Openrocket stability puzzle

[DeltaVee]

I've been using OR 22.03 for a bit now... and it's mostly working. I did find that 3d rendering (which was ok in 22.02) isn't working so well for me now (I did file a bug report, and I suspect it may be my JVM possibly) but that's not why I'm starting this. I've been bungling around with an idea for a rocket with a cone as a stabilizer.... reminiscent of the "Scott-B" plans from an old MRN dom. I have produced two ork files with *slightly* differently configured tail cone structures. Below one should find "Scotty.ork" and "Scotty_stable.ork". The main difference is Scotty has a blunt reverse cone at the bottom end while Scotty_Stable does not...

Here are the differences graphically speaking:



As you can see the cp is off some 21.272" to the left! Then, if I delete the trailing cone....




Everything looks fine! I find it hard to believe that such a thing could cause this level of a problem (ps: I would drill a hole thru the cone/centering ring for the launch rod, so don't worry too much!)

I don't quite get why this is a problem.... again, attached are the ork files. Any insight is useful... I can accept that I'm doing something less than clever here.

 

[neil_w]

Transitions push CP back… tail cones move it forward.

I’m surprised that the transition on its own pulls CP that far back.

In reality I’m guessing the two rockets wouldn’t behave that differently, if the tail cone is short enough. But I wouldn’t include one in a rocket that is to be cone-stabilized.

 

[DeltaVee]

In reality I’m guessing the two rockets wouldn’t behave that differently, if the tail cone is short enough. But I wouldn’t include one in a rocket that is to be cone-stabilized.

That's what I'd expect... But it's puzzling how such a blunt cone would be have such a strong effect.

 

[bjphoenix]

Doesn't that sim need the base cone hack?

 

[DeltaVee]

Doesn't that sim need the base cone hack?

I was unaware of what that hack is... I'll try it out this afternoon... after my root canal!

 

[neil_w]

Start here: https://openrocket.info/tutorials/base-drag.html

 

[lakeroadster]

Root Canal... ugh.

 

[neil_w]

I'm generally a proponent of using base drag CP correction, but in cases like this, with pure cone stability, the resulting CP location seems very optimistic to me.

 

[wonderboy]

I've never thought about it much before, but doesn't the CP need to be constrained to the physical extent of the rocket? Can a CP really be out in space ahead of the rocket? That doesn't make sense to me. I get that it will push CP forward, but that result above doesn't pass the laugh test.

(EDIT: laugh test referring to the results in post #1)

 

[neil_w]

I've never thought about it much before, but doesn't the CP need to be constrained to the physical extent of the rocket? Can a CP really be out in space ahead of the rocket? That doesn't make sense to me. I get that it will push CP forward, but that result above doesn't pass the laugh test.

Typically, out in space ahead of the rocket suggests that no valid CP is really being calculated. Rather than treating it as a precise value, consider it a qualitative indicator meaning "bad".

CP can for sure be behind the back of the rocket due to base drag, as is the case with saucers.

 

[BigMacDaddy]

This would be easy to swing test -- maybe make the back cone removable and try a swing test with and without it?

I do wonder: if OR does not take into account base drag how do boattail / rear transitions move CP backwards? Would an alternative be to incorporate a base drag cone and reduce it appropriately when a boattail / rear transition is used?

I also wonder how does the angle of the boattail / rear transition affect base drag? Does it drop off rapidly past a certain angle or is it some type of continuous base drag curve.

Ah found a bit of an answer for a specific set of circumstances - An interesting analysis of this for boattail shapes / lengths: https://apps.dtic.mil/sti/tr/pdf/AD0474352.pdf



By my read of the results there a boattail less than about .6 calibers long actually increases base drag. So your short boattail / transition should actually increase stability. After .5 caliber long the base drag rapidly reduces through around 1.5 calibers and then levels mostly.

The V-2-esque rocket used in the test is 7 calibers long counting the Nosecone and Boattail. That seems to fit into the short-stubby/fat definition (Big Daddy is 20" long and 3" wide so 6.66 calibers long which is not that much different than 7 - unless base drag also follows some type of log curve for number of calibers of length).

 

[neil_w]

I do wonder: if OR does not take into account base drag how do boattail / rear transitions move CP backwards?

Boat tails move CP forwards, working against stability. Has nothing to do with base drag.

Would an alternative be to incorporate a base drag cone and reduce it appropriately when a boattail / rear transition is used?

Exactly how to adjust the size of the base drag cone when a boattail is used is, as far as I know, an unsolved problem. At least I haven't seen anything written on it.

I also wonder how does the angle of the boattail / rear transition affect base drag?

I would think it must. Not sure how accurately it is modeled in OR, though.

By my read of the results there a boattail less than about .6 calibers long actually increases base drag. So your short boattail / transition should actually increase stability.

I don't know if I'd draw that conclusion. I think it is more likely, that a very short transition in back will have little or no effect when placed behind a sufficiently steep forward transition right in front of it (as in the proposed design). But the little hump in Cd for very short boattails is certainly interesting (and puzzling).

 

[BigMacDaddy]

Boat tails move CP forwards, working against stability. Has nothing to do with base drag.

Why do boattails / rear decreasing transitions move CP forward if it is not related to drag? Sorry, quite naive about aerodynamics - I did not make it that far in UG engineering before switching to psychology (I cannot even come up with a joke about the psychology of boattails).

 

[BigMacDaddy]

Boat tails move CP forwards, working against stability. Has nothing to do with base drag.

Exactly how to adjust the size of the base drag cone when a boattail is used is, as far as I know, an unsolved problem. At least I haven't seen anything written on it.

I would think it must. Not sure how accurately it is modeled in OR, though.

I don't know if I'd draw that conclusion. I think it is more likely, that a very short transition in back will have little or no effect when placed behind a sufficiently steep forward transition right in front of it (as in the proposed design). But the little hump in Cd for very short boattails is certainly interesting (and puzzling).

Likely different situation with standard boattail vs the cone to transition described here.

 

[JoePfeiffer]

The calculations assume a relatively gentle angle on transitions, with the airflow following the line of the body. At some point (I don't know where that point is) that ceases to be true, and the transition acts like a discontinuity. The calculations don't take that into account, so you end up with the transition pushing the calculated (as opposed to real) CP 'way far forward. I'm going to guess the design without the transition is the more accurate; also that since you've got a cone back there the base drag hack is not applicable.