Forward Fins

[nyancatrules]

We are a rocketry team competing in the Team America Rocketry Challenge. We are in need of advice on forward fins. The rules this year require two eggs, we put the eggs in the front of the rocket. I'm sure you know this pushes the CG forward. To push the CP back up to ratio with the CG we were thinking about adding forward fins. We have read that forward fins are not that stable, but with the extra forward weight would the rocket become more stable? Thank you in advance for the help!

 

[shreadvector]

DO NOT ADD FORWARD FINS!!!!!!!!!

Trip barber has posted about this in the past on the TARC YahooGroup. We had lots of reports and I personally saw many TARC models built with this faulty logic.

There is nothing dangerous about an "overstable" rocket unless you have underpowered it andf then launch it in a large crosswind. As long as it leaves the launch rod at a safe speed (much faster than any cross wind), then it will not "weathercock" and it will fly pretty much exactly where the launch rod is pointed. With slow speed off the rod, it will rotate and point into the wind like a weathervane, hence the term "weathercocking".

If you add forward fins, the rocket will almost definitely fly all over the sky as any tiny misalignment will cause them to 'steer' the rocket in random directions and it will skywrite across the sky.

Making the rear fins smaller is another faulty logic approach as the entire rocket will then tend to wobble and wiggle on the way up since the tiny fins will need to move (along with the aft end of the rocket) out into the airstream before they produce a corecting force to push the aft end of the rocket backwards.

Big fins sticking a decent amount out into the airstream (ouot of the turbulent boundary layer) combined with a motor that can acellerate the rocket off the rod quickly are the way to go for safety sake.

These tips were publicly given by Trip in past TARC competitions and are not 'designing the rocket for you'. These tips are preventing safety problems and massive wastes of precious time and money that most TARC teams cannot afford to squander.

 

[shreadvector]

https://groups.yahoo.com/group/NARTARC

https://groups.yahoo.com/group/NARTARC

https://groups.yahoo.com/group/NARTARC

 

[powderburner]

..... with the extra forward weight would the rocket become more stable?

With added forward weight, yes, the rocket becomes more stable.

With added forward fin area, no, the rocket becomes less stable (as has already been made abundantly clear in the other posts).

It sounds to me like you and your team desparately need to get the Barrowman stability equations (they are free, online) and work through them by hand. You seem plenty smart enough to run through the calcs if you are thinking about entering the contest. YOU DO NOT NEED A COMPUTER to do this, although after running the calcs once or twice to see what is going on, THEN it might be helpful to program an EXCEL spreadsheet to make things a little easier (reduce the number crunching).

If you run through Barrowman, you should quickly see the stability effect of adding fin area in the aft end of the rocket. The stability effect of adding weight in the front end is a different calc but you probably have already had classwork in how to sum up component weights to get a total center of gravity.

If you have more qstns you can come on back here, we'll try not to poke too much fun at you, but we will make sure you grab the basics. In the mean time, check out:
https://my.execpc.com/~culp/rockets/Barrowman.html

 

[bjphoenix]

It is possible to add forward fins and still fly OK- there are lots of scale models of military missiles with forward fins, but you definitely have to be careful that you still have the proper stability margin. And as mentioned above if you want to reduce the amount of overstability you can just reduce the sie of the rear fins.

There is no harm in being overly stable except for the aforementioned wind problem, and if you adjusted your fins to have a lower stability margin when loaded with a payload, the rocket might be unstable if you try to launch it without the payload.

 

[luke strawwalker]

IMHO, this is adding another complication to an already difficult task.

You need to fly your payload and get it back intact, to the designated altitude or time "target".

IMHO the simplest and most straightforward design with the most robust margins is the one most likely to achieve the desired goal. Overcomplication simply adds another layer of possible failure or problems resulting from unforeseen circumstances messing up the flight.

Design your rocket, keep it simple, fly in various conditions, keep scrupulous records of as many parameters as you can (weather, wind, engines, launcher angles, weights, etc.) and get the data and experience you need to 'dial in' your launches to hit the desired targets as closely as possible.

That's the point of the exercise and IMHO the most likely to win... focus on the GOALS, not the design to achieve the goals...

Later and good luck! OL JR

 

[JStarStar]

This seems to be a "solution" in response to a barely-existent (if at all) problem.

Adding forward fins also increases drag by increasing overall surface area, lateral cross-section, surface joint boundary turbulence, and unnecessarily increases overall vehicle weight with fins you don't need.

The problems with potential fin misalignment have also been noted. Even slight misalignment between forward/rear fins can cause major problems.

In addition, most military missiles or sounding rockets with substantial forward fins have active guidance systems which allow the vehicle to correct destabilizing motions in flight which your TARC rocket likely will not.

A good piece of advice as others also note, is to stick to the KISS principle. If you want something to make your rocket stand out, give it a really kick-butt paint job.

Although TARC, of course, is not a beauty contest -- it is a goal-based competition. Come closest to hitting the flight objectives, you win. Whether you or anybody else thinks your rocket looks cool or not is incidental (although it can be fun).

 

[powderburner]

It is possible to add forward fins and still fly OK- there are lots of scale models of military missiles with forward fins, but you definitely have to be careful that you still have the proper stability margin.

You're right, it is possible, but the part you left out of your advice is that if you add fixed forward fins you will have to add even larger aft fixed fins to make up for it. There is no beneficial purpose in a model rocket to adding fixed forward fins, unless you are building a 'scale' model.

The military missile designs with forward fins are an entirely different animal. On those missiles, the forward fins are moveable and steered and actively controlled. Do not confuse those designs with our model rockets, it simply does not work the same way for our stuff.

 

[GDJ]

Question:

Are the rockets a fixed predetermined design or is the design of the rockets completely free, or are you able to only pick from already manufactured kits?

 

[luke strawwalker]

Question:

Are the rockets a fixed predetermined design or is the design of the rockets completely free, or are you able to only pick from already manufactured kits?

For TARC, the goals are to carry a certain payload to as close as possible to a specific altitude with as close as possible to a specific flight duration. There have been limitations on the recovery device (streamer versus parachute) and on the motor size, but other than that it's a pretty open field. The idea is to design as reliable and predictable a rocket as possible to perform the mission successfully. The eggs have to be flown and recovered uncracked, and points are taken off for every foot of altitude above OR below the target altitude and points are also deducted for every second less OR more than the specified target flight duration from liftoff to touchdown. The team with the highest score wins. The rocket design itself isn't "judged" other than in the ability to hit all the "target" requirements as closely as possible and still deliver the payload successfully.

Since design criteria are pretty open, good design parameters are to focus on reliably achieving the technical requirements in the most straightforward way possible, minimizing 'complicating factors' which can lead to failures in any of the particular flight regimes and accounting for environmental factors that can affect the final judged flights such as wind and other weather factors, etc. Anything extra that doesn't substantially contribute to achieving the goals as stated or improving reliability or simplifying operations is just another thing that can go wrong and possibly lead to failure to achieve any one of the specific goals or lead to complete mission failure or disqualification... (for instance, parachute entanglement with the forward fins leading to a hard landing and cracking the eggs, or failure to deploy the chute and a crash that leads to a DQ, or misalignment of the forward fins or undesired flight effects from the forward fins in differing conditions at the contest caused by wind or weather leading to an erratic flight or crash due to the different conditions).

That's why the KISS principle really pays dividends here... reliability and repeatability is the name of the game...

Later! OL JR

PS. I'm not part of TARC but I've read quite a bit about it and observed some test flights along with our club senior advisor who was mentoring some teams a couple years ago...

 

[RandyT0001]

For TARC, .... eggs have to be flown and recovered uncracked, and points are taken off for every foot of altitude above OR below the target altitude and points are also deducted for every second less OR more than the specified target flight duration from liftoff to touchdown. The team with the highest score wins.

Actually the team with the lowest point total wins. Points are added for every foot above or below the target altitude and for time over or short of the specified time range.

Canada should start it's own TARC event. Right now there are similar competitions in the UK, France, and Japan. I think that each of those country's winning team (plus the US team) will meet at the international airshow in Europe for the international competition launch.

 

[GDJ]

Okay, so to do this, we need to know the mass of the 2 eggs plus pick a engine that has a low enough thrust to keep the G-forces down to a reasonable level, but have enough thrust/burntime to have a good flight, right?

 

[RandyT0001]

Okay, so to do this, we need to know the mass of the 2 eggs plus pick a engine that has a low enough thrust to keep the G-forces down to a reasonable level, but have enough thrust/burntime to have a good flight, right?

You can go to www.rocketcontest.org and download the 2012 rules to see the contest goals, specifications, and limitations. For example each egg has to have a mass from 57 to 63 grams, inclusive.

 

[JStarStar]

Okay, so to do this, we need to know the mass of the 2 eggs plus pick a engine that has a low enough thrust to keep the G-forces down to a reasonable level, but have enough thrust/burntime to have a good flight, right?

If your eggs are securely packed and effectively padded so they cannot rattle around inside the payload compartment, fracturing the eggs on launch is not real common unless you were to use a massively high-thrust motor (probably not compatible with the altitude objective).

Most egg-scrambling (not involving an overall vehicle crash) is recovery-system related, at least in the egglofting contests I have seen or been in.

For design and planning purposes you should probably assume the eggs will be 63 grams since the eggs are issued by contest organizers and you have no control whether you get a 57 or a 63.

 

[GDJ]

Interesting competition. Almost tempted to try this solo just for s**ts and giggles.

 

[luke strawwalker]

If your eggs are securely packed and effectively padded so they cannot rattle around inside the payload compartment, fracturing the eggs on launch is not real common unless you were to use a massively high-thrust motor (probably not compatible with the altitude objective).

Most egg-scrambling (not involving an overall vehicle crash) is recovery-system related, at least in the egglofting contests I have seen or been in.

For design and planning purposes you should probably assume the eggs will be 63 grams since the eggs are issued by contest organizers and you have no control whether you get a 57 or a 63.

Plus you can always add the necessary amount of ballast (sand or extra packing materials or whatever) to account for weight deficiencies in the actual eggs... Say you get a 62 gram egg and a 58 gram egg, you'd add 6 grams of ballast to make up for the difference between those eggs' combined weight of 120 grams and the maximum egg weight of 126 grams...

Just another way to minimize differentiation...

Later! OL JR

 

[GDJ]

Hey! What's the target apogee for this competition? I'm asking because a dead-stock Magnum Sport Loader has enough room for two eggs @ 60 grams each, and has a apogee of 510 feet on two C6-5 engines. A E9 version can do just shy of 1100 feet with a E9-6 engine.

 

[JStarStar]

Hey! What's the target apogee for this competition? I'm asking because a dead-stock Magnum Sport Loader has enough room for two eggs @ 60 grams each, and has a apogee of 510 feet on two C6-5 engines. A E9 version can do just shy of 1100 feet with a E9-6 engine.

I believe the contest calls for 800 feet for the target altitude.

I also believe you are not supposed to use a stock kit "with the only modification being the addition of an altimeter compartment." This could raise some interesting disputes as to what degrees of kit-bashing are necessary (or allowable) to fulfill this requirement.



Here's the link with the complete rules:

-- Max 650 grams gross weight at launch

-- One stage only

-- "F" motor maximum -- total impulse limitation 80 n-sec

-- No pyrotechnic charges other than those contained in the motor

-- Altittude to be determined by use of approved altimeter (list included)

-- Section of rocket containing eggs and altimeter must be recovered by parachute only.

-- Other sections may use other recovery methods but must not fall freely

-- Payload: 2 eggs as discussed above. Must be recovered without damage.

-- Duration scoring: Target duration range is 43-47 seconds; point penalties to be deducted for deviations above or below this range

-- Altitude scoring: 800 feet (244 meters) as discussed above. Penalty points for deviation above and below the target

-- No external (i.e. R/C) electronic controls. Any onboard flight control systems must be self-contained, also may not use any kind of pyro charges as noted above.



There are a lot of additional rules regarding qualification flights, return of vehicle, etc etc, but those are the main ones that have to do with how the rocket should be designed.


https://rocketcontest.org/pdf/2012_rules.pdf

 

[GDJ]

I believe the contest calls for 800 feet for the target altitude.

I also believe you are not supposed to use a stock kit "with the only modification being the addition of an altimeter compartment." This could raise some interesting disputes as to what degrees of kit-bashing are necessary (or allowable) to fulfill this requirement.



Here's the link with the complete rules:

-- Max 650 grams gross weight at launch

-- One stage only

-- "F" motor maximum -- total impulse limitation 80 n-sec

-- No pyrotechnic charges other than those contained in the motor

-- Altittude to be determined by use of approved altimeter (list included)

-- Section of rocket containing eggs and altimeter must be recovered by parachute only.

-- Other sections may use other recovery methods but must not fall freely

-- Payload: 2 eggs as discussed above. Must be recovered without damage.

-- Duration scoring: Target duration range is 43-47 seconds; point penalties to be deducted for deviations above or below this range

-- Altitude scoring: 800 feet (244 meters) as discussed above. Penalty points for deviation above and below the target

-- No external (i.e. R/C) electronic controls. Any onboard flight control systems must be self-contained, also may not use any kind of pyro charges as noted above.



There are a lot of additional rules regarding qualification flights, return of vehicle, etc etc, but those are the main ones that have to do with how the rocket should be designed.


https://rocketcontest.org/pdf/2012_rules.pdf

Hmm.....so the Magnum modded with a D engine mount and a slightly longer bodytube (like the one I built) might pass?
If I read the rules correctly, the egg capsule that the Magnum uses can be purchased from Quest as a separate off the shelf item, therefore a version of the Magnum would be in compliance.

 

[nyancatrules]

Thanks everyone for your help, any suggestions on what we can do? what has worked in the past for you all.

 

[RandyT0001]

Thanks everyone for your help, any suggestions on what we can do? what has worked in the past for you all.

Suggestions for solutions to what problem? Reading your original post it seems that you are afraid that you will have a very large static margin that will lead to weathercocking. Have you designed the rocket yet? If so, what is the static margin? Once we know that we can determine if your rocket is prone to severe weathercocking.

 

[JStarStar]

Hmm.....so the Magnum modded with a D engine mount and a slightly longer bodytube (like the one I built) might pass?
If I read the rules correctly, the egg capsule that the Magnum uses can be purchased from Quest as a separate off the shelf item, therefore a version of the Magnum would be in compliance.

Well, I am not a contest judge but it certainly sounds like you would be doing more significant modifications to the rocket than simply adding an altimeter section, so in my view it would be OK.

A separate D motor mount and longer body tube are fairly significant changes, so IMO it would qualify as sufficient to distinguish it from the stock kit.

I think the motivation for the rule is just so that people don't just go out and buy kits and launch those -- to require some original design work by contestants.

But ... judges can be funny people sometimes so if you were actually going to fly in a contest I would contact one of the judges before you put glue to paper to make sure you wouldn't get red-flagged at the check-in table.

If you're doing it solo just as a fun project, of course you can play as fast and loose with the rules as you want.

But the way I read the rules, you should be OK using a kit as long as you do MORE to modify it than simply add altimeter compartment. If they wanted to ban all kits from the competition completely, they would have said so.

 

[powderburner]

I think the motivation for the rule is just so that people don't just go out and buy kits and launch those -- to require some original design work by contestants.

You probably don't even need to go buy another sheet of balsa (or basswood), you could very likely find enough leftover acreage on the finstock that comes with the kit to add a little strake at the leading edge of the root, or at the trailing edge. Also, you could clip the fin tips at a new angle. It wouldn't take much to quickly become unrecognizeable when held up against the pages of a catalog.

As a last resort, you can find a sheet of 1/8 stock at your local hobby or craft stores for like $2-3, cut some new fins at a new sweep angle, and they'll never connect your modified bird with the catalog picture.

 

[GDJ]

You probably don't even need to go buy another sheet of balsa (or basswood), you could very likely find enough leftover acreage on the finstock that comes with the kit to add a little strake at the leading edge of the root, or at the trailing edge. Also, you could clip the fin tips at a new angle. It wouldn't take much to quickly become unrecognizeable when held up against the pages of a catalog.

As a last resort, you can find a sheet of 1/8 stock at your local hobby or craft stores for like $2-3, cut some new fins at a new sweep angle, and they'll never connect your modified bird with the catalog picture.

That or just make rectangular fins out of 1 mm 3 ply birch plywood then wrap it. The design is pretty flexible that way. As long as they are tabbed and glued to the motor mount, 1mm fins are plenty strong enough. A bit flexy, but that's not necessarily a bad thing on landings.

 

[JStarStar]

You probably don't even need to go buy another sheet of balsa (or basswood), you could very likely find enough leftover acreage on the finstock that comes with the kit to add a little strake at the leading edge of the root, or at the trailing edge. Also, you could clip the fin tips at a new angle. It wouldn't take much to quickly become unrecognizeable when held up against the pages of a catalog.

As a last resort, you can find a sheet of 1/8 stock at your local hobby or craft stores for like $2-3, cut some new fins at a new sweep angle, and they'll never connect your modified bird with the catalog picture.

Well since GDJ says he is only thinking of doing the TARC project himself for fun/the challenge, the only judge he really has to satisfy is himself.

And if the contest rules specifically lay out one type of modification (adding only an altimeter compartment) as not being sufficient to make a stock kit eligible, it would follow by logic that making other modifications would indeed fulfill the requirement.

But yeah, I would think fiddling around with the fins would do it. Changing the motor mount and BT length for sure.