Charles_McG
Ciderwright
So just what do you folks do with the data from a logging flight computer, like an eggtimer Quantum or Proton?
I compare it to the sim, and see how I can improve the sim.
Now, I know that my sims aren't perfect to begin with. I enter the major components, with the shapes and profiles reasonably close. Some things are hard - the fins on my Malemute are single side wedges, for instance - sharp top, fat bottom - I don't know how to sim that. And I measure the final weight and Cg, and use the overrides. And I set the appropriate launch conditions, rail length, ignition and deployment delays.
But after the first flight, i still end up with something like this.
Yes, I'm using non-standard tools to graph. I'm using JMP from SAS. It handles data sets with non-matching x axis values (different time increments in this case) handily.
You can see that the apogee was way lower than predicted. Velocity was lower and Gs were slightly low. In fact, I think there's a clue there - it looks like the difference in real v sim G might be increasing with velocity. and the coasting deceleration starts a little higher than sim. Sounds like my drag might be off.
Normally, I would tweak the surface finish and mass a bit to make the apogee match - but I'd got a lot of datapoints here, and besides, the mass change to tune the apogee is unreasonable - it makes a big mismatch during the burn and staging portions.
This rocket has a decent amount of fine surface detail, pretending to be a NASA sounding rocket payload, but done as bas relief to be better visible. So a drag issue doesn't surprise me.
I've had limited success playing with surface finish to change Cd. I can find an old Crazy Jim post talking about a Cd tab - but I think that's Rocksim.
My approach is to add a 'drag plate'. I insert a transition in the body of zero length and mass. It doesn't affect the Cp (at least the static one), but does make an element that I can tune by adjusting the diameter.
So I adjust the diameter of the drag plate to match the measured apogee.
The drag plate is just above the fins.
This time I'm plotting Alt, V and Gs together:
Very close. But there's a notable different in approach to apogee, and a slight difference in V and Gs. Since the sim is high, I do one more tweak, adding 10% mass (I didn't have the dog barf in the measured weight) and trimming the drag plate to rematch apogee. The final drag plate was 3.6" on a 3" body.
And the result:
The green line is the second tuning. It's a little under for Gs, but spot on for V, especially in the sustainer coast. And a somewhat better fit for approach to apogee.
The proof will be the next flight, of course. Got to have a second datapoint. The sustainer will get an H53 instead of a G126.
And the booster? Well, it didn't have a logging altimeter on board, so I can't do the same coasting analysis. But I did look at the ground based video a lot, and as far as I can tell, it's matching it's sim close enough. Much simpler surface and fins.
I compare it to the sim, and see how I can improve the sim.
Now, I know that my sims aren't perfect to begin with. I enter the major components, with the shapes and profiles reasonably close. Some things are hard - the fins on my Malemute are single side wedges, for instance - sharp top, fat bottom - I don't know how to sim that. And I measure the final weight and Cg, and use the overrides. And I set the appropriate launch conditions, rail length, ignition and deployment delays.
But after the first flight, i still end up with something like this.
Yes, I'm using non-standard tools to graph. I'm using JMP from SAS. It handles data sets with non-matching x axis values (different time increments in this case) handily.
You can see that the apogee was way lower than predicted. Velocity was lower and Gs were slightly low. In fact, I think there's a clue there - it looks like the difference in real v sim G might be increasing with velocity. and the coasting deceleration starts a little higher than sim. Sounds like my drag might be off.
Normally, I would tweak the surface finish and mass a bit to make the apogee match - but I'd got a lot of datapoints here, and besides, the mass change to tune the apogee is unreasonable - it makes a big mismatch during the burn and staging portions.
This rocket has a decent amount of fine surface detail, pretending to be a NASA sounding rocket payload, but done as bas relief to be better visible. So a drag issue doesn't surprise me.
I've had limited success playing with surface finish to change Cd. I can find an old Crazy Jim post talking about a Cd tab - but I think that's Rocksim.
My approach is to add a 'drag plate'. I insert a transition in the body of zero length and mass. It doesn't affect the Cp (at least the static one), but does make an element that I can tune by adjusting the diameter.
So I adjust the diameter of the drag plate to match the measured apogee.
The drag plate is just above the fins.
This time I'm plotting Alt, V and Gs together:
Very close. But there's a notable different in approach to apogee, and a slight difference in V and Gs. Since the sim is high, I do one more tweak, adding 10% mass (I didn't have the dog barf in the measured weight) and trimming the drag plate to rematch apogee. The final drag plate was 3.6" on a 3" body.
And the result:
The green line is the second tuning. It's a little under for Gs, but spot on for V, especially in the sustainer coast. And a somewhat better fit for approach to apogee.
The proof will be the next flight, of course. Got to have a second datapoint. The sustainer will get an H53 instead of a G126.
And the booster? Well, it didn't have a logging altimeter on board, so I can't do the same coasting analysis. But I did look at the ground based video a lot, and as far as I can tell, it's matching it's sim close enough. Much simpler surface and fins.
