Getting started with clusters and the math for them

[RodRocket]

Getting started with cluster engines and have a few questions.

The rockets are a 3x18 Bertha, and I have two cluster mounts with one a 4x24 and a 5x18 for BT-80 to go into soon to be built rockets.

So if I understand engines right, one "b" engine is equal to two "a" engines and one "d" is two "c" engines.

So taking the 4x24 mount, if I use four "d" engines, two "d" will be equal to an "e" so two "e" will equal an "f".

So four "d" equal an "f"

Am I doing this right?

Thanks,
Rod

 

[MClark]

yes

 

[fyrwrxz]

Just add up the Nsecs

 

[pepe]

Just a thought ,won't four D engines add a lot more weight then a single F thus having lower performance?

 

[qquake2k]

Just a thought ,won't four D engines add a lot more weight then a single F thus having lower performance?

Yeah, but clusters are just so cool!

 

[New Ocean]

Just a thought ,won't four D engines add a lot more weight then a single F thus having lower performance?

Indeed it is part of the fun. It is worth noting that since the rocket will not go as high, and it is quite possible that a motor or even two will fail to light, you should always err on the side of a short ejection delay with BP clusters. For example, I would go 4XD12 - 3 on my Viper even though with all three burning, a 5 second delay would be normal. This has saved me twice already when a motor did not light.

Also, depending on the rocket, be sure you maintain stability for that low and slow takeoff, again with possibly fewer motors than planned. Nothing worse than a land shark running on two D motors! (Actually there is one thing worse, that is the same land shark suddenly igniting the other two motors...)

 

[jadebox]

Am I doing this right?

Yep, you have it right.

At RocketReviews.com, you can see the stats and time-thrust curves for clustered or staged motors. For example: Cluster of Four Estes D12s.

As you can see on that page, four D12s are equivalent to a "68% F motor" - that is to say, the cluster has a total impulse which maps at 68% in the range from a minimum F motor to a maximum F motor.

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-- Roger