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Zeta

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Hello Rocket forum members

I am a new level 1 with just a few flights under my belt. I have a couple PML rockets, and about $4000 worth of rockets saved in shopping carts with LOC, and MAC so Christmas might be good this year ! He He He.

Got a question about open rocket since it overestimates the flights I am getting and I can't figure out why. I launched a PML Explorer on a Aerotech H115 and logged 1665 feet. The OR sim was predicting 2700 feet.

Tonight I tried to sim an H195 with the explorer (42oz.) and it is giving me apogee at 5200ft which I know is not realistic.
Thoughts ?

Zeta
 

dhbarr

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What level of finish have you selected in your sim? And did you weigh your final as-built?
 

Zeta

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I ran the sim with 3 different finish levels and judging from how much or rather little that affects the final apogee the sim is still way off. The rocket is 42oz. It just does seem to make sense to me that an H195 can put 42 oz. to 5200ft. I have even added mass to the sim (16oz) which only took about 500 feet off the apogee.
 

tsmith1315

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Hi Zeta, welcome to the forum!

If you were to post your Open Rocket file here, someone would certainly take a look at it.
 

Zeta

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Hi Tim!
So I have a little more data. A flight yesterday
H195NBT - 10 (Aerotech DMS)
Predicted 2642 ft. from Open Rocket
Actual on 2620 ft.
So this makes sense a 22 ft difference is totally acceptable

This is the simulation that had me scratching my head.
H182-14
Predicted 5388 ft.
I did not fly this motor, I was using O.R. to select motors. Or at least better understand average thrust.
Is it possible the thrust curve is not correct for this motor in O.R. ?
Average thrust for the H182 should be less than the H195NBT, I can’t see how it would produce apogee at twice the altitude all other things being equal.

Thanks in advance for your thoughts,
Zeta
 

tsmith1315

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Average thrust for the H182 should be less than the H195NBT, I can’t see how it would produce apogee at twice the altitude all other things being equal.
More importantly, the total impulse for these motors are similar mid H at 218Ns & 236Ns. Unless one thrust profile was extreme and just didn't work well for the rocket (primarily thrust/weight issues), the altitudes should be fairly similar.

I couldn't quickly find a file for the Explorer, so I scaled one of my "average" rocket files to 2.56 PML tube and the weight came out to 35oz. Close enough. Those two motors sim to 2400' and 2600' for me in this rocket. (BTW, look at those total impulses again- 8.3% more impulse, and 8.3% more altitude) I'd say replace your questionable motor file. Looking at the H115 makes me think there is either an additional issue or there were rocket changes between your sims. That's a baby H at 170Ns, and sims to 1900' on my "test" rocket

Your posted file looks like an exported data file. Do you know where your rocket (.ork) files are stored?
 

Zeta

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H195 O.R. file
More importantly, the total impulse for these motors are similar mid H at 218Ns & 236Ns. Unless one thrust profile was extreme and just didn't work well for the rocket (primarily thrust/weight issues), the altitudes should be fairly similar.

I couldn't quickly find a file for the Explorer, so I scaled one of my "average" rocket files to 2.56 PML tube and the weight came out to 35oz. Close enough. Those two motors sim to 2400' and 2600' for me in this rocket. (BTW, look at those total impulses again- 8.3% more impulse, and 8.3% more altitude) I'd say replace your questionable motor file. Looking at the H115 makes me think there is either an additional issue or there were rocket changes between your sims. That's a baby H at 170Ns, and sims to 1900' on my "test" rocket

Your posted file looks like an exported data file. Do you know where your rocket (.ork) files are stored?
Hi Neil, I think you are right the questionable motor file is the culprit. I did put a lot of epoxy into the rocket at the nose and tail so it's heavier than your sim, however your sims would give me confidence in the motor I was picking. I did change a parameters from one sim to the next but honestly don't find surface finish or 1 degree of fin chant to make double or half the resulting apogee so I was pretty sure the 5200 ft estimate was not right ... but with the all being new to me just really appreciate the help!

O.R. is giving me a warning when I am drafting rockets "Discontinuity in rocket body diameter" which I find confusing as I was using the files from the O.R. data base for body tubes and nose cones. ????? ideas?
 

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tsmith1315

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Yes, that file. Disclaimer: I'm no OR expert.

The discontinuity in body diameter warning- look at your nose cone diameter (2.7"), and then look at the Quantum Tube diameter (2.684")- that's discontinuous. I don't think it matters for simulation. If it annoys you, just make them match.

When I selected your H182 configuration and ran the sim, I got a similar altitude prediction at 4568', with deployment at 346mph.
When I added a new configuration and selected H182-14 (I assume this used my motor file), the prediction is 2374'.

A couple of things I notice:
Fin cant and fin rotation should be zero:
Zets PML Explorer fins.png


I haven't messed around with adjusting recovery options, wind speed, etc. I'm old school, and just want to be sure it looks stable with a delay time estimate. and something to print in case an RSO wants to have a look.

That said, I notice that your recovery options shows ejection at 500'. The sim showed the rocket coming in ballistic, until high speed ejection at 500'. Which confused me for a bit until I realized that you had bypassed motor ejection.

With those items changed for a predictable ejection and using whatever H182 file it is that I have with a 10 second delay, I get this:

Zets PML Explorer H182 plot.png


Edited .ork file attached
 

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Zeta

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Hey Tim,
Thanks! I did mess around with a few things (fin cant) to see if it made any difference. Looks like your file is good. I need to learn how to load in thrust curve files to OR. Ignore the chute deploy stuff.... use the apogee prediction to decide how much to take off the delay on the DMS motor. The jolly logic goes off at 500. So ya OR thinks I can going to rip my chute up at 168 mph ... maybe there is a way to tell it hey man the ejection charge is at 10 sec. but the chute opens at 500 feet .... still trying to find all the buttons.
Thanks Man!
Zeta
 

Mike Haberer

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Hey Tim,
Thanks! I did mess around with a few things (fin cant) to see if it made any difference. Looks like your file is good. I need to learn how to load in thrust curve files to OR. Ignore the chute deploy stuff.... use the apogee prediction to decide how much to take off the delay on the DMS motor. The jolly logic goes off at 500. So ya OR thinks I can going to rip my chute up at 168 mph ... maybe there is a way to tell it hey man the ejection charge is at 10 sec. but the chute opens at 500 feet .... still trying to find all the buttons.
Thanks Man!
Zeta
FYI, fin rotation won't make any difference. You use it when you have two fin sets at one location on the rocket that have different geometries so that they don't sit on top of one another. In a single fin set model, it just rotates the fins around the z axis, so they appear at different locations on the 2D and 3D representations, but have no effect on the rocket performance or sim.
 

neil_w

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Hey Tim,
I need to learn how to load in thrust curve files to OR.
 

Mike Haberer

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Hey Tim,
Thanks! I did mess around with a few things (fin cant) to see if it made any difference. Looks like your file is good. I need to learn how to load in thrust curve files to OR. Ignore the chute deploy stuff.... use the apogee prediction to decide how much to take off the delay on the DMS motor. The jolly logic goes off at 500. So ya OR thinks I can going to rip my chute up at 168 mph ... maybe there is a way to tell it hey man the ejection charge is at 10 sec. but the chute opens at 500 feet .... still trying to find all the buttons.
Thanks Man!
Zeta
Since the sim doesn't understand beans about a chute release, I always add either a (dummy) streamer or drogue parachute to the model so that the rocket descends non-ballistic at roughly 50 mph. That way the descent phase is more realistic. Ideally, Rocksim and OR should understand that there is an ejection at apogee (or whatever time you set it at) and the rocket descends drouge-less until the main event, but they don't because the physics of that is ridiculously difficult to calculate and very model specific.
 

Kelly

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It's not the motor that is the issue.
In the simulation options for the questionable flights, instead of selecting "Standard Atmosphere", you have selected a custom atmosphere with 4mbar of air pressure. Normal air pressure is around 1000 mbar. So, essentially, you are simulating this flight in a near vacuum, are seeing no drag, and as expected you get a lot more altitude. You can see this if you plot the flights, and include "drag" as one of the plotted values. Fix the atmosphere, and you should get more reasonable values, as Tim showed.
 

Zeta

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Thanks Tim, I questioned the atmospheric pressure but didn't take the time to figure it out.
Just made the adjustment to 1018 mb and it brought the apogee to a more realistic 2300 ft. Thanks !

And to all the other contributors ...you guys are great !
 
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