Simulation Question

My wife bought me a Wildman Sport for birthday and I launched it three times on Saturday at our club's model launch. Two launches were on F20-4W that I have had for a couple years. Then the third was on a G76. All three great flights and seemed a good shakedown prepping for high power launch next weekend.

But... the two flights on F20s were much lower then simulations would suggest. Thrustcurve comes up with 736 feet and OpenRocket puts it at 716. (One note is that OpenRockets indicates F20EJ not sure if it is the same.) Anyway, my actual altitude according to an Estes altimeter was 475 and 510 feet. Unfortunately I didn't get a reading on the last flight.

I really don't mind if the rocket is that much more draggy then the initial guess. But the Cd calculates out to over 2 as compared to initial estimates of .6 or .75 supplied by the programs. My only real concern is that I would like to get the delay as close as possible.

Anyway, would the age of the motors explain the difference in altitudes? I would gladly put another load in to test, but not sure if there is much point since the delay might still be off quite a bit and ejection at high speed could occur.

Can't answer the question about the motors or altitude, but I launched my Sport on an F52-8T... My OR sim said it would need about a 7 second delay, but I definitely could have gone with the 5 second delay. Definitely did not get the height I thought I would. I chalked it up to the strong winds kicking it over, but I'll be watching this thread to see what others say...

2.0 does seem like a really high CD for a 3FNC rocket. Another possibility is that a short delay would have shortened the flight by truncating the coast phase.

the current AT F20(econojet) engine data shows a total impulse of 55newton seconds while the one in OR lists 61 Ns, that alone will make a difference. weathercocking also throws delay timing off.
Rex

Rex R said:

the current AT F20(econojet) engine data shows a total impulse of 55newton seconds while the one in OR lists 61 Ns

Click to expand...

The simulation document on the AT site is the old one (from 1996). I have updated the info in ThrustCurve.org to match the newer certification on the NAR site (from 2005).

So, you may want to update the motor data you use for OR to the most recent one on ThrustCurve.org:
AT F20W

I agree: Cd of 2.0 on a 3FNC seems ridiculously high. I see you mentioned you ran a sim on the motors--my question is how accurate is the sim? Did you measure and modify to reflect the built model? Another possibility to explain the severe discrepancy which has not yet been discussed is the altimeter itself. Is there a calibration procedure or anything for it (I've never used one of the Estes/Jolly Logics)?


Later!

--Coop

Did you weigh the rocket after construction and paint?

Was your launch pad rod/rail angled off of vertical, and of so, did you enter that angle into the sim?

Did you set the finish to polished or equivalent? Typically this is nearly impossible to actually achieve and could have an effect on actual drag.


Mark Koelsch
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markkoelsch said:

Did you weigh the rocket after construction and paint?

Was your launch pad rod/rail angled off of vertical, and of so, did you enter that angle into the sim?

Did you set the finish to polished or equivalent? Typically this is nearly impossible to actually achieve and could have an effect on actual drag.


Mark Koelsch
Sent from my iPhone using Rocketry Forum

Click to expand...

I agree with Mark. Definitely weigh the rocket and then compare it with the OR simulation weight. When using Rocsim and if the weight is higher thane the Rocsim value, I put the difference in weight as an added mass value roughly at the calculated c.g. Then I run Rocsim again to get the new calculated altitude.

aerostadt said:

I agree with Mark. Definitely weigh the rocket and then compare it with the OR simulation weight. When using Rocsim and if the weight is higher thane the Rocsim value, I put the difference in weight as an added mass value roughly at the calculated c.g. Then I run Rocsim again to get the new calculated altitude.

Click to expand...

No need to create an additional mass part. Just use the mass override tab to adjust the entire rocket mass and CG to your measured values.

Anbody want to create a sticky that answers the oft-asked question: "Hey, how come my flight doesn't match the simulation?"

I started to do that for RP, but never finished it. Let me see if I can find my notes, and I will gladly post them.

Probably will not happen until next week though.


Mark Koelsch
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I've been using the F21W in OR, or F21WL in RASP even earlier, as an approximation of the F20 Econojet. The F21 was actually 24 mm! If in doubt, compare with other references, such as the last Aerotech catalog.

Rocksim is showing 660 feet on the stock file wildman made. It also shows 33 MPH off the rail which is a bit slow. I'd assume the low power led to weathercocking. the G76 should have left the rail at about 60 MPH and been much straighter, I'm showing 1500' for that and I bet it hit close to that. G76 shows 20G's, F20 only 5. may have led to the reading error.

At any rate check the real weight and compare.

Sorry, let this get old. The weight on the rocket is after completion with paint. Also, I knew that it would be on the edge of launch conditions so used an 8 foot rail with almost no wind.

Using a CD of .8 places the altitude well within 10% of what I experienced. If I change the CD to .7 it is almost perfect with experience on a G53 and H165 motors. All the same, I really don't know how much variance to expect between motors. And the F20s have been sitting around the house for a few years as I just couldn't bring myself to stuff them into a rocket. (I purchased four of them with my first mid power rocket. On the first launch flames blew out the side of the rocket on the pad. Aerotech did replace the rocket and sent two motors to replace the one that failed. But that day left a big mark.)

Temperature, Humidity and Baro Pressure can affect flight too. If you don't have the observations at hand, you can usually get a history from Wunderground.com. Plug those in and see how the sim does.

I will keep that in mind. At our next launch though I will probably launch a couple more of the old F20s I have then launch on a new G75 and compare results to sim. I understand that if the motor is old it might not light as easily. I wonder if age might also affect total thrust.

I have a couple motors that came second hand that are older yet. Won't mater a whole lot because their delay is long enough that I will need to use an altimeter anyway.


Kirk