Active Altitude Control

[boatgeek]

I found this in the TARC rules...

"The two sections must separate after apogee and recover separately, both by parachute"

Does this mean we can't use our method of deploying the parachute before apogee to add drag?

Check with Trip Barber to be sure, but I would be very surprised if anyone would interpret this rule to mean that you can't deploy chutes before apogee. It's so hard to tell when a rocket in flight hits apogee that it would probably be unenforceable anyway.

 

[LithosphereRocketry]

I found this in the TARC rules...

"The two sections must separate after apogee and recover separately, both by parachute"

Does this mean we can't use our method of deploying the parachute before apogee to add drag?

That seems like it's intended to ban that - I'd assume TARC wants to discourage people from deploying parachutes at speed since that can cause parachute shreds, etc.

 

[BEC]

The rule is simply that it must come down in two parts, both recovered by parachute, this year. Some years it all needs to come down together, sometimes it is this way. This is just one of the variables that gets tweaked each year so that the prior year's designs can't be reused directly.

It says nothing about when the 'chutes are deployed. As someone who, like @boatgeek, sometimes acts as an official observer for qualifying flights, this year we have to check off that the model comes down in two parts, each under a 'chute. I would have no issue with one of them (or both?) deployed while the model was still going up, as long as it recovered safely and in two parts.

 

[diyaerospace]

If anyone is interested,
Here is what Trip said

"
When two sections of a rocket come apart the huge increase in drag makes it stop going up (if it was still going up then) and it has its “apogee” then, even if that was not the plan for that flight. Most TARC rockets roll over the top, have apogee, then start coming down before the ejection charge goes off and separates them.
"


Trip Barber

NAR TARC Manager

 

[diyaerospace]

At this point we have 5-6 launches planned before TARC.
This probably won't be enough time to tune and test the active control system to get it working reliably.
We need to have a accuracy of +-20 ft based on last years qualification cutoff of 47.28.
I think for this year going with a regular rocket design will have a higher chance of passing qualifications. Assuming we launch and tune 10 times over 5 trips to the desert do you think that getting an accuracy of +-20 ft is possible with a non-active controlled rocket?
Any other recommendations?

I still wan't to use active control in TARC(if the rules permit it) but I think this year we will have better chances with a traditional design. What do you think?

Walter

 

[BEC]

Reliable and repeatable is what you want. Your altitude assumptions appear to assume that you'll hit the duration window every time. That means reliable, repeatable parachute deployment and opening (at least on the egg/altimeter portion of the rocket) with the right descent rate. Remember each second outside it costs 4 points.

It certainly can be done. You will just have to control your variables very well and understand what changes cause what to happen. Finding a way to have similar enough motor performance from flight to flight is important. I probably shouldn't say what I've seen teams do to work this aspect since I'm not a student on your team. But there are strategies to do this.

 

[waltr]

Was looking through and read NAR NARAM reports and found one on active altitude control.
https://www.nar.org/member-only-rep...s-Air-Braking-Precision-Altitude-NARAM-60.pdf
This is exactly what you are trying to do.

 

[boatgeek]

At this point we have 5-6 launches planned before TARC.
This probably won't be enough time to tune and test the active control system to get it working reliably.
We need to have a accuracy of +-20 ft based on last years qualification cutoff of 47.28.
I think for this year going with a regular rocket design will have a higher chance of passing qualifications. Assuming we launch and tune 10 times over 5 trips to the desert do you think that getting an accuracy of +-20 ft is possible with a non-active controlled rocket?
Any other recommendations?

I still wan't to use active control in TARC(if the rules permit it) but I think this year we will have better chances with a traditional design. What do you think?

Walter

The TARC handbooks say that a typical team that makes it to finals will launch their rocket ~20 times to get it dialed in and flying repeatably. While getting to nationals is possible on 10 flights, it would be challenging. It would really help if you could make 3 flights in a day. That gives you two launches to test performance and then three available for scored flights.

Your rocket will not be exactly like the simulation (~10% error is completely reasonable!), so you need to determine what your actual performance is so that you can tune the simulation.

 

[diyaerospace]

We launched our (non-active) TARC rocket for the first time yesterday. the results are not good.
The rocket went unstable despite having a margin of 1.1.

Does anyone have any ideas as to why this happened?

I also wanted to launch the active controlled rocket yesterday but the battery connecter broke at the last minute so we had to scrub

 

[boatgeek]

I can't ell for sure because the video image quality isn't great, but it looks like your fin span (distance from root to tip) is quite a bit less than your body tube diameter. If that's true, the fins may be mainly in the boundary layer and not all that effective. Good practice is to have your fin span be at least one body tube diameter.

 

[AllDigital]

Do you have a sim file? 1.1 is cutting it very close. There are a lot of factors that could come in to play. It could be the fin design, but it could also be that actual did not equal sim (did you check the CG with the motor in it?) or weight shifted aft at launch (what was in it?) or not enough rail or wind gust or "statically stable" vs. "dynamically stable".

 

[diyaerospace]

Launched again!

This time the bigger fins helped and the rocket was stable. We flew twice with only 4ft of difference between the flights!

The first flight had an apogee of 644ft and the second has an apogee of 648ft. This means the Cd is 1.17 so we need a more powerful motor. I am currently looking at the F51NT.

 

[waltr]

That is good news.
Does this apogee measurement and Cd match Sim?
Once you adjust sim to match then can be used for further predictions.

 

[diyaerospace]

Just launched twice more last weekend.

The first launch on an F42T motor was supposed to fly to 850 feet but actually flew to 1126 feet.

After doing some digging I found that the previous flights preformed two weeks ago were dubious because the rocksim (and openrocket) file for the E30T motor give a much higher total impulse than the certified motor thrust.

I did some more digging and it looks like almost all the motors in the rocksim and openrocket database have some serious differences from the certified thrust.

Does anyone know how I can fix this?

Thanks.

 

[waltr]

In the newest Oen Rocket in the motor selection window there is a "Show Details" tab (upper right). this will show a thrust curve.

Then, some motors have more than one thrust curve available. There can be pretty big differences.
Example is H238T...

I have also found that some motors sim apogee that matches actual flight very close whereas another motor sim apogee that is way off actual on the same rocket. No idea what to do about this.

One option if you have Accelerometer data is use the motor thrust (accel) curve and create your own thrust curve file to use in the sims.

Of course you can also use the accel data to compare two of the same motors for variations and to check against the thrust curve files.
For the H238T example (I used this motor for my L1 cert flight), I picked the curve that best matched the shape of the curve on the motor package. Sim then was pretty close to actual flight.

 

[diyaerospace]

One option if you have Accelerometer data is use the motor thrust (accel) curve and create your own thrust curve file to use in the sims.

Do you know how to do this? NAR publishes the "certified" motor thrust curve so making a motor file with that data would solve the problem.

 

[waltr]

Thrustcurve.com has the NAR or Tripli cert curves which are then used by Open Rocket & Rocsim.

I think the issue is the thrust curves for newly made motors seem to not match the original Cert curves. Besides variations from motor to motor or by batch.

Actual measured thrust would be a little bit of math to convert the Acceleration curve to a thrust curve.
A constant is Earth's Pull downward. A variable is the Drag as the rockets velocity increases.

Without considering drag, the thrust in Newtons = Accel (m/s/s) * mass of rocket (kg).
This will be accurate as rocket is lifting off pad due to V is low (low drag force).

Now we need to calculate the Drag Force and add to thrust.
Drag force is dependent of Velocity so integrate Acceleration to Velocity.
Plug V into the Drag force equation at each time step. Air density, front area & Cd are constants.
Watch the Units and ensure the Drag force is in Newtons.
Add to the thrust per time step to get the finial thrust curve.

 

[waltr]

Just went through the above calculations for a flight with the 10DOF sensors on an F67W.
The Thrust curve I got is close to the published thrust curve but has slight differences.
Also calculated the Total Impulse and this come out very close to published value.

Seems to be a very good exercise to verify motor performance matches published values and thrust curves.
.

 

[diyaerospace]

Hello everyone,

At this point the active control part of the project is on-hold because of the TARC qualifications approaching. I will now focus on conventional TARC methods for this years competition. However, I am still open to doing active altitude control next year.
I will now post updates on the non-active controlled rocket in a new thread.

Thank you so much for helping me on this build, I definitely learned a lot and am excited to continue next year!

-Walter