Why Landers?

[Kuroki]

Last night I decided I'd try out an Apollo-style Mun landing, complete with CSM and LM and I pulled it off (yay!) but it got me thinking. My craft would have been much more efficient if I'd skipped the LM entirely and slapped some landing legs on the CSM.

I would have left less fuel behind on the Mun in the landing stage of the LM and wouldn't have been carting around a redundant lander can mk.2 and set of engines, let alone docking ports and stability struts. It seems that having a separate lander is only more efficient for interplanetary trips, and even then only to leave the main drive in orbit without redundant crew pods etc.

Am I missing something aside from extra crew safety that makes the separate lander worth the extra delta-V? On a side note, does anyone know the reasoning behind why NASA went with the LM instead of a landing CSM? Google doesn't tell me much on the why front...

Also, hi! New to the forums

Edited June 18, 2015 by Kuroki

[Tourist]

Actually direct ascent was preferred by many including Von Braum for some time. Also earth (as opposed to moon) orbit rendezvous. Lunar orbit rendezvous was a dark horse. Can't remember reason why it won out precisely, ultimately cheaper in terms of fuel/weight requirements because you don't have to take your return fuel down to the surface and back. Doesn't work in KSP though because everything is smaller... The smart kids could explain it better. I think Scott Manly discusses it in his new beginners series.

[winged]

People do this because they want to make Apollo-style mission. You're right it's very inefficient in stock system.

So you wondering why NASA did it in real life. This is very easy to explain. In real life Moon is 10 times bigger than Mun so you need ~1600m/s to make descent/ascent instead of ~700m/s, also you need 800m/s if you want go back to Earth from low Moon orbit (300m/s in stock system). With much bigger delta V numbers Apollo-style landings will be more efficient.

I'm using Real Solar System mod and I can tell you that Direct style with CSM will need to build 50% heavier rocket than Saturn V.

Edited June 18, 2015 by winged

[Tourist]

There is a podcast called 'space rocket history' on iTunes. Discusses that very question in extreme detail.... And I mean extreme detail.

[georgTF]

In fairness real engines and tanks are much more efficient than those in KSP. Since KSP has a smaller planetary system the real engines would have been overkill. Probably the main reason would be to not risk the return vehicle getting damaged on the Moon. It was a first landing afterall and nobody knew for sure if they'd land safely or return after landing so in the worst case at least someone would make it back. Someone close enough to help understand what went wrong.

Reliability and crew safety are comically ignored by KSP. I don't mean that as a knock on KSP, just saying it's something our flights never have to factor in.

[Kuroki]

Thanks for the answers guys.

People do this because they want to make Apollo-style mission. You're right it's very inefficient in stock system.

So you wondering why NASA did it in real life. This is very easy to explain. In real life Moon is 10 times bigger than Mun so you need ~1600m/s to make descent/ascent instead of ~700m/s, also you need 800m/s if you want go back to Earth from low Moon orbit (300m/s in stock system). With much bigger delta V numbers Apollo-style landings will be more efficient.

I'm using Real Solar System mod and I can tell you that Direct style with CSM will need to build 50% heavier rocket than Saturn V.

I can understand this, and you're probably right, but at the very least couldn't you skip the extra pod with its avionics, life support, docking equipment, etc. and just leave the extra fuel in orbit?

There is a podcast called 'space rocket history' on iTunes. Discusses that very question in extreme detail.... And I mean extreme detail.

I wonder if there's a text version, sounds interesting

[RCgothic]

There were three modes considered for the Apollo mission.

Direct Ascent, in which the entire spacecraft goes to the moon and lands before returning to earth.

Earth Orbit Rendevous, in which the components of a Direct Ascent are assembled in earth orbit by multiple smaller individual launches.

Lunar Orbit Rendezvous, which is the classic Apollo mission.

Of these, Direct Ascent was the front-runner because in 1960 nobody had even achieved a rendezvous, let alone a docking.

Lunar Orbit Rendezvous was by far the more efficient, but very much less safe conceptually. Once it was chosen, The Gemini program was launched with the main objective of pathfinding these technologies.

Earth Orbit Rendezvous was a compromise between the two.

Interestingly the Service Module engine was sized for a Direct Ascent (though the module itself would have been bigger), and was massively overpowered for Trans-Lunar Injection burns. They were stuck with the engine developed for Direct Ascent! Note that the Orion engine is much smaller proportionately.

Direct Ascent works best in KSP because proportionately the required DV is a lot less and rendezvous is a hassle. Larger missions both in KSP and planned IRL benefit from both Earth/Kerbin Orbit Rendezvous and Target Orbit Rendezvous efficiencies.

Edited June 18, 2015 by RCgothic

[Corw]

Op, you are missing the point. You have overdesigned your crafts so you are taking wrong conclusions out of it. You left fuel behind which means you wasted all the resources you've burnt to bring it there etc. Apollo mission profile is bringing minimum mass required for each part of the mission. Less mass on top means less mass in each stage before it, which is most visible in your first lifter stage.

If you go with the lander you are not wasting fuel landing and lifting back to space return stage (Mun->Kerbin). You also need smaller return stage because you have discarded the mass you needed to land etc.

Try building two crafts, one Apollo style, other direct descent and compare them. They will be able to do same mission, but Apollo style one should always be smaller and cheaper. If you do not overbuild them, that is

Edited June 18, 2015 by Corw

[Dawnkeeper]

Having a landing and an ascent stage also allows you to leave extra mass behind and thus to build a smaller return craft.

[Harry Rhodan]

http://flyonbudget.one-giant-leap.info/chapter3.html

[Dilli]

Lander approach allows the lander have TWR set according to the body and then a lower TWR but higher efficiency "CSM" for orbit transfers. It also avoids the waste of accelerating / decelerating fuel during the landing and ascent that is to be used to return the craft to Kerbin, send the craft elsewhere, or land a second time.

IMHO unless you are planning mutliple landings on the mun direct return of the Lander to Kerbin is easier but not more efficient than a "CSM" and "LM" approach. Once interplanetary or multiple landings are involved the LM and CSM advantage over and all-in-one craft is significant (even if it is just to refuel the lander which will do the return)

[Rhomphaia]

Op, you are missing the point. You have overdesigned your crafts so you are taking wrong conclusions out of it. You left fuel behind which means you wasted all the resources you've burnt to bring it there etc. Apollo mission profile is bringing minimum mass required for each part of the mission. Less mass on top means less mass in each stage before it, which is most visible in your first lifter stage.

If you go with the lander you are not wasting fuel landing and lifting back to space return stage (Mun->Kerbin). You also need smaller return stage because you have discarded the mass you needed to land etc.

Try building two crafts, one Apollo style, other direct descent and compare them. They will be able to do same mission, but Apollo style one should always be smaller and cheaper. If you do not overbuild them, that is

Does not really work like that, in KSP Engines are far heavier, so an Apollo style mission with separate service, decent and ascent propulsion systems will lose out to a single engined direct ascent, also in KSP re-entry protection is still unnecessary coming back from Mun, so you don't have to worry about landing and returning heavy heat shields.

[Corw]

Does not really work like that, in KSP Engines are far heavier, so an Apollo style mission with separate service, decent and ascent propulsion systems will lose out to a single engined direct ascent, also in KSP re-entry protection is still unnecessary coming back from Mun, so you don't have to worry about landing and returning heavy heat shields.

In principle it mostly does, as I do not have separate descent and ascent engines. I'm not 100% sure as I can't test it right now (at work).

And the heat shield part is bit moot as it is a bug that should be fixed. And it is one of the reasons why I'm not playing since 1.0.2 has came out...

[FancyMouse]

And in fact KSP's mk1-2 pod and mk2 lander can are as heavy as hell - you save more than half the mass even when you just stick mk1 lander can for the same crew capacity. This probably accounts for your underestimate of lander approach.

For me, other than aesthetic reason, I don't see any use of mk1-2 pod and mk2 can. And if it's for aesthetic, I don't care efficiency...

[Kuroki]

http://flyonbudget.one-giant-leap.info/chapter3.html

http://flyonbudget.one-giant-leap.info/_images/lander.jpg

This is exactly what I was looking for, thanks! Lots of people pointed towards this answer, but the detailed explanation and maths on that link answer my question perfectly.

So it is usually not worthwhile to have a lander in an efficient KSP design, but is in the real world depending on circumstances.

And in fact KSP's mk1-2 pod and mk2 lander can are as heavy as hell - you save more than half the mass even when you just stick mk1 lander can for the same crew capacity. This probably accounts for your underestimate of lander approach.

For me, other than aesthetic reason, I don't see any use of mk1-2 pod and mk2 can. And if it's for aesthetic, I don't care efficiency...

This is also very true, and these days I prefer pretty rockets over ultra-efficient ones.

I'm still not sure the real Apollo missions wouldn't have been more efficient by cutting the LM and leaving the SM with return fuel and heat shielding in orbit, but I can see the advantage of the LM over just landing the whole CSM now.

[Scarecrow88]

Whilst in KSP it is quite easy to make a lander to go to the surface of the Mun and then return to Kerbin, I just find it more engaging and fun to have a 2-stage lander and to do a Mun mission Apollo style. Frankly, there's no right or wrong way, just do whatever you are happiest doing.

[Laie]

What Harry Rhodan said: it's basically a question wether the additional comand module and engine outweighs the fuel savings.

In KSP, equipment is heavier than in reality, while dV requirements are lower. Therefore it's often better to just add legs to your vessel and use the pod and engine you have, rather than bring along a second vessel with all that entails.

Also I suspect in RL it's not as simple as just adding legs to any vessel and declare it a lander.

[carlorizzante]

Does not really work like that, in KSP Engines are far heavier, so an Apollo style mission with separate service, decent and ascent propulsion systems will lose out to a single engined direct ascent, also in KSP re-entry protection is still unnecessary coming back from Mun, so you don't have to worry about landing and returning heavy heat shields.

Interesting. I'm new to the topic and this thread is gold.

So, what about leaving the fuel in orbit around the Mun, landing with the strictly necessary, and getting back to it later on the way home?

That way one won't need two engines (for lander and mother ship), but just one. What's left in orbit is the fuel for the trip back home.

[beabop]

In the Kerbol system, I think it makes more sense on bigger bodies that are further away in delta V from Kerbin.

For Eve, I think it's pretty much a requirement to have a separate lander.

[Rhomphaia]

What Harry Rhodan said: it's basically a question wether the additional comand module and engine outweighs the fuel savings.

In KSP, equipment is heavier than in reality, while dV requirements are lower. Therefore it's often better to just add legs to your vessel and use the pod and engine you have, rather than bring along a second vessel with all that entails.

Also I suspect in RL it's not as simple as just adding legs to any vessel and declare it a lander.

Indeed not, KSPs liquid fuel rocket engines are all infinitely restartable and can throttle control all the way from zero to max. Makes it a lot easier to use one rocket for all jobs.

[Whirligig Girl]

In real life, you definitely want the Lander/CSM model because of the fact that Moon is 10 times bigger than Mun. In KSP, you have to be really efficient to get a Lander/CSM to be more efficient than a direct lander. The nerfs to engine Isp in 1.0 helped with making mass ratios bigger, so landers were more required. Once you do things like going out to Dres or Duna or the Joolean Moons, then you will want to do the lander/CSM stuff.

It's worth noting that the first time I returned from the Mun was an Apollo-style mission, despite having landed on the Mun several times before.

[xtoro]

Not worth a lander: Moons like Minmis, Gilly, Bop etc, low grav. If I'm supposed to land and collect science from Bop, I just send a probe, not even any landing legs, just drop onto the engine...

[monstah]

So, what about leaving the fuel in orbit around the Mun, landing with the strictly necessary, and getting back to it later on the way home?

The fuel you need to eject from the Mun to LKO/aerobreak is so small compared to landing/taking off from Mun, that the hassle to dock with it is not worth it.

Hell, if you land on the Mun's east(?) side, you can just launch straight up and never bother orbiting the Mun at all!

[m00nchile]

The fuel you need to eject from the Mun to LKO/aerobreak is so small compared to landing/taking off from Mun, that the hassle to dock with it is not worth it.

Hell, if you land on the Mun's east(?) side, you can just launch straight up and never bother orbiting the Mun at all!

Launching straight up, aka radial out is inefficient, because you're fighting gravity all the way. Following prograde will always cost less delta v.

[mikegarrison]

Launching straight up, aka radial out is inefficient, because you're fighting gravity all the way. Following prograde will always cost less delta v.

I don't think that's true.

If you want to get into an orbit, then yes, you want to burn sideways and generate that velocity vector. But if you just want to get out of the Mun's gravity well, it seems like going straight up would be best. Why generate a velocity vector sideways that you aren't going to use?