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John Carmack: Rebuilding, Engine Development

Published by Cathleen Manville on Mon Aug 23, 2004 2:46 am
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From Armadillo Aerospace News, John Carmack updates:

Rebuilding
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 doesn’t have the acceleration to merit the more expensive “High Dynamics and Missile Applications” GPS.

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 won’t 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 doesn’t seem to work at all, so I will probably have to send it back for replacement.

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 that’s 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 sized correctly.

Engine Development
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 haven’t 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 didn’t 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 isn’t at the top of our priority list until we get more flight experience with the 48” diameter vehicle.

http://media.armadilloaerospace.com/2004_08_22/bigCatalyst.jpg

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.

http://media.armadilloaerospace.com/2004_08_22/mininozzleAssembly.jpg

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 columbium (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.

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