second time to the mun harder for some reason?

[bjb406]

So I did all the early stuff without any addons, just sort of using trial and error to figure how big of a rocket i needed. I managed to land on both mun and minmus that way. Now I downloaded kerbal engineer so I can plan how much delta-v I am going to need. I am trying to now send another ship to the mun to leave a rover, scientist, and mobile processing lab there, and bring the pilot back. Anyway, I sorta through a rocket together I thought might be enough, kerbal engineer said it had about 5700 delta-v, an after trying it out it was going to be a few hundred short of getting back. Looking at the delta-v charts online that seems pretty consistent, but then I loaded up my old lander from the first time I went to the mun without trying to bring so much with me, and it was barely over 4000 delta-v according to kerbal engineer.

So am I going crazy? The new rocket has a bigger paylod, but shouldn't that be factored into the delta v calculation already? I know the way I designed it with my rover at the bottom of the final stage, about a third of the way from the top of the total rocket, it can be a little wobbly in stage two, but it shouldn't affect the efficiency THAT much. Also, according to what I have read it shouldn't even be possible to land on the mun and come back with barely 4k delta v. Maybe the save of my old rocket got altered somehow without me realizing it? I am confused.

[Empiro]

There's no way to make it to the Mun, land, and and come back with 4000 m/s delta-v. That's just enough to make it into orbit and maybe perform a few orbital maneuvers in LKO. Maybe when you were new at the game and just testing things out, you enabled infinite fuel? For the first few landings on the Mun, it would make sense to not worry about fuel and just figure out how to get the controls right.

[bjb406]

I don't even know how to enable cheats. I think I figured it out though, maybe its just cause I wasn't breaking it down by which stage is going to be active where? I haven't really played around much with the options at the top o the kerbal engineer window. For the rocket that initially got me to the moon, with body: kerbin and atmospheric checked, it gives me a total dalta-v of 4061, but if I uncheck atmospheric, it shoots up to 7497 delta-v. With the new one with the rover on it, it starts at 5718 delta-v for atmospheric, and only increases to 6295 delta-v when I uncheck atmospheric. Should I just always have atmospheric unchecked?

[RocketPropelledGiraffe]

You should check your values again and pay attention to the "atmospheric" setting in Kerbal Engineer. If it is activated it will calculate the delta-v based on the atmospheric density at sea level. If you use the "vacuum" engines like the 909, Poodle, or Nukes, which have very low atmospheric ISP, your atmospheric delta-v calculations will be far off the correct value for your ship in space.

[FancyMouse]

Another example that a picture would show a lot more

[Arkalius]

For all these values, atmosphere is unchecked in KER.

I like to allow for about 3500 m/s for getting into LKO. Then you need about 1200 m/s to get to the Mun and insert into a low orbit. Landing and getting back into Mun orbit is about 600m/s each (more if you haven't perfected your Mun landing skills or have to hunt for a safe spot). Then you need like 320 or so to escape the Mun into a direct re-entry back to Kerbin. That's a total of just over 6200 m/s. That's all assuming you can fly the mission with decent efficiency with your maneuvers. You'd probably want a little extra wiggle room there. You could probably do it with less if you're really good.

With the 6295, is entirely conceivable that you were simply inefficient at some point and ended up short some delta-V in the end. Efficient ascent to LKO is probably where most people struggle the most. Mun landing can also come with a good bit of waste, especially if you're trying to land in a specific spot.

As for the atmospheric selection on KER, I usually only use that to check my TWR with my launch stage. The delta-V values with that mode selected aren't terribly useful unless you plan to do a lot of flying around a specific altitude range in the atmosphere.

Edited June 22, 2015 by Arkalius

[Kryxal]

Practically speaking, you might burn the first 1k or a bit more of fuel at atmospheric ISP, the rest at vacuum values. You can probably use the vacuum values but subtract a bit for the reduced ISP in the first stage or two.

[Mic_n]

Heavier rockets are also going to be burning a lot more of their dV in atmosphere and suffer higher gravity and especially drag losses.

[Sharpy]

Calculate your dV from LKO to the Mun and back using KER. These are pretty consistent. Add some 100-200m/s for landing maneuvers, you're unlikely to make a perfect lunar landing especially if you're new.

Then launch the contraption into LKO using some generic launch stage similar to what you intend, that will get the job done. Check how much dV you used to the orbit, revert flight (or load the quicksave) and create the launch stage that provides as much more or less as you fell short or over your goal.

The atmospheric drag (dependent on the shape of your ship), engine inefficiencies and trajectory done without MechJeb make for so many variables there is no way for you to design your launch stage to land you in the orbit with precisely as much dV left as you intend. The space flight requirements are constant, but the launch is a big variable you really need to develop experimentally; KER will just give you a rough advice.

[Mic_n]

so many variables there is no way for you to design your launch stage to land you in the orbit with precisely as much dV left as you intend. The space flight requirements are constant, but the launch is a big variable you really need to develop experimentally; KER will just give you a rough advice.

Add to that the exponential growth in mass required (and the rate at which it is converted to energy) as you get heavier, and that variation is greatly amplified. That's one of the reasons I like spaceplane launches. I can put my transfer stage up there and just launch it from an 80km LKO, then whatever fuel I have leftover in my launch vehicle comes back down with me to be recovered. It's streamlined & the payload's carried internally, so the only variable in there is the overall mass, and I can keep that in a narrow envelope determined by different launchers.

[tutike2000]

Having Atmospheric checked means that high-efficiency engines (swivel, terrier, poodle, etc) will show less delta-v than they actually will have in space.

Typically people refer to vacuum delta-v.

Look at your atmospheric stage with atmosperic turned on, your transfer/capture/landing stages with atmo off.

[Empiro]

For most mission planning, people only talk about vacuum delta-V, since you spend most of your time in space. In addition, the engines that have good TWR and are ones you want to lift your rocket off Kerbin all have similar ISP in both atmosphere and vacuum, so it's just more convenient to stick to vacuum delta-V