August 22, 2004 notes:
All the electronics parts are in. I was incorrect last week in saying that the vehicle rebuild will
be more expensive than the old one I decided to go with a standard G12 OEM
instead of a G12-HDMA for the GPS, which dropped the price from $10,000 to
$2,500, more than covering all the more expensive machining. Our vehicle just doesnt have the
acceleration to merit the more expensive High Dynamics and Missile
The major parts we are still waiting on are the tank,
machined manway, and the nose cone structures.
We are going to a 15 degree cone instead of a 10 degree cone to make the
vehicle a little more compact at the expense of increased air drag.
We are going to use a dragster parachute as the emergency
drogue for the new vehicle. It wont
slow the vehicle down all that much, but it will make sure that it plows into
the ground nose first, which should allow the thin nose cone to crumple and
absorb quite a lot of energy. We tested
our pneumatic cannon with the chute, and it works fine.
We are planning on using a differential pressure transducer
to measure the propellant level in the vehicle, but unfortunately the sensor we
got just doesnt seem to work at all, so I will probably have to send it back
There are tons of little things that will be improved in the
next vehicle, like making the engine thrust structure and landing gear a single
structure, and using one step smaller tubing for the mounts so we can get a
ratchet on the bolts by the mounting flanges.
One trivial little thing that I only learned recently: I had always been
milling flange o-ring grooves so that the o-ring was flush with the OD of the
groove, because thats where it is supposed to seal. The o-ring would always fall out in handling, so we would use a
little Teflon sealant to glue it in place.
That worked, but the standard (and obvious) solution is to just have the
o-ring be a slight stretch around the ID of the groove, so it holds itself in
place, and it just stretches under load to the other side if the groove is
We have been having a rather frustrating time with our 7
test engine. We want to make sure we
have a solution for the full-throttle instability on the 12 engine, but we
havent been able to make the 7 engine run stable at full throttle yet. We had lots of 5.5 and the first 12 engine
run fine at full throttle, but the current builds start chugging at about half
throttle. We added a couple more pressure
taps and have been collecting more data over several different
modifications. It looks like the key
difference is that the new engines with the 1 900 cpsi monolith and
precompressed hot pack have a reversed set of pressure drops from the old
engines the new ones have less cold pack drop and more hot pack drop. We hope to have this resolved next
week. One thing we did learn was that
our dedicated flameholder plate was completely unnecessary, the water jet cut
support plate for the cold pack was a more than adequate flameholder all by
itself. Removing the extra plate didnt
change anything, and one of our other tests with adding a thin plate below it
showed clear heat effected zones nicely mirroring the water jet cut support
plate structure above it.
The catalyst monolith for our 24 motor arrived. We can make a 16,000 lbf engine now, but it
isnt at the top of our priority list until we get more flight experience with
the 48 diameter vehicle.
The mini-nozzles have been assembled for the next 12
engine. This has slightly more throat
area than the single big nozzle, but is much shorter. We used the thick ¼ support plates on both sides, so this is a
touch heavier than the single nozzle, but our primary reason for using them is
to keep the jet vanes well engaged in flow even at very low chamber pressures.
It took James about two hours to weld them all in place,
thanks largely to having the mini-nozzles machined with built in V-grooves
where they meet the support plates, so the weld went in place very efficiently.
The big question is if the nozzle plate will warp or stretch
significantly after firing the engine a few times.
We have started doing some tests with Haynes 230 alloy (http://www.haynesintl.com/mini/230Site/230.htm)
for possible engine work. This is
probably the best of the superalloys for our type of engines, and would let
us cut the weight of our engines about in half. The advantage of alloys of this class over the refractory metals
like niobium and molybdenum (which have significantly higher melting points) is
that they are inherently oxidation resistant at their operating temperatures
without requiring a coating, and they are also generally easier to form. Entek machined a mini-nozzle from a 2
billet of H230 as a test, and reported that machining costs will be about 4x
the costs for 316 SS. The raw material
cost is about 20x higher, at $40 / lb.
Basically, it looks like it will cost twice as much (catalyst is the
largest cost for a 316 engine) to make an engine half the weight.