Having trouble building a launch/ascent stage for my lander.

[notasurgeon]

Hi!

I suck at designing spaceships, apparently. I really like the look of this lander, but I haven't been able to build a booster that can get it into a stable Kerbin orbit with enough fuel left for an interplanetary transfer. My biggest problem seems to be that no matter what I do it becomes highly unstable at higher altitudes as it becomes more and more top heavy. Any ideas?

https://dl.dropbox.com/u/37537505/Spectre1.craft

https://dl.dropbox.com/u/37537505/Spectre2.craft

I've tried at least a half-dozen different approaches, but little success so far.

[HYuy]

Well, first off, you shouldn't ever need this many NERVA engines. One, Two tops, is plenty to get a lander of that size where it needs to go, even to the farthest reaches if you give it enough fuel. The NERVA's proportional thrust-weight ratio just isn't worth using for much else beyond navigating in space since it's rather inefficient in the atmosphere, even moreso for the first few km on Kerbin. Even if you were intending to use the whole NERVA stage as part of your lander, it'd be extremely difficult to get down in one piece even with mechjeb help.

When it comes to lifting stages, most people just make the fair mistake of not putting enough fuel tanks per each engine. With the 1200 thrust 2m engine, typically 4-6 double tanks is optimal, and the same number applies to 1m engines.

I've attached a version I made using your lander (with a few modifications) that can get into orbit for you to see what I mean. All I changed on your lander stage was I reduced the NERVAs from 9 to 3 (still a bit excessive, but it keeps the symmetry this way) and added an ASAS module.

Steps to get into orbit manually (since I didn't put a mechjeb module on it): Set throttle to full, fire first stage. Upon reaching 15000m, angle 45 degrees in whichever orbit direction you want (you should be going roughly 350-400m/s at this point). When your Apoapsis is at the desired altitude (check via Map view), rotate to 0 degrees and continue burn until the stage finishes. Then, when you're a minute or two out from your Apoapsis, fire the second stage aerospikes to circularize your trajectory into a complete orbit. It should be MORE than enough fuel to do it if you're aiming for a 100-200km orbit.

[notasurgeon]

Interesting, and thanks! The reason I had so many NERVAs was just for the convenience of more thrust. I figured if I could get 9 of them up there, then my interplanetary burns could be shorter and more accurate. I'm not used to using so many fuel tanks. Why do you say more engines per fuel tank is a mistake? Is it just an efficiency thing? I downloaded a rocket from here last week that had a HUGE thrust to weight ratio and was still more stable than most of my designs. Doesn't mechjeb come with ASAS capability? Is there a need to add an actual module if I'm using it?

[HYuy]

Mechjeb's ASAS capability stems from utilizing whatever modules you have on the ship. So yes, you still need at least one. What you have to realize is you want DeltaV, not necessarily thrust to weight ratio when it comes to engines and fuel tanks. It's incredibly inefficient to have so many engines with only a comparably small amount of fuel to feed them. And in regards to your ship with 9 NERVAs, each of those weighs a LOT, so by reducing it to 3 from 9, you're lightening the load the lifter has to carry CONSIDERABLY, and thus, you don't have nearly as much trouble getting it into space. Thrust to weight ratios are kind of weird..... you don't want to have too little, but having too much usually means you're sacrificing fuel efficiency big-time. And that means you won't get very far since you can't burn nearly as long.

[notasurgeon]

Thanks again. Bob took her out for a spin, got to Ike with TONS of fuel left in the NERVA stage. Is it possible (with mods?) to leave that stage in orbit and re-attach after landing, similar to what the Apollo missions did? Because I could definitely get back to Kerbin with how much fuel I had left over.

[MaverickSawyer]

Try the Kerbal Engineer Redux. Using the thrust to weight ratios, and the dV charts, make sure your TWR is better than 1.2 on your ascent stages, and have at least 3800m/s dV total on the same stages. This will guarantee a clean ascent using Mechjeb or by hand. The reason for the high dV is that you suffer around 50% losses due to gravity and drag.

[HYuy]

Thanks again. Bob took her out for a spin, got to Ike with TONS of fuel left in the NERVA stage. Is it possible (with mods?) to leave that stage in orbit and re-attach after landing, similar to what the Apollo missions did? Because I could definitely get back to Kerbin with how much fuel I had left over.

It's possible, however I don't have any of those mods so I don't know much about how practical it is. If you were to land on any of the low gravity moons like Tylo, Bop, Gilly, Ike, or Minmus (or maybe even the Mun), you should have more than enough deltaV left in your tanks to get back even using the NERVA's small thrust output. It doesn't have enough to overcome most planets gravity (especially those with atmosphere), but low gravity celestial bodies shouldn't be a problem. To be able to land on one of the main planets and be able to return however, you'll likely need a drastic re-design of your ship and lander, and knowledge of bizarre but super-efficient configurations like the "asparagus engine fueling model".

It's definitely possible to make a landing and get back without going too insanely large even with stock parts, it's just difficult depending on the planet. Eve is by far the most difficult to land and still be able to return from. In it's case, simply returning back into orbit even with a 1-man capsule is an impressive feat. Just for a frame of reference and size comparison, my first rocket that managed to pull it off looked like this:

Launch Pad: http://i.imgur.com/AwC2c.jpg

On Eve's Surface: http://i.imgur.com/n6PIh.jpg

In Eve Orbit with barely fuel to spare: http://i.imgur.com/xSaYc.jpg

EDIT: Oh, and here's the gallery of a guy who managed to land his ship on Eve and have enough fuel to return. Note the significantly larger size of his ship: http://imgur.com/a/HHPog#0

Edited October 11, 2012 by HYuy