Lifter-Payload Mass Percentage

[Solivagant]

I'm wondering, what percent of the total mass should my payloads be? The lifters I'm designing carry between 13 and 15 percent, is that an okay amount? What sort of tips do you guys have for improving this?

Edited January 19, 2015 by Solivagant

[UmbralRaptor]

Depends on details of the craft design. Given recent part buffs, 13-15% is plausible for 2 stage (and sometimes single stage) liquid fuel rockets. Single stage ones can run anywhere from ~5-15%, depending on choice of engines, tankage, and ascent profile. Mixed solid/liquid designs might well get down to or below 10%. If you go the asparagus route with 3-4 stages, I would aim for 15-20%. Turbojets make a mockery of this, with payload fractions of up to 70%.

[Mad_Maelstrom]

Between 13 and 15 percent sounds pretty good to me, especially if it is reliable.

[OhioBob]

With a well designed 2-stage liquid fuelled rocket I typically get about 16%. You're not too far off from that so you're doing pretty good. Regarding tips, you should check out the following thread, in which, by coincidence, we're having a very similar conversation.

http://forum.kerbalspaceprogram.com/threads/107763-Designing-Launch-Vehicles

(edited to add)

With an all-solid first stage my payload drops to about 13%. With a combination of a liquid first stage and SRB strap-ons I can still get close to 16%. In all cases these percentages require flying a very precise and efficient ascent profile with little margin for error. A poorly flown ascent will reduce these percentages. This is also assuming a low orbit of about 75 km.

I also find that my designs using large and extra large parts generally get better payload fractions. When I build rockets from small parts I usually don't do any better than about 13%.

Also note that these percentages are when pushing the launch vehicle to its maximum performance. Sometimes it is necessary to overdesign simply because we're forced to round up to the next biggest available part. It's not always possible to construct the most ultra-efficient rocket for a particular payload size.

I should also note that my comments are based on the stock game without any of the aerodynamic mods.

All things considered, your 13-15% sounds about right.

Edited January 19, 2015 by OhioBob

[GoSlash27]

I agree with the above posts.

15% is totally reasonable for rockets. Turbojet spaceplanes can exceed 60% and turbojet VTOL lifters can exceed 100%.

Best,

-Slashy

[Pecan]

And again, agreed 13-15% payload ratio is an entirely respectable range.

For liquid-fuelled rockets 10% is just about acceptable, 15% starts to look efficient and anything over 17% good. Best I've seen, IIRC, was just over 20%.

The four things that dictate payload ratio are structural and fuel mass, TWR and staging so the tips for improving it are:

Structural mass - On a launch vehicle remove everything apart from the fuel and engines and, if you can't fly without it, a reaction wheel/SAS unit.

Fuel mass - Use higher-Isp engines, provided they deliver enough thrust, so less fuel. (Some experiment required - heavy LV-Ns usually too low-thrust to be effective launch engines).

TWR - Doesn't directly affect payload-ratio, but a high TWR means more engine mass. Beginner recommendations are for launch TWR 1.6 - 2 but good results can be had with a ratio as low as 1.2 or so. A final circularisation stage can also work well with a low TWR, possibly even below 1.

Staging - More stages, with the appropriate engine(s) for the speeds/altitudes at which they'll be used, are roughly more efficient but the parts for decouplers, etc. add to structural mass so there are still diminishing returns. A launch vehicle with three (serial) or four (asparagus) stages is about as far as it's worth going. Checkout esoteric staging strategies like twisted candle too.

You're doing well enough that you probably won't get much benefit from those though. Jets are probably the only way you'll get any significant improvement.

[cicatrix]

A well-built rocket can have 18-22%. 15-18% is a good result. <15% is somewhat poor

Check out this calculator - it will help you in your designs. There was a time I used it a lot right until the moment I learned to do this myself:

KSP Optimal Rocket Calculator for Kerbal Space Program 0.90

Edited January 19, 2015 by cicatrix

[Red Iron Crown]

13-15% is respectable. I consider a rocket lifter reasonably efficient if it can get one sixth of its mass into orbit (~17% payload fraction), chasing more than that requires overly complex staging for me.

If you want better payload fractions, use liquid fueled rocket engines and onion or asparagus staging.

[WuphonsReach]

Best I've done in FAR + KW with a 140t launchpad limit is about 23-24t payload into LKO. That's about 17% fraction.

SRBs with about 1.5k of DV (I use KW's SRBs)

main stage of another 2k DV (Mainsail)

orbit stage to push it the final bit into a circular orbit (Poodle)

[OhioBob]

The best I've been able to do in stock using two stages is 17.5%. I like to keep things simple, so 2 stages or 2 stages + SRBs is about all I've experimented with. It doesn't surprise me that more complex staging designs can get to 20% or more.

[SRV Ron]

If you are not doing better then 15% with conventional staging, you are pushing too much fuel in your boosters.

25% on a SSTO is possible.

Edited January 20, 2015 by SRV Ron

[cantab]

That's apples and oranges though, since you're counting the lifter as payload. In some cases that's relevant, for example if you use the same stage for circularising and for trans-Munar-injection (a la the Saturn V), but it's not really fair to compare the figures to a "pure" lifter and payload design.