Not enough fuel during launch.....options?

[Joe_Bender]

So my quest to build the ISS has hit a snag.

I cannot get my third module up into orbit. I run out of fuel before reaching escape velocity.

This is what I have now. I left the MJ data fields on so you can see what I am working with.

http://i.imgur.com/4nSWxD8.jpg

This is a basic asparagus launcher with a poodle stage to get the module up to the ISS. I am using the poodle because my main engine runs dry just seconds before I reach a stable orbit and I want it to burn up and not trash up my space. This launcher is the same thing I used before but I believe the payload this time is a couple of tons more. The other flights had almost a third of a fuel tank left after I had reached the ISS.

I am launching for a 150km orbit with my ISS being at 200km orbit.

I have added a third tank to the boosters but it lowers my TWR.....Didnt launch it to see if it worked or not. I am under the assumption that I need about 2.5 TWR or greater on my launch vehicles to be viable.

Suggestions? Comments? Anything from the Peanut Gallery?

[Nibb31]

Try using mainsails with orange tanks for your first stage radial tanks. In an asparagus arrangement, firing 5, then 3, then 1 engine, you can orbit some truly heavy loads.

[Joe_Bender]

K, let me try a 2+2 combo of orange and small tank first just to see what happens. I think the four orange setup might be overkill.

Thanks!!!!!!

o7

[Hacktech]

How about using solid fuel boosters, just to add that extra liftoff kick?

[Joe_Bender]

Well, that was interesting.

I try replacing two liquid fuel boosters with solids but the the liquids burnt out before the solids did and I didnt have the staging in the correct order. Eek!

I will do it again on the next module with the correct staging and see how it goes.

I then went with two orange/mainsail boosters with two smalls. Not very stable......I think that the booster mainsails were gimbaling too much and causing the wobble.

I also added four fins to the lower stage cuz a rocket without fins is like a woman without a bra, they are going to be all floppy. lol

[Noticeably FAT]

The orange tank + mainsail ought to work, but yeah, it's gonna flex. Strut the crap out of the side tanks, And put some sepotrons on them. Also, assign the outside engine gimbals to an action group and shut them off before you launch (leaving the center gimbal on should help). Then just launch it and watch it wobble. There's really no such animal as a solid jumbo and mainsail ship.

[ThatBum]

Yeah, multiple gimbaling engines are a bad idea. Last time I tried that, I had four Atlases in a square, and they eventually started fighting each others' corrections as the rocket got more top heavy with burning fuel. It gyrated out of control and ripped apart from the g-forces. Gimbaling engines always work best when in the center of non-gimbaling engines, or alone. Try using an action group to disabling the outer gimbals at launch without having to do it manually.

[shand]

my contribution would be to remember that mechjeb calculates deltaV for in atomosphere and out seperately. so your ship has 4400 m/s in vaccum only. when i was using mechjeb for stats i tried to make the first 2500 m/s from the atmosphere number, and then 2500 m/s from the vaccum number. you are running out of fuel and you have the tools to detect it before you get off the launchpad

other then that theres some nice info in the other posts about how to GET that extra fuel, i wont repeat it.

[Temstar]

Ideal TWR is 2, but since your TWR climbs during a stage turn you want to lift off with a TWR of 1.7. Lift off with 2.5 TWR is far too high and you're wasting fuel early on fighting air drag. This goes for each stage higher up as well, though later stages have even lower requirement in terms of TWR after gravity turn as it's no longer fighting much air and the fact that the stage is flying nearly horizontal means it's no longer fighting against gravity much as well so you can take your time. For example have a look at TWR for each stage in this rocket of mine:

Generally you cannot scale up rocket to greater payload by just adding more fuel. However in your case since you have such a high TWR at lift off you can actually afford to load a second stage on top. But still, a new upper stage is not a very efficient way to increase payload as the upper stage engine is dead weight during the early part of ascent. Much better to just build a larger asparagus.

[Sovek]

4.46km/s of ÃŽâ€V can get you into orbit, provided you fly your ascent really well. However it wont be enough to intercept anything unless what you are intercepting is at 75km or so.

You're T:W is also a bit higher than it should, even if the 4 radial engines are the LV-T45s, 2.1 is overkill. Look to add fuel, no more engines. 1.5 T:W should be your target. In fact I have a 250t+ rocket that has 5.5-6km/s of ÃŽâ€V with a T:W of about 1.8, with a 25t payload. in fact I can add more fuel if I wanted/needed any more ÃŽâ€V. I may even be able to get it to Mun if wanted.

Edited April 28, 2013 by Sovek

[Joe_Bender]

Thanks for all the great info!!!!!

Yea, my first attempt in getting the second module of the ISS into orbit was meet with a shower of broken parts flying through the sky and falling oh so lovely into the sea.

I knew I had derped that launch when the report stated that my mainsail engine crashed into my first docking port at the very top of the rocket. LoL