Open Rocket Variations - G77 & G78

[TZ250]

Everyone,
I made a file in Open Rocket to get some estimates for my Dark Star Lite. The DSL is built, but not painted. I haven't flown it and I haven't yet bought an altimeter.

When I select G78 for a motor, the estimate is 2,142 ft., max. velocity of 308 MPH and max. acceleration of 388 ft./sec^2. When I select G77, which is what I have, the estimates are 1,507 ft., 244 MPH & 357 ft./sec.^2.

Why the difference? It's not a case of drastically different total impulses, like with the H250 and H268. I checked the total impulse for each motor, they are only 3 Ns different from each other.

I hope to fly it with the G77 reload this week. That will be my practice motor, for a test flight, to see how it acts. After that, I'll attempt L1 with a H128.

 

[ScrapDaddy]

Openrocket motor files were prolly pulled from some old book, and are a bit off sometimes, has anyone noticed the average thrust of the quest C6? It's around 3N in real life, unlike the OR number.

Generally OR will have more than one file per motor, just pick the most recent one, and check the source of that file.

SD out!

 

[Rocketman248]

If you compare the thrust curves of each motor, they really are quite different. The G78 has a much higher initial thrust. It has about 22lbs for the first .75 second then drops down to 18 at 1.25 seconds before tapering down for another .5 second. That last taper brings the average thrust down a bit.

The G77 has a more gentle curve. It peaks at around 22 pounds, but only briefly. It also has a much shorter burn.

 

[KerryQuinn]

TZ -

I looked up the data on thrustcurve.org on the G77 and G78,
these engines are quite a bit different....(the total impulse is much
different from what you stated above - where did you get your comparison?)

..avg...........total...........burn
..thurst.....impulse........time
..78 Ns.....141 Ns......1.8 sec..... G78 Cesaroni
..77 Ns.....105 Ns......1.4 sec..... G77 AeroTech

Based on these diferences, I would expect significantly different altitudes using these engines.

-Kerry

 

[Rocketman248]

Aerotech also makes a G78. It's a Mohave Green single use motor.

 

[KerryQuinn]

Rocketman - you are correct - I had forgotten about the G78 Green.
I added it to the table below and put a plot up here for side by side comparison. The two aerotech engines are quite similar and I would NOT
expect a large difference between these (in simulation or real life).

All THREE of these engines are included in the OpenRocket default download
library, so I would make sure that the original simulations were actually run with the two AeroTech engines. If the sims were run with the two aerotech motors and showed large differences in peak altitude, upload the ORK files to TRF and maybe we can figure out what caused the difference.

..avg...........total...........burn
..thurst.....impulse........time
..78 Ns.....141 Ns......1.8 sec..... G78 Cesaroni
..77 Ns.....105 Ns......1.4 sec..... G77 AeroTech
..78 Ns.....109 Ns......1.4 sec..... G78 AeroTech Green

-Kerry

FYI - I have recently seen the OpenRocket developers working on the next release of OR. They are including a very nice engine thrust curve plotting tool as part of this release.

 

[Rocketman248]

Hmmm...There seems to be some mixup on Thrustcurves site. There are two RASP files for the AT G78. One of them looks to be the new G80. Although, looking at the G80 page, there are 4 different curves depending on which file you look at.

For Rob, my guess is that you used the mixed up G80 file. Try your sim on the Jul 29, 2008 file.

 

[bobkrech]

Check the manufacturers and the certification websites first for the correct thrust curves. (NAR provides a .eng file with the certification paperwork. You can copy and past them into a .eng file.)

It's not always clear where the thrustcurve.org files come from.

Bob

 

[TZ250]

Thanks guys. I'll do some more checking. Of course a flight with real data is better than a simulation, but I was just comparing some things before the first flight.