Number of fins for a rocket

[Spaceman.Spiff]

9 hours ago, Teilnehmer said:

Soyuz has only 4 fins.

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It mostly uses vernier engines for control. 

[Dientus]

12 hours ago, Teilnehmer said:

It's the other way around. The higher the center of mass is, the longer is the lever fin forces are applied to, the smaller the fins can be.

True, it was a bit late at night here at the time and I was tired but enough for excuses

I may have been thinking of center of pressure,

 

[peridoot]

i personaly use 24329834273143137 fins

[Vanamonde]

2 fins leaves an axis unaccounted for. With 3 when you pitch or yaw there are 2 fins on one side of the COM but only 1 on the other, which tends to cause unwanted attitude wandering. 4 is nicely balanced. More than that generally does not help much. 

[magnemoe]

3 hours ago, Spaceman.Spiff said:

It mostly uses vernier engines for control. 

Think fins on space rockets are mostly for separation. You need to keep first stage or the boosters stable after separation.  Guess this is the reason for the fin on the Soyuz boosters. Know this is the reasons for the small fins on Saturn 5 and pretty sure this is the reason for the huge grind fins on the N1. I assume they would be opened just before  separation. 

[Cavscout74]

I'll use three sometimes in the early career when limited to 30 parts, but any other time I'll use four if I use any.   When I do use just three, it is almost always fixed fins for passive stability rather than controllable fins.

[MetricKerbalist]

Hi KSP colleagues,

I thank all of you for your instructive answers.

Stanley

[SpaceFace545]

The specific formula to find the number of guns required is:

((L x D) + W / G) / <a^t

[wumpus]

On 7/7/2021 at 8:42 PM, mikegarrison said:

For most modern rockets the answer is "zero".

Pegasus (1990-not much longer) has 3 fins plus a wing.  Virgin Orbital's rocket (taking a similar role as Pegasus) has 4.  Pegasus uses solid rockets (3 stages), so absolutely needs fins for at least initial control.  Virgin Orbital has at least one kerolox engine, but presumably a dropped rocket benefits more by having fins (no idea if they are used for control).

As far as KSP is concerned, for less expensive rockets I like to use two control fins aligned North/South with two (or more*) fixed fins aligned  East/West.  Since your primary control needs involve modifying your inclination in that particular direction, it can be enough (you can eventually modify other directions by rolling and then altering the direction as needed, but expect a certain delay).  I don't recommend this for any situation  that doesn't include a "revert to launch" option.  Also, since KSP "smallest fins" are dirt cheap, adding more of them than a larger fin may make sense in game while making no real sense in the real word.

On 7/8/2021 at 1:32 AM, kerbiloid said:

Uncle Wernher was very tenacious in his habits.

Das works on V-2, warum shouldn't on Saturn?

I think for post V-2 rockets, he included the fins as a personal signature on the rocket.  I think he called it his trademark or similar.  I also suspect that between the simplicity and effectiveness on his earlier rockets, he wasn't about to give them up when moving to more complex rockets (when was the last time a fin failed on a rocket?).

[GoSlash27]

Assuming you're going to use fins at all, IRL 3 should be considered the minimum and in KSP 4 (for the reasons @Superfluous J mentioned).

Best,

-Slashy

[kerbiloid]

1 if put it on a circular rail and move along enough fast.

[sevenperforce]

On 7/12/2021 at 12:06 PM, wumpus said:

On 7/8/2021 at 1:32 AM, kerbiloid said:

Uncle Wernher was very tenacious in his habits.

Das works on V-2, warum shouldn't on Saturn?

I think for post V-2 rockets, he included the fins as a personal signature on the rocket.  I think he called it his trademark or similar.  I also suspect that between the simplicity and effectiveness on his earlier rockets, he wasn't about to give them up when moving to more complex rockets (when was the last time a fin failed on a rocket?).

Fins were included on the human-rated Redstone, Saturn I(B), and Saturn V to help with launch abort. If things went south and the first stage shut down during boost, you’d want a way to keep the first stage passively pointing in the proper direction as long as possible to give the launch escape system time to act, rather than have the stage begin tumbling immediately. The concern was that without fins, a first-stage shutdown could lead to a tumble and a rapid conflagration with a fireball too large for the launch abort system to escape.

Atlas-Mercury lacked fins because the bulky sideboard engines provided enough tail drag to maintain pointing in an abort. Titan-Gemini lacked fins because the Gemini capsule didn’t have a tractor abort system at all (and was also basically a death trap).

Also IIRC Redstone may have had actuated fins........

[magnemoe]

On 7/12/2021 at 6:06 PM, wumpus said:

Pegasus (1990-not much longer) has 3 fins plus a wing.  Virgin Orbital's rocket (taking a similar role as Pegasus) has 4.  Pegasus uses solid rockets (3 stages), so absolutely needs fins for at least initial control.  Virgin Orbital has at least one kerolox engine, but presumably a dropped rocket benefits more by having fins (no idea if they are used for control).

As far as KSP is concerned, for less expensive rockets I like to use two control fins aligned North/South with two (or more*) fixed fins aligned  East/West.  Since your primary control needs involve modifying your inclination in that particular direction, it can be enough (you can eventually modify other directions by rolling and then altering the direction as needed, but expect a certain delay).  I don't recommend this for any situation  that doesn't include a "revert to launch" option.  Also, since KSP "smallest fins" are dirt cheap, adding more of them than a larger fin may make sense in game while making no real sense in the real word.

I think for post V-2 rockets, he included the fins as a personal signature on the rocket.  I think he called it his trademark or similar.  I also suspect that between the simplicity and effectiveness on his earlier rockets, he wasn't about to give them up when moving to more complex rockets (when was the last time a fin failed on a rocket?).

An air dropped rocket need to turn upward asap after drop, guess their fins are steerable like the more advanced KSP ones,