Why is my SLS lifter payload fraction so low?

[WhiteWeasel]

It might be me, but it does not seem like a theoretical max payload of ~250t (TWR => 1 and >~4500 m/s dv) is enough for a rocket of that size. I mean it's a 6 symmetry asparagus lifter with three big tanks a stage!

Aspargus S3-14400's & SLS Engine Clusters.

Edited April 8, 2014 by WhiteWeasel

[ScottyDoesKnow]

Your TWR is incredibly low, so it takes you longer to launch and you lose efficiency to gravity. I'd say you want at least 1.5 at all times, though the last stage can be around 1. If you show all stats you can check your payload mass / total mass, it should be at least 15%.

[quasarrgames]

How much does the whole thing weigh? Normally lifting stages can lift payloads between 15 and 20% of their own weight. If its within that threshold it's a good lifting stage.

I don't know what would be causing it to underperform (if it is) other than a small thrust-to-weight ratio.

EDIT: Was i just... Ninja'd?

This is my first time. it feels... strange.

It makes me feel all cold and tingly inside.

*shivers*

Edited April 8, 2014 by quasarrgames

[WhiteWeasel]

It's weight is 1941 tons without the payload. And mind you the second picture with the mechjeb info has three big SLS tanks (246 t) as a dummy payload. Soo, that's a 12.6% payload fraction.

Edited title for new issue.

Edited April 8, 2014 by WhiteWeasel

[sojourner]

Too much fuel. Instead of using 3 big tanks per stack, use 2 tanks per stack. Most of your performance is being lost trying to overcome the low TWR.

Also, fire the SRBs in stage 5. carrying them to altitude is just dead weight. Heck, you might not even need them on the rocket at all.

Edited April 8, 2014 by sojourner

[Aedile]

Umm, too low TWR.

[Kasuha]

Mainsail can lift four jumbo tanks. Scaled to SLS, the new 4-engine does not even lift four large SLS tanks until it consumes some fuel. So yes, two per engine is about right. Maybe even a little less, especially the middle one holding the payload.

[WhiteWeasel]

Too much fuel. Instead of using 3 big tanks per stack, use 2 tanks per stack. Most of your performance is being lost trying to overcome the low TWR.

Also, fire the SRBs in stage 5. carrying them to altitude is just dead weight. Heck, you might not even need them on the rocket at all.

Ummm, the six big srbs all ignite at lift off.

[capi3101]

So...you're shooting for a 250 tonne payload to orbit on a 4STO asparagus booster?

Lessee....basic principles of asparagus here - 15% payload fraction of your rocket to orbit guesstimate, say 1660 tonnes total rocket mass to keep the numbers clean.

You want a 1.6 to 1.7 TWR on liftoff, so you'll need somewhere between 26,055-27,684 kN of thrust at takeoff, ideally with about a quarter of that in the center stack (which ain't happenin'). At 3200 kN of thrust a pop, you're going to want nine engine clusters...so you need to be constructing a 5STOa booster if you want to have enough thrust for the job.

After nine engines are on, you've got about 1320 left for fuel, divvied up in nine stacks that works out to 146.92 tonnes per stack, give or take. Try stacks with a -14400, -7200 and -3600 in them. If that doesn't give you what you need to make orbit, you can try a combo of one -14400 and two -7200s; you should still have enough thrust then, it'll just be sub-optimal.

Basically, what everybody else has been telling you is correct - you've got too much fuel and too little thrust.

[SRV Ron]

As others have said, too much effort is being used to lift the three stacks of fuel instead of some of that thrust going into lifting the payload. The SRBS could go into a ring configuration that gets the entire rocket up to 5 to 10 k for additional efficiency. I did that with this mod setup.

And, this was an experiment with stock in 0.23

Experiment with getting a higher TWO. And, onion staging is an option when adding large number of liquid boosters.

[zarakon]

TWR is low

Also try using a KR-2L engine for your center core, and only light it up once you drop your first stage.

[Kasuha]

For inspiration, here's my SLS-based lifter. The whole middle column is payload, 350 t of it (348.5 to be accurate). The lifter (6 asparagus stages around central column) delivers it to almost orbital trajectory (Ap 75 km, Pe 20 km) from which the payload takes over while parts of lifter safely burn in atmosphere.

Yes it is ugly, I did not spend much time making it pretty. But it does the job.

[WhiteWeasel]

Is this better?

[sojourner]

better, but I would try it without those half tanks. You've got more delta-v than you need by about 900, which means you're carrying too much fuel.

[falofonos]

An initial TWR of at least 1.5 on each stage is desirable. Seriously.

Anything below 1.3 and you're buring fuel to overcome gravity and drag with zero net acceleration.

As you're using mechjeb I'd recommend limiting to terminal velocity to minimise fuel wasted in low atmosphere.

[Zourin]

The nonscientific rule of thumb is no more than two full-sized tanks per engine being used as a booster, or you're giving up a lot of bonus TWR that's supposed to be going towards getting the payload into orbit.