Landing engine for 1.9t Mun lander?

[Joe_Bender]

K, I built a lander to land on Mun with no intent of it returning home.

Here is a pic of the lander. It weighs in at 1.9t fully loaded

http://i.imgur.com/FJIUWkR.jpg

I am using only stock parts. Fuel supplied by two Oscar B tanks and one Round 8 fuel tank. My first build had four Rockomax 24-77 engines mounted midships as the picture shows. No RCS on any build.

With the booster stage (tank and nuclear engine), I am using it to put the lander into a 60km orbit around Mun, brake the lander to a landing position and then jettison it right before I need to do the final burn for landing. I was hoping it would plode when it crashed onto the Mun surface.

My first attempt at landing was a speculator Failtastic! I ran out of fuel.

My second build, I used only three 24-77 mounted on the bottom fuel tank with no other changes.

My second attempt at landing was a speculator Failtastic! I ran out of fuel.

My third build is only using one LV-1 engine at the very bottom.

Will my third attempt at landing be a speculator Failtastic cuz I ran out of fuel again?

Before I find out, I just wanted to know if there is a standard engine or engine/fuel tank layout for small landers?

Comments? Suggestions? Heckling from the peanut gallery?

[Aeshi]

My 1.7-1.8 ton drone lander uses a set of 4 'ant engines' radially mounted via struts, that might be worth trying since I doubt that extra 0.01 tons will make much difference given how well said lander has worked for me.

[sal_vager]

I think what I'd do is put I-beams and legs on the nuclear stage and land the whole thing, then fly the probe from the (new) descent stage to the surface, you could hover around a bit to find a nice bit of grey dirt to look at

[EndlessWaves]

Will my third attempt at landing be a speculator Failtastic cuz I ran out of fuel again?

No, it'll be a glorious crash because you don't have enough thrust to counter the mun's gravity.

The mun's gravity is 1.6m/s² so it exerts the equivalent of 1.6 x 1.9 = 3.04 thrust on your rocket. As such you need at least a bit more to slow down and a bit more again if you want to slow down fast enough from orbital speed.

On the other hand you don't want too much thrust or it becomes difficult to control your rate of descent because the throttle is so sensitive.

If you're running out of fuel then changing engines isn't likely to be the answer. A smaller engine will burn less fuel but it will burn that fuel for longer and overall it doesn't make much difference for engines of similar ISP.

EDIT: At 1.9 tons your best choice of engine is an LV-N, that'll give you more DeltaV than anything else, approximately 460m/s

Edited May 5, 2013 by EndlessWaves

[Joe_Bender]

EEK!!!!

Math equations!!!!

Well. I guess I just failscaded another mish cuz I just took off with this.

http://i.imgur.com/NnlA6kg.jpg

Instead of starting over, I am going to see this ends in a ball of fire or many balls of fire. Lutz

[Joe_Bender]

\o/

Epic fail of momentous proportions!!!!

Failed to check to see if landing gear pads would be lower than engine nozzle at landing, besides failing to light landing engine pretty well finished the deal.

Back to VAB for another design modification.

[Atanar]

Your initial problem was that you used the worst fuel tanks. Regular fuel tanks have a 720fuel/mass unit ratio while oscars have only 382,33per and octagons are even worst with 73,529. Stick with regular tanks.

[Boiler1]

If you don't want to do the math add a small extra engine or two and then toggle them on/off with a 0-9 number key.

[foamyesque]

Your initial problem was that you used the worst fuel tanks. Regular fuel tanks have a 720fuel/mass unit ratio while oscars have only 382,33per and octagons are even worst with 73,529. Stick with regular tanks.

Problem there is that the -200 is too big for the job. A -100'd be better, but we don't have one.

My 1.7-1.8 ton drone lander uses a set of 4 'ant engines' radially mounted via struts, that might be worth trying since I doubt that extra 0.01 tons will make much difference given how well said lander has worked for me.

Precisely my solution as well, as it happens. I did my best to recreate Joe's lander, although I modified it slightly (I have no idea why he had a stack decoupler in there, so I put a stack battery instead so's to run the lights and probe etc) and I cut the nuke engine for a 909 to save weight. Here it is in orbit:

Quite frankly by using the 909 to deorbit and brake at the Mun you could probably cut out the torodials altogether, but this way your lander can perform the deorbit and landing itself.

[Joe_Bender]

Thanks for the direction.

For the lander in the second picture, this is how it is built, starting from the top.

1. PB-NUK Battery Generator.

2. MGS XL - Long Lattice box with four SP-B solar panels, two RCS bubble tanks, MJ module, and two antennas just for the giggle factor.

3. MGA - Base lattice mount.

4. Small ASAS with four RCS thrusters.

5. Large octagon command pod (OKTO) with scientist parts on it, for more giggles.

6. Octagon strut box

7. X200-16 tank with four small box struts and LT-1 gears

8. LV-909 motor

Then a decoupler, tank, and Nuc motor.

So it seems that there is some math skillz needed to determine if my boat will land on a planet or just become another hole in the ground.

I understand the formula to determine how much juice I need to land but how do I calculate the time factor?

For a silly example, lets just say I need 400 thrust per the vehicle weight and planet gravity, how do I find out how much fuel I need for the decent and and landing? The time from orbit burn to touchdown....more or less, I believe.

/joe opens Excel to do the formulas.

[EndlessWaves]

I would grab the kerbal engineer redux mod as that'll calculate DeltaV (how much change in speed you have the fuel for) automatically. You can do it yourself but for more than one stage it's can be a lot of work.

[foamyesque]

There are two key numbers you need: A TWR > 1.5 on the Mun (about 0.25 on Kerbin), which is easily achievable on a lightweight lander like this, and around 800-900m/s of deltaV to execute a landing from a low orbit (I like to bomb in from <10km).

[Joe_Bender]

We have touch down!

Replaced tank and motor with the tuna can fuel tank and 909 motor.....landed on Mun with 60% fuel left.

Now to figure out a rover to send up there so I may 'investigate' the arch right over the hill.

o7