I want to use Wrasp to sim altitudes, delay times and launch rail exit speeds for a a number of mid power rockets kits that will fly E-G motors so that my Christmas presents will be ones I can fly successfully! Wrasp defaults to a CD of .6 but I have been told to use .75. If I were going to use one CD in Wrasp to sim a number of different rockets, all mid power - what would you suggest? If one CD won't work, how do I determine the appropriate CD for a rocket? Thanks.
Real-world flight experience has shown me that the .6 CD in wRASP is too draggy. This also of course depends in the design on the rocket. Most 3FNC designs have a CD of less than .6
My wRASP sims always came out low compared to the readings from my altimeter, usually by 600-800 feet - but sometimes more. Adjusting the CD down until the sim altitude matched the altimeter is the method that I used to find the CD. Not super-accurate, I know, but better than guessing .6
Orange is a TFNC based on a full length body tube. All others but the Alpha are full length tubes.
In my own study, the biggest determinant of a well trimmed standard model was length to width ratio. Stubbier rockets have *lower* Cd's, but higher cross-sectional areas. Long, thin models have high Cd's. If you over-fin a model, you pay, of course.
Suggest you measure the Cd in flight tests, based on altitude or ascent (not delay) time. In this way, you base your simulations on observables, rather than on magic numbers. Use short burning motors for the tests; otherwise your results are too confounded by motor variability. (Actually, the procedure is a fun academic exercise in Stat 101 ... if such a thing were possible :-) Use motors with enough impulse that drag accounts for a lot of altitude loss. (One popular example of a configuration to avoid is a 60-gram Alpha powered by an A8. Cd makes so little difference in such a flight that you cannot measure it.) Finally, do this on dead calm days.
For initial estimates, would suggest that, instead of a single Cd, you use upper and lower bounds. This is normally enough to get a good handle on optimal mass, for example. After measuring these things, your eyeball gets calibrated enough that the Cd interval gets you altitude estimates within ~20%.
Rail exit speeds will normally be affected very little by Cd. In addition, rail friction can outstrip Cd. (Recommend ChapStick on the launcher.) Delay times are affected by wind speed and launch angle, neither of which WRASP can help you with.
Thanks, but a couple of points. 1) I can't fly and calculate because I don't have the rockets yet 2) Rockets of interest are midpower - LOC , PML, Aerotech, Binder Design - 1.5 to 4" diameter, flying on E thru G composite motors - generally 2-3" diameter - not Estes rockets or motors.
Prior rmr postings said CDs below .5 are suspect, and from the postings so far, maybe the default value of .6 is the best to use. Let me know what you think, group.
My experience is that as you get to higher power rockets, CD values drop. .6-.7 might be appropriate for an Alpha, but my 3FNC level 2 flight backtracked to something in the 0.3 range.
In fact, *all* Cd's are suspect. The Cd value you derive from flight is very sensitive to your measurements. Your measurements are error prone and there is much variability in conditions and motor performance. In addition, the expreimental design is key. I mentioned the underpowered problem, and the necessity for calm and short burning rockets. Also, repitition!
All error is attributed to Cd. Thus averages are important. It is possible to get
95% confidence intervals, as I did below.
Bottom line - it isn't the source you prefer or the rumor that too low a Cd is suspect. It's the data, the analysis, and the experiment.
Regards,
-Larry C
A good result: ESTES ALPHA III (MRC B6, 2.48 CM DIAMETER, 90 DEGREES F)
Mass Time Altitude Cd Adj Alt Adj Time
50.5 5.84 552.00 .4400 558.35 5.86
50.3 5.59 551.57 .5482 518.75 5.60
50.4 5.52 504.61 .5795 508.65 5.53
49.3 5.38 490.01 .6639 483.77 5.36
48.9 5.53 514.68 .5950 503.18 5.50
49.3 5.74 544.16 .4982 536.10 5.71
Adjusted altitudes and times are from simulations using the single-flight backtracked drag coefficients
with a launch masses of 50.00 grams. These are the numbers to be used for overall Cd determination.
Mean Adjusted Altitude =
518.24 feet
Median Adjusted Altitude =
513.70 feet
Mean Adjusted Time =
5.5933 sec
Median Adjusted Time =
5.5650 sec
Cd from Mean Adjusted Altitude = .
0.5498
Cd from Mean Adjusted Time =
0.5511
Mean Cd =
0.5541
Cd from Median Adjusted Altitude =
0.5637
Cd from Median Adjusted Time =
0.5641
Median Cd =
0.5638
95% Con-fidence Interval for Mean Adjusted Altitude = 490.81 -
545.67 feet
95% Confidence Interval for Cd (Asymmetric) =
0.4724 - .6388
95% Confidence Interval for Cd (Symmetric) =
0.4731 - .6394
These are simulations from closed form approximations. Using a full simulation with thrust curve
and air density depreciation, I get a mean Cd of 0.544 for the Alpha III. The difference is well
within the confidence interval. I doubt that this Cd would extrapolate well for larger motors,
because the ALPHA III fins are extraordinarily flexible. Larger motors might cause them to buzz.
It is undoubtedly usable for stiff-finned ALPHA's. Any actual difference is likely within experimental
error.
A poor result:
ESTES QUICKSILVER/(ESTES C6, 3.43 CM DIAMETER, 80 DEGREES F)
Mass Time Altitude Cd Adj Alt Adj Time
89.6 6.59 679.11 .6517 690.31 6.60
87.7 7.27 814.21 .4380 815.03 7.27
87.8 5.52 503.24
1.2762 503.78 5.52
87.6 6.73 713.94 .6032 713.94 6.73
Mean Adjusted Altitude =
680.77 feet
Median Adjusted Altitude =
702.13 feet
Mean Adjusted Time =
6.5275 sec
Median Adjusted Time =
6.6600 sec
Cd from Mean Adjusted Altitude = .
0.6726
Cd from Mean Adjusted Time =
0.6806
Mean Cd =
0.7404
Cd from Median Adjusted Altitude =
0.6269
Cd from Median Adjusted Time =
0.6289
Median Cd =
0.6274
95% Con-fidence Interval for Mean Adjusted Altitude = 474.22 feet - 887.30 feet
95% Confidence Interval for Cd (Asymmetric) =
0.3504 - 1.4415
95% Confidence Interval for Cd (Symmetric) =
0.1524 - 1.3284
Clearly, this drag coefficient isn't well nailed down, and the values are all carried to far more
precision than we actually have. The Cd values from means vary considerably; those from
medians are virtually identical. They would be precisely identical if I had had one more motor
that day. Since there were an even number of values, I had to average the two middle values
to get the median. That accounts for the small differences, which are well within precision.
(I quote my best Cd value as 0.65, and apologize that it is rough.)
0.3 is suspiciously low for a 3FNC, even one built by a master. To get good estimates of Cd from flight performance you need a high burnout velocity. and a low weight or ballistic coefficient. Did your two models have similar burnout velocity and ballistic coefficients? Higher Rn flights should have slightly lower Cd, but not as much as your example shows.
I wonder if your HPR engine data file over estimates motor performance more than the MR motor file. Since the HPR motor files and simulation software are often used to ensure that flights do not exceed waiver altitudes, an optimistic motor file (more total impulse than actually delivered) would be conservative, meaning safer for the intended use. However, this would also cause Cd estimates based on actual flight performance, using the simulation software to estimate to the Cd, will bias the Cd estimate much lower.
And they have a ballistic coefficient that makes accurately back tracking Cd more difficult.
You may be able to get Cd from the kit manufacturers. Still, Cd of a kit can vary substantially with different builders. What ever Cd you decide to use, run the sims with +/-20% of Cd values, so you know how sensitive your flights are to Cd, and to decide what might be a worst case estimate.
Jerry - what do you think is a reasonable CD to plug in wrasp for the calculations I want to run? I am not looking for precision (shoot, I have seen two altimeters in the same rocket give readings that were surprisingly different - in rocketry, even the altimeters can't agree I guess!), I am looking for approximations that are reasonable and will give me ballpark numbers that will not be significantly misleading. So far, Joe Michel says .6 is probably too draggy for my purposes.
Here is a rule of thumb I use. For "under BT-50" rockets with launch lug I use 0.75 (the GHS number).
For rockets 1-2" with launch lug I use 0.60 and 2.25-4" 0.45.
This is for rockets with an aspect ratio between a USR All Weather and a USR Banhee. The wide variance in numbers is due to the launch lug being a large fraction of total drag on smaller rockets and a smaller fraction on larger rockets.
The CD(r) for a USR Aero-Roc with 1/4" launch lug is 0.42 or so and this figure has been backtracked as well as calculated within a band of about
0.07 CDr or so and has an almost 35 year track record.
They are considerably lower drag than lugs on average. Subtract
0.08-0.10 CDr (2-4" rockets) for buttons and 0.15 CDr for tower launch.
I saw a rocket that was waxed at a records trials I hosted (Dave Cook IIRC) in the mid-80's and it was "considerably slicker" than any regular paint, even buffed paint I have seen. So after the lug and assuming no boattail, I would say surface friction is the largest factor.
Back in the "good old days" Dave and I used to build some really slick finished altitude models. Many coats of Aerogloss, rubbed and polished to a mirror like finish. Haven't done that in years. Decades :-(
BTW, I think base drag might beat surface friction.
No Jerry, you can't take credit for that. Altitude was never hugely popular, due the need for skilled tracking personnel. However, Pink Book revision cycles trashed altitude events by giving them relatively lower weighting factors. Eggloft Altitude was still popular for a while, until ELD was created to virtually kill off ELA. Some people want to compete on Barometer altimeter results, with no altitude tracking, but that's another can of worms.
The real tragedy is not the decline of altitude competition, but the lack of tracking at club launches. Rocketeers could always get any of their models tracked for altitude, and also for R&D flights, at a club launch where a contest tracking event was being flown.
Agreed, but in the west we (Irvine, Frost, Barber, Stine, Gracey) were certainly giving it the college try. We had alot of altitude events.
One of the claims made against me by NAR President Miller as a basis for my removal was tracking a "sport rocket" using "club gear". It was my gear and the NAR event was well over and done with.
Hey guys, don't hijack this thread to rehash different views of past events - start a new topic if you want to do that. Jerry - thank you for your posts on CDs to use in wrasp - very helpful and very clear and simple - I appreciate your sharing your knowledge and experience. Thanks to all the others as well. I like Larry's comment that all errors are attributed to CD (the only variable you change to match the altitude recorded). If anyone else wants to chime in on the wrasp CD topic, I am still watching and learning.
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