Best Engines for Lander?

[LuckySl7vin]

First of all, I'm a noob. So try not to laugh at me too hard when answering this.

I am having a problem returning from the Mun once I make a landing. I am currently using the liquid fuel engine that has 200 thrust output and using the long, skinny fuel cans for the source. My problem is that I burn up too much fuel trying to decelerate to the surface and have none left to get back. My question is twofold. First of all, is there a landing trajectory that will decrease my speed without using as much thrust? Second, would a nuclear powered engine be a more efficient choice and still get the job done (make orbital maneuvers and decelerate enough to land safely. Also I would appreciate any plans for decelerating to the Mun (altitude-speed indicators). Thanks!

[tavert]

Take your thrust (in kN), divide by your mass (in tonnes). You don't need more than 3.26 m/s^2 or so to land on the Mun, you can even get away with as low as 1.63 m/s^2 but at a cost of taking more delta-V to land. The best way to fly low-thrust landings is demonstrated in this video:

It takes a bit of practice to pull this off though, keep an eye out for bumpy terrain.

Nuclear engines are nicely efficient and give you a lot of delta-V to use for a given amount of fuel, but unless your lander is very large they can be unwieldy (and heavy) to use. Which command pod are you using? The best engine choice depends primarily on the payload (mass that isn't engines or fuel tanks) of the lander in question.

Edited July 30, 2013 by tavert
numbers off by order of magnitude, whoops

[dave4002000]

Nuclear engines(LV-N) are extremely efficient but they are also very heavy. So, if you have a small lander the LV-909's are usually going to be a better choice for low gravity bodies. They are 4.5 times lighter than a LV-N but they also produce 3 times less thrust. If you have a big, heavy lander, then the extra thrust of the LV-N's will usually be needed. And at that point, the extra weight is trivial compared to the rest of the craft.

[Immashift]

LV-Ns are great if you have a low mass craft or have a ton of time to kill doing burns.

Aerospikes are also good for ISP in the stock game, even after the nerf. Especially if it's the only engine on your craft.

On my landers I use (for small ones) the LV-909 engines, and for larger ones it depends. My Tylo lander for example used a mainsail - even though it's got HORRIBLE ISP, simply because TWR for Tylo is a bit more important than ISP.

At the end of the day deciding what engine to use is dependent on how much thrust you need, vs how much fuel your vehicle carries. Bear in mind that less fuel means less thrust required, and therefore you need a smaller rocket.

[Anton P. Nym]

It all depends upon the lander size and the world you're landing on; I'm not rocket scientist enough to be authoritative but here's my view.

First the easy one; the best lander engine for Kerbin is a jet. All that free oxidiser from the atmosphere makes for a lot of weight savings. Of course on Kerbin you could just use parachutes, which is easier.

For landing on low gravity worlds (Mun, Minmus, Ike, etc) you can use the low-thrust (but more efficient/controllable) rocket engines. I use the Poodle for my big landers with size 2 fuel tanks; for size 1 tanks I use either a pair of the white radial engines (for the heavier loads) or either a pair or a quad of the orange radial engines (for the light stuff).

I haven't worked out how to land on the heavier worlds with atmospheres; right now I'm experimenting with a big lander can with a descent stage of 4 radially-mounted aerospikes (heavy, but nice I(sp) throughout the profile) and an ascent stage of a Poodle. If I've got it right, I can use the descent stage's remaining fuel to help boost the ascent stage through the lower atmosphere (taking advantage of the aerospikes' efficiency in atmosphere) and into the thin air where the Poodle excels; but it's only tested on Kerbin, and it can't quite make it back to orbit there with enough fuel to rendezvous with my space station at 150km. So it's still a big question mark for me whether it'll work or not.

I'm looking forward to figuring out the answer, myself.

-- Steve

[Skorpychan]

I use either the LV909s, the little Rockomax radials for anything that needs to carry stuff underneath, or the big Rockomax radials for heavy stuff being hauled under a skycrane.

[Xeldrak]

Well, as the other have allready told you, it realy depends on the size of your lander.

When landing on the Mün, the LV-909s are usualy a good choice. They are very efficient, only nuclear engines are more efficient in vaccum (the are on par with aerospikes and poodle). They are also very short, which is great when trying to build a lander.

Here is a picture of one of my really basic landers that should get you to the surface and back to orbit:

[MarkusA380]

Yes, you can use NERVA's. I used them at a Duna landing AND return mission, so you can use them for Mun/Minmus/Duna/Bop/Ike... landings too.

Here's a picture:

It took long to ascend but it had half fuel left when it reached stable Orbit!

[Ahmed]

Hi,

I usually add Rockomax 24-77 for all my landers. That kind of engines have nice Thrust-to-weight ratio and I can always add them more if it`s needed because they are radial. If my lander is rly large I prefer to use Rockomax Mark 55 Radial because it have more thrust. You can laugh on me,but for me Mun landing is one of hardest landings in game. Mun don`t have atmosphere and have surface gravity 1.63 m/s2 what makes it much more than most (tylo or bust :}) of other natural satellites in game. If You master Mun landing, it will be good lesson for other landings.

[Cartz]

Take your thrust (in kN), divide by your mass (in tonnes). You don't need more than 3.26 m/s^2 or so to land on the Mun, you can even get away with as low as 1.63 m/s^2 but at a cost of taking more delta-V to land. The best way to fly low-thrust landings is demonstrated in this video:

/snip

It takes a bit of practice to pull this off though, keep an eye out for bumpy terrain.

Nuclear engines are nicely efficient and give you a lot of delta-V to use for a given amount of fuel, but unless your lander is very large they can be unwieldy (and heavy) to use. Which command pod are you using? The best engine choice depends primarily on the payload (mass that isn't engines or fuel tanks) of the lander in question.

I have a hard time believing that is anywhere close to optimal in terms of approach path. He spends a good 3-4 minutes at 45 degrees pitch in order to maintain his altitude while he decelerates. Thats literally 'wasting' half of his fuel. I would think you'd be better served killing that horizontal velocity off much quicker, so you can get down quicker, therefore reducing the effects of gravity on your velocity.

Start a bit higher then that guy, and accept some negative vertical velocity due to gravity as you brake. Once you have brought your horizontal velocity under control, pitch over and control your descent the way he suggests.

Thats the way Apollo did it.

[DChurchill]

Well, as the other have allready told you, it realy depends on the size of your lander.

When landing on the Mün, the LV-909s are usualy a good choice. They are very efficient, only nuclear engines are more efficient in vaccum (the are on par with aerospikes and poodle). They are also very short, which is great when trying to build a lander.

Here is a picture of one of my really basic landers that should get you to the surface and back to orbit:

If you're a noob, try this one. It's real simple and it will have plenty of dV to get you down and back. You can leave off the SAS on the top to save some weight. I'm assuming that's a .20 design, Xeldrak?

Edited July 30, 2013 by DChurchill

[Xeldrak]

Yeah, it's a .20 design - although the ASAS offers a good mounting spot for the lights

[tavert]

I have a hard time believing that is anywhere close to optimal in terms of approach path. He spends a good 3-4 minutes at 45 degrees pitch in order to maintain his altitude while he decelerates. Thats literally 'wasting' half of his fuel. I would think you'd be better served killing that horizontal velocity off much quicker, so you can get down quicker, therefore reducing the effects of gravity on your velocity.

Start a bit higher then that guy, and accept some negative vertical velocity due to gravity as you brake. Once you have brought your horizontal velocity under control, pitch over and control your descent the way he suggests.

Thats the way Apollo did it.

Real-life landers have many other considerations than we do in KSP. If you run the numbers, it is actually more efficient, maybe 20% or so if your TWR is low, to point off-retrograde and maintain constant altitude. The issue with burning retrograde is that gravity speeds you up as you descend, and it turns out those gravity losses are larger than the steering losses incurred by maintaining constant altitude. You're not wasting half your fuel at 45 degrees pitch, you're wasting 1 - cos(45 deg), so 29.3%. And in the end it isn't really wasted, since you don't gain the extra speed from gravity that you'd have to counter later.

Edited July 30, 2013 by tavert