On Fins

[Scrogdog]

I'd like to talk about fins on the mid to upper parts of rockets. Basically, not being an expert of course, I'd like to explore the topic for two reasons; I've seen it explicitly NOT recommended, and more importantly, because I do it. So far anyway.

I've only been playing for a few days and this is, by far, my most successful design. Very stable, doesn't spin, and can get the one-man capsule or light sat package in to orbit just fine.

Maybe it's my imagination, but I could swear the upper fins did a lot to stabilize this puppy, especially the spinning.

Now, I can't remember where I read it recently on these forums, but someone who advised against doing this said that you only see arrows fletched at the end. Made sense at the time, but the more I thought about it, and because of my experiences with this particular rocket, the more I might disagree. I think.

Arrows are not ballistic, but this baby is!

So, what do you think? I am way off on this (and probably many other things)?

[paleorob]

Arrows are ballistic but their flight path is relatively short so it seems to an observer to be a straight line.

[Scrogdog]

Good point. I stand corrected!

Ok, how about arrows are not powered by engines?

[marko dash]

for steering control surfaces up front tend to work better, but if you don't put equal or larger fins on the rear the rocket will act like a weather vane and try to put the finless point first.

[numerobis]

As long as fins are behind your center of mass, they'll stabilize your rocket. The most effective place to put them is where they get the most torque, which is at the back for pitch and yaw, and outward for roll.

Since fins have mass, I tend to avoid them to the extent possible: vectoring engines can deal with pitch and yaw, so I just have a pair or a triple of winglets out as far as possible, to control the roll. Also, I tend to use the other winglet since it has more control authority than the delta-deluxe (conversely, it's not as good at just keeping you straight).

[Read have Read]

Fins above the centre of mass will move the same direction as fins below the centre of mass. As an experiment, try moving those fins up to the first stage.

[DerekL1963]

Arrows are ballistic but their flight path is relatively short so it seems to an observer to be a straight line.

Only to the inexperienced observer or over very short ranges... In fact, historically speaking, bows were among the earliest artillery. The English longbowman, so feared of yore, was in fact was feared for his ability in large formations to rain down arrows on enemy formations at range as much as for than his accuracy in direct fire.

If you want to avoid the weight of fins, and want use the high thrust non vectoring engines like the LV-T30, you can try using the Rockomax 24-77 verniers.

Edited May 23, 2013 by DerekL1963

[Scrogdog]

Great stuff, folks! Thanks!

I'm going to experiment with some different placements and configs. You guys will make a bargain basement rocket scientist out of me yet!

[numerobis]

Fins above the centre of mass will move the same direction as fins below the centre of mass. As an experiment, try moving those fins up to the first stage.

Fins ahead of the center of mass are canards, which are unstable: when the aircraft gets a non-zero angle of attack, tail fins in the neutral position apply force to zero out the angle of attack, whereas neutral-position canards apply force to increase the angle of attack.

So you definitely don't want a delta-deluxe up top, you'd want the canard part or the other winglet that moves completely.

The other reason to avoid fins on upper stages is that they have so little effect past about 20 or 30 km, you're just carrying dead weight (unless you plan to return).

[magnemoe]

Only to the inexperienced observer or over very short ranges... In fact, historically speaking, bows were among the earliest artillery. The English longbowman, so feared of yore, was in fact was feared for his ability in large formations to rain down arrows on enemy formations at range as much as for than his accuracy in direct fire.

If you want to avoid the weight of fins, and want use the high thrust non vectoring engines like the LV-T30, you can try using the Rockomax 24-77 verniers.

True, longbow tactic was to start firing at long range using an ballistic path around 45 degree angle, fins was designed to make the arrow rotate.

However in KSP we use controllable fins, they can be placed anywhere, I usually add them at top of boosters. Benefit is that the drag after separation rotates the top of the booster outward.

Bottom placement rotate the bottom inwards where it might hit the core stage.

[drakesdoom]

Canards at the top will give you more control but will also make your rocket inherently unstable and tumble.

The trade off is advantageous for a computer controlled missile to deliver a payload onto a fast moving target. However if you want to hand fly a large rocket into space you will find that fins further back make you stable but with less control even to the point where straight is the only option.

Where you want your fins is up too you just be aware of the tradeoff you are engaged in.

[Godot]

Arrows are ballistic but their flight path is relatively short so it seems to an observer to be a straight line.

At 70 and 90m (which are the largest distances in regular tournaments) the fact that arrows are ballistic definitely becomes evident

As for fins ... I regularly use them in my rockets. They seem to help ASAS stabilize the rocket

Edited May 23, 2013 by Godot

[Scrogdog]

You guys rock!

Your great advice has lead to this modification. Essentially, I decreased the number of fins and added ones with better control ability. I also lowered them, so they are in about the same position relative to the second stage engines as the fins on the bottom stage. Makes all the sense in the world!

The bottom stage is "brute force", more or less. But now the middle stage has improved by quite a margin as I drop the bottom stage at about 12 kilometers. When number two takes over it is much more maneuverable now.

Great! Thanks again!

Edited May 23, 2013 by Scrogdog
Typo

[numerobis]

It's a pretty rocket. I'm so used to building overoptimized designs

[NeoMorph]

If you have a vehicle that goes wonky around Max Q I have found that four canards around a third of the way down works wonders.

[Scrogdog]

It's a pretty rocket. I'm so used to building overoptimized designs

Thanks!

I figured I was going to learn something about fins, and I did, but moreover, I was stunned at the effect that such a small modification had! The second stage requires a steady hand now; it's almost too maneuverable! Also, I was averaging an orbital insertion altitude of around 150km, now it's closer to 200 with the change! Now I can reasonably launch a heavier package.

Another thing I did was to make the hold down clamps its own event so I can release a split second after ignition. Seems to help with immediate off the pad stability because of momentum I guess.

I never wanted to be the guy who spent 3 times as much time in the VAB as he does flying, but even if that does not happen, I sure understand it!

[lazarus1024]

Only to the inexperienced observer or over very short ranges... In fact, historically speaking, bows were among the earliest artillery. The English longbowman, so feared of yore, was in fact was feared for his ability in large formations to rain down arrows on enemy formations at range as much as for than his accuracy in direct fire.

If you want to avoid the weight of fins, and want use the high thrust non vectoring engines like the LV-T30, you can try using the Rockomax 24-77 verniers.

Actually for both accuracy, penetration and range.

The English Yew Longbow plus English and Welsh longbowmen were rightly feared because of their long range, but also penetration and accuracy.

As described by contemporary sources as well as some reconstructions, period longbows were AWESOME (which we've been able to determine seemed to range from 100 to as much as possibly 200lbf drawstrength at 30". A modern longbow is more like 50-60lbf at 28" as we are weak and sissymen who don't train from an early age hundreds or thousands of hours per year). The best longbowmen could fire an arrow four hundred yards! Sources vary on typical range of a longbowman company in battle, but estimates are from around 270-350 yds. By comparison a modern longbow with a 60lbf 28" draw can send an arrow about 200yds.

Even at those long ranges a bodkin or broad arrow is generally effective at penetrating 3 or so inches through a jack coat (leather and padded coat typical of infantry without great means). A bodkin arrow would penetrate more than 2" through maile armor and would even be effective against poorly maintained steel plate or wrought iron armor. They'd also be capable of penetrating joints or eye holes of even the best steel plate armor...and keep in mind, thousands of arrows are being rained down on an army from (likely) several companies of longbowmen. So even the best armored, which would be VERY few, of an opposing army would stand a modest chance of taking an arrow in a joint, eyehole or other weakness in their armor. Let alone the vast majority of an army who would not be wearing the best armor.

Even at Againcourt where the charge on the English army (composed mostly of longbowmen) was composed almost exclusively of plate and mail armored French knights, most of the knights were dismounted by arrow fire and many were injured or killed by arrow fire (or the dismounting) and then slaughtered while mired in mud.

Some other modern tests show that the heaviest bows with the heaviest arrows and bodkin points ARE capable of penetrating even some of the better steel breast plates of the late medieval and early reniansance period at point blank range (average bows likely would only penetrate a very small amount, well less than an inch, meaning you MIGHT get hurt, but probably not at all as the point wouldn't penetrate far enough to punch through the padding you were wearing under the armor).

Then you factor in that the average longbowman was provided between 50-60 arrows during a battle; then that the sustained rate of fire (factoring fatigue and a long battle) was 6 arrows per minute and you have an awesome, awesome spectecal.

The average longbowman was expected to be able to hit a lone man some of the time at 200yds and an army all of the time. Lets not consider their accuracy at, say, 50yds (which was probably greater than a 95% chance of striking a man let alone a horse). Even figuring a typical sustained rate of fire a charging army on foot would take about 2 minutes to cross 300yds at a speed of around 8 feet per second (roughly 5mph. Probably about as fast as a full army could run in armor with weapons over open ground for nearly a quarter mile and have even a modecrum of cohesion). That means an army on foot would have to suffer at least 12 arrows per archer. If the force has a company of archers of only 100 men in it, that is 1,200 arrows. Figure an army of 3,000 might have 400 archers, which would be nearly 5,000 arrows. Then you also factor in that the archers will probably fire faster at first, especially seeing a charging army coming at them, they could probably fire off 20 arrows in those 2 minutes.

Even charging knights on horse back could be riddled. Figure knights on Destriers and full armor on level, unbroken ground might be able to charge at 25mph (which is probably on the high end) over 300yds. That still takes them close to 30 seconds to cover that ground, or at least 3 arrows per archer and since they'd probably be firing as fast as they possibly can, maybe 6-8 arrows per archer. I am a weak and sissyman (actually I am not), but using my 60lb oak longbow trying to fire as fast as I can in to a straw target at 40yds with my arrows point down in the dirt in front of me like an English longbowman would do during battle and firing accurately enough to put every arrow in to the target (not necessarily a bullseye, but accurate enough to hit a man sized target basically at 40yds) I can fire roughly 14-15 arrows in one minute, though admitedly my rate of fire does start dropping off from my arms getting tired after that. Even with heavier bows, I'd assume these guys with their life on the line and a decade or decades of training and stronger, even if using heavier bows could at least be able to manage a similar rate of fire in a short burst.

Yeowch.

Guns only won out for two reasons. One, they did penetrate better than arrows and as heavier armor became cheaper, they had an advantage as things like steel cuirasses started to become wide spread in mercenary armies during the reniassance. Next was training. A couple of hours could train a musketeer (not the book/movie kind. I mean the kind that used a musket) sufficient to load, fire and care for their weapon and a couple of weeks could make then rather proficient. It took years and years to train a good longbowman. Longbows continued to have a significantly better rate of fire, more accurate and longer ranged until the advent of rifling and continued to have the advantage rate of fire until the advent of breechlock rifles and repeaters in the mid 1800's (though longbows went out as a weapon of war in the early 1600s basically).

[NeoMorph]

The only downside with bows was having to fire them up into the air to get distance. If you have ever seen a master bowman aim, raise and pull all in one you just know how much skill it takes (none of this pulling, holding then aiming lark lol). The bow is under tension for such a short amount of time that you wonder if they aimed in the first place... and then the arrow comes down from the heaven like the proverbial bolt from the blue.

Falling from the sky also took out those knights because they came down into their necks a lot of the time. Was another reason they had extra shoulder armour I think. The power those arrows had would punch through mail right down to the heart. Sheeesh.

[Scrogdog]

Hmph. I feel like I should hand over my avid wargamer credentials for not knowing that stuff.

But then, I'm more a WWII guy anyway.

[Conarr]

On the advanced missile systems, the fins up front are stabilization and spin control. The fins in back are for steering. That particular rocket system has to make incredibly (relative to velocity) tight turns. In my experiments with fins in KSP, putting them at the top works pretty much in the same way it does for the missile - spin control. Just like for the missiles, you don't want it spinning like mad because it's hard to control. IMO

[lazarus1024]

The only downside with bows was having to fire them up into the air to get distance. If you have ever seen a master bowman aim, raise and pull all in one you just know how much skill it takes (none of this pulling, holding then aiming lark lol). The bow is under tension for such a short amount of time that you wonder if they aimed in the first place... and then the arrow comes down from the heaven like the proverbial bolt from the blue.

Falling from the sky also took out those knights because they came down into their necks a lot of the time. Was another reason they had extra shoulder armour I think. The power those arrows had would punch through mail right down to the heart. Sheeesh.

Very much so. At Againcourt the biggest deciding factors were weather and battlefield shape. The French knights effectively got funneled down toward the narrower front of the British army and the muddy ground both slowed horses and men once they were dismounted. Of course the English and Welsh longbowmen very much did their part, but if it had been an open dry battlefield the British probably would have lost in the end.

One could say similar things about many battles through history though "If just X were changed, the other side totally would have won".

Anyway, the vast majority of the French knights were killed or captured on the ground after being dismounted. A heck of a lot were captured. The Ransom a knight of even modest means could bring for a commoner longbowman was SUBSTANTIAL. With no horse riding experience I can't say from personal experience how nasty getting dismounted would be...but both watching modern jousting and then trying to imagine several hundred to a couple of thousand heavy horse all charging together and the effect that having your horse go out from under you...with all those OTHER horses charging right over/past you would have makes me cringe.

[pizzaoverhead]

That still takes them close to 30 seconds to cover that ground, or at least 3 arrows per archer and since they'd probably be firing as fast as they possibly can, maybe 6-8 arrows per archer.

It's possible that they could be firing considerably faster than that for short ranges:

Correct aerofoil placement is a good skill to learn and experiment with, especially when you're making spaceplanes! So many craft torn apart by spins at 20km... Always pack an escape system!

[lazarus1024]

Also a technical point, watch an arrow in slow motion video. It is NOT a straight path really, even over or especially at short distances. Ignoring the ballistic piece of the puzzle, an arrow off a bow (not a crossbow, but a bow) wobbles like a drunken sailor, especially over the first few dozen yards. The wobble does tend to straighten out the further the flight path. Of course it is wobbling back and forth along the same general trajectory, it isn't like it is weaving like a drunk sailor. However, at just a yard or two out from the bow, the arrow head (okay, whole thing in a sine pattern) is probably traversing back and forth over a 1-2" oscillation that dampens with time/range. The fletcheting of the arrow (the feathers) helps to stabalize the arrow by inducing spin along its flight.

It is the arm of the bow that introduces the wobble as the string is basically pushing the arrow straight ahead, but the arrow body "curves" around the arm of the bow and resumes the straight trajectory on the other side of the arm. You do not want arrows that are too rigid as they would have poorer accuracy (why solid steel arrows would be a bad choice, ignoring the weight factor for a minute).

[Catullus]

That AIM-9X missile has thrust vectoring for steering, too. You can see the ports just behind the front fins.

[NeoMorph]

Also a technical point, watch an arrow in slow motion video. It is NOT a straight path really, even over or especially at short distances. Ignoring the ballistic piece of the puzzle, an arrow off a bow (not a crossbow, but a bow) wobbles like a drunken sailor, especially over the first few dozen yards. The wobble does tend to straighten out the further the flight path. Of course it is wobbling back and forth along the same general trajectory, it isn't like it is weaving like a drunk sailor. However, at just a yard or two out from the bow, the arrow head (okay, whole thing in a sine pattern) is probably traversing back and forth over a 1-2" oscillation that dampens with time/range. The fletcheting of the arrow (the feathers) helps to stabalize the arrow by inducing spin along its flight.

It is the arm of the bow that introduces the wobble as the string is basically pushing the arrow straight ahead, but the arrow body "curves" around the arm of the bow and resumes the straight trajectory on the other side of the arm. You do not want arrows that are too rigid as they would have poorer accuracy (why solid steel arrows would be a bad choice, ignoring the weight factor for a minute).

That all depends on the fletching and the quality of the body of the arrow. Of course in the old days they didn't have carbon fibre so having a perfectly straight body was near impossible.

With regard to fins and KSP though, each rocket is going to have its own flight characteristics. I've learned not to overpower my launchers so that they are more stable but even some of them need those 1/3 way down stabilising canards or fins. I do hate them on my final payload though so if I do have to use them I fit them on detatchment manifold and blow them off before I circularize my orbit. That way they don't get left in orbit.