Staging efficiency question or those who have worked this out already.

[Moonfrog]

I've been having trouble with a conundrum in my head about when to stage. This occurred after working out in a small rocket, that it actually had more delta V for the same total mass, than using as many stages as possible. I was clearly under the mistaken impression that staging always helps.

After doing a bit of research, in form of a spread sheet, now looks like a mess of numbers that gives me a headache . Discovered a couple of things.

The spread sheet calculated delta V, total mass, and fuel mass and the ratio that you Ln in the calculator to get the delta V.

I increased its increments by the 0.5625 mass fuel container. As the increments increased I noticed the difference between each increment in Delta V was always getting smaller. Each 0.5625 container gave less delta V each time I added one, which I kinda expected.

I created another column next to it which calculated delta V of a rocket that had a new stage added, using a decoupler and engine. Then incrementally put this next to the column with difference in delta V for each added 0.5625 container. I paste some of the spread sheet here. It didn't copy very well, but if u look at the number 618.3, then column next to it that's what I'm referring too. Its very rough, I didn't take TWR into account, or really proper engines ISP, just used the same ISP. I think optimum staging point is roughly at 65% of the total mass as fuel, for when you should stage. If u do it before that, the staging creates less delta V increase than simply adding the container. It might be different with smaller parts, I only tried it with early career ones. Certainly anything that is less than 50% of total mass as fuel would be less efficient. Sort leads me to believe that the asparagus pancake design I see a lot is actually quite wasteful. I imagine the fuel pumps are not the problem just how the staging is done, with each section being the same size container as previous stage. This seems to suggest each stage should get proportionally larger. I'm not that great at maths or this stuff, so maybe someone else has already figured this out, and could save my brain a massive headache? Or just tell me I'm wrong i'll live with it.

pod/engine fuelpod totalm Fuelm ratio dv difference stageddv TMstage Ratiostage

1.49, 0.5625, 2.0525, 0.49, 1.3136, 856.8078875,

1.49, 1.125 2.615, 0.98, 1.599388379, 1475.136836, 618.3289489, 420.0112728, 3.915, 1.143065693

1.49, 1.6875, 3.1775, 1.47, 1.86090776, 1950.837845, 475.7010082, 364.0570231, 4.4775, 1.122884013

1.49, 2.25, 3.74, 1.96, 2.101123596, 2332.194425, 381.3565803, 321.2701385, 5.04, 1.107692308

1.49, 2.8125, 4.3025, 2.45, 2.322537112, 2646.896716, 314.7022912, 287.4891401, 5.6025, 1.095843521

1.49, 3.375, 4.865, 2.94, 2.527272727, 2912.260346, 265.36363, 260.1399357, 6.165, 1.086343612

1.49, 3.9375, 5.4275, 3.43, 2.717146433, 3139.807716, 227.5473698, 237.5445066, 6.7275, 1.078557114

1.49, 4.5, 5.99, 3.92, 2.893719807, 3337.574455, 197.7667394, 218.5620629, 7.29, 1.072058824

1.49, 5.0625, 6.5525, 4.41, 3.058343057, 3511.374132, 173.7996769, 202.3899111, 7.8525, 1.06655348,

Edited June 22, 2014 by Moonfrog

[Red Iron Crown]

For very small rockets, staging may actually decrease delta-V rather than increasing it, because of the additional dry mass of the decouplers (and engines if you're using different engines for each stage). Especially if you're staging radially, the lightest radial decoupler is fairly massive in relation to the 0.625m tanks.

[SRV Ron]

In a challenge, cheapest rocket to orbit, the lightest simplest designs brought out the high efficiency of few parts and single stage. You can easily reach Mun orbit with simple small designs.

With mods, such as NovaPunch and KWRocketry, one can build small probes capable of exploring almost everywhere.

[GoSlash27]

I had worked out the same thing a couple weeks ago using a spreadsheet. While staging does generate better mass ratios, there is a hard limit to how many staging events in a single phase of a launch will yield positive results.

Usually 2-3 is optimal for a single phase and I've never found an instance where 4 was an improvement.

The most efficient method I've found is 7-5-3-1 asparagus staging, followed by 4-2-1 layer cake staging (which yields over 90% the efficiency of asparagus staging). The least efficient method is a single stage, which yields approximately 55% the DV for the same mass.

To answer your question, the asparagus pancake design you see used is actually very efficient, although a poorly- executed asparagus setup is a lot less efficient than other methods when properly executed.

Where this analysis leads you astray is you're not fully simulating each stage working lifting a payload. The mass ratios get worse with each stage you add, which would lead you to believe that less is more, *but*the mass ratios of early stages have no negative effect on the mass ratio of the entire assembly other than the additional decouplers. The DV is additive, not the mass ratio.

So while the additional stages contribute less to the total DV of the vehicle the more you add, your total DV is still improved.

A couple different ways to look at it are to:

-compare a simple 3 stage rocket to an SSTO. While the SSTO has superior mass ratio, the 3 stager needs a tiny fraction of the mass.

-Mathematically, doubling a DV requirement requires 7.4 times the mass ratio. (e^2) Therefore splitting up the DV requirement into 2 stages will yield a mass roughly 27% the original. Likewise, 3/e^3 for 3 stages yields 15%; 56% the mass of a 2 stager. (N+1)/e^(N+1)/ N/e^N shows diminishing returns until the added mass of decouplers finally exceeds the gains.

HTHs,

-Slashy

Edited June 21, 2014 by GoSlash27

[Pecan]

How to decide when to stage:

1. Use the most efficient engine(s) for the flight-stage = ASL or Vac ISP.

2. Give it/them as much fuel as they can push with an 'acceptable' TWR = wherever you're launching from or how long you can cope with in-space burn times.

3. If deltaV is sufficient, stop. Otherwise, stage.

[UmbralRaptor]

The basic asparagus layout tends to be good for S1 vs S2, increasingly off of optimum size (for the total amount of fuel) for later ones. As best I'm aware, an ideal rocket would have the same mass ratio in every stage if the engines operated at the same Isp and TWR was a non-issue. In practice, other factors mean that lower stages tend to have less ÃŽâ€V than upper ones.

The mass penalties for staging (especially asparagus-style) aren't that large in present KSP, but were a major factor in 0.15 and earlier, which lead to some seemingly paradoxical results.

[Streetwind]

I've been having trouble with a conundrum in my head about when to stage.

The nature of your conundrum is something called "fuel mass fraction". It's that thing in logarithm brackets in the rocket equation. Even though it looks like it's just a simple, inconsequential division operation, it actually carries a number of important implications. And that's something that takes most people a while to truly understand - myself, I played KSP obsessively for months before it suddenly clicked (and then only after asking and reading on the forums).

I wrote a wall of text about it here: http://forum.kerbalspaceprogram.com/threads/81919-PURE-rocket-SSTO?p=1194322#post1194322

Slightly different topic (SSTO design), but same general gist, especially the first half.

The TL;DR version is:

Your fuel mass fraction can never exceed a certain number, even if you attempt to add the entire observable universe's mass as fuel. This in turn means your dV will never exceed a certain number either. And that, finally, means that the more fuel you add, the less you get from each additional unit of fuel (which is exactly what you observed). Eventually the point comes where you practically cannot gain any more dV by adding fuel. This is called "the tyranny of the rocket equation". The only way around that problem is staging, because this "law" applies to individual stages only. A staged vehicle can have a theoretically infinite amount of dV. The smaller each stage is, the less diminishing returns you face from the fuel mass fraction, and thus the higher your efficiency becomes and your total amount of dV goes up despite your mass on the launchpad staying the same.

However, as you continue to make stages smaller and smaller, you need more and more of these stages to achieve your intended total dV. Also, the amount of fuel in each stage becomes smaller and smaller, and thus it becomes more and more dominated by things that are not fuel. Such as engines. If you keep downsizing, your gains from downsizing get smaller and smaller - just like the diminishing returns a single stage faces from adding more fuel, just in the opposite direction. Eventually the point comes where you cannot gain any more dV by having a larger number of smaller stages. Additionally, there is a point even before that where you can build a less staged vehicle with the same dV but with less total weight on the launchpad, because you're lifting less dry mass.

Thus the ideal point of "least mass on the pad, largest dV in flight" lies somewhere in between these two opposing scales of diminishing returns.

Finally, in the real world, each staging operation is also a potential "single point of failure", which is a thing you really want to avoid in engineering. Thus having many stages is often undesired, even if additional stages could improve the efficiency of the launcher. Most contemporary rockets go to low earth orbit (or even geostationary orbit) in two stages. The Saturn V carried the Apollo lunar lander into lunar orbit in 3 launch stages plus a fourth transfer/return stage.

Ultimately, what I took away from all of that for myself is a set of three "rules of the thumb" for designing my rockets: 1.) all stages should have as similar dV as possible; 2.) individual stages should aim for a fuel mass fraction of between 50% and 66%; and 3.) this needs to be achieved while still maintaining useful TWR for all stages.

Edited June 21, 2014 by Streetwind

[Pecan]

...Finally, in the real world, each staging operation is also a potential "single point of failure", which is a thing you really want to avoid in engineering. Thus having many stages is often undesired, even if additional stages could improve the efficiency of the launcher...

In KSP as well more stages = more parts = more likely to fall apart on the pad or in flight (= moar struts!).

Nice summary Streetwind.

[GoSlash27]

How to decide when to stage:

1. Use the most efficient engine(s) for the flight-stage = ASL or Vac ISP.

2. Give it/them as much fuel as they can push with an 'acceptable' TWR = wherever you're launching from or how long you can cope with in-space burn times.

3. If deltaV is sufficient, stop. Otherwise, stage.

With one caveat: Often the most efficient engine (in terms of Isp) isn't actually the engine that will yield the lightest overall stage. A lot of the time you can get better results by choosing a lighter engine with worse Isp. When the additional fuel it needs to generate the same DV weighs less than the additional mass of the more efficient engine, it's better to go with the smaller one.

Regards,

-Slashy

[DeepOdyssey]

ASPARAGUS, enough said. Keep TWR in 1,2-1,6 region, Don't use engines with ISP lower than 350 for vacuum, unless they have exceptional weight.

As for rocket staging i usually use my first stage to get me to ~500m/s velocity, then second stage to get me to eccentric orbit 100km AP and 30km PE, and third stage to circularize and have delta V for near kerbin injections or interplanetary flights.This way i don't generate spacejunk, if you use second stage that reach stable orbit.

[bobisback]

 

I know this is a dead thread, so sorry for the revival, but for context I decided against starting a new thread. 

I am currently trying to build more efficient stages for rockets in KSP. And I am sure things have changed in the game over the years but I am guessing most of the math stays the same. Anyway, onto the quote, early in this thread this quote was mentioned and along with this quote was a link that is now dead, I found it and will repost the link for others who come across this thread. 

On 6/21/2014 at 12:05 PM, Streetwind said:

Ultimately, what I took away from all of that for myself is a set of three "rules of the thumb" for designing my rockets: 1.) all stages should have as similar dV as possible; 2.) individual stages should aim for a fuel mass fraction of between 50% and 66%; and 3.) this needs to be achieved while still maintaining useful TWR for all stages.

 So basically, I am wondering a couple of things:

1. Does this still pretty much apply to current ksp?

2. How do I calculate the fuel mass fraction for a rocket? A follow up to this is when calculating it for each individual stage should I take the mass of the stage plus payload, or should I take the total rocket mass from the pad?

3. Is the fuel mass ratio basically a curve of amount of fuel vs dV, if so would there be a certain "Fuel Mass Fraction" you should ideally have no matter what?

As another follow up, what is the benefit of keeping the dV as similar as possible between stages?

 

Thanks,

Bob

P.S

On 6/21/2014 at 8:07 AM, GoSlash27 said:

The most efficient method I've found is 7-5-3-1 asparagus staging, followed by 4-2-1 layer cake staging (which yields over 90% the efficiency of asparagus staging). The least efficient method is a single stage, which yields approximately 55% the DV for the same mass.

Could you elaborate on what you mean by "7-5-3-1 asparagus staging" and "4-2-1 layer cake staging"? I could not find any information on the layer cake, but I am familiar with asparagus staging. How did you calculate "efficiency" in these examples?

Edited August 18, 2019 by bobisback

[Streetwind]

@bobisback Wow, that was a long time ago.

A lot has changed since then. Technically, the way the rocket equation works has not - but the way we build rockets, that sure changed as the physics model of the game was improved over time. Asparagus staging, for example, is very rarely used nowadays, as the "new" aerodynamics model from 2015 actively works against pancake shaped rockets. Sequential staging (as we see in real life) works best in many cases.

As for the three rules of the thumb you quoted me mentioning? Those aren't bad, but the first is quite unfortunately misstated. Stages shouldn't have similar dV; they should have similar mass fractions. That is the same thing if the engine Isp is identical, but Isp can vary quite a bit between lower and upper stages.

A few years later I wrote a different post on the same topic that I consider far better put:

Even that is now outdated in parts, as the tank mass fractions have changed for the xenon parts since, but the gist stands. It directly answers most of the questions you have asked. If something is still unclear after that, ask again and I'll try to word it better.

[bobisback]

Oh man, that is perfect, yes it completely answered my questions. The examples are just want I needed. Thanks!

I wish I had found that thread in the first place.

So would you say that Asparagus staging still works really well in space because of no drag?

 

P.S so it would seem as a follow up, the big limiting factor when it comes to actual mass in space is the amount of thrust an engine can produce (based on tech of course, I am playing a career game). Since you want to keep a similar Mass fraction,  and you want to minimize pancake design (tho I play with FAR and deadly reentry so not sure how this effects things) then the TWR is your limiting factor on keeping a similar mass fraction. So it is a balancing act between keeping launch TWR and mass fraction good with respect to your mass you are delivering to space. 

 

So with this in mind it would make perfect since to design the payload first, so you can get a idea of total weight of the payload. Then with some fancy math you could easily come up with the amount of fuel needed for each stage below that. Then you just keep adding/removing stages until your TWR is good and you target dV is good (for me it is always low kerbin orbit). Then bam you have a rocket that can deliver the need goods with a good TWR, mass fraction, and dV to get to orbit. I wonder if someone has already built a spreadsheet for this lol

 

Edit: Ya that worked perfectly, no second guessing, I basically just loaded up 50 tons of cargo, then added a bunch of fuel tanks to get roughly 2.5 mass ratio, staged it off with engines, then raise and repeat until I had 3.5-4k dV (not including my cargo, I usually turn there engines off so I can get an idea of dV for low orbit). made sure my lift off TWR was close to 1.2-1.3 by adding SRB's as needed. Got to orbit on the first try with fuel left over hahahaha. Coolness! Thanks for the tips!!

Edited August 18, 2019 by bobisback

[Zhetaan]

18 hours ago, bobisback said:

So would you say that Asparagus staging still works really well in space because of no drag?

Not as such, no.  Putting aside the question of how you get an asparagus-staged rocket to space in the first place, once you are there, the rules are a bit different from the situation that would necessitate asparagus staging on the ground.

The main advantage of asparagus staging is that it was essentially a way to run the core along with the boosters while still expending only booster propellant.  This does two things:  it gives a lot of thrust, and it also leaves you with full propellant tanks after each staging event.

In space, there is little need for high thrust.  It helps in certain applications--the Rhino is a vacuum-rated engine and it has a proper niche--but it's not strictly necessary in the way that it is necessary to have high thrust to get off the pad at launch.

However, having full propellant tanks after each staging event (or, more accurately, staging away empty tanks rather than carrying the tankage dead weight) is useful in space.  That's already a concept; it's called drop tanks.  The idea is that you have one engine (or cluster, or however you're moving your rocket) feed from a series of tanks, and as each tank empties, you stage away the empty dead weight.  This works in applications that need it, but in many cases, it's just not necessary.

Part of the reason for that is that it doesn't take a literally astronomical amount of propellant to get anywhere in KSP.  Another part is that staging away the empty tank improves your mass fraction by the mass of an empty tank--i.e., not much if it's a large rocket.  Small rockets benefit more, but in those cases, it's trivial to add significant propellant mass fraction.  A small probe on an FL-T800 tank with a Spark engine can go nearly anywhere.  The same probe on an S3-14400 tank can go anywhere ten times.  There's no need for staged tanks when you can simply use a larger tank.

[Streetwind]

2 hours ago, Zhetaan said:

A small probe on an FL-T800 tank with a Spark engine can go nearly anywhere.  The same probe on an S3-14400 tank can go anywhere ten times.  There's no need for staged tanks when you can simply use a larger tank.

I know what you're trying to say, but be wary of leaning too far out the window with your examples. Any rocket stage is hard limited to the mass fraction of the tankage, even if it can devote infinite mass to fuel. A Spark engine with 320 seconds of specific impulse will be hard pressed to give you more than 6,000 m/s dV, even if you put a super-size tank on a tiny payload. And that's not quite "ten times anywhere". In fact, it won't even get you a Moho return trip without staging if you use standard Hohmann transfers.

Beware the diminishing returns!

 

Edited August 19, 2019 by Streetwind

[bobisback]

So I guess the takeaway is Asparagus staging is a bit better in space but just not by much. When I was using the nuke engines in Asparagus staging it seemed to work really really well as far as extra dV goes, guessing because the engines were just so damn heavy. I guess though that same situation but with drop tanks would probably be more efficient dV wise, but not time wise. 

 

2 hours ago, Zhetaan said:

Not as such, no.  Putting aside the question of how you get an asparagus-staged rocket to space in the first place, once you are there, the rules are a bit different from the situation that would necessitate asparagus staging on the ground.

As far as getting them to space, I have had no issue with my final stage being Asparagus staged. Most of my last sage rockets I have sent up have been those. Why are they so difficult in your mind, Am I missing something? But that is kinda my point is I am wondering just how efficient they are, more than anything. It is interesting how much documentation there is around KSP about how amazing Asparagus staging is, and I am just wondering how much of that is current. 

I could easily see an example of a small probe that is like 3-4 tons, with 3-4 tons of drop tanks/Asparagus staging, were dropping the mass fraction by a factor of 2 each stage would create many times more dV then if you just had one large tank and having to push it all the time. Am I wrong here?

Edited August 19, 2019 by bobisback

[Zhetaan]

4 hours ago, Streetwind said:

I know what you're trying to say, but be wary of leaning too far out the window with your examples.

For your information, I never lean out the window in space.  I'd get my helmet stuck on the frame!

4 hours ago, Streetwind said:

Any rocket stage is hard limited to the mass fraction of the tankage, even if it can devote infinite mass to fuel. A Spark engine with 320 seconds of specific impulse will be hard pressed to give you more than 6,000 m/s dV, even if you put a super-size tank on a tiny payload. And that's not quite "ten times anywhere". In fact, it won't even get you a Moho return trip without staging if you use standard Hohmann transfers.

Theoretically, the limit for a rocket with infinite fuel in KSP tanks and a massless payload is 6,895.2 m/s of delta-V, to be exact.  Also, I made no provisions for your choice of transfer trajectory or destination; my theoretical rocket can't lift off from Eve, either.  That wasn't really the point.

Neither was my point to say that staging has no value in the face of larger tankage.  My point is that larger tankage, in space, solves the same problem that asparagus staging solves on (or very near to) the ground:  making more propellant available to the engines.  I treat the asparagus-stage need for more engines, as well, to be a specific use-case application of drop tanks in a gee field too strong to otherwise support the increased fuel load.

That said, drop tanks, as I indicated, do have a use, and they work very nicely in those applications where manipulation of the dry mass portion of the mass fraction of the rocket is necessary to achieve a desired outcome--namely, the one where every element of the payload is essential across two or more stages.  Being able to remove dry mass from a rocket is the only way to overcome the problem of diminishing returns:  you diminish the rocket, instead.

Nevertheless, the point is well taken; I will endeavour to be more precise in the future.

4 hours ago, bobisback said:

So I guess the takeaway is Asparagus staging is a bit better in space but just not by much. When I was using the nuke engines in Asparagus staging it seemed to work really really well as far as extra dV goes, guessing because the engines were just so damn heavy. I guess though that same situation but with drop tanks would probably be more efficient dV wise, but not time wise. 

It's more that asparagus staging isn't necessary in space.  You don't need fuel lines and you definitely don't need extra engines because you're not trying to go straight up against gravity.  If you were seeing bad results from the nuke engines, then there are two likely reasons for that.  First is that your rocket was too light--three tonnes of engine on one tonne of payload is a lot.  Three tonnes of engine on thirty tonnes of payload is a lot less of a dry mass penalty, and gives the nuke a chance for its higher efficiency to work for you.  Second is that you were using rocket propellant tanks instead of liquid-fuel-only tanks.  That not only gives you a reduced fuel load, but also several tonnes of dead weight in oxidiser.  Even draining the oxidiser would still leave you with less than half the fuel in a too-large tank.

4 hours ago, bobisback said:

As far as getting them to space, I have had no issue with my final stage being Asparagus staged. Most of my last sage rockets I have sent up have been those. Why are they so difficult in your mind, Am I missing something? But that is kinda my point is I am wondering just how efficient they are, more than anything. It is interesting how much documentation there is around KSP about how amazing Asparagus staging is, and I am just wondering how much of that is current. 

Let me ask you this:  are you asparagus-staging rockets that have only two or three stages, or are you speaking of a ten-stage monstrosity?

Every staging scheme has its benefits and its costs.  Traditionally, there are inline staging and side-along staging.  There are some variants such as onion staging, and asparagus staging is a sort of hybrid.

Inline staging (like the Saturn V) has the benefit that when each stage empties and is jettisoned, the remaining upper stage is fully-fuelled.  This works well for the Saturn V because it gives the benefit of full burn time for each booster but at a cost of reduced thrust on each stage.  However, the reduced thrust is okay because the upper stage engines are optimised for the region of the atmosphere in which they were meant to work; there is no benefit from burning all of the stages at once (assuming that it could be done without rapid unplanned disassembly).

Side-along staging (like the Delta IV Heavy) has the benefit that all of the engines (core and two boosters), burn at the same time, which increases thrust (by about three times, assuming my arithmetic is still good).  Putting aside the throttle trickery used by the actual Delta IV Heavy, the net effect is that when the boosters empty and stage away, the core is left partially empty and thus suffering from both a higher dry mass in the mass fraction and reduced burn time because of the lack of fuel.

Onion staging (I don't know of any rockets that actually use this, but I don't think there's any reason they can't) is actually a variant of inline staging.  Envision a rocket like the Delta IV Heavy, but instead of lighting all three engines, imagine that only the outer two are lit, and when they are empty, they stage away and the core is lit.  The name comes from the idea that the outer boosters fall off and leave the inner ones to burn anew, much like peeling layers of an onion.  Note that it looks the same as the side-along-staged Delta IV Heavy,   but it merely appears similar to side-along staging because strapping the boosters to the sides helps with symmetric thrust.  You get the same result when you attach the boosters to a bicoupler or the like and achieve a more inline look.  In essence, we can even go so far as to say that inline staging is the degenerate case of onion staging when there is only one additional booster in each stage.  Its benefits and costs are the same as for inline staging, except that when there are multiple outer boosters, they do have the advantage of some increased thrust (though technically the same effect can be achieved with clustering and taller or more numerous propellant tanks).

Asparagus staging works by the boosters sharing their propellant with all engines inward from them.  Practically, it is a series of powered supplementary propellant tanks--meaning that the tanks lift their own mass against gravity rather than your core stack needing to lift their mass in addition to the rest of the rocket.  This means that, in the Delta IV Heavy example, all three engines are lit at the start, but you'd see the outer side boosters stage away much sooner.  The core engine, however, would have the benefit of full burn time because until that point, it would be running on propellant flowing in from the outer boosters rather than in its own tank.  In this way, it combines the advantages of high thrust and high second-stage mass fraction, but at the cost of increased drag (there's no inline form) and much-reduced stage burn time.  Because the core stage burns (usually with a full propellant load) the entire time, this is a problem with efficiency in the new aerodynamics model because first-stage engines get to be quite inefficient compared to their vacuum-rated counterparts once they get over ten kilometres.  The old model had an atmosphere thick enough to require high-thrust engines quite a long way, and it also calculated drag in a weird way that actually justified pancake-shaped rockets (drag was calculated per-part and paid no attention to orientation, so nosecones, for example, increased the part count and thus increased drag).  It made sense in some cases to have many-stage rockets that were wider than the launchpad just to get to orbit.

Now, the atmosphere is thinner (such it takes 1,000 m/s less delta-V to reach orbit), and the aerodynamics reward long, thin rockets.  If you need extra thrust in the first stage, then solid rocket boosters provide that at a bargain.  Asparagus staging, on the other hand, requires a wider rocket and involves extra parts that are also extra-draggy parts.  It still has applications (Darts enjoy good efficiency from the surface to orbit, and high thrust, high second-stage mass-fraction rockets are a practical application for a lot of situations--Eve ascent is the crown of them all), but it is no longer the champion Über-rocket that it once was.

However, in space (which was your original question) it is normally a waste to haul engines that you do not absolutely need.  As with anything, there are applications, but few to none for asparagus staging once in orbit.

Edited August 19, 2019 by Zhetaan

[bobisback]

8 hours ago, Zhetaan said:

It's more that asparagus staging isn't necessary in space.  You don't need fuel lines and you definitely don't need extra engines because you're not trying to go straight up against gravity.  If you were seeing bad results from the nuke engines, then there are two likely reasons for that.  First is that your rocket was too light--three tonnes of engine on one tonne of payload is a lot.  Three tonnes of engine on thirty tonnes of payload is a lot less of a dry mass penalty, and gives the nuke a chance for its higher efficiency to work for you.  Second is that you were using rocket propellant tanks instead of liquid-fuel-only tanks.  That not only gives you a reduced fuel load, but also several tonnes of dead weight in oxidiser.  Even draining the oxidiser would still leave you with less than half the fuel in a too-large tank.

 

I think you were misunderstanding what I said. I was saying that asparagus-staging worked really well with the nukes (especially since they were so heavy). Not sure why you are being so rude about it and assuming things that are just not true. I am also not sure how you can tell me how my rocket was built, you have never seen it haha. First it was meant to move a asteroid that was 300+ tons so having the extra engines made perfect since, but at the same time I did not want to move these engines with me after moving the asteroid so asparagus-staging seemed like a great choice. So your point about 1 ton with 3 ton engine is just flat wrong. Second I was using only LF tanks, that would be kinda dumb to use the non LF tanks.  So again you second point about " Second is that you were using rocket propellant tanks" is also just flat wrong.....

I am just trying to have a friendly discussion about the efficiency of certain designs over others while in space or otherwise. I am not sure the point of your above comments but I am not sure they are constructive in any way to the discussion at hand. my question is about if it is more efficient or not, I do not care about if it is "necessary" or not. if it is more efficient then I can look at the numbers and make a decision on rather it is necessary. If it is not then we can move on. Based on my limited experience with it though, it seemed to worked really well in space. Though i do take the point that just using drop tanks would probable be more "efficient" in terms of fuel use and fuel mass ratio, but it would also mean much longer burns which is personal preference but I do not care for long burns that much. 

8 hours ago, Zhetaan said:

Let me ask you this:  are you asparagus-staging rockets that have only two or three stages, or are you speaking of a ten-stage monstrosity?

Every staging scheme has its benefits and its costs.  Traditionally, there are inline staging and side-along staging.  There are some variants such as onion staging, and asparagus staging is a sort of hybrid.

Inline staging (like the Saturn V) has the benefit that when each stage empties and is jettisoned, the remaining upper stage is fully-fuelled.  This works well for the Saturn V because it gives the benefit of full burn time for each booster but at a cost of reduced thrust on each stage.  However, the reduced thrust is okay because the upper stage engines are optimized for the region of the atmosphere in which they were meant to work; there is no benefit from burning all of the stages at once (assuming that it could be done without rapid unplanned disassembly).

Side-along staging (like the Delta IV Heavy) has the benefit that all of the engines (core and two boosters), burn at the same time, which increases thrust (by about three times, assuming my arithmetic is still good).  Putting aside the throttle trickery used by the actual Delta IV Heavy, the net effect is that when the boosters empty and stage away, the core is left partially empty and thus suffering from both a higher dry mass in the mass fraction and reduced burn time because of the lack of fuel.

Onion staging (I don't know of any rockets that actually use this, but I don't think there's any reason they can't) is actually a variant of inline staging.  Envision a rocket like the Delta IV Heavy, but instead of lighting all three engines, imagine that only the outer two are lit, and when they are empty, they stage away and the core is lit.  The name comes from the idea that the outer boosters fall off and leave the inner ones to burn anew, much like peeling layers of an onion.  Note that it looks the same as the side-along-staged Delta IV Heavy,   but it merely appears similar to side-along staging because strapping the boosters to the sides helps with symmetric thrust.  You get the same result when you attach the boosters to a bicoupler or the like and achieve a more inline look.  In essence, we can even go so far as to say that inline staging is the degenerate case of onion staging when there is only one additional booster in each stage.  Its benefits and costs are the same as for inline staging, except that when there are multiple outer boosters, they do have the advantage of some increased thrust (though technically the same effect can be achieved with clustering and taller or more numerous propellant tanks).

Asparagus staging works by the boosters sharing their propellant with all engines inward from them.  Practically, it is a series of powered supplementary propellant tanks--meaning that the tanks lift their own mass against gravity rather than your core stack needing to lift their mass in addition to the rest of the rocket.  This means that, in the Delta IV Heavy example, all three engines are lit at the start, but you'd see the outer side boosters stage away much sooner.  The core engine, however, would have the benefit of full burn time because until that point, it would be running on propellant flowing in from the outer boosters rather than in its own tank.  In this way, it combines the advantages of high thrust and high second-stage mass fraction, but at the cost of increased drag (there's no inline form) and much-reduced stage burn time.  Because the core stage burns (usually with a full propellant load) the entire time, this is a problem with efficiency in the new aerodynamics model because first-stage engines get to be quite inefficient compared to their vacuum-rated counterparts once they get over ten kilometres.  The old model had an atmosphere thick enough to require high-thrust engines quite a long way, and it also calculated drag in a weird way that actually justified pancake-shaped rockets (drag was calculated per-part and paid no attention to orientation, so nosecones, for example, increased the part count and thus increased drag).  It made sense in some cases to have many-stage rockets that were wider than the launchpad just to get to orbit.

Now, the atmosphere is thinner (such it takes 1,000 m/s less delta-V to reach orbit), and the aerodynamics reward long, thin rockets.  If you need extra thrust in the first stage, then solid rocket boosters provide that at a bargain.  Asparagus staging, on the other hand, requires a wider rocket and involves extra parts that are also extra-draggy parts.  It still has applications (Darts enjoy good efficiency from the surface to orbit, and high thrust, high second-stage mass-fraction rockets are a practical application for a lot of situations--Eve ascent is the crown of them all), but it is no longer the champion Über-rocket that it once was.

However, in space (which was your original question) it is normally a waste to haul engines that you do not absolutely need.  As with anything, there are applications, but few to none for asparagus staging once in orbit.

Thanks for the staging info, this was actually very helpful information. I was not familiar with the other staging techniques and the details around them. Based on your descriptions most of my current launch stage designs are the side along designs.

So if the best designs are the long thin rockets (I am guessing this is for KSP stock only?) then it would seem the limiting factor for getting large payloads to space is limited by the biggest engine you can put on the bottom of the rocket and still keep good TWR (plus SRB's of course). But I am wondering how much dV is actually lost to drag? is it actually a huge amount? It seems like drag is only really an issue when under 20km on kerbin, or am on wrong here?

In the game I am currently playing I am finding the big issue is getting enough TWR, even with SRB's it is a challenge when you are trying to lift 50+ tons with only skiffs hahahaha.

 

Edited August 20, 2019 by bobisback

[magnemoe]

18 hours ago, Zhetaan said:

Not as such, no.  Putting aside the question of how you get an asparagus-staged rocket to space in the first place, once you are there, the rules are a bit different from the situation that would necessitate asparagus staging on the ground.

The main advantage of asparagus staging is that it was essentially a way to run the core along with the boosters while still expending only booster propellant.  This does two things:  it gives a lot of thrust, and it also leaves you with full propellant tanks after each staging event.

In space, there is little need for high thrust.  It helps in certain applications--the Rhino is a vacuum-rated engine and it has a proper niche--but it's not strictly necessary in the way that it is necessary to have high thrust to get off the pad at launch.

However, having full propellant tanks after each staging event (or, more accurately, staging away empty tanks rather than carrying the tankage dead weight) is useful in space.  That's already a concept; it's called drop tanks.  The idea is that you have one engine (or cluster, or however you're moving your rocket) feed from a series of tanks, and as each tank empties, you stage away the empty dead weight.  This works in applications that need it, but in many cases, it's just not necessary.

Part of the reason for that is that it doesn't take a literally astronomical amount of propellant to get anywhere in KSP.  Another part is that staging away the empty tank improves your mass fraction by the mass of an empty tank--i.e., not much if it's a large rocket.  Small rockets benefit more, but in those cases, it's trivial to add significant propellant mass fraction.  A small probe on an FL-T800 tank with a Spark engine can go nearly anywhere.  The same probe on an S3-14400 tank can go anywhere ten times.  There's no need for staged tanks when you can simply use a larger tank.

You are mostly correct, one reason to use asparagus in space is burn times who is an gameplay issue 
TWR here is already very low. 

Yes its 1+8 asparagus with nuclear engines, started with above 20km/s dV but used 2K for the intercept burn. 

Contract to rescue an kerbal and his capsule in retrograde orbit outside of Dress so ions would not be very practical. 
Launched the upper stage part half empty then refueled in LKO

One place asparagus is genuine useful is on Eve.  High gravity so you need high TWR as you want to get out of the thick atmosphere fast. 

For normal launches its pretty pointless because the much larger parts we have now, parts are also much stronger so you can build up. back then the mainsail was the largest engine you needed lots of bracing for large ships and building wide made this easier. 
Aerodynamic matter but far less. 

[Zhetaan]

12 hours ago, bobisback said:

I think you were misunderstanding what I said.

Indeed, that is the case.  I am not quite certain why I interpreted this statement:

20 hours ago, bobisback said:

When I was using the nuke engines in Asparagus staging it seemed to work really really well as far as extra dV goes, guessing because the engines were just so damn heavy.

... as meaning that you were having problems with your nuclear engines, but I did.  That being said:

13 hours ago, bobisback said:

Not sure why you are being so rude about it

It was not my intention to be rude.  It may not account for much, but I did not perceive my remarks as being such when I wrote them.  Nevertheless, you do have my apologies for any slight.  Even unintentional hostility does nothing to further my purpose in posting, which is to provide help and information to those who ask for it (though I freely admit that my apparent inability to closely read your posts is quite frustrating to that effort).  Let me try this again, and please accept my advance apologies for anything that repeats what was said above.  My full reply is quite long, so I put a lot of it in spoilers:

Spoiler

If I understand your mission profile correctly this time, then you staged the nukes after you were finished moving the asteroid, yes?  In that case, then yes, you saw increased delta-V because you removed many tonnes of no-longer-needed mission equipment that was reducing your mass ratio.  However, you would also have seen increases in delta-V from staging away the claw, for example; the nature or use of the dry mass is not important, but the amount of dry mass is important.  That the engines were heavy was the deciding factor in your delta-V increase, but the fact that they were asparagus-staged did not matter.  In this respect, once you are in space, any reduction of dry mass will give you delta-V increases.

As I believe you are aware, delta-V for a stage in space is dependent on exactly two things:  engine efficiency and mass ratio.  If you add additional engines of the same type, then the efficiency is the same; there's no 'bulk discount' or similar concept at work.  Thus, with the caveat that your engine choice for a given rocket stage remains the same (not necessarily always the case:  some spaceplanes can switch from a Dart for ascent-and-circularisation to a Nerv for orbital manoeuvres, for example), the only way to gain increased delta-V while in space is to increase the wet-to-dry mass ratio, and the only ways to do that are either to add wet mass (via tanker or ISRU) or to get rid of dry mass.  Note that I am not mentioning tricks such as the Oberth effect and other things that allow you to get more out of the delta-V that you already have; I am strictly limiting myself to actual increases in available delta-V by manipulation of the rocket, not the trajectory.

The location of the discarded mass can be an important factor:  a reduction in dry mass improves the mass ratio of its stage and of every stage below it.  Thus, payload mass reduction is extremely helpful, but launch-stage mass reduction doesn't help so much.

The first key element of asparagus staging that separates it from other staging options is that it optimises the fuel flow to allow you to run several different stages' engines off of one stage's propellant supply, with the effect that you get the benefit of increased thrust but without needing to add any more boosters to your rocket than you'd need otherwise.  The second key element of asparagus staging is that it ensures that stages drain sequentially and are immediately discarded--tanks and engines both--once emptied, with the effect that it keeps the wet-to-dry mass ratio for each stage as high as possible.

However--and this part is critical--you may note that this is functionally identical to inline staging insofar as delta-V calculation is concerned, because the rocket equation doesn't care about thrust.  Only the second element of discarding tanks and engines when they empty is relevant to the rocket equation.  The only differences are in the arrangement of the tanks and the fact that with asparagus, the engines all burn together rather than one at a time.  Since additional engines of the same efficiency do not contribute additional delta-V, but do count as dry mass, removing those engines but keeping the propellant tanks (i.e., drop tanks) will yield a modest increase in delta-V.  The same thing is true of inline staging:  imagine, if you will, a rocket that has a long line of propellant tanks behind it, and rather than having the engine at the bottom of that stack, there is a pair of radial-mounted engines on the forward-most tank.  Each tank drains sequentially from the aft to the fore, and as each aft-most tank empties, it is decoupled and discarded, leaving the remaining tanks full.  The net effect is inline-staged drop tanks, and it sees delta-V values that correspond to the radial-mounted, asparagus-derived drop tanks.

13 hours ago, bobisback said:

Though i do take the point that just using drop tanks would probable be more "efficient" in terms of fuel use and fuel mass ratio, but it would also mean much longer burns which is personal preference but I do not care for long burns that much. 

Of course, when you remove engines, there's a difference in thrust and attendant burn time, as you well noted.  Part of our misunderstanding, I think, is the fact that the various staging options are generally regarded as launch configurations.

Spoiler

And on the subject of launch configurations, though thrust may not be especially important in space (except for, as you stated, personal preference), thrust is immensely important when trying to escape gravity and reach orbit.

Launch delta-V depends on three things, not two:  engine efficiency, mass ratio, and thrust, because the rocket has to contend with the gravity field of the body that it is attempting to orbit (which is a technical way of saying that it needs must be able to lift itself from the ground).  Though it is not technically accurate to say that the launch delta-V is directly dependent on thrust, it is true that without a redesign of the rocket, thrust is the only way you have to control those additional factors that do directly affect launch delta-V.  I can show you the equation for delta-V in a situation where external forces act on a rocket, if you like, but the short version is that (among other things) there is a subtraction of gravitational acceleration multiplied by flight time (acceleration multiplied by time is a velocity--but this is negative velocity that takes away from your available delta-V).  Gravity always pulls the rocket down, reducing the vertical speed, and the longer the rocket is not in stable orbit, the longer the force acts against it and the more the force needs to be overcome.  High thrust mitigates much of this loss because, although it cannot do anything about the acceleration, it helps you reduce the flight time.

Thrust, engine efficiency, and mass ratio also interact with one another to affect burn time, both in the sense of length of time the boosters run before they are emptied and in the sense of length of time required to complete a manoeuvre.

The various staging configurations are all designed to maximise something, always at the cost of something else, and the trade-offs are all to do with the three components:  mass ratio, burn time, and thrust.  Asparagus staging maximises thrust and post-staging mass ratio at the expense of burn time.  Inline (and onion) staging maximises burn time and post-staging mass ratio at the expense of thrust.  Side-along staging maximises thrust and burn time at the expense of post-staging mass ratio.

There may be specific elements unique to each staging scheme that weigh their relative value one way or the other but do not relate to what I have listed here.  I am not attempting to be exhaustive.

Side-along staging, although it provides good thrust and manageable burn times, is often a bad choice for spaceborne operations because of the mass ratio issue.  For launching heavy payloads into space in the first place, it can be excellent because its long burn times mean it does not suffer the decay in thrust that asparagus staging does:  you get full thrust for the boosters' full burn time.  The problem is that when the boosters are expended, you are left with a core stack that spent an appreciable fraction of its propellant pushing the stage that is no longer there.  This reduces the mass ratio, which in space is inefficient.  You may note that SRBs are almost purpose-built for side-along staging, but that mostly results from the fact that they usually don't have enough thrust on their own to loft a payload to space, and that their propellant cannot be pumped from booster to booster.

Once in space, asparagus staging and inline staging both preserve maximum mass ratio for upper stages, so the best ways to compare their usefulness involve a focus on thrust in space (as you do) or on how you get the rocket to space in the first place (as I do).

But that runs into the issue of personal preference, too:  I don't like draggy rockets, even though asparagus staging also gives higher thrust.  One of the specific elements unique to asparagus staging is that it works best with fuel lines, but fuel lines are drag-inducing parts.  Therefore, I design my payloads, where possible, for inline-staged rockets where the thrust, though lower, is still exactly what I need to do the job, and I don't need to concern myself with the drag of the lines, struts, radial decouplers, and so forth.

16 hours ago, bobisback said:

So if the best designs are the long thin rockets (I am guessing this is for KSP stock only?)

Technically, I understand that long, thin rockets are the best designs for any situation with an atmosphere, provided that they will do the job.  You may feel free to launch pancake rockets to your heart's content on Minmus and other airless bodies where drag does not exist.  But long, thin rockets are not a balm for every situation, either, because:

16 hours ago, bobisback said:

[...] then it would seem the limiting factor for getting large payloads to space is limited by the biggest engine you can put on the bottom of the rocket and still keep good TWR (plus SRB's of course).

That is exactly right.  Eventually, you will reach the limit of what one engine can do.  Even a Mammoth, with its 4,000 kilonewtons of thrust, won't lift a rocket that weighs 4,001 kilonewtons (until it drains that kilonewton of fuel, that is).  Clustering can assist with this, but there's only so much room for that.  The only true solutions are either to cut the payload to manageable pieces and assemble it in space, or to accept that the limitation of thrust is an inherent trade-off of inline staging and that to get anything larger into orbit, you'll need a different scheme.

Spoiler

 

The closest thing to inline staging is onion staging--a bicoupler with two boosters attached to it under an inline configuration is technically similar to an onion-staged scheme because the second stage does not light until the lower boosters are staged away, and this is usually what I choose.  Doubling the engines doubles my potential lifting power, but the bicoupler limits the aerodynamic losses (somewhat).  On the other hand, it makes the long, thin rocket even longer and still rather thin--there's a limit there, too, where wobble and oscillation can cause problems.  My landers, for example, are often more pancake-like because they won't tip and fall on slopes.

It is interesting that you mention solid rocket boosters.  As I mentioned above, they would technically constitute side-along staging.  This is another unique element that makes side-along staging attractive in the face of the mass ratio 'penalty':  SRBs are extremely cheap for the thrust and delta-V that they provide, but of course the efficiency and mass ratio are so terrible that they really are best suited to first stages.  If you need the initial push, then loading your first stage with awful (but awfully cheap) solid rocket boosters is a good choice for career mode.  They even attach radially, so you don't need decouplers for each one.  Mind the heat, though.  So far, this is the first time in this post that I've considered cost as an important element, but it is important:  don't ignore it!

 

16 hours ago, bobisback said:

But I am wondering how much dV is actually lost to drag? is it actually a huge amount? It seems like drag is only really an issue when under 20km on kerbin, or am on wrong here?

The amount of delta-V lost to drag depends on the cross-sectional profile of the rocket, its orientation into the air stream, the thrust with which it attempts to plough through the atmosphere (airspeed, essentially), and the pressure and density properties of the atmosphere itself.  There are probably other elements, as well.

Spoiler

I cannot give you a number because it is so difficult to calculate (and so dependent on the conditions of each individual flight), but you are also correct in that the amount is not especially large and is mostly limited to the lower atmosphere.  The lower atmosphere is where your rocket is moving the slowest, so it spends a lot of time there, but the gravity turn is mostly vertical in the lower atmosphere, and higher thrust also gets you out of that situation more quickly.  However, drag increases with thrust, so a staging scheme that increases thrust by lighting more engines (as asparagus does) also necessarily increases drag, first because the engines need to have clear paths behind them to prevent exhaust impingement and so must be arranged in a wide base that increases the cross-sectional profile, and second because high-thrust rockets also need to make sharper gravity turns and so spend even more time in the lower atmosphere.  None of these factors are prohibitively large until the rocket itself reaches ridiculous size, but if reducing the profile will still accomplish the task without that inefficiency, then it is worth examination.

However, it is also true that gravity losses account for far more inefficiency in initial ascent than drag does, under the current system.  This is why, for my own builds, even my inline rockets usually come equipped with high-thrust engines.

I will attempt to find what numbers I can, but I can promise very little.

Drag's importance is less of an issue for delta-V and more a matter of balancing the forces on your rocket.  If you imagine the thrust from the engine acting as a lever that works through the rocket's centre of mass, then you'll see that it is necessary for that thrust to point directly through that centre to keep the rocket from spinning out of control.  Since you've been moving asteroids about, you've experienced this first-hand.  The forces involved in that do not need to be large:  most engines that gimbal have a gimballing range of less than five degrees.  For longer rockets, five degrees is too much.  In like fashion, tiny drag forces can cause a rocket to flip out of control; this is why fins at the fore are normally a bad idea.  Fins at the rear can help keep a rocket stable, but there is also such a thing as too stable, because too much stabilising drag can make the rocket difficult to turn.

Under the old (pre-version 1.0) aerodynamics, drag was so much worse that it cost over a thousand more metres per second just to reach orbit (roughly 4,500 m/s versus the present roughly 3,400 m/s).  Furthermore, it was practically necessary to lift off vertically for the first ten kilometres and then make a sharp turn to forty-five degrees.  You cannot do that with a current-version rocket and expect not to flip out of control and crash.  But this is also the reason why so many posts and threads (especially old ones) sing the praises of asparagus staging:  under that aero system, a staging scheme that had high thrust and kept high mass ratio was the best way by far to plough through the atmosphere and still have enough fuel to get to orbit.  Under the current system, it is not that asparagus staging is bad, but rather that it is reduced to an equal footing from its original status as the clear winner over everything else.

On 8/19/2019 at 9:36 PM, bobisback said:

In the game I am currently playing I am finding the big issue is getting enough TWR, even with SRB's it is a challenge when you are trying to lift 50+ tons with only skiffs hahahaha.

I'm guessing that you mean fifty tonnes of payload delivered to space, not on the pad.  If so, then yes, that's a lot of Skiffs!  Consider looking at the Twin Boar; I think that it unlocks in the same node as the Skiff.  It's expensive, but it provides a lot of rocket for the price.  If it's a mission or a challenge to do it with only Skiffs, then you may consider clustering.  Do Skiffs surface attach?

[HebaruSan]

On 8/18/2019 at 8:20 PM, bobisback said:

Edit: Ya that worked perfectly, no second guessing, I basically just loaded up 50 tons of cargo, then added a bunch of fuel tanks to get roughly 2.5 mass ratio, staged it off with engines, then raise and repeat until I had 3.5-4k dV (not including my cargo, I usually turn there engines off so I can get an idea of dV for low orbit). made sure my lift off TWR was close to 1.2-1.3 by adding SRB's as needed. Got to orbit on the first try with fuel left over hahahaha. Coolness! Thanks for the tips!!

Yup, this is exactly what I've been advising folks to do for a while now. It even has the benefit that you can easily pre-ballpark each stage's Δv since if you're effectively making ln(m₀/m_f) constant, and I_sp doesn't vary too drastically among engines.

   ≈ 2800 m/s for m₀/m_f near 2.5, or 2000 m/s for m₀/m_f near 2