Difference between revisions of "Asparagus staging"

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[[File:Asparagus-staging.svg|right|thumb|All engines uses first S4's fuel, then S3's fuel and so on]]
 
[[File:Asparagus-staging.svg|right|thumb|All engines uses first S4's fuel, then S3's fuel and so on]]
[[File:AsparagusStaging-6x3.png|right|300px|thumb|Overhead view of a vessel using asparagus staging]]
 
  
Asparagus staging is a method to build very efficient rockets.
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'''Asparagus staging''' is a rocket design method. It involves using parallel engines and fuel crossfeeds to increase initial thrust and available fuel while reducing the dry mass of later stages. The stock vessel [[Kerbal X]] demonstrates this technique.
  
The idea is that you create a rocket with a lot of parallel rocket engines with fuel tanks on top of them. All engines ignite at the same time. The trick, however, is that each rocket engine isn't depleting its own tank, but they are all draining their fuel from the two outmost tanks. When these are depleted, the outmost tanks with their engines are decoupled and the next fuel tank takes over which is still completely full. The result is that the rocket always flies with the minimum number of tanks required to transport the fuel it has left while also constantly using all engines it has on board.
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Extensive asparagus staging was very effective in building efficient rockets in early versions of Kerbal Space Program. Several updates gradually made the drag model of the game more realistic, making asparagus staging less effective, and added larger, higher-efficiency engines, giving effective alternatives for building large rockets. In modern versions of the game, it is usually better to use a single large main engine per stage and a less complex arrangement of boosters.
  
This concept can be realized through fuel ducts which connect the stages in the order they will be dropped. It exploits the fact that engines will always take their fuel from the most distant fuel tank available.
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The technique is still useful for building exceptionally large rockets when no larger engines are available.
  
The stock vessel "Kerbal X" demonstrates this technique.
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== Function ==
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[[File:Asparagus staging sequence.png|thumb|left|upright|Kerbal X's seq.]]
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The idea is to create a rocket with a lot of parallel [[rocket engine|engines]] each carrying fuel. Firing these engines in parallel allows for higher initial thrust than sequential staging. The trick, however, is that each rocket engine isn't depleting its own tank, but they are all draining their fuel from the outermost tanks. When these are depleted, the outermost tanks and their engines are decoupled, and the next fuel tank takes over which is still completely full. The result is that the rocket always flies with the minimum number of tanks required to transport the fuel it has left while also constantly using all engines it has on board. Ideally, this can allow lifting heavier payloads.
  
== Current Disadvantages ==
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This can be implemented in the [[VAB]] by attaching [[fuel line|fuel lines]] in the order stages will be dropped, or by staging decouplers with crossfeed enabled.
  
The biggest disadvantage of this technique is that it has a very stupid name. Both players and developers have expressed their disdain for the term. Unfortunately, no better name has been found yet. 
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== Disadvantages ==
Chocolate box staging has been proposed as an alternative, as one tends to eat one layer of chocolate before cracking into the lower layer, while simultaneously jettisoning the unneeded stage, much like throwing out the gross chocolates nobody wants.
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[[File:AsparagusStaging-6x3.png|right|300px|thumb|Overhead view of a vessel using massive asparagus staging]]
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Extensive asparagus staging can be complex to build. Such designs cannot take full advantage of symmetry modes in the [[VAB]]. Placing only two mirrored engines and their fuel lines at a time can leave engines at subtly different heights, inducing a torque on the craft. It is preferable to add the boosters first with full symmetry, and then place each fuel line manually.
  
Although Asparagus staging appears to be de rigeur, for the foreseeable future as it is widely understood as a shorthand for a really complex system, much like the Kerbals love of calling nuclear fission by its more common name, atom popping.
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Since each engine stack carries its own fuel but only the central stack carries the payload, if stacks are otherwise identical, the asparagus staged rocket's thrust-to-weight ratio will be reduced with each subsequent staging. So it can be difficult to have efficient thrust-to-weight ratios at each point of a many-staged launch.
  
== Future Disadvantages ==
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More layers of fuel stacks and struts, which may be required for more massive payloads, lead to high part counts. High part counts can lead to slowdowns and lower frame rates.
  
Asparagus staging benefits a lot from the inaccurate way KSP is modeling atmospheric drag in the current version (0.18.4). Currently, each part of a vessel generates drag proportional to its mass, regardless of how it is placed and how much of its surface area is exposed to the direction of the airflow (yes, that means all the [[Aerodynamic Nose Cone]]s you put on your rockets just slowed them down even more). [[Planned_features#Physics_.2F_Dynamics|The developers have already announced that the aerodynamic model will be made more realistic in future versions]], which will likely mean that the wide, flat designs of asparagus-staged crafts will generate a lot more atmospheric drag than slimmer designs with vertical staging.
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Uneven reduction of mass at different angles around the craft can lead the rocket to start to [[roll]], which might result in a loss in control.  
  
The new atmospheric drag model is unlikely to make asparagus staging completely obsolete as there is a planned real-life rocket which uses this method with one stage <ref>http://en.wikipedia.org/wiki/Falcon_Heavy#First_stage</ref>, but it will likely make it a lot more challenging to create aerodynamic crafts which use this technique excessively.
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Beyond in-game functionality, the appearance of a "matchstick bundle" may seem un-aesthetic, and, as discussed below, the designs are less true to real-world rockets.
  
== References ==
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Using larger central engines may achieve equal performance with a lower part count, more stability in flight, improved aesthetics, and greater realism. Additionally, in career mode, using solid fuel boosters may be a cheaper way to achieve a given delta-v than using liquid boosters with asparagus staging.
  
<references/>
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=== Drag ===
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With the the release of [[1.0|version 1.0]], a more realistic drag and aerodynamics model was introduced, and in [[1.2|version 1.2]] trans- and supersonic drag was further increased for blunt objects. This made designing wide asparagus-staged craft far more challenging and reduced its efficiency relative to more conventional staging.
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Limited asparagus staging as seen in the [[Kerbal X]] can still be effective.
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Booster stages which are slanted away from central rockets reduce frontal drag, and help make limited asparagus staging more viable. [[Fuel_tank#Rocket_fuel_tanks | Three slanted adapter fuel tanks]] were introduced in [[0.90.0]], and the slanted [[FL-C1000 Fuel Tank]], modeled after the real-world [[w:R-7 (rocket family) | Soviet R-7 boosters]], is available with the [[Making History |Making History DLC]].
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== Real world application ==
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Asparagus staging, properly known as a propellant crossfeed system, has not yet appeared in real-world rockets, though it has been proposed multiple times.
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The "asparagus" name is found in a book on orbital mechanics by Tom Logsdon. According to Logsdon, an engineer named Ed Keith coined the term "asparagus-stalk booster" for launch vehicles using propellant crossfeed.<ref>[https://books.google.com/books?id=C70gQI5ayEAC&pg=PA143&lpg=PA143&dq=asparagus-stalk+booster&source=bl&ots=eXLhW_FLSQ&sig=WVZJZM1kpAzAXXCZRVVa_fwtYAI&hl=en&sa=X&ei=72rtUfybI8aayQHkhoHwCA&ved=0CH8Q6AEwDQ#v=onepage&q=asparagus-stalk%20booster&f=false Logsdon, Tom. ''Orbital Mechanics: Theory and Applications'' (Wiley, 1997), 143-144.]</ref>
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In 1947, [[w:Mikhail Tikhonravov|Mikhail Tikhonravov]] developed plans for parallel staging with propellant crossfeed, which he called “packet rockets”. This led to the development of the [[w:R-7 Semyorka|R-7 Semyorka]] and the rest of the  [[w:R-7 (rocket family)|R-7 rocket family]]. But none of those rockets ever used the original propellant crossfeed idea.
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While the [[w:Falcon Heavy|Falcon Heavy]] was originally going to feature a propellant crossfeed, this feature was cancelled by 2016.
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Engineering challenges have thus far prevented the adoption of fuel crossfeeds in the real world. Maintaining steady, reliable, very high flow rates of propellant at the correct pressures during rocket flight is already complicated. Pumping fuel between tanks and switching valves on and off introduces considerable further complications and real risks.
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[[Category:Tutorials|{{PAGENAME}}]]

Latest revision as of 00:40, 10 August 2023

All engines uses first S4's fuel, then S3's fuel and so on

Asparagus staging is a rocket design method. It involves using parallel engines and fuel crossfeeds to increase initial thrust and available fuel while reducing the dry mass of later stages. The stock vessel Kerbal X demonstrates this technique.

Extensive asparagus staging was very effective in building efficient rockets in early versions of Kerbal Space Program. Several updates gradually made the drag model of the game more realistic, making asparagus staging less effective, and added larger, higher-efficiency engines, giving effective alternatives for building large rockets. In modern versions of the game, it is usually better to use a single large main engine per stage and a less complex arrangement of boosters.

The technique is still useful for building exceptionally large rockets when no larger engines are available.

Function

Kerbal X's seq.

The idea is to create a rocket with a lot of parallel engines each carrying fuel. Firing these engines in parallel allows for higher initial thrust than sequential staging. The trick, however, is that each rocket engine isn't depleting its own tank, but they are all draining their fuel from the outermost tanks. When these are depleted, the outermost tanks and their engines are decoupled, and the next fuel tank takes over which is still completely full. The result is that the rocket always flies with the minimum number of tanks required to transport the fuel it has left while also constantly using all engines it has on board. Ideally, this can allow lifting heavier payloads.

This can be implemented in the VAB by attaching fuel lines in the order stages will be dropped, or by staging decouplers with crossfeed enabled.

Disadvantages

Overhead view of a vessel using massive asparagus staging

Extensive asparagus staging can be complex to build. Such designs cannot take full advantage of symmetry modes in the VAB. Placing only two mirrored engines and their fuel lines at a time can leave engines at subtly different heights, inducing a torque on the craft. It is preferable to add the boosters first with full symmetry, and then place each fuel line manually.

Since each engine stack carries its own fuel but only the central stack carries the payload, if stacks are otherwise identical, the asparagus staged rocket's thrust-to-weight ratio will be reduced with each subsequent staging. So it can be difficult to have efficient thrust-to-weight ratios at each point of a many-staged launch.

More layers of fuel stacks and struts, which may be required for more massive payloads, lead to high part counts. High part counts can lead to slowdowns and lower frame rates.

Uneven reduction of mass at different angles around the craft can lead the rocket to start to roll, which might result in a loss in control.

Beyond in-game functionality, the appearance of a "matchstick bundle" may seem un-aesthetic, and, as discussed below, the designs are less true to real-world rockets.

Using larger central engines may achieve equal performance with a lower part count, more stability in flight, improved aesthetics, and greater realism. Additionally, in career mode, using solid fuel boosters may be a cheaper way to achieve a given delta-v than using liquid boosters with asparagus staging.

Drag

With the the release of version 1.0, a more realistic drag and aerodynamics model was introduced, and in version 1.2 trans- and supersonic drag was further increased for blunt objects. This made designing wide asparagus-staged craft far more challenging and reduced its efficiency relative to more conventional staging.

Limited asparagus staging as seen in the Kerbal X can still be effective.

Booster stages which are slanted away from central rockets reduce frontal drag, and help make limited asparagus staging more viable. Three slanted adapter fuel tanks were introduced in 0.90.0, and the slanted FL-C1000 Fuel Tank, modeled after the real-world Soviet R-7 boosters, is available with the Making History DLC.

Real world application

Asparagus staging, properly known as a propellant crossfeed system, has not yet appeared in real-world rockets, though it has been proposed multiple times.

The "asparagus" name is found in a book on orbital mechanics by Tom Logsdon. According to Logsdon, an engineer named Ed Keith coined the term "asparagus-stalk booster" for launch vehicles using propellant crossfeed.[1]

In 1947, Mikhail Tikhonravov developed plans for parallel staging with propellant crossfeed, which he called “packet rockets”. This led to the development of the R-7 Semyorka and the rest of the R-7 rocket family. But none of those rockets ever used the original propellant crossfeed idea.

While the Falcon Heavy was originally going to feature a propellant crossfeed, this feature was cancelled by 2016.

Engineering challenges have thus far prevented the adoption of fuel crossfeeds in the real world. Maintaining steady, reliable, very high flow rates of propellant at the correct pressures during rocket flight is already complicated. Pumping fuel between tanks and switching valves on and off introduces considerable further complications and real risks.
  1. Logsdon, Tom. Orbital Mechanics: Theory and Applications (Wiley, 1997), 143-144.