Neutral and progressive burn from nozzleless motors

[calebkessinger]

I don't wax tubes... If they won't fly without it calm down the fuel.

and on the larger bore rockets the wax can be detrimental. They start having trouble holding in the bulkhead. The pressure can shove then entire upper core and bulkhead right out the top of the tube. No, I don't use clay for bulkheads in larger whistle rockets. I just use plenty of whistle.

If you video the rockets you can see what's really going on most of the time. whether it be the rockets cato's in the middle, the bulkhead gets pushed out, or instant bomb on a stick.

[NeighborJ]

I've pressed another motor and it is an improvement but still not something I feel comfortable putting a fuse in. Used the pulpy tubes with the shim support. Still got cracks in the grain though they aren't nearly as bad as previous versions. I can say that the shim support is not without issue, I tightened it up as tight as humanly possible but the tube expanded enough to allow fuel to squeeze up above the drift and between the wall. This would not be an issue to remove if I could use some type of wrench to hold back the support while I use the rammer removal tool but that would dent it. I had to cut the tube off the drift and start over. I tried one of my two remaining NEPT tubes but it got stuck as well. I feel the need to add that the stuck rammer was a tight fit and would not have fit at all if it was even a few thousands larger. The drifts need rebuilt in a way that will allow the comp which squeezes past the face to dump into a hollow space so it won't get stuck between the wall.

It has become clear to me that if I am to use the shim support I will need to use NEPT tubes, shim support, and re-designed drifts.

The drifts I propose on making are simple, I will tack a SS washer to a piece of SS pipe then bore out the appropriate dia hole and turn down the outer lip to the ID of my tube. This won't prevent the comp from squeezing past the head but it will prevent it from wedging between the walls.

[dagabu]

J, you are experiencing what we ALL have gone through. Two things that fix this problem. Use smaller increments. Don't press that hard. Every press, every tube, every fuel, every rammer needs to be tweaked to work well. This is why I use mineral oil and granulate (10 mesh) my whistle then make sure its dry before making rockets.

[NeighborJ]

I can see why there are no large cores on large whistle tooling, it's a b##ch to get it right and I haven't even gotten to the point which I can test it yet. I'm really tempted to give up on the idea and stick with the skinny spindles that work so well. Just because it works on paper doesn't mean it will work in real life.

Shrug.

[OldMarine]

Too many variables. I start at safe and work my way up to disaster.

I must admit that the danger zone is more fun when it comes to rockets.

I had a spectacular RUD this evening,a 3lb grenade!

[calebkessinger]

Getting fuel around the drift and sticking the rammer is "usually" a result of too big of an increment.

 

Rule of thumb for nice pressing..

 

1lb rockets. - 1 teaspoon increments
3lb rockets - 1.5 teaspoon
6 lb rockets - 1.5 tablespoon

 

I adjust up with each comp till the rammers start getting hard to pull then back off till they are easy again.

[NeighborJ]

Damn, I'm an idiot. I've finally figured out what was causing the cracks. It wasn't loading pressure or increment size though the tube support surely helped, it was caused by the excess fuel left behind around the spindle after the rammer is removed. I wasn't cleaning it off with a fuel leveler and because the spindle has such an steep angle, it simply was not getting compressed enough like a steeper spindle would do.

The first test motor is showing no signs of cracks so I think I feel comfortable putting a fuse and stick on it tonight. I've applied a coating of inhibitor on the lower bell and I will finally get to see if this thing will fly.

I think that three rammers are not enough to press the lower portion of this motor. It goes from a 1" core to a 1/2" core with three rammers so if I make another two rammers for this section and level after each pressing then the cracking issues will surely be eliminated.

[NeighborJ]

First test motor catoed at about 4-500'. It gave me enough info to make a decision as to what direction I need to take it. The fact that it over pressurized near the end of the burn tells me that either the fuel burned thin and could no longer help the tube contain the excess pressure or it is actually burning with a progressive profile.

The fuel was a hot RIO benzo mix. I will try the slower "sound" formula before I try trimming the spindle. It's not far off from being a working motor and I'd like to test it on a stand but I don't think an acme stand can test this high.