Reducing air bubbles in motors.

[Grog6]

OK,I was working on a new batch of the "sprint" mix, and I figured out how to extrude into a 29mm casing, While it's in the vacuum chamber, in a well controlled fashion.
It's in the cure box, should be solid by tomorrow.

 

[jderimig]

A couple thoughts...
Vacuum casting doesn't by itself guarantee no voids, just the voids will be vacuum. You still need a force to compress the voids. Gravity probably works most of the time...but less at the top of the cast.

 

[rocket_troy]

Vacuum casting doesn't by itself guarantee no voids

It certainly doesn't. I both mix and cast under vacuum and that still doesn't guarantee anything although vacuum processing does allow a greater versatility of propellant ingredients - like "potentially" higher solids loadings varieties. If you're utilising a PU chemistry like most probably are these days with HTPB, you need to ensure your ingredients and mix (in general) is dry of moisture otherwise your curative (diisocyanates generally) will likely react with any H2O it can find to produce CO2 and urea. The urea isn't so much an issue, but the CO2 can create hundreds or thousands (even) of gaseous voids during the curing process. Vacuum processing (in combination with entrapped air removal) can also assist in the removal of volatiles such as moisture.
Also, if you can cast under vac and your mix is runny enough to settle with a sealed surface on top, that sealed surface can act as a pneumatic ram when the casting is exposed to an atmosphere again thereby assisting with the compression of internal (low pressure) voids.

TP

 

[Kelly]

Also, if you can cast under vac and your mix is runny enough to settle with a sealed surface on top, that sealed surface can act as a pneumatic ram when the casting is exposed to an atmosphere again thereby assisting with the compression of internal (low pressure) voids.

Even without a sealed surface, releasing the vacuum exposes the casting to 15psi of pressure on top. Again assuming that the mix is flowable, this pressure should compress any vacuum voids to a very small volume, regardless of where they are in the casting.

 

[Grog6]

This casting is hardening nicely I may be able to cut it into sections tonight.
I added the mix to a used, empty tube of silicone caulk which is cut to match my casing, and extrude the contents of the tube into the casing in one push, which is performed by adding some vent air to the base of the tube, which is then pushed into the tube.

Usually I see a couple of bubbles rise to the top as it's drying, but not this time, it looks solid.

 

[Sandy H.]

I was doing a project involving casting under vacuum completely unrelated to rocketry. As part of the development, I would do various methods and then cut the parts into sections, count bubbles, sizes etc and record everything.

Once I got a good/stable method, I would make a batch using the new method and part of the same batch using the conventional method. After everything was done, I would measure the outside volume of the parts and weigh them with a precise scale. I was easily able to tell the difference in density. From that point forward, I made density measurement the key parameter for accepting the final product and it has worked out very well with minimal destructive testing required.

Not sure if this would be applicable to your pursuits or not, but I was surprised at how well it worked.

Sandy.

 

[rocket_troy]

Even without a sealed surface, releasing the vacuum exposes the casting to 15psi of pressure on top. Again assuming that the mix is flowable, this pressure should compress any vacuum voids to a very small volume, regardless of where they are in the casting.

Well, that very much depends on your binder. When you're pushing the solids content up to and beyond 85ish% you really need that settled surface on top to ensure good effective compression from the re-introduction of an atm pressure, although saying that a good hydrophobic binder/resin combination does help with that a lot.

TP

 

[rocket_troy]

Not sure if this would be applicable to your pursuits or not, but I was surprised at how well it worked.

Absolutely. Everyone casting their own propellant should be checking it for density or more specifically the ratio of measured to theoretical.

TP

 

[Grog6]

Anyone ever put a pressure gauge on a front closure?

I have a 2500lb gauge, and my casing material is rated to 2000.
I was thinking to add it to my test stand.
I also need to add a travel limiter to it; I broke a10kg load cell last week. Tthankfully, it was a surplus buy.

 

[0011001100]

Anyone ever put a pressure gauge on a front closure?

Electric pressure transducer:

Need pressure to be able to properly characterize propellant and get A and N numbers.

 

[Brian H.]

I

Anyone ever put a pressure gauge on a front closure?

I have a 2500lb gauge, and my casing material is rated to 2000.
I was thinking to add it to my test stand.
I also need to add a travel limiter to it; I broke a10kg load cell last week. Tthankfully, it was a surplus buy.

I use a 1ksi transducer connected to an Arduino that logs to an SD card.
Had to adapt the 1/8 npt to 1/4-20 threads for my 54mm Loki forward closure.

If the transducer is connected directly to the closure, make sure fittings are filled with light grease or the transducer will be damaged.