Reducing air bubbles in motors.

[Grog6]

I found muchly by accident, that a cake decorating tool
makes a perfect extruder, useful for filling casting casings.
Some additional effort is required to make it match your casings,but adding the degased mixi nto the extruder'
is better than trying to degas the loaded casing tube.
If this

 

[Brian H.]

Whatever works for you, however, experimental belongs in the research forum. Not a big issue getting access. Simple request to moderators.

 

[Grog6]

I don't live close enough to a club to get acredited, but I'm an engineer, and have been flying rockets since the 60's.

I'll request access to the research forum, I have a few things I can contribute.

 

[jimzcatz]

you degas BEFORE you fill the casting tube. Straight from the mixing bowl. How do you even do it in a filled tube?

 

[Grog6]

In the same chamber; the final degas makes sure you didn't add air bubbles putting it in the tubes. I have a Varian that will go to 150 millitorr.

 

[jimzcatz]

We just draw a vacuum on the mixing bowl. Whatever works is key here

 

[rocket_troy]

Jim,
I've been told that what Grog6 is suggesting is (to some extent) industry standard practice for degassing larger motors ie. extrude the mixed propellant into the casing directly under vac. Whether that's extruded through a head of multiple small nozzles or some other geometry varies with manufactures.

TP

 

[Brian H.]

I could see extruding under vacuum working very well, however, I am having a hard time picturing a good setup to do this.

 

[jimzcatz]

It is, I agree. But we are not mixing large amounts of propellant.

 

[rocket_troy]

I agree and I should've caveated what I said with a general agreement with the point you're making ie. it's not necessarily practical with HPR scale batches.

TP

 

[Brian H.]

I have a fully characterized propellant that has worked reliably and predictably for 5+ years using repeatable manufacturing procedures without any type of vacuum degassing. The only reason I would consider adding degassing is to improve ISP, which would also involve re-characterization. I'm not a performance seeker, so current methods are acceptable.

I am not saying increased ISP, performance gains, and possible reliability improvements are bad. They just do not fit my criteria right now.

 

[rocket_troy]

We generally vacuum degas to increase *density* - which is also an important contributor to performance as well as (obviously) reliability . Vacuum degassing can influence ISP via more steady combustion (nozzle efficiency), but it's a secondary influence. It's very rarely the primary reason to vacuum process.

TP

 

[Brian H.]

If a propellant is characterized with included "air bubbles", is reliable, and repeatable, the only reason I can see to de-gas is ISP performance.

 

[rocket_troy]

Put it this way, say your delivered (measured) density is 5% under theoretical - by increasing that delivered density to 100% (via vac processing or whatever means) you can expect a 5% increase in *volumetric* performance ie. 5% total impulse increase from a unit volume of propellant and a 1-2% increase in Isp from increased nozzle efficiency (at sea level). Yes, there are probably some instances (smaller motors) where ISP might increase more with steadier combustion and yes ISP generally does have a more significant contribution to the rocket equation, but also the *effect* the increase in nozzle efficiency has is reduced with altitude or lower ambient pressure conditions whilst the engine is operating,

TP

 

[Brian H.]

Put it this way, say your delivered (measured) density is 5% under theoretical - by increasing that delivered density to 100% (via vac processing or whatever means) you can expect a 5% increase in *volumetric* performance ie. 5% total impulse increase from a unit volume of propellant and a 1-2% increase in Isp from increased nozzle efficiency (at sea level). Yes, there are probably some instances where ISP might increase more and yes ISP generally does have a more significant contribution to the rocket equation, but also the *effect* the increase in nozzle efficiency has is reduced with altitude or lower ambient pressure conditions whilst the engine is operating,

TP

I agree 110% reaffirming my belief that current production procedures are sufficient without de-gassing.

Edit: Unless you are seeking absolute performance from a propellant.

 

[rocket_troy]

If you can obtain acceptable reliability without vacuum processing, then absolutely - we're all in complete agreement

TP

 

[Brian H.]

If you can obtain acceptable reliability without vacuum processing, then absolutely - we're all in complete agreement

TP

The key to success is repeatable results from repeatable production. Any change to production affects multiple aspects of results.

Your ultimate goal, along with acceptable side-effects, will affect how you decide what is important when designing a propellant.

 

[Blast it Tom!]

I got nuthin', you guys are way over my head. But @Grog6, welcome! This stuff sounds fascinating. 150 millitorr? Yow! (yes I avoided making a crude comparison to a living VP... oops, wait, no I didn't!)

On the other hand, my wife wishes I'd find alternate methods of degassing!
I'll get my coat... I can see myself out, thanks...

 

[Grog6]

I rebuilt a real roughing pump for a vacuum system that I got from ebay. these are normally ~1200 bux,
I built my chamber out of a flux-off can and a sheet of rubber glued to an aluminum plate.
It pulls bubbles out of RTV like crazy.

 

[Blast it Tom!]

Well, all kidding aside, that takes some skilful work. 14.7 psi builds up a lot of force on a cylinder before too many square inches build up!

 

[Grog6]

A 5 gallon plastic bucket from lowes Does not work, lol.
But the fail was spectacular.

 

[Grog6]

Can I post pix of the setup without getting banned?

 

[G_T]

Vacuum levels aren't exactly always the more the better. Depends. Not a subject for here though I suspect.

 

[rocket_troy]

But the fail was spectacular

I've never had a vacuum chamber implode, but I know someone who had - 6" PVC pipe about 1m in length. Very spectacular indeed. Certainly gave him a wake up and the surrounding neighbourhood.

TP

 

[Blast it Tom!]

I've never had a vacuum chamber implode, but I know someone who had - 6" PVC pipe about 1m in length. Very spectacular indeed. Certainly gave him a wake up and the surrounding neighbourhood.

TP

Yes, and it happens at a lower pressure than what corresponds to the compressive yield strength of the material. Usually these are relatively thin rings, but even if they are heavier you have to consider the buckling of the ring, not simply its compressive strength. Under too much vacuum, it buckles into an elliptical shape, or if the material is too brittle, fractures in a spectacular fashion.

 

[rocket_troy]

Also, high vacuum implosions can result in significant shocks as the incoming air (filling the volumetric void) can be accelerated supersonically very quickly hence the famous ping-pong ball vacuum cannon. I can imagine this would be a significant contributor to the often spectacular consequences of a high vac implosion.

TP

 

[Grog6]

I saw a glass bell jar fail, once, at work.
We used it for degassing rtv.
We found pieces of glass 50 feet away.

 

[Alan15578]

I've never had a vacuum chamber implode, but I know someone who had - 6" PVC pipe about 1m in length. Very spectacular indeed. Certainly gave him a wake up and the surrounding neighbourhood.

TP

Back in 73 or so I built a vacuum forming machine. Being poor and stupid I used a steel drum for the vacuum reservour, instead of an expensive pressure vessel. Actually it was a 30 gallon or so grease barrel. To save space and suppress noise, I mounted the vacuum pump inside the barrel. I did not even have a gauge on it, but I started pulling some parts and just used a watch to estimate the vacuum pressure. Then I started increasing the time, because a guy just has to know...
The failure was stunning, but did not even crack nearby glass. My father came running in, in his underwear, expecting to find part of the house blown away. I had to have the vacuum pump professionally repaired. I replaced the reservoir with a stouter standard 55 gallon steel drum, kept the pump outside the barrel and added a vacuum gauge. I still have it, but I would never suggest it should be replicated! I think 55 gallon steel drums are pressure tested to 7 PSI, but I have attained about -10 PSI without failure.

 

[Grog6]

My chamber is thick steel one gallon container; It fits a painter's cup perfectly.
I try to reuse as much as possible.

I have crushed a 55 gal drum like a beercan, lol,I worked at a place that did leak finding machines, using helium.
We had a "Drum machine".
One of the sequences got messed up, and it vented the outside, while the inside was at about 1 millitorr.
It was about a foot tall coming out, perfect little triangles...

 

[Grog6]

I have a lot of respect for physics. Gravity alone has taught me many lessons.
And I can say particle physics has too.