First Scratch Rocket

I recently ordered the Estes Designer Special Kit which is basically just a bunch of parts that allow you to design your own rocket. I've designed one on Open Rocket that I plan to build but wanted to see if anyone had any other suggestions for things to improve or things to do when actually building. I also had a question about whether or not the ejection charge on a standard Estes engine would be enough to deploy the parachute on this rocket? I've heard of people adding more black powder to the ejection charge but was wondering if that would be necessary. I plan on launching with either a D12-5 or D12-7 depending on which I can find cheaper. If you need to know the exact parts I am using just ask below. Thanks everyone!
View attachment First Rocket Out of New Kit.ork

Looks like a great D12 rocket. A fun flyer for sure. Think of a name yet?

The ejection charge on a D12 will be plenty for a standard BT-60 body tube. Even the ejection charge of an 18mm motor, which is half the ejection force of a 24mm, is still fine for a full 18" BT-60.

I checked out your .ork file. Looks pretty good. Making use of the parts database is a great start. Once it's built though, override the CP and CG and re-run the sims. I personally would feel more comfortable with the fins about 10% larger - if the chute and shock cord sink down the tube at liftoff, your stability could drop to the 0.7 range upon rod clearance. Better to leave some room for variability.

Also, make sure you space the engine block a bit farther aft so the motor sticks out enough to grab(1/4" or so). I built one where a D12 would sit flush with the motor mount tube, had a heck of a time trying to get the spent motor out. Had to tap it through from the front with a dowel.

Looks nice! The chute and shock cord shouldn't shift down the tube too much, but it could drop your stability down close to 1. To play it safe, I would consider putting about 4 g of modeling clay in the nose. That bumps stability up to about 1.5 calibers if the chute slides down a bit.

Also, looks like you have an 18" chute in the model. The sim has it coming down at 3-4 m/s which is really slow! I'd expect quite a bit of drift on something like that when it deploys at apogee. I'm not sure what chutes are in the builders kit, but you might consider using a 9" chute. That will bring it down at a more reasonable 6.4 m/s. On a rocket that small, be prepared to lose it on an 18" chute if it drifts too far.

Finally, I'd go with the D12-7. Engines are allowed to have a 20% variance in specs. So, if you go with the D12-5, it's reasonable to expect the deployment charge going off around 4-6 seconds. If it goes on the early side, you're robbing yourself of altitude since optimal delay according to the sim is 5.6s. The D12-7 will pop at 5.6 - 8.4s. If it goes off at the upper end, the deployment speed is only falling at 11 m/s. Not enough to harm it. On the early side, it will go off right at apogee.

I'm a fan of the long delays. I love my F15-8 with the dramatic, long coast time. With my Trajector, it falls for a good 2-3 seconds after apogee before deployment. Never had an issue on those light rockets with that.

Looks great. Agree on adding a little weight in the nose. I got a huge tub of modeling clay from Walmart for a few dollars.
One thing I do with all my rockets is drilling a hole thru the motor mount centering rings and tie the shock cord there. IMO it makes it so much easier to pack than gluing the paper to the side of the tube.

+1 What Justin said. I dislike having the shock cord glued to the top of the tube. If you attach it to the motor mount, that will shift your mass aft, so add a couple more grams of clay to compensate. Here's an updated ork with the engine hanging out 0.5 cm, a smaller chute, a shock cord mounted off the motor mount, and 6g of clay. Looks like it would be a good flier.

Bonus points if you try it on an E30-7. :grin:

View attachment First Rocket Out of New Kit.ork

For the benefit of those of us who don't need sim software to do our designs, would one of you kind folks publish a screen print or something so we can see what this design look like?

Thanks.

LW Bercini said:

For the benefit of those of us who don't need sim software to do our designs, would one of you kind folks publish a screen print or something so we can see what this design look like?

Thanks.

Click to expand...

I finally had the chance to read all of your suggestions and they all seem to be very good. I've never thought of tying the shock cord further back but that would make packing a bit easier so I will definitely try it. I also will make the rocket more stable by increasing the fin size some and also adding weight to the nose. I've never really looked at descent speed before so I just kind of guessed what would be okay as to not harm the rocket and using a smaller parachute will definately help so I'll be able to hopefully recover it (without running too far). I'll build the rocket over the next couple days and upload photos of it built and after I fly it. If it works well I may try on the E30-7 just for fun. Thanks everyone!

With a little clay in the nose, the fins you have are good. Don't make it too over-stable, or you might get too much weathercocking.

Not bad... In addition to making the motor stick out, I would make the mount 4" long so it could fly on an E... You would have to shift the motor block a bit, but the Designer's Special comes with 4" tubes as well.

On my 24mm rockets, I like to make the motor stick out 3/4" so I can use the thick coupler as a motor block. That way the motor is *really* easy to get out. Of course, the CG shift means more fins, but the convenience is worth it for me.

The airfoil is ambitious, rounding is much easier and is maybe a 5% difference on this size of rocket. Also, the fins you have are 0.118" thick. If I remember right, the Special only has 3/32, 1/8, and 5/32" stock. I would personally use 5/32, but that's just me - you can probably get away with 1/8".

Don't forget the launch lug - I still forget it sometimes...

You don't have to take any of this seriously if it doesn't fit your plan. Either way, I want to see how this goes! My first scratchbuild CATOed (darn A10s), so I hope you have better luck. +1 on the E30!

Here's an OR file with my suggestions...

View attachment First Rocket Out of New Kit.ork

Now that I look, you can fit an AeroTech F44W baby F with 41Ns... Fun times, and no HAZMAT fees! An E15 might be better than an E30 for those flimsy CR's if you want to go composite E...

For those favoring attaching shock cord to motor mount tube, are you putting shrink tube over the cord or something?

I seem to have a routine issues with my Kevlar shock cords burning through just above the engine. Most of is have probably seen a test fire of a BP Motor and there is a flame that stays lit for a few seconds after ejection, which I think may be a contributing factor.

Personally, I like the standard teabag arrangement on LPR. For my MPR, I usually have a baffle to attach the cord to. I don't know what Aidan likes though.

I know what you mean about burning after ejection though - I think it's the last fragments of the delay grain.