Saucer Stability

[jqavins]

I'm building a Frik-'n'-Frack, amd considering a mod. But I don't understand saucer stability nearly well enough to know if it will affect stability too much. I'm pretty sure it won't, but not really sure. I want to add a small nose cone. The shoulder will take the place of the engine block, leading to a little more overhang, which is not the concern. The concern is that, on a "regular" rocket the nose cone raises the CP, and if such a rocket were this short that CP move could easily be an issue. But then, on the other hand, if this were a regular rocket I could easily sim it to see if I need either nose weight or larger fins. With a saucer I don't know.

So here's the wat it looks stock:


And here's how it looks with the nose cone:


To head off some other questions:

• I will give the bottom of the nose cone an epoxy coating for flame and heat resistance.
• I will epoxy in the nose cone so that no gas gets betweem the shoulder and the body tube.
• I will use a good, tight friction fit of the engine, expecting the ejection gas to vent out the nozzle.

 

[neil_w]

Mindsim says that should be noooooo problem. Just think of the Estes Twin Factor: it has a nose cone and much less base drag than Frik 'n' Frak.

 

[MALBAR 70]

I've seen people put an Estes Mosquito or Swift 220 on top and make it a three stage. They fly just fine.

 

[BABAR]

• I will use a good, tight friction fit of the engine, expecting the ejection gas to vent out the nozzle.

No.

Motor nozzle too small to handle ejection charge, even on zero delay motor which technically doesnt have one. Will likely cause structural failure (yes I have been watching too much Star Trek Voyager)

Options

Best option. Decorative removable cone. Take it off before flight. Trust me, on that rocket adding a nose cone is not going to do much to increase aerodynamic efficiency

Nose blow. Use a short replaceable kevlar cord to keep from losing it. Just let it eject.

Motor eject. IMO the second best option. But some people frown on it

Side vents. These can work, but often get charred

 

[KenECoyote]

I like saucers! I'm no expert tho, but from my understanding, adding weight forward/upwards brings the CG up, much more than moves the Cp up, which is actually better for stability.

Think of it almost in extremes...stretch the nose to like 3' tall and the saucer is like fins on a regular rocket. Take a regular rocket, enlarge the fins and shorten the nose and it's a saucer! (Kinda)

Here's an example where I took a package kit saucer and added a lot of weight up top. Yes, I flew this one fine albeit altitude was maybe half normal due to all the extra weight.



Also the ejection charge is considerable and not to be taken lightly. The first flight of this saucer still blew off the top even with the big gap. I would expect it to blow up your nose cone unless the motor is very loose, which wouldn't be a good idea.

 

[Speaknoevil]

Look here: https://www.apogeerockets.com/education/downloads/Newsletter162.pdf

 

[jqavins]

I like saucers! I'm no expert tho, but from my understanding, adding weight forward/upwards brings the CG up, much more than moves the Cp up, which is actually better for stability.

Yes and no. In some sims I've done, the nose cone moves the CP by about the same amount no matter the rocket's length, but its effect on the CG is directy relatrd to the length it's sitting on top of. That's the reason for the concern. Also, I do know that the dynamics of the stability of saucers is quite different fom that of a rocket with regular fins, so making the nose longer (which increases the overall lenght) and comparing saicer panels to regular fins is a flawed analogy.

BABAR thanks for the benefit of your experience on (failing to) vent the ejection charge through the nozzle, and for never forgetting. I'd plug tje motor by pouring epoxy into the top if that dodn't count as modifying it. I don't want to leave it off for flight; I never was expecting any aerodynamic benefit, I just don't like having one configuration for flight and a different one on the shelf.

What I migth do is drill into the base of the cone and flameproof it, then glue the nose in, then drill a cross hole that intersects the core hole, and flameproof that.

 

[BABAR]

Hadn’t thought about plugging the motor, but that is certainly the neatest solution (I still like motor eject, it makes the saucer lighter so you got a longer return, but that’s just me.)

If you wanted to fly this at a NAR event or other sanctioned launch, Estes still has (and ACSUPPLY.COM still sells...

https://www.acsupplyco.com/estes/engines.htm
....)

The A10-PT. A stock plugged motor presumably RSO approvable .

The performance profile is not that different from the A8.

You can buy a 13 mm to 18 mm adapter (although with your experience you can easily build one certainly cheaper and probably lighter)

So there is a “legal” option for a plugged motor at a sanctioned launch.

 

[BABAR]

Another option which I just read about.

Tape a streamer to the motor and wrap the streamer around the motor, see if it will still fit loosely enough to eject.

Kind of adds to the fun of the flight, give a dollar to the kid who chases down the motor and brings it back!

https://www.rocketreviews.com/plans---estes-industries-flying-jenny-mike-salter.html

 

[jqavins]

Hadn’t thought about plugging the motor, ... If you wanted to fly this at a NAR event or other sanctioned launch, Estes still has (and ACSUPPLY.COM still sells...

https://www.acsupplyco.com/estes/engines.htm

Ooh, thanks for that!

 

[mbeels]

Doesn't adding a nose cone generally move the CP back? (opposite effect of adding a tailcone.) I would guess that your stability would be improved with the addition of the nose cone. By streamlining the front, you should be reducing drag forces in the front part of the rocket.

Regarding the ejection charge, you could add enough venting holes below the nose cone. I would add enough vent to equal the cross section area of your motor. (at least that's what I did with Skunk'd and it worked).

Tape a streamer to the motor and wrap the streamer around the motor, see if it will still fit loosely enough to eject.

What streamer material would withstand the heat of the motor? I've tried this before, but even mylar streamers melt into oblivion.

 

[kuririn]

Great minds think alike!
I put a display only nose cone on my Trifecta.
Original intent was nose blow with a kevlar leader epoxied into the body.
But there is only about 1/4" between the end of the tube and the thrust ring.
In fact I had to cut down the nose cone shoulder to get it to fit.
So it's on the shelf with a display nose cone, on the field noseless.

 

[Nytrunner]

I highly doubt the addition of JQ's nose to the rocket will have any perceptible affect on the 'stability' of the rocket, or negatively impact the drag function that makes saucer's flyable.

No I haven't simulated the design, nor modeled it for a cfd run.
Just thinking about it, you're changing the cd from .9-1 (solid tube or hollow end tube depending on where your motor is) to ~.1 for the center .7 in^2 of rocket, while you've got the outer ~100 in^2 subject to whatever the drag of the inclined plane at that angle is.
Even when you remove half the area at staging, the nose effect is still overruled

 

[GlenP]

I am not sure of the rule of thumb, but the base drag of a saucer is usually a much larger contribution in the effective Cp location than the smaller contribution from a central stuffer tube & nose cone. You could visualize the base drag Cp contribution as if coming from a phantom cone behind the saucer, which represents the streamlines, or wake around the cone, albeit in a very simplified explanation. The effective profile shape that determines the Cp is the combination of the saucer and the wake behind it, which can extend pretty far behind the saucer. Again, this is a very simplified explanation, not a dissertation on the subject.

 

[BABAR]

I did a separate post on the watering hole. There are no specific Nar or Tripoli rules against ejecting motor casings.

Looks to me like this is the planned method of execution for a flight of the Estes twin Factor sustainer.

 

[jqavins]

Doesn't adding a nose cone generally move the CP back? (opposite effect of adding a tailcone.) I would guess that your stability would be improved with the addition of the nose cone. By streamlining the front, you should be reducing drag forces in the front part of the rocket.

Because the nose cone is at the opposite end of the rocket it has the opposite effect. In rough, hand-wavy terms, a section growing from forward to rear attracts the CP, so a nose cone raises it and a flare at the tail pulls it down. One that's shrinking repells the CP so a tail cone is destabilizing. Yes, I did say it's hand-wavy, but it's a good way to remember th rule of thumb.