Half scale BGM-71C ITOW

This thread is a continuation of one I started in the Software section looking for some guidance with OpenRocket. Original thread was started here, but it seems a little misplaced to be posting photos and flight reports there...

Thanks again to Buckeye for confirming how to handle the net reduced impulse from my canted thrust lines. I successfully modified the thrust profile of the D12 "flight motors" and simmed an apogee of 285 m, and more importantly the need for a 6 second delay time.

Good weather was with us today so I've also got a first flight report to make. But before we get to that, I took some obligatory "pre-flight" photos just in case something went ugly:


And one last one with the proud designer:


Despite the snow falling we had a warm (ish) day with negligible winds - perfect for a first flight. Motor config for flight one was a D13-7W (drilled back by 2 sec) to boost off the pad, and a pair of D12-0 "flight" motors on a 0.5 second air start delay. I lucked out with my camera today, and the overcast sky turned out to be really good for being able to see the flight motors burning:


JL A3 says I hit 278 m, which is close enough to what OR predicted to keep me happy.


Post flight revealed some light singing of the horizontal surfaces in the flight-motor blast, but nothing that can't be dealt with.

Now, on to painting!

That's a good flight!

I forget from your other thread, did you locate the forward motors just as Pods on the outside of the airframe?

D13.......Is that from the 18/20 hobbyline case?

I wouldn't really describe them as pods, most of the forward (flight) motors sit inside the main BT. The motor tubes are canted in at 25 deg to the centerline so the two motor tubes just touch each other at the front but the back end is flush with the BT. I posted a top-view arrangement drawing in the other thread that clearly shows the thrust angle.

Yes, the launch motor is an 18/20 reload. I'm looking forward to trading that out for a D24 for a more prototypical "punt" off the pad. (TOW has a distinct two-stage launch, with the central motor to get it moving away from the launcher, then the flight motor kicks in when it's some distance away.)

I was talking about the sim, yet I completely failed to state that explicitly.

Sorry, I didn't connect the dots on that one. :caffeine:

Yes, in the sim I made the forward motors with a 2x clustered motor mount with enough lateral separation to have them show up outside of the 3" airframe section that they're "children" to. I attached the OR model file in the other thread if you want to take a look. (The model file posted there has a lot of extra elements in it to try to make it look nice in the 3D Finished view like K'Tesh always does, but I have simpler model that's more focused on the parts build up if that's of any more interest.) The big trick of course was the special motor file that reduces the thrust appropriately for the angled mount so that the sim gives more correct numbers.

KevinM said:

Sorry, I didn't connect the dots on that one. :caffeine:

Yes, in the sim I made the forward motors with a 2x clustered motor mount with enough lateral separation to have them show up outside of the 3" airframe section that they're "children" to. I attached the OR model file in the other thread if you want to take a look. (The model file posted there has a lot of extra elements in it to try to make it look nice in the 3D Finished view like K'Tesh always does, but I have simpler model that's more focused on the parts build up if that's of any more interest.) The big trick of course was the special motor file that reduces the thrust appropriately for the angled mount so that the sim gives more correct numbers.

Click to expand...

No apologies necessary lol, I really should have specified.

My simulation files often stop at functional. I've never felt like recreating a cosmetic file that has dubious flight characteristics. Besides, the forum means I can lok at K'tesh and Neil_w's designs as much as I want!

Update!

It's only taken me a year, but I finally got paint on my iTOW. Special thanks go our to neil_w for sharing some of the surplus clear domes he acquired for his IRIS-T project for the lamp cover I needed on the aft face of the TOW.

That looks fantastic!

I was just talking about your project on Lakeroadster's Lifting Rocket thread. You were able to adjust motor files for canted thrust right?

Amazing detail.

Outstanding!

Nytrunner said:

That looks fantastic!

I was just talking about your project on Lakeroadster's Lifting Rocket thread. You were able to adjust motor files for canted thrust right?

Click to expand...

Thanks for the kind words.

Yes, I wound up making an alternate motor file for OR that just had the thrust value for each datapoint reduced by a factor of (sin 20) from the original. (20 deg being the cant angle on each of the motors from the long axis of the rocket.) It was all of 3 minutes work with a text editor and a pocket calculator and I was off to the races!

Care to describe your electronics package or other means for igniting all those motors?

Andrew_ASC said:

Care to describe your electronics package or other means for igniting all those motors?

Click to expand...

Actually that part is not so sophisticated. The "launch" motor is fired from the standard electric controller. The two "flight" motors are fired by a MiniTimer4 (onboard) that's set to kick 2.5 seconds after the initial impulse is detected from the "launch" motor. So far I've been using D13 for launch and 2 x D12 for the flight motors. (I'd love to use APCP for the flight motors, but so far I've elected to use only BP motors for reliable ignition.)

That is one wicked _ss build. Congrats.

Badass looking machine! Great work!

Amazing work . . . Truly exceptional !