Comp pressures

[TSO]

Calling all rocket gurus.

My 10 ton Grizzly Shop press is assembled and in the garage. Now on to figuring out pressures. I'm using the Wolter page http://www.wolterpyrotools.com/?page=rppf as my basis for calculations. They handily have all the common tooling measurements right there for you.

When figuring the area of the piston are we talking the head of the piston, or the main working part of the piston? The Grizzly piston is 1.5" wide but then has a head on the end of it that is 1.75" wide. Needless to say this makes a big difference for the numbers I should be shooting for to reach a certain PSI.

They also have posted PSI pressures you should be shooting for for Whistle comps for both end and core burner engines. My next question is what pressures should you be reaching for other common comps, say for instance a straight BP rocket. Or for pressing BP into pucks, or for pressing comets?

All these numbers are starting to make my head hurt. I suppose I could just get the pressure conversion gauge, but I don't really want to spend the cash if I can figure it out on my own.

Thanks for any info.

[AdmiralDonSnider]

I´m not really an expert here, just a few things:

 

- the value of interest is not PSI (pressure) but the force applied.

- pressing BP cakes: here you press until you reach the desired density. Density is weight per volume, so the weight of the powder and the final volume of the cake are the main variables of interest.

[nater]

I'm far from an expert, but I was wondering the same thing and found my answer on Passfire. If you have access to Passfire, search for a thread titled "Core vs End Burner Loading Pressure." I found it to be extremely helpful to understand what pressures to use with each comp and why. What I took home from that thread was that as you approach the red line for your fuel and casings, you need higher pressure to avoid CATOs.

[NightHawkInLight]

- the value of interest is not PSI (pressure) but the force applied.

The force applied can still be measured in psi, it's just not going to be the same psi as is read from the cylinders oil pressure. If you're pressing a rocket with a .75²inch bore area, whatever overall force is read should be multiplied by .75 to determine the psi the comp is being pressed at. In that way you would know what force is required in psi for that fuel, and be able to know exactly how much force is required for larger and smaller bore engines by multiplying the psi required by the area in inches² of the bore.

[Updup]

The force applied can still be measured in psi, it's just not going to be the same psi as is read from the cylinders oil pressure. If you're pressing a rocket with a .75²inch bore area, whatever overall force is read should be multiplied by .75 to determine the psi the comp is being pressed at. In that way you would know what force is required in psi for that fuel, and be able to know exactly how much force is required for larger and smaller bore engines by multiplying the psi required by the area in inches² of the bore.

 

 

I don't think it would be sqaured, since you arn't pressing sqare rockets. The overall PSI read*(3.14159 * (Rockets ID/2)²) would be more accuate would it not?

 

I could be totally wrong though.

[Mumbles]

The nice thing about the wolter p2f gauge is that it has a 1 square inch piston, so it reads out PSI on comp without any further calculations.

 

Here is the press, just for visualization purposes: http://www.grizzly.com/products/10-Ton-Floor-Shop-Press/H6232 The bench top one looks very similar, just shorter overall.

 

I'm no expert on this either. Perhaps Dagabu or other members with good mechanical skills can correct anything incorrect I am about to say. Use the ram face on your press. No matter what pressure is inside the jack, it is distributed across that 1.75" face. When calculating pressure on your tooling you have to make sure to take into account the surface area of the tooling face. IE cored rammers have smaller surface areas than flat rammers, and thus require less jack pressure to achieve the same force on the comp.

 

I either read that page wrong in the past, or they changed it. I swear it used to say 8800psi for whistle, and 6500 for BP. Then again, I hear people like Steve LaDuke talk about pressing BP to 9000psi. There's lots of hand waving, voodoo, and animal sacrifices involved in making rockets; you can't get hung up on these sort of things that will be part of the tuning anyway.

 

For BP pucks, yes it is a density thing. In very unscientific tests I did probably 5 or 6 years ago, I tested riced BP, and BP pressed to 1.4, 1.7, and 2.0 g/mL. This was density inside of the press, and expansion after pressing was not taken into account. The pucks were broken up, and screened on a 16 mesh screen to remove the fines. No upper bound on the particle size was used (probably a big mistake). I tested 5 shots of each and averaged the flight times of a wooden dowel plug in the side fused mortar. After 25 shots the hole in the mortar was noticeably enlarged. 1.7 was the best, but I think one of the 2.0 one went the highest overall.

 

I've often heard values of 1500psi on the comp for comets. I'm sure it depends on the specific comp, so no real consistent value can be given. Again, testing is in order. Most of the comets I've pressed are pretty firm after pressing, requiring some effort to crush them between your fingers. You could use that as a test. I use a 1 ton arbor press on comets from 3/4" to 1". There is still plenty of power left, the bigger tooling just wont fit into the gap the arbor press base. On the 1" tooling, that would be roughly 2500psi, but there is some expansion after pushing out of the tooling.

[Mumbles]

updup, I think nighthawkinlight just but the squared symbol in the wrong spot. The units on area are squared. If you read the rest of the post it's pretty clear he meant .75 sqin, not .75² (.5625 sqin). Other than that, you just restated pretty much everything he just said.

[NightHawkInLight]

I don't think it would be sqaured, since you arn't pressing sqare rockets. The overall PSI read*(3.14159 * (Rockets ID/2)²) would be more accuate would it not?

 

I could be totally wrong though.

Surfaces that are not square can still be measured in inches². For a circle it's quite simple: http://www.worsleyschool.net/science/files/circle/area.html

 

Edit: Yeah, Mumbles is right, I misplaced my ². It should have been .75 in²

Edited November 7, 2010 by NightHawkInLight

[TSO]

Thanks Mumbles!

As awesome as the gauge would be, I'm sure I can figure out the calcs for this. The only varible I was iffy on was wether to calculate based on the 1.75" on the end of the piston, or the 1.5" of the piston that is actuated in and out the hydraulic cylinder. On 3/4" tooling that a difference of 1628 PSI (1.5" piston) as compared to 1191 PSI (1.75" piston).

So I knew that the pressure gauge was for inside the piston and you had to do the math with the piston surface acting against the tooling face to translate into pressure exerted against the comp. No where did I see a mention of less pressure with core rammers as compared to the flat rammers. It makes perfect sense seeing as there is less suface area on the working end of the core rammer, but how do you calculate those numbers? Not the easiest trick to pull off. Yeah, I know. Test, and trial and error

Thanks for the BP measurements, g/ml I can calculate pretty easily.

[Mumbles]

The cored rammers really aren't too hard to calculate. It's area of the rocket ID minus area of the hole.

 

For instance, lets say you have 3lb tooling (1" diameter), and the first rammer has a hole of roughly .5" It's (pi*(0.5")²) - (pi*(.25")²). a pair of calipers is helpful. It's not one of those things you think about, but it makes sense when someone mentions it. The person who was showing me how to make rockets had me press every rammer to a different pressure on the jack and I had a short epiphany.

[Col]

The cored rammers really aren't too hard to calculate. It's area of the rocket ID minus area of the hole.

 

Hi Mumbles,

There is a hidden twist , the 1st rammer / drift is usually hollow and bevelled at upto 45 degrees. It takes a bit more head scratching to figure out the side area of a flat topped cone

[guntoteninfadel]

I know we are all a bunch of nerds but I think those numbers should be used as guides and testing(as has already been pointed out) taken from there. I know I don't press my riced bp nearly as much as my corned bp. At first I was all about the actual pressure on the comp but I noted the gauge readings and use that now. Sad to say I can't tell you what my comp pressure is but when my gauge hits 3k I'm done because that's where my testing led me to.

[Mumbles]

Certain approximations were made. If you're going to take the truncated cone into effect, you may also have to take into account force vectors. In this case, the gauges really shine.

 

The calculation to find the area of a truncated cone isn't too difficult, but it's one of those things that is easier with a calculator. A pair of calipers will help in getting the slant length or "depth" of the cone, but it can be calculated by geometrical relations too. In this case it's (outer radius - inner radius)*tan(cone angle) = depth, which is the easier of the two to calculate. As a hint if you have a 45 degree taper the height is the same as the difference between the two radii.

 

http://www.onlineconversion.com/object_surfacearea_trunc_cone.htm

 

With the same example I gave earlier for a 1" ID rocket and a .5" starting core diameter. A flat rammer has a surface area of .59 sqin, where as a cone with a 45 degree taper it has an area of 1.81 sqin. Compare this to the area of the flat rammer of .785 sqin. It makes a much larger difference than I thought. It seems odd, but those are the results I got.

[moondogman]

easy cheap p to f guage

 

Check this out It makes all the difference and doesnt cost an arm or leg.

Steve

[Col]

The bevel seems to make a big difference to the area so logically you may need more pressure to compensate. I don`t know anything about calculating force vectors but i guess with the bevel the pressure could be directed more outward than downward? Maybe even less pressure on the comp compared to a flat ended hollow ram of equivalent area?

Thats a great article, Moondogman, they work pretty good

http://i770.photobucket.com/albums/xx341/colinspyro/pressuretoforcemeter.jpg

Edited October 18, 2011 by Col