Is it possible for a standard rocket to be stable and not pass the swing test?

[cornyl]

Hi All,

Is it possible for a standard rocket ( 3 fins,tube,nosecone) to be stable and not pass the swing test?

I'm working on some basic designs for PD and SD for Naram next week.
I built a clone design from plans that is not stable at all according the swing test.
Have you ever seen designs that rocsim or calculations or cardboard cutout say they are unstable and they do not swing test well but actually fly well.

The cp is about 3 calibers ahead of the cp. Which is reaally unstable. I do not want to add wieght to the nose of this model but to get this model stable I will have to make the fins much larger.

All the PD and SD plans I have seen have really small fins. I suspect these plans call for adding lots of nose wieght.

Thanks,
CornyL

 

[TWRackers]

I've seen some rockets that were actually stable but failed the swing test, because when swinging a rocket it's sometimes subjected to high enough angles of attack that it won't turn forward like you'd expect. But such conditions aren't always very realistic. I had an Astron Alpha (the original) way back in the Second Millennium which flew fine but wouldn't pass the swing test.

IF (all caps, mind you) you have a design that fails ALL stability tests (RockSim, cardboard cut-out, swing test) like you stated, then there's a pretty good bet it's NOT stable. If RockSim showed it as stable while the other tests said unstable, I'd probably believe it is stable. That's my opinion, though.

You stated that the CP was ahead of the CP (sic). I assume you meant to say the CP was ahead of the CG, since you then stated it was "reaally unstable".

 

[luke strawwalker]

Hi All,

Is it possible for a standard rocket ( 3 fins,tube,nosecone) to be stable and not pass the swing test?

I'm working on some basic designs for PD and SD for Naram next week.
I built a clone design from plans that is not stable at all according the swing test.
Have you ever seen designs that rocsim or calculations or cardboard cutout say they are unstable and they do not swing test well but actually fly well.

The cp is about 3 calibers ahead of the cp. Which is reaally unstable. I do not want to add wieght to the nose of this model but to get this model stable I will have to make the fins much larger.

All the PD and SD plans I have seen have really small fins. I suspect these plans call for adding lots of nose wieght.

Thanks,
CornyL

The cardboard cutout method, or center of lateral area, is the 'worst case scenario' (most conservative) as it will show the CP the furthest forward of any of the ways you can determine stability. The reason that it's the most conservative is it shows the CP at an angle of attack of 90 degrees from the flight direction, which is HIGHLY unlikely in flight except with a completely unstable design. As angle of attack increases, CP moves forward because of lift/drag from the upper body and nosecone, acting in a direction counter to the restoring forces generated by the fins.

As was said, the swing test can show a model to be unstable that actually is stable in flight. For one thing, swinging the rocket around, it is likely to point in just any direction starting out, which means the CP is going to be forward of it's "normal location" in a rod takeoff. Additionally, the air directions are somewhat random (especially in a wind-- see the "Mu factor" as it relates to helicopter lift as the rotor is turning-- if the helicopter is going 150 MPH forward and the rotor tip speed is 150 mph, basically the rotors on the side of the aircraft swinging toward the rear are at 0 mph relative to the surrounding air, while the rotor blades on the other side turning toward the front of the chopper are at 300 mph relative velocity) Another thing is, the speed of a swing test is pretty low, and therefore the lift forces the fins are capable of generating are probably not going to be generated during the average swing test.

Now, if you run this through RockSim and it tells you the CP is WAY too far forward and it's unstable, especially by three calibers, then it's a fair bet it's unstable. You can switch between modes on Rocksim in how it calculates CP-- you can choose Barrowman, Rocksim, or Cardboard cutout methods. The selection box is on the first tab. You might also check the actual loaded CG of the rocket in flight configuration-- maybe there's something off with the weight distribution of the model as it's programmed into Rocksim??

Another thing that pops out... you said these were clones... How were the originals stabilized?? Where was the CG on the original models, how much noseweight, etc.?? If you clone an existing stable kit and keep the weights and weight distribution and fin sizes the same, you should expect the stability margins to be pretty close to the original...

Later! OL JR

 

[cornyl]

Another thing that pops out... you said these were clones... How were the originals stabilized?? Where was the CG on the original models, how much noseweight, etc.?? If you clone an existing stable kit and keep the weights and weight distribution and fin sizes the same, you should expect the stability margins to be pretty close to the original...

Thankyou for your valued opinions...much appreciation!!!

I really don't know how the originals were stabilized.
Possibly assumptions were made that adding nose weight was a given.
My gut feeling is that these rockets are marginally stable and flown like that just save weight.

As another thought... How much weight does an engine decrease by
before it leaves the launch rod or how many grams of propellant burn in 3 (if you use a 3 feet rod)feet after ignition. This would also help stability as the rocket would become more stable during flight. I would guess that a rocket is at it's least stable state immediately after clearing the rod.

CornyL

 

[georgegassaway]

The streamers tend to have some significant mass that shifts the CG forward. By contrast, parachutes do not tend to weigh much, or else one would be using an awfully thick chute.

Anyway, I hope the plans you have cloned are not among those I referred to for PD and SD in the NARAM tips pages. Go here and select either PD or SD:

https://www.psc473.org/naram51/event_tips.php

Both the ASTRE PD/SD model, and Trip Barber’s SD model (also suitable for PD) should be stable.

The models I fly are along the same lines.

Propellant loss in the very short time from ignition to leaving the pad is so tiny that it won’t make a difference in being “unstable” at ignition but “stable” leaving the pad.

Swing testing does have problems at times. It is because if the model is not kept pointed dead straight into the relative airflow as you start up the rotation, a very significant angle of attack can cause the model to act unstably.

It is also tricky to get a very light small model like these to stay pointed “into the airflow” as you start the rotation, as compared to say a Big Bertha.

I have to admit that while I never did a lot of swing tests, and have not done any since the 1970’s, I never did any with rockets as small and light as PD/SD models.

A lot of designs would flip unstable if, for example they got to see an angle of attack of 45 degrees to the airflow, because the CP will move forward as angle of attack increases. This is also why some marginally stable models often fly stably in calm weather, then can fly unstable on a very windy day (and usually pitching off downwind as it goes unstable).

- George Gassaway

 

[MarkII]

Some competition designs that I have found online are 10 years to 20 years old (or older), and were originally built using components from companies that are no longer in business. Some of those components have current production equivalents, and some don't. In the case of components (such as certain kinds of tubing) that have no current equivalent, you might be able to substitute something that is similar, but not exactly the same. Designs that pushed the performance and stability envelope when they were built with the original components may not work with contemporary parts. Neither one of the two PD/SD designs that George mentioned fall into that category, but I just thought that I would put that issue out there. Most published competition designs, regardless of their age, are still quite valid, but a small number may need to be checked to see if they are still buildable. Also, some designs that won awards some years back did so on motors that are no longer available.

MarkII

 

[Micromeister]

Cornyl:
I have to say something just doesn't ring true here!
The very short answer to your original question is yes a model can be unstable until ALL the internal as well as external components are added.

I have to take a little exception to your gut feeling that these models were just flown "hoping" they'd be stable. While most serious competitiors push the envelope, we did it with mostly "flight tested" models, at least the modelers I flew with anyway, and we generally knew when a Heads-up flight was going to be in order. Field conditions sometimes changed performance, particularly with Tiny finned SD, payload and eggloft models.

Are you attempting to swing test a competition style model in it's ready to fly configuration? Meaning with everything in place except the ignitor? that includes the tape holding the motor in?

To add on to the things George talked about in his post; He's right on in that many of the older designs were hand calculated back before personal computers. Yeah we really did do this stuff with pencil, paper and a slide rule. Further a competitor would never add noseweight for it's own sake unless it was to adjust for optimum mass. Rather we used the mass of the streamers as part of the calculations and Flight testing varing the mass and CG with the size of the Streamers or Chutes & Plugs. I don't know who's model you cloned but I'd be willing to bet if it's a Streamer duration model if you add the stated streamer your model will not only fly straight but will swing test also If it's a PD model, if prepared using the motor/ chute etc suggested it'll test positive.
Just about everything that was used back in the day, is either still around or Made lighter then back than now. Vacuum formed .010" styrene NC's and paper thin body materials are not only still out here, they are pretty easily found. Just ask George or just about any of the internat fliers. Some of the BEST streamer materials are no longer available but as far as "mass objects" go there are lots of other alternatives to fill the gaps. I've flown Paper Tigers and Taper Papers for years that would never swing test without having the Streamers, plugs, wadding and/or chutes in place along with one of the chosen motors for the evert.

I'm sure if you show or tell us which model you've cloned we can give you a much better explaination as to why your getting the results you reported.

 

[The EGE]

If a rocket is marginally stable (ie, under 1 caliber static margin), then it certainly can fail the swing test but still be stable in flight. My 29mm Machbuster has about .5 calibers of static stability when loaded with a G78 or G80, and it's very marginal with the swing test. Only at the absolute fastest I can swing a 10oz rocket does it orient pointy-end forward. However, on a swing test it's hard to get up to more than 15 mph anyway - lower than the accepted minimum of 30fps. When launched off a 6ft rod, it was perfectly stable all the way up to 4000 ft