TARC Questions

Hi everyone, I'm leading a TARC team for the first time this year and I had a few questions.

1. We are using OpenRocket to model the rocket and everything looks fine but the flight time looks a little unrealistic. We are using two body tubes and the weight of the rocket is around 300g right now. According to the software, it goes up to around 280m but has a flight time of around 40 seconds with only one 30 cm diameter parachute. The parachutes are set to come out at apogee and there is a shock cord attaching the two parts body tubes together. Is there something we have to specify to make sure that it thinks both parts are coming down together with the one parachute or what are we doing wrong?

2. We are also trying to find a good E- class rocket motor that's reliable and easy to find with an impulse of around 33. We can adjust the design to make sure that it fits a rocket motor that won't be difficult to find or use. Any recommendations?

firpisto34 said:

Hi everyone, I'm leading a TARC team for the first time this year and I had a few questions.

1. We are using OpenRocket to model the rocket and everything looks fine but the flight time looks a little unrealistic. We are using two body tubes and the weight of the rocket is around 300g right now. According to the software, it goes up to around 280m but has a flight time of around 40 seconds with only one 30 cm diameter parachute. The parachutes are set to come out at apogee and there is a shock cord attaching the two parts body tubes together. Is there something we have to specify to make sure that it thinks both parts are coming down together with the one parachute or what are we doing wrong?

2. We are also trying to find a good E- class rocket motor that's reliable and easy to find with an impulse of around 33. We can adjust the design to make sure that it fits a rocket motor that won't be difficult to find or use. Any recommendations?

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I will answer question 2:

https://rocketcontest.org/wp-content/uploads/2019/10/Rocket-Motors-Approved-for-TARC-2020.pdf

Estes E16 motors are simple, but are 29mm and weigh more than similar total-impulse Aerotech motors (E15/E20).

The Aerotech E15 and E20 are identical. They had an 'old' E15 with more total-impulse, but years go they redesigned the casings to use an integrally molded nozzle and aft thrust ring. Part of the redesign meant the casing wall got thicker and that meant less room inside for propellant, so the total-impulse wnt down and the average thrust went up. The result was the E20, but they also made 'new' E15 motors that were identical except for the label. Look at the NAR Standards & Testing test data sheets and you will see this is true.

E15 or E20 is 35 N-s total-impulse and only 24mm diameter.

The E30 is closer to 33 N-s
The Aerotech E30 is the Estes E30 because Aerotech made them for Estes and Estes glued on their label.

Do not be surprised if your rocket does not fly anywhere near what your simulation predicts. Almost everyone underestimates the drag coefficient in the real world. Build and fly a couple of times and then use the actual altitude data to change your drag coefficient to make the simulation results match the real world. Then you can continue with other variables, such as total-impulse, weight, and even add or reduce drag (which means changing the drag coefficient again...).

Have fun.

Oh, one more thing:

The Estes motor has more than 30 grams of propellant, which means that vendors who ship them must ship them HAZMAT, which is EXSPENSIVE.

The Aerotech motors have less than 30 grams, and can be shipped parcel post cheaply by vendors who have the appropriate shipping permits.

Answers to 1 & 2 can be similar : don't sim it with just one motor, sim it with all the motors you believe you can reasonably obtain.

Now refine the design to more exactly match your primary & secondary candidate designs. Iterate.

If you don't believe the flight time, look at the momentum plots. Are the numbers believable? More than one ground hit ?

Another reason to look at multiple motors - there will be periodic availability issues. Don't put all your eggs in one basket and then come close to qualification time and not be able to find a motor.

dhbarr said:

Answers to 1 & 2 can be similar : don't sim it with just one motor, sim it with all the motors you believe you can reasonably obtain.

Now refine the design to more exactly match your primary & secondary candidate designs. Iterate.

If you don't believe the flight time, look at the momentum plots. Are the numbers believable? More than one ground hit ?

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Also, you have lots of weight budget for ballast. If you want to standardize on a motor early, choose one that will take the rocket to well over the desired altitude. You'll need to make weight adjustments on the flights anyway, so you may as well get started early on that and make sure that your desired motor can get to the higher target altitude at nationals.

300g with a: motor, egg, padding, chute, shock cords and entire rocket is pretty amazing. Are you loading actual available parts into the sim? If you are, your parts must be pretty light, and that means delicate. You want your rocket to last. F motors are not that much more expensive than E's. I would recommend beefing things up and going with an F motor in the 50+ Newton range. You can always bring it down with more mass or drag, but if your rocket is low you will need a bigger motor.

FYI, my students flew their rocket yesterday: 530g, 2.56" body, Estes BT-80 nose cone, 57N motor, flew 952 feet. So they need to knock ~150 feet off with more mass or drag. That's the most "too high" any of my teams have ever been, but I'm not concerned. I would be if it was low.

Also, a 15" chute on a light TARC rocket might work, heavier ones may need a chute closer to 20". It's best to be a little big, and choke the chute size down with a knot that effectively shortens the chute lines.

Hi everyone, my TARC team had our first launch today and we did pretty, well but we have a few quick questions:
Our whole rocket is made out of BT-70 tubes, so we use a coupler with a bulkhead inside to separate the bottom section from the upper(payload bay). Because of this, two problems have appeared.

1. We attached the shock chord to the top section using the method in this video:
However, during the launch, the shock chord was ripped off of the coupler on both the attachment points causing the top and bottom sections of the rocket to come down separately. The parachute was attached to the shock chord, so top section came down in freefall. Is there a better way to attach the shock chord to the top section so it won't come off next time?

2. Also, after we recovered both parts of the rocket, we noticed that the nose cone came off during the flight. We don't think it came off when it landed though because neither the nose cone nor the top body tube were really damaged in the fall and the nose cone wasn't found near the tube. We suspect that, because we used the method in the above link to separate the top and bottom sections of the tube, we left a few holes when gluing the bulkhead to the coupler, so the pressure from the ejection charge made it to the top of the rocket and ejected the nose cone as well. Could this be the reason why the nose cone fell off or are there any other guesses. And if so, how can we stop this from happening next time?

Thanks for all the help and Merry Christmas!!!

Please don't post the exact same question in two different threads.

Attachment failures are a common mode of failure for TARC groups. I watched your attached Apogee video. I can make several recommendations. You could follow the approach in the video, but after letting the white glue or Carpenter's (yellow )glue dry (in general yellow glue should be better), mix up some 5-minute epoxy and put that on top. Another approach would be to stop using the cardboard bulkheads or centering rings and use plywood bulkheads or center rings.

Better yet for parachute attachments and/or payload attachments I would recommend thin plywood or model aircraft plywood about 1/8" thick. Eliminate this failure mode and you have one less thing to worry about. Another idea would be to epoxy something like a small length wood dowel (approx. 1/2" diameter) with a screw-eye in it to a plywood disk (bulkhead or CR)

I've attached some notes from previous TARC competitions

aerostadt said:

Attachment failures are a common mode of failure for TARC groups. I watched your attached Apogee video. I can make several recommendations. You could follow the approach in the video, but after letting the white glue or Carpenter's (yellow )glue dry (in general yellow glue should be better), mix up some 5-minute epoxy and put that on top. Another approach would be to stop using the cardboard bulkheads or centering rings and use plywood bulkheads or center rings.

Better yet for parachute attachments and/or payload attachments I would recommend thin plywood or model aircraft plywood about 1/8" thick. Eliminate this failure mode and you have one less thing to worry about. Another idea would be to epoxy something like a small length wood dowel (approx. 1/2" diameter) with a screw-eye in it to a plywood disk (bulkhead or CR)

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I didn't mention it in the question, but we did put 5-minute epoxy over the top after all the glue dried. But I have a question about using plywood bulkheads and centering rings. If we were to use them, would wood glue or epoxy work to effectively glue them to the coupler. Also, If we did that, can we attach the shock chord directly to the plywood? Lastly, can you elaborate on the wood dowel idea you had? I don't fully understand what you mean. Thanks

firpisto34 said:

I didn't mention it in the question, but we did put 5-minute epoxy over the top after all the glue dried. But I have a question about using plywood bulkheads and centering rings. If we were to use them, would wood glue or epoxy work to effectively glue them to the coupler. Also, If we did that, can we attach the shock chord directly to the plywood? Lastly, can you elaborate on the wood dowel idea you had? I don't fully understand what you mean. Thanks

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Wood glue is strong stuff and easier to work with than epoxy, but I think that the epoxy is stronger especially if it's applied a little bit thicker. I think attaching the shock cord to the screw eye is better and applying some epoxy to the screw eye threads before screwing it in. There is a drawing of the wooden dowel and screw eye idea in the attachment in post #11. When you think about it, the heavy payload (the number of eggs varies from one TARC year to another) puts a heavy load on the shock cord attachments when the ejection charge separates everything. Also, you probably want a good shock cord, either a long enough kevlar string on a wide enough elastic from the sewing section at Wal-mart.

One good thing is that you are launching early. Many TARC teams won't start launching for 2 or more months.