Gyro stabilization =advanced=

Originally posted by utahrc
How about fins that are curved to conform to the diameter of the rocket? They're static state is to be fully deployed but they could be designed such that aerodynamic forces would cause them to collapse around the body as speed increased.

The problem with such fins is that they induce roll at zero angle-of-attack due to their shape and can cause yaw-roll problems.

Ref MIL-HDBK-762 paragraph 5-3.2.1.2 "Wraparound Fins and Tangent Fins."

FWIW, such fins are used on the 2.75 folding-fin rockets - unguided "dumb" rockets used on ground-attack aircraft/helicopters.

 

[powderburner]

Originally posted by dosco
FWIW, such fins are used on the 2.75 folding-fin rockets - unguided "dumb" rockets used on ground-attack aircraft/helicopters.

Yes, except those fin mechanisms work a little differently. On the 2.75 FFAR the fins are closed for carriage inside the launch tube and pop open when the rocket clears the launcher. Those fins are spring-loaded with a metal coil spring on each fin, and remain open for the entire flight of the rocket.

I think what utahrc was suggesting was sort of the opposite, fins that were deployed at launch and during low speeds and then somehow retracted during high speed. This would be an interesting fin/device to design?

 

[denverdoc]

But the point made was interesting--that such fins roll the rocket and may even cause some corkscrewing, which might negate whatever drag advantage such a scheme might have.
JS

 

Originally posted by powderburner
I think what utahrc was suggesting was sort of the opposite, fins that were deployed at launch and during low speeds and then somehow retracted during high speed. This would be an interesting fin/device to design?

I understood what he was getting at.

My point is, though, that by their very shape such fins induce roll and therefore are problematic.

There are 2 techniques mentioned in the MIL-HDBK that strike me as interesting:

1. Put a slot at the intersection of the fin root and body tube.

2. Use the exhaust plume to affect the fin aerodynamics.

 

[prowlerguy]

What about a set of fins that are hinged at the top and can retract into the body tube by pivoting around the hinge. Hold the fins out with a bladder filled with water or air, and have the plug pulled on launch. The bladder empties, allowing the fins to retract. Depending on fin placement and BT/MM delta, you could have the fins retract almost all the way.

 

[denverdoc]

Originally posted by prowlerguy
What about a set of fins that are hinged at the top and can retract into the body tube by pivoting around the hinge. Hold the fins out with a bladder filled with water or air, and have the plug pulled on launch. The bladder empties, allowing the fins to retract. Depending on fin placement and BT/MM delta, you could have the fins retract almost all the way.

That would be too simple We are rocket scientists after all.
Surely some technically inclined guy with some radio control experience could rig things to have an R/C servo pull them in at a rate proportional to airspeed, then push em back out as it slows or maybe just let the rocket tumble at the top if it loses stability as the momentum poops out.

 

[bfo]

Someone in the UK has been working on this for a while he has a gyro stabalised finless rocket with a gimballed motor, more information at

https://www.ukrocketman.com/rocketry/guidance.shtml

and

https://www.ukrocketman.com/rocketry/gimbal.shtml

there is also video at

https://www.ukrocketry.com/index.php?option=com_weblinks&catid=15&Itemid=35

scroll down to the video called

Gyroc UKRA 2003 - MPG 3.3mb

Cath

p.s. none of these websites are written by the guy who has the rocket.

 

[rstaff3]

Just emembered ER Mag has a book on the subject
https://www.extremerocketry.com/Mer..._Code=ERWS&Product_Code=BKVT&Category_Code=EB

 

[benjarvis]

mmm.


Interesting....


I have a scale model with very tiny fins... and I want to fly it as slowly as possible... AND I need to (at present) add a lot of weight to it (50% again on top of the overall lift-off mass) to make it stable... anyone think a gyro would be a better use of that weight?.... in which case... I wonder what the best location would be for it? it's a moderately long rocket (12/1 L/D)... would the gyro be best near the nose (adding nose weight as well) or would it have most effect around the middle of the rocket?

This is a BIG rocket so I'd rather not get it wrong if I was gonna try this route... any ideas?



Ben

 

[11bravo]

but one that uses small fins to reduce spinning for video purposes.

LINK

VERY KEWL!

Greg

p.s. If I came up with that link through this thread, sorry for the repeat.

 

[Mike 8-{>]

Originally posted by rstaff3
Gyros are certainly common on satellites, and are used in conjunction with other attitude sensors. The only descritpion I've seen that uses the gyro directly is in rollerons. IIRC these were free spinning wheels, turned my airflow and are bounted in free-moving tabs at the end of the fins. Earlier I had wondered about mounting three motors in the fins to see what they'd do. No motors and too litle ambition stalled my ideas.

Kind of gyroscopes. Actually called reaction wheels which only spin up for short periods, as I understand it, to correct / adjust orientation. I dont think they run all the time as is the case with a gyroscope.

Mike 8-{>

 

[denverdoc]

11 Bravo,

Very interesting link, even though the spin rate is low w/o the gyro, the difference is remarkable. But the piece tha to me is even more interesting--Brian goes to absolutely extraordinary lengths to make sure he gets the fins on straight, yet finds the need for a gyro, while another video buff produces arguably better video with less sophisticated measures to get the fins on straight. ????

John S

PS: your signature leaves open to the user to fill in the ..... My own is: "practice uninformed by theory is the surest recipe for disaster."

 

[rstaff3]

Originally posted by Mike 8-{>
Kind of gyroscopes. Actually called reaction wheels which only spin up for short periods, as I understand it, to correct / adjust orientation. I dont think they run all the time as is the case with a gyroscope.

Mike 8-{>

Yea, reaction or momentum wheels work by gyroscopic action so I guess I actually know of several uses of direct gyro control in satellites. IIRC some move bidirectionally, some can be turned on or off, some move continuously. There are flavors of each that I don't understand. I could pull out an attitude control book I reckon. In rockets, I only know of rollerons.

These are opposed to gyros used as sensors.

I also don't understand one gyro control laws