TARC Question

[abw]

Last weekend my friend and I launched our TARC rocket a couple of times to calibrate it (750ft, 45sec) and did pretty well in terms of altitude on our first flight: 760.

The links to the YouTube videos are here: https://www.rocketryforum.com/showthread.php?t=1531

However, on our next two flights, the altimeter read 731 and 732. The conditions were exactly the same, and nothing had been altered. Can anyone think of a reason that there was such a consistent drop in altitude? The only thing we could think of was that the two motors had been out in the cold for a while and therefore didn't burn as efficiently...

Also, does anyone know what the cutoff score typically is to make it to the finals?

Thanks!

 

[Handeman]

Not sure about TARC scores, I seem to remember something about 18 to 20, but that could be way off.

As for the difference in altitude, it could be a couple of things. First there is a range of thrust for a motor. It can vary quite a bit and still be "in range". The other is the delay. Did the second two have consistant delay times? Was the first one long and coast more?

I would recommend that when you time the flight, you also time the "time to ejection". That will give you another data point.

 

[kjohnson]

The cutoff score has been around 20. Sometimes slightly below and sometimes slightly above.

Motors with the same designation and even within the same batch can have variability in the amount of thrust they produce. I'm sure there are others who can answer with more certainty, but 10% from the published values comes to my mind.

kj

 

[abw]

Thanks guys. I figured the qualification score would be around there somewhere. It's interesting to look at the scores of the top 100 teams from previous competitions and see how many of them don't get as lucky as they did during their qualification flights.

Handeman, all three launches used G53-5's from the same purchase, and the launches all appeared to be similar. The suggestion about timing the "to ejection" part of the flight seems like a good idea, though.

kj, thanks for the info on the variability. I had always wondered about that...

Would physical motor temperature at launch have any effect on the speed/efficiency of the burn?

 

[n5wd]

... Can anyone think of a reason that there was such a consistent drop in altitude? The only thing we could think of was that the two motors had been out in the cold for a while and therefore didn't burn as efficiently...

Lots of things come in to play here (as my three TARC teams are finding out)...

winds on the ground (easily seen and measured), winds aloft (not easily seen nor measured), differences in air temp/humidity as the day progresses, launch rod angle, the position of your left pinkie when it presses the GO button (I'm not scientifically positive about that last one, but it's the only thing that could account for a couple of our flights being off from a previous flight, so you're not the only team that's scratching your head about that!).

As others have mentioned, the way to minimize that is to take consistent data and, if you have RockSim, plug it into the 'puter and see what it says. Of course, if you re-run the same exact conditions in multiples of six times, you'll see that RockSim also introduces some predictable variability.

Also, does anyone know what the cutoff score typically is to make it to the finals?Thanks!

Last year's cutoff was right about 25 (24 point something), IIRC.

Yeah, it's funny how things at the finals don't always represent what you'd think the top 100 did. One of my teams last year, my only all-girls team, went to the finals where they placed 32nd with a 30.34. 11 of the top 18 teams flying had worse second-round scores than my girls did on the first round. Assuming that everything about the rockets was as close as possible to the first round, the only difference being time, you can see that there are some variables in there that aren't easily accounted for.

So far this year, one of my teams has flown their quals... a 24.24 was their low score. The flight right before that was sub 10. Go figure.

Good luck! And, hope to see you in Manassas!

 

[abw]

Thanks, n5wd.

Ughh; there's so much variability that we can't account for... Congratulations on your team getting a 24.24! We have had some really good flights (probably 15.0 ish), but replicating them is the problem. Hope to see you at the finals too!

 

[Handeman]

Winds, weather, temps, etc are all things you can make some compensation for before launch. What you can't account for is the variations in the motors. If it comes up to pressure slow, or chuffs even slightly you'll get less altitude. If the rocket is still nose up at ejection, then it was going up and the length of the delay will affect altitude. The tolerance of the machining of the nozzle will affect internal pressures slightly, and thus the burn rate.

I'm not sure you can determine what any motor will do, even with a very close inspection. Some of it is just luck of the draw.