Does the "one caliber" rule apply to stubby rockets

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J Blatz

J Blatz

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I am building a short and stubby Mini Magg-esque rocket with 5.5" OD. If I have the CG say 3" ahead of the CP rather than the traditional one caliber or more will it be stable?
 
Depends.

How many fins? How much fin area? How long of a motor?

As a reference, i have a Polecat Aerospace Goblin 5.5" which is 36" long. It has 4 large fins and a large fiberglass nose cone. The instructions say no weight needs to be added. Numerous flights have proved this.

A stubby rocket needs a big kick in the butt for stability so stay away from low impulse motors. Long motors will add mass ahead of the cp and as they burn the rocket become more stable. Quick links and swivels in your recovery train will also add weight in front of the cp.
 
I grabbed a MiniMagg file from Rocketreviews.com, added a J350, added nose weight to set the margin to 3, added the appropriate 'phantom cone' and the margin showed to be almost exactly one. If I use the 'rocksim method' vs Barrowman, the margin is better. I have used this method for low and mid-power rockets. For high power I give myself more wiggle room. That being said, you should get more opinions.
 
The Levison/Apogee method does work really well in my experience.

Before going that route, I always put WAY too much nose weight in some of my (shorter/stubbier) rockets. Sure they flew stable, but were pigs that wasted a lot of motor power and they required huge chutes. I also often had nose cone damage on hard landings due to the excess mass. Inelegant to say the least.

Once I found the Levison/Apogee article, and started designing/building/flying using the info therein, my short/stubby rockets flew GREAT..I had more/better motor choices to pick from....I didn't waste so much motor power (and $$) on just getting the dang thing up in the air....etc. All good.

s6
 
Thanks for the input guys. Rocket has six fins, and I plan to fly it with punchy motors like H242/I357/H550. I will read the Apogee article.
 
FYI, although it did not survive its maiden flight for other reasons, my MAC Performance 3FNC did not need any nose weight, even though the stability margin was, like, 0.2 cal.
 
Bat-mite said:
FYI, although it did not survive its maiden flight for other reasons, my MAC Performance 3FNC did not need any nose weight, even though the stability margin was, like, 0.2 cal.
Click to expand...

What was the length & diameter of that rocket?
 
In my book the trouble with cutting it too close is that, white one or two people may have a positive experience, others who just take one number on face value may use other motors and or fly in other conditions and the results may not match.
 
Given that Apogee newsletter 154 is 12 years old now, I was a bit worried that OR and/or RockSim may have been updated to account for this effect. But digging into this a bit in Open Rocket, it does look like the core assumption of the Levison article remains true: OR does not place the CP far enough back for a wide plate at the back of the rocket.

I played around with a tailcone on a simple rocket in OR, making the tailcone wider and narrower than the main airframe tube. Now, OR will show some impact of base drag - a wider tailcone will move the CP back, and decrease max altitude (a smaller cone has the opposite effects). Hence my worry that the base drag effect had been fully implemented and the "phantom cone" was no longer accurate.

However, using the component analysis feature, I could see that the CP of the tailcone never moved aft of the end of the tailcone itself - inaccurate according to flat plate aerodynamics. So the "phantom cone" is still needed to avoid significantly underestimating stability (and this wouldn't apply only to stubby rockets, just that that's where the effect matters the most). It's worth being careful though, the base drag effect will not be as pronounced under thrust (in simple terms, the motor plume "fills in" the separated flow behind the base plate, reducing the drag). So you probably don't want to fly a design that is just barely stable with the phantom cone included.

But now a question - has anyone had success matching both stability and predicted altitude / coast time with the phantom cone? Interested in some empirical results. My suspicion is that the phantom cone is appropriate for getting the CP right, but might end up double-counting the base drag and making the sim results less accurate. Certainly important for selecting delay lengths.
 
Oberon said:
But now a question - has anyone had success matching both stability and predicted altitude / coast time with the phantom cone? Interested in some empirical results. My suspicion is that the phantom cone is appropriate for getting the CP right, but might end up double-counting the base drag and making the sim results less accurate. Certainly important for selecting delay lengths.
Click to expand...

Yes. Your assumptions are correct. The trick is to put the cone in for CP calculations and then remove it for altitude/delay prediction.
 
Banzai88, thanks for the reply - have you had some success matching the final altitudes for stubbies without the phantom cone? The real trick would be if the sim was unstable without the phantom cone - then you wouldn't be able to sim it at all without it. This doesn't seem to be the case for most short stubby kits though, they at least have some notional stability without the phantom cone.

mpitfield, I'm not sure what you're implying. I've read both the previous thread and the Apogee newsletter. My question regarding simming for altitude was not, at a first read, included in either location. And in fact my whole point was that the Apogee newsletter is 12 years and several RockSim / OR releases old at this point, so I wanted to know if it was still relevant. This was the most recently updated thread on the subject and therefore seemed like the most reasonable place to ask.
 
Oberon said:
Banzai88, thanks for the reply - have you had some success matching the final altitudes for stubbies without the phantom cone? The real trick would be if the sim was unstable without the phantom cone - then you wouldn't be able to sim it at all without it. This doesn't seem to be the case for most short stubby kits though, they at least have some notional stability without the phantom cone.
Click to expand...

It's a fine line. I've been playing with OR on a sim for my MadCow Cowabunga, amongst others. The Cowabunga is the only HP stubby that I've done, though (along with a metric butt ton of the usual LP suspects). I've tweaked the Cowabunga sim close enough after 3 flights that I'm at the point now where the last 4 flights have all achieved altitude within 25 feet and timing on ejection to within 1 second of optimal as indicated by my JL ALT2.

There is no real 'magic formula'. Just continue to tweak your sim until it reflects what your real rocket is doing. I find that the form factor and weight are the most important things to get 100% in the sim, then input the REAL wind conditions and rod angle for the launch that you're about to do and run the sim for the motor that you want to fly.

One other thing in OR, I start with 'regular paint' for surface finish. Make sure you 'set for all'. Fly and get a baseline performance, adjust from there. So far, I find OR to be very user friendly and capable of giving you good data if you give it good data.

I can reasonably predict a delay and an altitude to well within 5% after the first base line flight of any of my rockets. That's well within the noise and rounding errors in the math and the variability allowed in the motor performance and delay timing. In fact, one of the games that we like to play is 'closest to the sim prediction'.
 
Oberon said:
mpitfield, I'm not sure what you're implying. I've read both the previous thread and the Apogee newsletter. My question regarding simming for altitude was not, at a first read, included in either location. And in fact my whole point was that the Apogee newsletter is 12 years and several RockSim / OR releases old at this point, so I wanted to know if it was still relevant. This was the most recently updated thread on the subject and therefore seemed like the most reasonable place to ask.
Click to expand...

I was not responding to your post but the OP's instead.

AFAIK the simulation packages mentioned, do not account for base drag on stubby rockets, and the physics discussed in the Apogee news letter remain the same.

To respond to your post I have not had simulation results match for AGL when using the Apogee method, but my sims for stubby rockets are limited to one. My other simulations are typically +- 5% accurate when compared to actual results, which is much better than to be expected. The most common number I have seen thrown around is +- 10%, but some have suggested that is even optimistic. However on my Madcow Tembo my sim was - 50ish% off. I can't recall the exact numbers as I cannot recall the motor I used, just that it was an Aerotech 38/480, possibly an I211. I believe it was simmed to somewhere 3100'ish AGL and I realized somewhere around 1,535' AGL (baro).

The biggest issue I have to take into consideration when considering why the sim is so off is not just the phantom cone, or base drag, but also the fact that I used a GLR bell retainer. So I have often wondered if the Krushnic effect played a role in the result. BTW the rocket and flight in my Avatar is the one I am discussing.
 
Oops, I read your post (along with the moderator badge) as implying I should have asked my question elsewhere. Sorry for misreading you.

Thanks for the data! I do think the sim programs have at least some accounting for base drag as I mentioned, there's a noticeable impact to changing the base diameter of the rocket. But they don't seem to account for base drag stability.

I hadn't actually noticed the bell nozzle on your rocket - but now that you mention it, it looks really cool. It definitely doesn't seem ideally sized for actual exhaust expansion, but doesn't seem deep enough for the physics of the Krushnic effect. And it doesn't seem to impact everyone with the nozzle retainer.

Really odd otherwise though - could it be that, on a wide-base rocket, the GLR bell sticks out just enough that it intereferes with the flow off the back of the rocket? You've got a separated flow zone right behind the base, and now you've injected another expanding body right where the flow would normally be remixing with the free stream. Could be you've trapped an unusual vortex / low pressure zone that is substantially increasing effective the base drag. That, and you've basically got two bodies producing base drag instead of just one. The extended Barrowman methods wouldn't account for these effects I wouldn't think.

@Binder - that's a good suggestion too. It's important to remember that "one caliber stability" is really just shorthand for "sufficient moment arm for the net aerodynamic restoring force to act on, plus margin for uncertainties in the calculation", and that depends more on the actual distance from CG to CP and the size of the fins more so than the body diameter itself.
 
Oberon said:
@Binder - that's a good suggestion too. It's important to remember that "one caliber stability" is really just shorthand for "sufficient moment arm for the net aerodynamic restoring force to act on, plus margin for uncertainties in the calculation", and that depends more on the actual distance from CG to CP and the size of the fins more so than the body diameter itself.
Click to expand...

Just to stir the pot in a different direction, I'll throw out my usual vote for "Don't measure Calibers, measure percentages!"

I can't find them at the moment, but I had a couple nasa/aero articles regarding passive stability of sounding rockets and similar designed to have the CP and CG separated by "8-15% of the rocket's length" regardless of body diameter.

That can make a lot of sense when you compare fat stubbies where the margin is pretty close (or under) 1 caliber, against needle rockets like the mean machine where the margin can fluctuate by 2-3(!) "calibers" with shifts in the angle of attack.

Like Oberon said, 1 caliber is a shorthand approximation which in my opinion is misleading in a good number of cases. Perhaps stemming from an older guideline that 10:1 L/D is a good starting point when designing a model rocket. In that case, ~10% of L is right around 1 body diameter or "caliber".


Science!
 
My Wildman 4" V2 sims with a stability margin of about 0.1. Not a typo. I asked about it in a build thread and they all assured me it would fly just fine without nose weight. I couldn't help myself and put 6 oz of steel in the nose.

I did not add a phantom tailcone to test the impact on the sim. Without it, it sims to 4490' on a J280 smoky. Flew it at NXRS to 4417', straight as an arrow:

2017-NXRS-104.jpg

Photo credit: Gary Goncher
 
I read a similar post a while back about a Minnie-Magg. A few posters had reported good luck with no weight in the NC due to short-body behaviors. I had 6oz of BBs and epoxy in the NC. I thought, wow, it would be nice to get a bit more altitude. So I bought a new NC and loaded a 4gr CTI I-303. The launch was a "sheet-show" on the away pad. One complete loop then 3/4 of another then tomahawk trajectory at about 100' AGL... After burn-out it gracefully arc'd into the ground and rewarded me appropriately for my folly, a pile-o-pieces. I felt really bad about that flight. I was reckless. So, I bought another one and tried to figure out a way to make weight in the NC adjustable so I could back into the minimum balance. I used BIG quick-links attached to the NC totaling 8oz. I then removed one at a time for each launch. By the time I was down to 2oz I noticed a "little wiggle" and decided 3 oz was just fine for me and my I-303... In went the dowel, BBs and epoxy in the front of the NC. She's flown great since then and I feel I have a pretty good stability margin. My challenge was a good way to make a lot of weight, adjustable... One thing with the quick-connects, I didn't like that the weight could move a bit thus changing the results at bit. I bet others have much better methods.

What really floors me is the LPR pie-pan rockets (and spools) that fly fine... That shows me that I don't fully understand everything that is going on during flight.
 
Is there a general consensus as to what qualifies as a "short and stubby"?
 

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