Thrust in rocket delay

[ballmill]

I've been firing some 3 lb whistle rockets using a core fuel of 76/23/1+3 salicylate fuel on Wolters Extreme Tooling. They take off very fast with lots of power, but as soon as delay begins they slow down and lawn dart. They do get up pretty high so I don't think the thrust phase is the problem. After I press the core I switch to a slow 70/30 benzoate fuel that is too slow to use for the core, and I'm trying to use it up. This fuel produces a nice sound, but not much power.

 

My question is, during the delay burning of the rocket, is there ANY thrust being generated to keep it flying or is all the thrust created in the core burn and it's just momentum keeping the rocket moving?

 

Also, for what it's worth, I noticed the rocket does a strange twisting movement in the air before arcing over. I'm using a 3/8" by 36 inches stick.

Edited March 24, 2012 by ballmill

[Seymour]

The real thrust is created as the cored section is consumed.

 

However yes, some thrust is certainly consumed in the end burning section. Even if this were not sufficient to accelerate the rocket it will compensate significantly to counteracting drag, thus your rocket will not loose speed so fast, if at all. You can see a small scale demonstration of this in consumer whistle items where an and burning whistle is lightly lobbed by a very small lift charge and the thrust keeps it going. On this smaller size the thrust often is enough for fairly impressive acceleration.

 

Because your lifting power is from the cored section you could easily replace the delay section with a large number of compositions for various reasons, but considering how effective the end burning section is at howling, I'd have reservations about abandoning this feature!

Edited March 24, 2012 by Seymour

[ballmill]

The problem is my rockets have become lawn darts and I'm trying to figure out why.

[dagabu]

Ballmill,

 

You have several issues with your rockets that lend them to the realm of "lawn darts". The first is to get as straight and stable of flight as possible. You achieve this by balancing the drag placed on the rocket by ensuring that you have a rounded nose and an equally balanced tail.

 

Think about what happens when the rocket takes off. The rocket motor is providing many pounds of thrust for a brief second and then it enters the coast phase. When it takes off, the motor actually bends the stick outward slightly as the stick provides lots of drag on the motor and if the stick is not exactly straight and if it is not mounted exactly in line with the motor, the motor will enter a powered spiral. This is the equivalent of throwing the speed brakes on as you bleed speed of rapidly in this condition.

 

Sometimes the wood causes this problem and even the best of the best will get a tight spiral from a well made rocket, it cannot always be avoided. Once the rocket enters the coast phase, all the forces that were trying to tear the stick off your motor are now relaxed but often times damage has occurred and now the motor shows the flaws in an even tighter and slower spiral. Now the lawn dart starts.

 

Here are a couple tricks I have found that are handy when you want a straight flight that doesn't turn into a lawn dart. Use two sticks instead of one. If you have a 6" motor, use two 24" sticks each mounted 180° of each other, make sure the bow points into the center like a bow legged cowboy and don't use a stick any wider then one 1/4 the OD of the motor.

 

The second is to use a nose cone. You cannot believe the air resistance that a blunt stick or rocket nose can have when accelerating from stand still to bullet speed in a quarter second and any help you can give the rocket nose will pay dividends in height. It is that height and the speed at which your rocket enters it's coast phase that determine if you will get a round trip or a five second delay from report to hearing the sound (about a mile).

 

In answer to your question about thrust from whistle, yes, they do have some thrust and are often used to drive a girendola around in circles prior to lift off. The range may be #2-3 and is not significant due to the fuel weight. this will not add to coasting time much.

 

You are better off to add 1.5" of whistle delay and see where that gets you and add increments and you have successful flights. Remember, these rockets are usually just report rockets with a few grams of flash in the body tube and don't carry shells very well do to extreme stresses on lift off.

 

-dag

[allrocketspsl]

The problem is my rockets have become lawn darts and I'm trying to figure out why.

 

3lb whitsle rockets are large and dangerous I know you have a press and blast shield yes?If your getting lawn darts from this size rockets I assume your way out from civilization.Maybe scale down start smaller unless you already have then that problem shouldnt be to hard to figure out.

[psyco_1322]

Your lawn dart issues comes from plainly, too much delay. It can happen with lots of fuel or with a slow, crappy mix of whistle, in your case. Just cut back on the delay section. If you use slow whistle, the small amount of thrust created will be nearly useless. If you burn a fast fuel for delay, it can get you a little more distance on the cost. I figure you are just making report rockets, not that its the only kind people make, lots of people fly shells regular on whistles, because of the higher power. In that case the delay should be enough to let the rocket cost to a little before it would start arching over. In the event you use the rockets for lifting purposes, as the tooling is ideal for, then a short delay is all that's needed. Just about 2-3 seconds depending on the header weight.

 

The spirally thing your motors do it because they are nose heavy and not moving anymore as they cost to a stop and fall. In a normal engine the tube would be near empty. Where your slow fuel is still burning and there is a little more weight because of all the remaining fuel. Easy fix is to terminate the rocket before they get to that point, with less of a delay obviously.

[ballmill]

Thanks. Should I try increasing the stick size and length to compensate for the heavy nose during coast too see if that will balance it out?

[dagabu]

You can try that but as most people find after a while, the stick means less and less and the forces acting upon the motor mean more and more. Psyco did mention something about spiraling at coast phase but there is actually something happening physically to the rocket that causes the spiral, it is not simply a case that it is not moving o that it has a heavy heading. The oscillation is caused by air rebutting the stick as it exits the slip stream and enters into clean air, this phenomena is seen on the trailing edge of kites, flags, etc.

 

In order to to stop the oscillation you can increase the thickness of the stick, increase the length of the stick or eliminate the stick leaving the slip stream altogether. For instance, Lawn Darts have metal points and are obscenely nose heavy, as they are thrown, the large fins rebut the air and force it into a slipstream. Take the same lawn dart and throw it by the point, the lawn dart will wobble like crazy as it straightens out but it will straighten and will enter the ground point first every time.

 

Rockets are susceptible to the same forces but at much higher speeds and at no time are traveling as slowly as a lawn dart does and even at apogee, the pyro rocket is still moving 40 to 50 MPH but looks to be standing still because of the tight arch it travels through. Only rarely does a pyro rocket reach a 0° (meaning straight up) apogee and even then, the rocket never stops moving. The fastest way to enter stable flight and then keep stable flight is to allow the tail, fins, streamer, drogue etc. to remain in the slipstream at all time and not allow for oscillation.

 

Rocketeers much more experienced then I have developed and perfected (as much as one can in pyro rockets) a dual stick method and provide for material standards that don't allow for stick whip (read: shorter sticks, not longer).

 

Testing this theory will only cost you a stick. Add a second stick to the other side of the motor, point the bow into the center to keep the rebutting down and fly it as normal. You will notice a higher climb and little if any oscillation.

 

-dag

[ballmill]

Dag,

I did as you suggested and added a second stick to my rocket. I also added a few grams of flash for a finish report, nearly no weight added. The delay was very long as you can see in the video. The rocket did however go very high but after looking at the video it did seem to arc over and travel downward towards the end. Probably too much delay here. The performance of this one far exceeded the last rockets I was having problems with.

 

http://pyrobin.com/files/3lb_1.wmv

[dagabu]

Ballmill,

 

Yes, all rockets do at some point come back down

 

That was one nice whistle rocket, a full 14 seconds of flight, almost 2 seconds from report to sound, about 2000' feet away. I would say that rocket was very successful. No need to reduce the delay as far as I am concerned. A bigger report would be fun though.

 

-dag

[ballmill]

Thanks. I forgot to mention the last one I fired that had issues was cupped at the top of the motor. I simply used a paper end plug to cap off the motor and didn't flatten out the top, if you know what I mean. This rocket was taped off and there was no cup shape at the top which probably helped its flight.