Why Landers?

[Archgeek]

I guess I'm going to have to find some time to do the calculus, but I think this is wrong. You have to "fight the Mun's gravity" no matter what.

Let's specify the question. You are already sitting on Mun, facing retrograde to your Kerbin orbit. (Ignore Munar rotation for now, because it is a small effect.) Is there any direction you can burn other than straight up which will give you a lower delta-v return back to Kerbin?

I'm not asking if there is any other location on Mun that has a lower d-v requirement to get back to Kerbin. I'm asking, from that spot (facing retrograde), is there any trajectory that gets you back to Kerbin better than just straight up? I think not.

You do fight the mun's gravity, but you don't have to fight it as much. I once thought as you did, but a couple of little experiments proved me wrong. I just tried both things from the same quicksave, and checked how much fuel I had left once I made good my escape, then calculated remaining delta-v using notepad and calc.exe (this was before I started using KE). The flat exit beat the direct ascent handily. Also, it seems I was wrong on the Oberth bit, there. Direct Ascent would get a weaker Oberth effect due to rampaging out of the gravity well as quickly as possible rather than burning deep within it.

Ohoh, moving the goalposts, eh? That's not fair, but my advantage is less fair so I'll allow it. The answer's simply east -- you just burn up clear of local terrain, burn east until you've an apoapsis clear of the mountains, then whip around the Mun until you're a bit past 180 degrees of your starting point, then burn hard prograde (nice and near the surface, you'll note) into an ejection angle that's nicely retrograde to Kerbin until your Kerbin PE is at your favourite re-entry height, 'same as you would with having burned straight up earlier.

The dealyotrope is simply that energy put into an orbit is never lost, you can add to it whenever you like, and by burning closer to the surface you get a nice Oberth effect savings on your delta-v. It is true you're still "fighting gravity" by having to put enough energy into it to escape, but by opposing it directly, you wind up moving slower at the end of your burn than you would be burning near the surface (which is kinda how the Oberth effect works). It's a small savings on the mun, pointless on Minmus, but nigh-mission-critical on Tylo.

[RCgothic]

Just draw a force vector diagram. Say your TWR is 2.

Vertical Ascent goes up 2 and down 1 due to gravity. Resultant up acceleration is 1.

Alternatively, burn at an angle just enough to hover, but pick up horizontal velocity. (Assume launch from a mountain summit with no obstacles to slam into). With a vertical thrust component of 1 and total thrust of 2, by Pythagoras the horizontal thrust is 1.73. Note the vertical component cancels out with gravity, but the horizontal component remains 1.73!

So that this is nearly twice as big as the Vertical Ascent case! You'll reach escape velocity on almost half the burn time and therefore half the fuel.

Exact figures vary depending on TWR, but at any reasonable TWR burning horizontally will always be significantly more effective than simply fighting gravity.

Edited June 19, 2015 by RCgothic

[Orbital Vagabond]

This is kind of a pet peeve, but the terms "Direct Ascent" and "Vertical Ascent" aren't interchangable. Direct ascent describes the mission profile for landing the whole ship on the Moon, not how that vehicle takes off. Direct ascent is in contrast to Earth Orbit Rendezvous (EOR) and Lunar Orbit Rendezvous (LOR) lunar mission profiles. Vertical ascent is a way to get off the surface. A vessel on a "Direct Ascent" mission could take off into a vertical ascent trajectory, or a go into a parking orbit first.

As a note: The LOR profile won out for the Apollo missions because the direct ascent would have required a vessel larger than what NASA could build (Von Braun's 'Nova' rocket) and the EOR mission was too complex (one study estimated it would require 10 or more Saturn 1 launches). The Soviets probably would have used the EOR profile. In KSP, the weight savings in fuel of the LOR mission to the Mun is really ouweighed by the increased weight of the lander you have to shuck out to the Mun. Hence, the direct ascent profile has become a popular choice.

Also, I found the thread where I saw those videos posted. I'm NOT going to provide a link to the thread though, because it got extremely toxic. Best to leave that trash buried.

Edited June 18, 2015 by Orbital Vagabond

[Archgeek]

This is kind of a pet peeve, but the terms "Direct Ascent" and "Vertical Ascent" aren't interchangable. Direct ascent describes the mission profile for landing the whole ship on the Moon, not how that vehicle takes off. Direct ascent is in contrast to Earth Orbit Rendezvous (EOR) and Lunar Orbit Rendezvous (LOR) lunar mission profiles. Vertical ascent is a way to get off the surface. A vessel on a "Direct Ascent" mission could take off into a vertical ascent trajectory, or a go into a parking orbit first.

Oops, good point there. I was getting a bit fast and loose with the terminology. Also, I clearly confused myself a little and attributed too much to the Oberth effect. There is some oberthage going on, as an orthagonal burn escape does do its burning close to the surface and thus deep in the gravity well, but yeah, the main savings is in mostly ignoring gravity. Once you're safely falling around the mun, every kN of thrust goes into your escape, while on a vertical ascent, you're spending your might on holding yourself up on a pilar of flame, and only get to keep the difference.

[monstah]

I didn't mean to spark such a fire!

Anyway, to put it briefly, yes, straight up there are gravity losses. But those are minimal on Mun, so if you're on the side that's retrograde with respect to Kerbin orbit, the amount you lose to gravity is less than would be needed to orbit before ejecting.

I didn't do any math, mind you, just experimented a bit. May vary with TWR, for instance.

[Eric S]

I didn't mean to spark such a fire!

No worries. To be honest, the only reason it matters to me is that I don't want anyone taking incorrect statements as gospel. Which of course means that we need to determine which ones are incorrect.

Anyway, to put it briefly, yes, straight up there are gravity losses. But those are minimal on Mun, so if you're on the side that's retrograde with respect to Kerbin orbit, the amount you lose to gravity is less than would be needed to orbit before ejecting.

I didn't do any math, mind you, just experimented a bit. May vary with TWR, for instance.

Can you be a little more specific on your experiments? I'm just trying to figure out where the discrepancy is between your experiments and this experiment

I did a quick experiment with a probe that landed on Mun. With a single 48-7S engine, the initial TWR was 5.94 on the surface. A direct vertical ascent until the escape velocity required 833 m/s, while a horizontal ascent took just 777 m/s.

and the description on how the orbital energy worked that I posted, which agrees with Jouni's results. I think I'm right, and if I'm wrong, I'd like to know before I spread the wrong ideas any further.

[Orbital Vagabond]

Oops, good point there. I was getting a bit fast and loose with the terminology.

Just had to say it. I thought it was especially relevant to make the distinction since the thread started on one (LOR/EOR/DA) and moved to the other (Vertical ascent).

Also, I clearly confused myself a little and attributed too much to the Oberth effect. There is some oberthage going on, as an orthagonal burn escape does do its burning close to the surface and thus deep in the gravity well, but yeah, the main savings is in mostly ignoring gravity. Once you're safely falling around the mun, every kN of thrust goes into your escape, while on a vertical ascent, you're spending your might on holding yourself up on a pilar of flame, and only get to keep the difference.

Yeah, the difference is ultimately not that great in the case of the Mun. Other bodies aren't so forgiving though.

Anyway, to put it briefly, yes, straight up there are gravity losses. But those are minimal on Mun, so if you're on the side that's retrograde with respect to Kerbin orbit, the amount you lose to gravity is less than would be needed to orbit before ejecting.

I didn't do any math, mind you, just experimented a bit. May vary with TWR, for instance.

I would refer you to the videos I linked in my previous post. They provide evidence that directly contradicts your statement that gravity loses are less than "wasted" prograde dV.

You lose more going straight up than you lose going into orbit first. The statement you're making (Vertical ascent suffers less losses than a parking orbit) doesn't stand in either theory or practice. Differences are small, but the vertical ascent is not the more efficient of the two, and it never will be. TWR will reduce the difference, but it will never cross the line.

Edited June 19, 2015 by Orbital Vagabond

[NecroBones]

Usually the most efficient "burn" is one that is firing along your current velocity vector. However, during ascent, lateral (horizontal) acceleration is kinetic energy that you get to keep without gravity losses.

On an airless world, you want to pitch over toward horizontal as early as you can without hitting a mountain or something. Then, performing your escape burn with a prograde (relative to your velocity vector) orientation, ideally horizontally to minimize gravity losses, will give you the most efficient results, whether you have circularized first (easiest), or you're still ascending on a suborbital trajectory. This means that yes, you can efficiently burn straight into an escape from the ground, but not vertically, and it's more difficult to get the timing and the escape vector correct.

To get the most out of a single-burn ascent-to-escape, you would need to time your lift-off (in terms of your landing site's position and Mun's orbital phase) such that a horizontal acceleration will put you on a trajectory that curves around the Mun and places your escape perfectly retrograde with respect to Mun's orbit around Kerbin. Effectively this lets you take advantage of the Oberth Effect at a very low altitude, by burning horizontally as early as possible.

Having said that, circularizing first is a lot easier and doesn't cost much more energy in comparison. It will probably cost less dV than a poorly executed single burn escape. And it will definitely cost less than a vertical ascent.

EDIT: One last thought-- If you have an extremely high TWR, your time spent fighting gravity losses while burning vertically is reduced. With an overbuilt lander, the efficiency gap is reduced. And burning straight up is pretty easy too. But just know that brute-forcing a vertical ascent (fighting gravity) is always less efficient than working with gravity. KSP is very forgiving with it's smaller distances and velocities than reality, so you can do a lot of things "wrong" and get adequate results.

Edited June 19, 2015 by NecroBones

[Superfluous J]

To throw my thoughts in on the "why a lander?" question, if you're going to Mun and coming home no, a separate lander is overkill.

However, if you're trying to land 2 or more times (to, say, snag multiple biomes) then having a separate lander that can get back into orbit to refuel and go back down quickly becomes far more efficient than bringing all that fuel with you. You can snag 9+ Mun biomes in 3 landings if you pick your spots well, and instead of sending 3 full missions (and therefore 3x the cost and mass) that can each land once, send one mission that masses less than twice as much (as all you're bringing along is some extra fuel) and get it all done in one mission.

[Corw]

I didn't mean to spark such a fire!

Anyway, to put it briefly, yes, straight up there are gravity losses. But those are minimal on Mun, so if you're on the side that's retrograde with respect to Kerbin orbit, the amount you lose to gravity is less than would be needed to orbit before ejecting.

I didn't do any math, mind you, just experimented a bit. May vary with TWR, for instance.

There is additional point. If you have high TWR, means you have bigger engine then required for the mission. So your vessel is overbuilt to start with and competing vessel would be smaller and required less fuel.

[RCgothic]

Comments on Vertical Ascent vs Direct Ascent noted and edited.

The experiment that showed 833 vs 777 ms-1 is a smaller difference than the maths for TWR of 5 would suggest by about 2/3rds. Then I realised that of course TWR increases during a burn, and thus would naturally reduce the difference between the two escape profiles. For a small payload fraction with a high TWR, the difference may not be much.

However, an angled burn will still always win, and for a low TWR (optimised) craft it is especially wasteful to burn straight up.

[Raptor9]

All the engineering and conceptual points aside, an Earth Orbit Rendezvous is what I have found to be the cheapest in my career mode. Obviously costs will vary based on a players design technique, how much scientific equipment he puts on board, and other factors based on play style.

An all in one shot Apollo-style rocket I started with cost me ~156,000 funds.

Doing two separate launches with the upper stage of the lander rocket providing the Munar transfer burn, cost me ~115,000 funds total. But I also have a lot of redundancy in the capsule and lander, I could drive the costs down further if I wanted to get to barebones what is needed. (removing the LES tower, getting a cheaper less capable probe core on the lander's launcher, more solid-fuel propulsion/less LFO engines, etc.)

Edited June 19, 2015 by Raptor9

[John FX]

my opinion is that they just wanted to make sure that one man returned from the mission and if the guys died on the Moon at least Michael Collins would have returned.

[tutike2000]

Am I missing something

You could also just have a fuel tank instead of the transfer module, and use the lander engines exclusively.

Leave a whole bunch of fuel in orbit so you'll have enough to return. But yes, it's not really worth it since you can return to Kerbin cheaply anyway.

Also, the bigger the transfer stage, the more likely it is that a lander will be worth dragging around. You could also abandon the lander and only return with the transfer stage. Lots of combinations to play around with...

[Terwin]

Something else to consider for missions outside the Kerbin SOI: ISRU

I have an up-coming Duna/Ike mission and I intend to land the whole thing on Ike to refill the fuel tank.(fully fueled the Transfer/Ike refinery stage can do ~8km/s, which is good as I am not using a transfer window, in fact I'll probably leave Minmus with an additional 1500 ore to refine in-flight, just in case. The only time I made it to Duna before was in the sandbox and there was not enough fuel to do more than pass through the edge of the Kerbin SOI on the return[0.90])

(If I can also lift off from Duna with enough fuel to get to Ike, I may not need to bring a lander at all)

[Nich]

Reasons to use a lander in KSP

1) Multiple landings greatly increases the efficiency (my lander lands on the moon and rendezvous with the mother ship for only 180 LF) I think it is funny someone suggested this on Minimus as it is total overkill)

2) If you play with life support most of that can stay in the mothership

3) If you play with deadly reentry that can stay in the mother ship

4) In career mode attempt building a craft capable of landing on the moon with 30 parts that has 2 of each science and no 90+ tech. Cant be done I may make an official challenge.

Reasons not use use a lander in KSP

1) Lander cans are WAY to heavy I believe .77 tons (1670 lbs) when they should really be .077 (160 lbs)

2) Only doing a single landing

3) There are no small ultra light landing gear

4) you can land on your engine without a care in the world (personally I wouldn't do that to my only ride home)

5) No good RCS choices for 1 ton ships, wish they have something 10 times smaller/lighter

6) The spark is just a little to small for my taste and the terrior is to powerful.

Reason to use a LM CSM in real life

1) redundant life support

2) redundant power supplies

3) redundant propulsion systems

Honestly I think accelerate straight up guy is a TROLL stop feeding him

[Archgeek]

Honestly I think accelerate straight up guy is a TROLL stop feeding him

Nah, I used to think like that myself. Orbital mechanics is kind of easy in patched conics land, but it's still a little unintuitive for creatures who grow up this deep in a gravity well. It's to the point that those of us who know better sometimes only store the result in our heads and run around in circles when trying to support it. </self-deprecation>

One has to latch on to the fact that with the right timing, horizontal takeoffery can have the exact same ejection benefit as vertical ascent, meaning they're on equal footing save for gravity drag, which is usually enough to get the logic engine to turn over in the face of one's false intuition.

[wumpus]

A few other issues:

"lander cans" vs. "command chair, fuel, engine.

Pros [lander can] You can store science in a lander. You can also do a crew report in a lander.

Pros [command chair]: .2T per Kerbal does wonders to your delta-v. Put two chairs symetrically across the fuel tank. Let Jeb do the flying [no SAS module needed], Bob do the science ]land multiple places and reset the science]. You will still be able to bring back two sets of science (less the crew report) [in Jeb and Bob's hands, er arms.] and send more reports from different biomes with an antenna (with the materials bay, mystery goo, [and, I think] eva reports and soil samples [the biggest return].

Also remember that the *real* issue is Direct Ascent vs. Lunar Orbit Rendezvous. Even if there is no reason to build separate landers (you haven't unlocked command chairs or are avoiding them for a different reason), you can still gain from Lunar Orbit Rendezvous. Just leave a fuel tank with a docking port in orbit and refuel with it for the journey home.

Cons: You need the lander tech tree item plus another 90 point item for the clamp-o-tron jr. You might also need an upgraded building somewhere to transfer fuel.

[Kuroki]

Thanks for the answers all, I think I understand both the situation in KSP and during Apollo 11 and onwards much better now. The thread kind of meandered off in to orbital physics which is only tangentially relevant, but I'll never say no to some orbital physics talk

I'll quickly summarise my thoughts using this handy post from the thread:

Reasons to use a lander in KSP

1) Multiple landings greatly increases the efficiency (my lander lands on the moon and rendezvous with the mother ship for only 180 LF) I think it is funny someone suggested this on Minimus as it is total overkill)

2) If you play with life support most of that can stay in the mothership

3) If you play with deadly reentry that can stay in the mother ship

4) In career mode attempt building a craft capable of landing on the moon with 30 parts that has 2 of each science and no 90+ tech. Cant be done I may make an official challenge.

1) True, but multiple round-trip landings are much worse than rovers, and usually a short biome hop is more efficient if your landing site is well picked. (I don't do that either and limit myself to one biome worth of science per celestial body to stretch career mode out a bit anyway.)

2) Yes, but you're still carting around the weight of the equipment itself, not just the resources. The life support mods I've tried don't model this all that well and seem to assume the command pod weights include the various pumps and other necessary equipment.

3) Heat shielding? Yeah, this is a big one and I'd leave my heat shielding with the mothership. In the real world this probably wouldn't work too well...

4) Wouldn't a 30 part limit preclude landers?

Reasons to use a lander in KSP

Reasons not use use a lander in KSP

1) Lander cans are WAY to heavy I believe .77 tons (1670 lbs) when they should really be .077 (160 lbs)

2) Only doing a single landing

3) There are no small ultra light landing gear

4) you can land on your engine without a care in the world (personally I wouldn't do that to my only ride home)

5) No good RCS choices for 1 ton ships, wish they have something 10 times smaller/lighter

6) The spark is just a little to small for my taste and the terrior is to powerful.

1) Definitely too heavy. Especially as size increases.

2) I tend to do this so I'm probably a bit biased against them anyway.

3) Cubic octagonal struts are pretty light and sturdy

4) Yep

5) RCS is a waste for anything not docking in my experience. I leave RCS and docking responsibility up to the mothership. Obviously not applicable to the real world...

6) I've had some good results with ants.

Reasons to use a lander in KSP

Reason to use a LM CSM in real life

1) redundant life support

2) redundant power supplies

3) redundant propulsion systems

This is all true, but Tom Hanks taught me that redundancy only gets you so far

[mikegarrison]

1) True, but multiple round-trip landings are much worse than rovers

Rovers are a PITA. When I first started playing (0.23, maybe?), I built rovers. I drove Jeb from the Munar Midlands down into one of the major craters, all the way through it, and (very carefully) up and out the other side. It was way more trouble than it was worth, and involved lots of F9 action because of all the times the rover would get out of control going downhill and fly, fly, fly, tumble, crash....

It is so much easier to just bring a little more fuel and hop your ship in a little suborbital loop.

[cfds]

A further reason for the Apollo-style moon landings in the real world is the engines. Contrary to what SQUAD makes you believe, good transfer vehicle engines and good lander engines are not the same. The first ones are build for high Isp which means a large nozzle. They must be started a handful of times at worst, do need no throttle control and are not limited by TWR concerns. A lander engine needs to have a low profile and provide some minimum TWR. Additionally you probably want to have a large throttling range and / or arbitrary restart capability.

[monstah]

Can you be a little more specific on your experiments? I'm just trying to figure out where the discrepancy is between your experiments and this experiment

Not conducted on purpose; I had a lander that happened to be there low on fuel, and after many quicksaves and loads trying to orbit and then eject, I tried going straight up and it worked.

I was not doing following an experiment protocol, mind you! I was just trying to get back home and failing to achieve it, until I tried something desperate and it worked. Let's just call it anecdotal evidence, not hard at all.

The point to be taken here is that (as many have said already), in KSP the system and the bodies are tiny, so you can do all sorts of things that would be impractical IRL.

[Superfluous J]

Reasons to use a lander in KSP

1) Multiple landings greatly increases the efficiency (my lander lands on the moon and rendezvous with the mother ship for only 180 LF) I think it is funny someone suggested this on Minimus as it is total overkill)

Not sure if you're talking about me here, but hopping through 9 biomes on Minmus with the ship you got to Minmus with, without refueling in an orbiter, is a waste of fuel. As much a waste as Mun? Of course not, but it's still a waste.

Reasons not use use a lander in KSP

1) Lander cans are WAY to heavy I believe .77 tons (1670 lbs) when they should really be .077 (160 lbs)

The Mk1 lander can is the best capsule in the game. It weighs 0.66 tons and you can't get a Kerbal anywhere for less mass without using a chair (or a claw or ladder or something like that )

[magnemoe]

Not sure if you're talking about me here, but hopping through 9 biomes on Minmus with the ship you got to Minmus with, without refueling in an orbiter, is a waste of fuel. As much a waste as Mun? Of course not, but it's still a waste.

The Mk1 lander can is the best capsule in the game. It weighs 0.66 tons and you can't get a Kerbal anywhere for less mass without using a chair (or a claw or ladder or something like that )

Yes you waste fuel, however the science payoff of doing it early, before rcs and docking ports is significant. My way was to have lander with four outriger drop tanks where top stage had 4 legs so it could be used independently. this also held material lab, goo, batteries and other stuff on other side, the two other outriger tanks was dropped then empty, then 4 tanks below who was dropped then empty or before landing.

With this I did all biomes on Minmus except poles, did the uneven terrain first before dropping first outrigger

My mun rover refueled at transfer stage, grabbing the last science on Mun and Minmus was done from an surface base with ISRU.

[abowl]

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

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

How do i find out the weight of the fuel in my main ship?