L3 Project Design/Build Thread: Warp Zero

I'm starting work on my L3 project, which I've decided will be a little bit more ambitious than is really necessary, in an effort to minimize costs and maximize the value of the flight (my school is also in the early stages of planning stuff for another project that will achieve similar performance, and I wanted to test some hardware for it ahead of time). The basic idea is a 4" diameter rocket with a 75mm motor mount, and I'm planning on using an M1780, which should give an apogee of around 14,000' and max velocity of 1440 ft/sec.

The openrocket and rocksim files for the rocket are attached (my L3CC recommended I use both, and for something of this magnitude, that seems rather prudent), though I'm really not inclined to put a huge amount of stock in their predictions until I've got a better handle of the Cd for the rocket after a flight test.

Beyond the standard 'do you see anything wrong?' I'm wondering if any of you can point me in the direction of someone who can do custom CNC'd and anodized aluminum bulkheads for the project (I'd like this rocket to last a while, and would like the additional corrosion resistance on the bulkheads).

Current plan for recovery is drogueless from 14k' with either a 60" SkyAngle Classic or a Cert 3 Medium as my main (depending on how heavy it ends up being - hoping I can get away with the 60"). Altimeters are an SLCF and an RRC3 as backup - tracking is a custom-built Arduino based GPS/accelerometer/barometer unit with a 2.4 Ghz radio, and a backup MissileWorks T3 (900 Mhz).

I'm at work and can't look at your ORK; also, I'm sure someone else will have rec's for anodized aluminum. However, I just wanted to say that my L3 project was almost exactly the same. I used a MAC Performance Radial Flyer and a CTI M1101. My apogee was just over 13k'. If my L3 doc would be of any help to you at all, it is contained the L3 documents sticky thread.

Bat-mite said:

I'm at work and can't look at your ORK; also, I'm sure someone else will have rec's for anodized aluminum. However, I just wanted to say that my L3 project was almost exactly the same. I used a MAC Performance Radial Flyer and a CTI M1101. My apogee was just over 13k'. If my L3 doc would be of any help to you at all, it is contained the L3 documents sticky thread.

Click to expand...

Thanks! I'll definitely take a look over it.

jjwb22101 said:

The openrocket and rocksim files for the rocket are attached (my L3CC recommended I use both, and for something of this magnitude, that seems rather prudent), though I'm really not inclined to put a huge amount of stock in their predictions until I've got a better handle of the Cd for the rocket after a flight test.

Click to expand...

What do you plan to do with this flight test? Are you going to fly a smaller motor and fudge the Cd until it matches your altimeter? I am not a big fan of using Cd as a catch-all for all your sim/measurement discrepancies, especially when most of the variation is due to the motor. Reverse engineering one flight does not make it a universal truth.

For one, the small flight will be in a different velocity regime than your big flight, so the Cds may not equate, especially if one is subsonic and the other supersonic.

Secondly, RockSim may let you enter a constant value for Cd, but Cd is not constant, especially going through Mach. In OR, you can't enter your own Cd, you must futz around with finish or fin parameters, and you may luckily land on your altimeter number. Do you trust that to be a repeatable process?

Lastly, your barometric altimeter has it's own 5%-15% errors, that vary day by day with the weather, especially in the altitudes you are shooting for.

Honestly, If you are careful with the inputs, OR (and RASAeroII) will give you perfectly good simulations, no fudging required. RockSim Cd gets wonky at Mach 1, so I suggest avoiding that for your 1400 ft/s flight. All 4 of your readings (2 sim, 2 alts) can easily agree within 1000' on your planned 14k flight without much effort. That's about the best you can ask for and good enough to make engineering decisions. Actually, your two GPS (that's 6 altitude readings!) are probably the most accurate of all

Now, if your test flight is in a wind tunnel or CFD simulation, then I am with ya!

My L3 rocket was the Maddog kit - that started out very similar.

You expressed a desire to have the rocket last a long time. Making a rocket last a long time is mostly about having really reliable ejection and recovery elements.
When submitting your L3 packet to your L3 committee, remember that the goal isn't to nail an altitude, but to design, build and fly a rocket that hangs together at the kinds of worst-case forces expected. So modelling to a Cp is not as important as designing so that the first all-up weight and balance doesn't cause needed modifications, and the first flight doesn't result in any "oh crap" moments.

Some tips on your ORK file:
The fins look a bit small. I had the same problem with the kit I used. Kit came with a 54mm mount and the fins were sized for that weight of motor. Since I had a 75mm case, and I didn't want to make 3 larger fins, I had to add a fin to achieve desired stability margin. You, of course, can do whatever you want since you are making the fins.
Fin cans end up with a fair amount of epoxy and paint on them. You could add a mass component for fillets, et. al. Maybe 6 ounces?
Your tracker is heavy even considering it includes a battery, sled and hardware.
Main avionics bay likewise. These may be leading you to a false margin of stability. Or you can use massive connection hardware and achieve these weights.

One thing about fiberglass fin cans with fiberglass bulkheads is they are solid. Is a machined thrust plate necessary? Take a look at the commercial 4" kits with 75mm motor tubes to see if the plate is even necessary. Similar comment about aluminum bulkheads. [I am assuming here that your centering rings are fiberglass since you have adjusted their weight to approximate fiberglass.]

Of course, we do things because we have our heart set on them, like machine thrust plates. So find a way, ground test a bunch, and keep us who no longer have time to build rockets posted on progress.

You are so close to Black Friday sales...just buy a good 4 in glass kit. I guarantee you will not be able to assemble the parts ale-cart cheaper!!! Just replace things like bulk plates with your custom CNC'd ones...although stock G-10 will hold up fine. My L-3 is 14yrs old with 22 flight on it & everything is as good as spank'n new.....just a thought to save you very big bucks.
My guess you will find something in the 200-250range that can do the trick!

Talk to Chris A at APE about CNC'd Al bulkheads.

Igotnothing said:

My L3 rocket was the Maddog kit - that started out very similar.

You expressed a desire to have the rocket last a long time. Making a rocket last a long time is mostly about having really reliable ejection and recovery elements.
When submitting your L3 packet to your L3 committee, remember that the goal isn't to nail an altitude, but to design, build and fly a rocket that hangs together at the kinds of worst-case forces expected. So modelling to a Cp is not as important as designing so that the first all-up weight and balance doesn't cause needed modifications, and the first flight doesn't result in any "oh crap" moments.

Some tips on your ORK file:
The fins look a bit small. I had the same problem with the kit I used. Kit came with a 54mm mount and the fins were sized for that weight of motor. Since I had a 75mm case, and I didn't want to make 3 larger fins, I had to add a fin to achieve desired stability margin. You, of course, can do whatever you want since you are making the fins.
Fin cans end up with a fair amount of epoxy and paint on them. You could add a mass component for fillets, et. al. Maybe 6 ounces?
Your tracker is heavy even considering it includes a battery, sled and hardware.
Main avionics bay likewise. These may be leading you to a false margin of stability. Or you can use massive connection hardware and achieve these weights.

One thing about fiberglass fin cans with fiberglass bulkheads is they are solid. Is a machined thrust plate necessary? Take a look at the commercial 4" kits with 75mm motor tubes to see if the plate is even necessary. Similar comment about aluminum bulkheads. [I am assuming here that your centering rings are fiberglass since you have adjusted their weight to approximate fiberglass.]

Of course, we do things because we have our heart set on them, like machine thrust plates. So find a way, ground test a bunch, and keep us who no longer have time to build rockets posted on progress.

Click to expand...

My concern about altitude isn't so much with nailing a specific one, as it is making sure I don't bust the waiver - I'd like to fly at URRG if possible (nearish school for me), so I want to make sure there's no chance I'm going over 15k. I'd also like it to be a somewhat similar launch profile to the other projects my university rocketry club has coming through the pipeline (data collection is always a plus), which is why I'm not doing this with a 7.5" cardboard rocket.

Yeah my tracker bay is going to be rather heavy. The custom primary tracker (ok honestly it's almost a flight computer, since it's got a barometer and an IMU as well) is closing in on 50 grams, plus the battery to power it (this is going to need more than a 9V - the radio alone pulls over 200 mA) - it's very much a prototype. The backup tracker also weighs ~25 grams, plus the 1s lipo to power it. That weight also includes the all-thread (I'm debating about using nylon all-thread here to minimize interference - if anyone has any experience/advice with this I'd appreciate it), which is going to be around 200g alone, the sled, random electrical bits (wires, switches, etc), as well as the eyebolt on the back of the nosecone. I'm using 1/4-20 hardware for everything, which is a lot heavier than I think a lot of people realize.

Similar story on the main avionics bay - I've got two altimeters and hopefully a camera mounted in there - it's not particularly light (and the weight is also accounting for the hardware on both ends).

I'll definitely add some weight at the rear there for the epoxy in the fin can - slipped my mind in the design phase. As for the thrust plate, I'm just mildly concerned about the low surface area I'll have with 3/32" fiberglass centering rings (also I kinda want one because they look cool). The aluminum bulkheads are honestly about half for ease of cleaning and machining, and half for longevity - I won't have to worry about them delaminating over time like I've had happen on some fiberglass av-bay lids - I know they're completely unnecessary structurally (ok fine they're also partly there for looks, but I do actually prefer them for those other reasons too).

Thanks for the advice!

blackjack2564 said:

You are so close to Black Friday sales...just buy a good 4 in glass kit. I guarantee you will not be able to assemble the parts ale-cart cheaper!!! Just replace things like bulk plates with your custom CNC'd ones...although stock G-10 will hold up fine. My L-3 is 14yrs old with 22 flight on it & everything is as good as spank'n new.....just a thought to save you very big bucks.
My guess you will find something in the 200-250range that can do the trick!

Click to expand...

I actually got the tubing and nosecone a few months back when MadCow was doing one of their sales on components - I think it ended up being under $200 all told, for the 60" piece of G12, a 1" switch band (didn't want to have to try to cut that myself), the av bay coupler, the nosecone, the motor mount tube, and the centering rings. It might have been worth it to wait for the DX3 XL sale they had last Friday, but I can't say I got a bad deal.

Buckeye said:

What do you plan to do with this flight test? Are you going to fly a smaller motor and fudge the Cd until it matches your altimeter? I am not a big fan of using Cd as a catch-all for all your sim/measurement discrepancies, especially when most of the variation is due to the motor. Reverse engineering one flight does not make it a universal truth.

For one, the small flight will be in a different velocity regime than your big flight, so the Cds may not equate, especially if one is subsonic and the other supersonic.

Secondly, RockSim may let you enter a constant value for Cd, but Cd is not constant, especially going through Mach. In OR, you can't enter your own Cd, you must futz around with finish or fin parameters, and you may luckily land on your altimeter number. Do you trust that to be a repeatable process?

Lastly, your barometric altimeter has it's own 5%-15% errors, that vary day by day with the weather, especially in the altitudes you are shooting for.

Honestly, If you are careful with the inputs, OR (and RASAeroII) will give you perfectly good simulations, no fudging required. RockSim Cd gets wonky at Mach 1, so I suggest avoiding that for your 1400 ft/s flight. All 4 of your readings (2 sim, 2 alts) can easily agree within 1000' on your planned 14k flight without much effort. That's about the best you can ask for and good enough to make engineering decisions. Actually, your two GPS (that's 6 altitude readings!) are probably the most accurate of all

Now, if your test flight is in a wind tunnel or CFD simulation, then I am with ya!

Click to expand...

Part of the analysis I'm hoping to do pre-flight is some CFD (going to have to get one of my aerospace engineering major friends to teach me how), and I've been playing around with RASAero II a bit - hoping to get a good enough handle on it where it's producing good numbers. When it says 'Nozzle Exit Diameter,' what does that actually mean? Does it need the actual diameter of the diverging part of the nozzle of the motor I'm using, and if so, how do I find that (I don't exactly have an M1780 lying around to measure)?

As for the test flight, it's going to be mostly about confirming all the electronics work as intended - the trackers both transmit locations to the ground stations, and I haven't screwed up my altimeter wiring. However, I was also planning to use accelerometer, barometer, and GPS data to back out my Cd as a function of velocity, and make sure it agrees with what the sims are saying it should be (at least in the subsonic regime where the test flight is being conducted).

Thanks for the advice!

Yeah, that's the nozzle exit RAII is asking for. It impacts the thrust-on drag coefficient, I think. Probably minor effect. The manual explains it.

jjwb22101 said:

However, I was also planning to use accelerometer, barometer, and GPS data to back out my Cd as a function of velocity,

Click to expand...

^ This I like. Look forward to the results.

I would hold off on purchasing your 60” until your close to the final weight numbers

Nick@JET said:

I would hold off on purchasing your 60” until your close to the final weight numbers

Click to expand...

That's the plan - not going to buy the chute till after the rest of the rocket is built. Unfortunately, I can't find much info on the Cert 3 Medium, which will make it interesting if it ends up being heavier than about 8 kg empty.

For comparison, I use an 84" Top Flight in my 14.5-15lb (dry) L2 rocket. Great touchdowns on both it's K flights.

The L3 that checked me out said upgrading from the 60 I originally had was exactly the right thing to do.