Congrats !
Did you post pictures on TRF ?
I especially want to see your boom lift recovery
-- kjh( trees suck )
Let's put an end to the "Base Drag Hack"
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Did you post pictures on TRF ?
I especially want to see your boom lift recovery
-- kjh( trees suck )
lakeroadster
When in doubt... build hell-for-stout!
Every rocket you show violates the base drag hack length to dia. must be less than 10:1 specification set by Bruce Levinson.
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Yes. I know.
I still think it's good to have more details and data.
The Levinson article quotes the 10:1 ratio a few times but it gives no explanation of where those numbers come from, what physical mechanisms might cause there to be different results for different aspect ratios, or why the drag forces it supposes are acting on the rear of the rocket would be dramatically different depending on what's going on much farther towards the front.
This situation leads to a lot of discussion about what, if anything, the 10:1 rule might mean. Does the hack actually work for long rockets as well? Can it actually be used everywhere not just for short rockets? If not does it suddenly turn off, such that it applies to a 9.9:1 rocket but not to a 10.1:1 rocket?
So, just for the record: no, the Base Drag Hack does not work as written for long rockets, it's too strong in that case. But the unmodified Barrowman equations also don't give great answers when body forces dominate.
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With ~20:1 L:W aspect ratio for the rocket overall, I think this is clearly a case where you want to use the percent of airframe length rule, rather than calibers. Your initial 2.32 calibers as reported by OR is not as conservatively safe as it seems, it's only in kinda the mid range of the rule of thumb for percent of airframe length at 11.3 percent.
FWIW, I'd target about 10% of airframe length for stability margin at burnout, or around 6x the margin your 0.35 cal will provide. Based on exactly no experience flying stuff like that, but just logicing my way through it.
Interesting that your CP position appears to get back to the original location at around Mach 1.55, rather than the Mach 2.0 or so that some literature predicts. I'm curious what OR is doing in calculating that.
Yes, the aspect ratio of the rocket is outside the 10:1 range for the 1-cal RoT.
I also forgot that I sim'd the rocket in 10 mph wind which is the norm for central TX.
When I set the wind speed to 0 mph the CP -vs- Mach Number looks more like the expected curve.
These are the same sims with 60 grams of nose weight:
This is the Stability -vs- Mach Number with NO WIND for the stable rocket with 60 grams of nose weight:
And this is the rocket without nose weight with NO WIND:
There is a fairly recent, related post here on TRF where the same question came up: CP Shift of Supersonic Rocket
Chuck Rogers gave a great explanation and @G_T posted plots from RAS Aero:
This is Gerald's Plot of CP -vs- Mach from RASAero and a profile image of his rocket:
Note that the shape of the curve for Gerald's Rocket is kinda-sorta similar to the OpenRocket curve for mine.
The event timing and the absolute values are different but so are the two rockets.
I am curious as to how the calculations work too.
Anyhow, more anecdotes ...
-- kjh
p.s. Can you think of a rocket kit with a 10:1 apsect ratio ?
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Chuck's post 9 in the thread you linked was posted earlier in this thread.
For BT-50, you'd need about a 7 1/4 inch tube with no fin overhang and an Alpha nose cone. I will likely build something close to that, but I'm not aware of any kits like that.
Or about a 4.7-in. BT-20 tube with the Wizard nose cone and no fin overhang. Doable with a 13mm motor, but not 18mm.
Checked Baby Bertha, but it's only 12.8 inches, so it's under. And seriously not in need of the BDH.
For BT-50, you'd need about a 7 1/4 inch tube with no fin overhang and an Alpha nose cone. I will likely build something close to that, but I'm not aware of any kits like that.
Or about a 4.7-in. BT-20 tube with the Wizard nose cone and no fin overhang. Doable with a 13mm motor, but not 18mm.
Checked Baby Bertha, but it's only 12.8 inches, so it's under. And seriously not in need of the BDH.
I need to post the flight to youtube. The treed Journey 75 I have launch and the treeining but not recovery from the tree. Giant halfburied burnt trash pile and broken glass, had to climb into the basket from the outrigger because of a enormous wild rose. I just wanted to get out of there.Congrats !
Did you post pictures on TRF ?
I especially want to see your boom lift recovery
-- kjh( trees suck )
Like you, I seek knowledge and not an argument, and like you I am a fan of the monocle emoji.Mine is a question, not an argument. I'm looking for knowing and don't have a stick in the game.
. . .
Here's a rephrase: "Rocket design software does not model Fat Boys/Warlocks/Mini-Maggs/etc. satisfactorily.
Regarding the second bit I quoted, I am not sure this is so. While not perfect, the weaknesses of our common simulation programs seem minor for the level of fidelity they provide and seem to be applied to rockets of all shapes. It seems to me more likely that people are simply trying to apply a poorly reasoned rule of thumb to the results of software. I have yet to see anyone in this thread explain why applying the hack in order to get to a 1 caliber result is better than simply recognizing that the 1 caliber rule of thumb is far too strict for short, fat rockets and thus probably not a good rule of thumb. (It's also far too lenient for long, thin rockets. Coincidentally I saw one of those go sideways today, though I didn't talk to the flier so don't know what assumptions they made about stability. It was bending like a banana on the RSO table, so its problems may have lain elsewhere.)
No doubt when taken to extremes the percentage of length rules of thumb (8-18% some say, 10-20% according to some, and no doubt there are others) break down too, but the extremes it takes to break them down seem far greater than the one caliber rule, which seems to work for a very narrow range of shapes and sizes.
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Stumbled upon another one that's darn close: "Part-of-Zon." One Estes 2.5-inch nose cone, one Estes 2.5-inch body tube segment, and a set of Partizon fins. (These are 75% size because 4 x 3/4 = 3, and they look better this way.) It's ~25.5 inches long. Looks cool. Needs a little nose weight, depending on what motors you mount. Thanks to @K'Tesh for the sim to use as a starting point.
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Looking for comments about the Apogee Catalyst. Not trying to (re)start any arguments, I'm just trying to make sense of it all. The Catalyst is 11:1 len/dia ignoring the fin overhang, or 12:1 considering the fins. In OpenRocket, I'm using the .rkt file on Apogee's website.
In particular, OpenRocket and Apogee's website - which undoubtedly uses RockSim - show a fairly large difference in Cp. Nearly an inch on a short-ish rocket. Using a G64 motor, OpenRocket shows stability at 0.712 cal / 5.89%. Which might raise eyebrows at the RSO table. But using Apogee's number, it's about 1.03 cal / 8.55%. So that's OK. I guess.
If I add a short base drag cone of the dimensions 3" dia and 1" length, it is near spot on with Apogee's Cp.
Just experimenting, when I do sims with wind speeds above what is prudent (like 15 - 20mph), I get instability on some heavier motors using OR's values. But if I use the base drag hack to simulate the Apogee Cp, the sims are stable.
Which makes me wonder what is different with RockSim, and which set of numbers do I trust?
(I'm going to try and attach screen shots. Let's see if it works....)
Hans.
In particular, OpenRocket and Apogee's website - which undoubtedly uses RockSim - show a fairly large difference in Cp. Nearly an inch on a short-ish rocket. Using a G64 motor, OpenRocket shows stability at 0.712 cal / 5.89%. Which might raise eyebrows at the RSO table. But using Apogee's number, it's about 1.03 cal / 8.55%. So that's OK. I guess.
If I add a short base drag cone of the dimensions 3" dia and 1" length, it is near spot on with Apogee's Cp.
Just experimenting, when I do sims with wind speeds above what is prudent (like 15 - 20mph), I get instability on some heavier motors using OR's values. But if I use the base drag hack to simulate the Apogee Cp, the sims are stable.
Which makes me wonder what is different with RockSim, and which set of numbers do I trust?
(I'm going to try and attach screen shots. Let's see if it works....)
Hans.
lakeroadster
When in doubt... build hell-for-stout!
It's 11:1... don't use the Base Drag Hack, it's for 10:1 and shorter. 0.712 caliber is fine, that's stable.
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Yes, I know.
My point was that using the Base Drag hack gives the same result as RockSim (presumably). So if BDH is not correct for the Catalyst, then RockSim's comparable result is also invalid.
Hans.
Edit: At our HP launches, most of the RSOs seem fixated on the 1 caliber rule. So therefore the Catalyst wouldn't be allowed. Unless I marked the body tube using Apogee's Cp number.
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Thanks for this Hans.
Do you mind posting your OpenRocket .rpt file so I can play along ?
Thanks !
-- kjh
NTP2
Well-Known Member
Maybe apogee changed the number, I don’t think that they would do that though.
Sure. Like I said, it's the .rkt file from Apogee. But here is the .ork file (unmodified), and the .ork with the mini-BDH:
Hans.
Edit: Seems I forgot to give Apogee's Cp. It's 27.9"
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There are lots of forum threads and Peak of Flight articles about the RockSim method, and why it is different. It is mostly about relieving Barrowman constraints on fins. The fins are sizeable on the Catalyst.
Your BDH and Rocksim results are similar for different reasons. RockSim method is presumably logical and rigorous. BDH, not so much.
Apogee designs are very robust and flight tested. There are 4, huge, swept fins on the Catalyst. It will fly great.
Those Catalyst fins are almost "clown like". I have the kit, just haven't put it together yet.
I read over some of TVM's comments about RockSim/Barrowman. The only thing I found is that RickSim allows more complex fin shapes. He said the simplified Barrowman that is commonly in use by others is designed for trapezoidal fins, which the Catalyst has. In the back of my mind, I was wondering if there is an ethical situation placing the Cp mark on the tube at a location other than where my OR sims say it should be. But TVM is a pretty sharp guy, I'll place the mark where Apogee suggests.
Hans.
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The Catalyst (0.712 caliber/5.89%) is just fine, while Levison's FatBoy (0.67 caliber/13%) needs a cone stuck to the ass end?
Ugh.
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More info here:
https://www.apogeerockets.com/downloads/Technical_Publications/Tech_Pub_17.pdf
I haven't checked, but it looks like the Rocksim Method will still end up computing a trapezoidal fin differently than original Barrowman.
lakeroadster
When in doubt... build hell-for-stout!
Follow the rules for the Base Drag Hack... less than 10:1.
It's a hack... follow the rules
As much as I like OR (and it's price!), I might have to break down and buy RockSim. I have seen a few other edge cases, like the LOC Forte that shows questionable stability in OR, but is apparently fine in RockSim.
Hans.
ReynoldsSlumber
Well-Known Member
If it was only to evaluate stability and knowing the air flow velocity isn't critical, you could basically build a static swing test apparatus. Remember a rocket is effectively a wind vane: instead of having a hard axis to swivel around, its swivel point is its CG - so that's where you place your swivel point for a static flow test.
Then you need a high velocity air flow: compressed air supply is probably the cheapest source for that. Depending on how far you need to go with the velocity will depend on your throughput requirements unless you're releasing the flow into a vacuum chamber - but that's for much higher mach numbers.
Valve off your compressed air with a decent throughput & fast acting valve and pass the flow out of a de laval nozzle with a low expansion ratio (say x1.7 - x2) for regular compressed air (100psi) expanding to 1 atm. Perfect expansion from 100psi to 1 atm should provide you with Mach 1.5ish exhaust. That will obviously get smashed with shock interactions with stagnant/slow air unless your testing chamber was small enough to match the nozzle expansion 1:1; nevertheless, you should still be left with some decent flow velocity if your chamber cross-sectional area is anything close to the expansion area.
TP
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Thanks Hans.
I am not a rocket scientist but ...
One thing I did was change the Mass and the CD of your BDH Cone to 0 so that the BDH Cone does not affect sim'd altitude.
See the official Open Rocket Doc: Open Rocket Tutorial > Implementing Base Drag CP Correction
After making that change, the altitudes for your original BDH sim about the same as the rocket without the BDH.
I am curious about the length of your BDH Cone ( 1.00 inch ) instead of Levinson's 'standard' Length( BDH Cone ) = Rocket Circumference ( in this case, 3 * Pi = 9.425 inch ) ?
Did you set it that way so that the final OR CP matches the Apogee / RockSim CP = 27.9 inch ?
When I appended a 'standard' Pi * D ( 9.425 inch ) Cone, the CP moved back to 'too far back' to 28.77 inch but the altitudes and velicities match the unmodified rocket when I adjusted the mass and CD for the BDH Cone( see below my sig ).
As for applying the BDH to this rocket ...
Like @Buckeye said, it is stable without it ( I don't think Apogee would sell an unstable rocket either
)The AoA -vs- Time plot for the unmodified Catalyst looks plenty stable to me with your original wind speed of 8 mph at 430 ft MSL and an ISA atmosphere:
And the OR Stability -vs- Mach Number Plot for the unmodified Catalyst looks OK too:
IMO, the Catalyst sims fine without the BDH ... even though the CP is 'only' 0.71 cal / 5.89 % for the 'unbuggered' rocket
What you're really after from the CP location is the length of the lever arm for the restoring normal force when the rocket's motion is disturbed.
But that is not the whole story ...
This is a fairly fat rocket at 3-inches and the normal force calculations factor in the reference area of the rocket ( 3-inch diameter = 7.07 in^2 ) and q ( 0.5 * Rho * v^2 ).
There is quite a bit of moment there ...
But what is the rule of thumb for that ?
I really am confused by the BDH !
-- kjh
This is the rocket with a standard BDH Cone where Mass = CD = 0
In this case OR moves the CP back beyond the RockSim CP !
lakeroadster
When in doubt... build hell-for-stout!
Here's my P-40 Warhawk. It's a 3" diameter rocket that is 10-3/16" long, which is a 3.4:1 length to dia. ratio.
I used the Base Drag Hack and stability is 1.04 caliber. Without the base drag hack stability is 0.002 caliber.
I never would have attempted the build without the Base Drag Hack.
I used the Base Drag Hack and stability is 1.04 caliber. Without the base drag hack stability is 0.002 caliber.
I never would have attempted the build without the Base Drag Hack.
Thanks Hans.
I am not a rocket scientist but ...
One thing I did was change the Mass and the CD of your BDH Cone to 0 so that the BDH Cone does not affect sim'd altitude.
See the official Open Rocket Doc: Open Rocket Tutorial > Implementing Base Drag CP Correction
After making that change, the altitudes for your original BDH sim about the same as the rocket without the BDH.
I am curious about the length of your BDH Cone ( 1.00 inch ) instead of Levinson's 'standard' Length( BDH Cone ) = Rocket Circumference ( in this case, 3 * Pi = 9.425 inch ) ?
Did you set it that way so that the final OR CP matches the Apogee / RockSim CP = 27.9 inch ?
When I appended a 'standard' Pi * D ( 9.425 inch ) Cone, the CP moved back to 'too far back' to 28.77 inch but the altitudes and velicities match the unmodified rocket when I adjusted the mass and CD for the BDH Cone( see below my sig ).
As for applying the BDH to this rocket ...
Like @Buckeye said, it is stable without it ( I don't think Apogee would sell an unstable rocket either
The AoA -vs- Time plot for the unmodified Catalyst looks plenty stable to me with your original wind speed of 8 mph at 430 ft MSL and an ISA atmosphere:
View attachment 632991
And the OR Stability -vs- Mach Number Plot for the unmodified Catalyst looks OK too:
View attachment 632990
IMO, the Catalyst sims fine without the BDH ... even though the CP is 'only' 0.71 cal / 5.89 % for the 'unbuggered' rocket
What you're really after from the CP location is the length of the lever arm for the restoring normal force when the rocket's motion is disturbed.
But that is not the whole story ...
This is a fairly fat rocket at 3-inches and the normal force calculations factor in the reference area of the rocket ( 3-inch diameter = 7.07 in^2 ) and q ( 0.5 * Rho * v^2 ).
There is quite a bit of moment there ...
But what is the rule of thumb for that ?
I really am confused by the BDH !
-- kjh
This is the rocket with a standard BDH Cone where Mass = CD = 0
View attachment 632988
In this case OR moves the CP back beyond the RockSim CP !
Yeah, I forgot to adjust the mass of the BDH. I was just interested in it's affect on Cp. As you surmised, I adjusted the length to correspond to Apogee's published number after trying the "standard" length. I also checked AoA and stability plots, but then wondered about the edge case of high wind, setting the wind speed to 20mph just to see what would happen.
As mentioned, probably my biggest concern is getting rejected at the RSO table by an official who is stuck on the 1 caliber rule. Using Apogee's Cp number, that wouldn't be the case.
Hans.
drewnickel
Resident Hybrid Nut
Not enough static pressure from a box fan to accelerate air to those speeds without also pulling a vacuum on the other side, and at that point you don't even really need the fan. One of the tunnels I've worked with just had a giant propane tank used as a vacuum reservoir on one end and used atmospheric pressure on the inlet going through a CD nozzle to get to like, Mach 4 ish for a couple seconds at a time.
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Levison never explained the origin of using Pi*D for the length. He says many aerodynamic sources show that the flat disk CP is 2.2 calibers behind, yet doesn't bother to reference any of them. He also goes on to say that the CP measurement varies widely. There is not much engineering rigor in the article. People can write anything they like in the POF newsletter. TVM will publish it, but he may put a disclaimer on it, as he did for the Levison articles.
In contrast, the Barrowman and Galejs work show mathematical derivations, and in the case of Barrowman, probably some level of peer review by NAR R&D judges. Rogers plots Barrowman and modified Barrowman against some flight data on the RASAero website.
Gotcha !
So how about this ole-timey, tried -n- true method for determining the CP of a rocket that you KNOW is stable for those pesky RSO's:
Tools needed:
PENCIL
ERASER
RULER
FINGER
In addition, you will need:
A LOADED ROCKET, ready to present to the RSO and fly
Before approaching the RSO, say back at your Prep Area:
1. Measure the diameter of your LOADED ROCKET
That measurement is called One-Caliber.
2. If there is already a CP marker on the rocket, use the ERASER to remove it.
3. Balance the LOADED ROCKET on your extended FINGER.
That is the CG
4. Use the RULER to measure One-Caliber backwards from the CG, toward the Flamey End of the Rocket
5. Mark that point on your LOADED ROCKET with your PENCIL
That is the CP
6. Proceed to the RSO and if the RSO asks, show them the CP marker.
7. Fly your stable rocket
-- kjh
I scanned through that paper, and the gist of it was to calculate an equivalent trapezoid for complex fin shapes and then use that equivalent shape in the conventional (Centuri TIR-33) Barrowman equations. To me.... It looks like the equivalent trapezoidal shape for a simple trapezoid is the original trapezoid. So it ends up using the original Barrowman method. Meaning that the RockSim method should agree with the OpenRocket method, assuming that the original Barrowman method is implemented the same in both cases. I'm wondering if these "equivalent trapezoid" calculations are introducing an inaccuracy. But determining that is above my pay grade.
So now I'm even more confused.
Hans.
