Base drag correction vs. stability margin vs. speed off the rod

[neil_w]

I'm toying with a rocket that is somewhat Mars Lander-ish, but a bit closer to a cone shape (close enough for this discussion I think). In OR, before I apply the base drag correction, stability margin is close to zero or slightly negative. After base drag correction, CP moves very far back, so much so that precise CG location hardly matters anymore. The correction I'm using is a weightless cone pi diameters long, with diameter equal to the widest point at the base of the rocket.

I'm uncertain about a few things:

1) Base drag is a dynamic phenomenon. At what speed does it take full effect? What speed should I be looking for off the rod to be convinced that it'll be stable once it departs? It's easy to imagine that once it reaches full speed it'll be fine, but what about when it's going 40 mph coming off the rod?

2) With the base drag correction in place, what distance do I look for between the CP and CG? Stability margin as reported by OR isn't really relevant, so I just need a reasonable formula to evaluate CP->CG distance.

3) Base drag is a pretty powerful effect. So why would something like the Mars Lander need nose weight? It should be a pretty good base drag machine, unless it's construction makes it *really* tail-heavy. I was accidentally reading a Mars Lander thread yesterday and several folks said it needs nose weight or can be unstable. This makes me wonder if it's just not moving fast enough off the rail for base drag to kick in fully, but I don't know.

4) For my launch simulations, I've been (a) removing the base drag correction, and (b) overriding the CG to be forward enough that OR thinks it's stable. Does that sound correct? I'm not expecting a highly accurate sim, but just trying to get a rough idea so I can plan for appropriate motors.

Thanks!

 

[Bat-mite]

op:

 

[neil_w]

Sorry Bat-mite, I guess everyone's in the theater next door seeing Batman vs. Superman.

The manager will issue you a refund if you wish.

 

[dixontj93060]

I'm not sure I'm understanding your concern, but I'll comment based on what I do. With a "zero" mass cone in place as referred to in your point #1 above you will get a shift in CP. This new CP is your effective CP for stubby and/or high base drag rockets. From there simulate as normal with the cone in place (don't move your CG). Same launch/stability parameters apply, e.g., speed off rail, one caliber margin, etc. Additional ballast is only needed if your shifted CP does not get you to the one caliber minimum.

 

[neil_w]

What is the definition of a caliber for a cone-shaped rocket? Again, not *exactly* a cone, but roughly, sort of. The base drag correction puts the CP behind the rocket, so in that case it would seem I don't even need to worry about CG.

Also, I'm uncertain about simulating with the base drag correction in place (I just tried and it costs me about 30% in altitude). I think OR is already correctly factoring in the drag of the wide bottom (without the correction); the correction is just there to provide a more accurate CP point for stability calculation. So simulating with the base drag correction is just putting in a lot more drag that isn't there in the model.

I think.

 

[dixontj93060]

What is the definition of a caliber for a cone-shaped rocket? Again, not *exactly* a cone, but roughly, sort of. The base drag correction puts the CP behind the rocket, so in that case it would seem I don't even need to worry about CG.

Also, I'm uncertain about simulating with the base drag correction in place (I just tried and it costs me about 30% in altitude). I think OR is already correctly factoring in the drag of the wide bottom (without the correction); the correction is just there to provide a more accurate CP point for stability calculation. So simulating with the base drag correction is just putting in a lot more drag that isn't there in the model.

I think.

It depends on the length/width ratio of your cone. Base drag of a saucer or cone shape as in one of Art Applewhite's designs most certainly puts the CP aft of the rocket (see: https://www.artapplewhite.com/cp.html). For a cone that is fairly long base drag becomes less of a factor and the CP approaches 2/3rds distance from the tip (see: https://www.ninfinger.org/rockets/ModelRocketry/Model_Rocketry_v02n01_10-69.pdf referenced here: https://www.rocketryforum.com/showthread.php?10360-Cone-Rocket-Stability). And finally, yes, base drag does curtail altitude (see: https://www.apogeerockets.com/education/downloads/Newsletter103_sm.pdf), but you are right, it is taken care of in Rocksim (don't know about OR) already when you simulate the rocket. To clarify though... I just use the "zero" cone aft "attachment" to validate static and dynamic stability whereas I use the rocket without the cone to simulate the "actual" flight.

 

[Gary Byrum]

Neil, sometimes you have to toss OR to the wind and let your gut stability instincts prevail. If you're trying to understand the reason "WHY" then I get it. Otherwise, let your eyes be the judge in the matter.

[video=youtube;FO26EXvbiFc]https://www.youtube.com/watch?v=FO26EXvbiFc&feature=youtu.be[/video]

 

[neil_w]

Trying to understand the "why" here, haven't dealt with a model like this before. Part of my work with OR is learning what it can tell me and what it can't.

Looks great in full warpaint, by the way! Would love to see some better pics if you have them.

 

[Gary Byrum]

Honestly, I have no real great pics of it. What I do have is the one below, shelved with a bunch of other models.



Then there's some coloring concepts I've been tinkering around with recently. These are not a done deal by any means. It's just how I toss a bunch of colors around, looking for that balance. Later would come the striping and any aesthetics I choose.

 

[bill_s]

If your method of calculating base drag gives results that no one can believe and contradicts experience of that design, the method is useless.

As I have said here before, I don't believe in base drag stabilization. Some claimed examples of it regard rockets that are stable without any such effect. Others, such as saucers, it looks to me the shape of the forward side has a stabilizing effect. In any case where I have been tempted to use a base drag correction, the results would have been disasterous, as experience showed that the rocket was the same stability as originally calculated or worse.

Seems to me that the needed stability margin is more a function of rocket length than diameter, except again for extremely blunt shapes also needs some margin due to their diameter. The desirable thing about using calibers of stability is it can be eyeballed if the CG and CP are known by comparing that to the rocket diameter. It is a convenient unit. However, since the needed number of calibers is dependent on rocket length, it is highly variable.