Is there a work around for this large ship issue?

[Magicide]

http://imgur.com/a/DqZqK#0

I am trying to launch my heavy lander to send Jeb to Laythe and back. However I am having a frustrating time getting my launch vehicle to actually launch. The pictures show the story just as well but here is my issue:

I like launching stupidly large ships and was running into physics issues trying to use a 4 layer asparagus launcher made of stock tanks. So I downloaded KW Rocketry to use the larger tanks/heavier engines. Except no matter how much I reinforce my ship, it usually falls apart on the launch pad once the physics initialize. I've found the work around to that by launching the second the physics come online and throttling up before the rocket falls to the ground.

Except once the rocket gets to 3000-4000 feet (different depending on the rocket design I use) I get hit with a sudden heavy jolt out of nowhere. If I throttle down through that altitude, the ship is still hit with a jolt but it usually survives and I can go back to full throttle. Most of the time I can then happily carry on to orbit.

Except... about 1 in 5 launches, when a stage runs out it causes one of the now empty tanks to jolt up and down like it's attached with bungee cords. If it bounces the wrong way it knocks something important loose and the kraken feasts. I can solve this by killing thrust and dumping tanks a second or two before the fuel runs out. But between throttling down at 3000-4000 feet and killing thrust every asparagus stage drop I'm wasting a lot of delta V.

I know I'm pushing against the limits of what the engine wants to work with but is there anything I can do to prevent this?

[700NitroXpress]

As long as your ship isn't over 250 tons try using mine.

The Leviathan super max lifter.

Download it here: https://drive.google.com/file/d/0B-vTRL2n8wvzQkEtUTBCNHpIMW8/edit?usp=sharing

See how to operate it here:

A slack tank asparagus design doesn't suffer from the sudden deceleration jolt like asparagus with engines attached to them. Once the orange tanks are all dropped, throttle down to 3/4 and that way the jolt will be less intense. You'll have plenty of power to get anything into orbit that isn't over 250 tons. If you're ship is over 250 tons, just add more fuel on top of the orange tanks and add nose cones to reduce drag. Structurally reinforce all of the tanks to the center and your payload to the lifter. You can also add 8 of the Mark 55 radial engines to the center for more power if the outer engines experience structural failure.

Edited November 11, 2013 by 700NitroXpress

[nhnifong]

Strut the engines to the tanks they are connected to.

[Monkeh]

It looks to me like maybe you just have too much power! Getting all that mass up to over 230 m/s before you hit 4000 m high sounds like you have one hell of a thrust to weight ratio going on there along with one hell of a weight, that thing must weigh a helluva lot. Just keep yourself throttled down.

Alternatively, open up your parts folder and change the strength of the strut to 3000 or something, that should help. Alternative alternation, B9 aerospace has some strong, (and invisible), struts to use that kept this baby of mine together really nicely.

[allmhuran]

Are you using asparagus (or onion )staging? If so, read on...

When your radial tanks are running low on fuel they will be light. If you are using asparagus staging, when your first set of tanks run dry the rest of the rocket will still be full of fuel. This means there is an enormous difference in thrust to weight ratio between the now-almost-empty columns and the still full columns. As such, the almost-empty columns tend to tear themselves off their decouplers. It looks like that is happening in your fourth picture.

Solution:

Or, yeah, just hack the parts to increase strut strength. Heck, while you're at it why not just triple the fuel capacity of all the tanks and quadruple the thrust of every engine. *rolls eyes*

Edited November 11, 2013 by allmhuran

[capi3101]

230 m/s by 3,000 m? Yeah, your launch TWR is too high; you want it somewhere closer to 1.6-1.7 for an asparagus-staged rocket. By 3,000 meters you only want to be going about 130 m/s or so.

Looks like you've got four LV-T30s per asparagus booster - that's 860 kN of thrust. Try ramping it down to three apiece (645 kN) thrust or just use a Skipper if you have them (slightly higher thrust, lower Isp, smaller part count) and see if that helps. You want the gee meter to be right at the top of the green zone just before stage separation.

Can't really tell the mass of your payload; pics are too dark. You wouldn't happen to know it, would you?

[Magicide]

The TWR at launch is 2.95 with a mass of 1184.5 tons and 34300 kN of thrust according to MechJeb, if it doesn't explode it easily carries the payload to high Kerbal orbit.

The lander itself is 106.5 tons and carries enough fuel to make it to Kerbal orbit which should be enough to make it to the return ship from Laythe.

[capi3101]

Definitely too much thrust at launch then. For an 1184.5 tonne rocket, an optimal amount of thrust is somewhere between 18,592 and 19,754 kN. You need to bring it down almost by half. Or just use 1/2 throttle.

[Magicide]

Does engine efficiency in KSP scale 1:1 with throttle? I've read that most rockets work at maximum efficiency at full throttle and that they can only be throttled down to about 50% before there are issues with the propellant burning/pumping correctly.

Also why would you not want to reach orbit as quickly as possible? Every second you spend thrusting to orbit is another 9.81 m/s^2 of of downwards acceleration you are having to counter with more fuel. Isn't it then most efficient to thrust at the maximum rate your craft can sustain?

[phunk]

Also why would you not want to reach orbit as quickly as possible? Every second you spend thrusting to orbit is another 9.81 m/s^2 of of downwards acceleration you are having to counter with more fuel. Isn't it then most efficient to thrust at the maximum rate your craft can sustain?

Air resistance is highest at low altitude, you waste fuel going too fast too soon.

[Specialist290]

Does engine efficiency in KSP scale 1:1 with throttle? I've read that most rockets work at maximum efficiency at full throttle and that they can only be throttled down to about 50% before there are issues with the propellant burning/pumping correctly.

As far as I'm aware, this isn't an issue in KSP out of the box as shipped. An engine's Isp values scale only with respect to atmospheric pressure, not throttle settings.

Also why would you not want to reach orbit as quickly as possible? Every second you spend thrusting to orbit is another 9.81 m/s^2 of of downwards acceleration you are having to counter with more fuel. Isn't it then most efficient to thrust at the maximum rate your craft can sustain?

What you've mentioned is indeed correct -- and that effect is known as gravity drag. However, especially lower in the atmosphere, during your ascent your vessel also encounters drag from the atmosphere pushing back against your rocket. Past a certain point, if you try to "brute-force" your way through the air, you're also wasting fuel, because more of your fuel is being spent fighting drag than it is to actually move the rocket upward. Ideally, you want your rocket's thrust to be as close to terminal velocity at any given point as possible, because that's the point where gravity drag and air drag balance one another out.

You can find details about how the game calculates drag here, and a chart with Kerbin's atmosphere's terminal velocity at given altitudes can be found here.

Hope this helps

[capi3101]

And that point where gravity drag and air drag balance one another out is generally somewhere around a TWR of 2.2 while you're still in atmo. Consider then that atmo goes up to 70,000 meters...

That same drag force in the lower atmosphere, incidentally, is probably what's causing your stack to disintegrate when you go full burn. Between the thrust and the drag there's more dynamic pressure being placed upon your rocket than it an handle. Solution's the same either way - either change the design to generate less thrust, or throttle back.

Next question: why do most folks say "shoot for 1.6 or 1.7 at launch" with asparagus? It's so they can average out their ascent somewhere around that magic 2.2 TWR mark without having to adjust the throttle.

[Eric S]

Does engine efficiency in KSP scale 1:1 with throttle? I've read that most rockets work at maximum efficiency at full throttle and that they can only be throttled down to about 50% before there are issues with the propellant burning/pumping correctly.

KSP doesn't model this, an engine running at half throttle will produce the same total thrust given the same amount of fuel as one running at full throttle, it will just take twice as long.

[Magicide]

Actually that is quite helpful, I read before that for max efficiency in the low atmosphere you don't want to be moving more than 250 m/s below 10 km. By that chart though, once you are above about 15 km the atmosphere starts to become negligible. Why would you not want to have a 3 or 4 TWR at that point so that your burn to apoapsis is as short as possible? It seems like the extra engine weight needed for that acceleration is justified by spending significantly less time requiring vertical thrust to counter gravity. Also with a high TWR you can thrust prograde closer to the apoapsis to circularize your orbit which is more efficient that having to burn for a longer time before/after hitting apoapsis.