Advice for Interplanetary Mothership bound for Duna and Nucelar Rockets

[aeromike221]

Greetings fellow KSP'ers,

To those who have mastered interplanetary flight I ask for your advice, inquiry and assistance. I have successfully put a kerbed lander on Duna, however my previous design was limited and my lander doesn't have enough fuel to return back home or even get back into Duna Orbit. So I am designing a new interplanetary mothership that will not only rescue the crew from Duna, will also serve on other interplanetary explorations to other worlds.

Here are a couple of photos, attached to the post. (Note, I have yet to assemble the first and second stages...for now please focus on what I have.) As you can see below the payload I am trying to take into orbit is composed of two parts. One the powerful "mammoth" engine block which will give the final push into orbit and perhaps give some delta-v for the Duna burn assuming enough fuel is left. The other half, which is the one I'd like to focus on is the "Endurance Module". It is two be powered by 6 nuclear rockets, with 1 jumbo x64 orange fuel tank, a smaller x16 white fuel tank, a equipment storage container, a "hitchhike" contanier for possible crew, two large batteries, 6 fins, 12 1x6 solar pannels and two dockports(one on the top and one on the bottom.) Also note I plan to attach a fuel lander that will go to the surface and rescue the boys. The Module weighs about 100 tons and total configuration(with the rocket stage below) is about 225 tons.

I've previously tried to design a mothership using mainsail rockets and that failed catastrophically. I realize now that nuclear rockets provide the best bang for the buck(they have the best efficiency), however I'm a little uncertain how they will preform due to their low-thrust so I might have to rethink my burn strategy.

So for all those who are pros at getting to other planets can you spare some advice on my design as it is. Should I add more tanks, add more rockets, get rid of some containers, scrap it completely, keep it as it is? Anything I should do?

Also I would greatly appreciate advice for nuclear rockets because I have never used them before and I don't know what maneuvers I should do because I know they will behave differently then the conventional rockets. Please spare any constructive criticism or advice you have to helping me get of the launch pad, and rescue my guys stranded on Duna currently. Thank you so much!!

[everything-is-nominal]

Well i think that 6 nuclear engines are a bit too much for this spacecraft, 2 or 3 should give you a TWR greater than 1 and will be more efficient.

Second thing, you need to install at least one mod : http://kerbal.curseforge.com/ksp-mods/222685-kerbal-engineer-redux

It's kerbal engineer and it will give you 2 usefull information for your travel : your Thrust to Weight Ratio and your Delta V.

To get in orbit of Kerbin, you need in 1.0 at least 3500 m/s, to make an injection from kerbin to Duna you need 1100 m/s, from there, you can aerobreak or make a retro burn (wich will cost you then 400 m/s). To get back on kerbin, you will then need at least 700 Dv (if you aerobreak, otherwise you need to add 950 Dv).

To sum everything up, if you are already in orbit around to kerbin, to get to Duna and coming back, you will need at least 2000 m/s, and if you add the 3500 to get in orbit around kerbin, that's 5500 m/s (let's call it 6000 for safety).

So maybe what i wrote doesn't help you much right now, but if you download and install the mod, everything will be a lot more easier, you will be able to know if your ship is able or not for a journey around another planet.

Now for the nuclear engines, don't forget they only use liquid fuel, so you can dump all the useless oxydiser from every tank.

Also if i don't make any mistake, you should maybe add a probe or something like that on your rocket because the hitchhiker don't give any control on your rocket (but i might be wrong on this one)...

P.S : here is a little cheat sheet to know the Dv needed to get somewhere http://i.imgur.com/9vVZMxq.png

Edit : i changed some value in the Dv thing, i used an old Dv chart...

Edited May 16, 2015 by everything-is-nominal

[Fizbanger]

Any chance of some of you guys posting screenshots of your own successful interplanetary craft?

I use to have the Duna trip down but now I'm really struggling to get a workable craft and am in desperate need of some inspiration.

[cybersol]

For nukes, which run on liquid fuel only now, you really want to be using LF only tanks like the Mk3 parts for efficiency.

[everything-is-nominal]

Here are some of my creations, all of them are at least able to go to Duna, some others can go beyond. If you look at the big one with the two cargo bay, he never went to space because i never lauched it (i will do it after my finals).

The others where used on 0.90 but will still work like a charm on 1.0.2

P.S i tried to make a [imgUR] [imgUR] thing, but it looks like i am doing something wrong...

EDIT : tried to make an imgur album in the post (and it worked ! thanks cybersol)

Edited May 16, 2015 by everything-is-nominal

[cybersol]

P.S i tried to make a IMGUR thing, but it looks like i am doing something wrong...

Its [*IMGUR*]ZIc7q[*/*IMGUR*] with all the * removed

Edited May 16, 2015 by cybersol

[Capt. Hunt]

It might be easier to get that monstrosity into orbit if the booster is staged, you'll save some delta-V if you're not lugging all the empty tanks and the Mammoth. Also, you may save some weight by launching your Duna ship to orbit unfueled and fueling it on orbit with smaller tanker rockets. Also, you don't need the fins and nosecones on the NERVA nacelles, you might have better aerodynamics during launch with a fairing. Also, you should replace the fins on the booster with steerable fins for better in atmo control, and add a fer verniers for orbital maneuvering, especially if you are hoping to use the launcher for an extra boost during the trans-duna burn.

[Botch]

you really only need one nuke if you're already in orbit. So use your booster stages to push you, your liquid fuel, your lander and of course your one nuke into orbit. Wrap the unaerodynamic parts in a fairing to get into orbit. Gently aerobrake when you get to duna. If you can set up a very close approach very early in your flight (like as your launch) Consider leaving parts of your craft, like empty tanks and some, most or all of the lander behind for the return trip. Watch out for martian pox.

[Brofessional]

You only need one nuclear engine. You might want 2 or maaaybe 3 for something like a Moho ship that will need to slow down fast before it zooms past the target body, but for Duna any more than 1 is a waste.

As others have said, either use the spaceplane LF tanks, or drain the oxidizer from the rocket LFO tanks, because the NERVA's don't use oxidizer anymore.

[DBowman]

++ re Kerbal Engineer & the deltaV 'hints' from everything-is-nominal.

Also you don't even need thrust to weight of 1, less than 1 is fine just for orbital transfers. The lower the TWR the longer you have to burn for, if you are in orbit and try to do a very long escape burn you can end up in the atmosphere. If that's the case you can split the burn into 2 (or more), for example burn 600 m/s 'at the right time' then wait till you orbit once and burn the rest. Why bother with that - less engines = less mass = more deltaV from your fuel = less fuel = less mass = ....

You could try my new first mod: [removed link to defunct website] It gives you some fuel only variants of the big orange tank. Also in the 1.0 version the nukes make a lot more heat which means you cannot run them for as long as you used to be able to. They will transmit heat to a tank they are directly connected to, if that is a small tank it will heat up fast and you have to throttle down. The mod will let you surface mount the nuke - so you could mount 4 (say) directly to the back of the big orange tank.

[merendel]

You only need one nuclear engine. You might want 2 or maaaybe 3 for something like a Moho ship that will need to slow down fast before it zooms past the target body, but for Duna any more than 1 is a waste.

As others have said, either use the spaceplane LF tanks, or drain the oxidizer from the rocket LFO tanks, because the NERVA's don't use oxidizer anymore.

I'd still use 2 myself just so I can outrigger them instead of haveing to stack a largish craft ontop of a large launcher with that long spindly thing in the center. I'll take the slight efficency hit to have it look better and not have to strut as much to keep the stage joint stable. Cuting burn times nearly in half is nice too. I do agree that 6 is a tad overkill though.

[Hannu]

It seems that your booster is inadequate. You should use 2 stage booster. Interplanetary ship have unnecessarily many engines. LV-Ns are heavy. TWR of about 0.3 is enough to interplanetary burns. It depends on initial orbit and dv, but if you use 250 km orbit and burn to Duna even 0.15-0.2 may be enough. Of course if you transport massive landers, stations or fuel/ore tanks with your interplanetary ship you may want to more force to avoid annoyingly long maneuvers. You should really get some engineering mod for interplanetary trips. It is not fun (IMO) to make them with eyeballs.

LV-Ns are like any engines except they does not consume oxidizer (you should take it away from tanks (except what your lander needs)) and they produce heat. Heat should not be problem in your design if you use normal transfer orbits (about 1000 m/s burns).

Nerfing of LV-Ns encourage to use local resources. There are plenty of ore in Ike. I sent a refinery ship there before manned operations. Now I can get return fuel from Duna system. Of course you can do an Apollo-like mission with one landing without resource utilization, but with local resources you can stay months and land to every biome on Ike and Duna.

[Kuu Lightwing]

NERVAs work well and look cool with mk 2 fuselages. 6x symmetry around a 2.5m core looks nice and has decent fuel capacity. It also allows to attach up to 6 NERVAs so that TWR would be quite decent (craft at the pic has TWR about 0.5 on NERVAs).

[Acet]

Also you should probably make sure you start your burn to Duna during a transfer window (the period in time when it's possible to transfer from one celestial body to another with the smallest burn).

There are some mods that help you find a transfer window (MechJeb allows you to plan a transfer taking it into account and Kerbal Alarm System lets you set an alarm for the next available transfer window) and some sites allow you to find the best time to do a transfer.

[winged]

As others have said, either use the spaceplane LF tanks, or drain the oxidizer from the rocket LFO tanks, because the NERVA's don't use oxidizer anymore.

Draining oxidizer from LFO tanks will not help much because wet mass/dry mass ratio will be still poor. Spaceplane LF tanks is now only solution for NERVA's. With LFO is better to use Poodle or Terrier.

[Warzouz]

Here are some of my creations, all of them are at least able to go to Duna, some others can go beyond. If you look at the big one with the two cargo bay, he never went to space because i never lauched it (i will do it after my finals).

The others where used on 0.90 but will still work like a charm on 1.0.2

Did you tried the 2 last ones until reentry in 1.0.2 ?

I used some sort of MK3 ship bus to get XP from orbiting around the sun. I plugge a MK3 cabin and passenger cabin and adapater then heatshield. 3 LVN and a droppable tank.

I did numerous pass in high atmo to reduce speed as I could (from 3000 to 2400), but as I was reducing periaps, even with 4 aerobrakes, everything blasted very fast. I'm not sure your ship would reenter atmo alive.

But maybe your ship is designed to depart and arrive to a space station ?

[KerikBalm]

I disagree.

First part:

Draining oxidizer from LFO tanks will not help much because wet mass/dry mass ratio will be still poor.

Consider the orange tank: 36 tons full, 32 tons of LFO.

Of those 32 tons of LFO, 14.4 are LF, and 17.6 are O.

There is a big difference between a tank that is 4 tons when the LF is gone, and a tank that is 4+17.6= 21.6 tons

The mass ratio for a tank emptied of oxidizer is 4.182

The mass ratio for a tank not emptied of oxidizer is 1.667

HUGE difference

With LFO is better to use Poodle or Terrier.

No... lets assume no payload (payloads only skew things further in the direction of the LV-N)

Best mass ratio a poodle can achieve with LFO tanks is 9:1

The best mass ratio a LV-N can achieve with LFO tanks drained of O is 4.18:1

max dV from a LV-N stage with those tanks: 9.81*800*ln (4.18) = 11,225 m/s

max dV from a Poodle stage with those tanks: 9.81*350*ln (9) = 7,544

As you can see, the LV-N is still way way way better than the poodle.

Adding a payload makes the mass fraction worse for both, and the extra tank weight needed for the LV-Ns becomes more and more insignificant.

Useing spaceplane tanks is still preferable... but LV-Ns still outperform the other engines with standard rocket LFO tanks, provided you empty the O - which does have very very large benefits.

[Pax Kerbana]

You may be over engineering...

I find that a single nuke with a medium white tank is sufficient. That along with 2 hitchhiker a number of docking ports and of course solar panels should work fine. I keep my ships light and with only enough fuel for a one way trip so they can haul cargo (landers, station modules) easily and change orbits. Fuel for return is drained from any left over from the other mission craft If you're only sending one ship, make it twoâ€â€one for your Kerbals and lander, one for your rescue pod. Just make sure you send over enough fuel... A separate launch for small fuel tanker may be in order.

Edit: I don't always keep my ships light...

Edited May 19, 2015 by Pax Kerbana

[haltux]

Useing spaceplane tanks is still preferable... but LV-Ns still outperform the other engines with standard rocket LFO tanks, provided you empty the O - which does have very very large benefits.

I don't know how you can reach this conclusion. Terrier is way better in terms of pure Delta-V for any ship except massive ones, unless Kerbal Engineering is completely wrong in its computation.

- - - Updated - - -

About your ship, I think the main issue is that you have only one stage under your interplanetary spaceship, which is unlikely to be optimal. You save the weight of one engine, but I guess that once in orbit you have already burnt a significant part of the fuel of your interplanetary ship. That might explain as well why you need so much nukes, you use them to go to orbit.

You might want to have a smaller interplanetary ship but a larger, two-stage launcher, so that you use only the launcher's fuel to get to orbit.

Another suggestion: as you plan to have your spaceship in two parts (spaceship + lander), which is a good idea, I suggest to have more balanced parts. For example, you could have engine + fuel in one part and everything else (hitchiker + lander + some fuel?) in the other. They will be easier to put in orbit (without burning fuel dedicated to interplanetary trip).

[Wjolcz]

My old Duna design:

http://forum.kerbalspaceprogram.com/threads/36214-Manned-Extraplanetary-Vessels

It's pretty ugly compared to what I designed after that:

Javascript is disabled. View full album

It's from before 1.0 so it would probably overheat a lot.

Edit: Found some others I completely forgot about!:

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Gotta rebuild those sometime!

Edited May 19, 2015 by Veeltch

[lajoswinkler]

You don't need more than one LV-N engine.

This ship had an equivalent of 4 LV-N engines. TWR was initially below 0.05 I think. And it went 2 times farther than Jool.

I suppose you're running sandbox mode. To reach Duna you don't even need nuclear thermal propulsion because delta v required for the whole trip is relatively tiny compared to going to Moho.

Space ships do not need high TWR values. 0.1 is more than enough.

[winged]

I disagree.

First part:

Consider the orange tank: 36 tons full, 32 tons of LFO.

Of those 32 tons of LFO, 14.4 are LF, and 17.6 are O.

There is a big difference between a tank that is 4 tons when the LF is gone, and a tank that is 4+17.6= 21.6 tons

The mass ratio for a tank emptied of oxidizer is 4.182

The mass ratio for a tank not emptied of oxidizer is 1.667

HUGE difference

No... lets assume no payload (payloads only skew things further in the direction of the LV-N)

Best mass ratio a poodle can achieve with LFO tanks is 9:1

The best mass ratio a LV-N can achieve with LFO tanks drained of O is 4.18:1

max dV from a LV-N stage with those tanks: 9.81*800*ln (4.18) = 11,225 m/s

max dV from a Poodle stage with those tanks: 9.81*350*ln (9) = 7,544

As you can see, the LV-N is still way way way better than the poodle.

Adding a payload makes the mass fraction worse for both, and the extra tank weight needed for the LV-Ns becomes more and more insignificant.

Useing spaceplane tanks is still preferable... but LV-Ns still outperform the other engines with standard rocket LFO tanks, provided you empty the O - which does have very very large benefits.

ok, you're right but there is still no sense to use LVN with LFO if you have Spaceplane tanks.

@lajoswinker really cool ship

[ArchGaden]

For nukes, which run on liquid fuel only now, you really want to be using LF only tanks like the Mk3 parts for efficiency.

This is good advice. LV-Ns do not use oxidizer, so you do not want to pack it for LV-N stage. The Lf/O tanks will have wasted mass when you take the O out, so you can shave off a fair bit of mass by going with liquid only tanks. They are shaped a bit odd for the purpose, but they'll get the job done. Hopefully we'll get the ability to fill oxidizer space with more fuel in the future. You can do that with a mod right now if you want to deviate from stock.

http://wiki.kerbalspaceprogram.com/wiki/Mk3_Liquid_Fuel_Fuselage

You definitely don't want to carry so many LV-Ns. You really only need one, but I generally like to keep the TWR at about 0.3 (with payload) so that I don't have to spend 20 minutes burning. From the look of your design, it looks like you're using the LV-Ns to finish your orbital burn, and thus need higher TWR. I'd recommend cutting it down to one LV-N and then adding another stage below that with a poodle or 909. That second stage can finish your orbital burn and probably still give you a kick towards your destination.

For my own part, I generally create dedicated interplanetary tugs now. My current design is pretty simple. I use one or two of the big Mk3 5000 fuel tanks and one or two LV-Ns along with a bit of control (probe core + big reaction wheel + battery + RTG) and a docking port. Anything I plan to send to another planet will have a matching shielded docking port at the top. Then I'll just dock the LV-N tug up with it for the journey and drop it when we're in orbit at the destination. With mining, the tug can be refueled for a return trip and used for the next mission. Waste not, want not!

[Kuu Lightwing]

I don't know how you can reach this conclusion. Terrier is way better in terms of pure Delta-V for any ship except massive ones, unless Kerbal Engineering is completely wrong in its computation.

What craft did you test? Did you made the same mistake of giving NERVAs less than a half of the propellant compared to LFO engines, because you just removed the oxidizer, but didn't add more fuel instead?

[Sky_walker]

Why would you use LV-Ns for simple Duna trip? There's no reason. It can be safely reached with regular rockets.