Monday, February 19, 2007

A New chamber design

I put together a short outline of my vehicle and the safety systems to present to the faa/ast when I was in DC. There are some typos, and a couple of things missing, but its probably worth publishing. astpresnt.pdf.

I have a bunch of new hardware that I will post pictures of in the next few days. I've designed a 250 lb thrust chamber. Its a copper tube with an outer aluminum shell. I've designed the pintile injector and and I built one this weekend. The hard part for a motor this small is keeping the annular alignment between the pintile and the fuel plate the gap is nominally 0.0062" all around. This seems insanely small,(The 25 #69 lox holes in the pintile aren't exactly large) but I did the calculations a bunch of times and it seems to be correct. I machined an alignment step on the pintile and a corresponding recess in the plate that forms the annulus. Goal was to hold the annulus size to +/0.0005" and I think I got it. That is the limit of my measuring equipment.
The alignment recess and step are close enough that if you heat the the plate to slightly warm and cool the pintile to slightly cool they slide together. At the same temperatures they are a tight press fit.

The O-rings I had on hand were not correct to seal in my injector test fixture so I did not get the satisfaction of seeing the injector work. (the o-ring sealed enough for my to try it and get all wet... again.) In the next two days I will be flow testing the injector and I'll post pictures of the hardware.

On the left is a split rendering of the chamber design. To give an idea of scale the inner copper tube has an ID 0f 2 inches. I still have not decided if the throat will be copper or Graphite.
I will try both. The chamber needs to throttle from 80 lbs to 250 lbs. so I set this up to be 50 to 150 % of nominal. Thus the chamber is nominally laid out for 166 lbs of thrust at 166 psi expanding to 12.5 PSI. (the ambient pressure at Las Cruces.) Theoretical ISP at this low pressure is 216 frozen, I'm hoping to realize 180+. The pintile orifices are designed for 32 PSI of pressure drop at the desired flow rate. The orfice sizes were calculated with a discharge coefficient of 1.5. We will see how close I got when I do the flow testing this week. I'll gladly post my notes and spread sheets for all of this, alas I fear they probably won't make any sense to anyone but myself.

I resurrected the lrd (liquid chamber design program ) from the rocketworkbench. I modified it with two additional parameters. water_percent to allow you to add water to your fuel/oxidizer and exit_pressure to set an exit pressure other than 14.7 psi. The program executable is here and an example input is here. If anyone wants the modified source code I'll send you the microsoft visual C 6.0 project with all the files in it. (this also builds cpropep, but the cpropep I built seems to go bboom when I try to run it. I haven't taken the time to figure out why. In the end it turned out easier to use the output from cpropep and a spread sheet.

1 comment:

Anonymous said...

A few questions...

(1) How does the lower disc in the injector stay put? It doesn't seem like the radial force of the o-ring will be enough to keep it in place. I know you said that it is almost a press-fit, but the minute the high pressure fuel fills that region and before there is pressure in the combustion chamber, there will be a great deal of force trying to push that disc out the motor.

(2) Where do your pressurant tanks go in your design? Are they the spherical stainless steel vessels you welded up, or your composite tanks for your pnuematic lander?

--- Carl T.