Asparagus question regarding final center section.

[LarryWallwart]

I am well aware of how to build a very elaborate asparagus system. What kills me is figuring out how many tanks to put above each engine, especially the final center engine. I can get past 20,000 but then that final engine doesn't have enough oomph to push me into orbit. I am assuming I am stacking too many tanks on the center section. Is there a standard tanks per engine reference?

[irongut]

You're center section is either too heavy or its engine is too weak. You need a Thrust to Weight Ratio of at least 1.4 to get off Kerbin. I recommend using the Kerbal Engineer Redux plugin, it will tell you TWR, deltaV, etc while building.

[AlamoVampire]

Id also recommend mechjeb, it can give you dV for each stage. Also sometimes more can be less

[capi3101]

Try http://forum.kerbalspaceprogram.com/showthread.php/28248-Is-asparagus-the-best-staging-system-%28might-contain-science%29?p=346702&viewfull=1#post346702 for general guidelines on building asparagus.

In general, you want to work Tsiolkovsky backwards - with asparagus, you have the decouplers and all the engines that have mass, you add that to your payload mass (this "dead mass" gets added to both mass values in the equation) and calculate the total atmospheric Isp of the engines. You also use the 9:1 wet-to-dry mass ratio for most of the liquid fuel tanks in the game (say M=9Mo) so you get your final value in terms of dry mass. Multiply the final result by 9 to get how much fuel you need, and then divvy it out evenly among all your asparagus stages.

[Dispatcher]

I'm not aware of a "standard" quantity of tanks/ fuel mass per booster ring. Too many variables with regard to total dV, engine(s) used, number of stages, etc. However, if each of your boosters is the same (same engine, same amount of fuel), then as you move inward, the dV actually takes a hit and your central engine may struggle, relative to the dV of the other rings of boosters. While I usually just bludgeon my way into orbit, I do know that you could try making each succeeding ring of booster stages have more fuel than the inner rings. I've made some with 4 and 5 rings; so in this case the outer ring has the "tallest" set of tanks and each succeeding ring inward is less tall (has less fuel). This will help get you to a greater altitude than if all rings are of same sized boosters, as the outer engines get to work longer before time for jettisoning. That center engine and its fuel may be more payload than help, but it should finally be able to get you into a circular orbit after the boosters have done their thing.

The ship might look more like a scoop, but it should give you good results.

[Sunsparc]

Id also recommend mechjeb, it can give you dV for each stage. Also sometimes more can be less

So does Kerbal Engineering, like the previous poster mentioned.

[-Velocity-]

To "solve" the asparagus center section problem, I've actually used some extra engine "nacelles" that attach to the top of the asparagus center section, and are above the other asparagus sections. When the last of the side sections are jettisoned, the extra engine nacelles on the center section start firing.

On the pad:

Just after lift-off (notice, the top engine nacelles that aren't firing- they would, of course, destroy the rocket if they did):

In orbit, the center asparagus section separating from the payload (and the payload jettisoning the ascent attitude control engines):

Now, I know it's not technically a true asparagus rocket anymore, as the upper set of engines do not fire until the last "stage", but it's one solution to the problem.

Edited January 15, 2014 by |Velocity|

[AlamoVampire]

So does Kerbal Engineering, like the previous poster mentioned.

All the same, but MJ also needs love hehe

[Kasuha]

If your central section feels too weak, then the problem may be that your earlier stages don't give it enough initial impulse. My lifter has no problems lifting its nominal payload to orbit although its four nuclear engines in the last stage have fraction of TWR compared to earlier stages.

Technically the best approach is to aim for constant TWR of 2 after each staging.

Another option is "evoltionary approach", or simply put, trial-error.

My asparagus lifter (with some cargo), built using "evoltionary approach".

[Dispatcher]

Of course, asparagus staging is made possible because of the "fuel line". Note that I'm still playing Career mode. The first pics are before the fuel line part was unlocked for me. Everything was nicer once I got the fuel line part.

Yes, it was quite the slide show!

It was much more fun once I was able to use fuel lines. Asparagus staging was now possible.

Its not an optimized set up, but it does the job. Of course you know how to do the staging; I just thought you'd get a kick out of the shots.

[Tank Buddy]

I don't think I can give the same recommendations as everyone else. I think you should avoid becoming too dependent on mods to do thing for you, but I guess it's OK if you need the information badly. I almost never do ANY math for any mission, I just have a gut feeling and sense that I can tell what can and can't work. I would recommend watching YouTube videos to see how other people fly and build asparagus lifters.

Also, 20,000 meters if far from orbit, and you probably shouldn't be on your last asparagus stage that early.

All I can really recommend is practice and trial and error.

[Pecan]

Use Kerbal Engineer Redux (KER) or MechJeb so you can see your TWR and total deltaV in the VAB.

1. Build your payload.

2. Put any fuel tank and your favourite engine under it.

3. Replace the fuel tank with a larger or smaller one until the TWR for this, final, stage is 1.4 or so - by the time it's used you won't need the 'usual' ratio of 2ish.

4. Add an asparagus stage.

5. Replace the two fuel tanks on this until the TWR is 1.6+ (should reach over 2 by the time it's jettisoned).

6. Repeat 4 and 5 until the total deltaV is sufficient to reach and circularise orbit (or whatever you're intending to do).

7. Launch.

8. Add struts if it falls over instead of flying.

9. More struts (and possibly control surfaces, but I've never needed or wanted them).

[Vanamonde]

The amount of payload an engine can move depends upon a number of things, including where you are in the flight. For example, an ion engine can move a massive space station if it's already in orbit, but you may need a Mainsail to get a much smaller rocket off of the pad. At 20,000m, you need to be able to start building up lateral speed to achieve orbit, so if you're not able to do that, 1) lighten the upper stage, 2) switch to a larger engine, and/or 3) add an additional stage to the original launch vehicle so that you're calling on that engine later in the flight, where it may be sufficient.

[LarryWallwart]

...

Yes, it was quite the slide show!

http://farm3.staticflickr.com/2881/11973366443_fc7000acf1_b.jpg.

I'd like to keep my budget under 1 trillion. LOL

[Dispatcher]

I'd like to keep my budget under 1 trillion. LOL

Hehe! Yeah. That shot was definitely before I was able to asparagus stage. That was actually the flight which allowed me to get the science to unlock fuel lines, and other goodies (like larger diameter parts). The reason why solid booster fuel is shown in the resources is that I had sepratrons sitting on the top of each booster. I had problems before with them hitting the rest of the ship once jettisoned, and this solved the problem. Once costs/ budgets are enabled, it is gonna be a whole other challenge; and asparagus staging should actually reduce costs.

[blizzy78]

May I recommend my engine cluster calculator. (see signature)

[SRV Ron]

A design I used that got this probe ship in orbit with fuel to spare for the LV-N. Note, I staged the two engines after achieving orbit on this deep space probe.

[SuperBigD60]

--snip--You need a Thrust to Weight Ratio of at least 1.4 to get off Kerbin.

--snip--Technically the best approach is to aim for constant TWR of 2 after each staging.

As long as your TWR is above 1 you can make it to space.

some stages can even be lower than this if they come late enough and you are high enough (during part of its ascent, the space shuttle would often have a TWR <1 although once it burned enough fuel that number went up like a rocket --teehee--).

--snip--Also, 20,000 meters if far from orbit, and you probably shouldn't be on your last asparagus stage that early.

It depends on how fast you're going once you get there and how big the last stage is. sometimes I've got 2000dV or so in my last stage and its definitely the only thing going at that altitude.

I do know that a mainsail will lift at least 3 full orange tanks pretty easily, and a little more. But that just takes experimentation (or back of the napkin calculations based on the thrust of the engine and the mass of the payload/stage).

[bsalis]

For this style of staging, all stacks should have enough TWR to lift themselves, including the core.