons. 3 stages from LKO to Moho surface. Launching 150 tons in one launch is not unheard of, but you can also launch it empty (~48 tons) and fill it up in orbit, or launch each stage separately and dock them (though that will require adding extra mass in the form of RCS and probe cores and batteries and whatnot).

Not much room for mistakes, and certainly not enough to mess around with plane changes in Moho orbit (insert directly into a polar orbit so you can always hit your desired landing site no matter where on the planet it is). But it's economic If you feel unsure, the huge TWR of the Kerbin departure stage lets you add in more fuel without hurting, but the launch weight will increase noticably.

EDIT: Bonus method to increase dV for free! Just occurred to me over lunch break. Nowadays in .90 you can do symmetric fuel transfers involving multiple tanks in stock KSP. When the nuclear stage runs out of fuel, you can transfer over some from the lander's four tanks without causing a fuel amount imbalance in them. Despite the added dry mass of the spent nuclear stage, the superior Isp of the LV-N's still gives you approximately 60% more dV for your fuel than if you were to decouple and use the 48-7S's. Heck, the two LV-N's even have exactly the same thrust as the four 48-7S's! Hence, finish your orbit circularization burn this way - and, in fact, keep using the LV-N's for the first part of the landing too. Hang onto them until the spent stage gets in the way or it starts dragging your down too much, then (make sure you transferred all remaining fuel back symmetrically and) ditch it during descent and finish the landing normally.

Want me to try and do a Near Future equipped electric variant as well?

Edited March 5, 2015 by Streetwind

[Foxster]

shamelessly exploiting massless parts may earn you a ton or so less weight, but it'll also look dumb, leave a bad aftertaste and the small savings won't actually absolve you of the need to design a giant transfer vessel.

We might have to agree to disagree on that one .

These parts are deliberately made massless by the dev team. There is a property set to make them so and they have chosen to go that route with them. There has been lots of debate about these items and if the devs thought they were a problem they would have been changed a long time ago.

For that reason they aren't an exploit and I don't see why they should leave "a bad after-taste".

As for "small savings" - the OP has already halved the weight of his payload by using a few massless items and using a few design tips. That's a lot more than "small savings".

[Barefoot Friar]

Sorry for the delay in replying. I worked on this for a couple more hours yesterday morning and here's where we stand:

First, I played around with getting two LV-Ns to fit. I ended up trading back the LT-1 landing legs for the longer LT-2s, along with moving the engines up as far as possible. This gives my lander stage 1883m/s dv, for a slightly lower initial Moho TWR (2.37 vs. 3.something). The lander's weight went up a couple or three tons due to LV-Ns being so wicked heavy, but I think it will be okay.

Next, I used Streetwind's design (thanks!!!), with the only adjustment of running fuel lines from the bottom tanks to the lander engines; this will increase thrust for the insertion at the cost of increased fuel usage. However, MJ is claiming that the insertion stage has 3574m/s of dv by itself, giving me a total in of about 5300m/s so far. I've already set an action group to disable the lander engines in case I need to trade thrust for dv.

The first stage I left the same, although I experimented with (and photographed) an engine change. I ended going with the one Streetwind recommended. Given the weight savings I implemented yesterday, this stage gives 2893m/s at 1.91TWR in Kerbin orbit. Total vessel dv on the launchpad (figuring vaccuum) should be 8350m/s.

Launching that is no problem. I'll launch it with asparagus stages, and then refuel the core as needed once in orbit.

What do you think? I'll strut it all down right before launch, but I wanted to leave it free for now to play around with it.

[Streetwind]

Looking good! For such a big habitation module that needs to go all the way down onto Moho, that's a reasonably economic rocket, don't you think? With that much dV it could even be considered oversized.

I like the trick with volunteering the lander's engines for extra TWR during the flight. I rarely ever use radial engines due to needing streamlined rockets under FAR, but I may just have to steal that sometime down the road.

P.S.: whether the lander engines are running or not does not influence your dV budget, because all the engines are identical. Such a thing only works if some of them have lower Isp values than the others, so toggling them on or off actually changes the average vessel Isp*. Your action group toggle is kinda unecessary. Just keep 'em on!

* Calculated: (Sum of (thrust * Isp) of all active engines) / (Sum of thrust of all active engines)

[Barefoot Friar]

Ah, okay, that makes sense.

Thanks again for the help. I'm going to change the tag to answered now.