How to calculate increased drag due to spin

[Bill Hanson]

Hi all,

I'm working up for my L3, which is going to be a fiberglass Madcow DX3 XL (black Friday special), with (tentatively) an M1350 motor. Going with the KISS principle, I'm planning to do it "stock" with the addition of a thrust plate.

My problem is keeping the thing within our altitude waiver. Launching from Alamogordo, NM the max altitude waiver is 17,000' MSL, which works out to 12,664 AGL. Since the NAR L3 preflight inspection checklist mandates a predicted apogee of no greater than 90% of the FAA waiver restriction, that lowers my maximum altitude to 11,398' AGL. OpenRocket gives me a predicted apogee of ~13,400' (assuming regular paint). I tried adding weight, but even an addition 2 KG doesn't do the trick, and making the rocket that heavy gives different issues. Setting the component finish to "rough" gets me down to just under 11,000' -- which would work. However, given that the fiberglass is pretty smooth, that means I would have to add some sort of rough coating (resulting CD ~= 0.9). There's an Apogee newsletter that talks about mixing sand with the paint, which is one possibility. I suppose I could also spray it with chipguard.

Extrapolating from that method, I guess I could glue sequins all over the thing to increase the "drag" factor <rimshot>.

But, while I'm not the greatest painter in the world, I do like my rockets to look good. More important, with a predicted top speed of Mach 1.13 even with rough finish, I'm a little concerned about potential heating issues (or the coating just peeling off) and causing stability issues if it only partially peeled.

Given all that, I'm wondering if adding some trim tabs to the fins to make the rocket spin would do the trick. However, I don't know how to calculate the amount of increased drag (or reduced vertical velocity vector) due to the spin.

Can anyone point me in the direction of a source for that kind of calculation? Among other stuff I did in the AF, they sent me to school to learn modeling and simulation -- but those were nuclear weapons effects and nuclear exchange models, which don't help here. So, even if the calculations are gnarly, I can deal with it.

Alternatively, am I overthinking this? I suppose that a rough black exterior would look pretty mean, but I'm more worried about issues with it in the transonic/Mach region.

Any thoughts or ideas greatly appreciated!

Thanks in advance,
Bill H

NAR/L2

 

[Steve Shannon]

I would not try to add spin. I would just try a smaller motor. How does the M1297 sim?
See what thicker fins does for you too.

 

[boatgeek]

This is on a far smaller scale, but I have made a couple midpower rockets with canted fins for deliberate spin. Canting the fins made almost no difference in the predicted altitude. I don't have altimeter data, but the delays were spot on to predicted so they must have been close.

 

[afadeev]

I'm working up for my L3, which is going to be a fiberglass Madcow DX3 XL (black Friday special), with (tentatively) an M1350 motor. Going with the KISS principle, I'm planning to do it "stock" with the addition of a thrust plate.

My problem is keeping the thing within our altitude waiver. [...]
Extrapolating from that method, I guess I could glue sequins all over the thing to increase the "drag" factor <rimshot>.

In the spirit of KISS, how about dropping a little 5"-to-4" centering ring on top of the fins (exterior), something like the following:
https://www.madcowrocketry.com/5x98mm-black-g10-cr/

Just for fun, I threw it on my 4" OpenRocket model, and it lowered the M1350 altitude from 13.7K feet to 8.7K. Based on that, you can probably go with a lower profile (smaller outer diameter) "air brake".

Alternatively, you can launch at an angle...
... or consider flying at a field with a higher waiver!

I would not try to add spin. I would just try a smaller motor. How does the M1297 sim?

M1350 is the smallest "baby" 1% M.
M1297, while having lower average thrust, has higher total thrust due to longer burn time, adding up to a 5% M motor. It will go higher.

a

 

[Steve Shannon]

In the spirit of KISS, how about dropping a little 5"-to-4" centering ring on top of the fins (exterior), something like the following:
https://www.madcowrocketry.com/5x98mm-black-g10-cr/

Just for fun, I threw it on my 4" OpenRocket model, and it lowered the M1350 altitude from 13.7K feet to 8.7K. Based on that, you can probably go with a lower profile (smaller outer diameter) "air brake".

Alternatively, you can launch at an angle...
... or consider flying at a field with a higher waiver!



M1350 is the smallest "baby" 1% M.
M1297, while having lower average thrust, has higher total thrust due to longer burn time, adding up to a 5% M motor. It will go higher.

a

You’re right, the M1297 has greater impulse (I dislike the phrase “total thrust” but that’s probably just me: thrust is force, which is instantaneous. Impulse is the measurement of thrust over time). And of course you’re exactly right; the greater impulse and lower average thrust will almost certainly send it higher, although there is the extreme case with high impulse but low average thrust motors where all of the impulse is wasted because the thrust never exceeds the weight of the rocket. Obviously that’s not the case here, just me trying to salvage something from my suggestion. [emoji15]
I really like your idea of a drag plate. It could even screw to the bottom of the thrust plate.

 

[OverTheTop]

In sounding rockets like the Nike variants they add a wedge to the rear edge of each fin to induce spin. This obviously increases frontal surface area and base drag. What about extending the idea and putting a wedge on each side of the fin trailing edges. Less spin but more frontal area and base drag.

Centering ring idea is neat too .

 

[afadeev]

I really like your idea of a drag plate. It could even screw to the bottom of the thrust plate.

I like this evolution even better - add an over-diameter (e.g.: 4.5" for 4" airframe) rear disk/air-brake to the aft end of the rocket. Sandwich it between the aft centering ring and the thrust plate, secure with long but removable bolts. This should also move your CP way back, and reduce the need for ballast in the nose!

Fly on an M to lower altitudes, get your L3.
Then remove the air-brake and fly on K & L motors to equal heights, for pleasure.
May even be able to safely fly on short-burn 54mm J's (e.g.: J1799NP, J800T), if you rocket is not too heavy!

In sounding rockets like the Nike variants they add a wedge to the rear edge of each fin to induce spin. This obviously increases frontal surface area and base drag. What about extending the idea and putting a wedge on each side of the fin trailing edges. Less spin but more frontal area and base drag.

Increasing fin surface area should work too (akin to modeling thicker fins), but may be harder to model/optimize, and secure: 3 failure points instead of 1.
I'm sure it can be overcome, but if only one of the three fin air-brakes were to let go, you would induce massive coning that may tear the rocket apart.

a

 

[BrAdam]

Could be a dumb suggestion but is there a possibility of loosing weight? Might get less coast? I would assume if you’re going KISS this isn’t a likelihood but, just making sure it’s not an option for you.

Good luck with the L3 and whatever solution you arrive at.

 

[Bill Hanson]

I like this evolution even better - add an over-diameter (e.g.: 4.5" for 4" airframe) rear disk/air-brake to the aft end of the rocket. Sandwich it between the aft centering ring and the thrust plate, secure with long but removable bolts. This should also move your CP way back, and reduce the need for ballast in the nose!

Fly on an M to lower altitudes, get your L3.
Then remove the air-brake and fly on K & L motors to equal heights, for pleasure.
May even be able to safely fly on short-burn 54mm J's (e.g.: J1799NP, J800T), if you rocket is not too heavy!

Increasing fin surface area should work too (akin to modeling thicker fins), but may be harder to model/optimize, and secure: 3 failure points instead of 1.
I'm sure it can be overcome, but if only one of the three fin air-brakes were to let go, you would induce massive coning that may tear the rocket apart.

a

Hi Steve and Afadeev,

Thanks very much for the thought and suggestions. I'm thinking that the idea of a 4.5" disk at the end of the airframe between the aft CR and thrust plate is the way to go. That's much more straightforward for modeling than trying to spin the thing. Being able to undo that mod is a big plus as well.

A quick look at OpenRocket with the 4.5" CR on the back gives me a predicted apogee right at 11K ft -- right where it needs to be. I'll have to look at it, but I think that a G10 fiberglass CR will handle the stress with no problem, since there is only 1/2" sticking out.

Again, thanks to all for your ideas. This is one of the reasons I enjoy being a part of this community. Still new at this, so need all the help I can get!

Will have to bounce this off my L3 advisor, but this sounds like a plan.

Bill

 

[saadzmirza]

Just my 2 cents, but maybe not the best way to get the added drag. Spin can be risky due to roll resonance which leads to a nasty in-flight breakup.


Here's an example from the first UP Aerospace sounding rocket launch in 2006. Kinda cool to watch it spiral out of control, as the crowd continues to cheer, failing to realize that the mission has catastrophically failed.

Any other ideas to increase the drag?

 

[jsdemar]

Hi Bill,
It was good to meet you at the launch yesterday. I was the pad manager that gave you the extra igniter. It's great to hear you're doing your L3!

My first recommendation is to import your design into RasAero2 and simulate it there. OpenRocket underestimates transonic drag. RasAero does a better job and also uses dynamic stability and angle of attack. The program is free. If you need help send me a PM.

The other option is to use our 75k agl waiver near Las Cruces. The site is available for special projects and it does not have regular club events. The clear radius is >5 miles. Although the waiver in Alamo is still 12.5K, many more houses have been built nearby which makes it more challenging to stay within the clear radius. I personally won't launch anything higher than 7k there.

Let me know if I can help. Good luck!

 

[Bill Hanson]

Just my 2 cents, but maybe not the best way to get the added drag. Spin can be risky due to roll resonance which leads to a nasty in-flight breakup.


Here's an example from the first UP Aerospace sounding rocket launch in 2006. Kinda cool to watch it spiral out of control, as the crowd continues to cheer, failing to realize that the mission has catastrophically failed.

Any other ideas to increase the drag?

Hi, thanks for your thoughts and the video. After looking into it, I realized that (at least at my level of expertise), trying to spin the thing would likely end up just like the video.

I had also kicked around the idea of adding a drag disc -- see the thread at https://www.rocketryforum.com/threads/measuring-stress-on-drag-device.149949/ I think this would have worked, but eventually my L3 supervisor and I decided just to travel a bit and launch where we have a high enough waiver that we don't have to do anything exotic.

Bill

 

[Bill Hanson]

Hi Bill,
It was good to meet you at the launch yesterday. I was the pad manager that gave you the extra igniter. It's great to hear you're doing your L3!

My first recommendation is to import your design into RasAero2 and simulate it there. OpenRocket underestimates transonic drag. RasAero does a better job and also uses dynamic stability and angle of attack. The program is free. If you need help send me a PM.

The other option is to use our 75k agl waiver near Las Cruces. The site is available for special projects and it does not have regular club events. The clear radius is >5 miles. Although the waiver in Alamo is still 12.5K, many more houses have been built nearby which makes it more challenging to stay within the clear radius. I personally won't launch anything higher than 7k there.

Let me know if I can help. Good luck!

Hi John,

Good to "see" you again -- and many thanks for the igniter. First thing I did when I got home was to order a box of them so I would have some spares on hand.

I do have a copy of RasAero, and am climbing the learning curve with it. Denzil (who's supervising my L3) and I talked about it and decided against any drag devices or similar trickery. KISS is in effect. Right now, the plan is to launch up at the ABQ launch location (I'm a member there too), which has a 15,000 AGL waiver -- that gives plenty of headroom for the fiberglass MadCow DX3XL with an M1350. We did kick around going at the alternate 75K site as well, but given the situation it seems cleaner to go with the Rio Rancho site.

That's penciled in for May. For April, I'm planning on a test flight at Alamo with an L1000 in a 75-54 adapter-- should go about 7K. The motor choice is purely to save money -- I'm using DMS motors, and the 75 mm L875 is $350 vs $240 for the 54mm L1000, so I come out ahead even after buying the adapter, and the beefier motor is probably better for the test anyway.

In the meantime, I have 38mm I180 motor plus some other I and J's on order to launch my 3" at Alamo for Feb and March while I'm building the DX3. I will remember to connect everything -- can't believe I missed that yesterday (facepalm).

Bill