I'm just about done with the machining of the 250 lb engine. The only item left is the little cap
that fits over the bottom of the chamber to close out the cooling holes. I've
posted some new photos of the finished parts.
Progress is steady but slow given the lack of available time to work in the machine shop. I've switched
to using SAE straight O-ring fittings (SAE J1926/MS16142) in the hopes I can get away with fewer stripped
threads than I saw with the NPT fittings in aluminum. The port tools were expensive (~$130 ea) but the
results are really nice - very smooth and precise.
I still have a bit of hand work to do on the parts (debur the internal holes with sanding cord, install the
Cv expansion plugs, etc.).
After that are some retrofits to the test stand - a new chamber-load cell mount and larger tanks. I'll probably
do the cold flow tests with the existing setup however in the next month or so.
I've mostly finished with the chamber - photos are here. The
items remaining on the chamber are to drill the fuel inlet and outlet holes, pressure tap hole, and
thermocouple holes. Oh, and of course, seal the cross drilled holes at the start of the converging portion.
I've had a lot of trouble deciding the best way to seal the cross drilled holes. I had originally planned
on using SeelSkrews from APM Hexseal but I messed
up the design and didn't leave enough room for the fastener head. I considered
Lee Plugs but they are $4 ea and the
minimum order quantity is 100 (all I need is 12 plus some spares). I then purchased some
Expansion Cv Plugs but I've had a lot of trouble
getting the ball seated on my scrap piece. I'm using the hand install tool and even after beating the
heck out of them with a hammer, they don't want to seat properly. I was able to get it to work by drilling an
oversized hole (size F drill). The hole size tolerance for the 0.250 inch
plugs is 0.250 - 0.254. I wasn't able to make a 0.254 inch hole as all three 1/4 inch drill bits I possess
cut a hole around 0.252. So, I expect the installation force to go down somewhat if I can make a slightly larger
hole. A 6.4 mm drill should produce a hole around 0.254 or 0.255. If I can't make the Cv plugs work, I'm
prepared to modify a shoulder screw to make a removable plug seal. Actually the shoulder screw would work great
but would require me to create 12 special little parts.
I also spent yesterday and part of today fixing my lathe chuck. I had a little "incident" involving a parting tool while
cutting the chamber. No, I didn't hit the chuck with with anything but the parting tool dug into the part which
broke the blade and yanked the chamber out of the lathe chuck. I was really surprised an 8 inch chuck would let
the part loose like that. Luckily, the chuck marks left behind on the chamber were easily with some turning.
Afterwards, everything seemed to work but the chuck jaws took a lot more effort to
scroll. After taking the chuck apart, the 3rd jaw was really hard to move and it looked like the semi-steel chuck
body must have deformed a small amount. I put the chuck on my mill and took off 0.002 or so and it is back to
normal just with a little more slop than usual.
If all goes well, I hope to make the injector next weekend. Then I need to cut all the "special" features such
as fluid and instrumentation ports, etc.
The detailed design of the new 250 lbf chamber has been completed.
Designing in the O-ring seals took a bit more effort than I anticipated. I also ended up using a 12-pass cooling tube
instead of the 10-pass tube I was planning so the pressure drop is also higher than I had hoped. My estimate of the
pressure drop through the cooling jacket is about 300 psi which will keep the maximum fuel temperature at around 430 degF.
I'll post the thermal analysis estimates when I get a chance.
In the hopes that the rocket powered by this engine will
get some altitude, I designed the engine to have optimum expansion at 15000 feet. If I did the calculations
correctly, it should provide about 270 lbf at sea level (assuming no separation), 250 lbf at 15000 ft,
and 238 lbf at 30000 feet. The total engine weight not counting fasteners should be around 5.2 lbm but I should
be able to make some additional cuts and easily get it down to about 4.5 lbm.
I picked up the 5 inch diameter 6061-T6 bar stock last week and I'm just waiting on a new boring bar before
I start making chips. I'm trying some new cutting inserts this time (TCGT with AK chipbreaker) which are supposed
to work great with aluminum. I actually was considering a slightly larger flange than 5 inches on the top of the
chamber when I realized
that 5 inches was the max diameter I can turn on my lathe using the inside jaws. The outside jaws can turn much larger
but the chamber is fairly long so I want to have a good grip on it.
Instead of using NPT fittings, this time I'm using SAE O-ring ports. The only problem is
that the port reamers are very expensive (~$200 each!) but this should allow me to avoid stripping out the
aluminum threads with the stainless steel fasteners.
The test stand will need some upgrades to support a 30-40 second run time on this engine in addition to the higher
thrust. The load cell has a maximum capacity of 500 lbf but I need to add some linear slides or something similar to
keep the engine from being cantilevered out so much. I'm also considering adding a cascaded purge system to blow out
the extra propellants after each run.
I've been working on the design for the next engine
the past few months. It will be a
200-250 lbf regen engine with LOX/Kerosene as the propellants, essentially the
same design as the 100 lbf engine I've been testing but with some changes (O-rings for seals
instead of Grafoil, etc.). One of the parameters I wanted to determine from the 100 lbf regen
tests was the gas side heat transfer coefficient so I could use it to correct my design equations.
I've done that and used an hg correction value (x1.45) for the design of the new one but I'm wondering
whether I'm using the proper methodology to determine it. I'll go over the method
I used when I get a chance to write it up.
Since I plan to use this new engine to fly in a rocket, I'm trying to decide on the exact
thrust level I need. I don't have a lot of empirical data yet for empty weights of the
various parts in a vehicle but I suspect the empty weight will be heavier than I expect.
The vehicle will be designed for a 30-40 second burn time so I'm hoping the new amateur rocketry
regulations get approved in the next year or so which lift the 15-second burn time limit.
I finally got around to updating the website. So far, no new content, just a rearrangement
to make things easier to sort out. The old site was all maintained by hand and there
were rendering problems with some browsers so it was time for a change. I'm using a
nifty package called WebMake which is a set of Perl
scripts that implement a simple Content Management System (CMS). I looked at other tools such
as Mambo/Joomla but they were way overkill for what I need to do. I did like the look and
feel of the default Joomla template so I decided to use the template portion of it here ;)
All the content *should* be in the same place so if you run across a broken link that used
to work, email me and I'll fix it.
Getting back to rockets, I've been trying to decide on a flight vehicle configuration so
I can size my next engine. The intention is to take the next engine, put it in a rocket
and fly it. I'm looking at something in the 200 lbf range but I need to develop a good
empty mass model for the vehicle so I can vary the design parameters and see what size
engine I need.