My big KNSU model rocket attempt

[Yankie]

I decided to up the scale of my KNSU rockets and made some 3cm ID 10-15cm long rocket motors. I made a ghetto model rocket for one of the motors out of basic materials and made a parachute too.

 

Specs:

Length: 60cm

Width: 5cm

Total weight: 466 grams

120g of propellant

 

 

Inside the rocket:

http://img380.imageshack.us/img380/7813/rocketak7.th.pnghttp://img380.imageshack.us/images/thpix.gif

 

Left to right: box of matches, the motor, the rocket:

http://img235.imageshack.us/img235/1416/dsc00001so1.th.jpghttp://img235.imageshack.us/images/thpix.gif

 

Nearly finished rocket next to 30cm ruler:

http://img235.imageshack.us/img235/5412/dsc00004qw2.th.jpghttp://img235.imageshack.us/images/thpix.gif

 

The finished rocket:

http://img80.imageshack.us/img80/8003/dsc00002wf8.th.jpghttp://img80.imageshack.us/images/thpix.gif

 

Heres the video: http://uk.youtube.com/watch?v=I5KlbAsjEjo

 

 

What was supposed to happen is the motor consumes all of its fuel (by this stage it should have reached a high altitude) then the fire travels through the passfire hole lighting some BP which ensures ignition of the time fuse, this burns for 3-4 seconds before it hits some granulated black powder which forces the parachute out of the top and the motor out of the bottom of the rocket, leaving the empty rocket housing to sail back to the ground.

 

What went wrong: The rocket did not gain enough speed to remain stable and tipped. The motor was too long and built up too much pressure blowing the end plug and all of the fuel out of the bottom of the motor.

 

Apart from that the parachute worked perfectly (can just be seen popping out on the last frame of the video)

 

My diagnosis: Needs a shorter rocket motor to prevent CATO (I tested the 10cm long ones and they worked perfectly going almost out of sight)

Needs longer launching rod so the rocket has time to gain enough speed to spin stabilise itself.

 

Feedback and suggestions would be apreciated.

Edited October 11, 2008 by fredbert

[Yankie]

Generally, spin stabilized (Stinger) rockets don't use fins.

 

I dont mean horizontal spin I mean verticle spin like a bullet leaving a rifle. The fins are angled so that at high speeds the air causes the rocket to spin creating stability.

[Richtee]

I dont mean horizontal spin I mean verticle spin like a bullet leaving a rifle. The fins are angled so that at high speeds the air causes the rocket to spin creating stability.

 

I'm no rocket scientist... but is spin needed/wanted with sufficient fin area to keep it stable? I DO know strange things happen with even slightly unbalanced payloads when spin is employed.

[Yankie]

I'm no rocket scientist... but is spin needed/wanted with sufficient fin area to keep it stable? I DO know strange things happen with even slightly unbalanced payloads when spin is employed.

 

 

To be honest I dont know much about rockets, I just thought spinning makes things stable so I will make it spin. I will improve on the design over summer and make many more rockets (mostly smaller scale)

[oskarchem]

Here's my hypothesis:

 

I think that all of your fins are not at the same angle, because, let's say that you 3 fins are at 5°, 6°, and 7°, your rocket is not going to go straight. Try to get the exact same angle on all of your fins.

[Richtee]

Here's my hypothesis:

 

I think that all of your fins are not at the same angle, because, let's say that you 3 fins are at 5°, 6°, and 7°, your rocket is not going to go straight. Try to get the exact same angle on all of your fins.

 

And make that angle 0°- I really don't think you want to spin. Too many varibles, and with enough fin area, it will fly straight and true.

 

Another reason for straight fins...how's it supposed to spin off the launch rod? Stingers use a central post to launch.

Edited October 11, 2008 by Richtee

[oskarchem]

Yeah, as Richtee said, try with a 0° angle. And maybe make the fins longer, the longer they are, the more stable the flight should be, no? I'm guessing make them 30cm long.

[psyco_1322]

An idea to keep in mind next time before blowing up hours of working on your rocket. Do a static test of your engine and make sure you can make them repeatably with out any problems. If it has a core, light it at the top and see if you can intentionally blow it up like that. If the motor can be lit up there, it will get better take off and you might as well make use of it.

[oskarchem]

Not to keep hyping the NASA site and the simulator software, but that's the beauty of the site- fin size and number, rocket size, power, nose cone, placement of balance centers, etc. can all be manipulated to see just what will happen.

 

There's not a thing wrong with trial and error, mind you- I first used a few simple rules of thumb to get my rockets up and running, w/o benefit of any program. It just allows a large number of trials in a short period of time and answers a lot of "what if's".

 

 

Huh, I've gotta look at that site, could be interesting, I like rocket science and maybe I could start making some nice big model rockets.

 

Oww god, I drunk way too much last night... It's never been so bad..

[Swede]

Spinning a long, skinny object is a bad idea. You end up with a phenomenon called roll coupling, I believe. What this does... picture the very tip of the nose cone. Initially, it is a rotating point, but soon the tip begins to describe a circle, which grow, and grows, as the centripetal acceleration grabs hold.

 

The T-38 is a long, skinny jet with a phenomenal roll rate. The roll rate was so high, it would begin to roll couple after two turns, and the possibility of breakup became high. Thus, max-rate aileron rolls of more than two revolutions were prohibited.

 

I applaud your attention to detail and your experimentation. Take each step individually... get a motor that works, measure the thrust, do the calculations, etc. Then build the rocket!

[Arqwat]

Good for you, scratch building your rocket !

That is how one learns engineering techniques in building anything.

 

Looked slow to start - The fins didn't stabilize the rocket as there wasn't sufficient speed.

The fins should be OK providing the rocket has enough speed when it leaves the launch rod.

More power . . longer rod ??

 

Possibly too high of a Center of Gravity (CG) . . . too much weight towards the nose ??

If the nose is too heavy, the tail will tend to "blow around past the nose" from the propulsion.

By the same token too low a CG (too tail heavy) & the nose will tend to "blow around" to the back.

Both of these scenarios can be overcome when the rocket builds sufficient speed for the fins to stabilize it.

 

Check your CG . . . A generally safe CG is about 75%.

The rocket (tied to a string) should balance about 3/4 down from the top (with motor & chute installed).

If it's even "ballpark close", it should work.

 

Consider strengthening the nozzle area of the motor.

Going with a shorter motor may be sacrificing needed power.

 

Just a note, I tried using visco as time fuse with KNSU motors before .

The fuse did not burn normally, it exploded - there was no time delay at all.

I think this was because the visco was actually burning under internal motor pressure.

Visco is loosely bound BP. Under pressure, loose BP becomes more "explosive".

Looks like this didn't affect you as the visco is not actually enclosed within the motor casing.

 

If you can . . test fire a couple of motors on the ground, this may save you from a crashed rocket.

 

Figure out what happened & correct the problem and DO IT AGAIN !

 

For great tips on sugar motor building see - http://www.jamesyawn.net/index.htm

Jimmy Yawn is the East Coast Sugar Motor Guru & a personal friend.

He's got tons of great info on his site.

 

Cheers

Edited October 12, 2008 by Arqwat

[psyco_1322]

Why are you guys using fuse for a delay? Why no just press/ram in some more fuel. I was making whislte the other day that I wanted to try in a model rocket, and I decided to press in some plain smoke mix for a delay, seemed to work pretty good.

[Arqwat]

I didn't use fuse.

I was using an Aerotech E-18 Load.

The delay was a standard Aerotech 7 sec.

Meant to use a drilled delay down to 3 sec.

Believe I used the 7 sec delay by mistake.

 

Agreed - The most consistant delays for my sugar motors is a "propellant slug".

[DIYMark]

I second the smoke mix delay - it works good.

 

Also If you like you can try this for your motor sizes. This is derived from my KNSU "skyrocket" motors and it scales up and down great. It works for a 3/8" motor a 1/2" motor a 16mm and as well as a 35mm motor (I tried all of them).

 

This assumes a UN-catalyzed, KNSU motor made up of rammed dry mix.

 

First establish your motor ID - In your case it is 30mm.

The nozzle size is ID divided by 3.629 and for 30mm ID you need a nozzle 8.2mm wide.

Core length is 2.756 times your ID. So for 30mm you need a core 83mm long. This is the length of core in the propellant - do not include the clay cap in this.

Other specs are as pernormal - cap legths are 1 ID (30mm) and depending on the aerodynamics/coast time the delay should be ~1.25 ID's (37.5mm) above the core.

 

I have used these specs in a vast array of motors and they all perform great.

 

Now, to stabilize the rocket stick with straight fins - 2 rocket OD's high and 3 rocket OD's wide (3 fins). Also the problem with KNSU dry packed motors is the initial thrust. In order for your motor to not blow up later in the burn time the motor must have a low initial Kn. This means that there is low initial thrust which is a problem because you need that initial kick to get the motor off the launch guide and up to speed so the fins can do their job. To fix this issue you can make a longer launch guide to make a motor with proper BATES grains (but this loses fun fast when you have to make then continually).

 

As a result of experiencing all this Ive decided to make ESTES performance motors (well in some cases more performance) with BP. The end burning design is great because the initial peak get the rocket off the guide and up to speed while the continuous thrust later gets the rocket to a good altitude (less thrust for longer is better then more for shorter - the drag equation gas a velocity squared in it somewhere and this means to double the speed of the rocket it takes 4 items the force!).

 

But, if you do continue to make motors from KNSU dry mix, my advice is to use the above mentioned specs and to use a longer launch guide. Also making a "special" ignitor may help. Get black match and turn one part into quick match that is longer than your motor core. On the quick match end put a SMALL BP charge. Use slow burning (like thermite) BP grade. Insert this ignitor to the very back of the core and it will give you better initial thrust as your motor will light the whole core nearly simultaneously.

 

Lastly if the casing for the motor is made from PVC and you are ramming bentonite caps - use a drill bit or scissors to scrape/carve grooves all around the ends with the clay plugs.

Edited October 13, 2008 by DIYMark

[Arqwat]

Good info DYIMark . . .

 

A tip . . If you ram powdered KNSU with a core, you can insert a piece of Quick Visco (I use Cannonfuse.com) for ignition.

Use a piece of visco or even an ignitor to light the Quick Visco.

The Quick Visco is 4 sec per foot - so an inch or 2 is fast enough to get the whole core burning almost instantly.

This works well for me - for private launches . . . Can't do it at an NAR/TRA launch though . . . No fuses allowed . .

 

Cheers

[Yankie]

Thanks for all the advice, the reason I didn't static test my motors first was because I had 2 days to build the rocket in and the lauch site is a few hours from here. I took the risk of using a longer core hoping for more thrust but as it shows I should have used the shorter cored rockets (which worked very well by the way)

My new rocket will be loosely based on Jimmy Yawn's "Snapdragon" rockets (which are of much smaller scale) I will update on my progress during the construction.

For some reason the NASA simulator does not work for me, do I need toinstall some software such as flash?

Currently I am making 5 1lb rockets some with multiple stages and testing new things.

I might use some red iron oxide to catalyse the fuel and get a faster burn, also I will read into the different grain types as advised by J. Yawn.

 

Thanks again.

[Yankie]

I emailed James Yawn in regards to my rocket and I thought I would share his response as it was extremely useful to me and no doubt contains information that will be useful to many other forum users.

 

 

Here is the email:

 

Date: Sun, 12 Oct 2008 13:26:03 -0400

> From: jyawn@sfcc.net

> To: yankie_scholz@hotmail.com

> Subject: Re: Rockets

>

> Dear Yankie: Your rocket looks good! I think you are close to a nice

> flight with it.

>

> I see two possible issues that might have led to the "interesting"

> flight in your video.

>

> One is fin placement and other stability issues. Your fin size looks

> good. But they may be a bit too far forward, given other things. As

> someone on the forum pointed out, the center of gravity must be a little

> ways forward of the center of pressure. One "caliber" (body tube

> diameter) is recommended. Greater distance makes the rocket more stable

> (or overstable) Less distance makes it less stable, and having the

> center of pressure AHEAD of the center of gravity makes the rocket want

> to fly backwards. The motor makes it want to fly forwards, resulting in

> a "loopy" flight. (Since your rocket did not loop, this is my SECOND

> guess as too what went wrong, I'm just talking about it first. Motor

> design issue is my first guess, see below.)

>

> The motor, being heavy, moves the center of pressure rearward, and if CG

> and CP get too close together, instability results.

>

> Several ways to deal with this. One is to move the fins further

> rearward. Trailing fins often help. That's why my Snap Dragon rocket

> has trailing fins

> (http://www.jamesyawn.net/modelrocket/intro/index.html) as it is

> intended to be built by kids, and "quality control" may be lacking.

> Thus it is an overstable design, tolerant of such anomalies as crooked

> fins, off-center nose cones, etc.

>

> Larger fins might help your rocket, but only if they are shaped such

> that the majority of surface area is behind the CG. Delta fins are

> often used for this purpose.

>

> The launch lugs on your rocket are large. So large that the forward one

> may act as a forward fin, destabilizing the rocket. I'd suggest using a

> smaller launch rod, allowing the use of thinner launch lugs. And with

> the right CG, a forward lug may be unnecessary. Nowdays on the Snap

> Dragon, I'm making one long lug (rolled from an index card and white

> glue) which attaches at the root of one fin, and extends beyond the fin

> a ways so that it goes an inch or so beyond the center of gravity.

>

> You sometimes see forward fins on military rockets. Some amateur

> rocketeers copy this design, either because they are modeling such a

> rocket, think it will improve stability, or because it looks "cool."

> But forward fins tend to destabilize a rocket. The military uses them

> for that reason - these rockets have active guidance systems, and

> forward fins make them more steerable in flight.

>

> Another option is to move the center of gravity forward. This can be

> done by adding weight to the forward end of the rocket, such as washers

> attached to the nose cone, or lead shot mixed with epoxy in the nose

> cone. Sometimes I've filled a paper nose cone with fiberglass resin -

> gives it good weight, and makes it very strong!

>

> Using a smaller, thus lighter motor also moves the CG forward. An error

> some rocketeers make is to fly their rockets on a small motor first,

> note that it works, then fly it with a larger motor. The greater mass

> of the larger motor may destabilize the rocket, if its stability ratio

> (distance from CG to CP) is marginal.

>

> There was some discussion on the forum about spin. It usually isn't

> very helpful in this kind of rocket, as the rocket can't spin until it

> has left the launch rod, and it must achieve some degree of airspeed

> before it will start to spin. Thus one might get stability, but in an

> unknown direction. This can be a bit too exciting! Rockets usually use

> either fins or spin for stability, but rarely both. There are some

> exceptions, like the Super Loki which launches from a spiral launch

> rail, so that it gets good spin at the start. This is somewhat like the

> pyrotechnic "stinger" rockets are made to spin before they leave the

> launch pad. (Of course, professional rockets often have active guidance

> mechanisms, but that is beyond the scope of this discussion.)

>

>

> MOTOR DESIGN

>

> The other possible problem has to do with the motor design. From your

> drawing, it looks like you are using a core-burning motor. (Please let

> me know if I am wrong about this!)

>

> There are two issues with the core-burning design that makes it less

> than optimal for KN/SU motors. One is the progressive nature of the

> core-burner. It will start off with low thrust and pressure, and is it

> burns build up to higher and higher pressure. So you get the lowest

> thrust when you need high thrust the most, to get the rocket off the pad

> and up to stable airspeed before it leaves the guide rod.

>

> Then as the motor burns, the burning surface area increases, the thrust

> and pressures thus increase, and if not very carefully designed to

> accommodate this increase in pressure, the motor may burst.

>

> So you have the choice of too-little thrust at the beginning and

> adequate thrust toward to end, or adequate thrust at the beginning, and

> too much pressure at the end. Or in the case of your rocket, both.

>

> One more possibility, an issue with the core-burning KN/SU design is

> case expansion and propellant delamination. Under pressure, motor cases

> expand. This happens with all cases, some more than others. By

> pressing the propellant into the casing, you are counting on the outside

> of the propellant grain to stay attached to the wall of the casing, so

> that it burns from the inside out, right? Well, KN/SU is rigid, and

> does not expand much without either cracking, which is bad, or

> separating from the case wall, which is worse. Both increase the

> burning surface of the propellant grain dramatically, and are very

> likely to result in excessive pressure. This may be what happened to

> your rocket. As the grain burns, it gets thinner and thus weaker. And

> it produces more pressure, expanding the case wall even more, making

> cracking, separation, or both much more likely.

>

> The cure? A better grain design. I recommend using uninhibited grains

> for small motors - they provide a short, high-thrust burn that is

> moderately regressive. I use these in my model rocket motors

> (www.jamesyawn.net/modelrocket/mrmovies/index.html) and also for

> propellant tests in 38mm motors. The grains are easy to make, ignite

> easily, and burn with a vengeance. I think your motor would work well

> with uninhibited grains for starters.

>

> BATES grains would be another good choice, and good for larger motors.

> A BATES motor will burn for about twice as long as an uninhibited grain

> motor, but at about half the thrust. But BATES motors can be made

> longer, to generate equal thrust, still at twice the burn time. BATES

> grains would be a good next step for your motor - the fast burn of the

> uninhibited grain is very efficient, but not as entertaining as the

> longer burn.

>

> I use moonburner grains a lot nowdays, as they provide about twice the

> burn time of an equivalent BATES motor, again at about half the thrust

> for a given length. But I can then make the motor even longer, getting

> high thrust plus long burn time.

>

> BATES example:

>

> For comparison, here is a launch of the Sugar Rush using a BATES-grain

> motor:

>

> http://www.jamesyawn.net/Sentinel-9-20-08/index.html (its the last

> launch on that page, #31.)

>

>

> Moonburner example:

>

> Here is my favorite example of TWO Sugar Rushes using Moonburner loads:

>

> http://www.nefar.net/gallery/2008-01/index.html (its the second launch

> on that page.)

>

>

> Uninhibited example:

>

> I've only launched a few larger rockets using uninhibited grain motors,

> but have a video of one at:

>

> http://www.jamesyawn.net/nefar2-12-05/index.html (its the second launch

> on that page. The first launch is with a BATES motor. These are 38mm

> motors, the Sugar Rush flies on a 54mm motor)

>

> Feel free to post this to the discussion list if you think your pyro

> friends would find it interesting. They have made good suggestions, by

> and large, and might find this interesting too. The rocket simulation

> software on the NASA site is indeed pretty cool, I recommend it too.

> There are other simulation programs for motor design, so let me know if

> you have an interest in other designs, I'll be happy to send them to

> you, or direct you to them.

>

> Hope to hear from you soon!

>

> Jimmy Yawn

> jyawn@sfcc.net

[Yankie]

Heres my 1lb rockets (I will base the design of my 4lb on these)

 

http://img390.imageshack.us/img390/989/dsc00005iu8.th.jpghttp://img390.imageshack.us/images/thpix.gif

 

left to right: 4oz, 4lb, 1lb (x5)

http://img205.imageshack.us/img205/5222/dsc00006yn8.th.jpghttp://img205.imageshack.us/images/thpix.gif

[Swede]

Wow, that was really nice of James Yawn to create such a detailed reply. That information is worthy of saving somewhere.

 

Good luck with your rockets! They look great! I assume you have Sleeter's book?

[ns91]

Hi,

 

Well i made some rocket motors and i will shoot them for a day or two and post what i did...

[Yankie]

I assume you have Sleeter's book?

 

No I don't have his book... should I have?

[Swede]

If you are interested in homemade BP motors for serious rocketry, then I'd say it's a must-have. It's an excellent resource, full of tooling prints for dozens of sizes of rammed BP motors, as well as mucho theory. Sleeter is a rocket enthusiast, not a Pyro (as far as I can gather) and if you are interested in rocketry, then I'd give it a huge thumb's up.

 

"Amateur Rocket Motor Construction" by David Sleeter.

[MilkyJoe]

If you are interested in homemade BP motors for serious rocketry, then I'd say it's a must-have. It's an excellent resource, full of tooling prints for dozens of sizes of rammed BP motors, as well as mucho theory. Sleeter is a rocket enthusiast, not a Pyro (as far as I can gather) and if you are interested in rocketry, then I'd give it a huge thumb's up.

 

"Amateur Rocket Motor Construction" by David Sleeter.

 

Is the theory something that an amateur hobbyist is likely to understand? I'd like to get involved in a bit more of the theory behind rockets but I don't want to dive headfirst into the serious stuff.

 

Thanks!

[Swede]

I believe so! By theory, really, it isn't the math behind rocket propulsion... Sleeter goes into the "theory" of making excellent BP rocket motors. He also gets into KNSU motors a bit, but 90% of the book is about cored BP engines, the tubes, the tooling, nozzles, their diameters and construction, etc. He does not assume you have a lathe. he takes two approaches, the low tech "no fancy tools" AND the high tech "You have fancy tools and can make your own tooling. It is definitely geared more towards model rockets than pyrotechnics, although the principles will work for either.

[zappeling]

Good rocket..But it does a very short fligth.