TARC Team Has RMS Troubles

OK, my TARC team (well, the one I was observing for) had done some flight testing last weekend and wound up with some respectable times using F22-5 RMS motors from Aerotech.

There best time during flight tests last weekend were 38.4 sec and 749'. The rest of the test flights were very consistant and all resulted in less than a 10 or 15 point score. Very impressive!

Unfortunately I wasn't there so the those test flights weren't recorded.

Yesterday the winds were light (5 to 7mph) it was sunny and about 50 degrees. Perfect flying weather!

Here's the problem.

They made 5 flights using the same batch of F22-5 motors but honestly, I think something was wrong.

The flights all seemed to weather cock badly (two eggs in the nose is a lot of weight!) in fairly light winds. all flights arced over into the wind. Like they had weaker thrust than normal.

Maximum altitude out of 5 flights was 618' but most were in the 300 to 400" range.

The bad parts were the delays. The F22-5's all seem to have closer to 8 to 10sec delays. All heart stopping flights with deployment at close to terminal velocity and altitudes of 100 to 200'.

They tried a shorter 3 sec delay (that came with the reloads) and wound up with ejection at 8 seconds after burnout.

I can verify this because I had the stopwatch.

My question is, why all the bonus delays?

Any ideas?:surprised:

They didn't appear to be doing anything differently and assembling the RMS motors.

If my sim is correct, to get near 750' with a nominal 1.9" rocket on an F22J, the loaded rocket weight is around 750 grams. If the weight is correct, the max acceleration is ~2.8 G which explains the severe weather cocking in light winds. It's a really bad choice as a motor for this competition if the weight is as high as I think it is.

Regardless of the weight, as I look at the internals of the F22J I see lots of ways to get a bonus delay. The instruction place the short fuel grain at the nozzle end of the casing, and there is a huge gap between the top end of the fuel grain and the ejection charge. If there's any crud on the base of the delay grain column, (like excess grease) it's going to take several seconds to light the delay grain and as the casing pressure is lower at the end of the burn, you will get much longer delay than expected.

Bob

This is the common failure mode for every team I have observed that attempted to get away with minimum power or low thrust.

You must (in my opinion) use more powerful motors and higher initial and average thrust. REASON: You must leave the launch rod at a FAST speed relative to ANY wind you might encounter on launch day. You then have plenty of power to get to the target altitude in any wind and you can add ballast and/or switch parachutes to home in on the target altitude and duration for any launch conditions.

And make sure the launch rod is clean and lubed and there is no "crud" inside the lugs.

As a TARC observer I can make suggestions but the decisions on what motor to use was already made.

I would have used at least an F40-4 or an F52-5.

I was wondering about the difference between last evenings flight and last weekends flights.

sandman said:

As a TARC observer I can make suggestions but the decisions on what motor to use was already made.

I would have used at least an F40-4 or an F52-5.

I was wondering about the difference between last evenings flight and last weekends flights.

Click to expand...

Wind?

Also, do you know exactly how the flights went last weekend?

You said you were not at the previous launch.

Art Upton said:

Wind?

Also, do you know exactly how the flights went last weekend?

You said you were not at the previous launch.

Click to expand...

Yes indeed. A slow moving model will not "weathercock" if there is zero wind. The most a slow moving model can do if there is no wind is deviate from vertical because of imbalanced forces (eggs or altimeter or parachute off center, thrust off center, fins misaligned, launch lug drag, etc.) combined with the effect of gravity on the way-too-slow-moving-rocket.

Trip Barber has stressed the need for high speed off the end of the launch rod and even wrote several long messages on the TARC YahooGroup in previous years (which, some groups are not using and suffering as a result). Other mentors have posted on this topic on the TARC YahooGroup many, many times.

It is an important variable that can be elimintated.

Art Upton said:

Wind?

Also, do you know exactly how the flights went last weekend?

You said you were not at the previous launch.

Click to expand...

All of the info I got was from the Principal who is the Tarc sponser.