A new way of looking at Asparagus staging?

[GoSlash27]

I had an interesting thought earlier and I'd like to share it here and let you folks kick it around...

The advantage of asparagus staging in terms of efficiency is that virtually none of the weight involved is "dead" weight. The disadvantage is that it's payload capacity is ultimately limited by how much payload the last remaining stage can accelerate acceptably.

So what if you set up the center stage to accelerate the payload at a desired rate, while the radially dependent stages merely have enough thrust to lift themselves? Looked at this way, the radial stages become not "boosters", but actually "massles fuel tanks". They provide additional DV by donating their unburned fuel to the core booster, rather than the usual practice of providing additional thrust. If this line of reasoning is correct, an asparagus booster designed in this fashion should be noticeably more efficient than common asparagus designs.

As an example, say you're designing an asparagus booster as a transstage for a 50t payload/injection stage combo. You want to generate 2,000 DV at 1G. So you design the core stage to generate maybe 700DV while lifting 20T + whatever it's fuel is. The radial stages have the same amount of fuel as the core, but only enough thrust to lift themselves. Each stage pair would then be expected to keep the core booster running to generate another 200+450+650 and ultimately fulfill the DV requirement.

This should also cut down on the inherent instability problems since the bulk of the thrust is behind the payload instead of spread out.

Has anyone experimented along these lines?

Best,

-Slashy

[Vanamonde]

Since you're talking about rocket building rather than sharing a craft file, this thread has been moved to Gameplay Questions.

[sal_vager]

I did this way back in 0.13.2, had to use an addon for it though as there were no stock engines that were right for it.

It seemed to work but it was hard to be sure, the parts were pretty OP.

[diomedea]

Could be I am missing something?

When you say each radial stage should be just able to lift itself, it shows you want to launch from a body surface; in such a case, those radial stages will have TWR = 1.00; then if I get right your Core also has a desired acceleration of 1G, means the whole ship has the same TWR = 1.00; really too low, it will start moving only after having consumed some fuel.

Good TWR for a launch are in the range 1.5 - 2.0, so your aim should be to have radial stages able to give that much. Now, each body requires a different ratio boosters/payload, because of gravity and atmosphere density: let me say it takes a ratio of 4 in case (close enough to what required for a launch in LKO). If each of the radial stages had a own TWR of 2.0, the total TWR of the ship would then be: (boosterTWR * ratio + CoreTWR)/(ratio+1) = (2.0 * 4 + 1.0)/5 = 1.8, certainly fitting.

If instead you are designing asparagus staging for a ship already in orbit, you can't really tell about lift capability of the radial stages, but their acceleration. In that case, a same acceleration of 1G for each stage would certainly do.

[UmbralRaptor]

I'm not following, especially given the notable gravity losses for TWRs below ~1.5. (The outer stages would "drag down" the core into this range, giving especially bad losses for the first few kilometers.)

I suspect a more practical difficulty is sizing the stages and/or engines appropriately. But perhaps if we can see an actual craft...?

[GoSlash27]

Could be I am missing something?

When you say each radial stage should be just able to lift itself, it shows you want to launch from a body surface; in such a case, those radial stages will have TWR = 1.00; then if I get right your Core also has a desired acceleration of 1G, means the whole ship has the same TWR = 1.00; really too low, it will start moving only after having consumed some fuel.

Good TWR for a launch are in the range 1.5 - 2.0, so your aim should be to have radial stages able to give that much. Now, each body requires a different ratio boosters/payload, because of gravity and atmosphere density: let me say it takes a ratio of 4 in case (close enough to what required for a launch in LKO). If each of the radial stages had a own TWR of 2.0, the total TWR of the ship would then be: (boosterTWR * ratio + CoreTWR)/(ratio+1) = (2.0 * 4 + 1.0)/5 = 1.8, certainly fitting.

If instead you are designing asparagus staging for a ship already in orbit, you can't really tell about lift capability of the radial stages, but their acceleration. In that case, a same acceleration of 1G for each stage would certainly do.

I'm assuming a transstage for the example rather than a boost stage, hence the arbitrary "1G" specification. For a booster stage, you'd use 2G on the central core booster to lift the entire stack and each radial "feeder" would generate 2G, but only for itself.

Using completely invented numbers by way of illustration, you might be lifting a 20T payload +50T transstage/ injection assembly off the pad. Your central core booster has 30T of fuel, so it has an engine that generates 200T of thrust. The radial "feeder boosters", only have to lift themselves, so each requires only 60T of thrust.

Hope I'm explaining it right!

-Slashy

Edited June 22, 2014 by GoSlash27

[wasmic]

I had an interesting thought earlier and I'd like to share it here and let you folks kick it around...

The advantage of asparagus staging in terms of efficiency is that virtually none of the weight involved is "dead" weight. The disadvantage is that it's payload capacity is ultimately limited by how much payload the last remaining stage can accelerate acceptably.

So what if you set up the center stage to accelerate the payload at a desired rate, while the radially dependent stages merely have enough thrust to lift themselves? Looked at this way, the radial stages become not "boosters", but actually "massles fuel tanks". They provide additional DV by donating their unburned fuel to the core booster, rather than the usual practice of providing additional thrust. If this line of reasoning is correct, an asparagus booster designed in this fashion should be noticeably more efficient than common asparagus designs.

As an example, say you're designing an asparagus booster as a transstage for a 50t payload/injection stage combo. You want to generate 2,000 DV at 1G. So you design the core stage to generate maybe 700DV while lifting 20T + whatever it's fuel is. The radial stages have the same amount of fuel as the core, but only enough thrust to lift themselves. Each stage pair would then be expected to keep the core booster running to generate another 200+450+650 and ultimately fulfill the DV requirement.

This should also cut down on the inherent instability problems since the bulk of the thrust is behind the payload instead of spread out.

Has anyone experimented along these lines?

Best,

-Slashy

That's what I usually do, in the rare case where I do Asparagus.

[SRV Ron]

The core stage, while ideally should be capable of lifting the payload on its own, doesn't need to for efficiency in asparagus staging. It only needs to be placed almost to orbit. I have used nukes for the core stage efficiently. They don't even have to be firing either. I did one where half the fuel remained in the last pair once orbit was reaches and dumped the engines to take advantage of the high isp of the center nuke.

Ideally, the asparagus is made up of copies of the core section. Get a combo right and you can get very good launch to payload in orbit ratios.

And, experiment with different engine combos. One heavy lift stock design worked best with all skippers combined with a pair of mainsails on the last pair stage.

Example of a matched core to asparagus pair.

Another matched core example.

And, the stock mixed engine example;

All skippers was too low a thrust weight ratio while all mainsails was too high using fuel up too quickly. The one pair got the right combo of power and fuel efficiency for placing the fuel can payload in orbit with lots of fuel to spare.

Edited June 22, 2014 by SRV Ron

[GoSlash27]

I'm not following, especially given the notable gravity losses for TWRs below ~1.5. (The outer stages would "drag down" the core into this range, giving especially bad losses for the first few kilometers.)

I suspect a more practical difficulty is sizing the stages and/or engines appropriately. But perhaps if we can see an actual craft...?

I'm just kicking around the concept, so I don't have a craft to show as an example. The thinking is that copying the core booster basically makes the radial stages "boosters", while eliminating their engines entirely essentially makes them drop tanks. My suspicion is that the optimal efficiency lies somewhere between those two points. I think it's where they burn just enough gas to provide their own acceleration.

-Slashy

[diomedea]

I'm assuming a transstage for the example rather than a boost stage, hence the arbitrary "1G" specification. For a booster stage, you'd use 2G on the central core booster to lift the entire stack and each radial "feeder" would generate 2G, but only for itself.

Using completely invented numbers by way of illustration, you might be lifting a 20T payload +50T transstage/ injection assembly off the pad. Your central core booster has 30T of fuel, so it has an engine that generates 200T of thrust. The radial "feeder boosters", only have to lift themselves, so each requires only 60T of thrust.

Hope I'm explaining it right!

-Slashy

OK, what you mean is each radial stage has enough thrust to provide the required TWR at launch, instead of "to lift itself".

[GoSlash27]

OK, what you mean is each radial stage has enough thrust to provide the required TWR at launch, instead of "to lift itself".

No Sir.

In this model they do not contribute to the TWR of the stage as a whole. They merely generate enough thrust to make themselves "transparent" to the rest of the stack. Their job is not to provide additional thrust, but rather additional fuel. Simply carrying the fuel with no engines would incur a weight penalty on the engine that's doing all the work, so they generate just enough thrust to lift themselves so the core engine doesn't have to.

-Slashy

[diomedea]

Look, you keep using the wrong terms. Lift means to just be able to get higher, but without any knowledge about the performance. TWR gives that. What you mean is what I wrote.

You say "they do not contribute to the TWR of the stage". Instead, anything that has mass or thrust conributes to TWR, what you mean is those stages are to be seen as neutral, having the same TWR of the core stage so the total TWR does not change.

[GoSlash27]

"what you mean is those stages are to be seen as neutral, having the same TWR of the core stage so the total TWR does not change."

This. And my reasoning is that if you have radial stages providing as much thrust as the core stage, then you are either over-accelerating off the pad or throttling back. Either way, you're carrying more engine than you need to. Likewise, if you are carrying radial drop tanks, you will either under- accelerate off the pad or else put in a bigger engine and throttle down at the end. Again, more engine than you need.

If the system is tailored so that you always have just the right amount of engine then you have maximized your mass ratio, allowing you to get more DV with a lighter stage.

And the reason I used a transstage as an example instead of a booster is because saving weight on upper stages is critical while saving weight on boosters isn't.

Best,

-Slashy

[Moonfrog]

I think I understand what your thinking. Your idea, uses the main engine to provide the majority of thrust and determine the TWR of the rocket. The radial rockets in asparasgus set up, initially act with an acceleration of 0. Since they have TWR of 1 themselves.

I do believe that there is a flaw though, when dealing with delta V or TWR, they all treat each individual stage as a whole entity.

As far as physics is concerned, its only interested in your total thrust provided by all your firing engines. so the TWR ratio of any individual part effectively does not matter. I do believe it would effect stability, I don't know how much physics is in the game, but in real life there would be differences in stress through the rocket depending on where the thrust came from, and was applied to the rocket.

In terms of delta V, I don't think it makes any difference, if u use smaller engines to achieve a twr of 1 with radial fuel tanks, you will in fact with each stage, be dropping less mass, since the central engine would need to be larger to provide TWR for takeoff. Goal usualy is to drop a high proportion of mass in your first stages than you do in the next stages. I always personally feel people do Asparagus wrong, because they simply copy the same engine and fuel tank then multiply it round. I think this is inefficient use of staging, the advantage of asparasgus staging is not delta V, its being able to use all your engines at once to provide thrust through the launch. Because you have all engines firing, you can use less engines overall, because you have less deadweight engines in upper stages. Extra delta V comes from using more small efficient engines and less deadweight engines, it does not come from using fuel more efficiency. Your simply reducing the mass needed, because more engines are being used than in an apollo style set up, that uses one engine at a time, every engine above is a deadweight. I'm not good at explaining but I hope that makes sense. only other way I can explain it this,

Lets say u have three engines, one main rocket, two boosters on the side. Without the fuel pumps, your max thrust assuming engines have a thrust of 200 is 400 for the boosters. But with fuel pumps, you can use the third engine as part of the booster stage. Provides total thrust of 600. Allows for extra 200 thrust of extra fuel, in that stage, equals more delta V. Main stage still has a thrust of 200. Of course if u didn't use fuel pumps, still fired all three engines, u would have 600 and the extra fuel from that 600 thrust. But you would only have one stage, not two stages. Assuming that its more efficient to stage with the given fuel mass and total mass your using, this would reduce your delta V. Do you see what I mean by, its not fuel efficiency, its engine use efficiency that provides the extra delta V, in asparagus. The way in which u were thinking of getting more fuel, by using your thrust efficiency is actually on the right track of how asparagus works. At least in my opinion anyway, not like I'm a rocket scientist, my instincts tell me that's why it works.

[Pecan]

I'm just kicking around the concept, so I don't have a craft to show as an example. The thinking is that copying the core booster basically makes the radial stages "boosters", while eliminating their engines entirely essentially makes them drop tanks. My suspicion is that the optimal efficiency lies somewhere between those two points. I think it's where they burn just enough gas to provide their own acceleration.

-Slashy

I'm taking this as the most succinct form of what you're saying and, broadly, I'd agree that booster (non-core) stages should not be simple replicas of the core. The situation is more complicated than you put it though because of at what phase of a manoeuvre each stage might be used. For space-only operation TWR is only relevant to burn-time, while at launch it is critical from the start. None of this is specific to asparagus staging though - it applies to all strategies except serial and radial (where only one stage can burn at a time).

In space: the core has to be able to provide 'acceptable' TWR on its own and, as you say, the role of any other stages is simply to provide additional fuel. The mass of that fuel reduces the TWR though, so add drop-tanks as long as you can, with additional engines only where the TWR drops below an acceptable level. Since engines are discreet units that's likely to give you more TWR than you want/need so add additional drop tanks again in any later stages. If that's what you mean then, yes, I entirely concurr - especially don't carry engines you don't need and jettison them as early as possible because their dead-mass is even worse than empty tanks.

For launch: As you have written in other places the TWR considerations are different at different launch-phases. From Kerbin you want a high TWR at the start but don't need so much for the final push to apoapsis and circularisation. As such the core should only be designed to give you the TWR for that final part of the flight, since that's where it will be used alone. Earlier stage(s) should have exactly those engines required to maintain the necessary TWR for their flight-phase but if you are splitting fuel-load to ditch used tank-mass and the 'inner' engines already provide enough thrust then, yes, make the stage drop-tanks.

Overall I don't think it's a case of trying to maintain some required TWR but balancing the fuel-load and engine-mass in each stage. As has been long noted if you make each stage a clone of inner ones the TWR must decrease all the time. As an antidote I try to put all the engine-power I can on later stages and make earlier ones comparatively underpowered or even slack tanks. (Chapters 4 and especially 5 of the tutorial).

[GoSlash27]

UmbralRaptor,

I knocked together an example of what I'm talking about. It was a rush job and I didn't do any prep work, so please pardon the crudity...

Here you see the complete stack. Almost 20T of payload at the top. The guidance/ docking unit below that. Further down is the injection stage surrounded by the transstage in an asparagus arrangement. You can't tell by the pic on the pad, but the injection stage actually has 4 engines while the transstage units have twice the fuel but only the one 48-7S engine per tank. The injection stage does the bulk of the lifting for the upper stages, while the transstage units are just along for the ride, feeding it.

Likewise, the boost stage sits below. A single Mainsail surrounded in an asparagus arrangement by 6 Aerospikes. They don't add thrust to the stack, but are merely there to lift their individual tanks. The Mainsail does the heavy work.

This results in a remarkably small assembly for such a massive payload.

Again, this is a crude example. I can do better with some prep time.

Edited June 23, 2014 by GoSlash27
Typo

[GoSlash27]

I think I understand what your thinking. Your idea, uses the main engine to provide the majority of thrust and determine the TWR of the rocket. The radial rockets in asparasgus set up, initially act with an acceleration of 0. Since they have TWR of 1 themselves.

I do believe that there is a flaw though, when dealing with delta V or TWR, they all treat each individual stage as a whole entity.

As far as physics is concerned, its only interested in your total thrust provided by all your firing engines. so the TWR ratio of any individual part effectively does not matter. I do believe it would effect stability, I don't know how much physics is in the game, but in real life there would be differences in stress through the rocket depending on where the thrust came from, and was applied to the rocket.

In terms of delta V, I don't think it makes any difference, if u use smaller engines to achieve a twr of 1 with radial fuel tanks, you will in fact with each stage, be dropping less mass, since the central engine would need to be larger to provide TWR for takeoff. Goal usualy is to drop a high proportion of mass in your first stages than you do in the next stages. I always personally feel people do Asparagus wrong, because they simply copy the same engine and fuel tank then multiply it round. I think this is inefficient use of staging, the advantage of asparasgus staging is not delta V, its being able to use all your engines at once to provide thrust through the launch. Because you have all engines firing, you can use less engines overall, because you have less deadweight engines in upper stages. Extra delta V comes from using more small efficient engines and less deadweight engines, it does not come from using fuel more efficiency. Your simply reducing the mass needed, because more engines are being used than in an apollo style set up, that uses one engine at a time, every engine above is a deadweight. I'm not good at explaining but I hope that makes sense. only other way I can explain it this,

Lets say u have three engines, one main rocket, two boosters on the side. Without the fuel pumps, your max thrust assuming engines have a thrust of 200 is 400 for the boosters. But with fuel pumps, you can use the third engine as part of the booster stage. Provides total thrust of 600. Allows for extra 200 thrust of extra fuel, in that stage, equals more delta V. Main stage still has a thrust of 200. Of course if u didn't use fuel pumps, still fired all three engines, u would have 600 and the extra fuel from that 600 thrust. But you would only have one stage, not two stages. Assuming that its more efficient to stage with the given fuel mass and total mass your using, this would reduce your delta V. Do you see what I mean by, its not fuel efficiency, its engine use efficiency that provides the extra delta V, in asparagus. The way in which u were thinking of getting more fuel, by using your thrust efficiency is actually on the right track of how asparagus works. At least in my opinion anyway, not like I'm a rocket scientist, my instincts tell me that's why it works.

Moonfrog,

The error in this train of thought is that more thrust does not equal more DV. Whether you have a single engine or a dozen, they will all yield the same DV for a given quantity of fuel given the same Isp, since the DV is actually a function of the Isp and mass ratio. Arguably, the total DV is actually *reduced* by adding engines, since they are mass that is not fuel.

The efficiency is mostly as you say; using engines instead of lifting them as cargo. It allows you to meet your thrust requirement for a given phase of a launch using less total engine mass.

Also, in this setup the radial rockets don't have a TWR of 1. They have a TWR appropriate for the tank they're lifting and the phase of the launch. An early stage might accelerate it's tank at 2 Gs, while a late stage might only lift it's tank at .7G. As you say, it depends on the requirement at the time...

Best,

-Slashy

Edited June 23, 2014 by GoSlash27

[bsalis]

So what if you set up the center stage to accelerate the payload at a desired rate, while the radially dependent stages merely have enough thrust to lift themselves? Looked at this way, the radial stages become not "boosters", but actually "massles fuel tanks".

IMO that is actually the correct way to think of this style of staging.

If you treat the outer stages as boosters that contribute lift to the core, the TWR will decline as stages are separated. Which is not what you want.

[allmhuran]

This is on the right track.

With an asparagus setup (or really any kind of parallel staging, but particularly asparagus) having every column provide the same TWR at launch minimizes shear stresses.... at launch.

However, keep in mind that with asparagus staging in particular, the outermost stages will be drained very rapidly, which means their TWR will increase very rapidly. Immediately prior to their ejection they will have all of the engine thrust but only the mass of the empty tank to lift. This radically increases shear stress.

Since we can't easily adjust engine throttle independently (yes you can click on them one at a time if you want your rocket to leave controlled flight) there's not much to be done about this other than having a higher TWR for the core than for the outer columns. This is obviously difficult to achieve, since the core is the column carrying the payload! This means a bit more stress at launch, but less during flight.

Additionally, as you get closer and closer to your apoapsis you usually want your TWR to decrease... not because lower TWR is better per-se, but because having a high TWR at this point means you've lifted a big engine (or big group of engines) all the way to space, whereas you actually only need enough thrust to circularize... and with a good ascent this can be quite a low TWR indeed, meaning you only need to lift a little engine all the way to space.

So a combination of high-thrust core, asparagus or other parallel boosters, and a one or two serial final stages for your last few km to apoapsis and orbital injection is usually an ideal approach.

[Red Iron Crown]

Additionally, as you get closer and closer to your apoapsis you usually want your TWR to decrease... not because lower TWR is better per-se, but because having a high TWR at this point means you've lifted a big engine (or big group of engines) all the way to space, whereas you actually only need enough thrust to circularize... and with a good ascent this can be quite a low TWR indeed, meaning you only need to lift a little engine all the way to space.

I agree with this, the falling TWR of asparagus designs as they stage is a feature, not a flaw. It's inefficient from a payload fraction perspective to lug enough engine up to have a TWR suitable for launching. I often circularize with a TWR of <1.

[rkman]

IMO that is actually the correct way to think of this style of staging.

If you treat the outer stages as boosters that contribute lift to the core, the TWR will decline as stages are separated. Which is not what you want.

It is usually what i want when launching to orbit: ~1.7twr @ takeoff, up to 3twr @ about 10 to 30km, about 1twr for final circularization. For interplanetary maneuvers 0.5 ~ 1 is sufficient.

Basically you always want sufficient twr (and d-v) for every phase of the mission (which is why it is good practice to "design backwards"), this applies to asparagus as much as is does to traditional staging. The twr of asparagus stacks follows from that, not from the generalization that asparagus stacks should lift no more than their own mass (they won't anyway, their twr starts increasing as soon as they start burning fuel).

[Moonfrog]

This is just really just an idea, but what if u could find a way to do asparagus style staging, but a different way to the 6 radial tanks round the core?

What if you used a large fuel tank, like the shuttle fuel tank that it is attached too. then add side boosters with smaller engines to that, which are attached vertically to the side, rather than in a circle. If the boosters and engines are going ot be smaller, be more room to position them. You could put the payload on the side of the rocket. Probably wouldn't work, be a nightmare putting it together, but thinking about your idea again, maybe the way your thinking would allow for rockets to be designed differently.

Anyone even tried having a payload on the side of the rocket? Or even in the middle of a rocket if that's possible.

Maybe you could have a Core fuel tank, but not a core engine. have the fuel pumped out to smaller engines around that core fuel tank. As your need for TWR becomes less, you could drop the engines. something something, think, where is that bee going with my honey. alright I've lost my train of thought, I thought I had something there.

[Pecan]

Are you thinking of bamboo or twitsted candle perhaps? http://forum.kerbalspaceprogram.com/threads/75669-Staging-Methods-Overview

[SRV Ron]

This is just really just an idea, but what if u could find a way to do asparagus style staging, but a different way to the 6 radial tanks round the core?

Experiment and see what works;

[GoSlash27]

This is just really just an idea, but what if u could find a way to do asparagus style staging, but a different way to the 6 radial tanks round the core?

What if you used a large fuel tank, like the shuttle fuel tank that it is attached too. then add side boosters with smaller engines to that, which are attached vertically to the side, rather than in a circle. If the boosters and engines are going ot be smaller, be more room to position them. You could put the payload on the side of the rocket. Probably wouldn't work, be a nightmare putting it together, but thinking about your idea again, maybe the way your thinking would allow for rockets to be designed differently.

Anyone even tried having a payload on the side of the rocket? Or even in the middle of a rocket if that's possible.

Maybe you could have a Core fuel tank, but not a core engine. have the fuel pumped out to smaller engines around that core fuel tank. As your need for TWR becomes less, you could drop the engines. something something, think, where is that bee going with my honey. alright I've lost my train of thought, I thought I had something there.

I'm actually doing something like that with my (hopefully) properly designed 20t lifter. The transstage is arrayed around the injection stage asparagus style, but the injection stage is merely cargo. I lose some mass ratio doing that, but I have a self- imposed rule against lighting nuclear engines in Kerbal atmosphere. Ironically enough, all the stages of that booster wound up using the same exact engine because I wanted the thrust to taper down from 1.5G to 1G over the course of the burn.

The boost stage looks to be a lot like my knocked- together example. single mainsail surrounded by aerospikes with radial engines. It should maintain a constant 2G throughout the boost phase.

We'll see how it goes...