I don't understand "asparagus" staging.

[strider3]

What I have read about it leaves me more confused rather than enlightened? I am sure it's a lack of knowledge on my part. One tutorial left me wondering why I continue carrying the mass of empty tanks and dead engines? Can someone point me to an explanation of why asparagus staging works (using the "K.I.S.S. principle"), and then a basic explanation of the "mechanics" that goes along with it ( how to build it, when to stage off, where to decouple, etc.)?

A side note from the "SRB's vs. Asparagus staging" thread: I have always thought that SRB's are more efficient (TWR wise) from Kerbin surface than any LFO launch stage? I don't have any math to support this, just my experiences in actual launches. It always seemed to me that the weight of an LFO launch stage, compared to the DV it provided, was less than strapping a bunch of SRB's on and firing away to get away from the ground. The SRB's always seemed to provide more "bang for the buck" when getting away from Kerbin's lower / mid atmosphere.

The reason I ask this is that my ships are getting taller and taller...and less stable, of course. I am doing Duna science, at this point, and my plan is to put an MPL, with some spare fuel, in orbit around Duna. Then I want to launch a Duna science lander, from Kerbin, to collect science on Duna's high, low and surface, launch back to the MPL orbiter, and transfer the data. I would then, hopefully, have enough spare fuel on the orbiter to refuel the lander and have a few more goes at Duna's surface. From what I have read, been told by you and experienced...I need to stop "building up" as getting out of Kerbin's atmosphere is becoming a major challenge without instability.

As always, your input is...invaluable.

Vic the Newbal

Edited October 12, 2016 by strider3

[Starhawk]

Here is a link to the KSP Wiki Page on Asparagus Staging.

Basically, an asparagus staged rocket has a core and a ring of boosters.  The boosters are dropped in pairs.  Fuel lines connect the boosters such that the last pair to be dropped has fuel lines that feed from the booster tanks into the main core.  This has the effect of allowing the boosters and the core engine to all burn but leaves the core with a full tank at the moment the last pair of boosters is dropped.  The second last pair to be dropped has fuel lines that feed to the last pair and so on.

This requires individual placement of fuel lines and splitting of symmetrically grouped decouplers into pairs to create the stages.  Part highlighting is a great help with this.

In flight, you must watch your fuel gauges in the staging display and drop each pair as soon as the tanks become empty.

Every Eve lifter I have built has used asparagus staging.

Happy landings!

[strider3]

12 minutes ago, Starhawk said:

Here is a link to the KSP Wiki Page on Asparagus Staging.

Basically, an asparagus staged rocket has a core and a ring of boosters.  The boosters are dropped in pairs.  Fuel lines connect the boosters such that the last pair to be dropped has fuel lines that feed from the booster tanks into the main core.  This has the effect of allowing the boosters and the core engine to all burn but leaves the core with a full tank at the moment the last pair of boosters is dropped.  The second last pair to be dropped has fuel lines that feed to the last pair and so on.

This requires individual placement of fuel lines and splitting of symmetrically grouped decouplers into pairs to create the stages.  Part highlighting is a great help with this.

In flight, you must watch your fuel gauges in the staging display and drop each pair as soon as the tanks become empty.

Every Eve lifter I have built has used asparagus staging.

Happy landings!

Here's the main part that was confusing me:

"The result is that the rocket always flies with the minimum number of tanks required to transport the fuel it has left while also constantly using all engines it has on board." It left out the key element, decoupling empty tank / engine pairs as the fuel in those engine's tanks runs out. Or, as you said..."In flight, you must watch your fuel gauges in the staging display and drop each pair as soon as the tanks become empty". That makes so much more sense now. I do foresee some roll and control problems as you drop pairs (as stated in the link you provided)...but that is a piloting / controls issue, in my mind?

This brings us to my next question  (you guys knew there would be one.........right?). Wings on vertical lift ships...soon to be posted in a new thread at a KSP forum near you!

Thanks Starhawk!

[Superfluous J]

Note in 1.2 you no longer need fuel lines. I built an Asparagus staged rocket (for Eve actually) and simply by enabling crossfeed on each decoupler and setting them up to stage in the correct order, the fuel flow priorities were automatically set to drain the tanks exactly as fuel lines would. One caveat, the engines will keep running even after their attached tanks were out of fuel, so you have to watch the tanks themselves.

50 minutes ago, Victor3 said:

One tutorial left me wondering why I continue carrying the mass of empty tanks and dead engines?

You either misunderstood that tutorial or it was wrong. The whole point of Asparagus staging is that you shed weight as the engines stop helping.

50 minutes ago, Victor3 said:

It always seemed to me that the weight of an LFO launch stage, compared to the DV it provided, was less than strapping a bunch of SRB's on and firing away to get away from the ground. The SRB's always seemed to provide more "bang for the buck" when getting away from Kerbin's lower / mid atmosphere.

This is incorrect. SRBs are very heavy for the lift they provide. Their benefit is they're super cheap, which matters a lot in career mode but not so much in sandbox or science mode. In those modes, about the only reason to use SRBs is aesthetics. That said, SRBs are very nice to give you a boost off the pad, even in non-career modes it's frequently easier to "just toss some SRBs on it" than try to figure out how to get a little more TWR and/or dV out of LFO stages.

50 minutes ago, Victor3 said:

I need to stop "building up" as getting out of Kerbin's atmosphere is becoming a major challenge without instability.

You likely want to look into multiple launches. There's a reason we (humans) didn't send the ISS up in one big launch, and it's not (only) because the stuff was built over time by different countries.

You may also want to look into auto-struts and the new rigid joints. They don't fix everything, but they fix a lot of things.

Edited October 13, 2016 by 5thHorseman
Edited for clarity

[Superfluous J]

14 minutes ago, Victor3 said:

I do foresee some roll and control problems as you drop pairs (as stated in the link you provided)...but that is a piloting / controls issue, in my mind?

Depending on how you set up your pairs, yes. But usually it's not too bad especially if you have gimbaling engines and your tanks are well-strutted.

But if you make sure all of your decouplers are exactly inline with the center stack (i.e., they push outward instead of radially) then no, there's no roll to worry about.

[Vanamonde]

My old tutorial is a little outdated now, but maybe it will help clarify some aspects. 

[DD_bwest]

a good way to think of it, is asparagus will in a way, allow your rocket to be fully fueled without boosters at a greater height.

think to the most basic, a core rocket with 2 liquid boosters.   if you cross feed into the core,  all your engines(boosters plus main) will be using the fuel in your boosters.   When you stage out, your boosters are empty and your core is fully fueled, but in the air and has velocity to help.

Now add 2 more boosters, with those cross fed into the first boosters.   So all your engines are running off the fuel in the last boosters you just added. when those are empty, they are discarded.   Now you have your rocket + boosters fully fueled, which will eventually discard and leave your core fully fueled and even higher and faster.

this cycle continues.

[Aegolius13]

A maybe more abstract way to think about it:  why do we stage at all?  To get rid of unneeded mass - namely the dry weight of empty tanks, the engines that were once required to lift the fuel we burned, and maybe a few other things to service the spent stages.  Staging is good, so we want to stage (drop that excess mass) as frequently as possible.  But it would make no sense to drop a stage when it's still half full, since there's still valuable fuel in there.  

Asparagus staging makes it so you drop stages as frequently as possible, while not wasting fuel.  Since the outer tanks feed the inner tanks, they run out of fuel sooner than if there were no fuel connections.  Thus, you're able to shed some excess weight more quickly, which improves your efficiency.  And there's no loss of fuel, since the inner stage will still be full of fuel once you drop the outer stage.  

[magnemoe]

Note that asparagus is not as needed as it was in old versions of KSP because of the 3.75 meter parts and larger solid fuel rockets. 
Three exceptions, you try to lift far over 100 ton, like I did with my Minmus mining rig, you try to get back to orbit from Eve, finally early in carrier if you have not unlocked 2.5 meter parts and want to make an Mun landing.

[strider3]

I should note here that I'm still on 1.1.3 and playing Science Sandbox (I will NOT comment on Contracts in Career mode...nope...not gonna do it...no, no, no...shut up Vic...be quiet).

[Superfluous J]

17 minutes ago, Victor3 said:

I should note here that I'm still on 1.1.3 and playing Science Sandbox (I will NOT comment on Contracts in Career mode...nope...not gonna do it...no, no, no...shut up Vic...be quiet).

If you're playing Science Sandbox then your only concern is aesthetics and ease. SRB's "look right" because "real rockets" use them. They are also far easier than setting up asparagus staging.

That doesn't make them better. Or worse. Just different.

[Warzouz]

Stock rocket "Kerbal-X" has a asparagus staging. you could check it and try it.

[strider3]

Is it possible to asparagus stage "triplets"...instead of pairs? In a 6 booster alignment dropping 3, instead of 2, would seem more balanced on drag, weight balance, etc. I understand that I lose some of the benefit this way but the balance worries me. I'm going to launch a craft with this set-up...I'll be back.

Edited October 15, 2016 by strider3

[ForScience6686]

Asparagus also works well for gaining extra dv on interplanetary trips.  Fuel tanks are worthless once empty, unless you plan to refuel.  But I often times will setup my ships to drop every tank to gain every bit of dv I can.  Plus, less mass equals more twr.

[Starhawk]

48 minutes ago, Victor3 said:

Is it possible to asparagus stage "triplets"...instead of pairs? In a 6 booster alignment dropping 3, instead of 2, would seem more balanced on drag, weight balance, etc. I understand that I lose some of the benefit this way but the balance worries me. I'm going to launch a craft with this set-up...I'll be back.

Yes, it certainly is.

However, the point of asparagus staging is that you want to drop mass as quickly as possible to improve efficiency.  Pairs are used to maintain balance.  If balance wasn't a factor, the most efficient way would be to drop single tank/engine combos quite often.

Balance is an issue but is usually resolvable with piloting (gentle touch, stay close to prograde) or engineering (struts, change of booster placement) solutions.

Happy landings!

[Plusck]

49 minutes ago, Victor3 said:

Is it possible to asparagus stage "triplets"...instead of pairs? In a 6 booster alignment dropping 3, instead of 2, would seem more balanced on drag, weight balance, etc. I understand that I lose some of the benefit this way but the balance worries me. I'm going to launch a craft with this set-up...I'll be back.

It's more the opposite. An X-shaped configuration is balanced. You can stretch the X in one dimension as much as you want, it's still balanced. Likewise if you drop 2 of 6 boosters: your balanced snowflake becomes a balanced stretched X.

However, 3-way symmetry in a coordinates-based system can easily give rise to rounding errors, where the rounding of 2-way symmetry would give equal errors on each side. Also, 3-way symmetry is an added complication for flight control: yaw or pitch controls will necessarily affect two of the three side stacks differently in at least one of those axes.
KSP has improved, I think, in dealing with the errors that can arise in 3-way symmetry, but there are few situations where it's actually better than 2 or 4-way.

[DD_bwest]

aslong as the boosters shed in a symetrical fasion, you should be fine. but in groups of 2 you shed the useless weight faster.

[ForScience6686]

Shedding two is more effective as you are dropping empty mass sooner.  You also won't take as big of a hit in twr.

[strider3]

HMMmmm...a few issues in my first attempt at "triplet" asparagus.

Do the fuel transfer ducts have a set rate of fuel transfer? Although I am using more fuel from my first "set" of 3 boosters (out of 6), all boosters and center stack are using fuel, albeit at a slower rate. I thought I would use fuel only from the first 3 boosters stack tanks? I have had issues with Jumbo 64 tanks when stacked "booster fashion", in the past, however. I have a feeling that all 3 tanks in each stack are not correctly "connected" and not transferring fuel. I will launch again checking all 3 tanks in each stack as fuel is burned. This problem may also explain the next one, below?

Decoupling the first 3 boosters has become an issue. I have my Hydraulic Detachment Manifolds as near centered on the booster stacks (AFAICT) as possible but my first attempt resulted in a massive explosion as the 3 I decoupled ran into the rest of my craft. I should note that I have struts at the top and bottom of each stack. I am using these HDM's as they have the highest ejection force...might take some tweaking to get the stacks to eject radially away from the rest of the craft. I might have misunderstood 5thH's comment as to what was required.

The experiment continues. I have read your above replies and will go back to "pairs" if things become unworkable...I just want to give this a shot, now that I'm into it.

[strider3]

Here's a picture of the craft I'm trying this with:

[strider3]

As I suspected...the Jumbo 64's are not transferring fuel correctly. Notice the middle tank is remaining full? It appears the lower tank is feeding the Mainsail rather than all 3 tanks feeding top to bottom. Some kind of "disconnect" between the middle and bottom tanks:

This is one of the first "triplet" stacks, and should be feeding fuel to all stacks. I would need another launch, this time viewing this stack and it's adjacent one, to see what is really going on. Obviously, however, there is the recurring problem I've had stacking Jumbo-64's. It appears to me the middle tank is doing nothing? Obviously a "build" issue when I stack these tanks as booster stages...never really had any luck with them working in unison. Maybe that's why I usually go with SRB's?

Not that SRB's are the answer...I need to figure out why this is happening.

[ForScience6686]

Where do you have the fuel ducts mounted?  My guess would be the central tank, which is why it isn't draining yet.  As for the booster separation, you want the decoupler to be above the booster com.  That way it ejects the top of the booster away and aero forces will do the rest in peeling it out and away.

[Aegolius13]

44 minutes ago, Victor3 said:

Do the fuel transfer ducts have a set rate of fuel transfer?

At least in my prior experience, if there is a fuel duct speed limit, it's not a bottleneck even when dealing with multiple Mammoths.  Sounds more likely there's an issue with the fuel flow / duct location.  I've generally had good results placing the duct on the lowest tank of each stack.  

However, not sure how this interfaces with 1.2 and the fuel flow logic.  You may not even need ducts now that you can crossfeed through the decouplers.  

47 minutes ago, Victor3 said:

Decoupling the first 3 boosters has become an issue. I have my Hydraulic Detachment Manifolds as near centered on the booster stacks (AFAICT) as possible but my first attempt resulted in a massive explosion as the 3 I decoupled ran into the rest of my craft. I should note that I have struts at the top and bottom of each stack. I am using these HDM's as they have the highest ejection force...might take some tweaking to get the stacks to eject radially away from the rest of the craft.

I'd recommend putting the decoupler a little higher than center on the booster.  That way the ejection force pushes the top away more strongly, letting the boosters angle away from your ship.  Even the TT-70 decoupler should be enough to get those boosters out of the way.  

[Plusck]

53 minutes ago, Victor3 said:

HMMmmm...a few issues in my first attempt at "triplet" asparagus.

Do the fuel transfer ducts have a set rate of fuel transfer? Although I am using more fuel from my first "set" of 3 boosters (out of 6), all boosters and center stack are using fuel, albeit at a slower rate. I thought I would use fuel only from the first 3 boosters stack tanks? I have had issues with Jumbo 64 tanks when stacked "booster fashion", in the past, however. I have a feeling that all 3 tanks in each stack are not correctly "connected" and not transferring fuel. I will launch again checking all 3 tanks in each stack as fuel is burned. This problem may also explain the next one, below?

If you're talking about the fuel levels visible alongside the staging icons: this is normal.

Each engine "sees" a total amount of available fuel. It then displays what portion of that total is unused. So your centre stack "sees" its tanks plus all boosters, and displays that a small portion of that (the first stage) is emptying. When you stage, the display should return instantly to "full" fuel.

The fuel ducts should always (if possible) be connected "from" the topmost tanks. That means that they'll drain that stack from the bottom up, improving stability. It doesn't really matter where they connect "to" as long as you're going in the right direction, since fuel ducts are always the priority path when the engine is looking for fuel.

 

If, on the other hand, you use the new fuel routing tweakables and no fuel ducts, you'll need to watch every set of booster tanks carefully since the fuel crossfeed goes both ways, and your boosters will empty their own tanks then start emptying the neighbouring ones, without ever flaming out.

[strider3]

3 hours ago, ForScience6686 said:

Where do you have the fuel ducts mounted?  My guess would be the central tank, which is why it isn't draining yet.  As for the booster separation, you want the decoupler to be above the booster com.  That way it ejects the top of the booster away and aero forces will do the rest in peeling it out and away.

The fuel ducts are mounted on the top tanks of each of the first to be used "triples".