What are the factors that vary when choosing optimal rocket designs for TARC?

[Logic]

Hi, Captain of a first-year team here with pretty mediocore knowledge of building rockets. I know how one is built, but for competitions like TARC where you need to reach specific height requirements and come back down in a specific amount of time, what factors or components of a rocket are usually changed to alternate altitude and time outcomes?

 

[dhbarr]

All of them?

Once you factor in the constraints for the year and compare it with the list of off-the-shelf components, the field of workable designs starts to look pretty similar.

Sim & fly & trim, sim & fly and trim.

 

[neil_w]

For the most part, three aspects of the rocket interact to determine altitude (excluding external factors like wind, temperature and humidity):
1) drag
2) mass
3) motor thrust curve

Those three can be tweaked and combined in infinite ways.

Sim & fly & trim, sim & fly and trim.

Yep, no substitute for experience. If motors and/or flight opportunities are limited, then play around in OpenRocket as much as you can to get a feel for how the above three factors affect your flight profile.

 

[bad_idea]

Experienced TARC mentors I've talked to have said the teams who excel are keeping close track of the barometric pressure on the field at every practice flight. Log that data, extrapolate curves, and adjust your mass and/or parachute to fly to them.

(Haven't tried it myself, so classify this as "some random guy on the internet's received wisdom.")

 

[Logic]

Experienced TARC mentors I've talked to have said the teams who excel are keeping close track of the barometric pressure on the field at every practice flight. Log that data, extrapolate curves, and adjust your mass and/or parachute to fly to them.

(Haven't tried it myself, so classify this as "some random guy on the internet's received wisdom.")

Why is barometric pressure so important? Also by log data do you mean just record the pressure (which I'm not even sure how you would do since from what I know an altimeter just gives you the maximum height of your rocket)?

 

[neil_w]

Why is barometric pressure so important?

Higher pressure = higher air density = more drag = lower altitude = longer descent

Also by log data do you mean just record the pressure (which I'm not even sure how you would do since from what I know an altimeter just gives you the maximum height of your rocket)?

Yes, just record the pressure so you can analyze and track your results against different barometric pressures.

Also: many altimeters record much more than maximum height of the rocket, but that's not what is being discussed here.

 

[bad_idea]

Why is barometric pressure so important? Also by log data do you mean just record the pressure (which I'm not even sure how you would do since from what I know an altimeter just gives you the maximum height of your rocket)?

@neil_w's post covered the reason and use. As to the instrument, I believe simple stand-alone barometers are being used at ground level.

 

[boatgeek]

Here’s a few more things to think about in the design phase:
Ease of assembly
Resistance to damage (from zippers to chipped fins)
Speed off the rail (faster means less weathercocking)
Motor budget (will you fly 10 flights or 40?)

There are an infinite number of designs that work. Even at finals, you won’t see a single type of rocket in the top 10. My advice to my teams is to design something that is well over the max altitude and time in the sims, and you can bring both down with ballast and reefing the chute.