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I don't understand why bigger isn't better


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The way to make bigger better is to use boosters that are lifting in substantial excess to their own weight. Asparagus or onion Staging makes sure that all engines are lifting for max efficiency.

Three examples;

Orange fuel can to orbit;

0x77eoW.jpg

HfUTTmt.jpg

Mod launcher;

HVDJ57X.jpg

hOY4zAo.jpg

Five fuel cans to orbit. This one used onion staging.

UgECGod.jpg

JspCvuz.jpg

The key, making all engines work without wasting fuel going too fast, or lifting too much of their own weight.

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A really good rocket, non-airbreathing (stock) can get in excess of 1/6 payload fraction. I think that 1/5 is doable, but it can be hard to hit. On my 5500 metric ton launcher (includes 800 ton payload), my payload fraction is substantially reduced- about 0.15- less than 1/6. In this case, the problem is that when you get to really, really big rockets, it's VERY hard to get the fuel to flow to flow correctly- if you're trying to use asparagus staging, that is. The largest rocket I could get asparagus staging to work correctly for was my 2200 launcher (includes 400 ton payload)- that's a payload fraction of about 1/5.5.

As I'm now playing the 0.24 career, I started playing around with air-breathing SSTOs for refueling an orbital fuel depot- the idea is to save myself having to launch single-use fuel tankers. I actually ended up having more luck in the VAB than the SPH. I basically make a rocket (with huge fins, parachutes and landing legs) that launches as an air-breather, climbs to around 11km, slowly makes a turn to level off at around 17 km, and slowly climbs and accelerates till at about 24 km and going around 1300-1400 m/s, it kicks into regular rocket mode to take it the rest of the way to low orbit. I believe my payload fraction of excess fuel I can get into orbit ends up being around 1/4, if I remember correctly. It might be able to do better with more tweaks. Unfortunately, the number of SABRE engines you have to use starts to become excessive if the rocket gets much more than like 200 tons in mass.

Edited by |Velocity|
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|Velocity|, I'm curious about the problems you had with asparagus in larger rockets. Fuel line set up for asparagus is the same no matter the size, what was the difficulty?

Largest asparagus I've done is the one I showed above, largest onion was a bit over 3600t. Does some effect happen when the rocket gets much larger?

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I hardly ever launch anything over 40t but I've got this fully-recoverable 100t-payload rocket for a forthcoming SSTO tutorial.

iCBdSA0l.png

740.925t wet / 196.925t dry. Launch TWR 1.38, 52 parts, cost 320866.

I make that a 15.6% payload ratio, but then it delivers just over 4,900m/s deltaV to allow for deorbit and landing.

Incidentally, I consider a payload ratio of 15% or more 'efficient' for a rocket and would normally aim for 17-18%. Although over 20% can be done (see challenges), it apparently can't be done by me ^^.

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Most stock 20%+ payloadfraction designs are extremely specialized and lack in other fields like cost or partcount. here are a few examples.

Look at all that candlestick staging!

Pecan, I have to wonder how that lifter would work with a real 100t payload, NRAP makes it small/dense. Have you tried lifting a normal payload with it?

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Oh, FAR makes bigger payload fractions much easier due to the lower dV requirement. Still seems like a lot of engine for the payload.

I find the gravity turn much easier to pull off in FAR as well, further reducing dV requirement and improving payload fractions.

|Velocity|, I'm curious about the problems you had with asparagus in larger rockets. Fuel line set up for asparagus is the same no matter the size, what was the difficulty?

Largest asparagus I've done is the one I showed above, largest onion was a bit over 3600t. Does some effect happen when the rocket gets much larger?

He must be having the same problem I've had with Asparagus: you run out of places to mount things securely. Out comes Teh Spaec Taep and up comes the part count.

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|Velocity|, I'm curious about the problems you had with asparagus in larger rockets. Fuel line set up for asparagus is the same no matter the size, what was the difficulty?

Largest asparagus I've done is the one I showed above, largest onion was a bit over 3600t. Does some effect happen when the rocket gets much larger?

1) The amount of engines you can stack underneath the rocket increases by the dimension of the rocket squared;

2) The amount of thrust needed is proportional to volume and thus increases by the dimension CUBED.

Therefore, your rockets become fatter and fatter the heavier they are, so that you can pack enough engines on the limited surface area of their back ends.

Now in KSP, fat isn't always a problem, because of the unrealistic way that drag is calculated. However, fat becomes a major problem when you want asparagus staging. This is because to get fat enough, you have to use multiple fuel tanks placed radially on each other. For a 4000+ ton rocket, it's a huge number of tanks you need to use. On each stalk, you need all the fuel tanks to drain out BEFORE the next stalk starts using fuel. And with the HORRIBLE way that fuel flow works in KSP- there's no way to tell KSP to treat two fuel tanks as one (no, running fuel lines between them does not do it)- that's VERY difficult to achieve when you start stacking together a lot of tanks. What you get instead is that the next stalk will start draining fuel before the previous one is finished :(

Oh it's not impossible at all. One solution I can think of is to use a LOT of stalks. You might have like 10 stages or something. But for big asparagus rockets, I've always tried to use six-sided symmetry and have just four "stages" (7 stalks - 5 stalks - 3 stalks - 1 stalk). It simplifies things.

So anyway, for my 5500 ton lifter (4700 ton rocket + 800 tons LKO) I just ended up using liquid boosters and vertical staging. Not as good as asparagus, but simple enough that it doesn't take an eternity to perfect. I think each launch with that thing is like 2.5 million kerbal dollars though, lol.

What is onion staging?

Edited by |Velocity|
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1) The amount of engines you can stack underneath the rocket increases by the dimension of the rocket squared;

2) The amount of thrust needed is proportional to volume and thus increases by the dimension CUBED.

Therefore, your rockets become fatter and fatter the heavier they are, so that you can pack enough engines on the limited surface area of their back ends.

I just build a core stage that has the desired TWR, then make boosters with that same TWR and add pairs until I meet the dV requirements. It does result in unnaturally wide rockets for heavy payloads, but as you say that's not an issue for KSP's stock aero model.

Now in KSP, fat isn't always a problem, because of the unrealistic way that drag is calculated. However, fat becomes a major problem when you want asparagus staging. This is because to get fat enough, you have to use multiple fuel tanks placed radially on each other. For a 4000+ ton rocket, it's a huge number of tanks you need to use. On each stalk, you need all the fuel tanks to drain out BEFORE the next stalk starts using fuel. And with the HORRIBLE way that fuel flow works in KSP- there's no way to tell KSP to treat two fuel tanks as one (no, running fuel lines between them does not do it)- that's VERY difficult to achieve when you start stacking together a lot of tanks. What you get instead is that the next stalk will start draining fuel before the previous one is finished :(

I have found that connecting the bottommost tanks in each stalk reliably flows fuel correctly for asparagus, I have never had it drain a tank out of sequence. I shall have to try with a larger rocket.

Oh it's not impossible at all. One solution I can think of is to use a LOT of stalks. You might have like 10 stages or something. But for big asparagus rockets, I've always tried to use six-sided symmetry and have just four "stages" (7 stalks - 5 stalks - 3 stalks - 1 stalk). It simplifies things.

I prefer a single ringed asparagus (sometimes six-way, sometimes eight-way) for simplicity of construction, too.

What is onion staging?

Close cousin of asparagus, except each ring of boosters is a single stage that feeds fuel inward. Diagram:

ELILpu7.png

Not as efficient as asparagus but much easier to construct using symmetry.

Edit: Ha, I dug up mhoram's tutorial to get the diagram, and here he linked while I was composing.

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1) The amount of engines you can stack underneath the rocket increases by the dimension of the rocket squared;

2) The amount of thrust needed is proportional to volume and thus increases by the dimension CUBED.

Therefore, your rockets become fatter and fatter the heavier they are, so that you can pack enough engines on the limited surface area of their back ends.

Now in KSP, fat isn't always a problem, because of the unrealistic way that drag is calculated. However, fat becomes a major problem when you want asparagus staging. This is because to get fat enough, you have to use multiple fuel tanks placed radially on each other. For a 4000+ ton rocket, it's a huge number of tanks you need to use. On each stalk, you need all the fuel tanks to drain out BEFORE the next stalk starts using fuel. And with the HORRIBLE way that fuel flow works in KSP- there's no way to tell KSP to treat two fuel tanks as one (no, running fuel lines between them does not do it)- that's VERY difficult to achieve when you start stacking together a lot of tanks. What you get instead is that the next stalk will start draining fuel before the previous one is finished :(

Yes, and building with Symmetry is a pain. Both getting everything aesthetically symmetrical (my OCD strikes again) and getting all the asymmetric drag off my craft so it doesn't pick up rotations. Thus the "onion staging" technique mentioned by others.

Fat is a problem for those of us working with FAR/NEAR. I stopped asparagusing my rockets the instant I made the switch, the insane amount of drag even on a tall, thin, noseconed asparagus assembly is noticeable. For me it got to the point where the efficiency gains of Asparagus/Onion were far outweighed by the drag. This, plus fuel flow rate restrictions (something I want implemented in the stock game) prevent asparagus from being used in real life rocketry, though the Soviets did think of the idea.

Oh, and the whole "two stacked fuel tanks do not act as one" thing has bit me in the ass more than a few times. Heavier rockets it's not a huge deal, but for my 14-ton Soyuz equivalent, that little rule has sent a few craft spinning out of control during the gravity turn.

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Actual payload fraction of my thing is roughly 10%, BTW.

OTOH, given that it hits orbit with the centre tank nearly full, you could arguably consider the core booster as part of the payload, which would bring it to about 25%.

Launch cost √733,102...so about 25 times the price of my normal Mun rockets. And roughly 145 times the cost of sending up my heavy tanker SSTO spaceplane.

Edited by Wanderfound
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...Pecan, I have to wonder how that lifter would work with a real 100t payload, NRAP makes it small/dense. Have you tried lifting a normal payload with it?

tWAn8Rvl.png

3 full orange tubes (108t) do it for you?

ETA: BTW the 52-part count does include the 4 fuel lines (base of core to radial stacks) and 8 struts tying the core/payload to the radial stacks so it's 40 parts if you like; 36 if you don't care about the aerodynamic adapters on top of the stacks. I didn't want the 'a rocket HAS to look like this' crowd to complain though.

Edited by Pecan
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http://i.imgur.com/tWAn8Rvl.png

3 full orange tubes (108t) do it for you?

ETA: BTW the 52-part count does include the 4 fuel lines (base of core to radial stacks) and 8 struts tying the core/payload to the radial stacks so it's 40 parts if you like; 36 if you don't care about the aerodynamic adapters on top of the stacks. I didn't want the 'a rocket HAS to look like this' crowd to complain though.

Nice, I figured it might be too wobbly to control. My mind still hasn't fully adjusted to the stronger joints in 0.23.5, I probably over strut all my stuff.

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...I probably over strut all my stuff.

;-0 4 struts tying the bottom of the radial stacks together are probably not necessary, but they did 'flare' a little on first tests. 4 struts tying the core/payload to the top of the stacks are very necessary as its TWR is >4 by the time it gets to the circularisation burn and had a 100% habit of folding in half at that point ^^.

(PS: If anyone looks at the picture and wonders how it's meant to deorbit with only 89m/s deltaV left - you're forgetting it's about to undock a 100t payload, leaving the launch vehicle itself with plenty to de-orbit, slow under (4) drogue parachutes and engine-brake for touchdown. Still an awful lot of fuel compared to jet-power though ... and if we're not being FAR-worthy that's 32 parts, taking off the adapters for the engines too, but leaving the probe core, SAS, solar panels and battery if you want to bring it back)

Edited by Pecan
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Fat is a problem for those of us working with FAR/NEAR. I stopped asparagusing my rockets the instant I made the switch, the insane amount of drag even on a tall, thin, noseconed asparagus assembly is noticeable. For me it got to the point where the efficiency gains of Asparagus/Onion were far outweighed by the drag. This, plus fuel flow rate restrictions (something I want implemented in the stock game) prevent asparagus from being used in real life rocketry, though the Soviets did think of the idea.

Is it even possible to make a Eve sea-level-to-orbit ascent vehicle in FAR that isn't ridiculously top-heavy- too top heavy to land? My Eve ascent vehicles have all been rather short and squat (and asparagus staged) so that they could land on the terrain- which unless you are REALLY lucky, will mean landing on at least a 5 degree slope. A vertically staged rocket would simply tip over on landing on like 99% of the land surface of Eve.

Hmm... I suppose you could make really wide landing legs out of those truss pieces that stretched way out to the side though. Hmmm....

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...I stopped asparagusing my rockets the instant I made the switch, the insane amount of drag even on a tall, thin, noseconed asparagus assembly is noticeable. For me it got to the point where the efficiency gains of Asparagus/Onion were far outweighed by the drag...

Are you saying that you can only build in a single stack, or that fuel-lines cause drag? Either seems very odd.

If you're not saying either of those things I fail to see how an onion arrangement is any less aerodynamic than radial/parallel, and asparagus is only the optimal special-case of onion using symmetry 2.

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Are you saying that you can only build in a single stack, or that fuel-lines cause drag? Either seems very odd.

If you're not saying either of those things I fail to see how an onion arrangement is any less aerodynamic than radial/parallel, and asparagus is only the optimal special-case of onion using symmetry 2.

He's saying that in FAR, each separate stack causes a huge amount of drag, so to minimize drag you want to put everything in one stack. That is realistic. In stock KSP, each part creates the same drag no matter where it is placed. That is unrealistic.

This (realistically) forces you away from massive asparagus staged rockets in FAR.

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This (realistically) forces you away from massive asparagus staged rockets in FAR.

Asparagus is just onion with symmetry-2 so if you can't use asparagus you can't use onion.

Onion is just side-boosters with fuel-lines so if you can't use onion you can't use radial/parallel.

If you can't use radial/parallel you must build in a single stack - which is very odd.

IF, on the other hand, you CAN use radial/parallel boosters then you can use onion.

If you can use onion you can use asparagus.

So the argument does not hold.

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Asparagus is just onion with symmetry-2 so if you can't use asparagus you can't use onion.

Onion is just side-boosters with fuel-lines so if you can't use onion you can't use radial/parallel.

If you can't use radial/parallel you must build in a single stack - which is very odd.

IF, on the other hand, you CAN use radial/parallel boosters then you can use onion.

If you can use onion you can use asparagus.

So the argument does not hold.

Uh, no.

Parallel triples the amount of drag you get; three towers instead of one. You can get away with that with enough power. going from 2 symmetry to 4 symmetry does not give you a doubling of dV, but it does double the drag.

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