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Improvement to asparagus design, optimizing efficiency and thrust


Jfischer
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Anyone, using a standard asparagus set up with 2-3 vertically stacked tanks, tried putting a fuel line from top most center tank back out into the top most outer tanks?

It allows ALL engines to keep burning if ANY tank has fuel, and you can still detach in standard asparagus to jettison mass if you have enough thrust.

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The point of an asparagus is to drop tanks and particularly engines as quickly as you can. If you find your asparagus lacking TWR for the payload it's suppose to be carrying then you should redesign it. Messing with fuel flow otherwise to try to keep engines with you to increase TWR is working against the very principle of asparagus staging.

If you find your asparagus staging rockets having acceptable TWR at left off but too low of a TWR later on in it's burn it means the core stage is too small. You generally want something like 20-25% of the total lift off thrust coming from the core stage. If your launcher is just seven identical boosters set up in an asparagus then clearly the core stage is not up to the job as it will only be contributing 14.29% of the lift off thrust.

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Interesting idea... I haven't tried this yet, but I'm guessing the theory is that you could choose to continue running on more engines to keep higher thrust if you needed it, as opposed to dropping a pair of engines too early and ultimately taking more delta-v to get somewhere because you're fighting gravity losses?

Again, without having tried it, I would think that to optimize it even more if you're running into a situation where more thrust is needed at a certain point during the ascent, it would be better to tweak the amount of fuel you have in each asparagus pair. For example, instead of running your extra fuel line to burn an extra couple of seconds before staging, you should instead add a FL-T100 tank to your first stage and subtract one from the following stage. The end result is you're dropping the dry mass of a FL-T100 tank when you stage, which is not the case in your initial suggested optimization. The downside however would be that it would take more math and/or trial and error to get right, so it's not as adaptable "in the moment".

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I understand that it can make you lose some dv compared to asparagus but my strongest engine available at this point in career is the 'reliant' so I'm pretty proud of my ssto that can reach 100,000 m circular orbit pushing a 2stage with 4500 dv, especially with my engines and driving.

Edited by Jfischer
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6 minutes ago, Temstar said:

The point of an asparagus is to drop tanks and particularly engines as quickly as you can. If you find your asparagus lacking TWR for the payload it's suppose to be carrying then you should redesign it. Messing with fuel flow otherwise to try to keep engines with you to increase TWR is working against the very principle of asparagus staging.

If you find your asparagus staging rockets having acceptable TWR at left off but too low of a TWR later on in it's burn it means the core stage is too small. You generally want something like 20-25% of the total lift off thrust coming from the core stage. If your launcher is just seven identical boosters set up in an asparagus then clearly the core stage is not up to the job as it will only be contributing 14.29% of the lift off thrust.

Well.... I disagree, I think it means you should shift fuel from the core stage to earlier stages. My entry to the lightest eve sea level lander challenge had a 7 aerospike asparagus lifter (lv-909 stage on top). The first two engines to drop also had the most fuel above them

If X engines can lift Y tons, then (X-2) engines can lift (X-2)/X * Y tons. By having the fuel capacity of a stage drop for later stages, the TWR stays more constant. It is true in this case that the core stage ultimately must either be the payload itself, or have a lower TWR, or a very low fuel load.

Having equally sized fueltanks for each asparagus stage means that as the engine number goes up, the staging comes really really fast.

1x FL=T800 tank feeds 3 engines a lot longer than it feeds 7 engines, for instance... and to me it seems really wasteful to have these asparagus stages with engines, that burn for really short times. You might as well just have drop tanks... and at the lower stages, the weight of an empty fuel tank is minimal compared to the rest of the fuelled craft. Then factor in the added drag, decoupler mass, etc.. and I figure its more efficient to take that "drop tank" and stack it on top of another stage.

You can see my asymetric stages here:

xZTmWgF.png

the bigger the stages are, the earlier they are in the staging order/ (well there are side drop tanks to be used for deorbit and propulsive landing, not during the ascent)

Having a really beefy core stage means your TWR may be much higher than needed at the end of the burn.

A lot depends on the mass fraction of the rocket's payload.

If 30% of the starting mass is payload, then your core stage should be strong enough to handle that.

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Yes it's true, you can distribute the wet mass of the rocket outwards to the boosters if you really want to have a weaker core stage. For special cases like Eve that might even be desirable since you need much more TWR at left off, yet final climb to orbit can still be done with upper stages that have <1 Kerbin TWR. In other words for an Eve rocket the optimum TWR curve has a steeper shape than Kerbin optimum TWR curve.

The fact that asparagus staging rockets jettison the first pair of boosters very early in flight is by design and intended. If you think about it, the rate that the rocket consumes fuel (ie how fast is the rocket becoming lighter) is almost linear relationship with number of engines firing (not exactly, as it's affected by atm ISP). So at the start of the flight the rocket burns through a vast amount of fuel every second, hence the engines needed to accelerate that fuel becomes rapidly redundant. Towards the end of its flight there are fewer engines so the rate that the rocket lightens is much slower, hence why you hang onto the engines for longer in later stages.

With Kerbin launch vehicles, making the core stage too small has the unfortunate effect of making the core stage shorter. There's only a finite amount that you can do this before the core stage becomes shorter than the boosters and the resulting "bowl" shape means there will be form factor limitation on the payload. You will no longer be able to put very wide payload on top of the rocket because the boosters will be protruding into the wide payload.

Edited by Temstar
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14 hours ago, Temstar said:

With Kerbin launch vehicles, making the core stage too small has the unfortunate effect of making the core stage shorter. There's only a finite amount that you can do this before the core stage becomes shorter than the boosters and the resulting "bowl" shape means there will be form factor limitation on the payload. You will no longer be able to put very wide payload on top of the rocket because the boosters will be protruding into the wide payload.

Only if you have all the engines at the same level.

When I use tweak-scale for larger solid boosters, I will often have those boosters shifted down to avoid interference with the payload cowl(especially when I add drop-tanks on top), and this often means that the boosters are the only engines resting on the launch pad.

 

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16 hours ago, Jfischer said:

I understand that it can make you lose some dv compared to asparagus but my strongest engine available at this point in career is the 'reliant' so I'm pretty proud of my ssto that can reach 100,000 km circular orbit pushing a 2stage with 2500 dv, especially with my crappy driving.

What purpose does your pride in your ssto have, in a thread about tweaking asparagus staging?

 

You can reach a 100 000 km circular orbit with a ssto?

That is only achievable around the sun! The Kerbin SOI is too small to fit such a huge orbit.

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Using a multi-dimensional asparagus design (yes I made that term up) was how I was able to lift a Mk1-2 capsule off of Eve (this was in 0.25 or 0.90, haven't tried since).

I had eight 1.25m boosters clustered around a 2.5m core.  The first two booster pairs to be jettisoned were powered by LV-T30's for greater TWR at launch, and the last two pairs were Aerospiked; with a Mainsail in the core.

However, each booster was sectioned by stack decouplers with an FL-T800 on the bottom with an engine mounted under it, FL-T400's mounted on top of it, and FL-T200's mounted on top of those.  At launch, every ~5 seconds the top mounted 200's would jettison a pair away, and then another, like any other asparagus rocket.  But the last two 200's had fuel lines feeding the first two 400's to be jettisoned, and on through the 800's.

So i had the asparagus jettison scheme wrapped around the rocket in 8 stacks, and 3 levels, so i maintained high TWR while dropping empty tanks for as long as possible before even losing any engines.  The bad side is I had additional mass for all the decouplers and Sepratrons used for the upper tanks, but I mitigated this by using 0.625m decouplers and reducing the solid fuel load in the Sepratrons so they only gave a brief burst to throw the tanks away.

It was a slow craw at first, but eventually the Mk1-2 capsule and all three Kerbals made it to a 100km orbit over Eve (caveat: launch was from a 6500m summit, no way that ship would make from sea level ha ha)

Edited by Raptor9
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I meant 100km or 100,000 m. My mistake. Im proud because I vame up with this when I was having trouble making ssto's that can lift larger payloads with the first available liquid engines. This gave me alot more dv available in orbit. 

 

The benefit of this design to me is that I wanted to maintain high thrust until I get high enough, then I can asparagus multiple tanks or none at all. I can see how it would sacrifice the efficiency of dropping all that mass, but it gains in flexibility/adaptability and control. I can have all engines burning as long as I want, and the CoM stays high because the last tank to drain is my top center rockomax 32. 

 

Raptor9, I tried using the same method for my launcher on kerbal, mainly because I my engine choice is very limited. I now use 3 vertically stacked flt800 with reliant attached as my boosters that I detach as I see fit and I leave my swivel on a center 3 rockomax stack, which gives me the gimbaling plus moderate thrust when I'm out of the atmosphere. I gave up on detaching top mounted fuel tanks because I kept blowing up my ship by hitting my engines with the tanks. It was frustrating to coordinate a launch while spinning to give them some force away from me, but I was using the 1 tier radial decouplers...

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I'm having trouble understanding what the OP is suggesting and trying to do, but it looks like increasing the thrust.

Once upon a time there was something called onion staging.  The idea was similar to asparagas staging, but included many stages burning through all at once, and then jettising them at once.  The idea was to maximize TWR.  Before 1.0.x and the aero pass, it made absolutely no sense: you could simply attach all the engines (or simply use larger ones) to the last stages jettisoned and simply ejected each fuel tank as it emptied (since you have exactly the same amount of thrust and less mass (due to jettisoned empty fuel tanks) you will always have both a higher TMR* and higher delta-v than strict onion staging.

It might make more sense now (you may have drag issues with the larger engines), but I suspect  that a better means would be to break up the engines and the fuel tanks.  You always want to jettison the tanks when empty, but it appears you want to manually control when to jettison the engines.  I *think* this will do what you described better than using extra fuel lines, but don't know how much drag it will add.

* this assumes you didn't explicitly use an adapter to add engines.  It also assumes the same number of radial decouplers (typically shown in onion staging, but personally I would just attach two and then simply attach all the boosters to each other).

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So what I needed was additional thrust in my later asparagus stages, it would start below a twr of 1 by the time I released my second tank. I still want to asparagus benefit of dropping mass of empty tanks and engines when the additional thrust isnt needed. I just found it harder to organize the amts of fuel needed in each asparagus release. Therefore I made a center stack of rockomax tanks.  Set up a smaller set of stacks of tanks in standard asparagus fashion. Then I tried something because I heard in a different post about unique drainage patterns with different fuel line loops. 

I put a fuel line from the center top rockomax out to every top outer tank. This causes a strange drainage pattern, which I find useful and I thought I would share it. Each outer tank drains as it would in standard asparagus, and can be dropped as it runs out of fuel just like asparagus. The unique thing, I think, is that the tank remains empty but the engine keeps thrusting.  And the top center big rockomax x200-32 is the last tank to empty, keeping CoM high so less chance of flipping. 

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About 1.3 but I hope that will go up when I get better engines. I haven't done any calculations for an optimal twr as it relates/changes with ascent profile but my instinct is that low twr when down in atm but higher as drag forces decrease. Just staying below terminal velocity until the air is thin enough.

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12 minutes ago, Jfischer said:

but my instinct is that low twr when down in atm but higher as drag forces decrease. Just staying below terminal velocity until the air is thin enough.

Yes it's true, from a pure aerodynamic pespective you want higher TWR as your climb, however that's not the whole story. Higher TWR is not free, it costs you dry mass to achieve higher TWR because you need to carry more engines.

Once you are above say 20km you are already above most of the atmosphere, and at this altitude your gravity turn would have placed your rocket to pretty close to horizontal. So in this phase of the flight you suffer very little aerodynamic drag (since there's little air) and very little gravity drag (since you're nearly horizontal) and almost all of the kinetic energy your engine is putting out goes into building up your horizontal velocity. Because you are suffering little drag there's no reason to punch through this phase of the flight to reach the coasting phase in a big hurry by having a high TWR. You can accelerate at a leisurely pace and enjoy the higher delta-V here by carrying up less engines provided that you can keep your time to AP around 30 seconds ahead of you while you keep thrusting at prograde. I find that a TWR of around one Kerbin G is enough at this phase of the flight.

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16 minutes ago, Temstar said:

Yes it's true, from a pure aerodynamic pespective you want higher TWR as your climb, however that's not the whole story. Higher TWR is not free, it costs you dry mass to achieve higher TWR because you need to carry more engines.

Once you are above say 20km you are already above most of the atmosphere, and at this altitude your gravity turn would have placed your rocket to pretty close to horizontal. So in this phase of the flight you suffer very little aerodynamic drag (since there's little air) and very little gravity drag (since you're nearly horizontal) and almost all of the kinetic energy your engine is putting out goes into building up your horizontal velocity. Because you are suffering little drag there's no reason to punch through this phase of the flight to reach the coasting phase in a big hurry by having a high TWR. You can accelerate at a leisurely pace and enjoy the higher delta-V here by carrying up less engines provided that you can keep your time to AP around 30 seconds ahead of you while you keep thrusting at prograde. I find that a TWR of around one Kerbin G is enough at this phase of the flight.

Ok thank you that makes alot of sense. I have alot of work to do on my gravity turn. I was always fighting my prograde to keep my ap further

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