100mm pvc KNDX SRM.

[Redrocketman]

Hello all. It's been a real long time since posting on APC, apologies things have been mad!! Progress in my experimental pvc sugar motors has been excellent, with 6 reliable class motors in the books, my favourite being a 5g KNDX J800.

Things as always are moving to bigger and better things with my current motor design being a 5g motor using 100mm dia casing. Basic propellant dimensions are 5 grains 100mm X 150mm, core diameter 26mm (grains 2-5) 30mm (grain 1) with a 19mm nozzle throat diameter. I haven't ran any accurate simulations but it will class - up there!! My hurdle at present is a suitable case liner. As the burn time will be much longer than my present motors it's a guarantee the case will fail without thermal protection. Extremely efficient thermal protection as with heatsink will come loss of all the PVCs strength. I'm putting this one out there as there's a huge variety of professions in this forum, perhaps something may pop up!!!

[Arthur]

Two turns of nomex paper inside the PVC tube should hold the heat away for a while. It's used in transformer windings.

 

https://uk.rs-online.com/web/p/thermal-insulating-sheets/7757469/

https://uk.rs-online.com/web/p/thermal-insulating-sheets/7248909/

 

May give you a starter for a search.

[Col]

I`d soak kraft paper in potassium silicate. Once dry it should cope with temps upto 500C for a short time.

[Redrocketman]

Spot on fellas, tho looking at the Nomex it can protect against flame, and temperature up to 300degC. Unfortunately that's not gunna make the grade. Especially when I looked at the price!! £28!!! That's heaps in AU dollars. I build a complete J class motor for less than $15 and the K for $25!! But nonetheless thanks heaps man. But we could be on to something with the potassium silicate, it reminded me of something a couple of years back. Could be it!

[Arthur]

Everywhere that asbestos was the preferred material, there has been a new material found or invented to replace it. It's just slightly hard to find all those replacements easily available.

 

Lot's/most of rocketry material suitability depends not on the peak theoretical temperature but the resistance to heat flow during the burn phase. One turn of paper might not be enough, but a second turn may protect the tube for long enough. The RS catalogue links were only to prove that the product exists. There may be an Australian equivalent company, or several american companies who can make you reasonable offers. Mouser is a US supplier of some similar material, CPC is another UK supplier. Is there a hobby/craft pottery industry in your area that might find you something suitable.

[Col]

I always look for the cheapest, most easily available solution. The more exotic something is, usually the more expensive and harder it is to find. If the cheap solution fails it`ll still be there as an option A lot depends on the space the material has to fit in, if its a fraction of a mm that will make things tougher vs a couple of mm. If the outside of the material can be held below 140C for the duration of the burn the pvc should be ok. A heatgun,IR thermometer and stopwatch is all you`d need for initial testing.

[stix]

Interesting to read how Richard Nakka deals with the issue (although with smaller 2"/50mm) tubes. No major thermal protection except for using masking tape for a snug fit.

 

http://www.nakka-rocketry.net/pvcmot10.html

. . . The segments are restacked back into the casing. However, before doing this, the top and bottom edge of each segment is wrapped with masking tape so the segments have a push fit into the casing. The amount of masking tape is determined by trial and error, but the fit should be tight enough so when the casing is turned upside down the grains do not fall out. The tape serves two purposes. First, it adds insulation to the inhibitor sleeve in an area, which could use some extra insulation. Second, the tight fit helps to reduce the volume of the gas flow between the sleeve and the casing that might otherwise cause problems.
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. . . There is also the possibility of building motors from 3" or even 4" PVC pipe, but the penalty paid by having to operate at much reduced pressure may not be worth the effort. It is believed that to achieve significant increases in impulse over what can now be achieved with PVC pipe will require motors with metal casings. There are re-useable rocket motor cases on the market for which Sorbitol grains can be made that will give impulses approaching 4500 Ns. If this proves to be possible, experimental reloads for these motors made from the sugar propellants would represent a significant cost benefit to the experimenter.

 

You can still attain J or K class by just going longer.

 

I'm not saying that you shouldn't experiment with 4"/100mm tubes, but it does seem like you may have issues getting the larger motors up to a decent operating pressure without over-pressurising the casing.

 

It'll be interesting to see how you go.

 

Cheers.

[Arthur]

How many seconds do you expect the motor to burn for? IMO it's essential to get only the necessary amount of insulator inside the motor to minimise the loss of space for fuel. So maybe it's essential to get just the minimum delay before the motor melts. There must somewhere be a sweet spot between the motor stopping at say 10 seconds and melting at 11 seconds, possibly tempered by cost and reliability issues. Maybe the motor could be reusable with a new heat shield. The advantage of the nomex sheet is it's thinness so you get more fuel inside.

 

I think we have all used paper or cloth to transitrily pick up something that's "too hot" to touch even when it's really too hot for the paper in the steady state but for a few seconds it's fine.

[dagabu]

Paper will give you a longer heat shield than cloth simply because of the weave in cloth. Virgin kraft soaked in Sodium Silicate (Na2SiO2)nO (Waterglass) and allowed to dry inside the tube (keep constant pressure on the paper pushing it outward toward the tube interior) completely, then load the grain/s, will give you a cheap heat shield for experimentation.

 

You can also use a self adhesive hi-tech material like ZircoFlex™ - Flexible Ceramic Heat Shield in two layers to keep the heat at bay. VERY expensive but is amazing in actual performance!

[stix]

. . . Virgin kraft soaked in Sodium Silicate (Na2SiO2)nO (Waterglass) and allowed to dry inside the tube (keep constant pressure on the paper pushing it outward toward the tube interior) completely, then load the grain/s, will give you a cheap heat shield for experimentation . . .

 

+1

 

Sounds like a relatively cheap and easy method worth trying. Sodium Silicate (Na2SiO2)nO (Waterglass) can be purchased from your local pottery supplies.

[Col]

3 layers of 60gsm pure kraft with 4 silicate layers (effectively 7 barriers in total) equates to 0.3mm thickness. Potassium silicate will provide better heat resistance than sodium where space is tight.

[stix]

. . . Potassium silicate will provide better heat resistance than sodium where space is tight.

 

Interesting info there Col. I couldn't find Potassium Silicate from my ceramics supplier - where does one purchase that? it doesn't seem obvious.

[PeteyPyro]

I believe that potassium silicate is available as a source of potassium, and especially silica, for horticultural use. Greenhouse, or rather, hydroponic supply houses may carry it in stock too.https://www.amazon.com/gp/aw/d/B00J2WVAM4/ref=mp_s_a_1_2?ie=UTF8&qid=1515932649&sr=8-2&pi=AC_SX236_SY340_FMwebp_QL65&keywords=potassium+silicate+liquid&dpPl=1&dpID=51QDly%2B83lL&ref=plSrch#

https://www.amazon.com/gp/aw/d/B003AUHO4M#

Edited January 14, 2018 by PeteyPyro

[Arthur]

A suggested method of making paper mortars flame resistant was to fill them with sodium silicate solution, stand them for a while, then drain and fill with Potassium chloride, then drain and dry. Effectively soaking them in Potassium Silicate formed in situ.

 

Consider that several layers of anything makes a better insulator than one layer of the same thickness. maybe two or three turns of bright white copier paper would work, the hot layer would be charred leaving the clay fillers but the next layers would then be protected for a short time and two or three short times is a burn time for a rocket. Very few materials are full strength when long term kept at rocket motor temperature

[Col]

 

Interesting info there Col. I couldn't find Potassium Silicate from my ceramics supplier - where does one purchase that? it doesn't seem obvious.

You could prolly make some using silicon dioxide and potassium hydroxide. The softening point of potassium silicate is 700-740C, sodium silicate around 650C so not quite as good but its very cheap at 60p/litre.

[Arthur]

The point when proofing tubes was that the potassium salt has lower solubility so doesn't migrate when damp. so you let the sodium silicate migrate into the paper thickness, then convert it to the potassium salt to fix it in place.

[Col]

it has greater solubility than sodium silicate with the same level of alkalinity As the paper thickness may only equate to a mm or two in this case, its neither here nor there.

[stix]

A suggested method of making paper mortars flame resistant was to fill them with sodium silicate solution, stand them for a while, then drain and fill with Potassium chloride, then drain and dry. Effectively soaking them in Potassium Silicate formed in situ . . .

 

Well that's very interesting Arthur. I have a similar method except that you end up with Calcium Silicate:

 

Rinse the tube internally with a strong solution of calcium chloride in water. Allow your tubes to stand until they are just damp. Calcium chloride is rather hygroscopic and will not dry past remaining damp.

 

Using a 25% w/w solution of sodium silicate in water rinse each tube internally. Pour out the excess into your bucket and let the tubes dry open end up. There is a chemical reaction that produces tough insoluble calcium silicate in the structure of the paper. This hardens and binds the surface making it resistant to damage by abrasion and alkaline residues. When dried give the tube a fresh water rinse to remove residual sodium chloride produced in the reaction and let dry open end up in a warm location.

 

I've tried the above method on a cardboard tube intended to be used as a mortar. Calcium Chloride can be purchased from the laundry section of your hardware store - sold as "Damp Rid" here in Aus.

Not sure of the resistance to heat though.

 

[EDIT] Just did some research and it looks like calcium silicate board is used as high temperature insulation. So that looks good for this application.

The hard part would be lining the PVC tube and ensuring it doesn't distort and leave voids.

Edited January 14, 2018 by stix

[Mumbles]

My notes say the same Stix. Given the higher solubility of potassium silicate according to Col, I'd expect KCl not to swap in for sodium either.

[Col]

Stix, the upside of silicate is it makes paper more dimensionally stable and less likely to distort. The downside is silicate wont bond to pvc so it`ll have to be a very snug fit.

[stix]

Whichever way, it's probably going to be difficult to implement. Maybe you could treat thin card (manila) then roll and insert it?

 

The main issue is always going to be the corner where the nozzle meets the casing which is the area under most attack.