U.S. Rockets Micro-Interroc (#1180) Build Thread

I am working with Rich O’Hara (raohara here on TRF) at Bellgate Distributors. We were looking through the inventory and discussed doing some build threads on some of the kits we distribute. First up is the U.S. Rockets Micro Interroc. I picked this kit for a couple of reasons. First, I’ve never built a U.S. Rockets kit. Second, I was intrigued by the specifications that indicated the kit could fly on 3x13mm, 18mm, 24mm and 29mm motors. Third, I could see plywood fins in the packaging.

My first dilemma with this build thread was deciding whether to call it an LPR or MPR build. Given the relatively straight forward design and the fact you could fly it strictly LPR there is certainly an argument you could call it an LPR build. On the other hand, it flies MPR motors and could be adapted to dual deployment.

Here are a few pictures of the product packaging



and the parts after unpackaging.



A couple of the parts drew my attention: the body tube and fins. First the body tubes.



"Other" body tube on left. U.S. Rockets body tube on right. Note both the exterior and interior layer are much thicker on the U.S. Rockets tube. The U.S. Rockets tube feels much more sturdy and solid. It has an ID of 29mm and OD of 31mm.




Plywood fins should add a level of performance and durability.

Fin preparation - 1/16" Plywood Fins​

The directions call for tapering the leading and trailing edges. I drew a line twice the thickness of the fin, in this case 1/8" from the edge to use as a guide. The double the thickness is just something that I've use and works for me




I started with a sanding block and 100 grit sandpaper. On one edge of the block, I use a strip of 3/32 scrap balsa so the block sands on an angle.




Color the edge with bright colored chalk (I picked this tip up from TRF). It is a great visual reference so you can see how much you have sanded or conversely how much you have left to sand. You can see the difference with these before and after pictures. Because these fins are so thin it doesn't take too long to taper the edges.




Tapered fins




I gave each fin a light sanding with 220 grit.

Looking forward to this build. I've wanted to get this kit for a while now.

Is Jerry I still running USR? If so, I wouldn't touch a USR kit with a ten foot pole, then then only with plastic gloves and the proper shots!

Maybe Bellgrade IS USR!

In my own experience, the USR kits I got were stained, warped, incomplete, and I could never get any resolution. But that was when Jerry ran things....

RecoRocket said:

Maybe Bellgrade IS USR!

Click to expand...

Bellgate Distributors is not U.S. Rockets. Bellgate Distributors is a second generation wholesale hobby distribution business selling worldwide to retail hobby, craft, and variety stores.

The instructions has a fin alignment guide that consists of a circle the size of the body tube with cross hairs to mark the fins. I have never had much success with this type of guide. I find a fin wrap is a little easier to use than horizontal fin alignment guide. The body tube is listed on the packaging as 1.22"/31mm. I looked up this tubing on semroc.com body tube chart to see if I could find something similar so I would have a part number that could be used to generate a fin wrap at www.payloadbay.com/page-tools.html. This body tube appears to be the same as the Series 115 BT (LT-11555) listed on the Semroc site. I used the LT-11555 on payloadbay to generate a fin wrap and it was a perfect fit.

The instructions also call for gluing the fins 1/2 inch from the rear of the tube with Titebond or other yellow wood glue. It is very important to glue them 1/2 inch up because you will be securing the motors with a wrap of tape around the bottom section of the body tube.

I used Titebond III. I placed a thin line of glue on both the body tube and fin and allowed the glue to tack up for a few minutes before putting the fin in place. One of the fins was very slightly not straight. You can see in the bottom picture that I used a small strip of tape to pull the front of the fin onto the line. This was very easy to do because the fins are not that thick.

The instructions then have you cut the included 1/8 launch lug in half. One half is glued against a fin and the other half 6 inches up the tube. I diverged from the instructions at this point. First, I did not want to have a launch lug in the way while adding fillets. I used the fin wrap to mark the body tube so the the launch lugs would be in between the fins.

I also had some concerns over the size of the launch lug. If you ever decided to fly this rocket at a club launch on an E or higher motor you might run into problems with your LCO. I've only seen 1/8 rod on the LPR flight line and not on the MPR flight line. I decided to replace with 3/16 launch lug. If you are not going to fly this rocket on much more than a D motor because of the size of the field where you fly then the 1/8 lugs should be sufficient.

I also glued the launch lugs on to a scrap of 1/16 balsa to give the lugs a stand off. This is something I do on all my builds to protect the finish from rubbing against the rod.



Fillets were next on the list. I used the Titebond III for my fillets. Here are some before and after pictures.

RocketGeekInFL said:

Looking forward to this build. I've wanted to get this kit for a while now.

Click to expand...

There are two dealers that are active on TRF where you could inquire about this kit: JonRocket and Countyline Hobbies. You can also see a list of other dealers here: https://v-serv.com/usr/dealers.htm

RecoRocket said:

Is Jerry I still running USR? If so, I wouldn't touch a USR kit with a ten foot pole, then then only with plastic gloves and the proper shots!

Maybe Bellgrade IS USR!

In my own experience, the USR kits I got were stained, warped, incomplete, and I could never get any resolution. But that was when Jerry ran things....

Click to expand...

I have built some of his HPR stuff , can't hurt them .

Motor Mounts​

While the glue was drying on the fins I started work on the motor mounts. There are three motor mounts that need to be built.

First up was the 3X13mm Mount. I place a little glue on two tubes and allowed it to tack up before joining together. I used a scrap block of wood to make sure the ends were even.



After the glue set I added the third tube



To finish the mount you must glue a "cluster alignment ring" to one end of the three tubes. The "cluster alignment ring" is nothing more than a 29mm thrust ring. The problem I encountered in dry fitting was getting the ring aligned with the triple tubes. It dawned on me to use the payload bay tube as a guide. I placed the triple mount into the payload bay until it was about ¼ - ½ inch exposed. I then put a little glue on the underside of the centering ring. I placed the ring on top of the triple mount and sited down the payload bay tube to make sure it was centered. After the glue dried I removed it and added a little more glue for a solid bond.



I then added the external thrust ring. The instructions then call for sealing the gaps between the triple tubes and centering to prevent exhaust gas from escaping. I glued in some Estes wadding instead of tissue.

18mm Mount - The instructions indicate you will get either a .94" x 2" centering tube (similar to what is used on the 24mm mount) or two 1/4 inch centering/thrust rings. This kit came with the the centering/thrust rings. I lightly sanded inside each ring so the would slide on the 18mm tube easier. I slide both rings on the tube past the 1/2" mark called for in the instructions. I added glue then slide the rings towards the mark and over the glue.



24mm Mount - The centering tube is identical in size and length to the tube coupler used on the payload bay. All the the tubes in this kit are very smooth. I roughed up the 24mm MMT with a little 150 grit so the glue would bond better. I wasn't sure how I was going to keep glue from being pushed out the other end of the centering tube when I inserted the motor mount. Have a paper towel handy before you glue. The 24mm tube extends 1/2" from both ends of the centering tube. The mount is finished with a tape thrust ring on the centering tube




Do not glue any of the mounts into the rocket.

Final Steps​

All that is left at this point is to mount the shock cord and build the payload bay. The shock cord mount is your standard trifold mount which is glued 1 1/2 inches down inside the the main tube. The shock cord provides is a fabric covered shock cord and not the typical elastic band.

To build the payload bay you need to install a screw eye into a bulkhead and then glue the bulkhead into the bottom of a tube coupler. The supplied screw eye is huge and I'm not sure why. The only thing I could come up with is added weight forward to help move CG forward. I had a little small screw eye in my spare parts bin so I used it instead.



The instructions call for either friction fitting the tube coupler into the payload bay tube or gluing in place. I went with glue.

I intend to fly a Jolly Logic altimeter in the payload bay. For me, this means positive retention of the altimeter to the payload bay and nose cone. I epoxied a loop of braided nylon into the nose cone. I also used a similar mount to epoxy a piece of nylon line inside the tube coupler at the bottom of the payload bay





The last steps before painting was applying a sanding sealer to the fins. I used Zinsser Bulls Eye Seal Coat. I applied one coat and then sanded down. I started with 150 grit and worked to 500 grit. I applied a second coat and again sanded with 150 to 500 grit.

Ready for paint​

RecoRocket said:

Is Jerry I still running USR? If so, I wouldn't touch a USR kit with a ten foot pole, then then only with plastic gloves and the proper shots!

Maybe Bellgrade IS USR!

In my own experience, the USR kits I got were stained, warped, incomplete, and I could never get any resolution. But that was when Jerry ran things....

Click to expand...

I've got a USR Sniper here at work tonight familiarizing myself with the parts. Everything about it looks to be first class.

The painting suggestions in the instructions were to paint the entire rocket gloss white. Then paint flat black on 2 fins and the payload section and fluorescent colors on the rest. I went a little different route and went with a gold body, red fins and black nose cone. The included decals were not much more than a USR logo so I decided not to use them.


This is a very straight forward and simple build. The instructions are very simplistic but then again so is the build. The biggest difference from other builds is the motor retention. The motor mounts are not glued into the rocket which eliminates a step in the build process. Motor retention is by means of masking tape thrust rings and a wrap of tape around the motor and body tube. There is an included “Information Report” in addition to the instructions that discusses motor installation and clustering. The information report lacks decent graphics and appears to have been typewritten many years ago but it does seem to contain useful information. With some planning, you could slightly modify the build and add an AV bay to make this a 29mm minimum diameter dual deploy.

The instructions list a variety of recommended motors (primarily MPR motors) that are not manufactured. Recently, I saw a similar U.S. Rocket kit fly on the Estes 29mm E16-8. It was a tremendous straight as an arrow flight. This rocket should do well with the Estes 29mm BP motors. According to the U.S. Rockets website, this rocket was designed to accepted up to 240 N-s motors. In any event, and as the instructions point out, you will likely need to add weight to the payload bay when using larger/heavier motors.

This rocket is lightweight so altitude should be easily attained. The painted rocket (including shock cord and streamer) weighed in at 3.58oz / 101.4gm. The motor mounts ranged from .15oz / 4.3gms to .42oz / 11.2gms.

I am hoping to test fly this weekend. However, because of the size for the field, I won’t be able to fly much more than a 24mm BP motor. If I am able to fly, I will post a final report.