Help Please..Rocket Size For Moon Orbit and back.

[sam1133]

I just cant seem to get it right, no matter how many tanks i stick on by the time i want to do my burn to leave orbit and head for moon, i am out of fuel...i am stumped,...please any help

thanks

sam

btw.. i am early in career, so no fancy bits yet.

[Vanamonde]

The third ship in my first post in this thread can make roundtrips to Kerbin's moons, and uses only parts from tiers 0-3.

[sam1133]

hey mate thats brilliant thank you

[godefroi]

It really doesn't take much to get to Kerbin's moons and back, especially if you're not landing. LV-T30, some fuel, a fin or three, and a solid booster or three, then an upper stage with an LV-T45, a bit of fuel, and finally, a capsule, a bit of fuel, and an LV-909 for the rest. Don't forget a parachute!

If you're getting into really big ships, then take a step back, and try a minimalist approach. I find it's VERY helpful to have something like VOID, MechJeb, or Kerbal Engineer Redux to get delta-V calculations in the VAB. For a munar flyby, figure ~5000 m/s for ascent (that gives you a very wide margin of error), ~1000 m/s for munar transfer (again, a plenty wide margin of error), and, a few hundred m/s for munar orbit, and return to Kerbin.

If you're actually trying to LAND, then I STRONGLY recommend Minmus first. It's MUCH more forgiving in terms of difficulty, as well as dV required for landing and takeoff. The dV required to get there is a bit more, but not much. Your 1000 m/s is still plenty.

[Taki117]

I just cant seem to get it right, no matter how many tanks i stick on by the time i want to do my burn to leave orbit and head for moon, i am out of fuel...i am stumped,...please any help

thanks

sam

btw.. i am early in career, so no fancy bits yet.

What's your flight profile look like? I have a feeling the problem lies there. Burning straight up and then turning is NOT the way to go

[sam1133]

You know Taki117 i think that is my problem, cause have tried suggestions and still i arnt able to do it, what do i do, or more to the point what am i doing wrong...yes up, then 45% then at 80km right angle for orbit, then maneuver mode to meet moon, burn to it but no fuel!!!

[EtherDragon]

You know Taki117 i think that is my problem, cause have tried suggestions and still i arnt able to do it, what do i do, or more to the point what am i doing wrong...yes up, then 45% then at 80km right angle for orbit, then maneuver mode to meet moon, burn to it but no fuel!!!

That's the issue - strait up to 80km and then turning right is not very efficient. Here - watch this:

Skip to 3:03 to see a good launch profile...

Watch the whole thing if you want to know why the profile matters. =)

[cantab]

Vertical to 10km, 45 degrees until apoapsis attained, then circularise at apoapsis isn't exactly efficient, but it's not that wasteful either. In my experience the bigger danger is going too shallow.

You should be using around 850-950 m/s of delta-V for the trans-Munar ejection. Certainly not more than 1000, if you are you're doing it wrong.

Assuming you use the 1-man command pod, one FL-T400 and any of the LV-T engines will do you a Munar orbit and return, even if you're taking a fair few Goo Canisters. Just get that into LKO without using any of its own fuel and you're set.

[SRV Ron]

This tier 3 ship easily does Mun orbit and return even with a pair of Goo canisters.

And, the landing was using aerobraking in a 15,000m reentry window to be gentle on Jeb with G forces.

[sam1133]

Hey people, you are so nice and kind heartd , thank you for all for the advice, and bits, have taken it all in and will do, i'll post back let you know how i get on, and again thank you all.

[SeventhArchitect]

Word of very friendly, insightful, and useful advice, It is a lot harder to make a rocket that can make a return trip to the mun than it is to make a rocket that can make a return trip to Minmus.

[Pecan]

It's easier to make a rocket that can make a return trip to Mun - as long as you don't land! DeltaV sequence is Mun orbit, Minmus landing, Mun landing. Minmus has the disadvantage of plane-change but advantage of much lower gravity and big flat areas.

[Dust]

I think you're falling for the fallacy of "more fuel means more delta v, right???". In fact, adding more fuel onto the same amount of thrust reaps rapidly diminishing returns, which is dictated by the Tsiolkovsky rocket equation.

Let's start with a Mk1 pod and use the rocket equation to see how we can get it to the Mun. If I add one FL-T400 fuel tank and 1 LV-T45 engine, you'll get a dV of 1818m/s (ASL). Add another FL-T400 and you get 2785m/s (a 53% increase). Add a 3rd - 3414m/s (+23%) - and a 4th - 3864m/s (+13%) - and you see where this is going. Meanwhile, your rocket has gone from weighing 4.55t on the launchpad to 11.3t, and your thrust to weight ratio (TWR) plummeted from 4.5 to 1.8.

Now, 3864m/s dV is not enough for low Kerbin orbit, let alone munar orbit and back, so you're going to need to add at least another 2000 or so m/s dV. If you try to add just fuel atop the same engine, you'll very quickly not have enough TWR to get off the ground: at 9 tanks your theoretical dV is 5025, but your TWR of .9 means you won't get off the ground until you've burned up nearly one tank of fuel.

A TWR of 1.8 is a good regime to be in, so let's go back to 4 tanks, with dV=3864 and TWR=1.8. Now, to effectively increase dV you're going to need to add thrust along with more fuel. To do that, let's add a couple of radial decouplers to the sides, stick FL-T400s to them, and stack another FL-T400 on top of each, with an LV-T45 underneath each one. You should now have 8 fuel tanks in a line (2-4-2) with 3 engines underneath.

Now, order your staging so that only the two side engines fire in stage 1, then stage 2 cuts the decouplers and fires the middle engine. On the pad your rocket will weigh 23.35t. When the two side engines fire you will have a TWR of 1.75 - nice. That 1st stage is good for 1316m/s dV (ASL), and when that is done you fire stage 2 with its 4467m/s (vacuum), which totals 5784m/s. And I believe that will get you to munar orbit and back (with practice, anyway). If not, add another tank on each side and fire all three engines in the first stage to maintain good TWR.

Finally, I very much recommend installing the Kerbal Engineer Redux mod, which does all this math for you.

Edited April 11, 2014 by Dust
Used the wrong Isp values