designing a heavy lander

[Halfdan]

hello

i am currently working on a heavy lander to take a crew of 20 to the surface of duna (or any other low gravity world for that matter) but i ran into some issues

this is ce current design i came up with, it can get of the ground and land again,but the fuel consumption is very high, combined with a thrust that seems rather low, and i highly doubt it will reach orbit

if i ditch the monoprop tank and go for quad clustered vernor engines, the result only fares slightly better

so i ask for your thoughts and ideas

wich engines should i use? LF/ox? nuclear? one big one, or several small ones?
 

should i scrap it all together and start from scratch?

[Leafbaron]

If you have the Main sail unlocked, I would replace the THUDs with that. It looks like you have 8 times symmetry on the THUDS so a total thrust of 960Kn and each one weighs .9 for a total max of 7.2 tons w/ 285 isp in atmosphere and 305 vacuum isp. The main sail has 1500Kn of thrust and only weighs 7 tons, additionally it has better isp at 285 in atmosphere and 310 in vacuum. even better yet would be the Vector engine, coming in at 4 tons for the engine and 1000Kn of thrust in vacuum with 295 atmospheric isp and 315 vacuum isp.

[Leafbaron]

Additionally you could swap out the Mk3 cockpit for a mk3 to 2.5 adapter with LFO and use the hitchiker can and mk1-2 cock pit. 

[Halfdan]

sounds solid, but mainsail and vector have ourtrageous fuel consumption, doesnt that inherit the risk of running out of fuel to soon?

[Snark]

Moving to Gameplay Questions.

[septemberWaves]

On Duna, a Skipper would be better than a Mainsail for a vehicle of that size, but better still would perhaps be a couple of Swivels. I think you could get away with three of them for Duna (and the Mk3 engine mount might be a good part to attach them to), and then it's also easier to add landing legs. In any case though, for heavy landers it's best to prioritize thrust over efficiency, since it's easier to add more fuel when you have a higher thrust to work with (most of my Tylo landers are probably going to use Swivels despite their less-than-ideal vacuum I_sp for this purpose, as will any particularly large landers for Duna).

Although this is all said without knowledge of your intended mission profile. Will the craft exclusively operate on Duna, or will it transport kerbals between Duna and Kerbin in a SpaceX-like manner? If the latter, I think the same engine cluster I suggested earlier would be able to land it on Kerbin with low fuel, but I would be doubtful about its ability to reach Kerbin from the surface of Duna with only the fuel tanks in the current design.

Also, you probably don't need that amount of monopropellant, unless either you have a mod that adds a monopropellant fuel cell or you're unfathomably bad at docking. A couple of smaller radial tanks should be fine, and they won't cause too much drag loss in Duna's thin atmosphere.

[AeroGav]

Are you testing that on Kerbin?

You won't get realistic results.    Use the cheat menu,  Set Orbit (Duna), land it with Infinite Fuel enabled, then turn off the cheat and try going to orbit.

Duna requires way less delta V than Kerbin and much lower TWR.

Also vacuum engines outperform all others even at ground level.

Nuclear engines might work , perhaps with a some discardable LFO engines to start the ascent?

That said, i've only ever landed spaceplanes on Duna.  Never been there in a rocket so can't speak from experience in that regard.

[Halfdan]

i now tried a redesign based on your advice with the vector engie and added an RCS based on the vernor engine, so far it works well on kerbin ( able to climb to 35 kilometer and perform a propulsive landing) witch means it should work on duna in landing and ascending back to orbit after surface refueling 

[Halfdan]

22 minutes ago, eloquentJane said:

Although this is all said without knowledge of your intended mission profile. Will the craft exclusively operate on Duna, or will it transport kerbals between Duna and Kerbin in a SpaceX-like manner? If the latter, I think the same engine cluster I suggested earlier would be able to land it on Kerbin with low fuel, but I would be doubtful about its ability to reach Kerbin from the surface of Duna with only the fuel tanks in the current design.

mission profile is kinda sorta mars direct based

the lander will go to orbit on a seperate booster, then rendevouz and dock with a space tug along with all necessary colony modules, wich will take care of the transit, there it will wait for the colony ship to arrive, dock with it, perform the crew transfer and land at the colony site

from then on it will serve as a ground - orbit shuttle

[septemberWaves]

It's looking pretty good for that purpose - although I think you could cut down rather a lot on part count. You're using a lot of RCS thrusters, and whilst that is good in terms of power it might be an issue if parts are a big deal for you (if part count is not an issue for you then feel free to ignore this). If you do want to cut down on RCS thrusters, I would suggest placing them as minimalistically as possible (for your spacecraft, there could be two thrusters for all 6 directions) and then using fine control when docking (toggled with caps lock) will ensure RCS efficiency. And you might want to drain the monopropellant from the command pod if you haven't done so already.

Also, your engine pods look quite good, I like the use of the Mk2 parts for them. Although I can't help thinking that 4 vector engines are somewhat overpowered. A single Vector has the thrust to lift a mass of approximately 300 tonnes from the surface of Duna. Four Swivels, by comparison, have the combined thrust for at least 200 tonnes (it's closer to 250 tonnes but I'm estimating) from the surface of Duna, whilst having less than half the total mass of four Vectors (a Swivel is 1.5 tonnes whilst a Vector is 4 tonnes). They also have much better vacuum efficiency, which matters more on Duna than on a Kerbin ascent because of Duna's nearly-negligible atmosphere. Although if you're using Vectors for aesthetics then you've certainly made the right choice here.

[Firemetal]

Take a couple of Aerospikes instead. As long as you can fly without gimbal and have enough TWR, Aerospikes have a nice ISP. For lower gravity planets, Nukes could be a good choice.

Fire

[Halfdan]

thanks for the tips, consider this as the Mk1 version, i will send the first one to duna, see how it performs and then maybe downgrade it accordingly, also, i put a thrust limiter on the vectors for reduction of fuel consumption

if all works well, this vessel will get a name, if not its some scrap on the dunes and i will have to make some contracts to get the funds back

wish me luck guys, and as elon musk said: "i would like to die on mars, just not in impact"

[W. Kerman]

Good tip- While this thread is for a heavy lander, remember that kerbals get to their destination if they have comfy quarters or have bare bones chairs in containers.

You could really light that craft up.

[AeroGav]

1 hour ago, W. Kerman said:

Good tip- While this thread is for a heavy lander, remember that kerbals get to their destination if they have comfy quarters or have bare bones chairs in containers.

You could really light that craft up.

The mk3 passenger cabin is actually the lightest pod on a per seat basis - 16 kerbals for 6.5 ton.   And it comes with in-flight entertainment !

[hms_warrior]

you have far to much engine+fuel.

to launch from Duna into a low orbit you need 1450 dV let's do 1800 to be sure.

Your "payload" is 10 tons (cockpit+cabin).

adding 6 tons for fueltank, engine+ stuff gives us an empty weight of 16 tons.

Using 5 terriers would give you a isp of 340 and a launch-acceleration of ~5m/s²

You would need about 12 tons of fuel for the mentioned 1800 dV.

Basicly a mk3 to 3.75 adapter whit 5 terriers below. That is all you need.

 

Edited December 7, 2016 by hms_warrior

[Sharpy]

Yeah. Four vectors would be an overkill for Tylo. Duna needs much, much less. Especially, that you don't need much thrust on landing if you take a parachute - and vacuum engines perform at nearly full thrust on Duna.

Also,

14 hours ago, Halfdan said:

sounds solid, but mainsail and vector have ourtrageous fuel consumption, doesnt that inherit the risk of running out of fuel to soon?

A terrible fallacy. Two parameters of the engines matter: Specific impulse and Mass. Thrust matters to some degree, but above TWR of 2 it's really moot.

Most of the options presented so far provide you with TWR of 4 or more, and a better or worse ISp at excessive mass. Only Terriers suggested seem reasonable.

Thuds have abysmal specific impulse. They will consume far more fuel much faster for the same amount of delta-V. Yes, Mainsail is fairly heavy. Vector is pretty heavy but has decent ISp and awesome thrust. Eight Thuds are damn heavy.

I'd check whether Poodle gives you enough TWR to launch from Duna surface and go with it. Alternatively, a couple Terriers - or nukes, if you can switch your tanks for LF only. Otherwise there's little point - they won't offer a considerable benefit on standard LFO tanks (even with oxidizer depleted) and they offer a whole slew of minor headaches that all add up to a headache worse than it's worth.

Terrier(s), Poodle, *maybe* nukes. A parachute or a couple, not too many - use the engines to slow down the last few m/s before touchdown. Also, MK2 fuselages have abysmal dry to wet ratio. Don't use them unless you need the lifting surface.

[AeroGav]

2 hours ago, Sharpy said:

Yeah. Four vectors would be an overkill for Tylo. Duna needs much, much less. Especially, that you don't need much thrust on landing if you take a parachute - and vacuum engines perform at nearly full thrust on Duna.

Also,

A terrible fallacy. Two parameters of the engines matter: Specific impulse and Mass. Thrust matters to some degree, but above TWR of 2 it's really moot.

Most of the options presented so far provide you with TWR of 4 or more, and a better or worse ISp at excessive mass. Only Terriers suggested seem reasonable.

Thuds have abysmal specific impulse. They will consume far more fuel much faster for the same amount of delta-V. Yes, Mainsail is fairly heavy. Vector is pretty heavy but has decent ISp and awesome thrust. Eight Thuds are damn heavy.

I'd check whether Poodle gives you enough TWR to launch from Duna surface and go with it. Alternatively, a couple Terriers - or nukes, if you can switch your tanks for LF only. Otherwise there's little point - they won't offer a considerable benefit on standard LFO tanks (even with oxidizer depleted) and they offer a whole slew of minor headaches that all add up to a headache worse than it's worth.

Terrier(s), Poodle, *maybe* nukes. A parachute or a couple, not too many - use the engines to slow down the last few m/s before touchdown. Also, MK2 fuselages have abysmal dry to wet ratio. Don't use them unless you need the lifting surface.

Since the crewed bits are mk3, how about mk3 liquid fuel fuselage short plus mk3 engine mount, which has four attach nodes?

Cockpit and Cabin - 10T

Nukes -  12T

LF - 12.5T

Mk3 tank dry mass and engine mount - 2.5T

Total Weight - 37T

Delta V  3738

TWR on Duna 2.16 to 1 !

That could probably double up as the transfer stage yikes.

 

If you wanted separate lander and Xfer stage,  a single poodle with a mk3 to 2.5m adapter should do as the lander.   Nose of the cockpit could carry a shielded docking port, which is used to dock to a mk1 fuel tank and a nuke, for the transfer engine (which can be left in orbit while visiting surface).  You could put a decoupler between the passenger cab and the landing engine and fuel so you can blow them off when done. 

 

[Halfdan]

44 minutes ago, AeroGav said:

 

If you wanted separate lander and Xfer stage,  a single poodle with a mk3 to 2.5m adapter should do as the lander.   Nose of the cockpit could carry a shielded docking port, which is used to dock to a mk1 fuel tank and a nuke, for the transfer engine (which can be left in orbit while visiting surface).  You could put a decoupler between the passenger cab and the landing engine and fuel so you can blow them off when done. 

 

i am planning on sending a whole package anyway, a remote operated huge spacetug that carries the lander in its front (Sr. docking port to avoid wobble and corresponding sr port on the underside of the lander hence also no use of the mk3 engine mount) and 4 colony modules on its side

and it already has a shielded docking port at the nose  

what you are proposing is a one way lander, but i want it to be reuseable after carrying the crew to the surface it is planned to operate as a ground to orbit shuttle as i said above, wich negates the use of parashoots and decouplers

Edited December 7, 2016 by Halfdan

[AeroGav]

7 minutes ago, Halfdan said:

i am planning on sending a whole package anyway, a remote operated huge spacetug that carries the lander in its front (Sr. docking port to avoid wobble and corresponding sr port on the underside of the lander hence also no use of the mk3 engine mount) and 4 colony modules on its side

and it already has a shielded docking port at the nose  

what you are proposing is a one way lander, but i want it to be reuseable after carrying the crew to the surface it is planned to operate as a ground to orbit shuttle as i said above, wich negates the use of parashoots and decouplers

OK,  you could radially attach four  stacks of mk1 liquid fuel fuselages around the outside of the pax cabin and put a nuke on the bottom of each.  

Or use LF/O tanks and Terriers instead.

Terrier option would be 2 tons of engine and 12 tons of fuel, nuke would be 12 tons of engine and a bit under 6 of fuel.

Terrier option therefore gives a lighter lander and makes job easier for transfer stage, but the surface base will have to mine more fuel with IRSU for the repeated trips.  That's not usually a problem though.

[foamyesque]

18 hours ago, AeroGav said:

The mk3 passenger cabin is actually the lightest pod on a per seat basis - 16 kerbals for 6.5 ton.   And it comes with in-flight entertainment !

 

Excluding chairs, yes. In principle you could stack a pile of chairs inside a cargobay and beat that. I think that's way more hassle than it's worth, though.

[Halfdan]

i continued the  redesign and testing based on the informations you guys gave me

changed the engines to swivels, remade the RCS clusters to an apollo style approach, ditched the MK2 engine pods and replaced them with mk1s

it handles pretty well, so far, stats are as follows:

Vac dV is 2588 m/s

TWR is 4,38 

based on data from mechjeb dV viewer set for duna 

i just pray the fuel is enough

Edited December 8, 2016 by Halfdan

[hms_warrior]

That looks far more resonable^^

regarding your dV concerns: launching into low orbit takes ~1500 dV. Deorbiting takes ~ 100 dV terminal-velocity whitout parachutes should be around ~ 500 m/s so you need 500 dV to stop+ extra for hovering 1 second of "hovermode" costs 3.8 m/s dV basicly you have 400 m/s dV for hover and "oh excrements" situations.

small improvements:

TWR is still a bit high wich means you carry more deadweight in engines than nessesary.

You can save dV on landing by adding 2 small and 2 big radial chutes on the cockpit (place them close to the exit-ports, than you can EVA with a engineer and repack them).

add 1-2 small solarpanels just in case.(or maybe you have allready and they are just not visible on the screenshot)

Mistakes to avoid:

Since the vernors run on LF/OX make sure you remove the monoprop from the cockpit. saves about 0.4 tons.

IF you add parachutes: remember to set to min.pressure.

Edited December 8, 2016 by hms_warrior

[foamyesque]

I don't think the fuel lines are needed. Tail connectors allow crossfeed and the new fuel flow model will allow them to transfer from things they are radially attached to.

[Halfdan]

2 hours ago, hms_warrior said:

That looks far more resonable^^

regarding your dV concerns: launching into low orbit takes ~1500 dV. Deorbiting takes ~ 100 dV terminal-velocity whitout parachutes should be around ~ 500 m/s so you need 500 dV to stop+ extra for hovering 1 second of "hovermode" costs 3.8 m/s dV basicly you have 400 m/s dV for hover and "oh excrements" situations.

small improvements:

TWR is still a bit high wich means you carry more deadweight in engines than nessesary.

You can save dV on landing by adding 2 small and 2 big radial chutes on the cockpit (place them close to the exit-ports, than you can EVA with a engineer and repack them).

add 1-2 small solarpanels just in case.(or maybe you have allready and they are just not visible on the screenshot)

Mistakes to avoid:

Since the vernors run on LF/OX make sure you remove the monoprop from the cockpit. saves about 0.4 tons.

IF you add parachutes: remember to set to min.pressure.

okay, thanks for the tip with the monoprop, forgot about that

i didnt use solar panels, since it is powerd by two RTGs

i am honestly not sure where i could save further weight

[hms_warrior]

You do not realy need to save more weight as your deltaV is inside that what you need, with a ~200 m/s margin. you could trade the engines for terriers (2 terriers + 1 2-to-1 adapter for each swivel) that would save 2 tons empty weight and increase your ISP by 25 should be good for an other 400 m/s extra deltaV. But since you have allready enough deltaV it would just mean more safety-margin.

Edited December 8, 2016 by hms_warrior