Impending Doom - Design, Build, Fly

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Active Member
Mar 9, 2009
Reaction score
Justin Lawyer and I were at Mudock last weekend and we really wanted to see the new Aerotech I49 fly. Gary and Karl were there and they had a demo motor, but we did not have an appropriate rocket. The I49 puts out about 10 pounds of thrust for over 7 seconds. If you want the 5:1 thrust to weight ratio you only get to fly a 2 pound rocket and since the motor weighs about 3/4 of a pound you only get 1.25 for the rocket. Add to that the fact that the weight of the motor moves the CG pretty far aft and you need to use some care to get a stable flight.

What really made it fun was that we were at Black Rock and had only what we had in our kits and what we could buy from the two on site vendors. Plus, I'm a very cheap guy and this rocket needed to not break the bank.

We decided that we should go with a MasterCard theme:

4 feet of 38mm cardboard tube - $6
lightest 38mm nose cone Jack had - $15
18" parachute - $7
odds and ends - $4
tube fins - free (thanks Jack)
MAWD altimeter - borrowed from my Wildman Jr
I49 motor - free (thanks Aerotech)

7 second burn on a tiny little cardboard rocket - priceless!

Last edited:
So here's what we ended up with:

[alright, so I need to learn to attach photos...]


Yup, in spite of the fact that Jack G's trailer is very well stocked, he had no small pre-made fins, no fin stock, and nothing that we could fake it with. No worries, we'll use tube fins. We were going to drop something like $18 on 6 quantum tube 38mm couplers until Jack offered to give us two 10" long sections of Estes tubing for free. A little work with an x-acto knife, a little sandpaper, a lot of epoxy and we had tube fins.

Note that this photo is from after we got home - there is no motor retention as that would be expensive, heavy, and a bit difficult on a minimum diameter rocket. We friction fit the motor and then since we set the fins so far forward we were able to add a wrap of tape around the back of the rocket and the at closure.

Th launch lugs are kinda cool too. The rod needed to go through a fin and so we needed standoffs to make the lugs clear the inside of the fin. Justin cut down some of the tube that larger Aerotech igniters come in and made little cradles for the lugs.


Note that the I49 is an end burner and was designed for plugged forward closure only - a special one that doesn't even have a recess for the delay even. So, you have to use electronic deployment to fly this motor. I like electronic deployment anyway.

We wanted minimum complication, weight as far forward as possible, quick build time, and we were limited to supplies we had out on the playa. So, we just cut a hole in the nose cone shoulder, loosely wrapped my MAWD in a bit of foam padding and stuffed the MAWD and a 9V battery up into the nose cone.

We hung the deployment charge off the bottom of the nosecone and used a bunch of masking tape to seal the hole where the wire went through the nose cone base to connect to the altimeter. We also sealed the hole in the center of the nose cone with masking tape and CA. By this time it had started to rain pretty hard, the wind had forced us to take down our easy up so Justin and I were building the rocket inside my Suburban. Somehow we didn't spill any CA or epoxy on the seats. But I hope I never get pulled over by a cop with a bomb sniffing dog as there is a fair bit of black powder in the seams of the drivers seat.

The power switch for the MAWD is just a scrap of wire stuck out the side of the nose cone - twist and go. And, at the last minute we remembered to make a baro port too - though the location is obviously far from ideal.

Once the thing is all loaded, the nose cone is held on by tape. The deployment charge blows the chute down and out at a coupler about a foot back from the nose. The tape just has to be strong enough to ensure that the rocket separates in the right place.



We put a coupler and bulkhead in a foot back from the nose and fastened 6 feet of kevlar thread from the bulkhead to the nose cone. Added an 18" nylon chute from Jack's trailer and a small kevlar pad from my stash and we had a recovery system.

The rocket weighed 1.2 pounds with no motor and just about exactly 2 pounds with the motor. It's 38mm diameter and about 56" overall length. We spent about $32 on it - less than the I49 reload was worth.

It rained again (they don't call it Mudrock for nothing) and this time while we holed up in the Suburban we entered enough data into Rocksim to calculate a CP. I was surprised by how far forward of those draggy tube fins the purported CP was and so marked it on the rocket with a sharpie and a big question mark. I'd like to believe that the true CP was further back than Rocksim claimed.
With the I49 you epoxy the fuel grain into the liner - presumably to keep it from shifting or maybe to ensure no burn up between the liner and the grain. You also inhibit the front of the grain with epoxy. We used 5 minute Devcon and so we had a little coffee break while it cured.

The grain goes in, I think there's a forward insulator disk and then the new no delay forward closure.

At the aft end there's a thin o-ring that goes inside the liner tube (the fuel grain is just a smidge shorter than the liner and is glued in flush with the front of the liner - leaving a little lip at the back). Then another fiber insulator goes on and you have to fit the ignitor since it lays flat against the bottom of the fuel grain between the grain and the aft insulator. Nozzle o-ring, nozzle, and aft closure, just like on a normal motor and you're good to fly.

Sorry, we didn't take photos.
With the motor loaded the rocket balanced almost exactly one diameter forward of the purported CP. I think the CP was pessimistic, the CG would move forward as the rocket flew, we didn't want it overstable and we didn't want to add nose weight. So we just made sure the LCO announced it as head's up and let her rip.

The rocket flew absolutely straight and vertical laying down a 7 second long column of pretty white smoke. The MAWD says it went to 5094 feet and much to my chagrin says it was doing 1100 fps when the motor burned out. I had not programmed a mach delay as I couldn't imagine this crappy little cardboard rocket doing mach 1. Apparently we weren't quite at Mach 1 as there is no sign of a baro increase in the flight data and we didn't get the dreaded early deployment - but I think we got really lucky. Another 100 fps and we'd have shredded that boy really good. I guess it's always safe to have a 8 second mach delay on a motor with a 7 second burn. Apogee was at something like 15.5 seconds.

We lost sight of the thing (might have entered low clouds but we wouldn't mention that here) and had to go an a rocket hunt. It was found for us by another team only about a quarter mile downrange. It's in like new condition and ready to go again just as soon as the motor is commercially available.

We're thinking we should build another one exactly like it but with traditional plywood fins. It would be interesting to see the performance difference.

Once again, many thanks to Gary and Karl - We both had a lot of fun with this cool new motor.

Mike and Justin
Last edited:
Awesome work on a last minute project guys.:)

I can't wait to try the I49N and I59WN when they're available but mine will fly in a saucer.;)