FINLESS ROCKET DEBATE why does the Aries1-x have no fins?

[ScrapDaddy]

The Aries1-x has no fins so why cant we do the same with our models? Iv read many articals about this concept and the most practical solution is to add a huge nose weight to a dr.zooch Aries 1-x and seeing where that goes so what do u this g

 

[TWRackers]

The big rockets (anything bigger than sounding rockets) don't need fins because they have active stabilization by steering the thrust. Usually this is done by gimballing (tilting) the engine(s), although it could be done with smaller vernier engines and/or varying thrust. The Atlas Mercury rockets had a pair of very obvious verniers mounted opposite each other just above the main engines and pointing downward at maybe 30-ish degrees from vertical. The Space Shuttle stack has gimballing on all five engines during the boost (3 main engines in orbiter and both solid rocket boosters). The Ares-1X first stage is a stretched Shuttle SRB and has the same gimballing action.

Since we don't have gimballed engines, we have to rely on something else for stabilization, which is almost always aerodynamics. In most cases that means fins, although some rockets like cones, saucers, and pyramids rely on drag stabilization. There have been a few that tried to go finless with spin stabilization, and even at least one model rocket actually had a gimballed motor mount.

 

[ScrapDaddy]

Buy wouldn't adding a large nose weight solve the problEm?

 

[The EGE]

In theory, yes. It would produce a statically-stable rocket.

However, it would also be a ridiculously heavy rocket. It also would not have the corrective force of fins, so any imbalance or wind could create problems.

That said, it's almost midnight and I've been awake 32 of the last 36 hours. I could be totally wrong here.

 

[BobH48]

I have a couple of models that I fly with no fins.

The strap on boosters provide enough drag along with a lot of nose weight to keep them stable.


They are paper models so even with the nose weight they can easily be flown with B6-4's

 

[Adrian A]

In theory, yes. It would produce a statically-stable rocket.

However, it would also be a ridiculously heavy rocket. It also would not have the corrective force of fins, so any imbalance or wind could create problems.

That said, it's almost midnight and I've been awake 32 of the last 36 hours. I could be totally wrong here.

The problem is that when you fly a cylinder with a nosecone, the airflow separates from the body near the nose even at a small angle of attack, so the center of pressure moves a lot farther forward than you would expect. It's nowhere near the geometric center of the rocket. A friend of mine demonstrated this (unintentionally) by flying a long, skinny, finless rocket with a very heavy nosecone. The result was a 90 degree turn shortly after leaving the pad. Unless you're using spin stabilization or active thrust vector control, there's really no amount of noseweight that will make a purely cylindrical rocket safe. The simulated solid strap-on boosters in the above examples are effective enough at acting like fins, that the rockets fly o.k.

 

[ScrapDaddy]

I'll run it through rocksim

 

[billspad]

Going in the other direction, why do the Saturns other than the Block 1 have fins? They all have active steering. The only explanation I've ever heard had something to do with keeping it going straight long enough for the escape system to fire if all the engines failed and I'm not sure I buy that. The only reason I can think of is that they make much cooler models with fins but I doubt that was a consideration.

 

[ScrapDaddy]

What does static stability mean

 

[WillMarchant]

Issue #248 at https://www.apogeerockets.com/Peak-of-Flight_index.asp#stability deals specifically with the lack of fins on Aries.

 

[WillMarchant]

Going in the other direction, why do the Saturns other than the Block 1 have fins? They all have active steering. The only explanation I've ever heard had something to do with keeping it going straight long enough for the escape system to fire if all the engines failed and I'm not sure I buy that. The only reason I can think of is that they make much cooler models with fins but I doubt that was a consideration.

I'd read an explanation and spent some time searching this morning without luck.

But I seem to recall that von Braun "thought a moon rocket should have fins." 8)

From an engineering perspective you may add fins to reduce the load on your thrust vector system. Fins add mass and testing complexity, of course. But you may get a payoff by reducing the capability and complexity of your main engine gimbal systems.

The engines on the side of the Atlas are verniers for performing fine control of speed.

 

[Stymye]

Buy wouldn't adding a large nose weight solve the problEm?

It does help to a degree,by keeping the nose forward, but thats about it.
Compare it to a A bottle rocket that has no fins and a heavy nose.

The problem is the flight path is totally unpredictable.The slightest puff of wind or variation in the nozzle will have a bigger influence than you might think.

 

[ScrapDaddy]

So I shouldn't waste my time trying?

 

[The EGE]

I wouldn't try to make a weight-stabilized finless rocket, no.

However, if you want to, try the Estes TAO. It's a 2-staged rocket with a finless spin-stabilized upper stage.

Plans from JimZ

 

[Commonwealth.Net]

The world's oldest rocket uses no fins!

A bottle rocket!

Unless you count the stick as a fin.

 

[The EGE]

True, true. I have a stick-finned rocket that flies on A10s. Always fun.

 

[MarkII]

Yes, but isn't a stick stabilizer, like those in Chinese Fire Arrows, British Congreve rockets and modern bottle rockets, also a type of aerodynamic stabilizer? Although it is not technically a fin, a rocket that possesses one could not be considered to be "finless" as that term is typically used.

Don't forget the Hale rockets of the 19th century. They were finless but they achieved stability with vectoring nozzles that caused the rocket to spin.

MarkII

 

[n3tjm]

Going in the other direction, why do the Saturns other than the Block 1 have fins? They all have active steering. The only explanation I've ever heard had something to do with keeping it going straight long enough for the escape system to fire if all the engines failed and I'm not sure I buy that. The only reason I can think of is that they make much cooler models with fins but I doubt that was a consideration.

Not sure the main reason is this, but one function was to keep the rocket anchored to the launch pad for stability. Each fin was bolted to the pad and after ignition they blew the bolts allowing the rocket to take off.

 

[WillMarchant]

Not sure the main reason is this, but one function was to keep the rocket anchored to the launch pad for stability. Each fin was bolted to the pad and after ignition they blew the bolts allowing the rocket to take off.

On the Saturn 5 there were four "hold down" arms that attached to the thrust ring. They went to the main cylindrical body of the rocket in between the shrouds for the F-1 engines.

 

[ScrapDaddy]

How do you create a gimblized mchanism for rockets

 

[JAL3]

How do you create a gimblized mchanism for rockets

I don't know how he did it, but Dave Hein built one and there's a thread about it around here someplace.

Search for Quad Pod

 

[billspad]

I'd read an explanation and spent some time searching this morning without luck.

Me too. There were several ideas but nothing that sounded like it came from anybody involved with the design.

But I seem to recall that von Braun "thought a moon rocket should have fins." 8)

Just about everything that Von Braun was involved with had fins. I think deep down he was a modeler and couldn't bring himself to design anything that couldn't be flown as a scale model.

 

[luke strawwalker]

Going in the other direction, why do the Saturns other than the Block 1 have fins? They all have active steering. The only explanation I've ever heard had something to do with keeping it going straight long enough for the escape system to fire if all the engines failed and I'm not sure I buy that. The only reason I can think of is that they make much cooler models with fins but I doubt that was a consideration.

Had something to do with relatively low speed stability as well... I've read that they put the fins on the Saturns to enhance stability from basically just past the tower until around Max-Q to minimize the engine gimballing, since more than one rocket was lost to a gimbal failure (hard over). I don't know that I totally buy that either, but you're right, it DOES make for cooler looking rockets, and sure makes life easier on us doesn't it?? (despite the fins usually being 'undersized' for our purposes).

I guess they had fins for the same reason CARS had fins in the late 50's... :roll::roll::roll:

OL JR

 

[luke strawwalker]

What does static stability mean

Static stability is the relationship of the CG and CP when the rocket is sitting still... ready for launch, with the parachute and motor(s) loaded and ready to go. It is generally considered to be considered 'safe' if the CG is AHEAD of the CP by at least one body diameter (one caliber stability). Usually stability of up to three calibers is better (safer) but going much over that and you start running into 'overstable' problems (which usually just rob altitude but can have bad effects like 'corkscrewing' and 'wobbling' in flight).

Now, the problem is, neither CG NOR CP are CONSTANTS!!! As the propellant burns, the rear of the rocket is getting lighter, so that moves the CG forward. With liquid propellant rockets, the opposite is true-- as the propellant burns, the TOP of the propellant tanks gets lighter, so the CG moves backwards, but this doesn't really apply to us modellers so we'll just keep that in the trivia dept.

Now, the CG moving forward helps to increase stability during flight, but the CG generally doesn't move a WHOLE lot... If you want to see the effect, balance the rocket in a loop of string at the CG with a loaded motor, remove the motor and install a spent casing, and rebalance-- that's how much the CG moves and where the CG is at the instant of ejection.

CP, on the other hand, is a VERY dynamic 'point' and moves around considerably more. Generally speaking, CP moves forward as the angle of attack increases. In other words, once the rocket starts to 'tip over' in flight, the CP moves forward more and more, which can put it ahead of the CG pretty quickly. The CP moving forward means that there is less and less corrective force to straighten the flight path back to 'normal' and if the CP passes the CG, the rocket INSTANTLY goes unstable because the corrective forces are now pulling the rocket OFF COURSE. Any wind at liftoff artificially creates an 'angle of attack' since the body of air surrounding the rocket is moving perpendicular to the rocket as it sits on the pad, and when the rocket is travelling at precisely the wind's speed, the 'effective angle of attack' is 45 degrees. As the rocket accelerates, this 'effective angle of attack' becomes less and less, since the rocket is moving upward MUCH faster than the wind is blowing sideways. This is why rockets weathercock and some marginally stable designs can go unstable in higher wind conditions, and overstable rockets can weathercock excessively, and why a higher power motor that really "kicks" a rocket off the pad at high speed reduces weathercock.

Now, since CP moves around, how do we know if the CP we calculated (or got from Rocksim) is correct?? When in flight is it correct?? That depends on the method used to calculate it, and the assumptions inherent in the method used. The Barrowman and "Rocksim" methods calculate the 'static' CP for a rocket sitting on the pad and moving in a more or less 'straight up' (low angle of attack) direction. SO, when you actually FLY the rocket, if it gets rod whip, or a sudden gust of wind at liftoff when the speed is slow causes high induced angle of attack, bad things CAN happen. The "cardboard cutout" method, on the other hand, 'calculates' CP as "the center of lateral area" (balance point of the area that would be exposed to the airflow if the rocket WERE FLYING NINETY DEGREES TO THE AIRFLOW (sideways). This is the MOST conservative method, since it will show the CP as far forward as it's basically possible for it to go! So it would be the best method to use for a finless rocket. It does tend to be 'overly conservative' for finned rockets though, because fins 'flat' structure create lift differently than the round body tube does at high angles of attack, but then you REALLY get into some deep math, as Barrowman (and Rocksim AFAIK) ignore the effects of body tube lift from the cylindrical tube moving through the airflow at an angle of attack, and the effects of fins 'stalling' at high angles of attack (which moves the CP forward).

All this helps explain why rockets sometimes do some of the crazy things they do in flight, like sometimes 'mysteriously' going unstable, yet then becoming 'stable' again, sometimes pointed in directions we'd rather they didn't.

Hope this helps! OL JR

 

[luke strawwalker]

Not sure the main reason is this, but one function was to keep the rocket anchored to the launch pad for stability. Each fin was bolted to the pad and after ignition they blew the bolts allowing the rocket to take off.

Hold-downs weren't on the fins per se... on Saturn I's/IB's they were on struts coming from the fin roots areas, but mechanically they were connected to the thrust structure at the base of the rocket which was actually supporting the weight. Saturn V's hold-downs were at the perimeter of the thrust structure well away from the fins.

Hold-downs kept the rocket immobile as the thrust built up. At some point, the thrust would build up until it was exactly the same as the weight of the rocket, and without hold-downs, the rocket could 'float' off the pad and go in unpredictable directions, which was highly undesirable. Hold downs kept the rocket immobile until the thrust built up to the desired thrust to weight ratio for liftoff, so when the hold-downs were released, the rocket would immediately begin accelerating UP and away from the pad and tower.

They also supported the incredibly massive weight of the rocket before liftoff when filled with hundreds of tons of liquid fuel... OL JR

 

[luke strawwalker]

How do you create a gimblized mchanism for rockets

There are some threads around that deal with it. It's rather involved to say the least.

I've seen John Pursley's gimballed motor rockets, and they're VERY neat! He has a large scale Vanguard (flopnik) with a gimballed engine and NO fins at all, and his Mercury Redstone with scale fins on it and a gimballed engine that he flew at NARAM years ago.

In both cases, he uses a model airplane anti-crash 'autopilot' system with four horizon sensors mounted in a ring 90 degrees apart, looking out from the nosecone of the rocket, and feeding their input to a control box which then steers a pair of servos 90 degrees apart in the gimbal to correctively steer the engine. It's a neat system.

"Real" rockets typically use guidance platforms with gyroscopes, but that's not really feasible for hobby use, so John tells me. Model gyros used in helicopters and stuff 'drift' and won't really work right.

Note also, the rocket MUST have SOME inherent stability, as the thrust vector control system (gimbals) don't work when you don't have THRUST, so you need 'inherent' stability to control the rocket during coast phase when you have no thrust for the gimbal to use to correct the rocket's path. The Vanguard uses it's larger lower stage/smaller upperstage inherent stability for this, and the Mercury Redstone of course has the small scale fins. A "straight tube" rocket (like say a Titan II) would need significant noseweight to create this 'inherent stability' to maintain course after burnout. Luckily the weight of the sensors/control box contribute to this (but are offset by the weight of the servos and gimbal!)

Later! OL JR

 

[bobkrech]

The Ares-1x has no fins so why can't we do the same with our models? Iv read many articles about this concept and the most practical solution is to add a huge nose weight to a dr.zooch Aries 1-x and seeing where that goes so what do u this g

Let's make it really simple.

There are two flight regimes of concern: A.) Initial lift-off before the LV have sufficient velocity to achieve aerodynamic stability; and B.) Flight after achieving sufficient velocity to attain aerodynamic stability.


A.) When at velocities below which aerodynamic stability is not possible.

1. Large finless rockets are bolted to the pad and are lift-off only after their engines have obtained full power. All of these rockets use some form of thrust vectoring to keep the rocket ascending vertically or on the desired trajectory. In other words, it can't fall over because the guidance system automatically vectors the thrust in the opposite direction of the deviation.
2. Smaller unguided rockets are launched on guide rails just like model rockets. They are not bolted down but they don't leave the rail until they have obtained a velocity where aerodynamic stabilizing forces come into play, just like model rocket.
3. Guns fire bullets without fins, and leave the barrel at very high speed. Bullets are aerodynamically stable because of their CG/CP relationship, their shape and their high spin rate. More later on spin.

B.) When above velocities where aerodynamic stability is possible.

1. Large finless rockets use thrust vectoring to keep the rocket on course. If they did not have thrust vectoring, any unsymmetrical aerodynamic force or off-axis thrust vector would cause the rocket to veer off course and crash. In other words, it won't fall off course because the guidance system automatically vectors the thrust in the opposite direction of the deviation.
2. Once smaller unguided rockets achieve velocity sufficient for attaining aerodynamic stability, their fins keep them going into the apparent wind. This works really well if the thrust vector is centered on the aerodynamic axis, but to balance out off-axis thrust, aerodynamic forces, and imperfect CG location, virtually all unguided rocket use spin fins to rotate the rocket at several RPS to minimize ballistic trajectory deviations.
3. Guns fire bullets without fins. Bullets are aerodynamically stable because of their CG/CP relationship, and their shape. Gun barrels are rifled to impart spin to the bullet. This is done because no bullet is perfect either aerodynamically or CG wise. Unless a bullet has fins, without spin it would begin to tumble immediately after leaving the barrel due to manufacturing imperfections.

Model rockets are not aerodynamically perfect, nor are they balanced so that the CG is on all 3 axises, nor is the thrust vector of the motor perfectly centered, and the wind velocity is never 0. All of these defects conspire again a straight flight path. If you were able to spin you rocket at several hundred RPS you could make it stable like a bullet, and that would be fine until you went to deploy your parachute, which would instantly wrap itself up and your rocket would crash.

Hope this simple explanation clarifies why unguided rocket need fins, or some other aerodynamic means such a tube fins, ring fins, aft mounted boosters, or even a long stick or lack fins, a high spin rate, to maintain trajectory stability.

Bob

 

[luke strawwalker]

Also one other thought occurred...

The Ares I is a TERRIBLE design to try to make finless... the larger diameter upperstage and smaller diameter lowerstage INSTANTLY moves the CP VERY far forward (see cardboard cutout method).

Rockets with larger LOWER stages are more amenable to finless stability because the lower stage is larger in diameter. Still not an easy proposition, but EASIER.

John Pursley's Vanguard was weighted and balanced so that basically it was stable without fins. The larger lower stage helps considerably with this.

Ares I, being a "hammerhead" rocket, actually works AGAINST this.

Later! OL JR

 

[Dr.Zooch]

I can answer 100% for the early and later Saturns- I've done a ton of research in that area.

First off- if ever you hear anyone who says the Saturn V did not actually need fins- they were just there for decoration- give them a big wet phbbbbbbt! right in the face. That's a myth. Keep in mind that all through the Saturn V program weight was a huge issue- right up to the point where they were actually reducing the number of bandaids in the first aid kit- so don't you think they'd have removed the fins if they were "just for decoration?"

In fact, the fins played a huge part in a specific segment of first stage boost if a manual abort had been required. In the SA-2 and SA-3 flights it was discovered that in this given mode of the flight, some abort required circumstances (such as an engine hard-over for example) would toss a finless booster to the point where the dynamic forces on the crew would have kept them from actually, physically, turning the abort handle. So to provide the needed few moments of extra stability needed, fins were added onto the Saturn I, Block II. Likewise the same was true in the Saturn IB and the Saturn V.

 

[billspad]

I can answer 100% for the early and later Saturns- I've done a ton of research in that area.

First off- if ever you hear anyone who says the Saturn V did not actually need fins- they were just there for decoration- give them a big wet phbbbbbbt! right in the face. That's a myth. Keep in mind that all through the Saturn V program weight was a huge issue- right up to the point where they were actually reducing the number of bandaids in the first aid kit- so don't you think they'd have removed the fins if they were "just for decoration?"

In fact, the fins played a huge part in a specific segment of first stage boost if a manual abort had been required. In the SA-2 and SA-3 flights it was discovered that in this given mode of the flight, some abort required circumstances (such as an engine hard-over for example) would toss a finless booster to the point where the dynamic forces on the crew would have kept them from actually, physically, turning the abort handle. So to provide the needed few moments of extra stability needed, fins were added onto the Saturn I, Block II. Likewise the same was true in the Saturn IB and the Saturn V.

Thanks. I find your explanation easy to believe. One of the articles I read mentioned that they were considering using it as a heavy lift booster without the fins. No escape system, no astronauts, no fins.