Cross feed capable rocket

[Guest doughbred]

From what I\'ve heard it\'s a good idea by saving weight on a higher stage. What I am wondering about however is what exactly is cross feeding in a rocket. Is it possible for us to recreate cross feeding in KSP at the time, with or without mods?

[SteevyT]

I believe that the fuel lines do what you are asking. You want to take fuel from outer stages to power inner stages so that you can have all of your engines run at once and dump the ones that run out of fuel correct?

[maltesh]

From what I\'ve heard it\'s a good idea by saving weight on a higher stage. What I am wondering about however is what exactly is cross feeding in a rocket. Is it possible for us to recreate cross feeding in KSP at the time, with or without mods?

Fuel crossfeeding is a technique where you use fuel lines run from tanks that you\'re planning to drop later, to improve rocket performance.

Imagine that each circle in the diagram represents a stack of liquid fuel tanks with a standard Liquid Engine at the bottom (Perhaps use a vectored thrust center engine for better control). The outer stacks are attached to the central stack by radial decouplers (not shown) and the yellow arrows are fuel lines.

The No Crossfeeding example is a basic setup. The six outer engines burn as S2, and then are dropped. The inner engine then ignites on a full rocket, and starts burning.

The Crossfeeding example runs fuel lines from the bottom tank of the outer ring to the bottom tank of the inner engine. All seven engines start burning on launch, and the inner engine is getting all its fuel from the outer tanks. This setup provides higher thrust off the pad than the No Crossfeeding example, but delta-V works out to be the same. However, because the atmosphere is thickest at the surface of Kerbin, it can provide better overall performance by getting out of the lower atmosphere quicker.

Asparagus Stalk Booster Staging is a subset of Crossfeeding , where again, all seven engines ignite on launch. However, instead of dropping the outer ring all at once, fuel lines and staging are set up so that all the fuel drains from the S4 tanks, and those are dropped, followed by fuel draining from the S3 tanks, and dropping them, followed by draining from the S2 tanks and dropping them, and finally burning the S1 fuel. Asparagus Staging combines the high thrust on launch advantage with an efficient mode of dumping empty tanks while keeping the rocket balanced. It provides a somewhat significant boost in delta-V over regular crossfeeding.

Three-tank-high Asparagus Staging is what I use in my standard stock Munlanders, and for a variety of other pursposes. It is a bit of overkill for a Munlander, to be honest; I often find myself chucking mostly-full boosters at the Mun to get them out of the way of the lander.

[Guest doughbred]

Fuel crossfeeding is a technique where you use fuel lines run from tanks that you\'re planning to drop later, to improve rocket performance.

Imagine that each circle in the diagram represents a stack of liquid fuel tanks with a standard Liquid Engine at the bottom (Perhaps use a vectored thrust center engine for better control). The outer stacks are attached to the central stack by radial decouplers (not shown) and the yellow arrows are fuel lines.

The No Crossfeeding example is a basic setup. The six outer engines burn as S2, and then are dropped. The inner engine then ignites on a full rocket, and starts burning.

The Crossfeeding example runs fuel lines from the bottom tank of the outer ring to the bottom tank of the inner engine. All seven engines start burning on launch, and the inner engine is getting all its fuel from the outer tanks. This setup provides higher thrust off the pad than the No Crossfeeding example, but delta-V works out to be the same. However, because the atmosphere is thickest at the surface of Kerbin, it can provide better overall performance by getting out of the lower atmosphere quicker.

Asparagus Stalk Booster Staging is a subset of Crossfeeding , where again, all seven engines ignite on launch. However, instead of dropping the outer ring all at once, fuel lines and staging are set up so that all the fuel drains from the S4 tanks, and those are dropped, followed by fuel draining from the S3 tanks, and dropping them, followed by draining from the S2 tanks and dropping them, and finally burning the S1 fuel. Asparagus Staging combines the high thrust on launch advantage with an efficient mode of dumping empty tanks while keeping the rocket balanced. It provides a somewhat significant boost in delta-V over regular crossfeeding.

Three-tank-high Asparagus Staging is what I use in my standard stock Munlanders, and for a variety of other pursposes. It is a bit of overkill for a Munlander, to be honest; I often find myself chucking mostly-full boosters at the Mun to get them out of the way of the lander.

So the method of increasing efficiency is by means of getting rid of as much weight as early as possible during flight.

[Bluejayek]

Three-tank-high Asparagus Staging is what I use in my standard stock Munlanders, and for a variety of other pursposes. It is a bit of overkill for a Munlander, to be honest; I often find myself chucking mostly-full boosters at the Mun to get them out of the way of the lander.

Then your not doing it right! You should land on those fuel tanks, lift off, then ladn on minimus with your real lander! Nice ship though. Except the decoupler before capsule. That irks me.

[TheBlaster]

Fuel crossfeeding is a technique where you use fuel lines run from tanks that you\'re planning to drop later, to improve rocket performance.

Imagine that each circle in the diagram represents a stack of liquid fuel tanks with a standard Liquid Engine at the bottom (Perhaps use a vectored thrust center engine for better control). The outer stacks are attached to the central stack by radial decouplers (not shown) and the yellow arrows are fuel lines.

The No Crossfeeding example is a basic setup. The six outer engines burn as S2, and then are dropped. The inner engine then ignites on a full rocket, and starts burning.

The Crossfeeding example runs fuel lines from the bottom tank of the outer ring to the bottom tank of the inner engine. All seven engines start burning on launch, and the inner engine is getting all its fuel from the outer tanks. This setup provides higher thrust off the pad than the No Crossfeeding example, but delta-V works out to be the same. However, because the atmosphere is thickest at the surface of Kerbin, it can provide better overall performance by getting out of the lower atmosphere quicker.

Asparagus Stalk Booster Staging is a subset of Crossfeeding , where again, all seven engines ignite on launch. However, instead of dropping the outer ring all at once, fuel lines and staging are set up so that all the fuel drains from the S4 tanks, and those are dropped, followed by fuel draining from the S3 tanks, and dropping them, followed by draining from the S2 tanks and dropping them, and finally burning the S1 fuel. Asparagus Staging combines the high thrust on launch advantage with an efficient mode of dumping empty tanks while keeping the rocket balanced. It provides a somewhat significant boost in delta-V over regular crossfeeding.

Three-tank-high Asparagus Staging is what I use in my standard stock Munlanders, and for a variety of other pursposes. It is a bit of overkill for a Munlander, to be honest; I often find myself chucking mostly-full boosters at the Mun to get them out of the way of the lander.

Standard crossfeed does increase delta v because it sheds the outer tanks slightly earlier than no crossfeeding.

[leopardenthusiast]

Then your not doing it right! You should land on those fuel tanks, lift off, then ladn on minimus with your real lander! Nice ship though. Except the decoupler before capsule. That irks me.

And what, he\'s supposed to just make a powered landing on Kerbin? Parachutes don\'t work so well when they have to slow more than a capsule.

[maltesh]

Standard crossfeed does increase delta v because it sheds the outer tanks slightly earlier than no crossfeeding.

As long as the specific impulse of the rockets, the stage composition, and the dry and fueled mass of each stage is the same, the delta-V calculated by the Tsiolkovsky Rocket Equation remains the same between the standard crossfeed and the non-crossfeed.

Performance can be better on the standard crossfeed from a Kerbin launch than a non-crossfeed because the Kerbin-launched rocket spends delta-V overcoming gravity and atmospheric drag, and the higher thrust that the crossfed rocket is capable of can get it out of the high drag zone quicker.

(There\'s also a fuel flow bug in v.0.15 that makes crossfed rockets somewhat more efficient than they should be, but we\'ll politely ignore that.)

[TheBlaster]

How is that possible? You shed excess weight earlier in the flight, is that not the mechanism that increases delta v with the asparagus method?

[Bluejayek]

And what, he\'s supposed to just make a powered landing on Kerbin? Parachutes don\'t work so well when they have to slow more than a capsule.

Sure, he could do that... Or since he has a decouple (Which is stupid and a massive waste of mass), he can just parachute down anyway.

[maltesh]

How is that possible? You shed excess weight earlier in the flight, is that not the mechanism that increases delta v with the asparagus method?

Delta-V is a measure of how much of a change in velocity your spacecraft could produce in ideal no-drag, no-grav conditions. And in Kerbal Space Program, all the stock Liquid Engines (when tied to the stock standard fuel tanks) have the same specific impulse at all times.

Which means that (in the above case) fuel mass expenditure rate is proportional to thrust expended, and it doesn\'t matter how quickly you burn X mass units of fuel on a single-stage rocket in the ideal no-drag, no-grav case, you wind up with the same change in velocity.

In other words, (assuming all identical engines on all stacks) the crossfed spacecraft is burning through its first-stage fuel in 6/7 the time that the non-crossfed craft is, but is getting 7/6 the thrust that the non-crossfed craft is for that time. The resulting delta-V for the first stage works out to be the same, and their second stages are identical. Similar ratios result when using different stock LFEs.

The reason asparagus staging works out better for delta-V is because you drop empty tanks /more often/, cranking your full mass/dry mass ratio upward each time, not because of the time when you drop them.

[LukeTim]

Fuel crossfeeding is a technique where you use fuel lines run from tanks that you\'re planning to drop later, to improve rocket performance.

Imagine that each circle in the diagram represents a stack of liquid fuel tanks with a standard Liquid Engine at the bottom (Perhaps use a vectored thrust center engine for better control). The outer stacks are attached to the central stack by radial decouplers (not shown) and the yellow arrows are fuel lines.

The No Crossfeeding example is a basic setup. The six outer engines burn as S2, and then are dropped. The inner engine then ignites on a full rocket, and starts burning.

The Crossfeeding example runs fuel lines from the bottom tank of the outer ring to the bottom tank of the inner engine. All seven engines start burning on launch, and the inner engine is getting all its fuel from the outer tanks. This setup provides higher thrust off the pad than the No Crossfeeding example, but delta-V works out to be the same. However, because the atmosphere is thickest at the surface of Kerbin, it can provide better overall performance by getting out of the lower atmosphere quicker.

Asparagus Stalk Booster Staging is a subset of Crossfeeding , where again, all seven engines ignite on launch. However, instead of dropping the outer ring all at once, fuel lines and staging are set up so that all the fuel drains from the S4 tanks, and those are dropped, followed by fuel draining from the S3 tanks, and dropping them, followed by draining from the S2 tanks and dropping them, and finally burning the S1 fuel. Asparagus Staging combines the high thrust on launch advantage with an efficient mode of dumping empty tanks while keeping the rocket balanced. It provides a somewhat significant boost in delta-V over regular crossfeeding.

Three-tank-high Asparagus Staging is what I use in my standard stock Munlanders, and for a variety of other pursposes. It is a bit of overkill for a Munlander, to be honest; I often find myself chucking mostly-full boosters at the Mun to get them out of the way of the lander.

Never used fuel lines ever... didn\'t really know when to use them.

After trying that out, I think I\'ll be using them a lot from now on. Thank you so much for this post!

[TheBlaster]

Delta-V is a measure of how much of a change in velocity your spacecraft could produce in ideal no-drag, no-grav conditions. And in Kerbal Space Program, all the stock Liquid Engines (when tied to the stock standard fuel tanks) have the same specific impulse at all times.

Which means that (in the above case) fuel mass expenditure rate is proportional to thrust expended, and it doesn\'t matter how quickly you burn X mass units of fuel on a single-stage rocket in the ideal no-drag, no-grav case, you wind up with the same change in velocity.

In other words, (assuming all identical engines on all stacks) the crossfed spacecraft is burning through its first-stage fuel in 6/7 the time that the non-crossfed craft is, but is getting 7/6 the thrust that the non-crossfed craft is for that time. The resulting delta-V for the first stage works out to be the same, and their second stages are identical. Similar ratios result when using different stock LFEs.

The reason asparagus staging works out better for delta-V is because you drop empty tanks /more often/, cranking your full mass/dry mass ratio upward each time, not because of the time when you drop them.

Very interesting. Thanks

[samstarman5]

Fuel crossfeeding is a technique where you use fuel lines run from tanks that you\'re planning to drop later, to improve rocket performance.

Well, I can testify to the effectiveness of the asparagus fuel system. Using three tanks per module, I was able to launch a rocket using only T-45 engines and place it into orbit with plenty to spare for a munar transfer and insertion. That practically takes away the need for an entire stage for a munar mission.

Write this one down in the notebook for economical rockets, kids, when campaign mode comes rolling around.