Number of fins for a rocket

[MetricKerbalist]

Hi KSP colleagues,

All other things being equal, how many fins should a rocket have -- two, three, four, or how many?

Thank you for your consideration.

Stanley

[Dientus]

It depends on the size of the rocket, the size of the fins, where its center of mass is, rockets trajectory, and the speed of the rocket. I am sure that there is a formula for it somewhere but I can't find it (I know, some help I am  ) all of this changes in flight.

 

If the rocket design limits fin size, then place more. If the center of mass is further up, make them larger. If you go faster, you could use less fins if you change trajectory less. The whole purpose of fins is to keep the back of the rocket pointed TO the back to provide thrust in atmospheric conditions and prevent loss of thrust by eliminating the wobbles which makes the rocket easier to fly and more steady during ascent.

 

I think?

[mikegarrison]

For most modern rockets the answer is "zero".

[cubinator]

More fins mean more drag. Less fins means less stability.

Usually triangular "delta" fins are good for supersonic flight. When you make something supersonic, you want to limit the locations where air vortices can form (like corners and edges) because they are really problematic as you cross the sound barrier (when SOME parts of your ship have supersonic flow, and OTHERS have subsonic)

As long as your center of drag is a little behind the center of mass, you can fly pretty stable. The rocket body has a certain center of drag even without fins, so if you balance it right you might not need any fins. Hobby rockets usually do need fins.

[sevenperforce]

In KSP, most engines have very limited gimbal range, so you need fins. Three minimum, four if you want fine pitch control on ascent. Anything more is probably overkill; if you want better stability, just use larger fins.

in the real world, engines always have sufficient gimbal range to maintain pitch and yard, so you never need fins unless your vehicle is intended to fly at some point without active gimbal control.

[Nuke]

back in high school i won a rocketry contest we had in science class by using 3 relatively large fins. they had kind of an ogive profile as i used a compass to draw a quarter circle on a strip of some kind of thick card stock, which i folded accordion style to keep it all together. i then cut out the curve and then used a paper guillotine to cut out maybe a third of the profile, and also to even out the trailing edge. somehow i screwed up and one fin was mangled in the operation, i had intended 4, but ended up doing 3 instead. i think we were using c motors and the contest was for altitude. i won by virtue of my rocket staying pointed at the sky for most of the flight. so many rockets spiraled out and exploded that they decided to never do model rockets again. interesting thing was that since science class was last hour, we got all the fat tubes as the earlier students used up all the choice materials. my rocket turned out to be the fattest rocket in the contest. but i did have the smarts to use a plastic easter egg as a nose cone, with a foam adapter i made with a file. of course after the contest we still had some rocket motors left over, and i tried launching with a d motor, which blew up my rocket, it was like ksp before ksp. moar boosters is not always the best option. 

[Teilnehmer]

2 hours ago, Dientus said:

If the center of mass is further up, make them larger.

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.

Edited July 8, 2021 by Teilnehmer

[Superfluous J]

In-game, I never use anything but 4.

1 is just crazy, 2 isn't enough to steer or stabalize in all direcctions, 3 is technically enough but (I think due to rounding) causes steering and rotational issues, and anything more than 4 is useless.

[kerbiloid]

Fins are useful for only small rockets, of V-2 size and smaller.

A square-cube law. The fin area  ~size², the inertial mass ~size³, so the bigger is the rocket - the higher is the speed when fin starts getting effective.
The same with gas rudders under the nozzle.

So, the only effective way to tilt or stabilize a heavy rocket is gimballing the engines.

On the other hand, the gimbal weights, so on a smaller rocket it's easier to use fins.

The small rockets usually have 4 fins, because it's technologically simpler and easier to manage the rocket in flight by treating it as a plane with "zenith-left-nadir-right" directions.

Also the fins can at the same time include the leg trusses, so be the legs (in V-2), and it's better to have 4 or 6 legs than 3.

[razark]

4 hours ago, MetricKerbalist said:

All other things being equal, how many fins should a rocket have -- two, three, four, or how many?

What do you mean "All other things being equal"?  There's a lot more that goes into it.  No single answer is going to fit every rocket.  The Redstone and Saturn V had four.  The STS had three.  The Falcon has none.

The answer is going to be "zero, three, or four, depending on the rest of the rocket design".

 

But, as we all know, the only true and correct answer is 42.

[Teilnehmer]

I like Saturn-IB with its eight big fins.

[razark]

1 minute ago, Teilnehmer said:

I like Saturn-IB with its eight big fins.

*glares at your avatar picture*

[Teilnehmer]

2 minutes ago, razark said:

*glares at your avatar picture*

Soyuz has only 4 fins.

Spoiler

 

[cubinator]

1 hour ago, Superfluous J said:

1 is just crazy,

Ooh, you know what this is useful for? Put one fin on the outer side of each side booster. When you separate them the fin's drag will guide the spent booster away from the rocket. I do this all the time, usually negates the need for sepatrons except on really large boosters.

Also, actually, if you had something called a "tube fin" that went all the way around the body there would technically be only one fin. 

[razark]

18 minutes ago, Teilnehmer said:

Soyuz has only 4 fins.

And yet you claim to like the eight fins of the Saturn-IB.

Eight, or four?  Which is it, huh? 

[kerbiloid]

32 minutes ago, Teilnehmer said:

I like Saturn-IB with its eight big fins.

Uncle Wernher was very tenacious in his habits.

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

[DDE]

1 hour ago, kerbiloid said:

Fins are useful for only small rockets, of V-2 size and smaller.

o_0

You just need more efficient surface area.

[Superfluous J]

54 minutes ago, cubinator said:

Ooh, you know what this is useful for? Put one fin on the outer side of each side booster. When you separate them the fin's drag will guide the spent booster away from the rocket. I do this all the time, usually negates the need for sepatrons except on really large boosters.

I actually use 2 tiny fins on the sides of side boosters to get them to fly away when I separate them. I never tried 1 on the outside and may try it next time, but I really like the way the boosters look as they "lift off" like planes, away from the main rocket.

[kerbiloid]

33 minutes ago, DDE said:

You just need more efficient surface area.

You just need to be OKB-1 and follow the previous samples.

***

Two wings and a fin per booster is what's actually needed.

Spoiler

 

[magnemoe]

 

3 hours ago, cubinator said:

Ooh, you know what this is useful for? Put one fin on the outer side of each side booster. When you separate them the fin's drag will guide the spent booster away from the rocket. I do this all the time, usually negates the need for sepatrons except on really large boosters.

Also, actually, if you had something called a "tube fin" that went all the way around the body there would technically be only one fin. 

This works well in KSP, at the time you drop the boosters you have less need for fins after all. 
The reason why modern rockets don't use fins is the length as some pointed out but also the advanced flight control system who correct for any  errors. 
The V2 rocket had no gimbal, just control vanes in the nozzle and an primitive analog control. 

[Nuke]

7 hours ago, Superfluous J said:

In-game, I never use anything but 4.

1 is just crazy, 2 isn't enough to steer or stabalize in all direcctions, 3 is technically enough but (I think due to rounding) causes steering and rotational issues, and anything more than 4 is useless.

3 also isnt very fault tolerant. with 4 you can have an actuator fail and still have 3 axis control. you can keep adding fins but at some point you are just introducing a lot of unwanted drag and structural weight.

Edited July 8, 2021 by Nuke

[CastleKSide]

I will also mention in passing that very few also rockets use rcs control on their first stage, while most modern rockets have some kind of active roll control. Think F9's cold gas thrusters.

[SOXBLOX]

If you're thinking of a model rocket, three is probably the best. The more fins you have, the harder it is to align them properly.

I never use fins in-game except for outsized payloads, when I need more control.

[DDE]

1 hour ago, CastleKSide said:

I will also mention in passing that very few also rockets use rcs control on their first stage, while most modern rockets have some kind of active roll control. Think F9's cold gas thrusters.

Some tend towards hot gas. If you have exhaust from a gas generator, you might as well direct it into a desirable direction.

[RCgothic]

3 equally spaced fins is the minimum for passive stability in both pitch and yaw.

4 is a simpler control scheme as pitch and yaw can be controlled independently.