Difference between revisions of "Asparagus staging"

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(→‎Name: Removing arguable assertion that there are no real world examples (see section above), and reference to "onion staging", which is often used to refer to something different)
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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]]
  
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'''Asparagus staging''' is a method to build very efficient [[rocket]]s. The stock vessel [[Kerbal X]] demonstrates this technique.
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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.
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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.
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The technique is still useful for building exceptionally large rockets when no larger engines are available.
  
 
== Function ==
 
== Function ==
 
[[File:Asparagus staging sequence.png|thumb|left|upright|Kerbal X's seq.]]
 
[[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]]s 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 outermost 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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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.
  
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This concept can be realized through [[fuel line]]s which connect the stages in the order they will be dropped. It uses the fact that engines will always take their fuel from the most distant fuel tank available.
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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.
  
 
== Disadvantages ==
 
== Disadvantages ==
 
[[File:AsparagusStaging-6x3.png|right|300px|thumb|Overhead view of a vessel using massive asparagus staging]]
 
[[File:AsparagusStaging-6x3.png|right|300px|thumb|Overhead view of a vessel using massive asparagus staging]]
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The biggest disadvantage of this technique is the complexity of building it. Usually the symmetry modes can only be used partially. One way is to only mirror each separate stage allowing to use symmetry for the complete setup. One drawback is, that the engines then are not always at the same height which may induce torque. Another way is to build everything except the fuel lines with the symmetry mode, but there each fuel line needs to be placed manually.
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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.
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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.
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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.
  
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With more massive payloads it may require more layers of fuel stacks and struts, making the technique challenging due to the high part count. Although a rocket without asparagus staging has a higher part count because it is more inefficient. The part count can be reduced by installing rocket part mods that provide fuel tanks and engines of larger sizes, allowing to achieve equal performance with less parts, more stability and better looks.
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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.  
  
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Another disadvantage is that rockets utilizing this design may be unrealistic, because in the real world it is impractical or impossible to move fuel as quickly as would be needed. It also might look not aesthetic especially with multiple layers, because there are then many small rockets bundled together.
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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.
  
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The rocket tends to start to [[roll]] when using this type of staging which might result in a loss in control of the craft. It also loses thrust with each staging event but only a lower mass compared to other staging methods. This can cause a serious drop of velocity reducing the efficiency.
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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.
  
 
=== Drag ===
 
=== Drag ===
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As rockets with asparagus staging may be wider than usual vertical stacked their real world [[drag]] would be higher. But as the drag depends on the mass instead of the cross-sectional area, there is no disadvantage of using asparagus staging when comparing the drag with other methods.
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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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Although this might change in the future — [[Planned features#Physics & Dynamics|the developers have already announced that the aerodynamic model will be made more realistic in future versions]]{{cn}}<sup>[''not cited in the link'']</sup> — this will most likely not a big issue, as the advantages outweigh this. But it will likely make it a lot more challenging to create aerodynamic crafts which use this technique excessively.
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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]].
  
 
== Real world application ==
 
== Real world application ==
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This system is currently only a feature of Kerbal Space Program. In the real world there might be one craft, the [[w:Falcon Heavy|Falcon Heavy]] by 2014/2015 which uses this type of staging, although only with one level. The problem with asparagus staging is, that transferring fuel to an engine is already complicated. With additional pumps transferring between tanks the rocket becomes more complicated which may reduce the reliability. Another — very important — important reason are pressure losses that occur because of additional piping and further valves. This can call for added propellant pumps, which negatively impacts the efficiency of asparagus staging.
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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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In 1947, [[w:Mikhail Tikhonravov|Mikhail Tikhonravov]] developed this kind of staging which he called “packet rockets”. This lead to the development of the [[w:R-7 Semyorka|R-7 Semyorka]]. Later developments of this rocket became then the [[w:R-7 (rocket family)|R-7 rocket family]]. But all those rockets never used the original “packet rockets”/asparagus staging.
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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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== Name ==
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[[Category:Tutorials|{{PAGENAME}}]]
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The name "asparagus staging" is a creation by the players of KSP and not an official word used in rocket science.
 

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.
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