Variable Drag Project

I'm working on a project to build a rocket with which I can adjust the altitude by varying the drag. Highest would be a straight tube fin. To reduce altitude, I am using plates attached to couplers that fit into the tube fins. The eventual plan is to build a 7.5" version light enough to fly on a N800 on the ICBM field
I started by building versions using BT60 and 24mm motors. All parts from BMS, 34" long tubes and couplers. The plates are 1/8" thick basswood.
Rocket dimensions:
All tubes are BT60 and matching couplers.
Main body length 17" (1/2 of a 34" tube).
Tube fins and payload section 2-13/16" (1/12 of a 34" tube).
All couplers 2" long 1/17 of 34" tube).
Nose cone BMS BT60 conical.
Recovery is Estes type trifold paper, 2 ft. of 1/4" elastic and a 15" nylon chute.

All wood parts had 2 coats of Pactra Balsa Fillercoat applied, sanding with 400 grit after each coat. No other paint.
Drag plates are based on an equilateral triangle 3-13/16 high. The tip is rounded off to match the BT60 coupler leaving 3" high from the base to where it touches the main body (a little less than twice the BT diameter). The triangle side length is 4.4" Parts were cut slightly oversize and sanded to fit.
Initial placement was in the front of the tube fins.
This proved unstable and all later flights are with the drag plates on the rear of the tube fins.
All carry an Estes 'How High' altimeter.

Weights, on pad with D12 motor.
No plates 160g, 2 plates 184g, 4 plates 204g, 6 plates 224g.

November 9-10 launch

All flights on D12 motors. See the video for the results (oops!)
I was never able to get the Estes altimeters to work so I got no data on these flights.

December 9-10 launch. Still having troubles with the Estes altimeters. I tested all in the motel Friday night and all were working. Got to the field Saturday morning and none were working. They were all working again in the motel Saturday night but again, none were working on the field Sunday. They were working again when I got home. Until I can find out what's causing the Estes altimeter problems, I will have to use something else.
Leo was kind enough to loan me one of his ALTDuino altimeters and helped with data recovery and I got good data from the set of 4 D12 flights.

No plates 560 ft.
2 plates 161 ft.
4 plates 113 ft.
6 plates 81 ft.
I'm not sure how much of the altitude difference is due to the weight or the drag.

I've ordered some Perfectflite Fireflys to replace the Estes altimeters. I will fly again at the Janurary launch on E12s (if its not too cold )
Fireflys are in. January launch cancelled, will have to wait for the February launch.

I will see what the results are with the E12s. Next step is to build a 4" using LOC parts and fly on J244s.

i don't know about the drag plates hanging on with the j244s
should be cool..

Cool project.

Rocketjunkie said:

No plates 560 ft.
2 plates 161 ft.
4 plates 113 ft.
6 plates 81 ft.
I'm not sure how much of the altitude difference is due to the weight or the drag.

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Without knowing anything else I'd feel pretty confident saying that the vast majority of the difference is due to drag.

It's absolutely almost all drag since you mention all-up weight being only half a pound or so.

What a fun project! Based on your "no plates" vs. "one plate" altitudes, I'd say most of the altitude reduction was due to drag. You might try each launch a second and third time to compare results of each- slight differences in the individual motor performance, wind speed differences, launch rod whip, etc. will most likely affect your results to some extent.

With an "N" motor, you'll really need to beef up both the plates and the attachment method. LOVE to see that launch!!!!!!

Yeah, that's a fun idea. If you have access to an accelerometer based altimeter, you can get a very good measurement of the Cd of your rocket configuration.

The Estes altimeter needs pretty good sized static ports in the model to work consistently. In that chunk of BT-60 you are using as a payload bay I’d put three 3/32 inch diameter holes (at least).

That said, you’ll find the FireFlys to be easier to deal with if a little harder to actually read.

BEC said:

The Estes altimeter needs pretty good sized static ports in the model to work consistently. In that chunk of BT-60 you are using as a payload bay I’d put three 3/32 inch diameter holes (at least).

That said, you’ll find the FireFlys to be easier to deal with if a little harder to actually read.

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3 1/8" holes in payload 3" long. I do expect the Fireflys to work. I test by covering all but 1 hole with tape, arming the altimeters, installing the nose cone then sucking on the remaining hole. When working, I get altitudes between 2000 and 4000 feet.

Sounds like you're building up something similar to the Bluefin Tuba that flew on G-M

Nytrunner said:

Sounds like you're building up something similar to the Bluefin Tuba that flew on G-M

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Whoa! How is that possible? What is this rocket? Is there a thread? What??

mbeels said:

Whoa! How is that possible? What is this rocket? Is there a thread? What??

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Search on this forum, there have been several threads for different-sized models. Here's the 4" which I *think* was the biggest: https://www.rocketryforum.com/threads/bluefin-tuba-4.125276/

In addition to being a great rocket, it is IMHO the best rocket name ever. Multiple simultaneous wordplays in action.

neil_w said:

Search on this forum, there have been several threads for different-sized models. Here's the 4" which I *think* was the biggest: https://www.rocketryforum.com/threads/bluefin-tuba-4.125276/

In addition to being a great rocket, it is IMHO the best rocket name ever. Multiple simultaneous wordplays in action.

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Oh my goodness, yes, that's clever, and very impressive. I did search, but apparently searched for "Bluefin tuna", my brain skipped over the correct name. Ok, I'll stop before I derail this thread. (Link to post with G-M results. Wow.)

mbeels said:

Yeah, that's a fun idea. If you have access to an accelerometer based altimeter, you can get a very good measurement of the Cd of your rocket configuration.

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Yes, do this. I don't know about a "very good" measurement of CD, but having accel numbers will be your best chance to compute it from flight data. Be prepared to smooth the accel data before working with it.

Wind tunnel or CFD simulations would be the best choice for drag prediction.

Rocketjunkie said:

I'm not sure how much of the altitude difference is due to the weight or the drag.

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I would like to take a crack at answering this question. I'm going to assume a D12 motor applies a constant 10.2 N force for 1.7 seconds (from ThrustCurve), that the force of drag from friction is proportional to rocket velocity squared, and the only other force on the rocket is gravity. So no wind, kick off the tower, or complicated thrust profile to integrate. I'm also going to use metric (not sorry ).

First I want to look at how high a rocket with no plates, mass of 0.16 kg, and no friction would go on a D12 motor. I get roughly 630 meters. The actual rocket with zero plates AND FRICTION went to 170 meters. I solved for the drag force that results in a flight of 170 meters. For drag force I'm using F(drag) = B * V^2. If you want it in terms of Cd, you have to take into account the shape of the rocket and that is for Rocksim/OpenRocket if you have a conventional rocket design.


Using the drag force found from the 0 plate flight analysis, I keep all the variable the same except change the mass of the rocket to match the mass from the 2,4, and 6 plate flight configurations. This is the simulated altitude assuming the plates only add mass, and no ADDITIONAL drag compared to the 0 plate configuration.


Each pair of plates adds 20 grams of mass and has a weight penalty of 10.7 meters per pair.

The upper set of curves are the same from the graph above. The lower set of curves show the measured altitude from the test flights. I solved for drag force to make the simulated data match the measured data.


And finally, these are the B terms for drag force, where F(drag) = B*V^2. To get to Cd, you will have to take into account surface area change from adding plates, and the shape of the plates interacting with the tube fins.

I'll wait for the (Estes) E12 results. Should get a better idea then. Then I'll go to a 4" on J244, J275, or 54-2G Rx motors. Half the fun of this project is flying them

How will you make sure the plates don't fold inward or snap off from the speed? Seens like it should be LOT of stress to me

Feb 8-9 Camden launch. Flew all 4 on E12-4s

No plates 955 ft.
2 plates 261 ft.
4 plates 192 ft.
6 plates 151 ft.
I was still not able to get the Estes altimeters to work, none of them got data on the field. All data was from the PerfectFlite Fireflys.
Time to construct the 4" from LOC parts, 54mm mount and power will be 2 grain 54 mm motors (J244, J275, or Rx J220).

Lukun7 said:

How will you make sure the plates don't fold inward or snap off from the speed? Seems like it should be LOT of stress to me

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Except for turbulence effects, the forces on the plates won't exceed the thrust of the motor. The 4" involves increasing the glue area and the 7.5" includes stepped bulk plates and threaded rods (in tension). The 7.5" test flights will probably be on Rx M1100s (98/7680).

No 3 plate? It'll compete the data set!

With six plates it's basically a saucer.

neil_w said:

With six plates it's basically a saucer.

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Yes, or a single plate spool. The objective is to be able to add plates depending on the field size and conditions.Its looking more like the normal configurations are 0 or 6 plates.

Was the 4 sec delay long enough for all designs to reach apogee?

Buckeye said:

Was the 4 sec delay long enough for all designs to reach apogee?

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The 4 seconds was a little short on the no drag plate model but not bad. Plenty long enough on the rest.

Ordered the LOC parts, enough for 2 boosters and 4 payload bays.

This concept, and the data derived from it, could be extremely useful for TARC teams.

The ability to "fine tune" altitude would be advantageous in Competition, wherever there is "set altitude" requirement.

Dave F.

Ez2cDave said:

This concept, and the data derived from it, could be extremely useful for TARC teams.

The ability to "fine tune" altitude would be advantageous in Competition, wherever there is "set altitude" requirement.

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I have wondered if an overpowered, high-drag design would be more or less predictable than a "normal" design. Certainly for the truly hard-core a deployable drag device would seem to be the ultimate; I believe some teams do that sort of thing already.

Dirk Gates flew their upscale Sumo at LDRS 29 with a huge drag plate. It was really cool to see in person.

Love this idea! I may have to build one! I don't have any tube-fin rockets but I love the idea of being able to slow it down! Very cool!

I'd love to try baffle plates with some holes.

If you don't want to slow it down as much, just make the drag plates smaller.

Can you add incremental equivalent weights to the payload in place of the plates so that all of the different plate configurations are the same weight?

BEC said:

The Estes altimeter needs pretty good sized static ports in the model to work consistently. In that chunk of BT-60 you are using as a payload bay I’d put three 3/32 inch diameter holes (at least).

That said, you’ll find the FireFlys to be easier to deal with if a little harder to actually read.

Click to expand...

wow, cool to hear that someone figured out how to make an estes altimeter work.. no one i know ever did