Sugar rocket pressure problem

[JMan]

So I'm new at making rockets. I have a for grasp on all the concepts and most all of the math (basic math at least) needed to get my rockets in the air. I have had a few successful flights. I am using (from top to bottom) 1 in pvc end cap 1x5 in pvc tube and 1 in pvc coupling with a washer inside as well as a 1x1 in pvc ring to hold the washer in place in the coupling.

 

My goal is to make a swappable rocket. Inspired by flight tests swappable motors I guess. But I want to make it reusable and versatile aka no pvc solvent to hold it together.

 

My washers are used for nozzle get the exhaust gas I have used 1/4 in holes 3/8 in holes 1/2 in holes and no nozzle. My problem is that any nozzling done to the rocket causes too much pressure and blows either the nozzle/end cap off, but as the size increases it looses pressure and thrust which prevents it from taking off. My question is how might I make a tight fit which can be removed and have parts replaced (switching nozzles endcaps or pipe containing fuel) I have tried set screws and I have also tried threading the pipe and using screw on couplings/endcaps. Do any of you guys got an idea on how to make a tighter fit so the pressure dosent blow the nozzle off resulting in no pressure. Maybe increese the friction somehow?

[JMan]

Forgot to add my fuel is 65:35 kno3 to sugar works great use 125 grams exactly in each motor I have tried glueing it all together and it works great but no parts are salvageable except the washers

[JMan]

Tried my rocket with full metal piping that screw together. Long story short is I need a new, more remote launch site void of any thing that a 3 pound rocket poses a threat to. I would like to make the rocket lighter. I'm thinking about making a pvc casing which has a coupling/nose cone and a nozzle and you can remove the top and/or bottom to insert a cpvc pipe full of fuel into. That way the casing contains the pressure and the cpvc gets burnt up and replaced instead of the whole motor. Trying to find a threader for pvc this big. My main reason for making the casing is so I don't have to thred new pipe every time, I'm looking for a quick drop in fuel and launch kind of rocket, customizable to try different fins/nose cones or different cores/grains of fuel. If anyone has made a motor casing like this (which contains the pressure and the nozzle) please let me know how you went about doing this.

[NeighborJ]

A lot of guys use the snap ring motors. They are made from a simple aluminum tube with an interior groove which received a snap ring. This is done at both ends and both the end cap and nozzle have recessed o-rings on them.

[JMan]

Bought a pipe threader and tapped the pipes rather than using a collage of connectors and couplings so these can be re used and no parts need "replaced" like clay used for nozzles in other makes. For anyone trying to recreate this the pieces are very simple. Screw on 1 in cap, 4 by 1 in pipe tapped by a pipe threader, 1 to 3/4 in adapter (screw to slip on) with a 10 x 60 (I think) metric washer inside the adapter. I used (about) a 1/4 in hole in the washer and same size core but, as all rocketteers encourage, try your own nozzle and core size maybe try a smaller or longer tube mine flew about 1000 feet up and 1000 feet out. I did not measure thrust I will try to get back on that soon. My plans are to make a rocket body that this fits into or make a screw on top that is a nose cone and the nozzle has attached fins to keep it more professional than a stick duct taped to it. Good luck with any deveations you guys try to make please let me know if you find out anything that's good I'd love to know

[NeighborJ]

I don't know how to post a link but google "snap ring rocket motors" and scroll thru the photos. This is the simplest and most indestructible motor design I've yet seen for amateur rockets and the snap ring serves as a failure point in the event of overpressurization. What you are building sounds like it will be a pipe bomb if it catos and it will happen eventually. I'm not trying to be a smart ass I just don't want you getting hurt or having any other kinds of legal or liability issues.

[lloyd]

Just highlight the link on your URL line, then ctl-C to copy the link to your clipboard.

Place your cursor here in your message, and do a ctl-V to paste the link into your text:

 

http://coastrocketry.com/Motor%20Retention.htm

 

Lloyd

[NeighborJ]

Umm Lloyd, smart phone. And no that's not the link and not a snap ring motor.

[chuckufarley]

Neighbor,

If its an android phone just tap and hold on the URL. You'll get a menu pop up with a copy option. When you make your post tap and hold where you want to place the link and you'll get another option to paste. Not sure if that works for the I phones though.

[lloyd]

I didn't say it was THE link to which you were referring. Those are AmRoc 'snap ring assembled' motors. I have a couple of shoeboxes full of them, here.

 

All I was trying to do was help in the posting of a link -- any link.

 

Sorry.

 

L

Edited March 12, 2017 by lloyd

[NeighborJ]

Sorry lloyd I hate computers I can't seem to understand them so I get a little frustrated. This is a photo of what I was looking at.

[dagabu]

These by Apogee are a good model to follow for good quality motor casings. I started with another big name but the machining left much to be desired.

[JMan]

Thank you all but I've found that threading the pvc (or using nipples) works fine but I'm trying to make a delay charge that dosnt blow out with the pressure but still pushes the top off I've gone with two very small holes in the cap near the outside of the pipe so when the fuel gets burnt the small holes transfer the flame up to the delay/charge but as soon as the rocket took off the charge blew. Needless to say that didn't work how do you guys do your delay charges without popping the top from the Motor pressure?

[JMan]

Also thank you for your concerns and suggestions.

1.) no I'm not making a pipe bomb, yes I'm careful but I know that the top or bottom will fly off before the pvc bursts. I also stand a safe distance away.

2.) the detainment rings arnt very avaliable to me nor is any mechining tools to make pipe, this makes it an expensive alternative and I think pvc will better suit my goals.

3.) I'd also like to make my own casings rather than buy them, if that was the case I'd just buy and esties rocket. Just a personal goal and something I like to do, build and make not just cook the fuel.

[NeighborJ]

Most people I know use a flight controller to ignite a charge in the upper portion of the rocket tube for parachute deployment and it is done in stages and with multiple chutes so the rocket comes down in the same county.

On the smaller motors a female threaded reducer is used with the fuel cast inside and held fast by the threads. The larger portion of the reducer is glued permanently to the casing. The delay is one solid grain. A small BP charge is then placed on top.

Note: the delay grain will burn fast while the casing is under pressure but slowly after the pressure drops so timing can be tricky.

[JMan]

Thank you so much neighborj. I'll try different sized reducers and do a little research on pressure/burn rates. I'll keep you guys updated.

[NeighborJ]

There is a rule of thumb way of measuring the length of the delay portion which will burn at the accelerated rate. Just measure the wall thickness of the fuel grain and that will be the rough amount of delay fuel which will burn fast. An atmospheric burn test will need to be done to determine how long the rest of the delay grain will need to be.

A second method which can be used as a delay is to use a Bates grain with only a partial core as the bulkhead but this is more tricky to keep from blowing out and I never had enough faith in it to use it.

[JMan]

Ok so you all talk about grains, I directly pack my fuel into the tube as a whole single grain then core it while it is still hot. Is there any atvantage to using grains? Does the space between them burn and make more surface area or just help with constantcy? I've looked it up but I can't find a real "benefit" over directly packing the fuel

[NeighborJ]

OK there are several reasons why Bates grains are used by many, not all.

1:the grain method starts with a large surface area and as they burn than surface area stays fairly constant because they are burning on all sides, slowly decreasing in size. This creates an almost neutral burn profile(constant steady case pressure). The single grain burns with a steadily increasing surface area and has a progressive burn profile (steadily increasing case pressure).

The closer a motor can get to a neutral burn the more consistent it will perform. The progressive burn will start with a low thrust but end with a high thrust and if optimized it will be close to failure. It will also only be capable of lifting a rocket which is easily lifted at its least amount of thrust otherwise the rocket may Vere off coarse at take off and shoot away in the wrong direction.

2:the Bates system allows for one to make lost of grains in one sitting which can be assembled on launch day in multiple motor configurations.

It is a genius system and may seem like a pain but it is well used and versatile.

Edited March 15, 2017 by NeighborJ

[JMan]

I have had problems with rockets turning sideways (espicaly my metal pipe rocket) I knew that was the proem but what is the recommended thrust to weight on model rockets? I know over kill will solve the problem but just for minimum requirements is it about 5:1? I've looked it up but most resulted are on larger scale model rockets. I know real rockets use much less on the pad but they also gimbal and have active stability assist

[NeighborJ]

I do not have an equation for maximum lift capacity for a motor. I never had access to a real test stand to get real usable data. If I am sure the motor had a full core ignition and it still had a sluggish lift off there are a few things which can be done to increase the motors initial thrust.

The most ideal solution is to make the core larger without making the nozzle larger. This is an easy solution and will be like igniting the motor later in its burn which creates a higher initial impulse but a shorter burn time. I have never been able to make a multiple Bates grain motor with the core being the same size as the nozzle without cato, it always needed to be a minimum of at least 1.5x the nozzle dia but there is no issue making the core a lot larger than the nozzle.

The single grain design you are employing may be capable of holding together at the 1:1 ratio because it has less surface area but that also means low initial thrust with an aggressive progressive burn rate.

The other options are not as attractive because they include building a stronger motor case, changing nozzle size or adding a catylist to your fuel. All of these options mean redesigning the motor To accommodate the changes.

Edited March 15, 2017 by NeighborJ

[dagabu]

OK there are several reasons why Bates grains are used by many, not all.

1:the grain method starts with a large surface area and as they burn than surface area stays fairly constant because they are burning on all sides, slowly decreasing in size. This creates an almost neutral burn profile(constant steady case pressure). The single grain burns with a steadily increasing surface area and has a progressive burn profile (steadily increasing case pressure).

The closer a motor can get to a neutral burn the more consistent it will perform. The progressive burn will start with a low thrust but end with a high thrust and if optimized it will be close to failure. It will also only be capable of lifting a rocket which is easily lifted at its least amount of thrust otherwise the rocket may Vere off coarse at take off and shoot away in the wrong direction.

2:the Bates system allows for one to make lost of grains in one sitting which can be assembled on launch day in multiple motor configurations.

It is a genius system and may seem like a pain but it is well used and versatile.

 

BATES grains are inhibited on the outside of the grain, it can certainly be a modified BATES grain without being inhibited on the outside but to be clear about the design, we should probably agree what we think a BATES grain is first. In the 60's, it was a single grain, now it is considered to be 2 or more grains abut all are exterior inhibited.

 

Sorry for being Mr know-it-all, I just don't want Jman to get confused about the terms.

[MrB]

Hey Dagabu. While on the topic... Why is it the exterior burn inhibited? If the inside and outside of a infinity long tube is lit, then the surface should remain the same until it is all consumed. (I suppose in reality when the two surfaces get close enough to one and other they will instantly consume whats left with a pressure spike, but that should be the case now to, burning inside out)

 

I make comets this way. Truth be told i only burn inhibit one side, the one mated to the shell, but "ring" comets give me a stronger effect then a "end burning" solid chunk. Also consumes more composition, since they burn fast. (only a quarter of the diameter (well, diameter, minus the center hole, and a quarter of what your left with) counts for the burn time. So the comet has to be 4 times as wide as a endburning one has to be thick. Tangent. Sorry.

B!

[NeighborJ]

Bates grains are Inhibited on the outside to prevent fire from getting between the case and the grain, this is important because the result is almost always CATO.

On Jmans motor it sounds as if he loads his fuel directly in the casing, he can get away with this because it is a single solid grain and has no pathway to the casing wall for fire to propagate.

Inhibitors also help protect the casings from being consumed as fuel or the veryleast acts as a heat shield for the millisecond of exposure it will have. Also, inhibitors are generally used in conjunction with a lubricant such as vaseline to help seal the grain against the case.

Dag does a good job of comprehensively explaining things when I may only explain if the question is asked.

[dagabu]

BATES grains were actually developed as an acronym for BAllistic Test and Evaluation System, the grain was actually a single grain and they found that inhibiting the grain (not sealed or lubed) but slip fitted into an case, not poured in place and therefore not a BATES grain in his case.

 

Sadly, B, this is not the case with BATES grains, they are progressive, the surface does not remain the same throughout the burn though 'rod and tube' and 'star' designs do offer relatively neutral thrust curves. This is why the BATES grains with a progressive burn is so desired in model rocketry but doesn't have the initial thrust to launch a ball shell on a pyro rocket.

 

A tapered spindle offers a fairly regressive burn with most of the thrust being used almost immediately to overcome its inertia.

 

Danny Creagan did some very cool testing some time back with different spindle designs. This may give some better ideas for a filled tube grain design.

 

http://www.wichitabuggywhip.com/fireworks/rockets/erich/index.html