Side boosters

[farmerben]

Arianne6, falcon heavy, the space shuttle, etc have two side boosters.  The Soyuz had four and was the workhorse of global spaceflight for quite a while.  KSP easily lets me have 6 or 8 side boosters using the same tech as I would need to do 2 or 4.  So why don't we see more side boosters in real life?   What are the best decoupling mechanisms?

[darthgently]

18 minutes ago, farmerben said:

Arianne6, falcon heavy, the space shuttle, etc have two side boosters.  The Soyuz had four and was the workhorse of global spaceflight for quite a while.  KSP easily lets me have 6 or 8 side boosters using the same tech as I would need to do 2 or 4.  So why don't we see more side boosters in real life?   What are the best decoupling mechanisms?

My guess is that fewer parts is usually better.  Less to go wrong and no advantage from  redundancy in this case. 

 Also consider that if SRBs are recovered and reused it is cheaper to recover 2 big ones than 6 small ones sprinkled over a few miles of ocean

Edited July 10 by darthgently

[cubinator]

If you have a core, and you want to add side boosters to it for the purpose of adding thrust, then the core has to be designed to take all that thrust into its structure. It might be able to handle two big boosters or six small ones, but six big ones might crush it.

Separating thrust out into many individual boosters also increases the dry mass of tank walls, number of parts you need to manufacture and test, etc. when you might be able to accomplish the same thing with fewer boosters. It's a big complicated tradeoff with many different answers, but I guess people who do it generally find they can get by best with smaller numbers of boosters.

[Nuke]

especially if you are going to dump all 6 at once. then its a lot simpler just to do 2 bigger ones. if you have six pairs of boosters with stepped burn times such that they burn out 2 at a time, then you add complexity and remove redundancy.

[AckSed]

1 hour ago, Nuke said:

Especially if you are going to dump all 6 at once. Then it's a lot simpler just to do 2 bigger ones. If you have six pairs of boosters with stepped burn times such that they burn out 2 at a time, then you add complexity and remove redundancy.

ULA Vulcan has its hands over its ears going, "Can't hear you, I need my boosters to be modular, la la la..."

[StrandedonEarth]

Delta II used up to nine SRBs, three of them airlit…

[mikegarrison]

It's a design strategy. You can make one launcher more flexible by giving it side boosters to adjust the thrust for each mission. Or you can choose other strategies that accomplish the same goal.

The pros of the side boosters are that the main booster (sustainer) does not have to have as much thrust, and therefore can be lighter and cheaper. But the structure has to be strengthened to carry the loads of the side boosters, and the side boosters themselves have costs, so that offsets the advantages of the lower thrust sustainer.

However, with for example Falcon 9, they don't care nearly as much if they have "too much rocket" because they aren't going to just throw it away into the ocean after using it. So it makes more sense for them to design the main booster to have more thrust and ignore the complications of side boosters.

[kerbiloid]

One rocket's "big" is another one's "small".