Efficient Ratio Booster/Payload Weight for LKO ?

[OhioBob]

I use very simple guidelines for 2-stage liquid-fueled launch vehicles:

Payload = everything that sits on top of the rocket, including decoupler, fairing, etc.
Second (upper) stage propellant mass = mass of payload
First (lower) stage propellant mass = 2 times mass of payload
Second stage TWR = 1 to 1.3
First stage TWR = 1.3 to 1.5

This will generally produce a rocket with about a 20% payload fraction and 3400 m/s delta-v.

(edit)

Here's an old post I made that details the method:

 

Edited November 19, 2018 by OhioBob

[FinalFan]

On 11/19/2018 at 8:53 AM, Pecan said:

Congratulations.  Presumably you had to twist the candle more to fit 3 engines in place of each of mine - which would have had the exhaust from the topmost thuds hitting the bottom ones.

If your launch mass is less for the same payload then that's perfect, after all the point is to maximise payload ratio.  I'm convinced though that, like multi-layer asparagus, any theoretical efficiency with twisted candle is lost to practical construction restrictions and difficulty. 

Sorry, but you presume incorrectly.  The Cubs are much smaller and narrower than the Thuds, allowing three to be crammed in approximately the same space as a single Thud.  

Maybe I'll try it again with dead weight to make up the difference; the 3-Cub formation is actually at a small launchpad thrust disadvantage versus a Thud, so it'll be interesting to see how that goes.  

[Pecan]

On 11/20/2018 at 9:36 PM, FinalFan said:

Sorry, but you presume incorrectly.  The Cubs are much smaller and narrower than the Thuds, allowing three to be crammed in approximately the same space as a single Thud.

Yes, I shouldn't have said "presumably".  I should have said "It doesn't work without very careful engine positioning, because exhaust from one will destroy another or other structure as they gimbal".  It's possible but just another indication of how hard it is to build twisted candle.

[KerbService]

 

On 11/19/2018 at 2:53 PM, Pecan said:

If your launch mass is less for the same payload then that's perfect, after all the point is to maximise payload ratio.  I'm convinced though that, like multi-layer asparagus, any theoretical efficiency with twisted candle is lost to practical construction restrictions and difficulty.  

This is based on your post with a few modifications...

The entire vessel is 507,350kg with a payload of 117,590kg for a payload ratio of 23.1% with a starting TWR 2.20. But I can not tell you what the total deltaV is because this type of staging does not register well with either K.E.R. or MJ. But I did launch it to a LKO of 80k. And it is simple with a total part count of 44, the booster using only 37 parts. Seems to fly pretty well too. And after a little experimentation I found that for this and asparagus the FTX-2 fuel ducts are not needed. Using the tank's flow priority settings and the decoupler's crossfeed settings results in the same effect. Here, the S4-128 tanks have from the bottom up priorities of 40-30-20-10 and the bottom 3 decouplers have enabled crossfeed. All engines fire until that stage is dropped and the fuel drains from the bottom up. So this is not quite the nightmare I thought.
Entire Mass - 507,350kg
Payload Mass - 117,590kg (decoupler, unusable fuel in top stage tanks, RGU, battery, pointy end)
Usable Fuel Mass - 304,000kg
9 S3 KS-25 engines and 3 RE-I5 engines

[Pecan]

9 hours ago, KerbService said:

Using the tank's flow priority settings and the decoupler's crossfeed settings results in the same effect...

Hoorah, congratulations and how nice to have someone who knows what they're doing for a change :-)  Having never looked at the (new-by-my-standards) fuel-flow system I wouldn't have known where to start, so thanks for that.

[KerbService]

Scales up quickly to a 360,000kg payload too!

(file under ludicrous lifters)

[Pecan]

2 hours ago, KerbService said:

Scales up quickly to a 360,000kg payload too!

So no more twisted nightmares?

[KerbService]

@Pecan ... Your implementation of twisted candle makes for an easy and fairly efficient booster build for payloads of up to maybe 500 tons. I would guess that with a bit more development of the idea (or maybe a lot) a payload ratio of 25% would become the norm. So this has mostly answered the question of my initial post.
The real advantage of Sensi's idea seems to stem from the ignition of all engines simultaneously while having the ability to dump them with the empty tank as a stage is used up. Everything on the booster is being used all the time from launch to orbit. Nothing wasted.
Some considerations:
-- Engine placement on upper stages can cause heat death of lower stage engines (BANG!)
-- Staging results in TWR dropping which might require the strongest engines on the upper stages instead of the lower, this might cap the stage count
-- K.E.R. and MJ would need updates to use those mods with this staging system (at least on my installation of KSP)

So my new booster targets, accept 20-25% and aim for 30.

Thanks man. Thanks to everyone who replied.

Edited November 24, 2018 by KerbService

[Pecan]

"The real advantage of Sensi's idea seems to stem from the ignition of all engines simultaneously ..."

Yes, the advantages of radial-type staging is having more engines firing plus using-and-discarding stages as quickly as possible.  Normal stack-staging has the limitation that upper-stage engines are just deadweight until their moment of fame.  Onion-staging uses fuel-lines to use and jettison outer stacks as quickly as possible.  Asparagus is just the optimal special-case for the minimum symmetrical number of stacks (everything takes from 2 stacks at a time), which is why it is most efficient.  As you say - everything is being used all the time, from launch to orbit.  The question always was - how can you drop just 1 tank at a time without unbalancing the vehicle?

Twisted candle is the answer to that but was always limited by i) the difficulty of placing lower engines so they aren't hit by the exhaust from higher ones, ii) the additional structure (with mass and drag) needed to side-mount most engines or the limited choice of radial ones, iii) similar awkwardness with the fuel-flow.

In my tests I also found a fair bit of structural heating just from engine gimballing but how important that becomes in practice is a matter of individual design.  Of course, once you've accepted the drag-penalty for side-mounting engines you can extend them further from the tanks to avoid that.  You have done the work (that I didn't bother with) to show how the fuel-flow system removes the need for fuel lines and struts so that makes the whole strategy much simpler - perhaps we'll see more of it thanks to you :-)

I wouldn't worry too much about TWR dropping as you jettison stages as that is also true with every other staging strategy and what you're aiming for anyway - TWR around 1.5 for launch but only 0.9 or so (initially) for the last ascent and circularisation stage(s).  It's also easy to vary the TWR by using different amounts of fuel in each stage - note my design had TWR 1.56 for several stages.  Also note that radial staging types also allow you to vary the engines and fuel-load in each stage so there is a lot of flexibility.

Have fun.