Zephyr build - Open Rocket concerns

[danielhv]

Finally got done painting. Installed the chute and got the weight coming in at 1508g. Hung it from a string and got a CG of 39.75". Updated the Stage item in Open Rocket to reflect those numbers, but the stability is now pretty low. Showing .665 cal, with a CP of 42.411in. Putting a motor in it makes it even worse. Not sure quite how this happened - followed the directions pretty close. Did I get too carried away with epoxy or something?

 

[ljwilley]

No, you did nothing wrong. Even if you did put on too much epoxy it wont hurt anything. Just take some metal BBs or something and epoxy them into the nose tip to add weight and counteract the tail weight. Add enough weight so that the stability in open rocket is better.

 

[Buckeye]

According to the Apogee product page, they list the CP as 44.89", likely using the Rocksim method. OR uses Barrowman or enhanced Barrowman which are very conservative, in my experience.

You probably didn't overdo anything, and I think the rocket will be fine as is. TVM's designs are excellent and good fliers. The Zephyr is a popular kit, and many people L1 with it. I have the Katana (Zephyr's big brother) and it flies straight and true on various motors.

If anything, cut open the nosecone and add a payload tube to house a GPS tracker. This added weight is more useful than BBs.

 

[thzero]

Where is the Cg measured from?

 

[danielhv]

Where is the Cg measured from?

tip of the nose.

 

[SkyFire]

Here is Rocksim configuration (without parachute or motor);

 

[waltr]

OR Base Drag Hack useful?

 

[Old School Doug]

I built the Zephyr according to the instructions and experienced the same issue in OR. Prior posts noted the difference between the CP in OR vs RockSim. Dropping in a base drag hack in the OR file to push the CP to the RockSim value and brought the stability into an acceptable range. I've flown the stock build Zeph numerous times and it has always been an excellent performing rocket with zero stability issues.

 

[neil_w]

I admit that sometimes I wonder if what Rocksim is doing is somehow incorporating base drag effects on CP (maybe or maybe not using calculations equivalent to the notorious base drag hack). That would explain quite a bit of the disparity between the two programs. Some careful testing could probably answer the question more definitively.

If this is the case, then implementing the base drag hack in OR would be the most effective fix for this issue (for now). Note, however, that there's a bug in Cd override in OR in the current version that doesn't quite get things right when overriding the Cd of the base drag cone to zero, so you have to be careful with your flight sims.

 

[Back_at_it]

Interesting. I built mine pretty close to stock with the only real change being the additional of an extra centering ring at the top end of the motor tube and moving the eye bolt up to that ring. DOing this moved the CG forward a bit. I balanced mine without motor and updated my SIM in Open Rocket. My CG is at 37.9 with a stability of 1.14.

Mine was built with BSI epoxy for reference.

If you're concerned with stability, you could add nose weight to get the CG where you want it. I can tell you that mine has flown on a G80 with a stability of .892 and the flight was dead straight and stable.

 

[SkyFire]

Built mine with a 54mmt and nose cone bay for tracker.
Flies great!

 

[Buckeye]

OR Base Drag Hack useful?

Funny, the technique was developed with Rocksim by Bruce Levison many years ago and documented in Apogee Peak of Flight newsletters. Now it's the "OR Base Drag Hack."

I admit that sometimes I wonder if what Rocksim is doing is somehow incorporating base drag effects on CP (maybe or maybe not using calculations equivalent to the notorious base drag hack).

I don't think it is solely base drag. There are some POF articles describing what the RockSim method does without giving away the secret sauce.

The Levison method was to counteract stupidly-stubby rockets like a Minie Magg. The Zephyr has a L/D of 14, hardly stubby.

I think this hack is now being over-used and abused. "I don't like the OR value for stability, so I'll add a fictitious cone behind the rocket and feel better about it."

 

[danielhv]

Built mine with a 54mmt and nose cone bay for tracker.
Flies great!
View attachment 599431
View attachment 599432
View attachment 599433

Really interested in learning more about this. Do you have any other pics you can send me so I can figure out how to do this?

 

[David_Stack]

Really interested in learning more about this. Do you have any other pics you can send me so I can figure out how to do this?

Looks like the nose cone bay kit as offered by Mac Performance:

https://macperformancerocketry.com/products/pinnacle-nosecone-bay-kits

 

[SkyFire]

Looks like the nose cone bay kit as offered by Mac Performance:

https://macperformancerocketry.com/products/pinnacle-nosecone-bay-kits

Yep!

 

[danielhv]

Nice! So is it just a tube in there? Or does a sled fit in there? Just curious what keeps the GPS module from bouncing around. Also, I don't really understand the part on their website that states:

Do you have a project that requires additional nosecone weight? Simply drill a hole in the pre-marked bulk plate and bolt it on the front of the bay. No need to epoxy lead shot or ????? into your nosecone ever again.

 

[JoePfeiffer]

I admit that sometimes I wonder if what Rocksim is doing is somehow incorporating base drag effects on CP (maybe or maybe not using calculations equivalent to the notorious base drag hack). That would explain quite a bit of the disparity between the two programs. Some careful testing could probably answer the question more definitively.

If this is the case, then implementing the base drag hack in OR would be the most effective fix for this issue (for now). Note, however, that there's a bug in Cd override in OR in the current version that doesn't quite get things right when overriding the Cd of the base drag cone to zero, so you have to be careful with your flight sims.

I just applied the hack to the Zephyr's .rkt file, and it moved the CP pretty close to where RockSim had it (45.822" vs. 44.89 cited above). So it does look like RockSim is incorporating base drag in its CP calculation.

 

[mikec]

I admit that sometimes I wonder if what Rocksim is doing is somehow incorporating base drag effects on CP (maybe or maybe not using calculations equivalent to the notorious base drag hack).

Rocksim uses additional "proprietary" terms not included in Barrowman having to do with fin interactions, AFAIK not base drag (see section 9.5 in the Rocksim manual, https://www.apogeerockets.com/downloads/PDFs/Rocksim.pdf ). I've never been convinced that they were correct, but Barrowman is often pretty conservative.

 

[SkyFire]

Nice! So is it just a tube in there? Or does a sled fit in there? Just curious what keeps the GPS module from bouncing around. Also, I don't really understand the part on their website that states:

Do you have a project that requires additional nosecone weight? Simply drill a hole in the pre-marked bulk plate and bolt it on the front of the bay. No need to epoxy lead shot or ????? into your nosecone ever again.

You just use some foam rubber to protect it in the bay.
In the top bulk plate you can bolt on weight if needed.

 

[Old School Doug]

I think this hack is now being over-used and abused. "I don't like the OR value for stability, so I'll add a fictitious cone behind the rocket and feel better about it."

In my case, adding the base drag was done simply to determine if I could bring the CP into line with what Apogee stated it should be, not to "feel better" about launching it. I saw more than enough stock built Zephyrs fly straight and true to know that stability would not be an issue when I put it up on it's 1st flight. I would have lit the fuse whether I added the hack to the OR file or not.

 

[mikec]

Hung it from a string and got a CG of 39.75".

The Rocksim CP per the Apogee website is 44.89 inches. Measure or sim where the rocket's CG is with the motor you plan to fly and see where you stand; the CG without a motor isn't interesting by itself. It does seem like you may have ended up more tail-heavy than Apogee assumes, though it may still be OK.

 

[RimshotRed]

Built mine with a 54mmt and nose cone bay for tracker.
Flies great!
View attachment 599431
View attachment 599432
View attachment 599433

That is a cool idea with the cap. what size did you use and did you have to cut out the center of the nose cone? I am looking at getting the Zephyr in the future for level 1 cert. Thanks for any info you can share. Very nice looking rocket.

 

[SkyFire]

That is a cool idea with the cap. what size did you use and did you have to cut out the center of the nose cone? I am looking at getting the Zephyr in the future for level 1 cert. Thanks for any info you can share. Very nice looking rocket.

I used the 4"-54mm nose cone bay; https://macperformancerocketry.com/...innacle-nosecone-bay-kits?variant=10395087815
Easy to install; simply cut off the end of the nose cone and glue in the bay.

 

[Buckeye]

I just applied the hack to the Zephyr's .rkt file, and it moved the CP pretty close to where RockSim had it (45.822" vs. 44.89 cited above). So it does look like RockSim is incorporating base drag in its CP calculation.

I don't think you can make this conclusion. I just re-read the POF article.

https://www.apogeerockets.com/education/downloads/Newsletter238.pdf
In summary, TVM says the RockSim method is the Barrowman method but with fewer restrictions on fin shapes. No mention of additional internal base drag magic. That's why the Levison method was developed as a supplement to the RockSim method for very small L/D rockets.

 

[neil_w]

In summary, TVM says the RockSim method is the Barrowman method but with fewer restrictions on fin shapes.

If that's actually true than it wouldn't explain the significant difference in CP calcs on this very simple 3FNC.

(I just scanned through the article quickly: interesting)

 

[mikec]

IMHO, Apogee does themselves no favors by not clearly documenting what the differences are in the Rocksim stability calculations, and Newsletter 238 doesn't do a great job. This has been debated many times on TRF over the years, see https://www.rocketryforum.com/threads/rocksim-vs-barrowman.111184/ for example.

Some people have gotten into trouble pushing the stability envelope with Rocksim. I would not fly something so on the edge that OR said it was unstable and Rocksim said it wasn't, but YMMV.

That said, the Zephyr design is likely hard to mess up unless you ignore Apogee's CP and end up with a way far aft CG.

 

[Buckeye]

If that's actually true than it wouldn't explain the significant difference in CP calcs on this very simple 3FNC.

(I just scanned through the article quickly: interesting)

The Rocksim method mentions properly handling fin root edges on transitions. The Zephyr has a funky notch in the root that overhangs the base. Maybe that is related, dunno. I honestly never read the Barrowman paper, nor OR's implementation, and nobody really knows what Rocksim's code is doing.

IMHO, Apogee does themselves no favors by not clearly documenting what the differences are in the Rocksim stability calculations, and Newsletter 238 doesn't do a great job. This has been debated many times on TRF over the years, see https://www.rocketryforum.com/threads/rocksim-vs-barrowman.111184/ for example.

With the looming threat of OpenRocket, do you really blame them for not divulging their method? Newsletter 238 mentions a technical paper available for purchase, but that doesn't seem to exist anymore, probably to protect their IP.

Thanks for the old TRF link. Again, no mention of base drag in the RockSim method. Just fins.

 

[RimshotRed]

I don't think you can make this conclusion. I just re-read the POF article.

https://www.apogeerockets.com/education/downloads/Newsletter238.pdf
In summary, TVM says the RockSim method is the Barrowman method but with fewer restrictions on fin shapes. No mention of additional internal base drag magic. That's why the Levison method was developed as a supplement to the RockSim method for very small L/D rockets.

I used the 4"-54mm nose cone bay; https://macperformancerocketry.com/...innacle-nosecone-bay-kits?variant=10395087815
Easy to install; simply cut off the end of the nose cone and glue in the bay.

Thank you for the information. cool idea I will be trying it out.

 

[Buckeye]

All right, I re-read the Levison articles in the Peak of Flight. His motivation for mimicking a flat plate CP in RockSim was to make the Estes Fat Boy stability margin above 1.0 caliber so that it jived with the rules of thumb. Without the imaginary cone, the margin was 0.6.

The Fat Boy was still stable with a margin greater than zero. 1.0 caliber is not a hard law of aerodynamics. It is just a slop factor.

This base drag hack now seems like a silly waste of time in any software. It is just easier to remember that smaller stability margins are OK on stubby rockets.

 

[JoePfeiffer]

Rocksim uses additional "proprietary" terms not included in Barrowman having to do with fin interactions, AFAIK not base drag (see section 9.5 in the Rocksim manual, https://www.apogeerockets.com/downloads/PDFs/Rocksim.pdf ). I've never been convinced that they were correct, but Barrowman is often pretty conservative.

That guide has a 2003 copyright, and Levison's article wasn't published until 2006. So the fact the guide doesn't mention it says nothing about whether RS is using it now.