What is rod whip..?

[[POW]Eagle159]

The title says it...What is rod whip, when does it happen, why?

Thanks for any help...
:handshake:

 

[Verna]

It usually happens when a. The rod is too small for the rocket as guaged by a rocket's weight or thrust being stronger than the rods rigidity. b. When a clustered rocket has 1 or more engines fail to ignite causing the rocket to launch at an angle. C. A dirty rod, making the rocket hang momentarily as it leaves the rod.

Verna
www.vernarockets.com

 

[KennB]

Rod whip happens when you put a sufficient side load on the launch rod that deflects it while the rocket is moving along it. If you have too heavy a rocket or too powerful a motor, even the short distance between the centerlines of the motor and launch lug can generate a torque that tends to yaw the rocket in towards the rod.

A larger diameter rod will be stiffer and resist the torque better; a rail has a huge section modulus and provides even more stiffness.

The result is a rocket that leaves the rod without the best stability or orientation.

At the other end, if you have a very light rocket, the yaw can happen but the rod will be stiff enough to resist it. However, the light rocket can hang up on the rod in that condition. That's why you see a lot of tower launchers for light contest rockets.

Hope that helps; I'm pretty sure some or most of it is correct.

 

[MarkII]

I'm sure that excessive friction caused by built-up crud on the rod can produce it in some situations, but I have seen rod whip discussed mostly in the context of rockets that are too large for the rod diameter or rockets containing motors with more initial thrust than the rod can handle. In either case as the rocket travels up the rod it exerts lateral forces that exceed the rod's ability to resist, causing the rod to momentarily bend to the side to some extent. Since the rocket usually goes in whatever direction the rod is pointing (ideally, straight up), this bending or deflection of the rod can cause the rocket to veer off in an unwanted, angled trajectory. A deflection of the launch rod that has an undesired effect on the rocket's path off the pad is referred to as rod whip. An example: a LOC Minnie-Magg that is launched on an I600 (or a G61, for that matter) from a pad that is equipped with a 5 foot long by 1/8" diameter rod can be expected to experience wicked rod whip. In an extreme example like this, the rod will most likely exhibit the rigidity of cooked pasta. If a launch rod failed to remain straight under the load at lift-off and therefore caused the rocket to veer off at an unintended angle, it is said to have "whipped."

 

[[POW]Eagle159]

So on the estes maxi 3/16 rod 36'' be bad for the estes SaturnV with some nose weight and an AT E20....That's what im planning to fly soon.

 

[MarkII]

So on the estes maxi 3/16 rod 36'' be bad for the estes SaturnV with some nose weight and an AT E20....That's what im planning to fly soon.

A final judgement would depend on how heavy the rocket was, but a 36" x 3/16" rod on a stable pad should be able to handle any E motor. I launch stuff on 24mm F motors off of 48" x 3/16" rods all the time and I have even launched G motors from them on a couple of occasions. (Using a 3/16" rod to launch Gs is not something that I would recommend doing routinely, though.)

 

[[POW]Eagle159]

A final judgement would depend on how heavy the rocket was, but a 36" x 3/16" rod on a stable pad should be able to handle any E motor. I launch stuff on 24mm F motors off of 48" x 3/16" rods all the time and I have even launched G motors from them on a couple of occasions. (Using a 3/16" rod to launch Gs is not something that I would recommend doing routinely, though.)

With the nose weight its like 14-15oz. or so, on the regular estes pad. It worked well with the E20 and no nose weight but wasn't stable.....:sad: IT WILL BE BETTER NEXT TIME :wink:

 

[MarkII]

With the nose weight its like 14-15oz. or so, on the regular estes pad. It worked well with the E20 and no nose weight but wasn't stable.....:sad: IT WILL BE BETTER NEXT TIME :wink:

Well if it is any help my Hercules weighed more than 16 oz. empty and I launched that successfully (when all 3 motors lit) on a 3xD12 cluster off of a 48" x 3/16" rod. With something as large as the Saturn V you also need to consider effective rod length, or rod travel. The rocket is longer than the 36" long rod, but as with any rocket the guidance provided by the launch rod is only effective for the distance that the lug is on the rod. If the Saturn on an E20 can get up to speed from a standing start before its lug runs out of rod then you should be OK with that length. You can get a 48" long x 3/16" diameter cold-rolled steel or plated rod at your local hardware or home improvement store and it will provide you with an extra margin of stability. Personally, that is what I would use if I was launching that rocket. The club I belong to uses 48" long rods for its 1/8" and 3/16" rods.

 

[Verna]

With the nose weight its like 14-15oz. or so, on the regular estes pad. It worked well with the E20 and no nose weight but wasn't stable.....:sad: IT WILL BE BETTER NEXT TIME :wink:

I think you'll be good to go. One of my clustered Saturn V's liftoff weight is 27ounces. I use a 4' x 3/8ths rod on a camera tripod. No whip at all.

Verna
www.vernarockets.com

 

[[POW]Eagle159]

Thanks all for the help... I will see what happens when I go to launch and hopefully get some video..:wink:

 

[MarkII]

I think you'll be good to go. One of my clustered Saturn V's liftoff weight is 27ounces. I use a 4' x 3/8ths rod on a camera tripod. No whip at all.

Verna
www.vernarockets.com

[POW]Eagle159 was asking about the advisability of using a 3/16 inch diameter rod for his Saturn V, wasn't he?

A 3/8" rod is really on the beefy end of the spectrum for mid-power!

 

[Pem Tech]

Rod whip?
That is when your significant other finds out how much you just spent on rockets and whips you upside da' head with your own launch rod.

Such a frightful experience....

 

[RandyT0001]

This sounds like personal experience.

Rod whip?
That is when your significant other finds out how much you just spent on rockets and whips you upside da' head with your own launch rod.

Such a frightful experience....

Trudy, I got a half inch diameter rod if you need to use it to keep Layne in line.

 

[Verna]

[POW]Eagle159 was asking about the advisability of using a 3/16 inch diameter rod for his Saturn V, wasn't he?

QUOTE]

:y: You are right Mark, I meant 3/16ths. Thanks for the correction!

Verna
www.vernarockets.com

 

[AstronMike]

A 3/8" rod is really on the beefy end of the spectrum for mid-power!

Actually, most F and G powered gliders I fly tend to use exactly that, a 6' by 3/8" launch rod. Regardless of weight, I have found that they tend to fly where I point 'em. Recovery is another matter though :lol:

 

[MarkII]

Actually, most F and G powered gliders I fly tend to use exactly that, a 6' by 3/8" launch rod. Regardless of weight, I have found that they tend to fly where I point 'em. Recovery is another matter though :lol:

The glider launchers that I have seen do indeed have pretty thick rods, now that you mention it. I'm venturing a couple of guesses as to why...[Caution -- completely uninformed excogitation follows]

that this because they are pitched so far off vertical and the thickness is to limit any sagging or bouncing while they are in that orientation?
​

Plus, perhaps:

as I alluded to in an earlier post, a large rocket that lifts off slowly due to a low-thrusting motor (or in the case of typical rockets, too little motor or too much mass) can induce rod whip. Since RBGs use long-burn low thrust motors by design, extra precautions are taken to reduce any bouncing or deflection of the guides.
​

Does any of this utter speculation have even a shred of truth?

 

[powderburner]

[POW]Eagle159 was asking about the advisability of using a 3/16 inch diameter rod for his Saturn V, wasn't he?

QUOTE]

:y: You are right Mark, I meant 3/16ths. Thanks for the correction!

Verna
www.vernarockets.com

Everyone else has already posted good advice, yeah, a 3/16th rod would be good, a 1/4inch might be better.

I say that only because you probably need a significantly longer length than 3 feet, and at 4 or 5 feet it is probably time to just go ahead to the 1/4inch size. Especially if you plan to keep working toward bigger and heavier rockets in the future, a 1/4inch is definitely going to come in handy.

As noted in previous posts, rod whip is complicated. It is a combination of rocket weight, rocket pitch inertia, lug placement, thrust axis offset from the launch rod, rod length and diam and material, the structural design of the anchor holding the bottom of the rod, and probably a few more things just make it all confusing. About the only simple thing you can say about launch rod selection is that if you are asking these questions, it's time for you to step up to the next size.

If you have an Estes Saturn V-sized model on a 'medium' thrust motor it will accel more gently and should have less rod whip, but then it will be separating from the launcher at lower speeds and will tend to be less aerodynamically stable. (It needs a longer rod to give it more time-on-the-rod to accel to safe, stable flying speed, and for rods longer than 3 feet you should be using 3/16th or 1/4inch.)

If you have an Estes Saturn V-sized model on a high-thrust motor it will accel better and should have higher velocity at the point where it separates from the launcher, but the higher thrust will tend to aggravate any rod whip problems you might have. (It needs a larger diam rod for more stiffness and less whip.)

And as Mark II pointed out back in #8, the distance by which the model's launch lugs are separated (as well as the position of the last lug) will make a difference on how much "usable" rod length you have before the rocket starts separating from the launcher. Again, longer is better, and anything beyond 3 feet cries for a bigger diam than 1/8th.

 

[AstronMike]

The glider launchers that I have seen do indeed have pretty thick rods, now that you mention it. I'm venturing a couple of guesses as to why...[Caution -- completely uninformed excogitation follows]

that this because they are pitched so far off vertical and the thickness is to limit any sagging or bouncing while they are in that orientation?
​

Plus, perhaps:

as I alluded to in an earlier post, a large rocket that lifts off slowly due to a low-thrusting motor (or in the case of typical rockets, too little motor or too much mass) can induce rod whip. Since RBGs use long-burn low thrust motors by design, extra precautions are taken to reduce any bouncing or deflection of the guides.
​

Does any of this utter speculation have even a shred of truth?

Are you referring to those multiple rodded RCRG type glider launchers commonly used at NARAM? Yeah, *those* do need thicker rods since they are supporting glider mass themselves as they sit on those launchers nonvertically.

My case is the opposite. I fly stuff like deltas and disc gliders at large sizes, and these are 'aimed' straight up. The launch rod doesn't support any of the gliders weight directly....but...since I run so much wing surface area on these, any wind *will* cause the vehicle to 'press' upon the rod. I have found in 25+ years of flying F+ gliders that standard 1/4" rods can 'give' just enough to create some serious rod whip if you get a wind gust just at ignition.

So, I went to 3/8" rod usage for anything powered by F or G motors. Even though I built large but light, I felt the stronger rod was needed, and from the dozens of launches using this, never had whipping issues. Now, anything that was HPR, of course, used 1/2" rods and probably really needed rails, which I did have access to 20 years ago.

I even do this 'scaled down'. My large/light BT80ish stuff always uses 1/4" rods, even though we are only talking around 8z and D12/E9 motors. Have even flown the occasional C6 or D5 this way. 3/16" by 4 foot is used for anything 'small', although that still encompasses up to 225 sq inches of wing area.

Now, any glider that uses a long burn low thrust motor....these really need solid guidance on the way up. However, any longer rod is more susceptible to whipping, and that is *KILLER* on such flights. I prefer a thicker rod and looser lug as any 'hangup' is BAD. In fact, that is how I treat any D5 flights in a max size/max wt vehicle. D5's need all the help they can get :fly:

 

[[POW]Eagle159]

So would the Estes E launch pad with the 1/4 4'' rod better to get in the future... I heard the the rod is not the best quality though.... any replacements??

 

[MarkII]

The only potential issue with Estes launch rods is that they come in two pieces, so there is the possibility of a slight hitch at the joint. You can avoid this by switching to a one-piece rod. Personally I have had three different Estes rods and I have never had any problem with the joint. The Porta Pad E doesn't look too bad, especially if you can get it on sale. It has a nice BIG deflector and it sits low to the ground, so it is less likely to be squashed down by a higher-thrusting motor or a cluster. Estes recommends using the Porta Pad E for launching the Saturn V.

 

[daveyfire]

Your question has been answered, but here is a really nice photo from Dale Emery's website showing some pretty intense rod whip.