Delta-v to [X] and lander design

[ibanix]

So I haven't been able to land on Mun and get back home, yet. What's the minimum delta-V I need to accomplish this? And is there a library of lander designs somewhere? My landers aren't very good.

Edited July 19, 2015 by ibanix

[Xannari Ferrows]

What type of lander are you looking for? Big, small, efficient, utile, mobile, ect? The Delta-V required depends on your lander. For transfer it is irrelevant, but the lower your TWR when landing and taking off, the more Delta-V it takes.

[ibanix]

What type of lander are you looking for?

For this stage of the game: Land with one Kerbal & scientific instruments, get out and walk around a little, blast off and return to earth. No rover, nothing fancy.

[Kryxal]

Just plan it out stage by stage:

4000 m/s to LKO, initial TWR 1.3-1.5 (you can probably do it on less, but better to be safe)

1100 m/s to Mun orbit

800 m/s to landing (value given is 640, but if there's one place for plenty of margin, it's HERE ... and you can use any excess in your transfer stage too)

900 m/s for takeoff and return

Total budget is 6800, with a lander carrying about 1700.

It's worth looking up delta-v maps and finding one you like...

[Snark]

Here's what you'll need:

1. Get to LKO (you're already doing that, I assume you need no advice)

2. Boost to go to the Mun, ~850 m/s

3. Enter munar orbit and land, ~1000 m/s. (escape velocity is a good rough measure of that, see http://wiki.kerbalspaceprogram.com/wiki/Mun; 800 m/s for Mun, then add a bit to account for not entering its SoI at zero velocity, and for some safety margin)

4. Take off from Mun, achieve orbit, launch back to Kerbin: this is just #3 in reverse, so another ~1000 km/s

5. Aerobrake and land on Kerbin; no dV needed.

So if you can build a craft that can get itself to LKO, and then have another ~3 km/s of dV on top of that, you should be reasonably well set.

Note: You may be tempted to want to do it somewhat Apollo-style and leave a return-module in orbit around the Mun (i.e. get to munar orbit, detach from orbiter, send a lander down and back, then on return, dock with the return module and leave the lander behind). You can certainly do that for the fun of it if you want to, but in my experience it's not really worth it. Getting from munar orbit back to Kerbin takes very little dV; the only savings you'd get would be saving that amount of dV for the lander's return, and it would likely be more than offset by the added cost of bringing along more hardware. So I'd suggest just a simple land-and-return.

Another thing to bear in mind: If you do the mission and end up finding that you've slightly miscalculated and don't have quite enough dV to make it home again: you can go home in a spacesuit. Kerbal EVA packs are ridiculously overpowered, they have several hundred m/s of dV. Also, kerbals on EVA are astonishingly tough-- they can handle reentry without any problem of overheating, and they can drop out of the sky and land without injury. So if you're ascending from the Mun and run out of juice, just go EVA, take all the science out of your ship, let go, and just EVA your way home. Hopefully it won't come to that, but it's nice to know you have the option.

(Fun fact: An EVA Kerbal standing on the surface of Minmus can go home without a spaceship, with dV to spare. Not from the Mun's surface, though.)

If you do end up EVAing your way home from the Mun, aim for a higher Kerbin reentry periapsis than you would for a spaceship; ~45km works well. (Kerbals have a lot more drag for their mass than ships do, so the high periapsis allows for more gentle aerobraking and avoids reentry heat. You'll enter as gently as a feather and won't even get any reentry f/x.)

Edited July 17, 2015 by Snark

[AaronLS]

I would verify you have enough delta V left after getting into Kerbin orbit. Depending on how well you get into orbit, the amount of deltav you use can be vastly different. So you might build something with enough deltav budget for the entire trip, but use up too much getting into orbit.

[AaronLS]

You can design your lander first, and calculate dv and fuel needed to get down to Mun and back up. Then determine what dv and fuel is needed to bring that back from the Mun to Kerbin. Lastly, we know all of that, the lander and the return fuel tanks, needs to be pushed out to the Mun, and since we know how much that payload is together, we can calculate the fuel needed to meet the dv from Kerbin orbit to Mun orbit.

For just the lander part identify the total weight of the "payload". Assuming we'll leave the command module in orbit for the return trip and just bring down the lander:

Mk1 Lander Can, .66t

Various other science and RCS, docking port, etc 1t

(you should do this more accurately

~Payload: 1.66t

Using http://garycourt.github.io/korc/ with Mun local gravity and 1500 minimum DeltaV is one way to get ideas, but alot of these are more complicated. I like to uncheck alot of the parts and limit it to a single stage for Mun landers. This isn't completely accurate but is an intersting starting point. It also verifies for me that the spark has enough thrust/TWR to get such a lander off the ground.

Simple lander might use a FL-T400 Fuel Tank and "Spark" engine with lots of wiggle room, but that's 2000 delta V and you might can get away with something smaller if you have toroidal or oscar fuel tanks, especially since my 1t estimate for "science/RCS/legs" is probably overkill.

I can verify this rocket design by taking it's wet and dry mass, ISP, switch radio button to specific impulse and Compute: http://www.strout.net/info/science/delta-v/intro.html

Full Mass, 4

Dry Mass, 1.91 (1.66 + .25 empty T400 + .1 spark)

ISP: 300

Computes DeltaV 2174, plenty more than enough.

Once you've decided the lander payload, consider you need to get back to Kerbin: ~230+80 to get back if you aerobreak into Kerbin:

http://i.imgur.com/NKZhU57.png

Determine how much fuel is needed to get back. This probably wouldn't be more than 1t fuel, assuming what part of the ship you are bringing back is small. If you are hauling lots of big empty fuel tanks around from Kerbin out to Mun and back, then that's really bad and a lot of dead weight.

Now consider, you need to get all of that, the lander+the fuel for return, out to Mun from Kerbin in the first place. That's 680+180+80+230=1170dv.

If the lander+command module+return fuel totaled 5t, then you'd need about 3t fuel to obtain that 1170 dv. I can put dry mass 6t into the DeltaV calculator, and guess around 9t wet mass, for a difference of 3t fuel burned, and the calculated DeltaV is 1192.

Of course you need to get that 9t of fuel and ship into Kerbin orbit.

This is a very rough example, and I'm really not as detailed as most people on this.

[Wemb]

Just plan it out stage by stage:

1100 m/s to Mun orbit

800 m/s to landing (value given is 640, but if there's one place for plenty of margin, it's HERE ... and you can use any excess in your transfer stage too)

The only thing I'd add, is that ^^^^ is cracking advice when you're learning - just because Apollo went all the way down in the Lander doesn't mean you need to - put on an excessive transfer stage - my first landers were de-orbitting dropping down to within a km or so from the Mun's surface on the Munar injection engine before I separated to leave just the lander - and even then I could well be ditching it with fuel still in it's tank.

You _will_ make a hash of landings, and you will want to make sure you have all the spare fuel you want when you take off and want to come home.

Wemb

[ibanix]

Thanks for all the help, guys!

So, about delta-v: How can I get an accurate count of how much I have, while I'm building? I have Kerbal Engineering System installed. It gives me delta-v for stages but that appears to be delta-v on the *ground*. Is there a way to get delta-v for each stage where it is going to be used (launch, orbit of Kerbin, orbit of Mun....).

[amorymeltzer]

So, about delta-v: How can I get an accurate count of how much I have, while I'm building? I have Kerbal Engineering System installed. It gives me delta-v for stages but that appears to be delta-v on the *ground*. Is there a way to get delta-v for each stage where it is going to be used (launch, orbit of Kerbin, orbit of Mun....).

If you click the atmosphere button it will toggle between delta-V in space and on Kerbin. Their are sliders for altitude and speed for in-atmo checks, and you can also change the reference planet/moon to anything else you like. Easy as click-click-click!

[Sharpy]

Note: You may be tempted to want to do it somewhat Apollo-style and leave a return-module in orbit around the Mun (i.e. get to munar orbit, detach from orbiter, send a lander down and back, then on return, dock with the return module and leave the lander behind). You can certainly do that for the fun of it if you want to, but in my experience it's not really worth it.

You can do a different Apollo-style trick to a good effect though: dump your legs and spent fuel tanks on the Mun.

The weight overhead is usually three radial decouplers or one stack decoupler or such. But to make this viable (don't dump any engines!) you need either the fuel pipes or resource transfer unlocked. (you move all the fuel from radial tanks into the central tank before launch).

The added benefit of adding extra fuel tanks radially (and legs to them) is a wider lander base, less prone to tipping over. (OTOH do NOT add the dumpable stage vertically, creating a tower-like lander. Tall, slender landers are a most common newbie mistake in KSP.

[ibanix]

So I got my Kerbal to the Mun... but am about 600 dv short of getting home. Time for a rescue mission!

How do I land exactly where I want? Is there a mostly-foolproof way?

Also, if I'm coming in to land and I see I've got a slope below me, is there an easy way to alter my landing area? Should I use RCS jets to just aim for a different place?

[Kryxal]

Adjusting your landing spot really depends on your descent profile. If you're dropping vertically, it's going to be sort of awkward unless you angle and burn ... if you're coming in more horizontally and burning as you go, just angle up or down.

Incidentally, if you have enough delta-v, you might be able to going fast enough to just have your Kerbal bail out (WITH science) and finish the orbit that way.